Scanning tunnelling spectroscopy of superconductivity on surfaces of LiTi2O4(111) thin films

Yoshinori Okada,Emi Minamitani,Ryota Shimizu,Susumu Shiraki,Yasunobu Ando,Taro Hitosugi,Satoshi Watanabe

doi:10.1038/ncomms15975

Abstract

Unique superconductivity at surfaces/interfaces, as exemplified by LaAlO3/SrTiO3 interfaces, and the high transition temperature in ultrathin FeSe films, have triggered intense debates on how superconductivity is affected in atomic and electronic reconstructions. The surface of superconducting cubic spinel oxide LiTi2O4 is another interesting system because its inherent surface electronic and atomic reconstructions add complexity to superconducting properties. Investigations of such surfaces are hampered by the lack of single crystals or high-quality thin films. Here, using low-temperature scanning tunnelling microscopy and spectroscopy, we report an unexpected small superconducting energy gap and a long coherence length on the surface of LiTi2O4(111) epitaxial thin films. Furthermore, we find that a pseudogap opening at the Fermi energy modifies the surface superconductivity. Our results open an avenue for exploring anomalous superconductivity on the surface of cubic transition-metal oxides, where the electronic states are spontaneously modulated involving rich many-body interactions.

Highlights

Unique superconductivity at surfaces/interfaces, as exemplified by LaAlO3/SrTiO3 interfaces, and the high transition temperature in ultrathin FeSe films, have triggered intense debates on how superconductivity is affected in atomic and electronic reconstructions
We found that a superconducting gap exists on large energy scale pseudogap states
We found that low-temperature film deposition using pulsed laser deposition (PLD) followed by post-deposition annealing increases Tc up to 13 K, together with improved surface flatness

Summary

Introduction

Unique superconductivity at surfaces/interfaces, as exemplified by LaAlO3/SrTiO3 interfaces, and the high transition temperature in ultrathin FeSe films, have triggered intense debates on how superconductivity is affected in atomic and electronic reconstructions. The surface of superconducting cubic spinel oxide LiTi2O4 is another interesting system because its inherent surface electronic and atomic reconstructions add complexity to superconducting properties Investigations of such surfaces are hampered by the lack of single crystals or high-quality thin films. A interesting system is the spinel oxide (AB2O4) superconductor In this system, the cubic pyrochlore sublattice of B-atoms provides large degeneracy (frustration) of charge, spin and orbital states in bulk[21,22], and a prominent degeneracy lifting at the surface is expected to lead rich electronic states on the surface. We found that a superconducting gap exists on large energy scale pseudogap states These results provide the spectroscopic evidence of spontaneous superconducting modification on the surface of cubic transition-metal oxides, paving an interesting path to explore exotic superconductivity involving rich interactions

Methods

Results

Conclusion