Signatures of bosonic 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, it remains unclear which mechanisms—spin fluctuations, electron-phonon coupling or mixed valency—are essential for superconductivity in LiTi₂O₄. Here, we use angle-resolved photoemission spectroscopy (ARPES) with molecular beam epitaxy (MBE) to interrogate the electronic band structure of LiTi₂O₄ thin films on MgAl₂O₄ (111) substrates. Our work shows the first-ever experimentally-determined band structure of LiTi₂O₄. The bands intriguingly show a kink resembling cuprate-like band renormalizations. Our data indicates the presence of strong correlations: the band centered at Γ shows a ‘kink’ at around E_B~40 meV and a quasi-particle peak and incoherent tail suggestive of coupling to a bosonic mode. We see that this mode is present at all values of k_F and k_Z and persists above T_c. We discuss the origin of the kinks in LiTi₂O₄, providing broader insight into the pairing symmetry present in this superconducting system.

[1] D. C. Johnston et al, Mater. Res. Bull. 8, 777–784 (1973).

[2] H. Xue et al, ACS Nano 16 (11), 19464 (2022).

[3] C. P. Sun et al, Phys. Rev. B 70, 054519 (2004).

[4] K. Jin. et al, Nat. Commun. 6, 7183 (2015)