Parallel and Perpendicular Susceptibility Above $T_c$ in La${}_{2-x}$Sr${}_x$CuO${}_4$ Single Crystals

Abstract

We report direction-dependent susceptibility and resistivity measurements on La${}_{2-x}$Sr${}_x$CuO${}_4$ single crystals. These crystals have rectangular needle-like shapes with the crystallographic "c" direction parallel or perpendicular to the needle axis,which, in turn, is in the applied field direction. At optimal doping we find finite diamagnetic susceptibility above $T_c$, namely fluctuating superconductivity (FSC), only when the field is perpendicular to the planes. In underdoped samples we could find FSC in both field directions. We provide a phase diagram showing the FSC region, although it is sample dependent in the underdoped cases. The variations in the susceptibility data suggest a different origin for the FSC between underdoping (below 10%) and optimal doping. Finally, our data indicates that the spontaneous vortex diffusion constant above $T_c$ is anomalously high.

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• Evidence for magnetic-field-induced decoupling of superconducting bilayers in La${}_{2-x}$Ca${}_{1+x}$Cu${}_2$O${}_6$
• The conductivity and the magnetization around Tc in optimally-doped YBa${}_2$Cu${}_3$O${}_{7-\delta }$ revisited: quantitative analysis in terms of fluctuating superconducting pairs
• Anisotropy of the upper critical fields and the paramagnetic Meissner effect in La1.85Sr0.15CuO4 single Crystals
• Two Superconducting Transitions in Single Crystal La${}_{2-x}$Ba${}_x$CuO${}_4$
• Anomalous elasticity in a disordered layered XY model