Dynamic Superconductivity Responses in Photo-excited Optical Conductivity and Nernst Effect

Abstract

High-Tc cuprate, A3C60 and other unconventional superconductors (SCs) have very high Tc with respect to the energy scale of superconducting (SC) charges inferred from the superfluid density (SFD). The observed linear dependence of Tc on the SFD can hardly be expected in BCS SCs while being reminiscent of Bose Einstein Condensation of pre-formed pairs. As additional non-BCS like behaviors, responses similar to those in the bulk SC states have been observed at temperatures (T) well above Tc in the vortex-like Nernst effect, diamagnetic susceptibility, and transient optical conductivity in photo-excitation (PEX) measurements. Here we propose a picture based on equilibrium and transient SFD to understand these unconventional behaviors. This picture assumes: (1) Dynamic SC responses in the Nernst and PEX measurements emerge at the formation of the local phase coherence (LPC) among wave functions of pre-formed pairs. (2) Its onset T (TLPC) is distinct from and lower than the pair-formation pseudo-gap T (Tstr). (3) The bulk SC Tc is reduced from TLPC, due to the competition between the SC and antiferromagnetic (AF) order. (4) The magnetic resonance mode (MRM) controls Tc in the SC-AF competition. (5) The transient optical responses can be attributed to a change of the balance between the competing SC and AF orders caused by PEX. The assumptions (1) and (2) explain the relationship between Tc and the transient SFD in PEX studies and equilibrium SFD in Nernst effect. (3) and (4) are inferred from the linear dependence of Tc on the MRM energy. (4) and (5) are consistent with the behaviors of the 400 cm-1 optical mode in equilibrium and PEX studies, whose intensity represents the MRM intensity. Unlike previous phase-fluctuation pictures which expect dynamic responses between Tstr and Tc, the present picture involving competing order indicates that dynamic SC responses are seen between TLPC and Tc.