Research Summaries

Condensed Matter Theory Group, Department of Physics

Complexity in Simple Dynamical Systems ( Kim Christensen )

The aim of the science of self-organised criticality (SOC) is to yield insight into the fundamental question of why nature is complex, not simple, as the laws of physics imply. The concept of SOC has been applied in fields spanning statistical mechanics, condensed matter theory, geophysics, economy and biology. Currently, we study models of granular media (sand and rice piles), earthquakes and evolution.

Quantum Monte Carlo Simulations of Solids ( Matthew Foulkes )

Using quantum Monte Carlo methods and parallel computers, it is now possible to simulate the quantum mechanical behaviour of up to 10³ interacting electrons. This allows us to simulate real solids with unprecedented accuracy, avoiding the crude mean-field approximations used in the past. Our computer experiments are as accurate and reliable as many real experiments, and can answer some questions that real experiments can't.

Theory and Simulation of Materials (Mike Finnis)

My research is about understanding some of the many ways, some quite extraordinary, that atoms arrange themselves in solid matter, and how these arrangements explain or predict the physical properties of a material. Materials, like people, are interesting because of their defects, which include vacancies, interstitials, impurities, dislocations, interfaces, surfaces and more. At Imperial, we theorists have the opportunity to interact with scientists who use state-of-the art microscopes and spectroscopies to study such defects right down to the atomic scale. The challenge is to apply our rapidly expanding knowledge and techniques of quantum mechanics, including the development and use of computer codes, to explain or predict what can now be seen, and how the structure at the atomic scale effects things like strength, stability or conductivity.

Quantum States of Matter ( Derek Lee )

Condensed matter provides a natural laboratory to study quantum many-body physics. Quantum effects lead to low-temperature phases with spontaneous broken symmetry such as superconductivity, superfluidity, and magnetism. In particular, the interplay of quantum statistics, interactions and disorder gives rise to exotic ground states in systems such as high-temperature superconductors, ultracold atomic condensates, and semiconductors in the quantum Hall regime.

Disordered & Mesoscopic Systems ( Angus MacKinnon )

Undergraduate Solid State Physics is based on the idea of an infinite perfect crystal. Real solids are often very different: they are neither infinite nor crystalline. The physics of disordered systems, on the one hand, and mesoscopic systems, on the other, displays several new phenomena including the Metal-Insulator transition, quantum interference and single particle effects.

Metamaterials & Negative Refraction ( John Pendry )

Negative refractive index is a new phenomenon which is creating great excitement not only in optics but also in materials science. Materials with negative index are not found in nature and can only be realised artificially. Research in this group has been central to realising the concept and to exploiting its extraordinary consequences, one of which is the prescription for a perfect lens with resolution unlimited by wavelength. Some reviews will be found elsewhere at my website.

Materials at the Nanoscale (Adrian Sutton)

As a materials scientist at heart, I like to apply fundamental theory and computer simulation to areas that have a potential application. For the past 14 years, I have worked on metallic nanowires - wires down to a single atom in width that can carry current densities up to 8 orders of magnitude greater than an ordinary electric light-bulb filament. With Tchavdar Todorov, I have developed the theory of current-induced forces on atoms, which is the fundamental origin of electromigration, to understand what the high current density does to the structure and mechanical stability of the nanowire. I am also interested in understanding the fundamental mechanisms of current-induced heating, namely the irreversible transfer of energy from electrons to ions. This work has led to the development of new non-adiabatic methods for the simulation of radiation damage in materials and transport phenomena in polymers. Another area of my research centres on the new states of matter found at certain diffuse interfaces between crystals, which are neither crystalline nor amorphous, but somewhere in between. These "interfacial materials" have already found useful mechanical, electrical and optical applications.

Epitaxial Phenomena ( Dimitri Vvedensky )

This research is concerned with developing a theoretical description of the various phenomena that occur when one material is deposited onto a surface of another (possibly different) material. A variety of theoretical techniques are employed, including atomistic simulations, continuum equations of motion, and modern techniques of solving nonlinear evolution equations. There is a close collaboration with the experimental groups in the Center for Electronic Materials, and with several groups worldwide.

Mathematical Physics Group, Department of Mathematics

Statistics of Quantum Spectra ( Yang Chen )

I study the behaviour of large random matrices through the average eigenvalue distributions and the distributions for the spacings between the eigenvalues by treating the eigenvalues as particles in a charged fluid. This theory has applications to quantum chaos, nuclear physics, and integrable systems.

Quantum Magnetism ( David Edwards )

Much of our everyday life relies on electronic devices which exploit the electron as a mobile charge. The magnetic aspect of the electron has recently given rise to the new field of spin electronics. We have been strongly involved in the theory of magnetic multilayers which exhibit giant magnetoresistance, and have application as sensors of magnetic fields, e.g. in reading a magnetic disk. Recent work on colossal magnetoresistance in manganite materials combines our earlier expertise with the theory of strongly correlated electrons.

One-dimensional Quantum Systems ( Alexander Gogolin )

Our main focus of attention is on one-dimensional quantum systems such as disordered spin chains and quantum wires. This also has applications to the multi-channel Kondo problem, and its realisation in quantum dots which exhibit peculiar, non-Fermi-liquid behaviour.

Renormalization Group Methods for Correlated Systems ( Alex Hewson )

I am particularly interested in the development and application of renormalization group methods, both analytical and numerical, to strong correlation problems (heavy fermion systems, high-temperature superconductors). Under active study at the present time using this approach is the Mott metal-insulator transition in the Hubbard model.

Nonlinear Dynamical Systems ( Roy Jacobs )

Complex systems display a wide variety of nonlinear dynamics including chaotic behaviour and various types of self-organisation, such as ordering phenomena. My main interest is in understanding how these phenomena arise from the underlying nonlinearity. Specific systems studied are nonlinear earthquake models and models of glassy systems. I am also interested in first-order phase transitions such as three-dimensional melting.

Statistical Physics of Dynamical Systems ( Henrik Jensen )

For me statistical mechanics is about understanding why the whole is different from the sum of the parts: identifying and understanding emergent properties; understanding how the combined effect of simple interactions and many degrees of freedom creates the never ending complexity we observe in our surroundings. Ongoing research include models of biological evolution, fungal growth, flux lines in type-two superconductors, and self-organised criticality.

Critical Phenomena at Interfaces ( Andrew Parry )

A fundamental problem in statistical mechanics is the development of a microscopic theory of phase transitions in systems that are strongly inhomogeneous. For example the interface between solid and liquid or liquid and vapour is characterised by an enormous change in density over a microscopic scale. In such systems, many new kinds of phase transitions are possible and are being investigated using techniques such as the renormalization group.