Event Details

Title: Non-conservation of the valley density and its implications for the observation of the valley Hall effect Speaker: Alessandro Principi, University of Manchester

We show that the conservation of the valley density in multi-valley insulators is broken in an unexpected way by the electric field that drives the valley Hall effect. This implies that time-reversal-invariant fully-gapped insulators, in which no bulk or edge state crosses the Fermi level, can support a valley Hall current in the bulk and yet show no valley density accumulation at the edges. Thus, the valley Hall effect cannot be observed in such systems. If the system is not fully gapped then valley density accumulation at the edges is possible. The accumulation has no contribution from undergap states and can be expressed as a Fermi surface average, for which we derive an explicit formula.

We demonstrate the theory by calculating the valley density accumulations in an archetypical valley-Hall insulator: a gapped graphene nanoribbon. Surprisingly, we discover that a net valley density polarization is dynamically generated for certain edge terminations.

About the speaker:

Dr Alessandro Principi is a Senior Lecturer in Theoretical Condensed Matter Physics at the Department of Physics and Astronomy in the University of Manchester.

His research focuses on the study of systems where strong interactions lead to the formation of exotic quantum phases. Examples are superconductors, magnetic systems, twisted bilayer graphene, quantum spin liquids and topologically-ordered systems. In these systems, novel (quasi)particles can emerge. Some of these, names "anyons, have no counterpart in high-energy physics and can find application to (topological) quantum computation. Principi's studies theoretical models that can be treated with (quasi)analytical techniques. With a wealth of quantum-field theoretical theoretical methods he addressed equilibrium and non-equilibrium properties of such systems. Amongst the others, the emergence of hydrodynamic behaviour of quantum particles.