Fluctuation induced diamagnetism in La 1.9Sr 0.1CuO 4 superconductors well inside the high-reduced temperature and the finite magnetic field regimes

Abstract

By using two grain oriented La 1.9Sr 0.1CuO 4 (LaSCO) samples, with masses as big as 48 and 110 mg, the fluctuation induced diamagnetism (FD) above the superconducting transition temperature, T c0, has been measured in the reduced temperature region bounded by 3×10 −2≲ ϵ≲0.5 and under reduced magnetic fields to within 8×10 −3≲ h≲0.2. Here ϵ≡ln( T/ T c0) and h≡ H/ H c2(0), where T c0 and H c2(0) are, respectively, the mean field normal–superconducting transition temperature at zero applied magnetic field and the upper critical magnetic field amplitude. These measurements deeply penetrate, then, in both the Schmidt and Schmid (or zero magnetic field)-limit ( h/ ϵ≪1) and the Prange regime ( h/ ϵ≳1). For the first time in a high-temperature cuprate superconductor (HTSC), these FD data in both fluctuation regimes cover all the reduced temperatures not too close to T co, including the high-reduced temperature region ( ϵ≳0.1), where the contribution of the short-wavelength fluctuations may be particularly important. Then, the existing FD calculations on the grounds of the Gaussian–Ginzburg–Landau (GGL) approach for 2D-layered superconductors are extended to the short-wavelength regime by introducing two possible cutoff conditions: The conventional kinetic energy (or momentum) cutoff and the total energy cutoff, this last taking also into account the quantum localization energy contribution associated with the shrinkage of the superconducting wavefunction. The differences between both cutoff conditions are expected to be appreciable mainly in the high-reduced temperature region, where the superconducting coherence length, ξ( T), becomes of the order of ξ(0), the coherence length extrapolated to T=0 K. The analyses of our experimental data on the grounds of these phenomenological approaches show that the different FD regimes may be explained in terms of the GGL theory for layered superconductors only under a total energy cutoff. In contrast, when regularized through a momentum cutoff condition the GGL approach fails to explain the high-reduced temperature region in both fluctuation regimes (Schmidt and Schmid-limit and Prange regime). The corresponding cutoff amplitude is found to be of the order of 0.6, in excellent agreement with our previous FD results obtained in the finite magnetic field regime but at low-reduced temperatures ( ϵ≲0.1). This cutoff amplitude is defined in units of ℏ 2/2m ∗ξ 2 ab(0) , where m ∗ is the effective mass of the superconducting carriers, ℏ is the reduced Planck constant and ξ ab (0) is the in-plane superconducting coherence length amplitude. These results strongly support, then, the adequacy of the mean field like GGL approach, regularized through a total energy cutoff, to explain the thermal fluctuations in HTSC at all the temperatures not too close to T c0 (above ϵ≃3×10 −2), including the high-reduced temperature region (above typically ϵ≃0.1). These results also show that, in contrast with the low-reduced temperature superconductors (LTSC), due to the shortness of their superconducting coherence length amplitudes the FD in the HTSC is not appreciably affected by non-local electrodynamic effects, at least for reduced magnetic fields up to 0.2. In addition, our present findings suggest that the presence of a normal-state pseudogap in underdoped LaSCO superconductors does not appreciably affect the behaviour of the fluctuation induced diamagnetism in all the accessible reduced temperatures above the superconducting transition.