Dphil Projects -Research Projects 2012-

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. The Berry group is trying to understand how these living machines work. We use a range of advanced microscopic techniques, including optical tweezers (3-D laser trap) and single-molecule fluorescence microscopy. For example, one of our current experiments aims to measure stepping rotation in individual flagellar motors, using a custom-built microscope that can track 200 nm gold particles with spatial resolution of <1 nm at up to 100,000 frames per second. More...
Email Dr Richard Berry at: r [dot] berry1 [at] physics [dot] ox [dot] ac [dot] uk

Designer Magnets and Superconductors: Selectively Controlling Interactions in Low-Dimensional Magnets and Unconventional Superconductors

In collaboration with some groups in the US we have been developing self-organised materials in which magnetic building blocks are linked by molecular ligands or highly metallic layers are assembled in
sandwich structures. We will measure the electronic and magnetic
properties of a variety of materials using high magnetic fields and muon spectroscopy while perturbing their structure using high applied pressures. Squeezing the crystal lattice adjusts bond lengths as well restricting electronic and magnetic channels, thus revealing the nature of the interactions that cause the material to become magnetically ordered and/or superconducting. The project student will develop the apparatus for performing high pressure measurements in the Nicholas Kurti Magnetic Field Laboratory in the Clarendon, as well as using the international high-field and muon facilities.

Web: http://correlated.physics.ox.ac.uk

Email: Dr Paul Goddard and Prof Steve Blundell at: p [dot] goddard1 [at] physics [dot] ox [dot] ac [dot] uk and s [dot] blundell1 [at] physics [dot] ox [dot] ac [dot] uk

Novel Electronic Order and Dynamics in Crystals

Systems of interacting electrons frequently exhibit subtle forms of order, examples being superconductivity and magnetism. In this project you will investigate electronic order and dynamics experimentally. Neutron and X-ray scattering will be the main techniques used, taking advantage of the new ISIS 2nd target station and the Diamond Light Source, plus other facilities in Europe. A willingness to travel is essential! Magnetic, transport and thermal measurements will be performed in the Clarendon Laboratory, and there is scope for theoretical modelling and numerical analysis. More...
Email Prof. Andrew Boothroyd at: a [dot] boothroyd1 [at] physics [dot] ox [dot] ac [dot] uk

Topological Insulators - A New State of Quantum Matter

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

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

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

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

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

Email Dr Yulin Chen at: yulin [dot] chen [at] physics [dot] ox [dot] ac [dot] uk

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

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

Quantum Magnetism and Quantum Phase Transitions

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

Designer Magnets and Superconductors: Selectively Controlling Interactions in Low-Dimensional Magnets and Unconventional Superconductors

In collaboration with some groups in the US we have been developing self-organised materials in which magnetic building blocks are linked by molecular ligands or highly metallic layers are assembled in
sandwich structures. We will measure the electronic and magnetic
properties of a variety of materials using high magnetic fields and muon spectroscopy while perturbing their structure using high applied pressures. Squeezing the crystal lattice adjusts bond lengths as well restricting electronic and magnetic channels, thus revealing the nature of the interactions that cause the material to become magnetically ordered and/or superconducting. The project student will develop the apparatus for performing high pressure measurements in the Nicholas Kurti Magnetic Field Laboratory in the Clarendon, as well as using the international high-field and muon facilities.

Web: http://correlated.physics.ox.ac.uk

Email: Dr Paul Goddard and Prof Steve Blundell at: p [dot] goddard1 [at] physics [dot] ox [dot] ac [dot] uk and s [dot] blundell1 [at] physics [dot] ox [dot] ac [dot] uk

Energy Transfer in Self-Assembled Organic Semiconductors

Organic semiconducting polymers and molecules have emerged over the last decade as cheaper and more flexible alternatives to existing semiconductor technology with light-emitting displays based on these materials now entering the market. But fundamental knowledge is still lacking on how intermolecular interactions in such systems are best tuned in order to optimize the mobility of charge carriers and diffusivity of photoexcitations. One highly promising approach to mimic self-assembly processes already occuring in nature (e.g. in DNA or natural light-harvesting systems) to arrange conducting molecular materials into desired geometries in solution prior to a casting process. During this DPhil project a range of new self-assembly approaches will be examined using femtosecond photoluminescence spectroscopy in order to probe the dynamics of photoexcitations in these nanostructures. This project offers exciting possibilities for work in a new interdisciplinary area of research.

It allows the exploration of physical phenomena in the increasingly popular area of organic semiconductors, and offers a high degree of training in the elegant and versatile techniques of femtosecond optical spectroscopy.

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

Investigation of Charge-Transfer Dynamics in Organic Materials for Solar Cells

Increasing world needs for electrical power have intensified research into materials suitable for cheap and efficient solar cells. Organic semiconductors offer great benefits in this area, as they can be easily processed into devices, e.g. via inkjet printing, allowing cheap production of large-scale solar panels. However, the fundamental mechanisms underlying both the process of charge-separation from photoexcitation, and the migration of charges to the electrodes are not yet fully understood. The aim of this project is therefore to probe the dynamics of charge-separation and carrier trapping in a range of semiconductor blend systems using a combination of ultra-fast optical techniques, e.g. photoluminescence upconversion and THz pump-probe spectroscopy. This work will be part of a recently funded activity in collaboration with other researchers within the UK and abroad.

This project allows the exploration of physical phenomena in the increasingly popular area of organic semiconductors, and offers a high degree of training in the elegant and versatile techniques of femtosecond optical spectroscopy.

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

Tailoring of the Magnetic Anisotropy

Magnetic materials with tunable magnetic properties are of great interest for data storage and logic device applications. Possible routes for tuning the anisotropy are compositional variations, structural changes, temperature, strain (magnetoelastic effect), and electric field (magnetoelectric effect). Our main focus is to synthesize and study novel multiferroic and magnetoelectric materials systems.

The materials synthesis will be performed by molecular beam epitaxy (MBE) and UHV sputtering in the Clarendon Laboratory and at Oxford’s new Thin Film Quantum Materials Laboratory at the recently opened RCaH (http://www.rc-harwell.ac.uk/). The magnetic and electronic properties will be studied at the adjacent Diamond Light Source (DLS - http://www.diamond.ac.uk/) by magnetometry, X-ray magnetic circular dichroism (XMCD), angular-dependent XMCD, locally-resolved XMCD (XMCD-PEEM), and ferromagnetic resonance (FMR) spectroscopy. A novel technique is under development at the DLS to probe element-specific FMR (XFMR). This work will be accompanied by micromagnetic simulations in collaboration with the University of Southampton.

Email Dr Thorsten Hesjedal at: t [dot] hesjedal1 [at] physics [dot] ox [dot] ac [dot] uk
Prof Gerrit van der Laan at: gerrit [dot] vanderlaan [at] stfc [dot] ac [dot] uk

Semiconductor Nanowire-Based Solar Cells

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

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

THz Conductivity of Graphene and Graphene-Like Materials

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

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

Terahertz Conductivity of Semiconductor Nanostructures and Devices

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

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

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

Controlling and Measuring the Polarisation State of Terahertz Photons: New Approaches to THz Spectroscopy and Imaging

In conventional optical spectroscopy only the intensity of light may be recorded, however in time-domain spectroscopy the electric fie ld of the light is recorded as a function of time giving complete amplitude and phase information of the light. This additional information allows the complete dielectric function of materials to be determined directly, thus allowing many physical properties of the matter to be extracted. The Oxford Terahertz group recently developed the first THz detector that can measure the full polarisation state of a THz pulse. In this project you will develop and exploit this technology into the areas of THz sepctroscopy and imaging. In particular this project will focus on spectroscopy of biological molecules where the polarisation state of a transmitted photon should reveal a wealth of information about the structure of the molecule.

Further Reading:
Development of a three contact photoconductive detector of THz radiation [more]
Characterisation of the frequency-dependent birefringence of quartz [more]
Email Dr Michael Johnston at m [dot] johnston1 [at] physics [dot] ox [dot] ac [dot] uk

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

Single Molecule Spectroscopy of Biomolecular Machines

Living cells contain thousands of molecular machines that process biomolecules. We study machines of gene expression, the path from "reading" genes on DNA to the synthesis of proteins. We seek to understand how machines work by observing individual molecules using cutting-edge single-molecule fluorescence methods (figure) which monitor nanometre distances in real-time and can "film" fascinating molecular movies of dynamic processes. Projects are multidisciplinary combining optics, spectroscopy, biochemistry, modelling and signal processing. More...
Email Dr Achillefs Kapanidis at: a [dot] kapanidis [at] physics [dot] ox [dot] ac [dot] uk

Remarkable Biological Machines

1. Reyes-Lamothe R, Sherratt DJ, Leake MC. Stoichiometry and architecture of active DNA replication machinery in Escherichia coli. Science. 2010, 328, 498-501.

The use of bespoke imaging tools and analysis can offer significant insight into the
living counterpart of soft condensed matter. The soft matter of biological systems consists of molecular building blocks, a staple of which is protein. Protein molecules, so small that 1 billion would fit on the full-stop at the end of this sentence, carry out most of the vital activities in living cells. Many of these processes require the assembly of multiple proteins into remarkable biological machines, and these are studied in my group. DPhil projects are available in the area of applying biological physics experiments on functional single-celled organisms using bespoke fluorescence microscopy imaging and analysis to monitor the number and dynamics of several different proteins at the nanometre length scale to a precision of single molecules.
www.physics.ox.ac.uk/users/leake
Email Dr Mark Leake at m [dot] Leake1 [at] physics [dot] ox [dot] ac [dot] uk

Quantum Dot Circuit QED

Solid state quantum systems such as quantum dots are good candidates for quantum bits in quantum computers, and can also be used to investigate interesting physics in the convenient environment of an electronic chip. In this funded DPhil project, new forms of cavity quantum electrodynamics will be investigated, in which artificial atoms in the form of electron charges and spins in quantum dots will be coupled to microwave photons trapped in electrical resonators. The research will teach us about decoherence in quantum dots, their potential as building blocks for a quantum computer, and involves the development of a new high frequency tool for investigating quantum dot physics.
Email Dr Peter Leek at: peter [dot] leek [at] physics [dot] ox [dot] ac [dot] uk

Quantised Surface Acoustic Devices

Confinement of photons in cavities allows an enhancement of their coupling strength to atoms, and control over the atoms’ electromagnetic environment. This is the idea behind the field of cavity QED, and can be achieved in a wide variety of settings, including on an electronic chip. In this funded DPhil project, the same idea will be applied to surface phonons on piezoelectric chips, confining them in electrical circuit cavities and coupling them to superconducting qubits. As well as providing new information about phonon decoherence and the solid state environment for quantum computing, the research may establish a new type of quantum memory and will be a step into the quantum regime for the long established electronic engineering field of surface acoustic waves.
Email Dr Peter Leek at: peter [dot] leek [at] physics [dot] ox [dot] ac [dot] uk

Quantum Transport in Graphene and Carbon Nanostructures

Carbon nanotubes and graphene (Nobel Prize 2010) are revolutionising the study of semiconductors and offer the potential for a new generation of devices which may supplant silicon. This project aims to study the electrical properties of graphene and carbon nanostructures in high magnetic fields where the electrical conduction is strongly influenced by quantum effects giving rise to phenomena such the Quantum Hall Effect and Coulomb Blockade. By studying nanostructures which are typically only a few nanometers in size the energy levels and transport can be controlled and modified to create new properties and behaviour. The structures to be studied will be made using a recently installed next generation electron beam lithography system capable of writing features down to a size of 20nm and it is planned to experiment with combining these with carbon nanotubes also. More...
Email Prof Robin Nicholas at r [dot] nicholas1 [at] physics [dot] ox [dot] ac [dot] uk

Carbon Based PhotoVoltaic Devices

Finding new ways to produce Photovoltaic (PV) cells with high efficiencies and novel manufacturing techniques is a challenge for semiconductor physics. This project will explore the use of graphite, carbon nanotubes and polymer semiconductors to produce new structures which can act as PV devices with responses in both the visible and infrared part of the spectrum. The project will combine optical spectroscopy of carbon nanotube/polymer nanostructures with studies of electrical properties and fabrication techniques.

The experiments will use techniques such as PhotoLuminescence Excitation (PLE) spectroscopy to study the transfer of energy and charge through different components of the cells and infrared spectroscopy to examine the behaviour of transparent conducting layers such as graphene and nanotubes. These techniques will be combined with electroluminescence and photoresponse to evaluate the potential efficiency of the devices. More...
Email Prof Robin Nicholas at r [dot] nicholas1 [at] physics [dot] ox [dot] ac [dot] uk

Modelling Multiferroic and Magnetoelectric Materials

Cu3Nb2O8 Polarisation: Magnetic structure (left) and induced ferroelectric polarisation (right) in the new multiferroic Cu3Nb2O8 . Published in R. D. Johnson, Sunil Nair, L. C. Chapon, A. Bombardi, C. Vecchini, D. Prabhakaran, A. T. Boothroyd, and P. G. Radaelli, Phys. Rev. Lett. 107, 137205 (2011)
Multiferroic and magnetoelectric materials respond to the application of both electric and magnetic fields. This affords a much greater degree of control than it is possible with present magnetic media, so that these materials could form the basis for the next generation of information storage and reading devices. Unfortunately, most known multiferroic and magnetoelectric materials only work below room temperature. This project is part of an EPSRC-funded effort to apply new concepts to the rational design and understanding of functional multiferroics and magnetoelectrics. The successful candidate will employ state-of-the-art ab-initio theoretical methods, such as those based on the Density Functional Theory, to model the static and dynamic properties of these systems, both at low temperatures and at room temperature. The results of these calculations will be compared quantitatively with electrical measurements and neutron and X-ray scattering experiments performed by our group in the Clarendon Laboratory and at the ISIS and Diamond facilities. This project is jointly supervised by Prof. Paolo G. Radaelli (Clarendon Laboratory) and by Dr Jonathan Yates (Materials Modelling Laboratory, Oxford Department of Materials). More....
Email Prof Paolo G. Radaelli at p [dot] g [dot] radaelli [at] physics [dot] ox [dot] ac [dot] uk and Dr Jonathan Yates at jonathan [dot] yates [at] materials [dot] ox [dot] ac [dot] uk.

Organic Based Photovoltaic Diodes

Solution processable organic and inorganic semiconductors represent an exciting new class of materials which are capable of delivering low cost large area optoelectronic devices. Our main focus is developing nano- and meso-structured material composites exploited as organic based solar cells. See images above. Probing the electronic processes occurring at the heterojunction between organic and inorganic semiconductors and understanding the structure-function and material-function relationships for these composites is also central to the research.
Email Dr Henry Snaith at h [dot] snaith1 [at] physics [dot] ox [dot] ac [dot] uk

Quantum Dots for Quantum Information Processing

I am currently collaborating with the Universities of Oxford, Cambridge, Sheffield and Imperial College London on several projects related to semiconductor quantum dots. One aim of the project is to try to achieve strong coupling between excitons and photons in quantum dot-based systems. The project will centre on the development of indexed quantum dots in a photonic bandgap structure. Materials will include InGaN quantum dots and InAs quantum dots. Time-resolved and coherent control techniques are employed. See my web page for details. More...
Email Prof. Robert A. Taylor at: r [dot] taylor1 [at] physics [dot] ox [dot] ac [dot] uk

DNA Nanostructures

Structures built from synthetic DNA, such as the tetrahedron shown, can be used as templates to create molecular devices with nanometre precision. The new science of synthetic molecular systems has applications from physics and computer science to structural biology and medicine. Current projects include: the physics of self-assembly; applications of methods of computer science to the design and control of self-assembly; design and fabrication of synthetic molecular motors, including single-molecule measurements and programmable automata; autonomous molecular machinery for chemical synthesis; molecular computation; DNA-templated protein arrays for molecular structure determination; hybrid DNA-protein structures to investigate the biophysics of motor proteins; nanostructures as sensors and intelligent drug delivery vehicles. More...
Email Prof Andrew Turberfield at: a [dot] turberfield [at] physics [dot] ox [dot] ac [dot] uk