Diode, Anisotropy Measurements, and Improvements of Exfoliation for Nanoscale Superconductor 2M-WS$_2$

Abstract

The quest for signatures of topological or broken symmetry states naturally intersects with engineering of 2D materials that offer a fantastic testbed for condensed matter physics. Delivering layer number control of key properties, local field-effect gating, heterostructure stacking and more, van der Waals transition metal dichalcogenides are promising candidates to explore the interplay between symmetry and novel quantum properties. A bottleneck towards observing tantalizing topological properties in thin materials remains the access to efficient multipurpose exfoliation methods. Motivated by the potential for topological superconductivity in bilayer 2M-WS$_2$, a newly discovered metastable superconductor with the highest T$_c$ (8.8K) in its class, I set out to develop a suite of fabrication tools that comply with heat, air and solvent restrictions. Along the way, using optical second harmonic generation and (scanning) transmission electron microscopy, I characterize the 2M to 2H polymorph transformation and observe structural ordering. I achieve high exfoliation yields both with a polymer-based liquid N$_2$ cooled cleaving, and Au-assisted surface preparation. These methods are used to systematically document the thickness-dependence of 2M-WS$_2$ electronic properties in single-layer increments down to the monolayer. Critical values (I$_c$, T$_c$, and H$c$) are aggregated for over 100 devices. The effect of a local electrostatic gating is also studied - a first. In another measurement, I realize a magnetic-field-free, tunable, and perfectly rectifying superconducting diode by applying an additional AC electrical excitation to a centrosymmetric 2M-WS$2$ sample with a tear running through it. I demonstrate diode efficiencies up to 30% at 90mT and calculate a magnetochiral anisotropy coefficient of $\gamma = 6.0\times10^{8}$ T$^{-1}$A$^{-1}$, the highest ever reported. When investigating a novel geometry, an AC drive applied perpendicular to the direction of current, I record at large AC drives, and under no external field, $I{c}^{+} = 0$ while $I{c}^{-} = 2 \mu$A, a 100% diode efficiency. Simulations suggest that a strongly asymmetric ratchet effect (enabled by the tear) is induced by the AC drive. They explicitly demonstrate increasing diode efficiency with increasing AC drive. Lastly, interested in correlated ferroelectric and superconducting states in T$_d$-MoTe$_2$, I also measure an unusual gate dependence in few-layer samples of this other candidate topological superconductor. Altogether, these observations are facilitated by the methodical optimization of 2D fabrication protocols, which, because they are heat- and solvent-free, could be applied to a range of other materials.