Anisotropic effect of appearing superconductivity on the electron transport in FeSe
Superconductivity in the most promising compounds, such as copper-oxide or iron-containing compounds, is usually manifested in the presence of non-stoichiometry of the chemical composition or doping. Superconductivity in such a systems, perhaps, initially arises in the form of small isolated superconducting islands [1], which become connected and coherent with decreasing temperature or with changing other parameters, such as doping or pressure. This behavior was recently discovered in [2] in the study of FeSe single crystals. Measurements of the conductivity of bridged structures oriented perpendicularly to the layers of FeSe single crystals showed the presence of excess conductivity, observed up to a temperature of 40 K. Detailed measurements of the magnetic susceptibility of FeSe single crystals made it possible to detect the appearance of a weak diamagnetic response at the same temperatures. Moreover, in transport measurements, the effect was anisotropic. The excess conductivity at high T was observed only in transport in a direction perpendicular to FeSe layers. Whereas in transport in the plane of layers the effect of excess conductivity was observed only in a narrow range of temperatures of superconducting fluctuations (no more than 1-2 K above Tc = 8 K). These data were interpreted as a manifestation of inhomogeneous superconductivity in the form of small inclusions with a volume fraction of ~ 10-3-10-4 [2], but not as an influence of superconducting fluctuations. We propose [3] a theoretical description of the conductivity in such an inhomogeneous system with superconducting islands of a spheroid shape with an arbitrary eccentricity. An analysis of the experimental results, obtained within the framework of this model, allowed obtaining both qualitative and quantitative agreement in estimating the fraction of the superconducting phase obtained in measurements of the magnetic susceptibility and electron transfer.
[1] V. Z. Kresin, Yu. N. Ovchinnikov, and S. A. Wolf, Physics Reports 431, 231, 2006.
[2] A. A. Sinchenko, P. D. Grigoriev, A. P. Orlov, A. V. Frolov, A. Shakin, D. A. Chareev, O. S. Volkova, and A. N. Vasiliev, Phys. Rev. B 95, 165120, 2017.
[3] P.D. Grigoriev, A.A. Sinchenko, K.K. Kesharpu, A. Shakin, T.I. Mogilyuk, A.P. Orlov, A.V. Frolov, D.S. Lyubshin, D.A. Chareev, O.S. Volkova, A.N. Vasiliev, JETP Letters 105, 786, 2017.