Peculiarities of performance of the spin valve for the superconducting current

The spin valve effect for the superconducting current based on the superconductor/ferromagnet proximity effect has been studied for a CoO _x /Fe1/Cu/Fe2/Cu/Pb multilayer. The magnitude of the effect ΔT _c = T _c ^AP ? T _c ^P , where T _c ^P and T _c ^AP are the superconducting transition temperatures for the parallel (P) and antiparallel (AP) orientation of magnetizations, respectively, has been measured for different thicknesses of the Fe1 layer d _Fe1. The obtained dependence of the effect on d _Fe1 reveals that ΔT _c can be increased in comparison with the case of a half-infinite Fe1 layer considered by the previous theory. A maximum of the spin valve effect occurs at d _Fe1 ～ d _Fe2. At the optimal value of d _Fe1 almost full switching from the normal to the superconducting state when changing the mutual orientation of magnetizations of the iron layers Fe1 and Fe2 from P to AP is demonstrated.     

μ+SR studies on superconducting YBa2(Cu1−xFex)3Oy system

Spin-glass like magnetic ordering of iron moments was observed in both orthorhombic and tetragonal YBa₂(Cu_1−xFe_x)₃O_y (x=0.08) by μ⁺SR measurements. In a “Tetra” sample, all the muons sense the superconducting transition at 60 K and magnetic ordering at around 15 K, while in an “Ortho” sample they reveal that two magnetically different parts exist in the sample: about 40% of the sample is superconducting withT _c ≈90K and the remaining part is magnetic withT _M≈33K. These phenomena can be explained in terms of clustering of the Fe atoms in the “Ortho” sample.     

Observation of the “Inverse” spin valve effect in a Ni/V/Ni trilayer system

An experimental study of magnetic and superconducting properties of a trilayer Ni/V/Ni thin film system grown on single-crystalline MgO(001) substrate is reported. The field dependence of the superconducting transition temperature T _c for samples comprising Ni layers with similar values of the coercive field H _c reveals no anomalies. However, in samples with different thicknesses of the nickel layers the difference in H _c amounts up to ΔH _c ∼ 1.8 kOe, thus enabling to manipulate the relative orientations of the layers’ magnetization by an external magnetic field. Surprisingly, for these samples the T _c for the parallel orientation of the magnetizations of the Ni layers is higher, in a certain magnetic field range, than for the antiparallel one, at odds with theoretical predictions. Possible reasons of this contradiction are discussed.     

Triplet proximity effect in FSF trilayers

We study the critical temperature T _c of FSF trilayers (F is a ferromagnet, S is a singlet superconductor), where the triplet superconducting component is generated at noncollinear magnetizations of the F layers. An exact numerical method is employed to calculate T _c as a function of the trilayer parameters, in particular, mutual orientation of magnetizations. Analytically, we consider limiting cases. Our results determine the conditions necessary for the existence of recently investigated odd triplet superconductivity in SF multilayers.     

Features of the melting dynamics of a vortex lattice in a high-T c superconductor in the presence of pinning centers

The phase transition “triangular lattice-vortex liquid” in layered high-T _c superconductors in the presence of pinning centers is studied. A two-dimensional system of vortices simulating the superconducting layers in a high-T _c Shubnikov phase is calculated by the Monte Carlo method. It was found that in the presence of defects the melting of the vortex lattice proceeds in two stages: First, the ideal triangular lattice transforms at low temperature (≃3 K)into islands which are pinned to the pinning centers and rotate around them and then, at a higher temperature (≃8 K for T _c 584 K), the boundaries of the “islands” become smeared and the system transforms into a vortex liquid. As the pinning force increases, the temperatures of both phase transitions shift: The temperature of the point “triangular lattice-rotating lattice” decreases slightly (to ≃2 K)and the temperature of the phase transition “rotating lattice-vortex liquid” increases substantially (≃70 K). Pis’ma Zh. éksp. Teor. Fiz. 66, No. 4, 269–274 (25 August 1997)     

Anisotropic distributions of electrical currents in high-<Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> grain-boundary junctions

We developed a self-consistent method for the calculation of spatial current distributions in high-T _c grain-boundary junctions. It is found that crystallographic anisotropy of high-T _c superconducting electrodes results in the effects, which previously were not taken into account for interpretations of experimental data. Among them is a significant redistribution of electrical currents in superconducting electrodes in the vicinity of a grain boundary. In particular in the case of [100]-tilt bicrystal junctions, this current redistribution results in a substantial focusing to the top or bottom part of a thickness of the grain boundary, depending on “roof”- or “valley”-type of the grain boundary. This redistribution is accompanied by generation of vortex currents around the grain boundary, which leads to self-biasing of grain-boundary junctions by magnetic field nucleated by these vortex currents. It is shown that twinning or variation of geometrical shape of the high-T _c electrode may also result in intensive redistribution of electrical currents and nucleation of local magnetic fields inside a high-T _c superconducting electrodes.     

Josephson effect in superconducting constrictions with hybrid SF electrodes: Peculiar properties determined by the misorientation of magnetizations

Josephson current in SFcFS junctions with arbitrary transparency of the constriction (c) is investigated. The emphasis is on the analysis of the supercurrent dependencies on the misorientation angle θ between the in-plane magnetizations of diffusive ferromagnetic layers (F). It is found that the current-phase relation I(φ) may be radically modified with the θ variation: the harmonic I ₁ sin φ vanishes for a definite value of θ provided that, for an identical orientation of the magnetizations (θ = 0), the junction is in the “π” state. The Josephson current may exhibit a nonmonotonic dependence on the misorientation angle both for realization of the “0” and “π” state at θ = 0. We also analyze the effect of the exchange field induced enhancement of the critical current, which may occur in a definite range of θ. The text was submitted by the author in English.     

Disordering effects in superconductors with anisotropic pairing: From Cooper pairs to compact bosons

In the weak-coupling BCS-theory approximation, normal impurities do not influence the superconducting transition temperature T _c in the case of isotropic s pairing. In the case of d pairing they result in a rapid destruction of the superconducting state. This is at variance with many experiments on the disordering of high-T _c superconductors, assuming that d pairing is realized in them. As the interelectronic attraction in a Cooper pair increases, the system transforms continuously from a BCS-type superconductor with “loose” pairs to a picture of superconductivity of “compact,” strongly coupled bosons. Near such a transition substantial deviations can be expected from the universal disorder dependence of T _c , as determined by the Abrikosov-Gor’kov equation, and T _c becomes more stable against disordering. Since high-T _c super-conducting systems fall into the transitional region from BCS-type pairs to compact bosons, these results can explain their relative stability against disordering. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 3, 258–262 (10 February 1997)     

An “EPR” study of YBa2Cu3O7 and related high-temperature superconductors

Besides ag ≈ 2 signal which disappears on cooling to the superconductingT _c , YBa₂Cu₃O₇ and related oxides show a near-zero-field signal in the superconducting state with certain unusual features attributable to a “superconducting glassy state”. Contribution No. 443 from Solid State and Structural Chemistry Unit.     

A simple theory of condensation

A simple assumption of the emergence in gas of small atomic clusters consisting of c particles each leads to a phase separation (first-order transition). It reveals itself by the emergence of a “forbidden” density range starting at a certain temperature. Defining this latter value as the critical temperature predicts the existence of an interval with the anomalous heat capacity behavior c _p ∝ ΔT ^−1/c . The value c = 13 suggested in the literature yields the heat capacity exponent α = 0.077.     

Segregation of impurities and vacancies on phase and antiphase boundaries in alloys

The equilibrium distribution of low-concentration impurities or vacancies is investigated in the region of a coherent phase boundary or antiphase boundary in a binary alloy. A general expression for the free energy of an inhomogeneous multicomponent alloy, which generalizes the expression previously derived for a binary alloy, is presented. Explicit formulas for the impurity concentration profile c _im(x) in terms of the distribution of the principal components of the alloy near a boundary are obtained from this expression in the mean-field and pair-cluster approximations. The shape of this profile is determined by a “preference potential” P, which characterizes the attraction of an impurity to one of the alloy components, as well as by the temperature T and the phase transition temperature T c. At small values of P/T impurities segregate on a phase boundary, and the degree of this segregation, i.e, the height of the maximum of c _im(x), in the region of the boundary increases exponentially as the ratio T _c/T increases. For P ≠ 0 the c _im(x) profile near a phase boundary is asymmetric, and as P/T increases, it takes on the form of a “worn step.” The maximum on the c _im(x) curve then decreases, and at a certain |P|≳T _c it vanishes. Segregation on an antiphase boundary is investigated in the case of CuZn ordering in a bcc alloy. The form of c _im(x) near an antiphase boundary depends significantly both on the form of the potential P and on the stoichiometry of the alloy. At small P impurities segregate on an antiphase boundary, and at fairly large P “antisegregation,” i.e., a decrease in the impurity concentration on the antiphase boundary in comparison with the value within the antiphase domains, is also possible. Zh. éksp. Teor. Fiz. 112, 714–728 (August 1997)     

Dependence of the critical temperature of high-temperature cuprate superconductors on hoppings and spin correlations between CuO<Subscript>2</Subscript> planes

The influence of interlayer hoppings on the superconducting transition temperature (T _c) in bilayer cuprates has been studied. The parameter of hopping between layers is expressed as t _⊥(k) = t _⊥(cos(k _x ) − cos(k _y ))² and treated as a small perturbation for the states of two CuO₂ planes described by the t-t′-t″-J* model. In the generalized mean field approximation for d_{x² - y²}{d_{{x^2} - {y^2}}} symmetry of the superconducting gap, neither the interlayer hopping or exchange interaction, nor the pair hopping between CuO₂ layers provides an additional mechanism of Cooper pair formation or an increase in T _c. In the concentration dependence of T _c, the bilayer splitting of the upper Hubbard band of quasi-holes is manifested as two peaks with temperatures slightly lower than the maximum T _c for a single-layer cuprate. Interlayer antiferromagnetic spin correlations suppress bilayer splitting.     

MgB2 thick films deposited on stainless steel substrate with Tc higher than 39K

Thick MgB₂ (magnesium diborate) films, ∼10 μm, with T _c (onset) = 39.4 K and T _c (zero) = 39.2 K have been successfully grown on a stainless steel substrate using a technique called hybrid physical-chemical deposition (HPCVD). The deposition rate is high, ∼6.7 nm/s. The X-ray diffraction (XRD) indicates that it is highly (101) and c-axis oriented. The scanning electron microscope (SEM) images demonstrate that the film grown is in “island-mode”. The uniform superconducting phase in the film is shown by the M-T measurement.     

Intrinsic Josephson junctions in Bi2Sr2CaCu2O8+δ single crystals

We report on thec-axis superconducting energy gap parameter Δ _c (T) of intrinsic Josephson tunnel junctions inBi ₂ Sr ₂ CaCu ₂ O _8+δ (Bi2212) single crystals. Δ _c (4.2K)≈10−13 meV, which is approximately a factor of two smaller than reported in the majority of tunneling experiments. Δ _c (T) deviates strongly from the BCS temperature dependence. These observations may be explained by a multilayer model of Bi2212 which assumes that theBi−O layers are superconducting due to the proximity effects. The Josephson tunneling then takes place between adjacentBi−O layers while there is a strong proximity coupling betweenBi−O andCu−O layers. The work is supported by Swedish Supercon-ductivity Consortium and NUTEK, and, in part, by Russian Foundation for Basic Research, grant #95-02-04307     

Rotating superconductors: Ginzburg-Landau equations

Superconductors put into rotation develope a spontaneous internal magnetic field (the “London field”). In this paper Ginzburg Landau equations for order parameter, field, and current distributions for superconductors in rotation are derived. Two simple examples are discussed: the massive cylinder and the “Little and Parks geometry”: a thin film of superconducting material deposited on a cylinder of normal material. A dependence of T _c on rotational frequency is predicted. The magnitude of the effect is estimated and should be observable. Received 28 May 2001     

Critical currents in polycrystalline thin films of high-T c superconductors

Summary It is shown that the behaviour of the temperature dependence of the critical current in polycrystalline thin films of high-T _c superconductors depends crucially on the assumption made concerning the nature of the intergranular material. The usual assumption of a superconductor-insulator-superconductor (=SIS) ?sandwich? between each grain leads to a crossover fromI _c∼(1−T/T _c) toI _c∼(1−T/T _c)^3/2, for temperatures nearT _c (whereI _c is the critical current,T the absolute temperature, andT _c the superconducting transition temperature). Instead, for a superconductor-normal metal-superconductor (=SNS) sandwich the dependenceI _c∼(1−T/T _c)² is found for all temperatures. Consideration is given to the effect of self-magnetic field on the analysis. The comparison between expressions for continuous and granular systems is extended. Due to the relevance of its scientific content, this paper has been given priority by the Journal Direction.     

Synthesis,structural and physical properties of <Emphasis Type="Italic">δ</Emphasis>’-FeSe<Subscript>1</Subscript><Subscript><Emphasis Type="Italic">-x</Emphasis></Subscript>

We report on synthesis, structural characterization, resistivity, magnetic and thermal expansion measurements on the as yet unexplored δ’-phase of FeSe_{1 -x} , here synthesized under ambient- (AP) and high-pressure (HP) conditions. We show that in contrast to β-FeSe_{1 -x} , monophasic superconducting δ’-FeSe_{1 -x} can be obtained in off-stoichiometric samples with excess Fe atoms preferentially residing in the van der Waals gap between the FeSe layers. The AP δ’-FeSe_{1 -x} sample studied here (T _c @\simeq 8.5 K) possesses an unprecedented residual resistivity ratio RRR @\simeq 16. Thermal expansion data reveal a small feature around ~90 K, which resembles the anomaly observed at the structural and magnetic transitions for other Fe-based superconductors, suggesting that some kind of “magnetic state” is formed also in FeSe. For HP samples (RRR @\simeq 3), the disorder within the FeSe layers is enhanced through the introduction of vacancies, the saturated magnetic moment of Fe is reduced and only spurious superconductivity is observed.     

Correlation between the electric-field effect and weak-link type in YBa2Cu3−x Oy and YBa2Cu3−x Oy /Agx high-T c superconducting ceramics

An experimental study is reported of the effect of an electric field E⩽120 MV/m and of temperature T on the critical current I _c and I-V characteristics of yttrium-based high-T _c superconducting ceramics. The materials studied were copper-deficient ceramics, YBa₂Cu_3−x O_y (D samples), and YBa₂Cu_3−x O_y/Ag_x ceramics [S samples with silver present in amounts equal to the copper deficiency (0⩽x⩽0.4)]. It has been established that in D samples at 77 K, the electric field increases I _c and reduces substantially R for I>I _c, whereas in S samples no field effect is observed. Measurements of the I _c(T) dependence near the critical temperature showed that they can be described for all samples by a relation of the type I _c =const(1−T/T _c )^α, where α≈1 for the D samples, and α≈2 for the S samples. The results obtained suggest that the electric-field effect correlates with the existence in the ceramic of SIS-type weak links at grain boundaries. Fiz. Tverd. Tela (St. Petersburg) 40, 1195–1198 (July 1998)     

Temperature of the superconducting transition in the TiV alloy at pressures up to 60.8 GPa

The dependence of the temperature of the transition to the superconducting state T _c on the pressure up to 60.8 GPa is measured for the TiV alloy. The dependence T _c (P) is increasing except for an anomaly in the form of a local minimum near P = 10 GPa. At the maximum pressure of 60.8 GPa, the superconducting transition temperature T _c reaches 18.2 K. The obtained curve T _c (P) is compared with the known dependences for pure vanadium, for which T _c (P) increases to 17.2 K at P = 120 GPa, and for pure niobium and the ZrNb alloy, for which the dependences T _c (P) also have anomalies in the form of local maxima at pressures of 5–10 GPa.     

Spectroscopy of quasiparticle excitations of superconducting bismuth cuprate at high pressures

We study the energy spectrum of Bi2223 (Bi_1.6Pb_0.4Sr_1.8Ca_2.2Cu₃O_x) at high hydrostatic pressures by Andreev-and tunneling-spectroscopy methods. We determine the gap anisotropy in the basal ab plane and find the following values for the parameters Δ(ϕ): Δ_max=42 mV, and Δ_min=19.5 mV (T _c =110 K and dT _c /dP=0.16 K/kbar). We detect an increase in the ratio R=2Δ_max/kT _c with pressure P; for Bi2223 cuprate, dR/dP≈0.017 kbar⁻¹. In the phonon-frequency region we detect a “softening,” due to pressure, of the high-frequency part of the phonon spectrum corresponding to “breathing” modes of oxygen, as well as other optical modes of Cu-O. The characteristic frequencies of the spectrum for ℏΩ>60 mV are found to decrease, with increasing pressure, at a rate d ln(ℏΩ)/dP≈−6.5±0.5×10⁻³ kbar⁻¹. This result explains the observed increase in the ratio 2Δ/kT _c (P) in the model of strong electron-phonon interaction. Zh. éksp. Teor. Fiz. 113, 1397–1410 (April 1998)