Quantum phase transitions in highly crystalline 2D superconductors

Recent technological advances of thin films fabrication, especially mechanical exfoliation, led to the successful preparation[N1] of highly-crystalline two-dimensional (2D) superconductors [1]; Examples include atomically thin NbSe₂ and ion-gated 2D crystals, which exhibit intrinsic properties of 2D superconductors with minimal disorder. In this talk, we provide a unified picture of temperature versus magnetic field phase diagram in highly crystalline 2D superconductors containing quantum phase transitions. According to the magneto-transport measurements in 2D superconducting ZrNCl and MoS₂, we found that the quantum metallic state (vortex liquid) [2] commonly observed at low magnetic fields is converted via the quantum Griffiths state to the weakly localized metal at high magnetic fields through a quantum phase transition with the diverging critical exponent. We discuss such evolution of quantum state is generic nature of highly crystalline 2D superconductors with weak pinning potentials.
[1] Y. Saito et al. Nature Rev. Mat. 2, 16094 (2016), [2] Y. Saito et al. Science 350, 409 (2015).