Gamayunova N. Point-contact investigation of iron-based superconductors

The dissertation presents the experimental results of the point-contact investigation of the following superconducting single crystals: FeSe (Tc = 9 K) belonging to the 11-group of iron-based superconductors (IBS), KFe2As2 (Tc = 3,8 К) and Ba1-xNaxFe2As2 with x=0.35 (Tc = 29 К) belonging to the 122-group of IBS, and ternary rare-earth copper antimonides LaCuSb2 and La(Cu0.8Ag0.2)Sb2 (Tc = 1K and 0.5 K respectively) which are related systems to the compounds of 112-group of IBS with the same structural type. The features of the electron-quasiparticle interaction and the superconducting state of these systems are studied by Yanson point-contact spectroscopy (PCS) and point-contact Andreev reflection spectroscopy (PCAR). The low-temperature superconductors KFe2As2, LaCuSb2, and La(Cu0.8Ag0.2)Sb2 have been investigated by Yanson PC spectroscopy method. The PC spectra (or the second derivatives of current-voltage characteristics d2V/dI2) of the PCs on the basis of KFe2As2 demonstrate a single 20 meV bosonic mode. Phonon and non-phonon (excitonic) nature of the maximum are discussed. According to the calculations of electron-phonon interaction and the specificity of the band structure of KFe2As2 an alternative model of indirect excitonic excitations is suggested to explain the experimental results. The d2V/dI2 spectra of the PCs on the basis of LaCuSb2 and La(Cu0.8Ag0.2)Sb2 demonstrate a pronounced maximum in the range of 10 ÷ 20 meV on the spectra of different contacts for both samples. The feature is caused by the electron-phonon interaction in these materials. The variation of the maximum position is likely connected with anisotropic phonon spectrum in these layered compounds. The calculation of electron-phonon interaction constant λ for LaCuSb2 gives the low limit of 0.20 ± 0.03 for its value. The superconducting state in Ba1-xNaxFe2As2 with x = 0.35 and FeSe has been investigated by PC Andreev reflection spectroscopy. The spectra of differential resistance (or the first derivatives of current-voltage characteristics dV/dI) of Ba0.65Na0.35Fe2As2 demonstrate the Andreev-like double minima structure in the range of low bias at the temperatures much lower than the superconducting transition temperature Tc. The calculations within the Blonder–Tinkham–Klapwijk (BTK) model gives the value 2Δ/kBTc = 3.6 ± 1. The temperature dependence of the superconducting gap is close to the BCS curve. Point-contact Andreev reflection study of FeSe is carried out using the contact formation by the needle-anvil and shear techniques as well as by the soft method. The analysis of PC Andreev reflection spectra of FeSe within the two-gap BTK model reveals the existence of two superconducting gaps. Using “soft” PCs the well-reproduced Andreev reflection spectra of FeSe have been obtained. The calculation within the two-gap BTK model gives the values of the reduced superconducting gaps 2ΔL/kBTc=4.2±0.9 and 2ΔS/kBTc=2.3±0.5,. The temperature dependence of the both gaps is close to the standard BCS behavior. The Andreev reflection double-minimum structure gradually decreases in magnetic field, but the position of the minima has a weak field dependence leading to almost field independent gaps value. The PC spectra of KFe2As2, Ba1 xNaхFe2As2 (x = 0.35) and FeSe in the thermal regime of current flow through the contact have been analyzed according to the existing theory. For all compounds the form of dV/dI spectra measured in the thermal regime is determined by the ρ(T) behavior of the samples. Such parameters as the Lorentz number L, the residual resistivity ρ0, and the diameter d of the contact are obtained for each sample. The values of L, ρ0, and d for KFe2As2 and FeSe are higher than expected ones, which could be due to surface degradation and the corresponding decrease in electron density.