Seminars 2019

In scanning tunneling microscopy one probes decaying wave functions of electronic states from a sample surface using a sharp needle tip located at very proximate distance from the surface. We usually do not care how the tip is close to the surface as far as the tunneling current flowing between them. There are, however, several cases where the tip-sample gap distance matters, and by carefully analyzing the gap variation one can extract information that cannot be accessible by other methods. Here some of such examples are demonstrated in my presentation.
When the surface has several electronic states whose decaying behavior into the vacuum is different, the relative intensity of each state should depend on the gap distance. On a cobalt-terminated plane of a cleaved CeCoIn5 surface, a square lattice of round-shaped Co atoms is observed in usual tunneling conditions, but at closer distances the atomic shape is transformed into a dumbbell whose orientation alternates x and y directions. The shape transformation of the Co atom is due to the switching of the probed states from s-derived states to d orbitals with the reduction in the gap distance. The alternating arrangement is explained by the ordering of Co d-orbitals induced by enhanced electron correlation at the surface. This is the first real-space observation of the orbital orderings, and was achieved by the selective probing of d-orbitals by setting the gap distance closer than usual STM operations.
At closer distances, the current cannot be described by the simple electron tunneling, and chemical interaction between the tipapex atom and surface atoms has to be considered. Because of the contribution from the conduction channels formed by the interaction, the contrast of atomically-resolved images taken on Pb(111) surface is enhanced or reversed depending the gap distance. The analysis of the conduction channels of well-controlled Pb atomic point contacts was performed based on the multiple Andreev reflection and Josephson current.

During the information technology (IT) revolution global capacity to compute information has grown at an astounding 60-70% per year, enabled by enormous gains in energy efficiency due to Moore’s Law advances in silicon technology. However Moore’s Law is ending, and the sustainable future of the IT revolution is uncertain. A new computing technology is needed with vastly lower energy consumed per operation than silicon CMOS. The recent discovery of topological phases of matter offers a new route to low-energy switches based on the conventional-to-topological quantum phase transition (QPT), a “topological transistor” in which an electric field tunes a material from a conventional insulator “off” state to a topological insulator “on” state, in which topologically protected edge modes carry dissipationless current. I will discuss our work on atomically thin films of Na3Bi (a topological Dirac semimetal) as a platform for a topological transistor. We study thin films of Na3Bi grown in ultra-high vacuum by molecular beam epitaxy[1], characterized with electronic transport, scanning tunneling microscopy (STM), and angle-resolved photoemission spectroscopy. When thinned to a few atomic layers Na3Bi is a large gap (>300 meV) 2D topological insulator with topologically protected edge modes observable in STM. Electric field applied perpendicular to the Na3Bi film, by potassium doping or by proximity of an STM tip, closes the bandgap completely and reopens it as a conventional insulator. Ultra-thin Na3Bi on sapphire can be probed by transport experiments, which show topological edge conduction as seen in non-local electronic transport and a giant negative magnetoresistance due to suppression of spin-flip scattering. The large bandgap of 2D Na3Bi, significantly greater than room temperature, and its compatibility with silicon, make it a promising platform for topological transistors.