Surface superconductivity and twinning-plane superconductivity in aluminum

The critical supercooling field H _sc is measured in aluminum single crystals and twinned bicrystals in a temperature range slightly below T _c0 (T _c0 ? 0.055 K < T < T _c0), where T _c0 is the critical superconducting transition temperature. It is found that, even in this small temperature range, the H _sc(H _c) dependence, which is considered to be identical to the H _c3(H _c) dependence for single crystals, is substantially nonlinear. The H _sc(H _c) dependences of the twinned bicrystals and single crystals are shown to be significantly different. The qualitative features of the phase diagram of the twinned aluminum bicrystals coincide with those of the phase diagram of twinning-plane superconductivity obtained earlier for tin in [1]. These findings allow the conclusion that the phenomenon of twinning-plane superconductivity also exists in face-centered cubic crystal lattices.     

Enhancement of superconductivity in disordered films by parallel magnetic field

We show that the superconducting transition temperature T _c (H) of a very thin highly disordered film with strong spin-orbital scattering can be increased by a parallel magnetic field H. This effect is due to the polarization of magnetic impurity spins, which reduces the full exchange scattering rate of electrons; the largest effect is predicted for spin-1/2 impurities. Moreover, for some range of magnetic impurity concentrations, the phenomenon of superconductivity induced by magnetic field is predicted: the superconducting transition temperature T _c (H) is found to be nonzero in the range of magnetic fields 0 < H* ≤ H ≤ H _c .     

Quantization and confinement effects in superconducting nanostructures

The confinement of the flux lines by a lattice of submicron holes (‘antidots’) has been studied in nanostructured superconducting Pb/Ge multilayers. By introducing regular arrays of sufficiently large antidots, multi-quanta vortex lattices have been stabilized. Sharp cusp-like magnetization (M) anomalies, appearing at matching fieldsH_m=mφ₀/Sin superconducting films with the antidot lattices having a unit cell areaS, are successfully explained. These anomalies are, analogues of the well-knownM(H) cusp atH_c1, but for the onset of multi-quanta (m+1)φ₀-vortices penetration at each subsequent matching fieldH_m. It is shown that theM(H) curve between the matching fieldsH_m<H<H_m+1follows a simpleM∝ln(H−H_m) dependence. These experimental observations have revealed an unusual expansion of validity of the London limit in superconductors with lattices of relatively large antidots. The successful high quality fit of theM(H,T) curves convincingly demonstrates that a new type of the critical stateB=const (‘single-terrace critical state’) can be realized in superconductors with the antidot lattices.     

Temperature Dependence of the Upper Critical Field in Disordered Hubbard Model with Attraction

We study disorder effects upon the temperature behavior of the upper critical magnetic field in an attractive Hubbard model within the generalized DMFT+Σ approach. We consider the wide range of attraction potentials U—from the weak coupling limit, where superconductivity is described by BCS model, up to the strong coupling limit, where superconducting transition is related to Bose–Einstein condensation (BEC) of compact Cooper pairs, formed at temperatures significantly higher than superconducting transition temperature, as well as the wide range of disorder—from weak to strong, when the system is in the vicinity of Anderson transition. The growth of coupling strength leads to the rapid growth of H_c2(T), especially at low temperatures. In BEC limit and in the region of BCS–BEC crossover H_c2(T), dependence becomes practically linear. Disordering also leads to the general growth of H_c2(T). In BCS limit of weak coupling increasing disorder lead both to the growth of the slope of the upper critical field in the vicinity of the transition point and to the increase of H_c2(T) in the low temperature region. In the limit of strong disorder in the vicinity of the Anderson transition localization corrections lead to the additional growth of H_c2(T) at low temperatures, so that the H_c2(T) dependence becomes concave. In BCS–BEC crossover region and in BEC limit disorder only slightly influences the slope of the upper critical field close to T _c . However, in the low temperature region H_c2 (T may significantly grow with disorder in the vicinity of the Anderson transition, where localization corrections notably increase H_c2 (T = 0) also making H_c2(T) dependence concave.     

High-field phase-diagram of Fe arsenide superconductors

Here, we report an overview of the phase-diagram of single-layered and double-layered Fe arsenide superconductors at high magnetic fields. Our systematic magneto-transport measurements of polycrystalline SmFeAsO_1-xF_x at different doping levels confirm the upward curvature of the upper critical magnetic field H_c2(T) as a function of temperature T defining the phase boundary between the superconducting and metallic states for crystallites with the ab planes oriented nearly perpendicular to the magnetic field. We further show from measurements on single-crystals that this feature, which was interpreted in terms of the existence of two superconducting gaps, is ubiquitous among both series of single- and double-layered compounds. In all compounds explored by us the zero temperature upper critical field H_c2(0), estimated either through the Ginzburg–Landau or the Werthamer–Helfand–Hohenberg single gap theories, strongly surpasses the weak-coupling Pauli paramagnetic limiting field. This clearly indicates the strong-coupling nature of the superconducting state and the importance of magnetic correlations for these materials. Our measurements indicate that the superconducting anisotropy, as estimated through the ratio of the effective masses γ =  (m_c/m_ab)^1/2 for carriers moving along the c-axis and the ab-planes, respectively, is relatively modest as compared to the high-T_c cuprates, but it is temperature, field and even doping dependent. Finally, our preliminary estimations of the irreversibility field H_m(T), separating the vortex-solid from the vortex-liquid phase in the single-layered compounds, indicates that it is well described by the melting of a vortex lattice in a moderately anisotropic uniaxial superconductor.     

Upper critical fields of Nb3Ge thin film superconductors

The upper critical field H_c2(T) of the highest T_c(～23K) Nb₃Ge superconducting films has been found to be ≈370kG at 4.2K. Measurements on lower T_c films show very broad transitions reflecting nonuniformity. The H_c2(T) characteristics are consistent with other Nb₃X type II superconductors.     

Magnetic and superconducting phase diagrams in ErNi2B2C

We present measurements of the superconducting upper critical field H_c2(T) and the magnetic phase diagram of the superconductor ErNi₂B₂C made with a scanning tunneling microscope (STM). The magnetic field was applied in the basal plane of the tetragonal crystal structure. We have found large gapless regions in the superconducting phase diagram of ErNi₂B₂C, extending between different magnetic transitions. A close correlation between magnetic transitions and H_c2(T) is found, showing that superconductivity is strongly linked to magnetism.     

Electron transport in diborides: Observation of superconductivity in ZrB2

We report on syntheses and electron transport properties of polycrystalline samples of diborides (AB₂) with different transition metals atoms (A=Zr, Nb, Ta). The temperature dependence of resistivity, ρ(T), and ac susceptibility of these samples reveal a superconducting transition of ZrB₂ with T _c =5.5 K, while NbB₂ and TaB₂ have been observed to be nonsuperconducting up to 0.37K. H _c2(T) is linear in temperature below T _c , leading to a rather low H _c2(0)=0.1 T. At T close to T _c , H _c2(T) demonstrates a downward curvature. We conclude that these diborides, as well as MgB₂ samples, behave like simple metals in the normal state with usual Bloch-Grüneisen temperature dependence of resistivity and with Debye temperatures 280, 460, and 440 K for ZrB₂, NbB₂, and MgB₂, respectively, rather than T ² and T ³, as previously reported for MgB₂.     

Enhancement of pseudogap anomalies induced by nanoscale structural inhomogeneity in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.93</Subscript> high-<Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> superconductor

The magnetization M(H) in the superconducting state, dc magnetic susceptibility χ(T) in the normal state, and specific heat C(T) near the superconducting transition temperature T _c have been measured for a series of fine-crystalline YBa₂Cu₃O _y samples having nearly optimum values of y = 6.93 ± 0.3 and T _c = (91.5 ± 0.5) K. The samples differ only in the degree of nanoscale structural inhomogeneity. The characteristic parameters of superconductors (the London penetration depth and the Ginzburg–Landau parameter) and the thermodynamic critical field H _c are determined by the analysis of the magnetization curves M(H). It is found that the increase in the degree of nanoscale structural inhomogeneity leads to an increase in the characteristic parameters of superconductors and a decrease in H _c(T) and the jump of the specific heat ΔC/T _c. It is shown that the changes in the physical characteristics are caused by the suppression of the density of states near the Fermi level. The pseudogap is estimated by analyzing χ(T). It is found that the nanoscale structural inhomogeneity significantly enhances and probably even creates the pseudogap regime in the optimally doped high-T _c superconductors.     

Influence of Fermi-surface topology change under pressure on two-band superconductors properties

An investigation is made of the influence of Fermi-surface topology change under pressure on the superconducting transition temperature T_c and on energy gaps at zero temperature γ_n(0) of a two-band superconductor. Two extrama in the pressure derivate of T_c and γ_n(0) are predicted.     

Vortex patterns and nucleation of superconductivity in mesoscopic rectangles and in hybrid superconductor/ferromagnet structures

Nucleation of superconductivity and vortex patterns have been studied in mesoscopic samples providing the crossover between square and rectangular geometries. The measured nucleation line, T _c (H), has been analyzed in the framework of the linearized Ginzburg-Landau theory. A very good agreement has been found between the theoretical T _c (H) boundary and the experimental data for rectangles with different aspect ratios. The superconductor/ferromagnet hybrids, such as magnetic Co/Pd dot in a superconducting loop and the dot on top of a superconducting disk have also been investigated. Pronounced effects of the dot on the T _c (H) boundary have been found, including strong asymmetry with respect to the field polarity.Received: 26 February 2004, Published online: 3 August 2004PACS: 74.25.Dw Superconductivity phase diagrams - 74.78.Na Mesoscopic and nanoscale systems     

Electronic structure of new multiple band Pt-pnictide superconductors APt3P

We report LDA calculated band structure, densities of states and Fermi surfaces for recently discovered Pt-pnictide superconductors APt₃P (A = Ca, Sr, La), confirming their multiple band nature. Electronic structure is essentially three dimensional, in contrast to Fe pnictides and chalcogenides. LDA calculated Sommerfeld coefficient agrees rather well with experimental data, leaving little space for very strong coupling super-conductivity, suggested by experimental data on specific heat of SrPt₃P. Elementary estimates show, that the values of critical temperature can be explained by rather weak or moderately strong coupling, while the decrease in superconducting transition temperature T _c from Sr to La compound can be explained by corresponding decrease in total density of states at the Fermi level N(E _F). The shape of the density of states near the Fermi level suggests that in SrPt₃P electron doping (such as replacement Sr by La) decreases N(E _F) and T _c , while hole doping (e.g., partial replacement of Sr with K, Rb or Cs, if possible) would increase N(E _F) and possibly T _c .     

Non-planar spin texture driven dissipation in a ferromagnet-superconductor bilayer

We report the observation of a pronounced dip in the in-plane magnetic field (H_∥) dependence of the critical current density J_c(H) and a peak in resistance R(H) of a NbN-HoNi₅ bilayer at temperatures below the magnetic ordering temperature (T_Curie ≈ 3.5 K) of HoNi₅, which is lower than the onset temperature (≈9 K) of superconductivity in the NbN layer. The extrema in J_c(H) and R(H) appear at fields much below the upper critical field of NbN. We attribute these features to a coupling between localized out-of-plane moments present in the magnetic film and Pearl vortices of the superconducting layer. A spin re-orientation transition of the localized moments by H_∥ breaks this coupling, leading to the observed excess dissipation.     

Suppression of superconductivity in YNi2B2C at the atomic disordering

The behavior of the electrical resistivity ρ(T), the superconducting transition temperature T _c , and the upper critical field H _c2(T) of a polycrystalline sample of YNi₂B₂C irradiated by thermal neutrons with the subsequent high-temperature isochronous annealing in the temperature interval T _ann = 100–1000°C has been studied. It has been found that the irradiation of YNi₂B₂C with a fluence of 10¹⁹cm^?2 leads to the suppression of the superconductivity. The final disordered state is reversible; i.e., the initial ρ(T), T _c , and H _c2(T) values are almost completely recovered upon annealing at up to T _ann = 1000°C. The quadratic dependence ρ(T) = ρ₀ + a ₂ T ² is observed for the sample in the superconducting state (T _c = 5.5?14.5 K). The coefficient a ₂ (proportional to the square of the electron mass m*) hardly changes. The form of the dependence of T _c on ρ₀ can be interpreted as the suppression of the two superconducting gaps, Δ₁ and Δ₂ (Δ₁ ～ 2Δ₂). The degradation rate of Δ₁ is about three times higher than that of Δ₂. The dependences dH _c2/dT on ρ₀ and T _c may be described by the relations for a superconductor in the intermediate limit (the coherence length ζ₀ is on the order of the electron mean free path l _tr) under the assumption of a nearly constant electron density of states on the Fermi level N(E _F). The observed behavior of T _c obviously does not agree with the widespread opinion about the purely electron-phonon mechanism of superconductivity in the compounds of this type supposing the anomalous type of superconducting pairing.     

Anisotropy of the superconducting transition in magnetic fields in a Nd1.85Ce0.15CuO4 single crystal

The anisotropy of superconducting properties of a Nd_1.85Ce_0.15CuO₄ single crystal is studied by resistance measurements over the temperature range 2–30 K in magnetic fields of 0, 1, 2, 4, and 6 T parallel to the a-b plane. A strong anisotropy of T _c (H) and H _c 2(T) is observed for different orientations of magnetic field in the a-b plane. This anisotropy leads to a twofold symmetry of T _c (H) and H _c 2(T), and the gap node direction is determined. An analysis of experimental data shows that this result can be attributed to a change in the local symmetry of the copper atom environment, which manifests itself as a reduction from tetragonal to orthorhombic symmetry in the low-temperature region. The comparison with La_1.85Sr_0.15CuO₄ suggests that the mechanisms of superconductivity in electron and hole doped superconductors are similar, and the difference observed in the experiment is related to the structural features of these materials.     

Superconductivity of (Sn1−z Pbz)1−x InxTe alloys

The temperature dependences of the electrical resistivity of (Sn_1?z Pb_z)_1?x In_xTe alloys with different concentrations of lead (z=0–0.60) and indium (x=0.03–0.20) were studied at temperatures T=0.4–4.2 K in magnetic fields from zero to H=15 kOe. A resistivity drop of no less than three-four orders of magnitude was observed in this range of alloy compositions. Application of a magnetic field above a critical level resulted in a recovery of the sample resistivity to the original value. The observed resistivity drop is identified with a superconducting transition. The critical parameters of the superconducting transition (T _c and H _c2) were determined at the drop to one half the normal resistivity level. Experimental dependences of the critical supercon-ducting-transition temperature T _c and of the second critical magnetic field H _c2 on the contents of lead (z) and indium (x) were measured. The data obtained confirm a strong localization of the In impurity states and are evidence of the extrinsic nature of superconductivity in the class of materials under study. It was established that as the Pb content in (Sn_1?z Pb_z)_1?x In_xTe increases, T _c and H _c2 decrease as the Fermi level E _F (fixed in the In impurity resonance band) leaves the Δ extremum and the superconductivity breaks down when E _F leaves the LΣ saddle point in the valence-band energy spectrum.     

Superconductivity in the Zr-D system under pressure

The superconducting transition temperature T _c of the ZrD_0.48 alloy is measured in the pressure range up to 41.5 GPa. The measurements are carried out in a high-pressure chamber with diamond anvils by the inductometric method. It is found that T _c(P) increases to 3.1 K at a pressure below 30 GPa, exhibits a sharp increase up to 8 K near 30 GPa, and then smoothly decreases to ～6.5 K at 41.5 GPa. A similar dependence T _c(P) is obtained for pure Zr. The similarity of the T _c(P) curves suggests that the dependence T _c(P) observed for ZrD_0.48 is due to the presence of ω-phase in this alloy at pressures P<30 GPa and the ω-β transition at P≈30 GPa, which leads to the establishment of new ratios between the phases in the Zr-D system. In the pressure range studied, no indications are observed for new superconducting phases similar to the phases of intermediate composition in the Ti-H(D) system, which are formed by the hydrogen transfer from tetrahedral to octahedral interstitials.     

Superconductivity and thermal properties of sulphur doped FeTe with effect of oxygen post annealing

Here, we report the synthesis and characterization of sulphur-substituted iron telluride i.e. FeTe_1?xS_x; (x = 0–30 %) system and study the impact of low temperature oxygen (O₂) annealing as well. Rietveld analysis of room temperature X-ray diffraction (XRD) patterns shows that all the compounds are crystallized in a tetragonal structure (space group P4/nmm) and no secondary phases are observed. Lattice constants are decreased with increasing S concentration. The parent compound of the system i.e. FeTe does not exhibit superconductivity but shows an anomaly in the resistivity measurement at around 78 K, which corresponds to a structural phase transition. Heat capacity C_p(T) measurement also confirms the structural phase transition of FeTe compound. Superconductivity appears by S substitution; the onset of superconducting transition temperature is about 8 K for FeTe_0.75S_0.25 sample. Thermoelectric power measurements S(T) also shows the superconducting transition at around 7 K for FeTe_0.75S_0.25 sample. The upper critical fields H_c2(10%), H_c2(50%) and H_c2(90%) are estimated to be 400, 650 and 900 kOe respectively at 0 K by applying Ginzburg Landau (GL) equation. Interestingly, superconducting volume fraction is increased with low temperature (200 °C) O₂ annealing at normal pressure. Detailed investigations related to structural (XRD), transport [S(T), R(T)H], magnetization (AC and DC susceptibility) and thermal [C_p(T)] measurements for FeTe_1?xS:O₂ system are presented and discussed.     

Temperature dependence of the lower critical field Hc1 in SmFeASO0.9F0.1 and Ba0.6K0.4Fe2As2 iron-arsenide superconductors

We report detailed measurements of the temperature dependence of the lower critical field H_c1 of the FeAs-based superconductor SmFeAsO_0.9F_0.1 (Sm-1111) and Ba_0.6K_{0. 4}Fe₂As₂ (BaK-122) by global and local magnetization measurements. It is found that the obtained H_c1 for both kinds of samples show a weak temperature dependence in low temperature region. We argue that this weak T-dependence of H_c1 of Sm-1111 does not indicate a conventional s-wave state, instead it satisfies a T² dependence up to a temperature of (0.5–0.6) T_c. In contrast, the data of H_c1(T) of BaK-122 superconductor clearly show a multigap behavior. Excellent fitting to the data can be reached with two s-wave superconducting gaps. Comparison of the absolute values of H_c1(0) between Sm-1111 and BaK-122 shows a relatively large superfluid density for the latter. This may indicate the distinction between the electron doped and hole-doped FeAs-based superconductors.     

Anomalous behavior of superconducting transition temperature of amorphous and crystalline Nb3Ge under pressure

The superconducting transition temperature (T_c) of amorphous Nb₃Ge was studied under both hydrostatic and quasihydrostatic pressure to 3.5 and 13 GPa, respectively. Whereas hydrostatic pressure causes T_c to initially decrease, T_c is found to increase under higher quasihydrostatic pressures. T_c(p) was also studied on an A-15 crystalline Nb₃Ge sample obtained from the amorphous sample by annealing.