Although the interests of the group have shifted towards biologically-inspired research, to date the majority of research activities have been targeted at the development of theoretical approaches to address problems in condensed matter and ultracold atom physics. Over the years, we have made contributions to the study of quantum interference effects in phase coherent (mesoscopic) structures, correlated electron, electron-hole and matter-light systems and, most recently, ultracold atom physics.

In the field of mesoscopic physics, we have studied quantum interference phenomena in weakly disordered normal and superconducting systems. These studies have been based on a general theoretical framework in which the quantum statistical properties of weakly disordered conductors are expressed as a field theory involving an action of nonlinear sigma-model type. Applying this methodology to the study of spectral and transport properties, we established a platform to describe a new and general class of mesoscopic correlations involving parametric dependences on external fields. These studies provided a generalization of the universal Wigner-Dyson energy level correlations in random matrix theory, finding applications beyond the realm of mesoscopic physics. The same field-theoretic scheme revealed a surprising connection between the eigenvalue correlations in random matrix ensembles, the dynamical properties of the Calogero-Sutherland model, and continuous matrix models in string theory.

Building upon this methodology, we generalized the nonlinear sigma-model scheme to target mesoscopic fluctuations in disordered superconducting compounds and proximity effect devices. These studies led to the identification of novel localization phenomena in symmetry-broken superconductors, and provided insight into the mechanism of Anderson localization in normal compounds. Finally, our investigations of weakly disordered systems led to the development of a general field-theoretic framework to describe spectral correlations of irregular ballistic quantum systems - a subject that has become knows as quantum chaos.

In the field of strongly correlated electron systems, the group has contributed to a range of topics, from studies of charge carriers in magnetic Mott insulators and high temperature superconductivity, to normal phase behaviour and quantum criticality in itinerant metamagnetic systems. Our studies of the recently-discovered graphite intercalate superconducting compounds were the first to identify the role of interlayer states in controlling electron correlations and facilitating the transition. Finally, in the field of strongly correlated electron-hole systems, we have published on coherence and Bose-Einstein condensation phenomena in highly excited semiconductors.

In recent years, advances in atomic laser cooling, trapping, and optical manipulation have allowed dilute alkali gases to be cooled below their degeneracy temperature. These developments have inspired new directions in quantum many-body physics, providing a platform to control and explore strong interaction phenomena in and out of equilibrium. As such, ultracold atom physics presents a new arena for solid state condensed matter theorists to engage in cross-disciplinary research. Over the past few years the group has addressed several topics within this area. In particular, we have explored the dynamics of condensate formation in two-component Fermi gases in the regime of BEC-BCS crossover, resonance superfluidity in the regime of population and mass imbalance, novel ground states of mutli-component Bose-Hubbard systems, and light-matter interaction in Bose-Hubbard mixtures - the atomic realisation of cavity QED. Current research activities embrace a range of topics from dynamical quantum phase transitions and superradiance phenomena in Bose-Einstein condensates coupled to optical cavities, itinerant ferromagentism and quantum criticality in resonant Fermi systems, and manifestations of phase coherence effects in disordered atomic gases.

Plain English:
In particle physics and cosmology, physicists are concerned with challenging and developing new fundamental physical laws to describe nature under the most extreme conditions. By contrast, in condensed matter, we are usually interested in applying known physical laws to understand the collective behaviour of matter under conditions that are accessible in the laboratory. In many cases, these fundamental physical laws of quantum mechanics and electromagnetism that govern individual particles (electrons, ions, and photons) and their interactions can be sufficient to define the physical properties of the system. However, in other cases, interactions can conspire to generate new correlated phases of matter, such as magnetism or superfluidity, in which new - equally fundamental - physical laws emerge. In the development of this programme, the field of theoretical condensed matter physics has drawn heavily on the recognition and exploitation of techniques and insights from disperate areas of research.

 

Selected publications:

• A. Berridge, A. G. Green, S. A. Grigera, and B. D. Simons (2009) Inhomogeneous magnetic phases: a LOFF-like phase in Sr3Ru2O7. Phys. Rev. Lett. 102, 136404

• M. M. Parish, F. M. Marchetti, A. Lamacraft, and B. D. Simons (2007) Finite temperature phase diagram of a polarised Fermi condensate. Nature Physics 3, 124

• G. Csanyi, P. B. Littlewood, A. H. Nevidomskyy, C. J. Pickard, and B. D. Simons (2005) Electronic Structure of the Superconducting Graphite Intercalates. Nature Physics 1, 48

• L. S. Levitov, B. D. Simons, and L. V. Butov (2005) Pattern Formation as a Signature of Quantum Degeneracy in a Cold Exciton System. Phys. Rev. Lett. 94, 013818

• A. Altland, B. D. Simons and M. R. Zirnbauer (2002) Theories of low-energy quasi-particle states in disordered d-wave superconductors. Physics Reports 359, 283

• A. V. Andreev, O. Agam, B. D. Simons, and B. L. Altshuler (1996) Quantum chaos, irreversible dynamics and random matrix theory. Phys. Rev. Lett. 76, 3947

• B. D. Simons, P. A. Lee and B. L. Altshuler (1994) Matrix models, one-dimensional fermions, and quantum chaos. Phys. Rev. Lett. 72, 64