Anisotropy of Thermodynamic Characteristics in Iron-Based Superconductors

Iron chalcogenides are of interest because of their extreme simplicity and although they have modest superconducting characteristics, they can be controlled by various modifications. The appearance of intercalated systems based on FeSe, in which Tc – 30-40K values were obtained and which immediately attracted attention. The interest became especially acute after the discovery of experimental superconductivity in FeSe Tc-80-100 monolayers. FeSe has a very complex zone structure, depending on the temperature, intricate shape of the Fermi surfaces, as well as at 90 degrees Kelvin occurs twinning – low-temperature effect of splitting the crystal lattice into a mosaic of two sublattices. The nature of the anisotropy and symmetry of the order parameter, the angular dependence of the upper critical field in iron-containing superconductors (and in FeSe in particular) is still a subject of debate and a rather controversial issue in the field of superconductivity, despite the vast amount of experimental and theoretical work on this subject. The experimental study of the anisotropy of the upper critical field in the basal plane has been an interesting task for several decades. The anisotropic behavior of Hc2 carries information about the order parameter and the conducting properties of the system. High-quality scanning tunneling microscopy and spectroscopy reveal a double energy gap. In this regard, a simple but effective method for estimating the anisotropy of the order parameter in a rather complex system of twinned FeSe single crystal from the angular behavior of the upper critical field in the basal plane of the crystal appears. The main idea is to use the Ginzburg-Landau theory using low fields and temperature near Tc. This is justified by the fact that in multi-slit superconductors in the superconducting state in each energy zone, a gap can arise, plus they can interact, but the upper critical field will be determined by the largest upper critical field in any of the zones (all the others are shunted).