Anisotropic electron-phonon coupling in the spinel oxide superconductor

TL;DR

This study reveals that electron-phonon coupling in LiTi2O4 superconductors is highly anisotropic and surface-orientation dependent, with implications for understanding its unconventional superconducting behavior.

Contribution

It provides experimental evidence of anisotropic electron-phonon interactions in LiTi2O4, supported by tunneling spectra and DFT calculations, highlighting the importance of surface orientation.

Findings

Bosonic modes observed in specific surface orientations

Modes persist beyond Tc and upper critical field

Electron-phonon coupling is highly anisotropic

Abstract

Among hundreds of spinel oxides, LiTi2O4 (LTO) is the only one that exhibits superconductivity (Tc ~13 K). Although the general electron-phonon coupling is still the main mechanism for electron pairing in LTO, unconventional behaviors such as the anomalous magnetoresistance, anisotropic orbital/spin susceptibilities, etc. reveal that both the spin and the orbital interactions should also be considered for understanding the superconductivity. Here, we investigate tunneling spectra of [111]-, [110]- and [001]-oriented high quality LTO thin films. Several bosonic modes in tunneling spectra are observed in the [111]- and [110]-oriented films but not in [001]-oriented ones, and these modes still exist at T = 2Tc and beyond the upper critical field, which are confirmed as stemming from electron-phonon interaction by DFT calculations. These modes only appear in special surface orientations, indicating that the electron-phonon coupling in LTO system is highly anisotropic and may be enhanced by orbital-related state. The anisotropic electron-phonon coupling should be taken seriously in understanding the nature of LTO superconductivity.