DPhil Projects - Research Projects 2018

Informal enquiries may be directed by email to the relevant potential supervisors. If project studentships are available, they are listed under the individual project.

Condensed Matter Physics is also associated with the following DPhil programmes:

DPhil Systems Approaches in Biomedical Sciences (EPSRC & MRC CDT)
DPhil Systems Biology (EPSRC CDT)
DPhil New and Sustainable Photovoltaics (EPSRC CDT)
DPhil Science and Application of Plastic Electronics (EPSRC CDT)
DPhil Synthetic Biology (EPSRC & BBSRC CDT)

If you are interested in the DPhil New and Sustainable Photovoltaics (EPSRC CDT), you can find more detailed information under the respective projects below and here.
And if you are interested in the DPhil Science and Application of Plastic Electronics (EPSRC CDT), you can find more detailed information under the respective projects below and here.

Dr Sonia Antoranz Contera

Magnetically controlled polymeric materials for biomedical applications

We are developing magnetically controlled systems for applications in e.g. cell culture and drug and cell delivery applications and to act as force transducers to interact with living systems.

Electromechanical coupling in neurons

The EPSRC funded Multidisciplinary programme NeuroPulse aims at developing and utilising state-of-the-art modelling approaches for the study of electrophysiological and mechanical coupling in a healthy and mechanically damaged axon, nerve and, eventually, spinal cord and brain white-matter tract. Collaboration with Prof Antoine Jerusalem (Engineering Science Oxford). Two teams of clinical project partners in Oxford and Cambridge will participate in the analysis of the results for direct applications in a clinical setting.

Physical aspects of cryopreservation of living systems

During cryopreservation, cells (e.g. stem cells, sperm, blood), tissues (ovarian tissues, umbilical cord, seeds, plants, fruit), and living organisms (bacteria, animal embryos) are preserved by cooling to sub-zero temperatures. A significant challenge of cryopreservation is to avoid damage caused by the formation of ice crystals during freezing. It is known that when cells are frozen they undergo a ‘‘cold shock'' which leads to mechanical damage to the plasma membrane and leakage of solutes across membranes. This project addresses the fundamental physics of ice formation and the role of biological interfaces and aims at the development of physical methods to increase the viability of frozen living systems. This is part of a new multidisciplinary collaboration OxfordCryo that puts together researchers from Physics, Mathematics, Engineering, Oxford University hospitals Zoology, Philosophy and Law departments, and external partners.

Prof Arzhang Ardavan

Quantum Coherent Phenomena in Molecular Magnets


Figure: A two-qubit device built by tying together individual molecular magnets in a controlled way, with the aim of generating entangled states and hosting simple quantum algorithms.

Molecular magnets are a class of zero-dimensional strongly correlated electron systems exhibiting a highly coherent quantum spin at low temperatures. By varying the molecular structure, properties of the spin such as its moment, anisotropy, etc., can be manipulated in a controlled way, offering a beautiful playground for experiments in quantum magnetism.

This project will investigate ways of exploiting the quantum properties of molecular magnets with a range of objectives. Through close collaborations with chemists who synthesise the materials, we will design structures using molecular spins as qubit candidates. In simple multi-qubit structures, we will use state-of-the-art electron spin resonance equipment to generate entangled states and perform basic quantum information algorithms. We will also develop new ways of measuring electrical transport through single molecules using innovative methods for device construction, and scanning tunnelling microscopy (via collaboration with IBM).
Email Prof Arzhang Ardavan at: arzhang.ardavan@physics.ox.ac.uk.

Dr Richard Berry

Biological Rotary Molecular Motors


Nature DID invent the wheel, at least 3 times! The bacterial flagellar motor is 50 nanometres across, spins at over 100,000 r.p.m. driven by electric current, and propels swimming bacterial cells. ATP-synthase is even smaller, about 10 nm across, consists of two rotary motors coupled back-to-back, and generates most of the ATP - life's "energy currency" - in most living organisms. We are trying to discover how these living machines work. We develop and use a range of methods in light microscopy including ultra-fast particle tracking, magnetic and optical tweezers and single-molecule fluorescence microscopy. Current projects in the lab: measurement of stepping rotation and mechano-sensing in flagellar motors, synthetic biology using ATP-synthase,single-molecule fluorescence microscopy of protein dynamics in bacterial motility and chemotaxis, high-torque magnetic tweezers, templated assembly of flagellar rotors using DNA nanotechnology scaffolds (collaboration with Turberfield group).
More ...

Email Dr Richard Berry at: r.berry1@physics.ox.ac.uk

Prof Stephen Blundell

Muon-spin rotation studies of correlated magnets and superconductors

Implanted muons give a unique local perspective on quantum materials. By measuring the rotation of the implanted muon spin in the local magnetic field, which in turn results from the dipolar field arising from nearby magnetic moments, one can follow the magnetic order parameter as a function of temperature. If there are a range of muonsites the technique can provide information about the internal magnetic field distribution. Even above the magnetic transition temperature, the measured relaxation of the muon spin can provide information about magnetic fluctuations and spin dynamics. In superconductors the technique is particularly valuable because the measured internal field distribution can allow us to infer the pairing mechanism. In iron-based superconductors, for example, competition between magnetism and superconductivity can be studied very effectively with this technique. However, there is a fundamental limitation of this technique which comes from the lack of knowledge of the implantation site of the muon and the uncertainty about the muon's perturbation of its host. We have found that this problem can be addressed using electronic-structure calculations using a technique we have developed which we call "DFT+mu", density functional theory with an included muon. This D.Phil. project will involve experiments using muons on new magnets and superconductors, as well as investigations with the DFT+mu method. There will also be collaborative work on
advancing the technique which will be carried out with the nearby Rutherford Appleton Laboratory who operate the world's most intense beam of pulsed muons.

Email Prof Blundell at: stephen.blundell@physics.ox.ac.uk.

Prof Andrew Boothroyd

Novel Electronic Order and Dynamics in Crystals


Systems of interacting electrons frequently exhibit subtle forms of order, examples being superconductivity and magnetism. In this project you will investigate electronic order and dynamics experimentally. Neutron and X-ray scattering will be the main techniques used, taking advantage of the new ISIS 2nd target station and the Diamond Light Source, plus other facilities in Europe. A willingness to travel is essential! Magnetic, transport and thermal measurements will be performed in the Clarendon Laboratory, and there is scope for theoretical modelling and numerical analysis. More...
Email Prof. Andrew Boothroyd at: a.boothroyd@physics.ox.ac.uk

Prof Donal Bradley

Dr Yulin Chen

Our group's research interest lies in experimental condensed matter physics: and specifically in understanding the behaviour of electrons in unconventional materials. Recently, we have focused our research activities in the following two directions:

I. Topological quantum matters
a) Topological insulators with better and richer properties
b) Topological Dirac and Weyl Semimetals
c) Topological superconductors
d) Low dimensional thin film and nano-scale topological quantum materials
e) Exploring application potentials of topological quantum materials

II. Research on complex materials with strongly correlated electron system
a) Unconventional superconductors
b) Colossal magneto-resistance (CMR) and other extremely large magneto-resistance (XMR) materials.
c) Heavy fermion systems

In addition, we’re also interested in developing advanced instrumentation that will drive the exploration of critical information on condensed matter systems with new degrees of freedom, such as: Lab, synchrotron and free electron laser based Spin-, time- and angle-resolved photoemission spectroscopy.

For more information of our recent research, please visit our website:
http://www.arpes.org.uk/Publication.html

Topological Insulators - A New State of Quantum Matter


Figure 1: Surface conduction of topological insulators (A) The spin of electrons on the surface is correlated with their direction of motion (B) The lattice structure of Bi2Te3 and the predicted relativistic "Dirac cone" like electronic structure formed by the surface electrons. (C) The electronic structure measured by angle-resolved photoemission (Figure generated from data in Ref(1)) that confirmed the theoretical prediction and the topological nature of Bi2Te3.

Topological insulators represent a new state of quantum matter with a bulk gap and odd number of relativistic Dirac fermions on the surface (Fig. 1). The bulk of such materials is insulating but the surface can conduct electric current with well-defined spin texture. In addition, the relativistic energy-momentum relationship of electrons in these materials provides a great opportunity to study the physics of relativity in a condensed matter system with the velocity of massless particles about 200 times slower than the light speed in vacuum. The unique properties of the topological insulators make them great candidate for energy and technology applications.

Unlike other materials where the fragile surface states can be easily altered by details in the surface geometry and chemistry, topological insulators are predicted to have unusually robust surface states due to the protection of time-reversal symmetry. These unique states are protected against all time-reversal-invariant perturbations, such as scattering by non-magnetic impurities, crystalline defects, and distortion of the surface itself, and can lead to striking quantum phenomena such as quantum spin Hall effect, an image magnetic monopole induced by an electric charge, and Majorana fermions (whose anti-particle is itself) induced by proximity effect from a superconductor.

Extracting the electronic and structural properties of topological insulators is essential for both the understanding of the underlying physics and potential applications. As a direct method to study the electron band structures of solids, ARPES can yield rich information of the electronic bands of topological insulators, as demonstrated in our recent results on the realization of the large gap single Dirac cone topological insulator Bi2Te3(Ref 1), the insulating massive Dirac fermion state (Ref. 2), and a topological superconductor candidate (Ref.3). For more information, please visit http://www.arpes.org.uk/

[1] Y. L. Chen, et. al., “Experimental Realization of a Three Dimensional Topological Insulator, Bi2Te3” Science, 325, 178 (2009)
[2] Y. L. Chen, et. al., “Massive Dirac Fermion on the Surface of a magnetically doped Topological Insulator” Science, 329, 659 (2010)
[3]. Y. L. Chen, et. al., “Single Dirac Cone Topological Surface State and Unusual Thermoelectric Property of
Compounds from a New Topological Insulator Family”
, Physical Review Letter, 105, 266401 (2010)

Email Dr Yulin Chen at: yulin.chen@physics.ox.ac.uk

Dr Amalia Coldea

Using High Magnetic Fields to Probe Unconventional States of Matter in Novel Superconductors and Topological Insulators


Understanding the behaviour of novel complex materials requires direct experimental access to their fundamental electronic properties. One such powerful method is to look directly at the quantum behaviour of electrons though quantum oscillations studies in high magnetic fields. A combination of highly sensitive experiments and predictions given by band structure calculations will aim to understand the newly discovered iron-based superconductors and the surface states of topological insulators. This knowledge will advance and help to predict future superconducting and multifunctional materials. The project will consist in using and developing highly sensitive tools to investigate micro-size single crystals and thin films. Thermodynamic and magnetotransport measurements will be performed in extreme conditions of high magnetic fields and low temperatures both in the Clarendon Laboratory and using high magnetic field facilities in Europe and the USA.
Email Dr Amalia Coldea at amalia.coldea@physics.ox.ac.uk

For more information and specific projects please follow this link

Further queries about these projects should be directed to Dr Amalia Coldea at amalia.coldea@physics.ox.ac.uk

Prof Radu Coldea

Quantum Magnetism and Quantum Phase Transitions

We explore experimentally materials where quantum correlation effects between electrons are important and often lead to novel forms of electronic order or dynamics dominated by quantum effects. Of particular interest is the phenomenon of "quantum frustration", i.e. how quantum systems resolve competing interactions, this is explored in frustrated spin-, orbital- and charge-ordered systems. Another focus is "quantum criticality" when the transition temperature to spontaneous magnetic order can be suppressed by high magnetic fields all the way down to zero temperature, thus realizing a regime where all ~10^23 electron spins in the material fluctuate strongly, but in perfect unison, a new regime for quantum matter that is only now becoming accessible experimentally and we plan to measure directly the quantum spin fluctuations via neutron scattering. The DPhil project will involve a mix of thermodynamic measurements, xray and neutron scattering experiments, data analysis and modelling.
More...
Email Prof Radu Coldea at: r.coldea@physics.ox.ac.uk

Prof John Gregg and Dr Alexy Karenowska

Magnon Spintronics

Magnons are the quasi-particles associated with electronic spin waves.
In certain magnetically ordered materials they are able to play the role of spin angular momentum carrying tokens which can be generated and transmitted over long distances (up to centimetres) and at high speed (many tens of kilometres per second).

A quartet of effects: spin-transfer torque, spin pumping, the spin-Hall effect, and the inverse spin-Hall effect permit the interconversion between magnon fluxes, spin currents, and charge currents. This functionality paves the way for magnon spintronics: the development of structures and devices which combine the rich physics and long-range spin angular momentum transfer capabilities of magnon systems with the established toolbox of conventional electron-based spintronics. However, the exploration of magnon spintronic systems is still very much in its infancy: improving our understanding of their underlying physics and its device applicability is essential if we are to exploit their potential to the full.

This project will combine investigations of fundamental aspects of magnon/spin/electron current interconversion in metallic and insulating magnetic systems with the development of new and exciting prototype magnon spintronic devices.

Email: Prof John Gregg and Dr Alexy Karenowska at:
j.gregg1@physics.ox.ac.uk and A.Karenowska@physics.ox.ac.uk

Prof Laura Herz

Charge generation dynamics in novel materials for solar cells

PVproject2_0.png

Metal halide pervoskites have emerged as an extremely promising photovoltaic (PV) technology due to their rapidly increasing power conversion efficiencies (PCEs) and low processing costs. Surprisingly, many of the fundamental mechanisms that underpin the remarkable performance of these materials are still poorly understood. Factors that influence the efficient operation of perovskite solar cells include electron-phonon coupling, charge-carrier mobility and recombination, light emission and re-absorption, and ion migration.

During this project we will advance the efficiencies of perovskite solar cells by gaining an understanding of fundamental photon-to-charge conversion processes using a combination of ultra-fast optical techniques, e.g. photoluminescence upconversion and THz pump-probe spectroscopy. These studies feed directly into collaborative efforts aimed at addressing remaining challenges in the in the creation of commercially available perovskite solar cells e.g. stability, band-gap tunability, lead-free perovskites, trap-free materials, material morphology control and alternative device structures. The project will be part of active collaboration with researchers working on solar cell fabrication within Oxford and the UK.

Applications for this project can be submitted through the University's Postgraduate Admissions Programme to the Department of Condensed Matter Physics - see http://www.admin.ox.ac.uk/postgraduate/apply/ for more information. Informal enquiries may be directed by email to Prof. Laura Herz at l.herz@physics.ox.ac.uk. Some useful information on funding for International Postgraduate Students may be found here.

Transitions from quantum confined to fully delocalized electronic states in semiconductor nanocrystal assemblies

Nanocrystal_0.jpg

The last decade has seen rapid progress in the fabrication and assembly of nanocrystals into thin layers of semiconducting material. Such systems may allow facile deposition of high-quality inorganic semiconductor layers through simple and scalable protocols such as ink-jet printing. However, these procedures raise fundamental questions on the nature of charge transport through such layers. While in sufficiently small nanocrystals, quantum confinement leads to the formation of discrete electronic layers that may exhibit "atom-like", energetically discrete states, increasingly electronic coupling between nanocrystals may induce the formation of mini-bands or bulk-like continuum states.

In this project, we will explore such transitions between fundamentally different regimes of electronic coupling and charge transport. We will spectroscopically investigate nanocrystal networks made of established lead chalcogenide inorganic semiconductors, but also explore more recently developed metal halide perovskite colloid materials. These studies will be interesting not only from a fundamental point of view, but also allow for development of such systems in light-emitting photovoltaics or transistor devices.

Applications for this project can be submitted through the University's Postgraduate Admissions Programme to the Department of Condensed Matter Physics - see the official Graduate Application Guide for more information. Informal enquiries may be directed by email to Prof. Laura Herz at laura.herz@physics.ox.ac.uk. Some useful information on funding for International Postgraduate Students may be found here.

Energy and Charge Transfer in Biomimetic Light-Harvesting Assemblies

Nanorings2_0.png

Photosynthetic organisms use arrays of chlorophyll molecules to absorb sunlight and to transfer its energy to reaction centers, where it is converted into a charge gradient. These processes are remarkably fast and efficient, because the excited states are coherently delocalized over several chlorophyll units. For natural scientists striving to create new molecular light-harvesting materials for applications such as photovoltaics, the designs nature has invented for us are fantastic templates to learn from. This project will explore energy transfer within and between large porphyrin nanorings that directly mimic natural light-harvesting chlorophyl ring assemblies. By creating interfaces with electron-accepting molecules we aim to create light-harvesting layers that rival their natural counterparts in photon conversion efficiency. This project offers exciting possibilities for work in a new interdisciplinary area of research in collaboration with Prof Harry Anderson at the University of Oxford.

This project allows the exploration of physical phenomena in the increasingly popular area of solution-processed and nanostructured semiconductors, and offer a high degree of training in the elegant and versatile techniques of femtosecond optical spectroscopy.

Applications for this project can be submitted through the University's Postgraduate Admissions Programme to the Department of Condensed Matter Physics - see the official Graduate Application Guide for more information. Informal enquiries may be directed by email to Prof. Laura Herz at laura.herz@physics.ox.ac.uk. Some useful information on funding for International Postgraduate Students may be found here.

Prof Laura Herz with Prof Michael Johnston

Pump-push-probe spectroscopy for identification and elimination of trap states in hybrid perovskites

Hybrid metal halide perovskites are promising semiconductors for next-generation solar cells now achieving power conversion efficiencies in excess of 22%. However, perovskite cells currently still suffer from sub-bandgap trap states that can act as non-radiative recombination centres, reducing device efficiencies. While the presence of traps in these materials has been inferred from charge-carrier recombination kinetics, their causes and nature are still largely unknown. Computational simulations have predicted the energies of specific point defects, however, matching experimental evidence for specific trap depths is more elusive.

This project will address this issue by implementing a new spectroscopic technique, optical-pump-THz-probe-IR-push transient photoconductivity spectroscopy, to monitor detrapping processes that are stimulated by a short laser pulse whose photon energy is matched to the trap depth. Standard optical-pump-THz-probe techniques already in operation will be extended by a third pulse of tunable photon energy that is used to "push" charges from trap states back into the conduction or valence bands. We will build on the resulting trap identification to develop processing protocols to remove specific hurdles to the adoption of perovskite as photovoltaic light-harvesters. We will target lead-free tin perovskites that currently exhibit dominant defect-related charge recombination, and mixed-halide perovskites for silicon tandems that suffer from trap-mediated halide segregation under illumination.

This project is supervised by Prof Laura Herz and Prof Michael Johnston in association with the EPSRC Centre for Doctoral Training in Plastic Electronics for start in October 2018.

Enquiries may be directed by email to Prof Laura Herz at laura.herz@physics.ox.ac.uk and Prof Michael Johnston at michael.johnston@physics.ox.ac.uk

Prof Thorsten Hesjedal

Topology treats higher-dimensional geometrical properties of matter that cannot be captured by symmetries.

Nowadays, condensed matter physicists become more and more aware of the fundamental implications of a material’s topological properties. The largely unexplored magnetic skyrmions carry rich topological physics and hold the promise of future applications in information technology.

This is an experimental project dedicated to the exploration and study of novel, low-dimensional skyrmion-carrying materials. It is part of the UK-wide, EPSRC-funded national research program into Skyrmionics, designed to achieve a step-change in our understanding of skyrmions in magnetic materials and engineer them towards application. A detailed description of the research topics and institutions involved can be found at http://www.skyrmions.ac.uk.

Email Prof Thorsten Hesjedal at: thorsten.hesjedal@physics.ox.ac.uk

Recent group publications on Skyrmionics:
[1] Zhang, S. L.; Bauer, A.; Berger, H.; et al., Resonant elastic x-ray scattering from the skyrmion lattice in Cu2OSeO3, Phys. Rev. B 93, 214420 (2016)
[2] Zhang, S. L.; Bauer, A.; Burn, D. M.; et al, Multidomain Skyrmion Lattice State in Cu2OSeO3, Nano Lett. 16, 3285-3291 (2016)
[3] Zhang, S. L.; Chalasani, R.; Baker, A. A.; et al., Engineering helimagnetism in MnSi thin films, AIP Adv. 6, 015217 (2016)
[4] Lancaster, T.; Xiao, F.; Salman, Z.; et al., Transverse field muon-spin rotation measurement of the topological anomaly in a thin film of MnSi, Phys. Rev. B 93, 140412 (2016)
[5] Lancaster, T.; Williams, R. C.; Thomas, I. O.; et al., Transverse field muon-spin rotation signature of the skyrmion-lattice phase in Cu2OSeO3, Phys. Rev. B 91, 224408 (2015)
[6] Zhang, Shilei; Baker, Alexander A.; Komineas, Stavros; et al., Topological computation based on direct magnetic logic communication, Sci. Rep. 5, 15773 (2015)

Angle-Resolved Photoelectron Spectroscopy (ARPES) Studies on Novel, Functional Materials Grown by Molecular Beam Epitaxy (MBE)

Angle-Resolved Photoelectron Spectroscopy (ARPES) is one of the key techniques for studying strongly correlated metallic materials. It can directly visualise the electronic band structure and Fermi surface, measure electron correlations and interactions with quasiparticles, such as phonons and magnons in the material. ARPES is extremely successful for the study of layered materials, where the required atomically clean surface is produced by cleavage and beamline I05 at Diamond Light Source (http://www.diamond.ac.uk/I05) is one of the best stations worldwide for such research (for a list of recent high-profile publications see http://www.diamond.ac.uk/Beamlines/Surfaces-and-Interfaces/I05/Publicati...).

A large number of highly relevant functional materials are, however, not layered in their nature, and the best quality samples can be grown by molecular beam epitaxy (MBE) and its variation of hybrid-MBE for oxides. In this case the samples have to be moved from MBE to the ARPES measurement station all in ultrahigh vacuum (UHV). This project makes use of the fully commissioned mu-MBE instrument to grow samples [1], and of beamline I05 to measure ARPES data from them. This facility has recently been fully commissioned and has proven its worth in conjunction with ARPES in studies of rock-salt structure doped EuO at variable doping [2].

Currently the mu-MBE is being expanded by a heated vapour inlet to a hybrid MBE system for the preparation of high quality (and high mobility) TiO2 films and related ternary compounds. Ultrathin films of Strontium Titanate (SrTiO3), homoepitaxially grown on SrTiO3 substrates with a highly doped intermediate layer, are already a fertile playground for interesting physics as the surface of these films inevitably develops a two-dimensional electron gas (2DEG). The density of this surface 2DEG can be controlled experimentally [3], including accessing the technologically important regime of the superconducting 2DEG of the LaAlO3/SrTiO3 interface [4]. By thin film growth it now becomes possible to also tune the thickness and penetration of the 2DEG by supplying charge carriers from the intermediate layer (delta-doping). The very promising titanate series will be further explored, with e.g. GdTiO3 (GTO) [5] being a system in which a 2DEG has been proposed as well [6].

Further characterisation will be carried out either in-situ, such as x-ray photoelectron spectroscopy, or ex-situ, such as x-ray diffraction, electronic transport measurements or the study of their magnetic properties. The student will be based at the Harwell Campus (http://harwellcampus.com/). The outcome of this research will be world-class data on novel functional materials that will allow interesting conclusions to be published in high impact journals.

This project will be jointly supervised by ARPES beamline staff and Prof. Thorsten Hesjedal.

References:
[1] A. A. Baker, W. Braun, G. Gassler, S. Rembold, A. Fischer, T. Hesjedal, "An ultra-compact, high-throughput molecular beam epitaxy growth system”, Rev. Sci. Instrum. 86, 043901 (2015) - http://scitation.aip.org/content/aip/journal/rsi/86/4/10.1063/1.4917009
[2] J. M. Riley, et al. manuscript in preparation.
[3] S. Mckeown Walker, et al. “Carrier-Density Control of the SrTiO3 (001) Surface 2D Electron Gas studied by ARPES”, Advanced Materials 27, 3894 (2015)
http://onlinelibrary.wiley.com/doi/10.1002/adma.201501556/full
[4] Z. Wang, et al., “Tailoring the nature and strength of electron–phonon interactions in the SrTiO3(001) 2D electron liquid”, Nature Materials 15, 835 (2016)
http://www.nature.com/nmat/journal/v15/n8/full/nmat4623.html
[5] S. Stemmer, S. James Allen, “Two-Dimensional Electron Gases at Complex Oxide Interfaces”, Annual Reviews Materials Research 44, 151 (2014)
www.annualreviews.org/doi/pdf/10.1146/annurev-matsci-070813-113552
[6] S. Nemsak, et al., “Energetic, spatial, and momentum character of the electronic structure at a buried interface: The two-dimensional electron gas between two metal oxides”, Phys. Rev. B 93, 245103 (2016)
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.93.245103

Email Prof Thorsten Hesjedal at: thorsten.hesjedal@physics.ox.ac.uk


Figure: Schematic view of the UHV station HR-ARPES, including the ARPES instrument on the right and the mu-MBE growth chamber and characterisation facility on the left.

Email Prof Thorsten Hesjedal at: thorsten.hesjedal@physics.ox.ac.uk

Dr Roger Johnson

Magneto-orbital physics of novel manganese oxides

The cooperative interactions of electrons in a crystal lattice can give rise to long-range ordering of charge, orbital and spin electronic degrees of freedom. Not only are such states of fundamental physical interest, but they also give rise to technologically important materials properties such as ferromagnetism, ferroelectricity, colossal magnetoresistance, and metal-to-insulator transitions – to name but a few. Furthermore, the different orders can be intimately linked, allowing in special cases the cross coupling of properties. The manganites – crystalline materials containing manganese and oxygen – are now considered to be textbook examples of charge, orbital, and magnetically ordered systems, owing largely to the electronic flexibility of the manganese ion and an ideal bonding geometry [1]. In our group, we recently focussed our research effort on quadruple-perovskite manganites [2-6]. This relatively new family of compounds significantly extends the phenomenology observed in the canonical single perovskites, including completely different forms of ordering such as magneto-orbital helices (Figure 1) [3,5]. The key question is whether there is a general unifying principle connecting all these different types of charge, magnetic and orbital ordering, in both canonical and novel compounds. Answering this question requires:
• Exploring complex magnetic states that arise due to interactions between rare-earth and transition metal ions.
• Manipulating the orbital states of manganese ions through direct isovalent cation substitution or indirect electron doping.
• Altering the metallic character of the compounds from strongly metallic to strongly insulating through manipulation of structural distortions.
• Investigating the role of strong spin-orbit coupling.

Johnson and Radaelli -Manganites Project GRAPHIC.docx__2_0.png

This EPSRC-funded DPhil project will give the successful candidate the opportunity to develop this line of research in different directions, including:
• Materials discovery (bulk and thin film).
• Development of laboratory-based characterisation techniques.
• Large-scale experiments with fully quantitative data analysis.

This project is likely to involve a combination of experimental techniques, such as:
• Elastic neutron scattering. We will perform experiments on bulk and thin film samples predominantly at the ISIS facility at Rutherford Appleton Laboratory.
• X-ray scattering, including resonant and non-resonant magnetic X-ray diffraction with hard and soft X-rays. We run state-of-the-art laboratory instrumentation in the Clarendon Laboratory, but we perform most of our high-end experiments at the Diamond Light source.
• Dielectric and transport measurements. One of our specialities is to perform measurements of ferroelectricity in extremely high magnetic fields (up to 65 T – a record in the UK), using the pulsed-magnetic-field facility in the Clarendon Laboratory, but a complete set of more standard measurements is also available.
• Advanced microscopy. We employ spectral microscopy (PEEM) at Diamond, Magnetic Force Microscopy (MFM) and the Magneto-Optical Kerr Effect to image magnetic domains.

For more information contact Dr Roger Johnson: roger.johnson@physics.ox.ac.uk
Supervisor: Dr Roger Johnson roger.johnson@physics.ox.ac.uk
Co-Supervisor: Prof Paolo Radaelli paolo.radaelli@physics.ox.ac.uk

[1] D. I. Khomskii, “Transition Metal Compounds”, Cambridge University Press (2014).
[2] R. D. Johnson et al., Physical Review Letters 108, 067201 (2012)
[3] N. J. Perks et al., Nature Communications 3, 1277 (2012)
[4] K. Cao et al., Physical Review B 91, 064422 (2015)
[5] R. D. Johnson et al, Physical Review B 93, 180403(R) (2016)
[6] R. D. Johnson et al., Physical Review B 96, 054448 (2017)

Electrical control of magnetism in oxide films and devices

This project will be co-supervised by Prof. Paolo G. Radaelli and Dr Roger D. Johnson, who holds a Royal Society University Research Fellowship in the Clarendon Laboratory

Radaelli  Johnston - oxide films graphic.jpg

Figure 1. a: a BiFeO3 epitaxial films with patterned electrical contacts, which was measured at ISIS and Diamond. b: image of a small area of the film, obtained using vector-mapped x-ray magnetic linear dichroism photoemission electron microscopy (XMLD-PEEM). c: Domains in an Fe2O3 thin film. Different colours represent different antiferromagnetic domains

One of the most promising routes towards a new generation of fast, low-power electronics is the electrical control of magnetism in insulators[1-3]. This approach exploits the ability to switch the antiferromagnetic state in several classes of oxides by applying a small "writing" voltage. The spin polarisation can then be transferred to a ferromagnetic material through an interface, and then 'read' in a conventional way, e.g., using a Tunnelling MagnetoResistance junction as in hard-disk reading heads. This scheme could be employed to produce fast and efficient non-volatile memories, with no 'writing' current to produce Joule heating and therefore dissipate energy.

We have recently developed a suite of techniques for simultaneous imaging of antiferromagnetic/ferromagnetic domains and their electrical switching in epitaxial oxide films and devices. We use a combination of synchrotron X-ray diffraction and microscopy, in-house Magnetic Force Microscopy (MFM) and neutron diffraction, which give us access to domains over length scales from 1 cm to < 100 nm. In the past two years, we obtained some very exciting results on thin films and devices of BiFeO3, grown by our collaborators at the University of Madison (Fig. 1)[4-6]. In very recent experiments, we directly imagined the process of electrical switching in BiFeO3 and demonstrated the coupling of the BiFeO3 domains with the ferromagnetic domains of a thin metal over-layer. Even more recently, we demonstrated similar effects in epitaxial films of Fe2O3, grown by one of our students in our lab (see Figure 1c). The domains in Fe2O3 display topological structures similar to nematic liquid crystals.

This EPSRC-funded DPhil project will give the successful candidate the opportunity to develop this line of research in different directions:

• Identify and grow new materials with electrically controllable domains
• Experiment with novel ways to switch the magnetic state of the domains, e.g., through the piezoelectric effect
• Build and test prototype devices using electron beam lithography and other clean room processes.

This project is likely to involve a combination of experimental techniques, such as:

• Elastic neutron scattering. We will perform experiments on bulk and films samples predominantly at the ISIS facility at Rutherford Appleton Laboratory.
• X-ray scattering, including resonant and non-resonant magnetic X-ray diffraction with hard and soft X-rays. We run state-of-the-art laboratory instrumentation in the Clarendon Laboratory, but we perform most of our high-end experiment at the Diamond Light source.
• Dielectric and transport measurements. One of our specialities is to perform measurements of ferroelectricity in extremely high magnetic fields (up to 65 T – a record in the UK), using the pulsed-magnetic-field facility in the Clarendon Laboratory, but a complete set of more standard measurements is also available.
• Advanced microscopy. We employ spectral microscopy (PEEM) at Diamond, Magnetic Force Microscopy (MFM) and the Magneto-Optical Kerr Effect to image multi-functional domains, which are the fundamental unit of information storage in oxides.
• Nanofabrication. We will be using electron beam lithography and other clean-room methods to design and build prototype oxide quantum materials devices.

Depending on the candidate's interests, the project may also include a computational element. In collaboration with the Materials Modelling Group in the Department of Materials, we employ Density Functional Theory methods and other computational techniques to model the functional properties of oxides and to predict their behaviour in different architectures.

For more information email Prof. Paolo Radaelli and visit the group webpages.

[1] Matsukura, F., Tokura, Y. & Ohno, H. Control of magnetism by electric fields. Nature Nanotechnology 10, 209–220 (2015). abstract pdf
[2] Eerenstein, W., Mathur, N. D. & Scott, J. F. Multiferroic and magnetoelectric materials. Nature 442, 759–65 (2006). abstract pdf
[3] Ramesh, R. & Spaldin, N. A. Multiferroics: progress and prospects in thin films. Nature materials 6, 21–9 (2007). abstract pdf
[4] Coherent magneto-elastic domains in multiferroic BiFeO3 filmsN. Waterfield Price, R. D. Johnson, W. Saenrang, F. Maccherozzi, S. S. Dhesi, A. Bombardi, F. P. Chmiel, C.-B. Eom, P. G. Radaelli Phys. Rev. Lett. 117, 177601 (2016)., abstract pdf
[5] Electrical Switching of Magnetic Polarity in a Multiferroic BiFeO3 Device at Room Temperature, N.W. Price, R.D. Johnson, W. Saenrang, A. Bombardi, F.P. Chmiel, C.B Eom, and P.G. Radaelli, Phys. Rev. Appl. 8, 014033 (2017).
[6] Deterministic and robust room-temperature exchange coupling in monodomain multiferroic BiFeO3 heterostructures, W. Saenrang, B.A. Davidson, F. Maccherozzi, J.P. Podkaminer, J. Irwin, R.D. Johnson, J.W. Freeland, J. Íñiguez, J.L. Schad, K. Reierson, J.C. Frederick, C.A.F. Vaz, L. Howald, T.H. Kim, S. Ryu, M.v. Veenendaal, P.G. Radaelli, S.S. Dhesi, M.S. Rzchowski & C.B. Eom, Nature Comm., 8, 1583 (2017).

Prof Michael Johnston

Semiconductor Nanowire-Based Solar Cells

III-V nanowires, such as GaAs and InP nanowires, are high aspect ratio cylinders of crystalline inorganic semiconductor, typically with diameters of a few tens of nanometres and lengths of several microns. These novel nanomaterials offer outstanding potential as nano-components of future electronic and optoelectronic devices, and are especially promising candidates for the light absorbing and charge transporting elements of solar cells. Nanowire-based solar cells could be produced at a fraction of the cost of conventional solar cells, and offer the possibility of unprecedented energy conversion efficiencies. This project will involve the fabrication of novel nanowire-based solar cells and studying charge dynamics in these devices using terahertz spectroscopy.
Email Prof Michael Johnston at m.johnston1@physics.ox.ac.uk

To apply for this project you need to submit an application through the University's Postgraduate Admissions Programme to the Department of Condensed Matter Physics - see http://www.admin.ox.ac.uk/postgraduate/apply/ for more information. If you are an international student there are a range of funding opportunities available to you.

THz Conductivity of Graphene and Graphene-Like Materials


Two-dimensional nano-materials such as graphene are currently the platform for fascinating fundamental science and have the potential to become the building blocks for a new era of fast, low-cost electronics. These materials are already inspiring new device concepts for applications including photovoltaics, sensing, high-speed communications and computing. However at present the electronic properties of many of these new materials are poorly understood, which severely limits the realisation of these electronic devices. Furthermore, nano-materials have a particularly bright future in ultra-high (THz) frequency electronic devices, yet to date their properties have almost exclusively been studied only at low frequencies. This project will develop non-contact tools to measure the electronic properties of nano-materials to frequencies well in excess of the operating frequency of the fastest electronic devices available today.
Email Prof Michael Johnston at m.johnston1@physics.ox.ac.uk

To apply for this project you need to submit an application through the University's Postgraduate Admissions Programme to the Department of Condensed Matter Physics - see http://www.admin.ox.ac.uk/postgraduate/apply/ for more information. If you are an international student there are a range of funding opportunities available to you.

Terahertz Conductivity of Semiconductor Nanostructures and Devices

As semiconductor devices become faster and smaller it is increasingly important to understand the fundamental nature of charge carrier dynamics in these systems on a sub-picosecon d timescale. Conventional electronic transport and magneto-transport methods are incapable of measuring dynamics on such short time scales. However, "Optical Pump, Terahertz Probe Spectroscopy (OPTHzP)" is a new non-contact technique that allows such measurements to be performed with a femtosecond resolution, thereby revealing novel ultrafast physical processes. When fully understood such processes have the potential be utilised in future generations of electronic devices. This project will make use of the state-of-the-art OPTHzP systems in the Oxford Terahertz Photonics group to help understand charge dynamics in novel semiconducting nanostructures, including semicondutor nanowires (lower Figure), carbon nanotubes and porous InP.

Further Reading:
Our recent study of ultrafast conductivity in GaAs nanowires [more]
Our THz study of polymer field effect transistors [more]
A description of the carrier dynamics model that has been developed in the group [more]
Email Prof Michael Johnston at m.johnston1@physics.ox.ac.uk

To apply for this project you need to submit an application through the University's Postgraduate Admissions Programme to the Department of Condensed Matter Physics - see http://www.admin.ox.ac.uk/postgraduate/apply/ for more information. If you are an international student there are a range of funding opportunities available to you.

Prof Achillefs Kapanidis

Fighting antibiotic resistance via nano-scale imaging of gene machines


Antibiotic resistance of bacteria is one of the major health challenges the world faces today, driven by overuse of our current antibiotic arsenal. To overcome this massive challenge, there is an intense effort to find new antibiotics and to understand how existing antibiotics work. In the past few years, we have been contributing to this field by developing single-molecule fluorescence imaging methods to study the function of the protein RNA polymerase, responsible for copying DNA into messenger RNA (a process known as “gene transcription”); this protein is also a major target of antibiotics.

Our current efforts focus on watching the nano-scale dynamics of RNA polymerase in real-time to identify new steps in transcription that can act as antibiotic targets, and to understand how existing and new antibiotics exert their function. These efforts depend on the ability to detect single molecules of RNA polymerase as they operate on DNA fragments that resemble expressed genes.

The project will involve single-molecule imaging and microscopy, measurements of 1-10 nm distances within transcription complexes (see Figure) based on dipole-dipole interactions between dyes attached to specific sites, molecular modelling, and advanced signal processing.

For more information, look at our website, or email Prof Achillefs Kapanidis at a.kapanidis1@physics.ox.ac.uk
To apply for this project you need to submit an application through the University's Postgraduate Admissions Programme to the Department of Condensed Matter Physics – see http://www.ox.ac.uk/admissions/graduate/applying-to-oxford for more information.

Super-resolution imaging of gene expression in living cells

During the past few years, a new family of microscopy methods is revolutionizing biology and medicine. These methods, known as super-resolution fluorescence imaging, rely on ingenious ways that circumvent the resolution limit imposed by the diffraction of light; the potential of these methods was recognized in the award of the 2014 Nobel Prize in Chemistry to three prominent physicists. Many of these methods rely on detecting single fluorescent molecules, and are known as single-molecule fluorescence microscopy, or simply localization microscopy. Localization microscopy bypass the diffraction limit by our ability to localize a fluorescent molecule with a precision up to 100-fold better than the width of its point-spread function (PSF; see figure). Provided that sufficient photons are collected, we can localize single fluorescent dyes with ~1 nm precision.

Using our extensive experience in localization-based super-resolution imaging, we study the 3-D organization and diffusion profile of DNA-binding proteins in single bacterial cells (see Figure at far right for detection of DNA polymerases in single bacteria). We are interested in relating the information regarding the transcription machinery in vivo with quantitative models of transcription. We are also interested in understanding how DNA binding proteins combine 1-D and 3-D diffusion to find their targets in cells and characterize the processes involved.

The project will involve single-molecule imaging and microscopy, single-molecule tracking in living bacteria based on localization microscopy, molecular modelling, and advanced signal processing.

For more information, look at our website, or email Prof Achillefs Kapanidis at a.kapanidis1@physics.ox.ac.uk
To apply for this project you need to submit an application through the University's Postgraduate Admissions Programme to the Department of Condensed Matter Physics – see http://www.ox.ac.uk/admissions/graduate/applying-to-oxford for more information.

DNA biosensors for detecting pathogenic bacteria and viruses


DNA is an extremely versatile material for build novel biosensors; molecules can be encoded with binding sites specific for proteins of interest, or for other nucleic acids (DNA/RNA), and can be easily modified to contain useful chemical groups such as fluorophores, biotin or quantum dots. Employing self-assembly properties of DNA, one can also design topological features to create different 2- or 3-dimensional shapes of sizes that can provide exquisite molecular recognition. This additional layer of freedom allows considerable flexibility when designing biosensors.

Our lab develops biosensors for the single-molecule detection of DNA-binding proteins (see Figure), nucleic acids, and viral particles. We are currently interested in detecting pathogenic bacteria (using E.coli as a model system) and viruses (using the influenza virus as a major pathogen target).

The project will involve design of DNA biosensors, single-molecule imaging and microscopy, chemical treatment of bacteria and viruses, theoretical modelling, and advanced signal processing.

For more information, look at our website, or email Prof Achillefs Kapanidis at a.kapanidis1@physics.ox.ac.uk
To apply for this project you need to submit an application through the University's Postgraduate Admissions Programme to the Department of Condensed Matter Physics – see http://www.ox.ac.uk/admissions/graduate/applying-to-oxford for more information.

Development of portable single-molecule imaging


We have recently developed a revolutionary novel portable optical microscope (“NanoImager”; see Figure) for advanced single-molecule imaging. The NanoImager is based on single-molecule fluorescence, which is the most popular single-molecule detection method, especially in vivo; single-molecule fluorescence has elucidated many biological mechanisms, allowed sub-diffraction cellular imaging (see Figure) and enabled biosensing applications such as next-generation DNA sequencing. The NanoImager has a highly efficient and robust optical/mechanical design that improves stability and substantially reduces complexity and size; these unique features offer a platform that can address large needs in research, diagnostics, and chemical analysis, and spark new applications in many sectors.

We have openings to work on further extensions of the NanoImager that will make it portable, increase its modes of operation (3D imaging, single-molecule tracking, detection of cells in suspension), interface it with microfluidics, and increase the molecular throughout. This project can be combined with the DNA biosensor project offered by the same group.

For more information, look at our website, or email Prof Achillefs Kapanidis at a.kapanidis1@physics.ox.ac.uk
To apply for this project you need to submit an application through the University's Postgraduate Admissions Programme to the Department of Condensed Matter Physics – see http://www.ox.ac.uk/admissions/graduate/applying-to-oxford for more information.

Dr Alexy Karenowska

Quantum Magnon Spintronics

Spintronics is the area of research dedicated to the study of how ‘spin’—the quantum mechanical currency of magnetism—can be used realize new types of information transport, storage, and processing system which surpass the capabilities of those currently found in our computers and other electronic devices.
The field of magnonics is the area of magnetics dedicated to the science of quasi-particles known as magnons. In certain magnetic systems, magnons are able to play the role of microscopic spin-carrying tokens which can be generated and transmitted over relatively long distances (up to centimetres) and at high speed (many tens of kilometres per second). Magnon spintronics, magnonics’ emerging sister discipline, is concerned with structures and devices which involve the passing of spin-information between magnons and electrons, the familiar workhorses of conventional electronics.
As appreciation of the interplay between magnonic and electronic spin-transport deepens, so excitement surrounding its possible contribution to next-generation information technology heightens. To date however, work in magnonics and magnon spintronics has focused on the study of room-temperature magnon and magnon/electron systems in the classical limit. As a result, the field of experimental quantum measurement and information processing has yet to explore what the magnonic theatre has to offer.
This project will involve working on the development of novel experiments for the investigation of magnonics and magnon spintronics at the quantum level. In particular, its goal will be to take some of the first steps towards accessing the new physics and technological opportunity at the interface between magnonic and magnon spintronic systems, and the techniques of contemporary quantum measurement and information processing.
Email Dr Alexy Karenowska at a.karenowska@physics.ox.ac.uk

Dr Karenowska has a further project, which is joint project with Prof John Gregg. Please see above 'Prof John Gregg and Dr Alexy Karenowska.

Dr Peter Leek

Prof Robin Nicholas

Graphene and carbon nanotube based PV electrodes

Both graphene and carbon nanotubes have very high electrical conductivities and mechanical strength, which makes them ideally suited for use as electrodes for flexible solar cells, replacing costly and brittle ITO and FTO. Over the past three years, we have been successfully working on these kinds of electrodes, investigating how to use graphene monolayer electrodes, graphene flakes and carbon nanotubes as both electron and hole collecting layers in PV cells. We have demonstrated the versatility of this approach by making PV cells using perovskite, organic semiconductor and colloidal quantum dot absorbing layers. For example, we showed that it is possible to reach more than 18% efficiency with carbon nanotube top electrodes in perovskite solar cells.

We propose an experimental project which will build on these results and continue with the development of nanotube/polymer/graphene/titania/tin
oxide combinations to produce the next generation of nanohybrid particles for electrodes. It will aim at exploring the possibilities of integrating carbon nanotubes based approaches into PV cells both as external contacting layers and as internal electrodes capable of implementing tandem cell designs for various PV technologies. In addition to spin-coating, we will explore the capabilities of a custom made spray-coater recently developed in the department which can deposit nanoparticles including carbon nanotubes, aiming to make this process compatible with roll-to-roll fabrication.

The three main objectives of this project are:

• Understand the wrapping of carbon nanotubes
• Make solar cells with nanotube/polymer/graphene/titania/tin oxide combinations to develop the next generation of PV electrodes.
• Demonstrate spray coating of these electrodes compatible with roll-to-roll fabrication.

Typical device structure:

RiedeNicholasDevice - Graphic_2.jpg  /><span class=

This project is supervised by Prof Robin Nicholas and Dr Moritz Riede in association with the EPSRC Centre for Doctoral Training in New and Sustainable Photovoltaics - see http://www.cdt-pv.org

Supervisor: Prof Robin Nicholas robin.nicholas@physics.ox.ac.uk +44 (0)1865 272250
Co-Supervisor: Dr Moritz Riede moritz.riede@physics.ox.ac.uk +44 (0)1865 272377

Prof Paolo Radaelli

Breaking symmetry with light: ultra-fast ferroelectricity and magnetism from non-linear phononics

A collaboration between Prof. Paolo G. Radaelli and Prof. Andrea Cavalleri, who holds a joint appointment between the Clarendon Laboratory and the Max Planck Institute for the Structure and Dynamics of Matter. (Hamburg).

Radaelli Cavalleri non linear graphic_0.png

Figure 1: schematic representation of non-linear photo-ferroicity. THz or far-IR ‘pump’ photons excite an IR-active mode (right), which is coherently coupled with a Raman mode (left). The rectified component of the Raman mode transiently generates ferromagnetism or ferroelectricity, which is probed coherently with a near-IR or visible light beam (top left).

The use of light to control the structural, electronic and magnetic properties of solids is emerging as one of the most exciting areas of condensed matter physics. One promising field of research, known as femto-magnetism, has developed from the early demonstration that magnetic ‘bits’ in certain materials can be ‘written’ at ultra-fast speeds with light in the visible or IR range [1]. More radically, it has been shown that fundamental materials properties such as superconductivity can be ‘switched on’ transiently under intense illumination [2]. Recently, the possibilities of manipulating materials by light have been greatly expanded by the demonstration of mode-selective optical control, whereby pumping a single infrared-active phonon mode results in a structural/electronic distortion along the coordinates of a second, anharmonically coupled Raman mode – a mechanism that was termed ‘nonlinear phononics’. Crucially, the Raman distortion is partially rectified, meaning that it oscillates around a different equilibrium position than in the absence of illumination. Very recently, it was realised that, under appropriate conditions, the rectified Raman distortion can transiently break the structural and/or magnetic symmetry of the crystal. Such symmetry breaking persist for a time corresponding to the carrier envelope of the pump, which can be less than a picosecond, and can give rise to the ultra-fast emergence of ferroic properties such as ferromagnetism and ferroelectricity. Through symmetry analysis and first-principle calculations, we have identified several promising ‘photo-ferroic’ materials that should display these effects, with potential applications in ultra-low-power information storage, ultra-fast electronics and many more.

This DPhil project will give the successful candidate the opportunity to pioneer this new field of research. Initial experiments on the candidate ‘photo-ferroic’ materials that we have already identified will be performed at the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, Germany. As a mode-selective pump, we will employ coherent laser radiation in the THz or far-IR range with sub-ps carrier envelopes, while the transient emergence of the ferroic properties will be probed with second-harmonic generation (SHG) and/or Faraday rotation of near-infra-red light. Later on in the project, changes in the crystal and magnetic structures of the materials will be probed with X-rays at free electron laser sources such as the European XFEL in Hamburg. Meanwhile, the candidate will develop search strategies for new classes of ‘photo-ferroic’ materials, based on symmetry and density functional theory calculations. He/she will develop the materials specifications in collaborations with crystal growers in Oxford and elsewhere, and will be involved hands on in all aspects of the design and realisation of the experiments and the data analysis.

The experimental part of this project will be predominantly based in Hamburg, so it is essential for the candidate to be willing and able to be based in Germany for extended periods during the DPhil.
[1]Femtomagnetism: Magnetism in step with light. Uwe Bovensiepen, Nature Physics 5, 461 - 463 (2009) abstract pdf
[2] See for example M. Mitrano,et al., ‘Possible light-induced superconductivity in K3C60 at high temperature’, Nature, 530, 461–464 (2016). [more at this link].
[3] Nonlinear phononics as an ultrafast route to lattice control, M. Först et. al., Nature Physics, 7, 854–856 (2011).

Electrical control of magnetism in oxide films and devices

This project will be co-supervised by Prof. Paolo G. Radaelli and Dr Roger D. Johnson, who holds a Royal Society University Research Fellowship in the Clarendon Laboratory

Radaelli  Johnston - oxide films graphic.jpg

Figure 1. a: a BiFeO3 epitaxial films with patterned electrical contacts, which was measured at ISIS and Diamond. b: image of a small area of the film, obtained using vector-mapped x-ray magnetic linear dichroism photoemission electron microscopy (XMLD-PEEM). c: Domains in an Fe2O3 thin film. Different colours represent different antiferromagnetic domains

One of the most promising routes towards a new generation of fast, low-power electronics is the electrical control of magnetism in insulators[1-3]. This approach exploits the ability to switch the antiferromagnetic state in several classes of oxides by applying a small "writing" voltage. The spin polarisation can then be transferred to a ferromagnetic material through an interface, and then 'read' in a conventional way, e.g., using a Tunnelling MagnetoResistance junction as in hard-disk reading heads. This scheme could be employed to produce fast and efficient non-volatile memories, with no 'writing' current to produce Joule heating and therefore dissipate energy.

We have recently developed a suite of techniques for simultaneous imaging of antiferromagnetic/ferromagnetic domains and their electrical switching in epitaxial oxide films and devices. We use a combination of synchrotron X-ray diffraction and microscopy, in-house Magnetic Force Microscopy (MFM) and neutron diffraction, which give us access to domains over length scales from 1 cm to < 100 nm. In the past two years, we obtained some very exciting results on thin films and devices of BiFeO3, grown by our collaborators at the University of Madison (Fig. 1)[4-6]. In very recent experiments, we directly imagined the process of electrical switching in BiFeO3 and demonstrated the coupling of the BiFeO3 domains with the ferromagnetic domains of a thin metal over-layer. Even more recently, we demonstrated similar effects in epitaxial films of Fe2O3, grown by one of our students in our lab (see Figure 1c). The domains in Fe2O3 display topological structures similar to nematic liquid crystals.

This EPSRC-funded DPhil project will give the successful candidate the opportunity to develop this line of research in different directions:

• Identify and grow new materials with electrically controllable domains
• Experiment with novel ways to switch the magnetic state of the domains, e.g., through the piezoelectric effect
• Build and test prototype devices using electron beam lithography and other clean room processes.

This project is likely to involve a combination of experimental techniques, such as:

• Elastic neutron scattering. We will perform experiments on bulk and films samples predominantly at the ISIS facility at Rutherford Appleton Laboratory.
• X-ray scattering, including resonant and non-resonant magnetic X-ray diffraction with hard and soft X-rays. We run state-of-the-art laboratory instrumentation in the Clarendon Laboratory, but we perform most of our high-end experiment at the Diamond Light source.
• Dielectric and transport measurements. One of our specialities is to perform measurements of ferroelectricity in extremely high magnetic fields (up to 65 T – a record in the UK), using the pulsed-magnetic-field facility in the Clarendon Laboratory, but a complete set of more standard measurements is also available.
• Advanced microscopy. We employ spectral microscopy (PEEM) at Diamond, Magnetic Force Microscopy (MFM) and the Magneto-Optical Kerr Effect to image multi-functional domains, which are the fundamental unit of information storage in oxides.
• Nanofabrication. We will be using electron beam lithography and other clean-room methods to design and build prototype oxide quantum materials devices.

Depending on the candidate's interests, the project may also include a computational element. In collaboration with the Materials Modelling Group in the Department of Materials, we employ Density Functional Theory methods and other computational techniques to model the functional properties of oxides and to predict their behaviour in different architectures.

For more information email Prof. Paolo Radaelli and visit the group webpages.

[1] Matsukura, F., Tokura, Y. & Ohno, H. Control of magnetism by electric fields. Nature Nanotechnology 10, 209–220 (2015). abstract pdf
[2] Eerenstein, W., Mathur, N. D. & Scott, J. F. Multiferroic and magnetoelectric materials. Nature 442, 759–65 (2006). abstract pdf
[3] Ramesh, R. & Spaldin, N. A. Multiferroics: progress and prospects in thin films. Nature materials 6, 21–9 (2007). abstract pdf
[4] Coherent magneto-elastic domains in multiferroic BiFeO3 filmsN. Waterfield Price, R. D. Johnson, W. Saenrang, F. Maccherozzi, S. S. Dhesi, A. Bombardi, F. P. Chmiel, C.-B. Eom, P. G. Radaelli Phys. Rev. Lett. 117, 177601 (2016)., abstract pdf
[5] Electrical Switching of Magnetic Polarity in a Multiferroic BiFeO3 Device at Room Temperature, N.W. Price, R.D. Johnson, W. Saenrang, A. Bombardi, F.P. Chmiel, C.B Eom, and P.G. Radaelli, Phys. Rev. Appl. 8, 014033 (2017).
[6] Deterministic and robust room-temperature exchange coupling in monodomain multiferroic BiFeO3 heterostructures, W. Saenrang, B.A. Davidson, F. Maccherozzi, J.P. Podkaminer, J. Irwin, R.D. Johnson, J.W. Freeland, J. Íñiguez, J.L. Schad, K. Reierson, J.C. Frederick, C.A.F. Vaz, L. Howald, T.H. Kim, S. Ryu, M.v. Veenendaal, P.G. Radaelli, S.S. Dhesi, M.S. Rzchowski & C.B. Eom, Nature Comm., 8, 1583 (2017).

Magneto-orbital physics of novel manganese oxides

The cooperative interactions of electrons in a crystal lattice can give rise to long-range ordering of charge, orbital and spin electronic degrees of freedom. Not only are such states of fundamental physical interest, but they also give rise to technologically important materials properties such as ferromagnetism, ferroelectricity, colossal magnetoresistance, and metal-to-insulator transitions – to name but a few. Furthermore, the different orders can be intimately linked, allowing in special cases the cross coupling of properties. The manganites – crystalline materials containing manganese and oxygen – are now considered to be textbook examples of charge, orbital, and magnetically ordered systems, owing largely to the electronic flexibility of the manganese ion and an ideal bonding geometry [1]. In our group, we recently focussed our research effort on quadruple-perovskite manganites [2-6]. This relatively new family of compounds significantly extends the phenomenology observed in the canonical single perovskites, including completely different forms of ordering such as magneto-orbital helices (Figure 1) [3,5]. The key question is whether there is a general unifying principle connecting all these different types of charge, magnetic and orbital ordering, in both canonical and novel compounds. Answering this question requires:
• Exploring complex magnetic states that arise due to interactions between rare-earth and transition metal ions.
• Manipulating the orbital states of manganese ions through direct isovalent cation substitution or indirect electron doping.
• Altering the metallic character of the compounds from strongly metallic to strongly insulating through manipulation of structural distortions.
• Investigating the role of strong spin-orbit coupling.

Johnson and Radaelli -Manganites Project GRAPHIC.docx__2_0.png

This EPSRC-funded DPhil project will give the successful candidate the opportunity to develop this line of research in different directions, including:
• Materials discovery (bulk and thin film).
• Development of laboratory-based characterisation techniques.
• Large-scale experiments with fully quantitative data analysis.

This project is likely to involve a combination of experimental techniques, such as:
• Elastic neutron scattering. We will perform experiments on bulk and thin film samples predominantly at the ISIS facility at Rutherford Appleton Laboratory.
• X-ray scattering, including resonant and non-resonant magnetic X-ray diffraction with hard and soft X-rays. We run state-of-the-art laboratory instrumentation in the Clarendon Laboratory, but we perform most of our high-end experiments at the Diamond Light source.
• Dielectric and transport measurements. One of our specialities is to perform measurements of ferroelectricity in extremely high magnetic fields (up to 65 T – a record in the UK), using the pulsed-magnetic-field facility in the Clarendon Laboratory, but a complete set of more standard measurements is also available.
• Advanced microscopy. We employ spectral microscopy (PEEM) at Diamond, Magnetic Force Microscopy (MFM) and the Magneto-Optical Kerr Effect to image magnetic domains.

For more information contact Dr Roger Johnson: roger.johnson@physics.ox.ac.uk.
Supervisor: Dr Roger Johnson roger.johnson@physics.ox.ac.uk
Co-Supervisor: Prof Paolo Radaelli paolo.radaelli@physics.ox.ac.uk

[1] D. I. Khomskii, “Transition Metal Compounds”, Cambridge University Press (2014).
[2] R. D. Johnson et al., Physical Review Letters 108, 067201 (2012)
[3] N. J. Perks et al., Nature Communications 3, 1277 (2012)
[4] K. Cao et al., Physical Review B 91, 064422 (2015)
[5] R. D. Johnson et al, Physical Review B 93, 180403(R) (2016)
[6] R. D. Johnson et al., Physical Review B 96, 054448 (2017)

Dr Moritz Riede

Understanding the Fundamental Efficiency Limits of Organic Solar Cells

The first wave of products using organic semiconductors has very successfully entered the market: organic light emitting diodes (OLED) are used in the displays of many mobile phones and TVs, featuring brilliant colours etc.. Key to their commercial success are vacuum processing of small organic molecules under precise control into well-defined multilayer stacks and the use of molecular doping, i.e. the modification of a semiconductor's properties by a controlled addition of "impurities". Both concepts are much less used in organic solar cells (OSC), but it can be applied here with similar benefits, enabling a cheap, efficient, light-weight and flexible renewable energy source. Best proof of this is that the current technology leader, the start-up Heliatek, is using both concepts.
Record OSC have achieved power conversion efficiencies (PCEs) of ~11-12%. However, this value is still low compared to those of commercial rigid and monochrome inorganic competitors (~20%) and the values for OSC have remained nearly the same since 2013, hindering a fast large scale commercial breakthrough. The main reasons for this are high energy losses at the open circuit voltage, of which much remains unclear. The goal of this DPhil is to get experimental access to the exact location in the OSC, i.e. the donor-acceptor interface, where these losses take place, characterise them and subsequently find ways to modify and control these processes. Only then will it be possible to significantly reduce these losses and to demonstrate PCEs exceeding 15% for OSC.
For the preparation of the samples we rely on vacuum processes, similar to those that are used for the production of OLEDs. To arrive at a better understanding of the loss processes, an extensive range of experimental methods to characterise the optical and electronic behaviour of our samples will be used. In this research, we will be collaborating with other groups in Oxford and with international experts from e.g. Canada, Germany and Switzerland.

Email Dr Moritz Riede at moritz.riede@physics.ox.ac.uk.

In-Situ Characterisation of Organic Thin Films using Neutron and X-Ray Scattering

This doctoral studentship is a collaboration between the University of Oxford (Department of Physics) and the ISIS Muon and Neutron Source (Rutherford Appleton Laboratory), partially funded through the Physics Department and an ISIS Facility Development Studentship (50% each). The academic supervisor is Moritz Riede, and the co-supervisors are Dr Robert Dalgliesh (ISIS) and Prof Chris Nicklin (Diamond Light Source).

Organic semiconductors (OSC) have demonstrated great promise for use in next-generation electronic devices, from the organic light emitting diodes (OLEDs) found in many mobile phone displays to other applications including solar cells, wearable health-monitoring sensors, and ultra low power computing. A major challenge in making these technologies a reality is understanding how these molecular materials – often as thin as 50 nm – arrange during fabrication and evolve from their initial fabricated state, during which they may be subjected to variations in temperature and pressure, and exposure to air. These processing changes can drastically alter the microstructure of the thin films, affecting the performance and lifetime of the devices.

This studentship will use X-rays and neutrons to characterise the microstructure of vacuum-deposited organic semiconductors, with a focus on bridging our understanding of thin film microstructure with underlying solar cell device physics. The student will carry out their scientific programme using our world-class X-ray characterisation facilities at the Oxford’s Department for Physics for ex-situ studies, as well as a recently commissioned deposition chamber at Diamond Light Source beamline I07 for in-situ X-ray scattering. Part of this studentship will also focus on exploiting techniques available at the new Larmor instrument at ISIS Neutron and Muon Source for the characterisation of small-molecule organic thin films, including small-angle neutron scattering, and spin-precession methods, with the goal of developing a similar deposition chamber for in-situ deposition measurements at Larmor. These unique sample environments will lay the foundation for new, world-leading capabilities across ISIS and Diamond for the study of OSCs and other vacuum-deposited advanced functional materials.

Email Dr Moritz Riede at moritz.riede@physics.ox.ac.uk.

Graphene and carbon nanotube based PV electrodes

Both graphene and carbon nanotubes have very high electrical conductivities and mechanical strength, which makes them ideally suited for use as electrodes for flexible solar cells, replacing costly and brittle ITO and FTO. Over the past three years, we have been successfully working on these kinds of electrodes, investigating how to use graphene monolayer electrodes, graphene flakes and carbon nanotubes as both electron and hole collecting layers in PV cells. We have demonstrated the versatility of this approach by making PV cells using perovskite, organic semiconductor and colloidal quantum dot absorbing layers. For example, we showed that it is possible to reach more than 18% efficiency with carbon nanotube top electrodes in perovskite solar cells.

We propose an experimental project which will build on these results and continue with the development of nanotube/polymer/graphene/titania/tin
oxide combinations to produce the next generation of nanohybrid particles for electrodes. It will aim at exploring the possibilities of integrating carbon nanotubes based approaches into PV cells both as external contacting layers and as internal electrodes capable of implementing tandem cell designs for various PV technologies. In addition to spin-coating, we will explore the capabilities of a custom made spray-coater recently developed in the department which can deposit nanoparticles including carbon nanotubes, aiming to make this process compatible with roll-to-roll fabrication.

The three main objectives of this project are:

• Understand the wrapping of carbon nanotubes
• Make solar cells with nanotube/polymer/graphene/titania/tin oxide combinations to develop the next generation of PV electrodes.
• Demonstrate spray coating of these electrodes compatible with roll-to-roll fabrication.

Typical device structure:

RiedeNicholasDevice - Graphic_2.jpg

This project is supervised by Prof Robin Nicholas and Dr Moritz Riede in association with the EPSRC Centre for Doctoral Training in New and Sustainable Photovoltaics - see http://www.cdt-pv.org

Supervisor: Prof Robin Nicholas robin.nicholas@physics.ox.ac.uk +44 (0)1865 272250
Co-Supervisor: Dr Moritz Riede moritz.riede@physics.ox.ac.uk +44 (0)1865 272377

Prof Henry Snaith

Perovskite Solar Cells


(Lee et al. Science 2012)

In the last 12 months we have had an unexpected breakthrough in the field
of emerging photovoltaics with the realization of over 15% efficient solid-state hybrid solar
cells based on organometal halide perovskite absorbers. These solar cells are fabricated from extremely inexpensive material and are remarkably efficient, with prospects for significant improvement in the next few years, setting them apart from other emerging PV technologies. We have DPhil projects centred around perovskite solar cells, with activity ranging from synthesising new perovskite absorbers, with a goal to understand the influence of structure and composition upon charge generation and electron-hole lifetime, through to thin film processing, device fabrication and advanced spectroscopy.

Email Prof Henry Snaith at h.snaith@physics.ox.ac.uk

Prof Snaith has a further project, which is a joint project with Prof Laura Herz. Please see entry above for 'Prof Laura Herz & Prof Henry Snaith'.

Prof Robert Taylor

Electrically-driven single photon sources in non-polar nitrides

This project involves the use of photon counting microphotoluminescence spectroscopy to investigate the optical properties of InGaN quantum dots grown and patterned into electrically driven single photon sources. The student will be involved in cryogenic experiments using pulsed and CW lasers and pulsed high-frequency electrical sources. The Taylor group has extensive experience in optical spectroscopy of semiconductor materials and the student would join a vibrant group which has an excellent track record of publication in high-impact journals.

A quantum light source is a device which generates either individual photons or pairs of photons in a regulated streami. Such devices are a key enabling technology for future quantum systems, enabling secure communication networks, new paradigms of computation and new metrological techniques. The project will seek to move away from impractical systems which require either large external pump lasers or cryogenic cooling (or both), towards compact voltage-driven devices, operational at room temperature or using only on-chip Peltier cooling. Moreover, if the source can provide deterministically polarised photons, this enhances the efficiency of quantum key distribution (QKD) systemsii. The figure below shows some promising recent results which underpin the work of this project.

Full details of Prof. Taylor’s research and publications can be found here.
Email Prof. Robert A. Taylor at: r.taylor1@physics.ox.ac.uk

Taylor, R graphic_0.jpgDeterministically polarised non-polar nitride SPSs have been demonstrated operating in electroluminescence (at 5 K) and in photoluminescence (at 220 K).

i Shields, A.J. Nat. Photon. 2007, 1, 215.
ii P. Bhattacharya Semicond. Sci. Technol. 2011 26 014002.

Prof Stephen Tucker

Ion Channels and Nanopores: From Structure to Function

Almost every single process in the human body is controlled at some level by electrical signals, from the way our hearts beat, the way our muscles move, to the way we think. These electrical signals are generated and controlled by a family of proteins called 'ion channels' which reside in the membrane of every living cell and which act as 'electrical switches' to control the selective movement of charged ions like potassium (K+) and Sodium (Na+) into and out of the cell.

Work in our lab uses a range of multidisciplinary approaches (molecular biology, electrophysiology, single-molecule fluorescence, molecular dynamics and crystallography) to study the structure and function of these channels. We have a range of projects available to people with physics, engineering, computing, biochemistry and physiology backgrounds.

In particular, we currently have an exciting new PhD project available to investigate how the unusual behaviour of water within the nano-sized pore of an ion channel influences its behaviour. This project would suit someone with a computational background and/or programming skills as it involves development of a new software tool to annotate membrane protein structures.

In collaboration with other colleagues in Oxford, we also have PhD projects available to work on the structural determination of ion channel drug binding sites using serial femtosecond crystallography and X-ray free electron laser (XFEL).

For further details, contact Prof Stephen Tucker at stephen.tucker@physics.ox.ac.uk or visit https://biophysics.physics.ox.ac.uk/tucker/

Prof Andrew Turberfield

DNA Nanostructures

Self-assembly of building blocks of synthetic DNA can be used to create molecular devices with nanometre precision, with applications from physics and computer science to structural biology and medicine. Current projects include: the physics of self-assembly; molecular electronics; molecular robotics; design and fabrication of synthetic molecular motors, including single-molecule characterization; autonomous molecular machinery for programmable chemical synthesis and molecular assembly; molecular computation; DNA nanostructures for structural biology; hybrid DNA-peptide structures; nanostructures to control gene editing and as intelligent drug delivery vehicles.
More ...
Email Prof Andrew Turberfield at: a.turberfield@physics.ox.ac.uk

Maria Sklodowska Curie Innovative Training Network in DNA Robotics

Two 'Early Stage Researcher' positions are available as part of the DNA Robotics Marie Sklodowska Curie Innovative Training Network www.DNA-Robotics.eu. These positions are suitable either for students who wish to undertake a first doctoral degree (Oxford DPhil) or as a first salaried position for researchers currently studying for a PhD that will be awarded after recruitment - see www.recruit.ox.ac.uk/pls/hrisliverecruit/erq_jobspec_version_4.jobspec?p...
The successful candidates will develop techniques for the design, construction, actuation and characterization of functional synthetic nanostructures and for their integration to create modular molecular robotic systems.
More ...
Email Prof Andrew Turberfield at: a.turberfield@physics.ox.ac.uk