Superconducting Fluctuation investigated by THz Conductivity of La$_{2-x}$Sr$_x$CuO$_4$ Thin Films

TL;DR

This study investigates the superconducting fluctuations in La$_{2-x}$Sr$_x$CuO$_4$ thin films using terahertz conductivity, revealing the fluctuation onset temperature and its dependence on carrier concentration, with implications for understanding high-temperature superconductivity.

Contribution

The paper provides detailed terahertz conductivity measurements showing the temperature and doping dependence of superconducting fluctuations in La$_{2-x}$Sr$_x$CuO$_4$, clarifying their relation to other phenomena.

Findings

Superconducting fluctuation onset temperature is at most ~2T_c^zero for underdoped samples.

Superconducting fluctuation is not enhanced by a 0.5 T magnetic field.

Temperature dependence of fluctuations varies with carrier concentration.

Abstract

Frequency-dependent terahertz conductivities of La$_{2-x}$Sr$_x$CuO$_4$ thin films with various carrier concentrations were investigated. The imaginary part of the complex conductivity considerably increased from far above a zero-resistance superconducting transition temperature, $T_\text{c}^\text{zero}$, because of the existence of the fluctuating superfluid density with a short lifetime. The onset temperature of the superconducting fluctuation is at most $\sim 2T_\text{c}^\text{zero}$ for underdoped samples, which is consistent with the previously reported analysis of microwave conductivity. The superconducting fluctuation was not enhanced under a 0.5 T magnetic field. We also found that the temperature dependence of the superconducting fluctuation was sensitive to the carrier concentration of La$_{2-x}$Sr$_x$CuO$_4$, which reflects the difference in the nature of the critical dynamics near the superconducting transition temperature. Our results suggest that the onset temperature of the Nernst signal is not related to the superconducting fluctuation we argued in this paper.