Quantum Materials Research

The thermoelectric effect is the conversion of waste heat into useful electric power. Conventional materials have an upper limit on how strong this effect can be. We showed that three-dimensional Dirac and Weyl semimetals can overcome this limit, and produce a thermoelectric effect that grows without bound as a function of applied magnetic field. See here for a popular news write-up. We share a patent on this mechanism together with Liang Fu at MIT.

Papers: Science Advances 4, eaat2621 (2018)

Nature Communications 11, 1046 (2020)

Phys. Rev. Materials 5, 024202 (2021).

In twisted bilayer graphene, the twist angle between the two layers determines the velocity of the electrons traveling through the graphene. Thus, variations in twist angle act like variations in the index of refraction for light traveling through an optical medium. We studied the transmission of electrons through a bilayer graphene sample with random wandering of the twist angle, using an analogy to disordered optics. We found a "transparent mirror effect", in which disorder of the twist angle leads to perfect reflection of electrons and vanishing conductivity.

Paper: Phys. Rev. Research 2, 043416 (2020).

Popular summary on Twitter

The crystalline material SmB6 caused a huge stir in condensed matter physics, because it somehow looks like both a conductor and an insulator simultaneously, depending on which property you measure. Here we showed how at least some of this controversy can be resolved by considering the behavior of impurity states. It turns out that constructing a proper theory of these states in SmB6 and other "mixed valence insulators" involves rewriting one of the basic rules of impurities in insulators, and solving for a new kind of "hydrogen atom".

Paper: Phys. Rev. Materials 3, 104601 (2019).

Popular summary on Twitter.

See also a recent workshop we hosted.

When current flows through a resistor, it exhibits random fluctuations (Johnson-Nyquist noise) that are proportional to the electron temperature. This phenomenon can be harnessed to measure the electron temperature inside a system and to gauge the flow of heat. Our group played a supporting role in the development of this technique, led by the group of Philip Kim at Harvard.

Papers: arXiv: 2101.01737 (2021).

Nature Nanotechnology 13, 797 (2018)

A recent experiment by Amir Yacoby's group at Harvard showed something puzzling: a 2D quantum spin Hall insulator behaved like a conductor, with a resistance that increased linearly with magnetic field. We were able to explain this data and show how both features are a signature of long-range correlated disorder arising from impurities in the substrate.

Paper: Phys. Rev. B 98, 214203 (2018).

Tunneling into a 2D electron system is complicated by the interaction between electrons. Attempting to insert an electron at low energy requires other electrons to "make way" for the new one, so that it can be placed in a location with low energy. This many-electron rearrangement process can be described by a kind of hydrodynamics, which becomes especially interesting when used to describe the composite fermion state at the half-filled Landau level. We produced such a theory, including the effect of spin polarization, which helped to explain recent experiments by Jim Eisenstein at Caltech.

Paper: Phys. Rev. B 97, (2018)