Dynamic superconductivity responses in photoexcited optical conductivity and Nernst effect

Abstract

High-T c cuprate, alkali-doped C 60 and several other unconventional superconductors have very high transition temperatures T c with respect to the energy scale of superconducting (SC) charges inferred from the superfluid density (SFD). The observed linear relationship between T c and the SFD can hardly be expected in BCS superconductors while being reminiscent of Bose Einstein Condensation of pre-formed bosonic charges. As additional non-BCS like behaviors, responses similar to those in the bulk SC states have been observed at temperatures well above T c in the vortex-like Nernst effect, diamagnetic susceptibility, and transient optical conductivity in recent photo-excited pump-probe measurements. In this paper, we propose a coherent picture based on equilibrium and transient SFD to understand these unconventional behaviors in cuprates, K 3 C 60 , and organic superconductors. This picture assumes: (1) Dynamic SC responses in the Nernst and photo-induced measurements emerge at the formation of the local phase coherence (LPC) among wave functions of pre-formed bosonic pairs. (2) Its onset temperature T LPC is distinct from and lower than the boson formation temperature often denoted as the "pseudo-gap temperature" T*, as T LPC is determined by the many-body boson density while T* represents attractive interaction between two fermions. (3) The bulk superconducting T c , signaling global phase coherence, is significantly reduced from T LPC , due to the competition between the SC and antiferromagnetic (AF) order. (4) The inelastic magnetic resonance mode (MRM) controls T c 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 photo excitation. The assumptions (1) and (2) explain the relationship between T c and the transient SFD in photo excited studies and equilibrium SFD in Nernst effect. (3) and (4) are inferred from the linear dependence of T c on the MRM energy. (4) and (5) are consistent with the behaviors of the 400 cm -1 optical responses in equilibrium and photo-excited studies and temperature dependence of the intensity of this optical mode and the MRM. Unlike previous phase-fluctuation pictures which expect dynamic responses between T* and T c , the present picture involving competing order indicates that dynamic SC responses are seen between T LPC and T c .

Photomolecular High-Temperature Superconductivity

“…Other mechanisms, different from the one discussed here, may also be at play [32][33][34], including the stabilization of preexisting Cooper pairs in the normal state [35], which have been revealed by superconducting Nernst effect measurements [36]. Finally, the possible appearance of a negative-U superconductivity through electronic squeezing in a molecular excited state should also be considered, especially in the context of a nearly frustrated electronic structure like the present one [37,38].…”

Evidence for metastable photo-induced superconductivity in K3C60

“…As argued in ref. 28 , the normal state of unconventional superconductors that are susceptible to being ordered with light may be that of a phase-incoherent bosonic fluid, in which superconducting fluctuations are present already. Hence, we consider a hypothetical situation in which phase synchronization is established by a light field, and we study how one such order is lost after the driving pulse has been turned off.…”

Unconventional superconductivity in highly-compressed unannealed sulphur hydride

“…As the result, unannealed H3S (P = 155 GPa) phase in all considered scenarios (Table 1) has 0.02 ≤ Tc/TF ≤ 0.05 and falls in unconventional superconductors band of the Uemura plot [16][17][18] in close proximity to other NRT counterparts [1,2,[12][13][14][15] (Fig. 2), including annealed H3S with transition temperature Tc ~ 190 K [1,38].…”

“…A plot of Tc versus TF where the most representative superconducting families are shown. Raw data is taken from[18,37,[39][40][41]. Characteristic lines for the Bose-Einstein condensate (BEC) and the Bardeen-Cooper-Schrieffer (BCS) superconductors are shown for clarity.Most of NRT superconductors have severe influence of thermodynamic fluctuations on the superconducting order parameter and, thus, on the observed superconducting transition temperature, Tc[39][40][41].…”