Diamagnetism, Nernst signal, and finite-size effects in superconductors above the transition temperature Tc

Abstract

Various superconductors, including cuprate superconductors, exhibit peculiar features above the transition temperature Tc. In particular the observation of a large diamagnetism and Nernst signal N in a wide temperature window above Tc attracted considerable attention. Noting that this temperature window exceeds the fluctuation dominated regime drastically and that in these materials the spatial extent of homogeneity is limited, we explore the relevance of the zero-dimensional (0D) model, neglecting local thermal fluctuations. It is shown that both the full 0D model as well as its Gaussian approximation mimic the essential features of the isothermal magnetization curves md(H) in Pb nanoparticles and various cuprates remarkably well. This analysis also provides estimates for the spatial extent of the homogeneous domains giving rise to a smeared transition in zero magnetic field. The resulting estimates for the amplitude of the in-plane correlation length exhibit a doping dependence reflecting the flow to the quantum phase transition in the underdoped limit. Furthermore it is shown that the isothermal Nernst signal of a superconducting Nb0.15Si0.85 film, treated as N∝−md, is fully consistent with this scenario. Accordingly, the observed diamagnetism above Tc in Pb nanoparticles and in the cuprates La1.91Sr0.09CuO4 and BiSr2Ca2CuO8−δ, as well as the Nernst signal in Nb0.15Si0.85 films, is in excellent agreement with the scaling properties emerging from the 0D model, giving a universal perspective on the interplay between diamagnetism, Nernst signal, correlation length, and the limited spatial extent of homogeneity. Our analysis also provides evidence that singlet Cooper pairs subjected to orbital pair breaking in a 0D system are the main source of the observed diamagnetism and Nernst signal in an extended temperature window above Tc.

Source-reported materials — not catalogue approval

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