Anisotropic Superconductivity Onset and Heterogeneous Superconductivity in FeSe and O.S.

March 6 at 15:00, Moscow time, ACTP will hold the seminar on "Anisotropic Superconductivity Onset and Heterogeneous Superconductivity in FeSe and Organic Superconductors" by Prof. Pavel D. Grigoriev (Landau Institute for Theoretical Physics)

Abstract

Superconductivity (SC) often onsets inhomogeneously in the form of isolated islands, for example, due to doping or the competition of various electronic instabilities, such as spin/charge-density waves. Then the relative resistivity drop i / i above Tc is anisotropic and maximal along the axis of least conductivity, perpendicular to the conducting layers [1-7]. This contradicts the naively expected layered SC structure and long remained a mystery. The transition temperature Tc is often also anisotropic and maximal in the interlayer direction, as observed in many organic superconductors [1-3] and by us in iron selenide [4-7]. We explained and described analytically and numerically the observed effects of anisotropic SC onset [4-10]. They originate from heterogeneous superconductivity. The anisotropic drop in resistance above Tc is described using the effective-medium approximation [11], generalized to anisotropy conductors by us [6-8]. We propose the origin of this heterogeneity both in FeSe [4] and in organic superconductors [12]. We also explain [4,10] the observed Tc anisotropy of the transition to zero resistance [1-5 by taking into account the finite size and flat shape of the samples. Our results explain many experiments [1–10] and allow us to estimate the volume fraction and typical shape and size of superconducting islands far from the sample surface using resistivity and susceptibility measurements [4-9,13], which is hard to make by other techniques. We also observe [4,5] and explain [4,10] that Tc in thin FeSe samples increases significantly, from 8K to 12K, as their thickness decreases from 300 to 50 nm. Our method of experimental study of heterogeneous SC structure deep inside the sample is applicable to various layered superconductors, including high-Tc.

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