Topological Surface and Bulk States in Dirac Semimetal α-Sn Thin Films

Mingzhong Wu (Principal Investigator)

Department of Physics, Colorado State University

Topological Dirac semimetals represent a relatively newly-discovered topological quantum phase.  In comparison with other topological Dirac semimetal materials, Dirac semimetal α-Sn is much more attractive because (a) it is a single-element material and is therefore relatively easy to grow and (b) it can transform to other topological phases, such as a Weyl semimetal or a topological insulator, under certain strain or field conditions.  The goal of this program is to advance the fundamental understanding of topological surface and bulk states in Dirac semimetal α-Sn thin films and pave the way to new technology.  The program consists of six focus topics: (1) film growth, (2) Fermi level tuning, (3) bilinear magneto-electric resistance, (4) quantum oscillations, (5) negative magnetoresistance, and (6) magnetization manipulation.  Our recent work shows that Sn films grown on Si substrates exhibit pure α phase only when the thicknesses are no larger than 6 nm.  Under Topic (1), we aim to overcome this thickness limitation through several strategies that include growth at lower temperatures, post-cooling, and metallic capping.  Under Topic (2), we will investigate how to tune the Fermi level via doping, metallic capping, and voltage gating.  Our preliminary data indicate the existence of the bilinear magneto-electric resistance in α-Sn thin films.  Under Topic (3), we will fully examine this effect through field angle-dependent, second-harmonic resistance measurements.  The goals of Topics (4) and (5) are to demonstrate surface states-associated quantum oscillations and the negative magnetoresistance associated with a topological Dirac-to-Weyl phase transition, respectively.  This will be done through the measurements of resistance as a function of a magnetic field at low temperatures.  Topic (6) aims to establish the high potential of topological Dirac semimetal α-Sn thin films for spintronics applications through two model experiments: spin-orbit torque-induced magnetization switching in small ferromagnetic islands and spin-orbit torque-driven domain wall motion in narrow ferromagnetic strips.  The program will demonstrate for the first time the bilinear magneto-electric resistance effect, the surface state-associated quantum oscillations, and the negative magnetoresistance in Dirac semimetal α-Sn thin films.  Such studies will promote the fundamental understanding of topological bulk and surface states in α-Sn in particular and advance the understanding and knowledge of topological quantum materials in general.  The work on (a) the bilinear magneto-electric resistance, (b) the quantum oscillation, (c) the negative magnetoresistance, (d) the spin-orbit torque switching, and (e) the spin-orbit torque-driven domain wall motion is also of great technological significance, in terms of potential applications in spintronic sensor and memory devices.  This technological significance is even more obvious if one takes into account the following two facts.  First, α-Sn thin films can be grown by sputtering, an industry-friendly thin film growth technique, on Si, a common substrate in industry.  Second, the topological Dirac semimetal phase is present at room temperature, in stark contrast to many other quantum materials where the topological phase exists only at low temperatures.