Non-conservation of the valley density and its implications for the observation of the valley Hall effect

TL;DR

This paper reveals that in multivalley insulators, the valley density is not conserved under an electric field, which affects the observability of the valley Hall effect, especially in fully gapped systems.

Contribution

It uncovers the non-conservation of valley density in insulators under electric fields and derives explicit formulas for valley density accumulation at edges.

Findings

Valley density is not conserved in the presence of electric fields.

Fully gapped insulators can exhibit a valley Hall current without edge valley density accumulation.

Edge terminations can induce net valley density polarization.

Abstract

We show that the conservation of the valley density in multivalley 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.