Higgs mode in the d-wave cuprate superconductor Bi2Sr2CaCu2O8+δ induced by an intense THz pulse
Jun 20, 2018
11:00 - 12:00
Department of Physics, The University of Tokyo, Tokyo, Japan
CFEL (Bldg. 99)
Seminar Room IV, O1.111
The nonequilibrium dynamicis of the superconductors has been intensively studied over decades. Among the viriety of nonequlibrium phenomena, the study of the collective dynamics of superconducting order parameter is of essential importance, as it provides deep insights into the properties of the order parameter. Recent developments of generating intense electromagnetic pulses in the terahertz (THz) frequency range have enabled the access to low energy collective modes without giving excess energy to the system. Particularly the amplitude fluctuation of the order parameter, referred to as the Higgs mode, has been observed in a conventional s-wave superconductor Nb1-xTixN [1,2]. Its extension to unconventional d-wave superconductors is intriguing, whereas it has been nontrivial whether the Higgs mode in d-wave superconductors is observable or not.
Here we investigated the Higgs mode in the d-wave cuprate superconductor Bi2Sr2CaCu2O8+δ (Bi2212) by THz pump-optical probe spectroscopy in reflection geometry. Upon entering the superconducting state, the transient reflectivity change exhibits an instantaneous component that follows the squared THz electric field. This instantaneous signal can be thought as the THz-pulse induced Kerr effect and described by the thirdorder nonlinear susceptibility. The polarization-resolved measurement reveals that the THz Kerr signal can be decomposed into the fully symmetric A1g component and the B1g component. While the ratio of the two components strongly depends on the hole doping concentration, the A1g component is always larger than B1g component. Comparison with a mean-field calculation of the nonlinear susceptibility indicates that the A1g component is associated with the Higgs mode of the d-wave order parameter .
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