Magnetic field induced transition from a vortex liquid to Bose metal in ultrathin a-MoGe thin film

TL;DR

This study demonstrates a magnetic field-induced transition in ultrathin a-MoGe films from a vortex liquid to a Bose metal, characterized by transport and spectroscopic measurements revealing loss of phase coherence without gap closure.

Contribution

It provides the first experimental evidence of a vortex liquid to Bose metal transition in a 2D amorphous superconductor using combined magnetotransport and tunnelling spectroscopy.

Findings

Identification of a critical magnetic field H_c^* where resistance behavior changes.

Observation of vortex lattice melting into vortex liquid at low fields.

Detection of persistent superconducting gap with suppressed coherence peak above H_c^*.

Abstract

We identify a magnetic field induced transition from a vortex liquid to Bose metal in a 2-dimensional amorphous superconductor, a-MoGe, using a combination of magnetotransport and scanning tunnelling spectroscopy (STS). Below the superconducting transition, Tc ~ 1.36 K, the magnetoresistance isotherms cross at a nearly temperature independent magnetic field, H_c^*~ 36 kOe. Above this field, the temperature coefficient of resistance is weakly negative, but the resistance remains finite as T --> 0, as expected in a bad metal. From STS conductance maps at 450 mK we observe a very disordered vortex lattice at very low fields that melts into a vortex liquid above 3 kOe. Up to H_c^* the tunnelling spectra display superconducting gap and coherence peak over a broad background caused by electron-electron interactions, as expected in a vortex liquid. However, above H_c^* the tunnelling spectra continue to display the gap but the coherence peak gets completely suppressed, suggesting that Cooper pairs lose their phase coherence. We conclude that H_c^* demarcates a transition from a vortex liquid to Bose metal, that eventually transforms to a regular metal at a higher field H* where the gap vanishes in the electronic spectrum.