Periodically Modulated Electronic States in Natural Superlattices

Abstract

The periodic arrangement of atoms in a crystal underpins our understanding of their electronic states. Since the development of the band theory description of such systems, it was realized that interaction effects and materials synthesis techniques could be exploited to introduce additional periodic modulations atop this underlying atomic periodicity. Over the years, efforts in this direction based on two-dimensional (2D) thin-films and van der Waals (vdW) heterostructures have realized a plethora of unconventional electronic states of matter. By virtue of their low-dimensionality, however, these states can prove fragile and inaccessible to a variety of experimental probes. Bulk materials exhibiting such periodically modulated electronic states could pave the way to incisive experiments and, potentially, new electronic states heretofore unavailable.

In this thesis, we present the discovery of a new family of natural superlattices formed by an alternating stacking of transition metal dichalcogenide (TMD) monolayers and insulating spacer layers, that host such periodically modulated electronic states. We present experimental results from three members of this family containing the group-V transition metal disulphides H-MS₂, M = (V, Nb, Ta). Across these materials, the TMD and spacers layers combine to form effectively 2D electronic states that experience a structure-derived periodic modulation. In addition to yielding single particle physics distinct from the parent compounds, for example topologically non-trivial bands for M = Nb and unusual quantum oscillations for M = Ta, these materials also host unconventional correlated states; we observe long-anticipated signatures of 2D Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductivity for M = Nb and preliminary evidence for a correlated insulator ground state when M = V. We conclude by discussing prospects of identifying new members of this family and, more broadly, new families of bulk superlattices.