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I am a condensed matter theorist interested in the dynamics of quantum systems away from equilibrium.
Recently, I studied the dynamics of 3D Kitaev quantum spin-liquids (QSLs). We calculated the dynamical structure factor for a range of lattices with low-energy density of states spanning those possible in 3D. The dynamical structure factor is measurable in inelastic neutron scattering (INS) experiments and our results reveal the different possible signatures in 3D and suggest INS as the spectroscopy of choice to illuminate the physics of Majorana fermions in Kitaev QSLs.
My current work concerns a simple model of localization without quenched disorder, either in the Hamiltonian parameters or the initial state. Disorder is generally thought to be necessary for localization – both Anderson (single particle) and many-body varieties – but we have demonstrated that this isn’t the case. Our work raises general questions about relaxation in isolated quantum systems through connections to Many-Body Localization and a recently suggested Quantum Disentangled Liquid – two alternate paradigms to eigenstate thermalization beyond integrable systems. Future directions of this work include higher dimension generalizations and the connection to lattice gauge theories.
In Plain English
Non-equilibrium behaviour is ubiquitous in nature and in fact the majority of physical systems of interest in the real world are not in equilibrium – a combustion engine, the weather and life itself are all examples. However, a proper theoretical understanding is usually only available in equilibrium, and only for very rare or fine tuned system that are not. Nevertheless, we are able to make progress and even use non-equilibrium physics to our advantage.
Scattering experiments are a nice example of where we can use the response to be being pushed away from equilibrium to deduce the structure, properties or excitations of a material. For my previous work I calculated the theoretical response to the inelastic scattering (energy is absorbed) of neutrons from 3D crystal structures that have been synthesised experimentally and are expected to be described by the Kitaev model. These results show the signatures that an experimentalist might hope to observe that signify a new phase of matter – the quantum spin liquid. Experiments have been performed on materials described by 2D structures where some of these signatures have been observed.
My more recent work concerns the phenomena of localization. Philip Anderson predicted that in a disordered material the quantum interference from scattering off the disorder would lead to an insulator and the localization of particles therein. There has been a lot of recent activity on showing this phenomena for interacting particles but it is generally believed that the disorder is a crucial ingredient for localization – our work shows that this is not the case. By coupling two species of particle we are able to dynamically generate the disorder to localize one of them. In quantum physics there is the eigenstate thermalization hypothesis, which is believed to hold for a generic many-body quantum system. It describes how a quantum system thermalizes (relaxes) in an analogous way to a classical system. Localized systems and our novel disorder-free mechanism provide important counterexamples and demand a new understanding of how isolated quantum systems can relax.