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[en] Recent experiments imply the presence of strong superconducting fluctuations above Tc in FeSe. It has been suggested that the observed effects could be due to the presence of a band with an anomalously small Fermi energy εF∝Δ. We report theoretical analysis of superconducting fluctuations in a two-band model with one of the bands having small εF. While close to Tc the system is described by a conventional Ginzburg-Landau theory (unless an accidental degeneracy between superconducting channels is present), the microscopic expressions for the coefficients differ from the ones obtained in the usual BCS (εF>>Δ) theory. We also present analytical expressions for fluctuation effects valid at elevated temperatures. On the basis of our results, we discuss the possible relevance of the BCS-BEC crossover to the phenomena observed in FeSe.
[en] We present quasiparticle interference (QPI) measurement in Fe-based superconductors as a robust way of determining the superconducting gap sign structure in experiment. We show that the bias dependence of the signed symmetrized and antisymmetrized QPI maps are useful to obtain a characteristic signature of a gap sign change or lack thereof, starting from two-band model up to ab initio based band structure calculation. The experimental realization of the suggested method was successfully realized in FeSe, where a sign changing gap sign structure was identified. We provide a motivation for the application to the LiFeAs compounds.
[en] Superconducting FeSe has been investigated by Moessbauer spectroscopy at various temperatures including strong external magnetic fields applied to the absorber. It was found that isomer shift exhibits sharply defined increase at about 105 K leading to the lowering of the electron density on iron nucleus by 0.02 electron a.u.3 . Above jump in the electron density is correlated with the transition from the P4/nmm to the Cmma structure, while decreasing temperature. Moessbauer measurements in the external magnetic field and for temperatures below transition to the superconducting state revealed null magnetic moment on iron atoms. The principal component of the electric field gradient on the iron nucleus was found as negative on the iron site. Superconductivity with the transition temperature Tc=8 K occurs for a compound being formed close to the FeSe stoichiometry. This compound has P4/nmm tetragonal structure at room temperature and transforms into Cmma orthorhombic phase between 100-80 K. It was found that Tc strongly depends upon applied pressure raising to 36.7 K at 8.9 GPa and subsequently dropping due to the induced phase transition into some hexagonal structure. Therefore, it is important to look upon phonon and electron density of states versus pressure in this unconventional compound to decide what kind of the boson field is responsible for the Cooper pairs formation. Calculations are performed for the stoichiometric compound within the density functional theory (DFT). Local density approximation (LDA) is used as better suited for the refinement of the crystal structure. A harmonic approximation is used to study lattice dynamics of the system. Results of the calculations are to be discussed in detail. Calculations show that the iron magnetic moment is zero in the ground state of both phases. (authors)
[en] I will discuss different scenario for time-reversal symmetry breaking in the superconducting state of Fe-based high Tc superconductors. I will review earlier works on s+id and s+is states and discuss recent theoretical and experimental results suggesting possible realization of time-reversal symmetry breaking nematic superconducting state in FeSe.
[en] The formation of the FeSe compound from a mixture of Fe and Se powders encased in a composite Cu/Nb sheath was studied in situ by means of high-energy synchrotron x-ray diffraction. Tetragonal β-FeSe does not seem to form directly from the starting elements. Instead, a sequence of FeSe2, Fe3Se4 and Fe7Se8 phases formed prior to the main formation stage of β-FeSe at 350-370 0C, although a small amount of this phase appears at 250 0C already during a heating ramp of 2 0C min-1. β-FeSe transforms to δ-FeSe around 480 0C and back to β-FeSe at 405 0C during cooling. There is no evidence for any interface reaction between the metal sheath and the superconducting core.
[en] We review a variety of theoretical and experimental results concerning electronic band structure of superconducting materials based on FeSe monolayers. Three type of systems are analyzed: intercalated FeSe systems AxFe_2Se_2_-_xS_x and [Li_1_-_xFe_xOH]FeSe as well as the single FeSe layer films on SrTiO_3 substrate. We present the results of detailed first principle electronic band structure calculations for these systems together with comparison with some experimental ARPES data. The electronic structure of these systems is rather different from that of typical FeAs superconductors, which is quite significant for possible microscopic mechanism of superconductivity. This is reflected in the absence of hole pockets of the Fermi surface at Γ-point in Brillouin zone, so that there are no ''nesting'' properties of different Fermi surface pockets. LDA+DMFT calculations show that correlation effects on Fe-3d states in the single FeSe layer are not that strong as in most of FeAs systems. As a result, at present there is no theoretical understanding of the formation of rather ''shallow'' electronic bands at M-points. LDA calculations show that the main difference in electronic structure of FeSe monolayer on SrTiO_3 substrate from isolated FeSe layer is the presence of the band of O-2p surface states of TiO_2 layer on the Fermi level together with Fe-3d states, which may be important for understanding the enhanced Tc values in this system. We briefly discuss the implications of our results for microscopic models of superconductivity.
[en] We explored the high-magnetic-field behavior of the thermoelectric and thermal-transport coefficients to probe the quasiparticle excitations in the normal and superconducting phase of FeSe. The small and compensated thermoelectric coefficients increase with decreasing temperature (T) due to the T-dependent increase of the charge-carrier mobility and the dominant role of the small hole Fermi-surface pocket. The measured longitudinal and transverse thermal conductivities imply that a highly anisotropic small superconducting gap forms at the electron pocket whereas an isotropic and larger gap forms at the hole pocket. Below 1 K, both thermal conductivities exhibit anomalies at the upper critical field, Hc2, and at a constant field H* around 14 T. These results support the existence of a distinct field-induced superconducting phase above H* which emerges with a presumed large spin-imbalance of the hole Fermi-surface pocket.
[en] Using linear-response density-functional theory, we obtain the magnetic interactions in several iron pnictides. The ground state has been found to be non-collinear in FeSe, with a large continuum of nearly degenerate states lying very close to the magnetic 'striped' structure. The presence of non-collinearity also seems to be a generic feature of iron pnictides when the Fe moment is small. At small RFe-Se the system is itinerant: strong frustration gives rise to an excess of spin entropy, long ranged interactions create incommensurate orderings and strong biquadratic coupling violates the applicability of the Heisenberg model. There is a smooth transition to more localized behavior as RFe-Se increases: stable magnetic orbital order develops which favors long range AFM stripe ordering with strongly anisotropic in-plane exchange couplings. The stabilization of the stripe magnetic order is accompanied by the inversion of the exchange coupling.
[en] We study the angular dependence of dissipation in the superconducting state of FeSe and Fe(Se1−x Tex) through electrical transport measurements, using crystalline intergrown materials. We reveal the key role of the inclusions of the non superconducting magnetic phase Fe1−y (Se1−x Tex), growing into the Fe(Se1−x Tex ) pure β -phase, in the development of a correlated defect structure. The matching of both atomic structures defines the growth habit of the crystalline material as well as the correlated planar defects orientation. (paper)