Session 4-3

Abstract

A superconductor-insulator transition (SIT) is a prototypical example of a quantum phase transition at zero temperature [1]. The SIT is induced by a magnetic field in disordered superconducting thin films. According to the localization theory, quantum fluctuations drive a direct transition from condensation to localization of electrons, with a well-defined quantum critical point. In contrast, in some weakly disordered or crystalline thin films, an anomalous metallic (AM) ground state appears, leading to a field-induced superconductor-metal-insulator transition (SMIT) [2]. The AM state has been predicted to originate from a quantum vortex liquid (QVL). However, convincing experimental evidence has not yet been obtained.

To detect the QVL and elucidate its mechanism, we employ the magneto-thermoelectric (Nernst) effect, which is highly sensitive to fluctuations of the superconducting order parameter [3], and investigate quantum fluctuations across the SMIT [4,5]. We measure the Nernst effect at temperatures down to 0.1 K in an amorphous MoxGe1-x thin film exhibiting the field-induced SMIT. From the Nernst signal, we demonstrate the existence of the QVL and identify a quantum critical point within the AM state. Our results suggest that the QVL is a quantum critical state arising from the broadening of the SIT.