Fluctuation conductivity in superconducting MgB2

According to the crystal structure of MgB₂ and band structure calculations, quasi-two-dimensional (2D) boron planes are responsible for the superconductivity. We report on critical-field and resistance measurements of 5.6-μm-thick MgB₂ films grown on a sapphire single-crystal substrate. Resistivity measurements yield a temperature dependence of the fluctuation conductivity above the critical temperature, which agrees with the Aslamazov-Larkin and Maki-Thompson theory of fluctuations in layered superconductors, indicating a quasi-two-dimensional nucleation of superconductivity in MgB₂.     

Influence of cation substitution on the magnetic properties of the FeCr2S4 ferrimagnet

The influence of cation substitution on the magnetic properties of single and polycrystals of FeCr₂S₄ spinel has been studied. The tetrahedral A-site substitution of the Fe by Cu in Fe_1−xCu_xCr₂S₄ was found to increase significantly the value of temperature T_m of the spin-glass like magnetization anomaly, whereas the octahedral B-site substitution of the Cr by In decreases T_m. This effect is suggested to result from a structural transformation influenced by variation of internal (chemical) pressure due to lattice contraction (Cu) or expansion (In). The observed reduced values of the Curie temperature for Cu-substituted single crystals compared to that of the polycrystalline samples are attributed to presence of Cl ions in samples detected by electron-probe microanalysis. The observed reduced value of saturation magnetization in the polycrystals compared to the single crystals is ascribed to the effect of surface anisotropy. Based on the experimental data the superexchange is concluded to be the dominant exchange interaction for 0≤x≤0.5 in Fe_1−xCu_xCr₂S₄, whereas the indirect exchange through the charge carriers is considered of minor importance.     

Critical temperature oscillations and reentrant superconductivity due to the FFLO like state in F/S/F trilayers

Ferromagnet/Superconductor/Ferromagnet (F/S/F) trilayers, in which the establishing of a Fulde‐Ferrell Larkin‐Ovchinnikov (FFLO) like state leads to interference effects of the superconducting pairing wave function, form the core of the superconducting spin valve. The realization of strong critical temperature oscillations in such trilayers, as a function of the ferromagnetic layer thicknesses or, even more efficient, reentrant superconductivity, are the key condition to obtain a large spin valve effect, i.e. a large shift in the critical temperature. Both phenomena have been realized experimentally in the Cu₄₁Ni₅₉/Nb/Cu₄₁Ni₅₉ trilayers investigated in the present work.     

Properties of a weak link with tunable strength in a quasi-one-dimensional superconductor

In a superconducting whisker a region of weakened superconductivity (weak link) has been induced by a second crystal carrying a control current and crossing the first one. By varying the control current the grade of weakness could be continuously adjusted and its influence on the voltage-current characteristics of the weak link could be investigated.     

The influence of the mean free path on the current induced superconducting/normal conducting transition of tin whiskers with indium impurities

The current induced step-like structure in the V-I characteristics of tin whiskers with indium impurities up to 4 at.% shows a zero voltage intercept I₀ of approximatively 0.5I_c. This current can be explained by the existence of phase slip centers above the critical current which carry a time averaged supercurrent of ≈0.5I_c. From the differential resistance associated with the first step a “healing length” L_n proportional to l^{$\text{1}\text{2}$} (l = mean free path) was obtained which can be related to the quasi-particle diffusion length Λ introduced by Tinkham et al. in the explanation of temperature independent nonequilibrium processes between pairs and quasiparticles at a phase slip center.     

Thermally assisted flux flow in MgB2: strong magnetic field dependence of the activation energy

The origin of the resistive transition broadening of superconducting MgB₂ thin films is investigated. Thermally activated flux flow is found to be responsible for the resistivity in the vicinity of the critical temperature. The activation energy for flux motion is observed to have an extraordinary strong dependence on the magnetic field. The results are discussed.     

Reentrant superconductivity in Nb/Cu1-xNix bilayers

We report on the first observation of a pronounced reentrant superconductivity phenomenon in a superconductor/ferromagnet layered system. The results were obtained using a superconductor/ferromagnetic-alloy bilayer of Nb/Cu(1-x)Ni(x). The superconducting transition temperature T(c) drops sharply with increasing thickness dCuNi) of the ferromagnetic layer, until complete suppression of superconductivity is observed at d(CuNi) approximately equal to 4 nm. Increasing the Cu(1-x)Ni(x) layer thickness further, superconductivity reappears at d(CuNi) > or =13 nm. Our experiments give evidence for the pairing function oscillations associated with a realization of the quasi-one-dimensional Fulde-Ferrell-Larkin-Ovchinnikov-like state in the ferromagnetic layer.     

Current-induced resistive state of superconducting tin whiskers with indium impurities

We have investigated the current-induced step-like structure in theV?T andV?I characteristics of tin whiskers with a mean free pathl for the electrons which is reduced by indium impurities (up to 3.6 at %). The extrapolation of theV?I characteristics beyond the first voltage step shows a zero-voltage interceptI ₀≈0.5I _c (I _c=critical current). From the differential resistance beyond the first voltage step a lengthL _An1 can be obtained which is proportional tol ^1/2. These results can be understood by assuming phase-slip centers in the whisker which carry a time-averaged supercurrent of approximately 0.5I _c above the critical current, and at which a voltage is developed by the “healing” of nonequilibrium quasiparticles.     

Orbital fluctuations and orbital order in FeCr2S4

Heat-capacity investigations on the ferrimagnetic spinel FeCr₂S₄ poly- and single crystals provide experimental evidence of orbital liquid and orbital glass states. The low-temperature transition in the polycrystals at 10 K arises from orbital order and is very sensitive to the sample stoichiometry. In the single crystals the orbital order is fully suppressed resulting in an orbital glass state with the heat capacity following a strict T² dependence towards zero temperature. At elevated temperatures, FeCr₂S₄ exhibits an unexpected large linear term of about 100 mJ mol⁻¹ K⁻² as the fingerprint of the orbital liquid.     

Quasi-one-dimensional Fulde-Ferrell-Larkin-Ovchinnikov-like state in Nb/Cu0.41Ni0.59 bilayers

In a ferromagnet (F) being in contact with a superconductor (S) an unconventional finite-momentum pairing of electrons forming Cooper pairs occurs. As a consequence, interference effects of the pairing wave function, leading to an oscillation of the critical temperature for increasing F-layer thickness in S/F bilayers, including extinction and recovery of the superconducting state, were predicted by theory. We observed experimentally all types of this behavior, calculated theoretically, in Nb/Cu_{1 ? x} Ni _x bilayers (x = 0.59) of nanometer film thickness, prepared by magnetron sputtering (utilizing a moving magnetron deposition technique to provide a superb homogeneity of the ultrathin Nb layers), including a double extinction of superconductivity, giving evidence for a multiple reentrant state.