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Colloquium – Yaxian Wang (Harvard University)
Unconventional transport phenomena in quantum materials
An ever-increasing societal demand calls for sustainable energy and quantum information. Elucidating exotic transport and response phenomena in quantum materials is needed not only in designing next-generation modern devices, but also in discovering new physics. In this talk, I will present two of my recent theoretical discoveries in goniopolar materials and hydrodynamic transport. More specifically, I constructed the theory of axis-dependent conduction polarity, termed “goniopolar”, where the same population of charge carriers can simultaneously conduct as n-type and p-type along orthogonal crystallographic axes, originating from the Fermi surface topology. I built a consolidated analytical model and proposed the chemical design principles in semimetals and semiconductors. My predictions enabled transverse thermoelectric devices with an unprecedentedly high figure of merit, which can serve as a promising route to improve energy conversion. Further, in quantum materials with nontrivial band topologies and strong interactions, electron scattering can give rise to unusual transport phenomena. For example, electrons can flow collectively, termed “hydrodynamic”, exhibiting classical fluid phenomena such as vortices and Poiseuille flow. I utilized first principles tools to investigate the hierarchy of electron-scattering lifetimes in layered semimetal tungsten ditelluride (WTe2) at different temperatures, among which the phonon-mediated scattering mechanism could give rise to hydrodynamic behavior at intermediate temperatures. The theory also shows quantitative agreement with spatial electron current profiles measured using cryogenic scanning magnetometry within an exfoliated WTe2 sample. This microscopic mechanism opens up new possibilities in the search for hydrodynamic flow and strong interactions in high carrier density materials, which I will elaborate on with generalized fingerprints of electronic and phononic structures. Finally, perspectives for the future research directions into emerging energy and quantum materials will also be provided.
Host: Hai-Ping Cheng