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Science magazine reports the discovery of quantum Griffiths singularity of superconductor-metal transition in Ga thin films

   More than 40 years ago, Robert B. Griffiths predicted that phase transitions can be dramatically changed by disorder effect and in particular the dynamical critical exponent can diverge. In the last 40 years, this theory has been applied to quantum phase transitions and developed into the theory of “quantum Griffiths singularity”. However, the major signature of the theory, the divergence of dynamical critical exponent, is very difficult to observe in experiments.   

   Recently, Prof. Jian Wang, in collaboration with Prof. Xincheng Xie, Prof. Xi Lin, and Prof. Fa Wang at Peking University, as well as Prof. Qi-Kun Xue and Prof. Xu-Cun Ma at Tsinghua University, observed for the first time the quantum Griffiths singularity in two dimensional (2D) superconducting system. They studied three monolayer thick Ga films in ultralow temperature regime, in which 2D superconductivity and superconductor to metal transition were detected. Furthermore, when approaching the zero temperature quantum critical point, they found the divergence of the dynamical critical exponent, which is consistent with the Griffiths singularity behavior. The superconductor-metal quantum phase transition in this 2D superconducting system with disorder could thus be explained by the theory of quantum Griffiths singularity, which is different with the previous understanding of quantum phase transition in 2D superconductors.   

  The paper was selected by Science Express and online published in Science on October 15, 2015 (DOI: 10.1126/science.aaa7154): http://www.sciencemag.org/content/early/2015/10/14/science.aaa7154.full.

   Prof. Jian Wang, Prof. Xi Lin at Peking University and Prof. Xu-Cun Ma at Tsinghua University are corresponding authors of this paper. Ying Xing, Hai-Long Fu, Dr. Haiwen Liu at Peking University and Hui-Min Zhang at Institute of Physics, Chinese Academy of Sciences contributed equally to this work.   

  The work was supported by National Basic Research Programs of China, National Natural Science Foundation of China, the Research Fund for the Doctoral Program of Higher Education (RFDP) of China, and Collaborative Innovation Center of Quantum Matter, China.



   Figure: The superconductivity in 3 monolayer thick Ga film and the divergence of the critical exponent approaching the quantum critical point (zero temperature limit).