# Anomalies of upper critical field in the spinel superconductor LiTi$_2$O$_{4-\delta}$

## TL;DR

This study investigates how oxygen impurities affect the upper critical field in LiTi$_2$O$_{4-\delta}$ superconductors, revealing a significant increase in $B_{c2}$ that exceeds the Pauli limit, linked to spin-orbit interactions.

## Contribution

It demonstrates that oxygen impurities induce a remarkable increase in $B_{c2}$ in LiTi$_2$O$_{4-\delta}$, suggesting a new mechanism involving spin-orbit interaction for enhancing superconductor performance.

## Key findings

- $B_{c2}$ increases with oxygen content.
- $B_{c2}$ exceeds the Pauli limit in oxygen-rich samples.
- Spin-orbit interaction is proposed as the cause.

## Abstract

High-field electrical transport and point-contact tunneling spectroscopy were used to investigate superconducting properties of the unique spinel oxide, LiTi$_2$O$_{4-\delta}$ films with various oxygen content. We find that the upper critical field $B_\mathrm{c2}$ gradually increases as more oxygen impurities are brought into the samples by carefully tuning the deposition atmosphere. It is striking that although the superconducting transition temperature and energy gap are almost unchanged, an astonishing isotropic $B_\mathrm{c2}$ up to $\sim$ 26 Tesla is observed in oxygen-rich sample, which is doubled compared to the anoxic sample and breaks the Pauli limit. Such anomalies of $B_\mathrm{c2}$ were rarely reported in other three dimensional superconductors. Combined with all the anomalies, three dimensional spin-orbit interaction induced by tiny oxygen impurities is naturally proposed to account for the remarkable enhancement of $B_\mathrm{c2}$ in oxygen-rich LiTi$_2$O$_{4-\delta}$ films. Such mechanism could be general and therefore provides ideas for optimizing practical superconductors with higher $B_\mathrm{c2}$.