The Fulde-Ferrell-Larkin-Ovchinnikov phase is the superconducting state for which the Cooper pairs have non-zero total momentum. From the time of conception of this phase, many groups have been searching for a realization of the state. Here we describe a proposal of experimental verification of this state in the case of multi-bands systems, by carrying out the specific-heat measurement.
Iron-based superconductors Ba_{0.7}Rb_{0.3}Fe_{2}As_{2} and CaFe_{1.92}Co_{0.08}As_{2} of the `122' family have been investigated by means of the 14.41-keV Mössbauer transition in ^{57}Fe versus temperature ranging from the room temperature till 4.2 K. A comparison is made with the previously investigated parent compounds BaFe_{2}As_{2} and CaFe_{2}As_{2}. It has been found that Mössbauer spectra of these superconductors are composed of the magnetically split component due to development of spin density wave and non-magnetic component surviving even at lowest temperatures. The latter component is responsible for superconductivity. Hence, the superconductivity occurs in the part of the sample despite the sample is single phase. This phenomenon is caused by the slight variation of the dopant concentration across the sample (crystal).
The resistivity, magnetoresistance, and magnetic susceptibility are measured in single crystals of FeTe_{0.65}Se_{0.35} with Cu, Ni, and Co substitutions for Fe. The crystals are grown by Bridgman's method. The resistivity measurements show that superconductivity disappears with the rate which correlates with the nominal valence of the impurity. From magnetoresistance we evaluate doping effect on the basic superconducting parameters, such as upper critical field and coherence length. We find indications that doping leads to two component superconducting behavior, possibly because of local charge depression around impurities.
We study the electronic structure of iron-based superconductors FeSe_{1-x}Te_x within the density functional theory. We pay particular attention to the pressure effects on the Fermi surface (FS) topology, which seem to be correlated with a critical superconducting temperature T_C of iron chalcogenides and pnictides. A reduction of the FS nesting between hole and electron cylinders with increasing pressure is observed, which can lead to higher values of T_C. The tellurium substitution into selenium sites yields FS changes similar to the pressure effect.
The discovery of high temperature superconductivity in iron pnictides and chalcogenides has resulted in surprising new insights into high temperature superconductivity and its relationship with magnetism. Here we provide an overview of some of what is known about these materials and in particular about the interplay of magnetism and superconductivity in them. Similarities and contrasts with cuprate superconductors are emphasized and the superconducting pairing is discussed within the framework of spin fluctuation induced pairing.
Iron-based superconductors exhibit features of systems where the Fulde-Ferrell-Larkin-Ovchinnikov phase, a superconducting state with non-zero total momentum of the Cooper pairs, is actively sought. Experimental and theoretical evidence points strongly to the Fulde-Ferrell-Larkin-Ovchinnikov phase in these materials above the Pauli limit. In this article we discuss the ground state of iron-based superconductors near the critical magnetic field and the full h-T phase diagram for pnictides in case of intra-band pairing, in a three-band model with s_{±} symmetry.
We report on the effect of interstitial iron defect and doping on iron physical properties and stability of iron telluride by combined experimental and theoretical study. We find that antimony doping and increase iron content in interstitial effect have both the effect to slightly decrease the temperature of the magneto-structural transition T_{trans}. From stability calculations and absence of change in lattice parameters, it is suggested that insertion of antimony did not occur. Large decrease of T_{trans} down to 32 K was observed with Ni doping and our stability calculations confirm that the Ni doping is most favorable in the stability point of view. First-Principles calculations of stability of defect using supercell technique for stoichiometric FeTe indicate that the most stable defect is iron interstitial defect, by far, confirming the proposal done in the literature. Our electronic calculations indicate the appearance of large peaks around the Fermi level in the case of this defect and not just simple doping effect.
We study the ab-plane resistivity and Hall effect in the single crystals of Fe_{1-y}M_yTe_{0.65}Se_{0.35}, where M = Co or Ni (0 ≤ y ≤ 0.21). In case of each dopant two types of crystals, with different crystalline quality, are prepared by Bridgman's method using different cooling rates, fast or slow. The impurities suppress the superconducting transition temperature, T_c, with different rates. T_c reaches zero at markedly different impurity content: only 3 at.% of Ni, and about 14 at.% of Co. In addition, the suppression is somewhat dependent on the crystal cooling rate. The resistivity at the onset of superconductivity rises only weakly with the Co doping, while it increases 10 times faster for Ni. The Hall coefficient R_{H} is positive for Co doping indicating that hole carriers dominate the transport. For Ni R_{H} changes sign into negative at low temperatures for crystals with the Ni content exceeding 6 at.%. The implications of these results are discussed.
We report on measurements of samples with nominal composition FeSe_{0.5}Te_{0.5}, crystallized by the Bridgman method. Magnetic and transport properties of the samples were examined. The measurements confirm the coexistence of ferromagnetism and superconductivity below the superconducting transition temperature. The ferromagnetic contribution to magnetization, estimated at 10%, might be caused by the presence of ferrimagnetic Fe_7Se_8, which occupies about 10% of sample volume. From the Andreev spectroscopy we found superconducting energy gap Δ = 2.6 meV at T = 4.2 K, and from magnetization measurements the critical temperature T_c = 15.8 K. The critical current density in magnetic field H = 4 kOe, determined from magnetization measurements, is j_c = (1-2) × 10^4 A/cm^2 and weakly depends on magnetic field intensity.
Syntheses of superconducting iron chalcogenides FeSe_{1 - x} (x = 0-0.15) and FeTe_{1 - y}Se_{y} (y = 0.3-0.55) were performed. Superconducting phase of iron selenide was obtained by the solid-state reaction and from liquid phase. The highest values of critical temperature (T_c = 8.2-8.7 K) exhibit FeSe_{1-x} obtained by the crystallization from a melt with excess of iron less than 1 mol%. The samples from a melt contain up to 78% of tetragonal phase, as estimated by the X-ray diffraction. Lattice parameters and unit cell volume for the samples exhibiting highest T_{c} and sharpest transition to superconducting state are limited to narrow range, with c/a ratio close to 1.469. The samples with excess of selenium contain higher amount of hexagonal phase than stoichiometric one. Superconducting single-crystalline samples of FeTe_{1 - y}Se_{y} (up to 100% of tetragonal phase) were obtained using Bridgman's method. When y value increases, the volume of unit cell decreases. The critical temperature T_{c} changes from ≈ 11.5 K for y ≈ 0.3 to ≈ 14.7 K for y ≈ 0.5.
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