The Theoretical Physics Research Group's interests span a diverse spectrum of condensed matter, ultracold-atom, and nonlinear-optical systems on length scales from the microscopic through mesoscopic to the macroscopic. From the theory of quantum critical points and the physics of nanodevices to the phase transitions of vortices in a Bose–Einstein condensate, the research has common threads of correlation, disorder and topology.
The effects of quantum mechanics and disorder on the phases of matter form the mainstay of our research. For instance, classical physics fails completely to account for the 'phases' of electrons in a solid. In many materials quantum effects ensure that the electrons remained gaseous down to zero temperature. However, quantum mechanics also saves solid-state physics from being merely the study of the electron gas – it provides non-classical phases such as magnetism, superconductivity, etc, where the electron liquid can be highly nontrivial.
Disorder plays a distinctive role in condensed matter physics. Strongly disordered systems can form glasses (from window glasses to Coulomb ones to spin ones), whose mechanical, electronic or spin properties differ drastically from those of solids or liquids. Even a weak disorder can completely change electronic and spin transport of low-dimensional materials, leading in some cases to metal-insulator transition. At a ‘mesoscopic’ scale (between micro- and macro-worlds) the quantum coherence plays a decisive role and a nontrivial interplay of disorder and interaction defines transport and thermodynamic properties of solids.
The recent creation and experimental study of ultracold quantum gases of atoms is providing startling phenomena for theoretical understanding. For example, light being slowed to 10ms^-1, or even stopped, and the creation of molecules at microKelvin temperatures present possibilities for forms of matter that are non-existent under normal conditions. These might be quantum 'condensates' of entities that are mixtures of light and particles and superfluid liquid crystals respectively.