Date of Award

2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Materials Science & Engineering

First Advisor

Ritesh Agarwal

Abstract

Weyl semimetals are gapless topological states of matter with broken inversion and/or time reversal symmetry. In this thesis, we will firstly discuss the observation of a novel photogalvanic effect in type-II Weyl semimetals including $\rm T_d-MoTe_2$, $\rm Mo_{0.9}W_{0.1}Te_2$ and $\rm Mo_{0.3}W_{0.7}Te_2$. A circulating photocurrent is obtained under the illumination of normally incident light with circular polarization and the circulating current direction is opposite with different light helicity. Through temperature induced phase transition of $\rm MoTe_2$, this effect is further confirmed to exclusively occur in the Weyl phase. Since this CPGE current is controlled by the spatially varying beam profile, we define the effect as a spatially dispersive circular photogalvanic effect (sCPGE) and current amplitude is proven to be proportional to the beam gradient. By performing frequency-dependent measurements on the Weyl phase, we observe a sign reversal of sCPGE current at high energy excitation and low energy excitation. Our theoretical derivation shows that sCPGE is controlled by a unique symmetry selection rule related to asymmetric carrier excitation and relaxation, explaining the difference between Weyl phase and trivial phase as well as frequency dependent properties.

Photoinduced anomalous Hall effect (AHE) is also observed in type-II Weyl semimetals. Longitudinal CPGE current is obtained under normally incident light while applying transverse bias, and the current magnitude is observed to be proportional to the bias voltage. Comparing the AHE conductivity in the 1T' phase and the $\rm T_d$ phase of $\rm MoTe_2$, photoinduced AHE is found to be much more significant in Weyl phase. This effect can be understood by symmetry arguments and is described by a Fermi surface modulation under the external electric field; meanwhile, the difference between two phases is evaluated. This model further predicts that under low energy excitation, Weyl points can be partially muted with tilted Fermi level, which provides a promising method to probe the band topology and Weyl nodes as well as encode more degree of freedom in device applications.

Our studies on sCPGE and photoinduced AHE in type-II Weyl semimetals provide a new idea of probing and controlling nonlinear optical responses of topological semimetals and will potentially promote the applications of those new material systems.

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