Interplay of spin-orbit coupling and trigonal crystal field enhances superconductivity in $LaAlO_3/KTaO_3$ (111)

TL;DR

This study reveals that reducing the thickness of LaAlO3/KTaO3 (111) heterostructures enhances superconductivity due to the interplay of spin-orbit coupling and trigonal crystal field, challenging conventional expectations.

Contribution

It demonstrates the enhancement of superconductivity in LAO/KTO (111) heterostructures through the interplay of SOC and trigonal crystal field, supported by experimental evidence.

Findings

Superconducting transition temperature Tc increases as the conducting layer thickness decreases.

Trigonal symmetry and spin-orbit scattering are enhanced with increased Tc.

Evidence of unconventional superconductivity and a quantum critical point near the superconducting phase.

Abstract

In conventional superconductors, bulk physical properties typically degrade as the film thickness approaches the two-dimensional (2D) limit. Here in the (111) oriented LaAlO3/KTaO3 (LAO/KTO) heterostructure, we demonstrate experimental evidence that reducing the conducting layer thickness at the interface significantly enhances superconducting transition temperature Tc, in direct contrast to conventional wisdom. From the sum frequency generation (SFG) spectroscopy and superconducting upper-critical field measurements, both the trigonal symmetry and spin orbit scattering are enhanced with the increased Tc. We attribute the enhanced superconductivity (SC) to the synergic interplay between spin-orbit coupling (SOC) and trigonal crystal field, resulting in an enhanced electron-phonon coupling. Furthermore, we show the existence of unconventional SC: the approaching linear temperature dependence of normal state resistance with increasing Tc and the existence of a quantum critical point (QCP) near the superconducting phase. Our findings provide important insight into the underlying mechanism of the strong orientation-dependent KTO interface SC.