|Probing Higgs and Leggett modes in superconductors with charge density waves|
Center for High-Pressure Science & Technology Advanced Research, Shanghai 201203
地点：Room 616,Physics Building
Collective excitation modes are a characteristic feature of symmetry-broken phases of matter. Their properties are of fundamental interest due to the distinguishing feature of any symmetry-broken phase such as superconductors, charge density waves or antiferromagnets. An amplitude Higgs mode and a phase modes of a superconductor are the radial and angular excitations in the Mexican-hat potential of the free energy. There exist an additional Leggett phase mode for a two-band superconductor. However, these Higgs and Leggett modes have never been simultaneously detected in any known two-band superconductors. There has been no clue for their evolution or development with tunable physical parameters such as temperature, doping, pressure, or magnetic fields. The great achievement in the study of high-temperature superconductivity in cuprates was the experimental findings of charge density waves and their competition with superconductivity in certain doping regime in the past 5 years. Since there is a close relation between the charge density wave and pseudogap in these superconductors, the complicated phase diagram is then simplified to the one solely including charge density wave and superconductivity. In this talk, I will first show our recent results of the emergence of the charge density wave in the overdoped region even at room temperature in a trilayer cuprate Tl2223 and the reduction of the superconducting transition temperature. Then a superconductor NbSe2, sharing the common simplified phase diagram with cuprates, is chosen to show how we detect Higgs and Leggett modes in this two-band superconductor by using Raman scattering technique and how these modes evolve with pressure upon heavy compression. The implications of such observations to the understanding of superconductivity in two-dimensional superconductors, cuprates, and our newly discovered molecule superconductors will be discussed.
Xiao-Jia Chen is currently a staff scientist of Center for High-Pressure Science and Technology Advanced Research in Shanghai. He was awarded as a Global Expert of the 1000 Talent Plan from the Central Government of China in 2014. He earned his Ph. D from Zhejiang University in 1997 and has expertise in the experimental and theoretical study of superconductivity, magnetism, and hydrogen-bearing materials at high pressures. He served as a leader of superconductivity program of U.S. Department of Energy’s Energy Frontier Research in Extreme Environments Center during 2009-2014 and a coordinator of high-pressure diffraction project at Spallation Neutron Source of Oak Ridge National Laboratory between 2007-2009. His current research focuses on the discovery of new superconductors and the improvement of the performance of known superconductors by understanding the fundamental physics of superconductivity through high-pressure study. He has published over 120 papers in peer-reviewed scientific journals, among which 10 appeared in Nature and its research journals and PNAS, 40 in Physical Review Letters and Physical Review B.