Weak coupling d-wave BCS superconductivity and unpaired electrons in overdoped La_{2-x}Sr_{x}CuO_{4} single crystals

TL;DR

This study measures the specific heat of overdoped La_{2-x}Sr_{x}CuO_{4} crystals, revealing weak-coupling d-wave superconductivity and increasing unpaired electrons with doping, suggesting phase separation.

Contribution

It provides experimental evidence that the superconducting gap follows weak-coupling d-wave BCS theory and identifies the emergence of unpaired electrons at high doping levels.

Findings

Superconducting gap approaches the weak-coupling d-wave BCS prediction.

Residual specific heat b3(0) increases dramatically beyond xrom 0.22, indicating unpaired electrons.

At high doping, the material transitions to a normal metallic state.

Abstract

The low-temperature specific heat (SH) of overdoped La_{2-x}Sr_{x}CuO_{4} single crystals (0.178=<x=<0.290) has been measured. For the superconducting samples (0.178=<x=<0.238), the derived gap values (without any adjusting parameters) approach closely onto the theoretical prediction \Delta_{0}=2.14k_{B}T_{c} for the weak-coupling d-wave BCS superconductivity. In addition, the residual term \gamma(0) of SH at H=0 increases with x dramatically when beyond x~0.22, and finally evolves into the value of a complete normal metallic state at higher doping levels, indicating growing amount of unpaired electrons. We argue that this large \gamma(0) cannot be simply attributed to the pair breaking induced by the impurity scattering, instead the phase separation is possible.