DPhil Research Projects 2020/21

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.
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 (with Grünewald group, Oxford STRUBI).
• 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

Marie Sklodowska Curie ITN: Artificial Molecular Machines

Two Early Stage Researcher (ESR) positions are available as part of the Marie Sklodowska Curie Innovative Training Network “Artificial Molecular Machines” (ArtMoMa) https://cordis.europa.eu/project/rcn/224720. ESRs will receive training through interdisciplinary research into the development of the new science and technology of artificial molecular machines and a coordinated European network-wide training programme. ESRs will visit partner institutions on secondment as an integral part of their individual training project. These positions are suitable for a student embarking on a first doctoral degree (an Oxford D.Phil.). They are also suitable as a first postdoctoral position for a researcher who, at the time of appointment, is close to completing a PhD at another university but has not yet been awarded this degree. When recruited, ESRs must not have been resident in the UK for more than 12 months in the past 3 years, not (yet) have been awarded a doctorate, and be in the first 4 years (FTE) of their research careers.
Artificial molecular machines are molecules which can perform controlled mechanical tasks (such as translation or rotation). The new field of research into the creation, manipulation, and applications of artificial molecular machines started in the mid 80's at leading research institutions and, in 2016, the first fundamental achievements were awarded the Nobel Prize in Chemistry. Complex biological machinery already exists in nature and is central to key cellular functions (such as replication, ATP synthesis, transport, and motion). Recent progress in the field of artificial molecular machines could provide the foundations for nanorobotics, an entirely new technology for, e.g., the synthesis of complex chemicals and pharmaceuticals and for medical diagnosis and treatment. Other promising applications include smart, dynamic materials and energy storage and conversion systems.

Individual research projects at Oxford

Project 1: “Actuation across length scales using DNA nanostructures”

Small-molecule actuators can achieve rapid conformation changes in response to chemical or photophysical stimuli. The ESR will work with collaborating groups in Manchester, Groningen and Strasbourg to integrate such actuators in self-assembled DNA nanostructures with characteristic dimensions in the range 10 ~ 100 nm, one or two orders of magnitude larger that the actuator molecules themselves, in order to scale up of the range of motion achieved and access new functionalities. Light-driven rotary motors will be coupled to DNA nanostructures to create rotary actuators and swimmers. Chemically responsive small-molecule actuators will be harnessed to open and close containers and to walk along self-assembled tracks, creating synthetic mimics of cellular molecular machinery. The resulting active nanostructures will be used to explore applications of chemically and optically powered molecular robotic devices as, for example, self-propelled drug delivery vehicles.


Project 2: “Optimisation by selection of oligonucleotide-based synthetic molecular machinery”

The ESR will develop methods of in vitro selection to create and optimize new functionalities for DNA-based synthetic molecular machinery. Random-sequence domains incorporated in molecular motors driven by DNA hybridization reactions will be used to optimize their performance – for example, using selection for rate of arrival at the end of a track. In vitro selection methods will also be used to develop a generic small-molecule chemical fuel that can be directly coupled to the operation of oligonucleotide-based synthetic molecular machinery. The new fuel – the equivalent of the biological energy currency ATP – has the potential to revolutionize research into synthetic molecular systems by combining, for the first time, rapid and efficient energy transduction with the flexible and programmable molecular construction achieved using DNA self-assembly. This would make possible the creation of biomimetic systems such as an active synthetic cytoskeleton, enabling macroscopic motion of a synthetic protocell. Fast-acting molecular machinery could also be used to control chemical synthesis and to enable transport over significant distances and at speed.

See ESR job advertisement https://www.jobs.ac.uk/job/BXM512/marie-sklodowska-curie-early-stage-researcher-2-posts

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.

Informal enquiries should be addressed to 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.

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

Dr Amalia Coldea

Projects available in our group for 2020 are listed below. Please get in touch with Dr Amalia Coldea amalia.coldea@physics.ox.ac.uk for further details.

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

1. 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.

2. 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.

3. Strain-Tuning of the Electronic Structure of Novel Quantum Materials Using Angle Resolved Photoemission Spectroscopy

Superconductivity is a unique state of matter with significant potential for practical applications due to the ability of electrons to form Cooper pairs that travel without dissipations in these materials. The electronic structure in the superconducting materials is strongly affected by the presence of certain interactions due to the lattice, magnetic, charge or orbital fluctuations. Iron-based superconductors are a new class of exciting material that offer a new prospective on the important routes towards high-temperature superconductivity. This project focuses on establishing experimentally the key ingredients that govern the electronic structure and the superconducting state found in different classes of iron-based superconductors in the presence of competing ground states. The project will use various external parameters that modify the electronic structure such as chemical pressure, in-situ electron doping and applied strain to build up a comprehensive picture of superconductivity towards high-temperature regimes. Experiments will be performed both at the Diamond Light Source and at the University of Oxford by complementing state-of-the art angle resolved photoemission spectroscopy and magnetotransport studies as well as theoretical first-principle calculations. A suitable candidate needs to have a good understanding of condensed matter physics and good computing skills as well the ability to work in an experimental team. This project will be joinly supervised by Dr Amalia Coldea and Dr Timur Kim at the Diamond Light Source.

4. Exploring the electronic structure and superconductivity of quantum materials under strain

This project will investigate the response of quantum materials, especially iron-based superconductors, to applied lattice strain. The fundamental reason to do this is that in these materials there are many electronic processes that occur simultaneously, which can be difficult to disentangle when looking at the unstressed system alone. One can gain much more information by understanding how elastic lattice distortion, applied through hydrostatic, biaxial, or uniaxial stress, affects the electronic properties of a material. For example, lattice strain may be used to suppress an electronic instability, or to enhance one or more of the processes in the material. One major area of interest in the iron-based compounds, which is likely to be a key part of this project, is electronic nematicity, a form of electronic order which breaks rotational but not translational symmetries. Nematicity may play an important role in high-temperature superconductivity. Uniaxial stress can be used to increase nematic polarisation in a material, by adding to the orthorhombic lattice distortion associated with nematic order.

Measurements to be performed may include quantum oscillations in high magnetic fields of up to 45T and potentially angle-resolved photoemission at the Diamond Light Source. The project also involves the development of high-precision, piezoelectric-driven stress apparatus, for example of a miniaturised stress cell that fits into a high-field magnet and therefore is suitable for students who want to develop their engineering skills along with their scientific skills. A suitable candidate should have a good understanding of condensed matter physics, good experimental and computing skills, and an ability to work well in an experimental team. This project will be jointly supervised by Dr Amalia Coldea at Oxford University, and by Dr Clifford Hicks and Prof Andrew Mackenzie at the Max Planck Institute for Chemical Physics of Solids, in Dresden, Germany. The student will be expected to spend approximately half of the time in Oxford and half in Dresden.

5. 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

Magnetic Skyrmionics

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.

Informal enquiries should be address to thorsten.hesjedal@physics.ox.ac.uk

Selected group publications on Skyrmionics:

[1] Zhang, S. L.; van der Laan, G.; Hesjedal, T.; Direct experimental determination of spiral spin structures via the dichroism extinction effect in resonant elastic soft x-ray scattering, Phys. Rev. B 96, 094401 (2017)

[2] Zhang, S. L. et al., Realisation and observation of room temperature helimagnetism in FeGe thin films, Scientific Reports 7, 123 (2017)

[3] Zhang, S. L.; van der Laan, G.; Hesjedal, T.; Direct Determination of the Topological Winding Number, Nature Commun. 8, 14619 (2017)

[4] Figueroa, A. I.; Zhang, S. L.; et al.; Strain in epitaxial MnSi films on Si(111) in the thick film limit studied by polarization-dependent extended x-ray absorption fine structure; Phys. Rev. 94, 174107 (2016)

[5] 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)

[6] Zhang, S. L.; Bauer, A.; Burn, D. M.; et al, Multidomain Skyrmion Lattice State in Cu2OSeO3, Nano Lett. 16, 3285-3291 (2016)

[7] Zhang, S. L.; Chalasani, R.; Baker, A. A.; et al., Engineering helimagnetism in MnSi thin films, AIP Adv. 6, 015217 (2016)

[8] 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)

[9] 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)

[10] 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 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). In this case the samples have to be moved from MBE to the ARPES measurement station all in ultrahigh vacuum (UHV). The MBE [1] has proven its worth in conjunction with ARPES in studies of rock-salt structure doped EuO at variable doping [2].

Further characterisation of the thin films 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. 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 I05’s principle beamline scientist, Dr. Cephise Cacho, and Prof. Thorsten Hesjedal. It will be partially RCUK funded, and so is restricted to applicants with a Home/EU fee status.

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, F. Caruso, C. Verdi, L. B. Duffy, M. D. Watson, L. Bawden, K. Volckaert, G. van der Laan, T. Hesjedal, M. Hoesch, F. Giustino & P. D. C. King, “Crossover from lattice to plasmonic polarons of a spin-polarised electron gas in ferromagnetic EuO”, Nature Communications 9, 2305 (2018).


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.

Informal enquiries should be addressed to thorsten.hesjedal@physics.ox.ac.uk

Intrinsic topological insulators (ITIs)

The Oxford-Diamond-ISIS joint project is aimed at exploring a new family of quantum materials, intrinsic topological insulators (ITIs), which combine non-trivial band topology and long-range magnetic order in a single material. The interplay of magnetism and topology offers great opportunities for the exploration of emerging physics, such as the quantum anomalous Hall (QAH) effect, axion electrodynamics, and Majorana fermions. However, the experimental realization of these exotic physical effects has remained a challenge due to the unavailability of suitable materials systems which combine the required robust magnetic order with low defect densities needed for transport. Recently, a new type of layered van der Waals material, MnBi2Te4, has been theoretically predicted to be an ITI with a ferromagnetic (FM) intralayer exchange interaction and antiferromagnetic (AF) interlayer coupling. While a single Te-Bi-TeMn-Te-Bi-Te septuple layer (SL) is a topologically trivial FM, thicker films are uncompensated (compensated) interlayer AF, depending on whether they contain an odd (even) number of SLs in the heterostructure. These third-generation magnetic TIs exhibit rich topological quantum states with outstanding characteristics, including an AF TI with the long sought-after topological axion states on the surface, as well as a collection of intrinsic axion insulators and QAH insulators in even- and odd-layer films, respectively. MnBi2Te4 features a massive Dirac surface state and relies neither on doping nor alloying, is only a first example of a wider emerging materials class which provide new pathways for spintronic devices.

This project aims at exploring the wider (Bi2Te3)n(TBi2Te4) (T = transition metal) ITI family of novel van der Waals magnets employing a multi-pronged, well-harmonised approach making best use of the combined capabilities of Diamond, ISIS, and Oxford Physics.

Workplace: Harwell Science and Innovation Campus

Supervison: Profs Hesjedal (Oxford), van der Laan (Diamond Light Source) and Langridge (ISIS/STFC)

Research techniques
• Sample Growth: Molecular-beam epitaxy in RCaH. Ex-situ study using XRD/XRR and SQUID. Additional bulk samples were obtained from our collaborators in Shanghai.
• XAS/XMCD/XMLD: X-ray spectroscopy on the Mn/Cr/V L2,3 edges. Understanding of the long-range-ordered ground state. Calculation of spectra (multiplet calculations).
• PNR: Polarised neutron reflectivity studies of the layer-resolved magnetism.
• REXS: Resonant elastic (soft) x-ray scattering study of the magnetic order.
• Magnetotransport: Anomalous Hall effect studies.
• ARPES: Determination of the electronic band structure. Band structure calculations.

Informal enquiries should be address to thorsten.hesjedal@physics.ox.ac.uk

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

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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 recently published in Nature Materials [4], an international team of collaborators lead by Professor Paolo G. Radaelli (Oxford Physics) presented a major breakthrough in this field [5,6]: they created, for the first time, small-scale hybrid oxide/metal topological magnetic objects, consisting of tightly-coupled spin vortices in antiferromagnetic iron oxide (-Fe2O3) and ferromagnetic metallic cobalt (Co). One particularly appealing feature of this system is that it employs cheap and readily available materials (-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 strcutures such as vortices and merons. 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.
[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] F. Chmiel et al., Nature Materials 17, pages 581–585 (2018)
[6] M. Fiebig, Nature Materials 17, pages 567–568 (2018)

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).

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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.

[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).
[4] “Breaking Symmetry with Light: Ultra-Fast Ferroelectricity and Magnetism from Three-Phonon Coupling”, P. G. Radaelli, Phys. Rev. B 97, 085145 (2018).

Semiconductor Materials, Devices & Nanostructures

Dr Marina Filip

First Principles Computational Modeling and Design of Low-Dimensional Hybrid Organic-Inorganic Perovskites

Hybrid organic-inorganic perovskites have recently emerged as highly promising materials for light-emission, in addition to their record breaking performance in photovoltaic devices. Quasi-two dimensional (Q2D) hybrid perovskites are particularly interesting in this context, since their chemical and structural heterogeneity translate into a rich variety of tunable photophysical properties. However, while the materials space of hybrid Q2D 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).

The premise of first-principles materials modeling is that the structural, electronic, optical, and transport properties of materials can be predicted starting from the basic laws of Quantum Mechanics. First principles, or ab initio methods, are implemented in massively parallel computational packages (we will be using, for example, the Quantum Espresso code, the BerkeleyGW code, the Yambo code, etc) 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 Q2D perovskites, develop computational frameworks for calculating the optoelectronic properties of these materials accurately and efficiently, and develop strategies to design and discover novel perovskite materials for optoelectronic applications.

For more details on research related to this project, please see here.

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 mrfilip@lbl.gov

Prof Laura Herz

Charge generation dynamics in novel materials for solar cells

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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

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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

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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

Terahertz Photonics with Semiconductor Nanostructures

Terahertz Photonics

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 Spintronics

Terahertz Spintronics

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 examine the physics of Inverse Spin-Hall Effect in thin film stacks of magnetic and non-magnetic materials. You will also develop emitters of single cycle THz radiation based on these layered materials.

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

This studentship is linked to the "prosperity partnership" project between Oxford PV Ltd and the University of Oxford. In this prosperity partnership, we have combined pioneering academic and industrial leaders in perovskite photovoltaics and will develop the underlying materials, science and technology, which will allow us to develop the next generation of multi-junction perovskite solar cells. The ambition of the project is to go well beyond the state-of-the-art, and deliver over 37% efficient triple junction perovskite solar cells, with good long term operational stability. This will be possible through a combined effort of new materials development, fundamental investigations, thin-film device engineering and interface modification, and significant effort on understanding and improving materials and device stability. The DPhil student working on this project will work collaboratively with the Postdocs, Scientist and Engineers, and probe deeply into understanding the photo physical and electronic processes which control optoelectronic operation, and long term stability of the solar cells.

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

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.

Informal enquiries should be address to henry.snaith@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.

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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)