DPhil Research Projects 2021/22

For information about how to apply, entry requirements, funding and costs please visit our main page on the University website.

Informal enquiries may be directed by email to the relevant potential supervisors.

Biological Physics

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 information can be found here.

Informal enquiries should be address to richard.berry@physics.ox.ac.uk

Prof Achillefs Kapanidis

Nanoscale imaging of molecular machines working on DNA

Typical custom microscope and DNA polymerase complex

As Richard Feynman noted back in 1959, “it is very easy to answer many of these fundamental biological questions…you just look at the thing!” Our research group is turning this bold vision into a reality by developing and applying cutting-edge methods to monitor sub-nanometre motions of single molecular machines with sub-millisecond temporal resolution. Our work focuses on the mechanisms of the protein RNA polymerase, which copies DNA into RNA during the process of gene transcription. This protein is a major target for antibiotics, making our work important for understanding and controlling antibiotic resistance of bacteria, one of the most pressing health challenges the world faces today.

Available projects focus on watching the nanoscale dynamics of RNA polymerase in real-time to find new steps and paths in transcription, and on understanding how antibiotics control nanoscale motions of RNA polymerase to control the spread of pathogenic bacteria. These efforts rely on our ability to detect single molecules of RNA polymerase as they copy DNA fragments that resemble expressed genes.

Projects will involve single-molecule imaging (see left Figure for a typical custom microscope in our lab), measurements of 1-10 nm distances within protein-DNA complexes (see right Figure for an example of DNA polymerase complex) based on dipole-dipole interactions between dyes attached to specific sites (via the method of single-molecule FRET), molecular modeling, quantitative modeling of reaction kinetics, and advanced signal processing of large data sets.

For a recent example of our work:
Duchi, Bauer… Kapanidis, Molecular Cell 2016, 63, 939-950
Dulin, Bauer… Kapanidis, Nature Communications 2018, 9, article number: 1478

For more information, look at our website, or email Prof Achilles Kapanidis at a.kapanidis1@physics.ox.ac.uk

Super-resolution imaging of DNA repair and gene expression in living cells

PSF Single DNA polymerases

A new type of optical microscopy is currently revolutionizing biology and medicine. Super-resolution imaging, as it is known, relies on ingenious ways that circumvent the resolution limit imposed by the diffraction of light; the potential of these methods was recognized by the award of the 2014 Nobel Prize in Chemistry to three physicists. A popular super-resolution method relies on detecting single fluorescent molecules, and is known as single-molecule localization microscopy. Localization microscopy bypasses the diffraction limit by finding the location of a fluorescent probe with a precision of 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, 250-times better than the optical resolution limit.

We offer projects on three main areas in super-resolution imaging. First, the development of new ways to break the diffraction limit either for immobile or diffusing molecules. Second, the adaptation of existing methods to study the 3-D organization, diffusion, and mechanisms of protein machines in single cells (see Figure far right panel for detection of single DNA polymerases in bacteria); we are interested in how DNA-binding proteins combine 1-D and 3-D diffusion to find their targets (a difficult problem: finding ~20 letters within 4,500,000 letters of chromosomal DNA), how cells repair their DNA rapidly and incredibly accurately, and how bacteria modulate their gene expression in a noisy environment. Third, the development of quantitative and stochastic mathematical models to describe the DNA-related processes we study.

For a recent example of our work: Stracy,… Kapanidis, PNAS 2015, 112, 4390-4399

For more information, look at our website, or email Prof Achilles Kapanidis at a.kapanidis1@physics.ox.ac.uk

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 D.Phil 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).

Informal enquiries should be addressed to stephen.tucker@physics.ox.ac.uk

Prof Andrew Turberfield

Self-Assembled Structures and Devices

Self-Assembled Structures and Devices

We are exploring applications of biomolecular nanofabrication at the interface between the physical and life sciences. DPhil projects are potentially available in any of the areas outlined below. We come from a wide range of scientific backgrounds, including physics, chemistry, biochemistry / molecular biology, engineering and computer science, and apply tools and techniques from these disciplines to important problems in science and technology. In 2021 we will be moving to a new interdisciplinary research centre, just across the road from our current home in the Clarendon Laboratory, which will greatly enhance our ability to undertake collaborative projects with colleagues in the biomedical sciences.

We create nanostructures from biopolymers – especially DNA and, increasingly, RNA. We control assembly by designing the base sequences of short, synthetic, strands of DNA to control their interactions. Programmed self-assembly enables rapid and flexible construction with near-atomic precision: designs can be developed in days and structures (sometimes!) made in minutes.

• Our work on synthetic molecular machinery includes molecular motors, a ‘synthetic ribosome’ (genetically-programmed molecular machinery capable of sequence-controlled synthesis of non-biological polymers), and molecular robotic devices that combine autonomous sensing, computation, and actuation. Some of the most exciting potential applications of molecular robots are in medicine: we are also developing “theranostic” devices, integrated systems that are capable of therapeutic intervention with single-cell resolution.
• We are developing nanostructures as intracellular sensors and markers, for ultra-high-resolution structural measurements by cryo-electron tomography.
• We study nanostructure folding, with the aim of understanding and designing assembly pathways in order to enable efficient construction. Particularly interesting, and challenging, is the creation of a nanostructure from a single strand of RNA as it is generated (transcribed) in vitro or in a cell.
• We use the nanometre-scale architectural control provided by DNA assembly to study the interactions between peptides (with the Woolfson group, Bristol Chemistry) and the assembly of protein components to make large molecular machines such as the bacterial flagellar motor (with the Berry group, Oxford Physics).
• We are using DNA templates to organise molecular components into circuits to create and study molecular electronic devices (with the Ardavan group, Oxford Physics).
Examples of publications from our wide range of interdisciplinary projects:
Peptide Assembly Directed and Quantified Using Megadalton DNA Nanostructures. J. Jin et al., ACS Nano 13, 9927-9935 (2019)
The Evolution of DNA-Templated Synthesis as a Tool for Materials Discovery. R. K. O’Reilly et al., Acc. Chem. Res. 50, 2496-2509 (2017)
An Autonomous Molecular Assembler for Programmable Chemical Synthesis. W. Meng et al.,Nature Chem.8, 542–548 (2016)
Guiding the folding pathway of DNA origami. K. E. Dunn, F. Dannenberg, T. E. Ouldridge et al.,Nature 525, 82–86 (2015)
Direct observation of stepwise movement of a synthetic molecular transporter. S. F. J. Wickham et al., Nature Nanotechnol. 6, 166-169 (2011)

For more information email Prof Andrew Turberfield at andrew.turberfield@physics.ox.ac.uk

Quantum Materials

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).

Informal enquiries should be address to arzhang.ardavan@physics.ox.ac.uk

Prof Stephen Blundell

Persistent spin dynamics in spin liquids

A muon is a spin-1/2 particle. When implanted in a solid, the muon behaves as a microscopic magnetometer. This is because its spin can precess in the local magnetic field. In various frustrated magnets it is possible for muons to probe low-frequency spin dynamics. By combining muon-spin rotation experiments and a.c. susceptibility one can learn a lot about these slow fluctuations which are known to persist to low temperature. But are these so-called “persistent spin dynamics” an intrinsic effect, or are they a highly subtle artefact of the muon implantation process? This project is designed to find out.

Informal enquiries should be address to stephen.blundell@physics.ox.ac.uk

High magnetic field studies of quantum magnets

The Nicholas Kurti High Magnetic Field Laboratory in the Clarendon Laboratory is capable of producing the highest magnetic fields for condensed matter physics experiments in the UK. This is achieved using pulsed magnetic fields which are short in duration and are a factor of 2 or 3 times higher than the ~20T limit set by superconducting magnets. This allows experiments in which quantum magnets are pushed into new regimes. Some of these quantum magnets are based on inorganic materials, but why not use molecules? This gives a much more flexible approach to understand low-dimensional quantum magnetism.
https://www2.physics.ox.ac.uk/sites/all/modules/bueditor/icons/x1.png

Informal enquiries should be addressed to stephen.blundell@physics.ox.ac.uk

Ultra-high magnetic fields for understanding complex quantum materials

Magnetic fields are a unique tool to explore and tune quantum materials towards extreme experimental regimes in which new phases of matter can be stabilized. Additionally, magnetic fields are essential to characterize the phase diagram of novel superconductors to identify suitable candidates for practical applications. This is an experimental project to explore and develop new experimental techniques for studying quantum materials in ultra-high magnetic fields. The quantum materials to be explored will include novel iron-based superconductors and molecular magnets, as well as systems in which the spin, electronic and lattice degrees of freedom interact strongly and can be disturbed by a magnetic field.

The student will be combining transport and thermodynamic techniques using superconducting magnets and pulsed field magnets available in Oxford. The pulsed magnetic fields are short in duration but up to a factor of 3 times higher than the field produced by superconducting magnets. Oxford has a long tradition in high magnetic field research having the largest magnetic fields in the UK for condensed matter physics both using pulsed field in the Nicholas Kurti High Magnetic Field Laboratory as well as superconducting magnets up to 21T as part of the High Magnetic Field facilities and the new Oxford Centre for Applied Superconductivity (http://www.cfas.ox.ac.uk/).

A suitable candidate needs to have a good understanding of condensed matter physics and good computational skills as well the ability to work well in an experimental team. The student will be co-supervised by Professor Stephen Blundell and Professor Amalia Coldea.

Informal enquiries should be address to stephen.blundell@physics.ox.ac.uk or amalia.coldea@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.

Informal enquiries should be address to a.boothroyd1@physics.ox.ac.uk

Prof Amalia Coldea

Projects available in our group for 2021 are listed below. Please get in touch with Prof Amalia Coldea (amalia.coldea@physics.ox.ac.uk) for further details.

We welcome applications from enthusiastic students who are excited and challenged by understanding the rich novel physical phenomena displayed by novel quantum materials.

Details about the application process can be found here and concerning the available scholarships can be found here.

1. Ultra-high magnetic fields for understanding complex quantum materials

Magnetic fields are a unique tool to explore and tune quantum materials towards extreme experimental regimes in which new phases of matter can be stabilized. Additionally, magnetic fields are essential to characterize the phase diagram of novel superconductors to identify suitable candidates for practical applications. This is an experimental project to explore and develop new experimental techniques for studying quantum materials in ultra-high magnetic fields. The quantum materials to be explored will include novel iron-based superconductors and molecular magnets, as well as systems in which the spin, electronic and lattice degrees of freedom interact strongly and can be disturbed by a magnetic field.

The student will be combining transport and thermodynamic techniques using superconducting magnets and pulsed field magnets available in Oxford. The pulsed magnetic fields are short in duration but up to a factor of 3 times higher than the field produced by superconducting magnets. Oxford has a long tradition in high magnetic field research having the largest magnetic fields in the UK for condensed matter physics both using pulsed field in the Nicholas Kurti High Magnetic Field Laboratory as well as superconducting magnets up to 21T as part of the High Magnetic Field facilities and the new Oxford Centre for Applied Superconductivity (http://www.cfas.ox.ac.uk/).

A suitable candidate needs to have a good understanding of condensed matter physics and good computational skills as well the ability to work well in an experimental team. The student will be co-supervised by Professor Stephen Blundell and Professor Amalia Coldea.

Informal enquiries should be address to stephen.blundell@physics.ox.ac.uk or amalia.coldea@physics.ox.ac.uk

2. Developing electronic tunable devices of thin flakes of iron-based superconductors

This project is to explore electronic and topological behaviour of quasi-two dimensional devices based on thin flakes of highly crystalline superconducting iron-based chalcogenides as well as Dirac and Weyl semimetals. The project will involve device preparation and a suite of physical properties measurements to study their electronic properties using high magnetic field and low temperatures. The aim is to search for quantum phenomena as well as for signature of topological matter in these highly tunable quantum material devices. The project will be hosted by the recently funded Oxford Centre for Applied Superconductivity (CfAS) in the Department of Physics. The student will investigate the phase diagrams of novel superconducting thin flake devices under different extreme conditions of high magnetic field, strain and pressure. Experiments using advanced techniques for transport will be performed using high magnetic field facilities available in Oxford and elsewhere. A suitable candidate needs to have a good understanding of condensed matter physics and good computing skills as well the ability to work well in an experimental team.

3. Tuning electronic ground states and superconductivity of iron-based superconductors under extreme experimental conditions

Applied hydrostatic pressure is a unique tuning parameter to study the characteristics of a nematic quantum critical point in the absence of long-range magnetic order in a single material and to gives access to the electronic structure and correlations of new magnetic and structural phases. FeSe is an unique superconductor that show a nematic electronic phase in which absence of magnetism at ambient pressure. However, a magnetic phase is stabilized at high pressure and superconductivity is enhanced four-fold. By combining the chemical pressure with the hydrostatic pressure in the series FeSe1-xSx, it is possible to separate the nematic and magnetic phases. This project will aim to understand the evolution of the complex Fermi surfaces and electronic interactions across the nematic phase transitions using applied hydrostatic pressure in different iron-based superconductors. High magnetic field and low temperatures will be used to access directly the Fermi surface by detecting quantum oscillations in different ground states tuned by applied hydrostatic pressure. A suitable candidate needs to have a good understanding of condensed matter physics and good experimental and computational skills as well the ability to work well in an experimental team.

4. Revealing topological signatures in the electronic behaviour of bulk quantum materials with Dirac dispersion

This is an experimental project combining electronic transport and quantum oscillations to detect unusual signatures of the manifestation of topology in single crystals of quantum materials with Dirac dispersions. The student will perform a series of studies in high magnetic fields and at low temperatures to search for evidence of non-trivial Berry phases and low temperature quantum transport. Studies will be also extended under applied pressure and strain to identify proximity to new toplogical superconducting phase. The work will be combined with first-principle band structure calculations to compare with experiments and disentangle trivial from non-trivial effects. A suitable candidate needs to have a good understanding of condensed matter physics and good computing skills as well the ability to work well in an experimental team.

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.

Informal enquiries should be address to radu.coldea@physics.ox.ac.uk

Prof JC Séamus Davis

Atomic-scale Visualization of Quantum Matter

2x D.Phil studentships are available from ERC - European Research Council

The studentships are part of an European Research Council funded project entitled "MILLIKELVIN VISUALIZATION OF TOPOLOGICAL ORDER (mVITO)" being undertaken by the University of Oxford, UK and University College Cork, IE under PI JC Séamus Davis.

The objective of this project is to develop and apply new techniques for atomic-scale visualization of electronic and magnetic quantum matter. Per interest and availability, the student will focus on one of the following sub-projects:

Visualization of electronic wavefunctions in topological Kondo insulators or heavy fermion superconductors, using millikelvin scanning tunnelling microscopy.

Visualization of Cooper-pair condensates or pair density wave states using scanned Josephson tunnelling microscopy.

Visualization of classical and quantum spin liquids using scanned magnetic-flux microscopy.

S/he will be a member of the Davis Group and will be supervised by Prof. JC Séamus Davis. Further information on the group and its research can be found here.

Prospective candidates will be judged according to how well they meet the following criteria:

• Demonstrates curiosity, creativity and courage in scientific research.
• A first class honours degree in Physics or equivalent.
• Experience in low temperature physics at liquid-helium temperatures T=4.2K or below
• Experience in ultra-high vacuum scanned probe microscopy.
• Experience in high-volume image-data management and analysis.
• Excellent English or Irish written and spoken communication skills;

The following skills are desirable but not essential:

• Ability to program in Matlab and/or Python.
• Ability in Labview-based experiment design and management.
• Ability in cryogenic operations with liquid helium, and ultra-high vacuum tech.
• Ability with modern theoretical techniques of quantum matter research.

Informal enquiries should be address to jcseamusdavis@gmail.com

Visualizing Quantum Matter at the Atomic-Scale

DAVIS GROUP RESEARCH OPPORTUNITIES

Davis Group research concentrates upon the fundamental physics of electronic, magnetic and atomic quantum matter. A specialty is development of innovative instrumentation to allow direct visualization (or perception) of characteristic quantum many-body phenomena at atomic scale. Among the fields of active interest today are:

Capture.PNG

Davis Group plans to operate two suites of ultra-low vibration laboratories, one in Beecroft Building at Oxford University (UK) and the other in the Kane Building at University College Cork (IE). Ours is as single research group conducting scientifically harmonized studies with complementary scientific instruments at Oxford and Cork. The overall objective is to exploit the distinct capabilities and facilities at both laboratories to maximize scientific efficiency.

Our immediate research objectives and associated collaborators include:

• Cooper-Pair Condensates
• Magnetic Monopole Fluids
• Magnetic Topological Insulators
• Topological Kondo Insulators
• Cu/Fe HT Superconductors
• Viscous Electron Fluids
• Macroscopic Quantum Interferometers
• Quantum Microscope Development

Please check here for further details.

Informal enquiries should be addressed to jcseamusdavis@gmail.com

Prof John Gregg

Magnonic Computing

The end of Moore's Law has long been prophesied, but its effects have been subtly present for over a decade: since 2004, computer processor clock speeds have been frozen circa 4GHz, as a desperation measure to prevent heat death of the chips. As increasing functionality is packed at ever higher density into semiconductor devices, the resulting heat dissipation yields multiple issues: device unreliability; inability of battery technology to keep pace with the power demands of portable devices such as phones and tablets; and the enormous heat generation of "server farms" - performing 30 Google searches is claimed to dissipate enough heat to boil a kettle.

Alternative computing technology based on magnons (waves of propagating angular momentum that exist in ordered magnetic materials) offers an elegant and viable room temperature solution to these problems. Magnonic processors use 1/1000 of the power of their silicon counterparts, are engineerable on the nanoscale and have clock speed ceilings that are potentially in the TeraHertz. Sophisticated logic devices such as XOR gates and half adders have already been demonstrated as has a magnonic equivalent of the field effect transistor. Moreover, magnonic computing paradigms offer functionality and economy of "real estate" that is impossible with silicon, such as the ability simultaneously to perform different operations on parallel datastreams using the same hardware. Recent work by our team demonstrates magnonic ability to perform the operations of time reversal and phase conjugation with a view to combining the speed of analogue computing with the versatility of digital.The D.Phil. project here described will involve further developing this new microwave science and its integration into conventional electronic hosts.

Informal enquiries should be addressed to john.gregg@physics.ox.ac.uk

Prof Thorsten Hesjedal

Reflectivity ferromagnetic resonance (RFMR) for layer-resolved dynamic study of multi-layered systems

image 3.jpg

Magnetic multilayers offer immense opportunities for the development of ultrafast functional devices. They are prime example of advanced materials where their functionalities can be precisely engineered through control of the layer properties, the coupling across the interfaces, and that between layers. They have enabled a massive downscaling of magnetic memory and contributed to exciting developments in fields such as skyrmions in topological magnetic materials, synthetic antiferromagnets, and spintronic devices. Most established magnetic characterization techniques aim at measuring the macroscopic properties, only sensitive to the total magnetization of a material, but are unable to discriminate between contributions from constituent atoms or layers. With increasing demand for materials for high frequency applications, techniques for probing both the depth- and time-resolved magnetization dynamics are urgently needed.

Very recently, we established a new technique in which ferromagnetic resonance (FMR) is combined with x-ray resonant reflectivity to reveal the depth-dependent dynamics within magnetic heterostructures. While FMR is widely used to probe magnetization dynamics in the frequency domain, x-ray reflectivity has become commonplace for characterizing the depth-dependent structure of layered materials, probing x-ray interference effects between layers. At resonance with an element-specific absorption energy, additional insight into the magnetic structure is obtained.

RFMR combines the best of both worlds, allowing layer-resolved insight into the complex dynamic behaviour of multilayers [D. M. Burn et al., Phys. Rev Lett. 125, 137201 (2020) – see also the Physics Viewpoint by Dario Arena and the Diamond Science Highlight entitled “High speed devices – What happens beneath the surface?”]. RFMR is a synchrotron-based technique, and Diamond with its unique technical facilities and beamline support is the ideal place to bring RFMR to full fruition.

The full development of this novel technique will provide a superb training and research environment for the student, who will be mainly based at the Harwell Research Campus. The outcome of the research project will establish the foundation of RFMR for the exploration of complex thin film and multilayer materials for future magnetic memory and processing device applications.

The student will be trained in the following research techniques:
• Sample growth: Magnetron sputtering Oxford and Diamond.
• Ex-situ characterization (SQUID, XRD/XRR, MOKE) at Diamond and ISIS.
• REXS: soft x-ray resonant reflectivity in RASOR.
• XAS/XMCD/XMLD: X-ray spectroscopy at Diamond.
• Ferromagnetic resonance (FMR) in Magnetic Spectroscopy Lab at Diamond

This studentship is funded through the Joint Diamond -ISIS Doctoral Studentship Programme.

Email Prof Thorsten Hesjedal at: thorsten.hesjedal@physics.ox.ac.uk Prof Gerrit van der Laan at: Gerrit.vanderlaan@diamond.ac.uk

Interfacial magnetism in topological insulator heterostructures

image 4_0.png

Topological insulators (TIs) can host dissipationless electrical transport channels without the need for low temperatures, high pressures, or high magnetic fields. This phenomenon is called the quantum anomalous Hall effect (QAHE) and was predicted by Douglas Haldane who won a Nobel Prize for his contributions to the field of topological materials. It requires that magnetic order is introduced to the non-magnetic topological insulator. The common approach so far had been to magnetically dope the topological insulator with magnetic atoms such as chromium or vanadium.

The challenge
At this early stage of research into the quantum anomalous Hall effect only a few successful experiments have been reported. The reason is that magnetically doped topological insulators are anything but perfect for this purpose. Consequently, carrying out experiments is challenging and requires very low temperatures, even lower than the ones required for superconductors. Thus, the main challenge is to improve the materials with the goal to increase the operating temperature. This will facilitate experiments and make them attractive for technological applications.

The approach
The research idea followed in this project is to combine a topological insulator and an antiferromagnet. In contrast to magnetic doping, no intermixing between the antiferromagnet and topological insulator will take place. The magnetic order in the topological insulator is induced due to the close proximity to the antiferromagnet. The benefit of this approach is that the structural, electronical, and, importantly, topological properties of the topological insulator are not detrimentally affected. Furthermore, in contrast to a ferromagnet, an antiferromagnet does not produce a magnetic stray field which could disturb the magnetic order in the topological insulator.

Theory predicts that a topological insulator can be magnetised more efficiently, and therefore up to higher temperatures, using this approach. The higher robustness of the magnetic ordering and the homogeneity of the underlying polarisation raise the expectation that the quantum anomalous Hall effect can be observed at higher temperatures. Combining topological insulators and antiferromagnets has the potential to be the platform for the next generation of energy-efficient electronic devices and will have applications in metrology and potentially topological quantum computing.

Student deliverables
The Oxford-Diamond-ISIS joint project will deliver ground-breaking experimental results on topological magnetic materials, in particular TIs proximity-coupled to an antiferromagnet, using an integrated, synchrotron- and neutron-based multi-tool approach. The significance of this project is to expand the operation of the QAHE towards even higher temperatures by antiferromagnet-TI proximity coupling. We will make use of the combined unique capabilities of Diamond and ISIS, carrying out a multi-method study of the magnetic properties (XAS/XMCD/XMLD, XPEEM, PNR) and electronic properties (HAXPES, transport).

The student will receive training in the following research techniques:
• Sample Growth: The TI thin-film samples will be grown by MBE in the Research Complex at Harwell (RCaH). The films will be monitored using in-situ RHEED and characterized structurally ex-situ using XRD/XRR and AFM (DLS/ISIS).
• Magnetometry: The samples will be investigated by SQUID magnetometry and MOKE (ISIS).
• XAS/XMLD/XMCD: X-ray spectroscopy on the Mn/Cr/V/Fe L2,3 edges. Comparison with calculated spectra. Understanding of the long-range-ordered ground state.
• XPEEM: X-ray imaging of the antiferromagnetic domain structure as a function of temperature. Materials optimisation to achieve a monodomain spin structure, important to observe the QAHE.
• HAXPES: Photoemission spectra reveal band-bending effects at interfaces between different semiconducting materials. Materials will be optimised to minimise these effects because they can trap topologically trivial electronic states, which can interfere with the topological states.
• PNR: Polarised neutron reflectivity studies of the layer-resolved magnetism. In contrast to top-surface sensitive soft x-ray techniques they allow to map the spin structure inside of the sample.
• Electric transport: Anomalous Hall effect studies of patterned TI samples in Oxford and at ISIS.

Workplace: Harwell Science and Innovation Campus

Supervision:
- Prof Thorsten Hesjedal (Oxford) - thorsten.hesjedal@physics.ox.ac.uk
- Dr Dirk Backes (Diamond Light Source) - dirk.backes@diamond.ac.uk
- Prof Sean Langridge (ISIS/STFC) – sean.langridge@stfc.ac.uk

This studentship is funded through the Joint Diamond -ISIS Doctoral Studentship Programme. .

Magnetic Skyrmionics

image 1_1.png

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 (thin film) growth, study, and device exploration of novel, low-dimensional skyrmion-carrying materials and heterostructures. 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 thorsten.hesjedal@physics.ox.ac.uk

Recent (2020) group publications on Skyrmionics:
[1] Y. Guang et al., Electron Beam Lithography of Magnetic Skyrmions, Advanced Materials 32 (2020), doi: 10.1002/adma.202003003
[2] S. L. Zhang et al., Robust Perpendicular Skyrmions and Their Surface Confinement, Nano Letters 20, 1428 (2020); doi: 10.1021/acs.nanolett.9b05141
[3] R. Brearton, G. van der Laan, T. Hesjedal, Magnetic skyrmion interactions in the micromagnetic framework, Phys. Rev. B 101, 134422 (2020); doi: 10.1103/PhysRevB.101.134422
[4] D. M. Burn et al., Field and temperature dependence of the skyrmion lattice phase in chiral magnet membranes, Phys. Rev. B 101, 014446 (2020); doi: 10.1103/PhysRevB.101.014446
[5] K. Zeissler et al., Diameter-independent skyrmion Hall angle observed in chiral magnetic multilayers, Nature Communications 11, 428 (2020); https://www.nature.com/articles/s41467-019-14232-9

Imagine 2.png

Leverhulme Project:
Nanocluster Engineering for High-Performance Topological Quantum Materials

About the Project:

A DPhil studentship is available in the Thin Film Quantum Materials group to explore the question if quantum confinement can help to resolve the bottlenecks in topological quantum materials, paving the way for novel quantum device applications. Up to now, our group has strived to synthesise ultra-pure materials of the highest quality, but this project is quite the opposite! You will be based in the Quantum Materials group in Oxford and at the Rutherford Appleton Laboratory in Harwell, supervised by Andreas Frisk and Thorsten Hesjedal.

You will use a range of methods to reach the project goal of establishing nanocrystalline topological quantum materials for applications:
• Thin film materials synthesis and systematic doping studies
• Lab-based structural (x-ray diffraction, transmission electron microscopy with energy-dispersive x-ray spectroscopy) and electronic (transport) characterisation
• Magnetic studies using polarised neutron reflectometry and synchrotron-based x-ray magnetic circular dichroism
• Exploratory functional device studies making use of tailored heterostructures

You have a background in physics, electrical engineering or materials science (with an excellent knowledge of condensed matter physics), you are a gifted hands-on experimentalist, an outstanding team player, and committed to becoming an extraordinary future scientist.

Funding Notes
A 3.5 year funded studentship is available for a UK/ROI student.

ThinFilm Quantum Materials.jpg

Please contact Thorsten Hesjedal (thorsten.hesjedal@physics.ox.ac.uk) for further questions.

Prof. Paolo G. Radaelli

Harnessing the power of topology in oxide electronics for future IT components

This project will be supervised by Prof. Paolo G. Radaelli

Fe2O3_Merons.png

In spite of its extraordinary success in fuelling the IT revolution, silicon (CMOS) technology is intrinsically energy-inefficient, because it relies on the movement of electrical charge, which is associated with Joule heating. One of the front runners among ‘beyond-CMOS’ technologies is spintronics, which relies on spins rather than charges to transfer and process information; however, much of the energy efficiency of spintronics is lost if spin flipping – the elementary spintronic operation – must in itself be performed by electrical currents. For this reason, voltage control of magnetic components is widely considered to be the key to large-scale commercialisation of spintronics [1-4]. The field of oxide electronics emerges precisely from the consideration that oxides, especially those containing magnetic transition metal ions such as Co, Mn and Fe, can display a multitude of intriguing electrically-controlled multi-functional properties in their insulating states, whilst integration with CMOS is already a reality.
The potential of oxide electronics can be further enhanced by exploiting the power of topology, which involves, quite literally, tying spins into ‘magnetic knots’. In work to be published soon in Nature [5] an international team of collaborators lead by Professor Paolo G. Radaelli (Oxford Physics) presented a major breakthrough in this field: they created, for the first time, a wide family of nanoscale antiferromagnetic topological spin textures – (anti)merons and bimerons at room temperature in iron oxide (α-Fe2O3).This follows their recently published work in Nature Materials [6,7], where they observed that these topological textures couple strongly with ferromagnetic metallic cobalt (Co). One particularly appealing feature of this system is that it employs cheap and readily available materials (α-Fe2O3 is the most abundant constituent of common rust!) and relatively simple fabrication, raising hopes that these systems could be deployed on a commercial scale in the future (for extended lay descriptions, see the Oxford Physics Newsletter – Autumn 2018 and this Diamond Research Highlight).

This EPSRC-funded DPhil project will give the successful candidate the opportunity to develop this line of research in different directions, both fundamental and applied:
• Identify and grow new oxides with topological magnetic states, study their fundamental properties and image the topological structures at the nanoscale using state-of-the-art microscopy techniques.
• Experiment with novel ways to control topological magnetic states, exploiting either intrinsic magnetoelectric properties or interactions with active substrates
• Design and test prototype devices, built using electron beam lithography and other clean room processes.
This project is likely to involve a combination of experimental techniques, such as:
Growth of thin films and devices – currently in collaboration with the groups of Prof. Thorsten Hesjedal (Oxford Physics), Prof. Chang-Beom Eom (Univ. of Wisconsin – Madison) and Prof. Venky Venkatesan (National University of Singapore).
Advanced microscopy. To image multi-functional domains, which are the fundamental unit of information storage in oxides, we employ spectral microscopy (PEEM – we invented and continue to develop many of the relevant data analysis methods at the Diamond synchrotron), Magnetic Force Microscopy (MFM – in house) Magneto-Optical Kerr Effect magnetometry/microscopy (new in house development) and Nitrogen Vacancy Centre Microscopy (new collaboration with the Max Planck Institute for Solid State Research in Stuttgart)
X-ray coherent imaging: this is a new direction, to be developed in collaboration with Prof. Laurent Chapon at the Diamond Light source, which should enable imaging of ferro- and antiferromagnetic domains with an unprecedented resolution of ~4 nm.
Dielectric and transport measurements (in house)
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.
Nanofabrication. In collaboration with National University of Singapore, 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 involve development of one or more of the above techniques (particularly the new techniques such as MOKE and N-V microscopy and X-ray coherent imaging) and may also include a computational element. In collaboration with Prof. Feliciano Giustino at the Oden Institute, Univ. of Texas, 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. We also employ micromagnetic simulations to study the formation and dynamics of topological structures such as vortices and merons [8]. There will also be an opportunity of extended stays at one of the collaborating institutes to acquire new skills.
For more information email Prof. Paolo Radaelli and visit the group webpages.
References
[1] Matsukura, F., Tokura, Y. & Ohno, H. Control of magnetism by electric fields. Nature Nanotechnology 10, 209–220 (2015).
[2] Eerenstein, W., Mathur, N. D. & Scott, J. F. Multiferroic and magnetoelectric materials. Nature 442, 759–65 (2006).
[3] Ramesh, R. & Spaldin, N. A. Multiferroics: progress and prospects in thin films. Nature Materials 6, 21–9 (2007).
[4] S. Manipatruni, et al, Nature 565, 35 (2019).
[5] H. Jani et al., arxiv.org/abs/2006.12699 [To be published in Nature] (2020)
[6] F. Chmiel et al., Nature Materials 17, pages 581–585 (2018)
[7] M. Fiebig, Nature Materials 17, pages 567–568 (2018)
[8] Radaelli P.G. et al., Phys. Rev B 101, 144420 (2020)

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).

photoferroicity.png

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. Recently it was realised that, under appropriate conditions, the rectified Raman distortion can transiently break the structural and/or magnetic symmetry of the crystal and hypothesised that such symmetry breaking would 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 [4]. Even more recently, this effect was experimentally demonstrated for the first time in our collaborators’ laboratory in Hamburg. Surprisingly, photo-ferroicity persisted for a significantly longer time than the carrier envelope (100s of ps). Although this is not yet fully understood, the most likely explanation is that magnetisation is being transferred to slower electronic/magnonic excitations.

This DPhil project will give the successful candidate the opportunity to pioneer this new field of research. Initial experiments on the ‘photo-ferroic’ materials that we have already characterised will be performed at the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, Germany. As a mode-selective pump, we are employing 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), Faraday rotation and dichroic absorption of visible/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 time-dependent 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.

References
[1] “Femtomagnetism: Magnetism in step with light”. Uwe Bovensiepen, Nature Physics 5, 461 - 463 (2009).
[2] See for example M. Mitrano,et al., “Possible light-induced superconductivity in K3C60 at high temperature”, Nature, 530, 461–464 (2016).
[3] “Nonlinear phononics as an ultrafast route to lattice control”, M. Först et. al., Nature Physics, 7, 854–856 (2011).
[4] “Breaking Symmetry with Light: Ultra-Fast Ferroelectricity and Magnetism from Three-Phonon Coupling”, P. G. Radaelli, Phys. Rev. B 97, 085145 (2018).
[5] Disa, A.S., Fechner, M., Nova, T.F., Tobia F. Nova, Biaolong Liu, Michael Först, Dharmalingam Prabhakaran, Paolo G. Radaelli & Andrea Cavalleri, “Polarizing an antiferromagnet by optical engineering of the crystal field”, Nat. Phys. 16, 937–941 (2020).

Semiconductor Materials, Devices & Nanostructures

Dr Marina Filip

Optoelectronic properties of hybrid halide perovskite semiconductors

Hybrid organic-inorganic perovskites have recently emerged as highly promising materials for a variety of optoelectronic devices, including solar cells and LEDs. They can be easily synthesized in a variety of chemical compositions, structures and dimensionalities through inexpensive solution processing methods, allowing for a broad tunability of their optoelectronic properties. However, while the materials space of hybrid organic-inorganic perovskites is continually expanding, their optoelectronic properties and structure-property relationships are not yet fully understood. In this project, we will study the electronic and optical properties of these novel materials from an atomistic perspective, using first-principles computational modeling methods, such as density functional theory (DFT), the GW method, and the Bethe-Salpeter equation (BSE).
First principles, or ab initio methods, are implemented in massively parallel computational packages which are highly optimized to operate efficiently on large supercomputing architectures, and tackle complex many-body problems in functional materials. As part of this project, we will use ab initio methods to understand many-body phenomena in complex semiconductors and insulators, and develop computational frameworks for calculating the optoelectronic properties of these materials accurately and efficiently.

Eligible candidates should have a Bachelor or Master degree in Physics, or related subjects, such as Chemistry or Materials Science. It is essential to have a background in Quantum Mechanics and Solid State Physics, as evidenced by your transcripts. Programming skills, experience with electronic structure codes, as well as any prior research experience are desirable but not essential.

Informal enquiries should be address to marina.filip@physics.ox.ac.uk

High throughput computational design of novel semiconductors for optoelectronic applications.

In the past decade, a tremendous mobilization of efforts in the Materials Science community has been directed towards the discovery of novel functional materials, to enable the development of new technologies. The task of designing a material that has never before been observed in real life requires the ability to predict how a certain arrangement of distinct chemical elements in the periodic table will react to form a material, and what physical properties this material might have. First principles computational materials modelling techniques are ideally suited for this task, because they rely on an atomistic perspective to predict materials properties. In this project we will use first principles methods to understand the fundamental physical properties of semiconductors and insulators, and design automated strategies to screen through a great variety of chemical compositions and structures in the search for semiconductors which are chemically stable, and have useful properties for optoelectronics. We will initially focus our attention on the heterogeneous and chemically diverse family of perovskites [PNAS, 115, 21, 5397-5402 (2018)]. First principles, or ab initio methods, are implemented in massively parallel computational packages which are highly optimized to operate efficiently on large supercomputing architectures. We will use and develop upon these techniques, as well as use materials databases, such as the Materials Project, to computationally design and discover new semiconductors and insulators.

Eligible candidates should have a Bachelor or Master degree in Physics, or related subjects, such as Chemistry or Materials Science. It is essential to have a background in Quantum Mechanics and Solid State Physics, as evidenced by your transcripts. Programming skills, experience with electronic structure codes, as well as any prior research experience are desirable but not essential.

Informal enquiries should be address to marina.filip@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.

Informal enquiries should be address to laura.herz@physics.ox.ac.uk

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.

Informal enquiries should be address to laura.herz@physics.ox.ac.uk

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.

Informal enquiries should be addressed to laura.herz@physics.ox.ac.uk

Prof Michael Johnston

Unveiling electron motion at surfaces and interfaces on ultrashort length and ultrafast time scales

project 1.png

Over the course of the project we will develop and implement a new instrument based on our recent advances in terahertz photoncs. The instraumnet will enable us to gain a deep understand nanoscale charge dynamics in metal halide perovskite semiconductors and semiconductor nanowires. The advances made will contribute to our active existing research programme in developing efficient multijunction solar cells.

Informal enquiries should be addressed to michael.johnston@physics.ox.ac.uk

Terahertz Photonics with Semiconductor Nanostructures

terahertz photonics.png

In this project you will develop novel photonic devices to enable a powerful form of ultrafast femtosecond spectroscopy at terahertz frequencies. The photonic devices will be based on ‘nanowires’, which are single crystals of semiconductors, with diameters of only tens of nanometre, but lengths of many microns. Owing to their geometry these nanowires have properties ideal for new and novel device applications. In particular the large surface area to volume ratio of nanowires allows these single crystals be grown in crystal structures that are not possible in the bulk forms of the materials, and allows for unusual light-matter interactions. The large surface area also makes nanowires ideal for applications such as chemical sensing and catalytic conversion.

During your D.Phil. the novel properties of nanowires will be exploited for spectroscopy at terahertz frequencies. The terahertz region of the electromagnetic spectrum encompasses a wide range of frequencies from the upper bound of microwave band to the lower bound of infrared light. The spectral region contains a wealth of spectroscopic information for a wide range of physical systems, with THz photons covering the characteristic energy scales of phonon, plasmons and excitons in semiconductors, and the correlations in solids that lead to phenomena such as superconductivity and magnetism.

In this project you will develop THz detectors and modulations based on nanowires, and implement them in state-of-the-art THz spectroscopy systems. You will also have the opportunity to exploit these new devices for investigating charge carrier dynamics in other novel semiconductors.

Informal enquiries should be addressed to michael.johnston@physics.ox.ac.uk

Ultrafast Terahertz Polarimetry

Ultrafast Teraherz .png

Single cycles of electromagnetic radiation are the ultimate tools for investigating light-matter interactions. According to the Heisenberg uncertainty principle (or indeed Fourier theory) a pulse of light very well localised in time will have a very broad frequency spectrum, thus single-cycle pulses are great tools for time-resolved spectroscopy. The spectral range of interest for many phenomena in Condensed Matter Physics is the terahertz frequency range, which corresponds to photon energies of ~1meV-20 meV. This is the energy range of the spectral features of charge transport in semiconductors, as well as the energies of quasiparticles associated with correlations, such as superconductor cooper-pair binding energies, phonon energies and exciton binding energies.

In this project you will utilise our recent development of cross-nanowire THz detectors (Science, 368:510--513, 2020) to examine the polarisation response of THz metamaterials, and the physics of semiconductors and magnetioc thin films via the THz Hall effect and Inverse Spin-Hall Effect respectively. You will also have the opportunity develop new THz devices as part of this project.

Informal enquiries should be addressed to michael.johnston@physics.ox.ac.uk

Vapour deposition of Perovskite Solar Cells

Vapour Disposition

Metal halide Perovskite (MHP) solar cells have emerged as promising semiconductor devices for next generation photovoltaics. Remarkably, the power conversion efficiency of single-junction solar cells has reached >25%. Efficient tandem solar cells based on perovskites have also recently been achieved. To date most research into MHP solar cells has focussed on solution processing, however this technique is challenging for multi-junction devices. This project will focus developing on highly efficient multijunction solar cells, using a vapour co-deposition technique.

The project will involve designing evaporation chamber components optimised of metal halide perovskite deposition, and devising new layer growth methodologies. The candidate will also gain experience in solar cell characterisation and a range of spectroscopy techniques.

Informal enquiries should be addressed to michael.johnston@physics.ox.ac.uk

Prof 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 inexpensive, efficient, light-weight and flexible renewable energy source made from earth abundant non-toxic raw materials. Best proof of this is that the current technology leader, the start-up Heliatek, is using both concepts.
There has been a significant improvement of power conversion efficiencies (PCSs) over the past years, enabled by novel materials, in particular non-fullerene acceptors, and record OSC achieved PCEs of ~17% by now. Previously unthinkable PCEs of >20% are within reach, which would get close to the main competing technology of silicon solar cells (~26%). For this, some loss mechanisms have to be overcome. The main loss mechanism 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 20% for OSC, i.e. a technology that in principle can be scaled to the required terawatt.
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.

Informal enquiries should be addressed to moritz.riede@physics.ox.ac.uk

In-Situ Microstructural Characterisation of Organic Solar Cells

The solar cells investigated in this project have the potential to allow us to harness the power of the sun clean and at cost lower than coal everywhere we are and go, even in the UK. They can be made, for example, flexible such they can be rolled up like a newspaper, and could become much cheaper than existing solar cell technologies. To achieve this, our solar cells are not based on silicon, the material of most solar cells currently sold, but on organic semiconductors.
The electrical and optical properties of organic solar cells (OSCs) and as result the performance critically depend on the molecular arrangement of the organic semiconductors and the domains they form in the thin organic layers used to absorb the light. This initial microstructure forms during the deposition process, can be tuned post-deposition and can evolve during the operation of the OSC.
The goal of this project is to work with and expand the capabilities of our vacuum deposition system we have recently installed at I07 (X-ray scattering) at Diamond (see DOI: https://dx.doi.org/10.1063/1.4989761), as well as our a currently developed setup at ISIS (neutron scattering). Combining these techniques along with extensive optoelectronic characterisation in Oxford allows to tackle questions about initial film formation, the effect of post-deposition treatments as well as the evolution of microstructure over time. Achieving and maintaining a favourable microstructure is crucial for many processes in OSCs. Thus, we expect that the results will improve our understanding of the OSC device physics and lead to improved device performance, which will be critical on the route towards commercialisation.
The work will be carried out in a well established collaboration between Diamond, ISIS and Oxford University, bringing together expertise in microstructural characterisation of these organic films, device fabrication and OSC device physics.

Informal enquiries should be addressed to moritz.riede@physics.ox.ac.uk

Prof Henry Snaith

All-perovskite multi-junction solar cells

Multi-junction perovskite solar cells promise to deliver much higher efficiency than existing PV technologies. However, many challenges exist in terms of development of absorber materials with the appropriate band gaps, developing the correct device structure with multiple layers of different semiconductor materials, and understanding optoelectronic processes occurring in the materials and at the interfaces. This PhD student will work on broad challenges associated with improving the efficiency and stability of these solar cells, and make use of the new EPSRC National Cluster Facility for Advanced Functional Materials.

Informal enquiries should be address to henry.snaith@physics.ox.ac.uk

Enhancing the long term stability and optoelectronic quality of metal halide perovskite semiconductors and optoelectronic devices.

The project will focus upon understanding and controlling the crystallisation of metal halide perovskites in order to deliver improved optoelectronic quality of the materials. This may include fewer defects, higher charge carrier mobility, longer chare carrier lifetime and ultimately improved performance in optoelectronic devices. The materials developed will be investigated by a range of microscopy and spectroscopic techniques and integrated into functional electronic devices including solar cells and light emitting diodes. The overall aim of the project is to deliver improved functionality and enhanced longevity of the materials and devices, whilst also expanding our understanding of the basic principles which govern both optoelectronic operation and degradation under environmental stressing conditions.

Informal enquiries should be address to clare.moloney@physics.ox.ac.uk

Perovskite Light Emitting Diodes

As well as being recognised as outstanding materials for solar energy conversion, metal halide perovskites can be tuned to be highly emissive, and emit light in the required visible emission channels for displays. There is much effort on developing perovskites as phosphors, where they absorb blue light from a conventional GaN LED, and reemit green of red light as desired. However, there it is also possible to create highly efficient LEDs, where charge carrier injection in a diode structure results in the emission of blue, green or red light. The efficiency of these LEDs have already matched commercial OLEDs. However, the long-term operational stability requires orders of magnitude improvement in order to match the tens to hundreds of thousands of hours required. This project will focus upon understanding instabilities in perovskite LEDs and devising novel routes to enhance the long-term stability. Research approaches can include materials chemistry, device physics and advanced spectroscopies, dependent upon the capability and desires of the preferred candidate.

We intend to secure a CASE studentship for this project, in partnership with Helio Display Materials.

Informal enquiries should be address to clare.moloney@physics.ox.ac.uk

Prof Robert Taylor

Carrier dynamics in Perovskite films and quantum dots

This project involves the use of photon counting microphotoluminescence spectroscopy to investigate the optical properties of thin films of various perovskite materials and perovskite quantum dots. These materials have great prospect for use in display technologies and solar cells. The student will be involved in cryogenic experiments using pulsed and CW lasers. 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 recent highlight is the observation of external cavity-less lasing in perovskite quantum dots, where clusters of quantum dots show emission peaks derived from transverse electromagnetic modes. The diagram below shows a typical spectrum seen.

Taylor_0.jpg

Informal enquiries should be address to robert.taylor@physics.ox.ac.uk

Single photon emission from two-dimensional monolayer films

Recent advances in the production and growth of monolayer films of materials such as WSe2 and MoS2 have prompted extensive research into these materials. Optically-pumped single photon sources have been measured and this project aims to produce single photons from such monolayers where nanolithography is used to produce electrodes that can make contact with these films. This work will take place in collaboration with Prof. Arzhang Ardavan.

Full details of Prof. Taylor’s research and publications can be found here.

Informal enquiries should be address to robert.taylor@physics.ox.ac.uk

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 Synthetic Biology (EPSRC & BBSRC CDT)