[Paper Review] Superconducting Diode Effect and Large Magnetochiral Anisotropy in T$_d$-MoTe$_2$ Thin Film

Wan-Shun Du, Weipeng Chen+15 more|arXiv (Cornell University)|Mar 16, 2023

Magnetic and transport properties of perovskites and related materialsMaterials Science3 citations

TL;DR

This study demonstrates a superconducting diode effect and large magnetochiral anisotropy in disordered T$_d$-MoTe$_2$ thin films under weak out-of-plane magnetic fields. The diode effect arises from a vortex ratchet mechanism enabled by noncentrosymmetric superconductivity, achieving a diode efficiency near 20% and a magnetochiral anisotropy coefficient up to $5.9\times10^8\,\mathrm{T^{-1}\,A^{-1}}$, with nonreciprocal transport governed by asymmetric spin-orbit coupling distinct from Rashba systems.

ABSTRACT

In the absence of time-reversal invariance, metals without inversion symmetry may exhibit nonreciprocal charge transport -- a magnetochiral anisotropy that manifests as unequal electrical resistance for opposite current flow directions. If superconductivity also sets in, the charge transmission may become dissipationless in one direction while remaining dissipative in the opposite, thereby realizing a superconducting diode. Through both direct-current and alternating-current measurements, we study the nonreciprocal effects in thin films of the noncentrosymmetric superconductor T$_d$-MoTe extsubscript{2} with disorders. We observe nonreciprocal superconducting critical currents with a diode efficiency close to 20\%~, and a large magnetochiral anisotropy coefficient up to $\SI{5.9e8}{\per esla\per\ampere}$, under weak out-of-plane magnetic field in the millitesla range. The great enhancement of rectification efficiency under out-of-plane magnetic field is likely abscribed to the vortex ratchet effect, which naturally appears in the noncentrosymmetric superconductor with disorders. Intriguingly, unlike the finding in Rashba systems, the strongest in-plane nonreciprocal effect does not occur when the field is perpendicular to the current flow direction. We develop a phenomenological theory to demonstrate that this peculiar behavior can be attributed to the asymmetric structure of spin-orbit coupling in T$_d$-MoTe extsubscript{2}. Our study highlights how the crystallographic symmetry critically impacts the nonreciprocal transport, and would further advance the research for designing the superconducting diode with the best performance.

Motivation & Objective

• To investigate nonreciprocal transport in disordered T$_d$-MoTe$_2$ thin films under magnetic fields.
• To understand the origin of enhanced superconducting diode effect and large magnetochiral anisotropy in noncentrosymmetric superconductors.
• To determine how crystallographic symmetry and spin-orbit coupling anisotropy influence directional charge transport in superconducting systems.
• To develop a phenomenological theory explaining the angular dependence of nonreciprocal effects under in-plane fields.

Proposed method

• Fabricated high-quality T$_d$-MoTe$_2$ thin films via chemical vapor transport growth and exfoliation.
• Performed low-temperature four-terminal transport measurements using a dilution refrigerator at ~10 mK.
• Applied both DC and AC current excitation with lock-in detection to measure differential resistance and harmonic signals.
• Used vector superconducting magnet to apply out-of-plane and in-plane magnetic fields up to 10 mT.
• Measured second harmonic voltage ($V_{2\omega}$) to extract the diode voltage signal and quantify nonreciprocity.
• Developed a phenomenological model based on asymmetric spin-orbit coupling to explain the angular dependence of the magnetochiral anisotropy.

Experimental results

Research questions

• RQ1 What causes the superconducting diode effect in disordered T$_d$-MoTe$_2$ thin films under weak out-of-plane magnetic fields?
• RQ2 Why does the largest in-plane magnetochiral anisotropy not occur when the magnetic field is perpendicular to the current flow, contrary to Rashba systems?
• RQ3 How does the asymmetric structure of spin-orbit coupling in T$_d$-MoTe$_2$ influence the angular dependence of nonreciprocal transport?
• RQ4 What role does the vortex ratchet effect play in enhancing the diode efficiency in disordered superconducting films?
• RQ5 Can a phenomenological model based on spin-orbit coupling parameters accurately reproduce the observed angular variation of the nonreciprocal signal?

Key findings

• A superconducting diode effect with a diode efficiency of approximately 20% was observed under an out-of-plane magnetic field of 10 mT.
• The magnetochiral anisotropy coefficient reached a large value of $5.9\times10^8\,\mathrm{T^{-1}\,A^{-1}}$ under weak out-of-plane fields.
• The strongest nonreciprocal response under in-plane fields did not occur at $\varphi = 90^\circ$ (perpendicular to current), contradicting expectations from Rashba systems.
• The angular dependence of the nonreciprocal signal exhibited two asymmetric lobes, which were reproduced by a phenomenological model with asymmetric spin-orbit coupling parameters.
• The vortex ratchet effect was identified as the key mechanism enhancing rectification efficiency in disordered T$_d$-MoTe$_2$ films.
• Theoretical modeling confirmed that a finite $\alpha_3$ term and non-equal $\alpha_1, \alpha_2$ coefficients in the spin-orbit coupling are essential for the observed asymmetric angular response.