Spectroscopic Signatures of Strong Electron-Phonon Coupling in Superconducting LiTi₂O₄ Thin Films

The mechanisms behind unconventional superconductivity have been intensely studied over the past few decades. Leading this thrust has been the high T_c cuprates, whose pairing ‘glue’ has been widely debated. LiTi₂O₄, a spinel oxide material, is an unconventional superconductor that preceded the cuprates [1]. However, despite having one of the highest T_c (~13.7 K) for a non-cuprate oxide, little is known about its' superconducting mechanism, with reports of both unconventional pairing [2] and traditional phonon-mediated BCS-like behavior [3]. There have also been signs of orbital and spin fluctuations persisting up to ~100 K, based on angle-dependent transport data [4]. Nevertheless, despite speculation about the nature of the superconducting mechanism in LiTi₂O₄, it remains clear that there is a strong presence of electron – phonon coupling in this material with phonon modes reported via different spectroscopic and scattering techniques [5,6]. Therefore, it becomes essential to investigate the nature and strength of these phonon modes and their relation to superconductivity. Our work combines the first ever MBE grown thin films of superconducting LiTi₂O₄ with spectroscopy to reveal the presence of strong phonon modes in LiTi₂O₄. Here, we present a comprehensive investigation of the strength and nature of electron – phonon coupling, visualized directly via Angle Resolved Photoemission spectroscopy (ARPES) experiments. We see clear spectroscopic signatures of coherent phonon oscillations for the first time in LiTi₂O₄ revealing the strength of electron – phonon coupling in this material. Finally, we discuss how the strength of the electron – phonon coupling in LiTi₂O₄ could potentially give rise to non- BCS behavior placing LiTi₂O₄ in an unconventional regime of superconductivity.<br/><br/>[1] D. C. Johnston et al, Mater. Res. Bull. 8, 777–784 (1973).<br/>[2] H. Xue et al, ACS Nano 16 (11), 19464 (2022).<br/>[3] C. P. Sun et al, Phys. Rev. B 70, 054519 (2004).<br/>[4] K. Jin. et al, Nat. Commun. 6, 7183 (2015).<br/>[5] Ge He at al, Phys. Rev. B 95, 054510 (2017).<br/>[6] M. A. Green, J. Phys.: Condens. Matter 9 10855 (1997).<br/>*This material is based upon work supported by the National Science Foundation under Award No. DMR-2339913. US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering. Platform for the Accelerated Realization, Analysis, and Discovery of Interface Materials(PARADIM). Paul & Daisy Soros Fellowship for New Americans.NSF Graduate Research Fellowship Grant No. DGE-1745303. Packard Foundation and the Gordon and Betty Moore Foundation’s EPiQS Initiative, grant GBMF6760