Gate-tunable superconducting tunneling spectra of Ti/LaAlO3/KTaO3 (111) planar junctions

Abstract

The KTaO3 (111) interface has attracted considerable attention due to its distinctive superconducting properties, which differ substantially from those of SrTiO3-based interfaces. Despite extensive investigations, the pairing mechanism of Cooper pairs at these interfaces remains elusive. Tunneling spectroscopy offers direct access to the pairing mechanism at buried superconducting interfaces; however, it requires ultrathin barrier layers that disfavor the formation of two-dimensional electron gas and interfacial superconductivity. Here, we employ titanium as the top electrode, enabling reduction of the LaAlO3 barrier thickness to 1.8 nm while maintaining stable superconductivity. This configuration facilitates the fabrication of planar tunneling junctions exhibiting well-resolved tunneling spectra. Our measurements reveal that the tunneling spectra of the LaAlO3/KTaO3 (111) interface are consistent with a single s-wave gap with a coupling strength ratio 2Δ/kBTc ranging from 3.5 to 4.0, slightly exceeding the Bardeen–Cooper–Schrieffer weak-coupling limit. The superconducting gap remains approximately 0.3 meV, independent of LaAlO3 barrier thickness. Moreover, back-gating simultaneously modulates transport properties and tunneling spectra, revealing no pseudogap up to electric fields of 3.6 kV/cm, in contrast to LaAlO3/SrTiO3 interfaces. However, increasing the gate field to 6.0 kV/cm induces an irreversible superconductor-to-insulator transition, demonstrating the fragility of interface conductivity with thinner LaAlO3 capping layers. These findings illuminate the pairing symmetry of LaAlO3/KTaO3 interfaces and advance our understanding of KTaO3-based interface superconductivity.