March 2023

Abstracts of the Quantum Center Lunch Seminar

Date: Thursday, March 2, 2023
Place: ETH Zurich, Hönggerberg, HPF G 6
Time: 12:00 - 13:30

Valley polarization in a 2D anisotropic electron gas

Agnes Valenti - Condensed Matter Theory and Metamaterials (Huber group), ETH Zurich

A 2D electron system in an AlAs quantum well has recently emerged as relevant platform for tuning of an electron’s valley degree of freedom:
At low densities, a spontaneous valley polarization has been observed. In addition, an effective low-energy description as an anisotropic electron gas with large effective mass renders the system an ideal playground for the study of the interplay between isospin flavours, anisotropy and strong correlations. We offer a qualitative and quantitative description of the phase diagram using Hartree-Fock theory, the random phase approximation (RPA) and variational Monte Carlo techniques. In particular, we find that valley polarization arises as a correlation effect, resulting from the valley-dependent anisotropic dispersion.

Evidence of the Coulomb gap in the density of states of MoS₂

Michele Masseroni - The Ensslin Nanophysics Group (Ensslin group), ETH Zurich

MoS₂ is an emergent van der Waals material that shows promising prospects in semiconductor industry and optoelectronic applications. However, its electronic properties are not yet fully understood. In particular, the nature of the insulating state at low carrier density deserves further investigation, as it is important for fundamental research and applications. In this study, we investigate
the insulating state of a dual-gated exfoliated bilayer MoS₂ field-effect transistor by performing magnetotransport experiments. We observe positive and non-saturating magnetoresistance, in a regime where only one band contributes to electron transport. At low electron density (∼ 1.4 × 10¹² cm⁻²) and a perpendicular magnetic field of 7 Tesla, the resistance exceeds by more than one order of magnitude the zero field resistance and exponentially drops with increasing temperature. We attribute this observation to strong electron localization. Both temperature and magnetic field dependence can, at least qualitatively, be described by the Efros-Shklovskii law, predicting the formation of a Coulomb gap in the density of states due to Coulomb interactions. However, the localization length obtained from fitting the temperature dependence exceeds by more than one order of magnitude the one obtained from the
magnetic field dependence. We attribute this discrepancy to the presence of a nearby metallic gate, which provides electrostatic screening and thus reduces long-range Coulomb interactions. The result of our study suggests that the insulating state of MoS₂ originates from a combination of disorder-driven electron localization and Coulomb interactions.