Superconductivity in monolayer FeSe and quantum Griffiths singularity in 2D superconductors

By direct transport and magnetic measurements, we provide first direct evidence for high temperature superconductivity in monolayer FeSe films on insulating SrTiO3 (STO) substrates with the onset Tc and critical current density much higher than those for bulk FeSe [1]. Besides, the thickness dependent superconductivity in ultrathin FeSe films on STO has been investigated [2,3]. By both in situ scanning tunneling microscopy/spectroscopy and ex situ transport and magnetization measurements, we find that the two-atomic-layer Ga film with graphene-like structure on wide band-gap semiconductor GaN is superconducting with Tc up to 5.4 K [4]. Furthermore, in three-atomic-layer Ga films, we observe for the first time the quantum Griffiths singularity of superconductor-metal transition in two dimensional (2D) superconductors [5]. References: 1. Chin. Phys. Lett. 31, 017401 (2014) (Editors' choice in Science 343, 230 (2014)) 2. Scientific Reports 4, 6040 (2014) 3. arXiv:1507.08431 (accepted by 2D Materials) 4. Physical Review Letters 114, 107003 (2015) (Editors' Suggestion) 5. Science 350, 542 (2015) (accompanied with a perspective paper: Science 350, 509)