Electronic phase separation close to the superconductor to insulator transition in thin TiN film

Orateur : Prasanna D. KULKARNI (Departamento de Fisica de la Materia Condensada Facultad de Ciencias Universidad Autonoma de Madrid, Madrid,Spain)

Superconductor to insulator transition (SIT) in very thin films is considered by many as a remarkable example of a c ontinuous quantum phase transition. The transition has been widely studied in macroscopic resistance experiments, which show how superconducting (R = 0 at T = 0 K) goes over into insulating (R = infinite at T = 0 K) behavior in these films when varying e.g. disorder level. Few imaging experiments have been made and the spatial variations of superconducting properties at scales of coherence length (typically few nms in these films) are found. However, the microscopic modifications in the electronic properties of the films close to the SIT transition are under topical debate and remain largely un-explored. In this talk, I will present our STM/S studies in ultra-thin disordered metallic films of Titanium Nitride (TiN). We obtain, for the first time, atomic resolution in the polycrystalline thin films (thickness of 5 nm) and make large scale conductance maps at 100 mK under magnetic fields. The film on the superconducting side shows over large scales the BCS-like tunneling condu ctance features, with quasiparticle peaks and small low energy conductance. However, we also find in the same film extended regions where the superconducting energy gap does not fully open. The images demonstrate that there is electronic phase separation close to the transition to the insulating state. Magnetic field restores the homogeneous phase. Peculiarity of the high field Density of States (DOS) is the reduced density of states at the Fermi energy. Atomic scale measurements evidence disordered charge density modulations on the film surface. Our measurements with unprecedented resolution provide unseen characteristics of disordered superconducting films.

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