Spin, strain, and superconductivity [electronic resource] : local probes of quantum materials between two and three dimensions

Silicon has served as the backbone of the electronics industry, but as conventional silicon transistors are scaled to their fundamental limits in an effort to continually improve computing power and expand the range of computing applications, alternative approaches have seen extensive research. Perhaps most straightforward is simply replacing silicon with another semiconductor material which has additional desirable properties such as mechanical flexibility. Utilizing the spin of an electron to carry and store information is another possibility which has the potential of great energy efficiency. Finally, some problems can be solved exponentially faster by replacing classical bits with quantum ones, forming a quantum computer. These are all promising ideas, but creating and characterizing potential materials with the required physics to implement them remains a key challenge. This thesis presents scanning tunneling microscopy (STM) and atomic force microscopy (AFM) studies of a number of materials which have potential applications in the ideas listed above due to the novel strain, spin, and superconducting physics that they exhibit. The first material studied, sodium cobaltate, is predicted to contain fully spin-polarized electrons on its surface, forming a half-metallic surface state. Due to spin-orbit coupling, these surface electrons are also predicted to have a chiral spin texture. These unique surface electrons can potentially support a topological superconducting state which could be utilized to achieve topological quantum computers. The surface state of sodium cobaltate is examined on atomic length scales using STM, AFM, and magneto-optic Kerr measurements in order to confirm the half-metallic nature of the surface state. Additionally, a superconducting tip is employed to induce superconductivity into the surface state, where signatures of nodal pairing symmetry are observed in addition to a zero bias peak, which suggests the possible observation of a Majorana Fermion bound state. The monolayer semiconductor material molybdenum disulfide is one example from a class of recently discovered two-dimensional materials, transition metal dichalcogenides, that contain unique physics. The band gap of this material is easily modulated by strain, which is possible over a large range of values due to its high strength. By straining molybdenum disulfide over a nanocone substrate, incident photons on these nanocones form strongly bound excitons. These drift towards the tip of the cone due to the varying strain profile, forming an artificial atom for excitons. Photoluminescence, Raman, and STM measurements of these devices confirm their electronic and photonic properties. Additionally, inducing sulfur vacancies into this strained semiconductor system allows it to act as a cost-effective catalyst for the hydrogen evolution reaction. The material properties of this system are studied using STM, and electrical measurements of the catalyst performance show top performance among methods for improving catalysis in molybdenum disulfide. Another way to modify monolayer molybdenum disulfide is to dope it with magnetic atoms, which is predicted to create a dilute magnetic semiconductor that could possibly operate at room temperature for high enough dopant concentrations. This would act as a voltage-controlled spin "switch", enabling efficient spin-based electronics. We study the effects of atomic manganese dopants on the electronic properties of molybdenum disulfide using STM, finding that these atomic dopants cause a strong energy splitting only visible at low temperatures, suggesting that the origin is magnetic in nature. Two superconducting materials are studied which exhibit unconventional behavior. First, BaPb1-xBixO3 is a three-dimensional superconductor which exhibits several signatures consistent with two-dimensional superconductivity. Second, observation of a tunnel junction between two conventional Pb superconductors using a tip/sample geometry reveals an electron-hole asymmetry in the phonon density of states, offering new insight into the superconducting pairing mechanism.

Keywords: Applied Physics.
Advisor: Fisher Ian
Manoharan Harindran C. (Harindran Chelvasekaran)
Devereaux Thomas Peter