Observation of anisotropic electronic correlations in the spinel oxide superconductor

Among hundreds of spinel oxides, LiTi2O4 (LTO) is the unique one that exhibits startling superconductivity at ~13 K. Previous investigations on polycrystals unveiled its complexity, such as spin and orbital degrees of freedom which should not be ignored for the origination of superconductivity besides the moderate electron-phonon coupling. After four decades, the first electronic phase diagram was presented on [001]-oriented single crystal thin films, whereas fundamental issues still remain obscure, e.g. the details on electron-phonon coupling. Here, we present a thorough study of electronic structure by probing tunneling spectra of [111]-, [110]-, as well as the [001]-oriented LTO thin films. Several bosonic modes are first observed in the [111]- and [110]-oriented films but not in [001]-oriented ones. Moreover, these modes retain prominent at twice of the superconducting transition temperature or in field up to 16 T. The selective appearance of bosonic modes indicates anisotropic electronic correlations, consistent with our DFT calculations that disclose an anisotropy of electron-phonon coupling in LTO films of different orientations, plausibly associated with Jahn-Teller distortions. These results urge the community to consider the anisotropic electronic structure in further exploring the nature of its superconductivity.