A nanoscale probe of the quasiparticle band structure for two dimensional electron systems

Abstract

The advent of a broad class of two-dimensional (2D) electronic materials has provided avenues to create and study designer electronic quantum phases. The coexistence of superconductivity, magnetism, density waves, and other ordered phases on the surfaces and interfaces of these 2D materials are governed by interactions which can be experimentally tuned with increasing precision. This motivates the need to develop spectroscopic probes that are sensitive to these tuning parameters, with the objective of studying the electronic properties and emergence of order in these materials. In the first part of this thesis, we report on spectroscopic studies of the topological semimetal antimony (Sb). Our simultaneous observation of Landau quantization and quasiparticle interference phenomena on this material enables their quantitative reconciliation - after two decades of their study on various materials. We use these observations to establish momentum-resolved scanning tunneling microscopy (MR-STM) as a robust nanoscale band structure probe, and reconstruct the multi-component dispersion of Sb(111) surface states. We quantify surface state parameters relevant to spintronics applications, and clarify the relationship between bulk conductivity and surface state robustness. At low momentum, we find a crossover in the single particle behavior from massless Dirac to massive Rashba character - a unique signature of topological surface states. In the second part of this thesis, we report on the spectroscopic study of charge density wave (CDW) order in the dichalcogenide 2H-NbSe2 - a model system for understanding the interplay of coexisting CDW and superconducting phases. We detail the observation of a previously unknown unidirectional (stripe) CDW smoothly interfacing with the familiar triangular CDW on this material. Our low temperature measurements rule out thermal fluctuations and point to local strain as the tuning parameter for this quantum phase transition. The distinct wavelengths and tunneling spectra of the two CDWs, in conjunction with band structure calculations, enable us to resolve two longstanding debates about the anomalous spectroscopic gap and the role of Fermi surface nesting in the CDW phase of NbSe2. Our observations motivate further spectroscopic studies of the phase evolution of the CDW, and of NbSe 2 as a prototypical strong coupling density wave system in the vicinity of a quantum critical point.