Anisotropic electron-phonon coupling in the spinel oxide superconductor LiTi2O4

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 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. I. INTRODUCTIONOxides with spinel structure have attracted a broad attention for their extraordinary features, such as spin fluctuations from frustrated magnetic sublattice [1], charge ordering by mixed valence [2], and orbital ordering [3]. Accordingly, these intriguing properties generate rich functionalities including ferrimagnetism, magnetostriction, multiferroicity, etc [4][5][6]. So far, only a few of spinels show superconductivity, among which LiTi2O4 (LTO) is the only oxide and holds the highest record of superconducting critical transition temperature, i.e. Tc ~ 13 K [7]. Therefore, it is interesting to study the nature of such considerable superconductivity, especially for a complicated system with frustrated Ti-sublattice and equal numbers of Ti 3+ and Ti 4+ ions.

The effects of oxygen in spinel oxide Li1+xTi2−xO4−δ thin films

“…Previous work on LiTi 2 O 4 polycrystals has disclosed that the existence of oxygen-site distortion induces prominent changes in the electronic states near E F 30 . In addition, tunneling spectra experiments on LiTi 2 O 4 films of different orientations reveal an anisotropic electron-phonon coupling in this system, which is regarded to originate from the Jahn-Teller distortions enhanced by oxygen vacancies 31 . Nevertheless, it is still unclear what happens in the microstructure of the sample during the transition from LiTi 2 O 4 to Li 4 Ti 5 O 12 .…”

Distinction between critical current effects and intrinsic anomalies in the point-contact Andreev reflection spectra of unconventional superconductors

“…These features are observed in various cuprate [48] and iron-based superconductors, [46,47] as well as in LiTi 2 O 4 . [49] In this situation, the spectra provides fingerprint information to resolve the nature of superconductivity. The energy scale can be exploited to discriminate between the possible origin of the features in the spectra.…”

Anomalies of upper critical field in the spinel superconductor LiTi2O4−δ

“…For ideal LiTi 2 O 4 structure, the valence of Ti is 3.5+ with low d-orbital filling and Jahn-Teller distortions, implying that large orbital freedom exists in this system [17]. Orbital order and spin-orbit scattering are unveiled in the anoxic films where amounts of ordering oxygen vacancies have been explicated [16,18]. As the vacancies are gradually filled by introducing oxygen atoms, the orbital order is suppressed locally and spin-orbit scattering is enhanced at low temperatures [19].…”