Magneto-transport and Photoresponse of 2D Semimetals

Abstract

A semimetal is a material with a small overlap between the bottom of the conduction band and the top of the valence band. Near the Fermi level, the density of state is almost negligible. Thus, in the two dimensional limit, the carrier densities can be substantially modulated by applying an electrostatic gate, affording new opportunities for tuning the electronic states as well as potential applications in next generation transistors and optoelectronics. On the other hand, the carrier mobilities in these 2D semimetals are usually extremely high, enabling observations of various quantum phenomena, such as the quantum Hall effect. In this thesis, two 2D semimetals, graphene and single layer tungsten ditelluride (WTe2), will be explored through optical and magneto-transport techniques. In the first part, we will introduce the discovery of a new type of photocurrent generated by the photo Nernst effect in graphene in both classical and quantum regimes. By generalizing the Song-Levitov result of the long-range photocurrent response to the case of nonvanishing magnetic field on strip shape devices, we were able to formulate the photocurrent and capture almost all features observed in the experiment. Hereafter, we will turn to the other semimetal, WTe2, which was recently found to be a rich system, hosting a lot of exotic phenomena and interesting theoretic predictions, such as large magnetoresistance, type-II Weyl points, pressure driven superconductivity and nontrivial topology. Through investigating the generic magneto-transport properties of WTe2 to the single layer limit, we discovered that the monolayer is likely a quantum spin Hall insulator near charge neutrality, while displays superconductivity for large electrostatic doping at sub-Kelvin temperatures.