Quantum geometry and anomalous transport in 2D materials


  • Datum: 09.07.2018
  • Uhrzeit: 11:00
  • Vortragende(r): Prof. Justin Song Chien Wen
  • Nanyang Technological University Singapore, Division of Physics & Applied Physics, School of Physical & Mathematical Sciences, College of Science
  • Ort: Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, 06120 Halle (Saale)
  • Raum: New Lecture Hall, Building B
Quantum geometry and anomalous transport in 2D materials

In a crystal, the “twisting” of electronic wave functions in momentum
space – as encoded in the Bloch band Berry curvature gives rise to a
wealth of interesting “anomalous” behaviors that typify a wide new
range of quantum materials. An emerging theme is how quantum geometry
enables coupling between electric and magnetic degrees of freedom. I
will discuss how coupled charge and magnetic degrees of freedom can
conspire to produce a variety of unusual transport phenomena.

A particularly striking example occurs in gapped graphene (a nominally
topologically trivial “vanilla” insulator). In vanilla/conventional
insulators, carrier transport is expected to be exponentially
activated, leading to a severely muted current response when an
electric field is applied. I will argue that this expectation fails in
gapped graphene (of with finite sample size) where its bulk free
carriers in valleys with non-vanishing Berry curvature give rise to
low-dissipation edge currents, which are squeezed within a distance of
the order of the valley diffusion length from the edge. This happens
even in the absence of edge states [topological (gapless) or
otherwise], and when the bulk equilibrium carrier concentration is
thermally activated across the gap. Instead, this behavior arises from
the unusual coupling between charge and magnetic degrees of freedom
afforded by Berry curvature.

If time permits, I will discuss another example of unusual quantum
geometric behavior. This occurs in monolayer 1T’-WTe2 where a
perpendicularly applied electric field induces a bulk band berry
curvature and magnetic moment with a rapid switch-like behavior. Due
to its low symmetry, Berry curvature and magnetic moment in 1T’-WTe2
possess a dipole-like distribution enabling (gate-tunable) quantum
non-linear anomalous Hall currents and current induced magnetization
(kinetic magneto-electric effect) respectively. Taken together these
render it a rich two-dimensional platform for all- electrical control
over quantum geometric effects as well as spin/magnetic texture.

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