Weak-couplingd-wave BCS superconductivity and unpaired electrons in overdopedLa2−xSrxCu

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 (0.178x 0.238), the derived gap values (without any adjusting parameters) approach closely onto the theoretical prediction ∆ 0 = 2.14k B T c for the weak-coupling d-wave BCS superconductivity. In addition, the residual term γ(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 γ(0) cannot be simply attributed to the pair breaking induced by the impurity scattering, instead the phase separation is possible. PACS numbers: 74.25.Bt,74.20.Rp, 74.25.Dw, 74.72.Dn c = 1 − 82.6(x − 0.16) 2 with T max c = 38 K. The SH measurements were performed on an Oxford Maglab cryogenic system using the thermal relaxation technique, as described in detail previously. 19 The temperature was down to 1.9 K and the magnetic field was applied parallel to c-axis in the measurements.

Doping dependence of the upper critical field in La 2−x Sr x CuO 4 from specific heat

“…From figure 1 and table 1 we can see the A essentially decreases with decreasing doping. Thus this suggests that the ∆ 0 becomes larger towards underdoping [10]. Actually it has been shown that in the underdoped region, the ∆ 0 quantitatively tracks the pseudogap in the normal state [9].…”

“…-We have performed the lowtemperature SH measurement in LSCO throughout the entire superconducting phase diagram [9][10][11]. The samples used for the study were single crystals prepared by the traveling-solvent floating-zone method.…”

“…-From the raw data, we have reliably separated the electronic SH C el = γT from other contributions such as the phonon SH and the possible Schottky anomaly [10,11]. In magnetic field H, for a superconductor, there is an increase of the electronic SH in the mixed state, denoted as γ(H)T .…”

Electronic Homogeneity in the Overdoped Regime of the Electron-Doped High-TcSuperconductors

“…Therefore, it has been concluded that the superconductivity in overdoped LSCO is pure and that the electronic state is inhomogeneous [2]. For dirty d-wave superconductors, on the other hand,  0 is proportional to HlnH [6].…”

“…For hole-doped La 2-x Sr x CuO 4 (LSCO), it has been suggested from the magnetic susceptibility measurements that both the superconducting (SC) transition temperature, T c , and the SC volume fraction decrease markedly with increasing x in the overdoped regime [1]. The low-temperature specific heat measurements have revealed the existence of a finite value of the Sommerfeld constant in the SC ground state [2]. In addition, from the magnetization-hysteresis-loop measurements, a so-called second magnetization peak has been observed in the overdoped regime, indicating the enhancement of the vortex pinning [3].…”