Abstract Body

A field(B)-induced superconductor-insulator transition (SIT) in disordered two-dimensional films is a famous example of a quantum phase transition (QPT) at zero temperature (T = 0) [1]. According to the localization theory, a direct transition from condensation to localization of electrons is driven by quantum fluctuations with a clear quantum critical point (QCP). In contrast, for some of weakly disordered [2,3] and crystalline thin films [4], an anomalous metallic (AM) ground state emerges, leading to a B-induced superconductor-metal-insulator transition (SMIT). We have recently shown from the Nernst effect, which has high sensitivity to fluctuations of the superconducting order parameter [5], that the AM state results from quantum liquid of vortices [3]. However, it remains an open question how the QPT picture of the SIT is modified in the SMIT. To answer the question, it is necessary to uncover quantum critical fluctuations developed around the QCP.

  In this work [6], we measure the Nernst effect for an amorphous Mo_xGe_1-x thin film exhibiting the B-induced SMIT down to 0.1 K. From a thorough contour map of the Nernst signal in the B-T plane, we have found a thermal-to-quantum crossover line of the superconducting fluctuations, a so-called ghost temperature line associated with the QPT [7], as well as a ghost-field line associated with a thermal transition [5]. By extrapolating the ghost temperature line to T = 0, the QCP is identified inside the AM state, which verifies the scenario that the AM state is a broadened critical state of the SIT.

References

[1] A. M. Goldman and N. Marcovi´c, Physics Today 51, 39 (1998).
[2] A. Kapitulnik, S. A. Kivelson, and B. Spivak, Rev. Mod. Phys. 91, 011002 (2019).
[3] K. Ienaga, T. Hayashi, Y. Tamoto, S. Kaneko, and S. Okuma, Phys. Rev. Lett. 125, 257001 (2020).
[4] Y. Saito, Y. Kasahara, J. Ye, Y. Iwasa, and T. Nojima, Science 350, 409 (2015).
[5] K. Behnia and H. Aubin, Rep. Prog. Phys. 79, 046502 (2016).
[6] K. Ienaga, Y. Tamoto, M. Yoda, Y. Yoshimura, T. Ishigami, S. Okuma (submitted).
[7] A. Glatz, A. Pourret, and A. A. Varlamov, Phys. Rev. B 102, 174507 (2020).

Acknowledgment

This work was supported by Grant-in-Aid for Early-Career Scientists (KAKENHI Grant No. 20K14413), Scientific Research(B) (KAKENHI Grant No. 22H01165), and Challenging Research (KAKENHI Grant No. 21K18598 and 23K17667) from the Japan Society for the Promotion of Science, and Yoshinori Ohsumi Fund for Fundamental Research and ASUNARO Grant from Tokyo Institute of Technology.