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Stefan Ilic

Quantum coherent phenomena in disordered transition metal dichalcogenide monolayers

Published on 4 October 2019
Thesis presented October 04, 2019

Abstract:
Transition metal dichalcogenide monolayers (TMDCs) are recently discovered two-dimensional materials. They host a strong intrinsic spin-orbit coupling (SOC), that acts as an effective Zeeman field with opposite, out-of-plane orientations in the +K and –K corners of the Brillouin zone (valleys). This SOC, and its interplay with disorder, strongly influences the behavior of quantum coherent phenomena in TMDCs. In this thesis, we investigate two such phenomena: superconductivity and interference corrections to the conductance, which include weak (anti-) localization and universal conductance fluctuations.
Several superconducting TMDCs have been experimentally found in both n-doped (MoS­2, WS2) and p-doped (NbSe2, TaS2) regimes. Here, the intrinsic SOC causes unusual “Ising pairing” of the Cooper pairs, formed of electrons from opposite valleys with strongly pinned out-of-plane spins. In-plane magnetic fields are thus not efficient in breaking the Cooper pairs by the paramagnetic effect, which results in a large enhancement of the in-plane upper critical field – the main signature of Ising superconductivity. In the first part of this work, we calculate the upper critical field as well as the density of states of disordered superconducting TMDCs. We show that intravalley scattering does not affect these properties, but that they strongly depend on intervalley scattering, which provides a depairing mechanism. In p-doped Ising superconductors, where multiple bands cross the Fermi level, we identify interband scattering as another important mechanism. We show that weak intervalley and interband scattering can explain experimental observations in n- and p-doped TMDC superconductors, respectively.
In the second part of this work, we calculate the interference corrections to the conductance in the normal state of TMDCs, which can serve as an independent probe of SOC of disorder. Because of the interplay between valley structure and SOC, these materials exhibit a rich behavior of weak (anti-) localization and universal conductance fluctuations, which is qualitatively different from other two-dimensional systems such as conventional metals or graphene. Our results can also be used to describe graphene/TMDC heterostructures, where SOC is induced in the graphene sheet. We discuss parameter regimes that can be used to interpret recent experiments and assess the strength of SOC and disorder. Furthermore, we show that an in-plane Zeeman field can be used to distinguish contributions of different kinds of SOC to the weak (anti-) localization.

Keywords:
Superconductivity, Disorder, Spin-Orbit coupling

On-line thesis.