All Research Projects

Quantum-mechanical simulations

Computer simulations are playing an important role as a complement to experiment in modern physics, chemistry, materials science and biology. Quantum mechanics describes the atomistic processes underlying all these areas, and while empirically-based methods may be sufficient in familiar situations, such assumptions cannot be relied upon when pioneering new fields or predicting the properties of new materials. In these cases the quantum-mechanical equations must be solved from first principles using only well-controlled approximations.

Our group is at the forefront of the development and application of revolutionary techniques including linear-scaling methods for density-functional theory and hybrid modelling schemes. The former has already enabled simulations of a few thousand atoms to be performed with unparalleled accuracy on systems ranging from biological macromolecules to nanostructures. The latter allows an accurate quantum simulation to be embedded within a fast empirical scheme dynamically, where the extent of the quantum-mechanical region is determined on the fly. Together these advances promise to bring the power of quantum-mechanical simulations to bear on systems of an unprecented scale.

Our goal for the next five years is to combine them and to use the resulting technology to tackle new classes of problems in biology and materials science that are currently beyond the reach of traditional simulation methods. Notoriously hard problems include defect motion and brittle fracture in solids, enzymatic catalysis and transport through cell membranes. In all of these areas, at least one novel aspect of our techniques is essential in order to embark on an accurate simulation that yields quantitative results and makes reliable predictions.

Sponsors: EPSRC, Leverhulme Trust, Royal Society

Phases of the electron-hole system in semiconductors

Optical excitation of carriers in a semiconductor generates bound electron-hole pairs, the solid state analogue of the Hydrogen atom. With increasing density these atoms unbind to form a metallic plasma. The goal of the project is to understand this prototypical metal-insulator transition. We wish to understand the conditions under which the insulating phase may become phase-coherent (i.e a Bose-Einstein condensate) and the physical properties - including coherent light emission - of such a state. A closely related problem is to understand the combined effect of interactions and disorder on luminescence and laser action.

*Department of Physics and Astronomy, Cardiff University

Sponsors: EPSRC, Lucent Technologies, Sidney Sussex College

Phase Diagram of the Rutheno-Cuprate Superconductor

The Rutheno-Cuprates describe a class of materials which are derivatives of the high temperature Cuprate superconductors. These are layered materials in which the superconductivity is believed to coexist with itinerant ferromagnetism in the Ru layers. Pair-breaking effects of ferromagnetic coupling to the superconductor order parameter usually makes such a situation untenable. At present we are exploring a simple phenomenology to explain this coexistence.

Sponsor: EPSRC

Spin-fluctutation Superconductivity

A systematic study of the problem of spin-fluctuation superconductivity is undertaken. This programme has a strong overlap with the experimental research done in the group of Professor Lonzarich as well as the Interdisciplinary Research Centre for Superconductivity.

Sponsors: Isaac Newton Trust, Cambridge European Trust, EPSRC

Computational Many-body physics

Non-perturbative computational techniques to tackle the problem of strongly correlated electrons are being developed. One is interested in simplified lattice models and real atomic or molecular systems.

Sponsor: Isaac Newton Trust

Strongly correlated electronic systems

Adding carriers to a normal semiconductor converts it to a simple metal, but in many insulators more complex changes are produced by doping. For example, some copper oxides become high-temperature superconductors and some manganese oxides become ferromagnetic metals; many other materials (e.g. nickel oxides) remain insulating. Our goal is to understand these novel materials with strong interaction effects, using a variety of approaches ranging from phenomenology of experiments to numerical and analytic models.

Sponsor: EPSRC

Correlated electrons in semiconductor microstructures

Geometrical confinement of carriers in semiconductor devices can quench the effective kinetic energy in comparison to electron-electron interactions and lead to a strong-correlation regime. The properties of the novel collective phases that emerge in such semiconductor structures are being investigated.

Zero Bias Tunneling Conductance in Bilayer Quantum Hall Systems

Theoretical predictions of a divergent tunneling conductance in bilayer quantum Hall systems are not borne out by experiment. Can damping by dilute localised quasiparticles account for the observed saturation of the tunneling conductance at low temperatures, or are the experiments limited by extrinsic effects?

*Imperial College, London

Quantum Phases of Vortices in Atomic Bose-Einstein Condensates

Under an imposed rotation, the groundstate of a Bose-Einstein condensate contains a large number of vortex lines. We are studying the transition between a conventional vortex-lattice phase, and unusual quantum-melted ``vortex-liquid'' phases that appear at large rotation rates, when the cores of the vortices overlap.

*University of Birmingham

Topological Solitons in Spinor Bose-Einstein Condensates

Bose-Einstein condensates formed in systems containing more than one atomic species are described by condensate wavefunctions with spinor structures. We are investigating the conditions under which there exist stable solitonic structures with non-trivial topological character (Skyrmions, monopoles, etc.).

*Department of Applied Mathematics and Theoretical Physics, University of Cambridge

Solitary waves in atomic Bose-Einstein condensates

Solitary waves occur in trapped gaseous Bose-Einstein condensates (BECs). In the usual case of repulsive interactions between atoms the soliton is actually a depletion of atoms in the condensate. For weak couplings this is of a quasi-1D nature but it becomes unstable to give a single vortex (so-called solitonic vortex) or a vortex ring for stronger couplings. These objects can be viewed as excited states of the condensate and have been created by engineering the phase of the wave function.

We study the relation between quasi-1D solitons, solitonic vortices, and vortex rings as the two latter bifurcate from the former. In line with current experiments we study different geometries: a cylindrical trap and a spherical trap. We also study travelling solitary waves and their dependence on velocity. The bifurcations revealed are rather unusual and stimulate further interest in the system.

*University of Crete.

Sponsor: EPSRC

Bright solitons in atomic Bose-Einstein condensates

In a BEC with attractive interactions among atoms a bright soliton or a train of solitons can be created when the number of atoms is sufficiently small. These are metastable states and for a larger number of atoms the condensate will inevitably collapse. We study the various possibilities for solitons, their formation process and the way to collapse.

Sponsor: EPSRC

Vortex phases in atomic Bose-Einstein condensates

The creation of individual vortices in atomic BECs and phases containing a number of vortices, e.g., a two-vortex state or a vortex lattice, are studied.

*University of Crete

Sponsor: EPSRC

Keldysh Field Theory, Bosonization and Quantum Transport in Quantum Wires

This project suggests merging two theoretical tools: Keldysh Field Theory and Bosonization in order to study the quantum transport of interacting particles in quantum wires. We aim to explain the effect of splitting the step of ballistic quantization and study shot noise in quantum wires.

Sponsor: ORS

Metal-Insulator Transition in Three-dimensional Semiconductors in a Magnetic Field

This is a long standing project. Some new and encouraging experimental data appeared recently.

Diffusion, Ballistics, Localization, Quantum Chaos

We are studying Anderson Localization and Quantum Chaos by the means of Supersymmetric Field Theory.

*University of Warwick

Sponsor: ORS

Quantization of Hall spin-mobility in two-dimensional systems

This is an attempt to find out the quantization rule for the Hall spin mobility similar to that of Hall conductance.

Large magnetoresistance in disordered narrow-gap semiconductors

Granular preparations of Ag_2+xSe have recently been discovered to show a very large positive magnetoresistance, essentially linear in magnetic field, and unsaturating to at least 10 Tesla. We are currently trying to explain this with a model of macroscopic inhomogeneous transport.

*Bristol University
**University of Chicago
§Argonne National Laboratory, USA

Phase Coherence Phenomena and Mesoscopic Fluctuations in Non-linear Media

This project concerns the investigation of the the manifestations of mesoscopic quantum coherence phenomena in non-linear dynamical systems. The identification of universal fluctuation phenomena has proved to be of great importance in linear problems (i.e. phase coherent transport in disordered conductors). This question has relevance to a number of topical issues ranging from non-linear optics to interacting bosons in a randomly screened potential. Here we are implementing a field theoretic procedure to express correlation functions of the non-linear Schrodinger equation as a supersymmetric functional non-linear sigma model.

Sponsor: EPSRC

Universal Phase Coherence Phenomena in Complex Structures

We study the manifestations of classical chaos in quantum dynamics. Motivated by parallel investigations of weakly disordered metallic systems, our approach involves the continuing investigation of a quantum statistical field theory which expresses average properties in terms of a supersymmetric non-linear sigma model. Current interest concerns the concept of "irreversibility" in the classical dynamics of irregular systems.

*University of Cologne, Germany

Electronic Transport in Disordered Conductors and Quantum Dots

The general role of electron interaction and mesoscopic quantum coherence effects in the transport and spectroscopic properties of disordered conductors and quantum dots is studied within the framework of statistical field theory. Of particular interest is the influence of mesoscopic coherence effects on the orthogonality catastrophe, and mechanisms of dephasing in interacting mesoscopic conductors. A second project concerns the investigation of the phenomena of adiabatic charge pumping in quantum dots.

Sponsor: EPSRC

Quasi-particle Properties of Disordered D-wave Superconductors

The low temperature properties of a d-wave are dominated by four Dirac nodes at the Fermi surface. Leaving aside pair-breaking effects, a weak impurity potential induces correlations in the spin quasi-particle transport properties. We investigate the phase diagram of the disordered d-wave superconductor as a function of the strength of disorder. We also investigate the influence of time-reversal symmetry breaking and phenomenon of the the spin quantum Hall effect.

*University of Cologne, Germany

Dissipation by Collective Modes in Quantum Hall Ferromagnets

Experimental observations indicate that the existence of a neutral Goldstone mode in the excitation spectrum of a quantum Hall system strongly affects the temperature dependence of the electrical conductivity. The mechanisms by which thermally-excited neutral excitations contribute to charge transport are being investigated.

Current-driven magnetisation dynamics

Applied electrical currents exert torques on magnetic materials and can drive non-linear dynamics. When the transverse dimensions of the active region are less than about 100nm, the magnetisation is driven by the transfer of spin angular momentum from the current-carrying electrons. The nature of the current-driven magnetisation dynamics in this regime is being studied.