Singular Temperature Dependence of the Equation of State of Superconductors with Spin-Orbit Interaction in Low-Temperature Region – II

We extend a previous study of the space and temperature dependence of the order parameter in an ultrathin superconducting film in a longitudinal magnetic field to the vicinity of the critical transition surface. In these films, the spin-orbit interaction can be sizeable. The calculations are restricted to the low-temperature regime. Special attention is given to the zero temperature limit where the state with the maximum value of the critical field is investigated for the given strength of the spin-orbit interaction.     

Impurity Bound States and Disorder-Induced Orbital and Magnetic Order in the s ± State of Fe-Based Superconductors

We study the presence of impurity bound states within a five-band Hubbard model relevant to iron-based superconductors. In agreement with earlier studies, we find that in the absence of Coulomb correlations there exists a range of repulsive impurity potentials where in-gap states are generated. In the presence of weak correlations, these states are generally pushed to the edges of the gap, whereas for larger correlations the on-site impurity potential induces a local magnetic region, which reintroduces the low-energy bound states into the gap.     

Finite-Element and Effective-Medium Calculations of the Electrical Behaviour Near the Vortex–Antivortex Binding Transition of Planar Superconductors with Critical Temperature Inhomogeneities

We present calculations of the electrical behaviour near the Berezinskii–Kosterlitz–Thouless (BKT) vortex–antivortex binding transition of planar type-II superconductors with critical temperatures varying in space according to a random Gaussian distribution. By using a finite-element mesh-circuit analysis we obtain as a function of temperature and intensity the spatial maps of local voltages. These calculations allow us to argument that in these inhomogeneous superconductors the occurrence of a global voltage-current curve V∝I ³ signals with good accuracy the average BKT temperature $\overline{T}_{\mathrm{BKT}}$ because of the appearance of percolation-like configurations when 50 % of the sample has experienced the vortex–antivortex binding transition. In addition, we also present an effective-medium calculation aimed to obtain the resistance V/I without any finite-element computation. It leads to results in agreement with the finite-element approach except in the close vicinity of $\overline{T}_{\mathrm{BKT}}$ . For instance, for the upper part of the transition both theoretical approaches coincide in all the ohmic region of temperatures, including also the average mean-field temperature $\overline{T}_{c0}$ .     

Influence of the Interaction Potential on the D↓1 Equation of State

The ground-state properties of spin-polarized deuterium with only one occupied nuclear spin state (D↓₁) at zero temperature are studied by means of the variational Monte Carlo (VMC) method. Influence of the backflow correlations on the ground-state energy of this Fermi system under investigation is also explored. The calculations have been performed using several interatomic potentials and the quantum phase of the D↓₁ system is discussed. Our results are compared with other previously reported variational results.     

Pseudo-gap and Vertex Correction of Electron–Phonon Interaction in High Transition-Temperature Superconductors

The strong-coupling Eliashberg theory plus vertex correction is used to calculate the maps of transition temperature (T _c) in parameter-space characterizing superconductivity. Based on these T _c maps, complex crossover behaviors are found when electron?Cphonon interaction increases from weak-coupling region to strong-coupling region. The doping-dependent T _c of cuprate superconductors and most importantly the emergence of pseudo-gap region can be explained as the effects of vertex correction.     

The Vortex State and Josephson Critical Current of a Diffusive SNS Junction

We study theoretically the electronic and transport properties of a diffusive superconductor-normal metal-superconductor (SNS) junction in the presence of a perpendicular magnetic field. We show that the field dependence of the critical current crosses over from the well-known Fraunhofer pattern in wide junctions to a monotonic decay when the width of the normal wire is smaller than the magnetic length $\xi_{\mathrm{H}}=\sqrt{\varPhi_{0}/H}$ , where H is the magnetic field and Φ ₀ the flux quantum. We demonstrate that this behavior is intimately related to the appearance of a linear array of vortices in the middle of the normal wire, the properties of which are very similar to those in the mixed state of a type II superconductor. This novel vortex structure is also manifested in a strong modulation of the local density of states along the transversal dimension, which can be measured with existing experimental techniques.     

Langevin Dynamics of Vortex Lines and Thermodynamic Equilibrium of Vortex Tangle

Langevin dynamics—stochastic motion of vortex filaments under action of random force is studied analytically and numerically. We introduce a Langevin-type equation of motion of the line with a stirring force satisfying the fluctuation-dissipation theorem. The respective Fokker-Planck equation for probability functional ?({s(ξ)}) in vortex loop configuration space is shown to have a solution of the form $\mathcal{P}(\{\mathbf{s}(\xi )\})=\mathcal{N}\exp (-H\{\mathbf{s}\}/T),$ where $\mathcal{N}$ is a normalizing factor and H{s} is energy of vortex line configurations. Numerical calculations are performed on base of the full Biot-Savart law for different intensities of the Langevin force. A new algorithm, which is based on consideration of crossing lines, is used for vortex reconnection procedure. After some transient period the vortex tangle develops into the stationary state characterizing by the developed fluctuations of various physical quantities, such as total length, energy etc. We tested this state to learn whether or not it the thermodynamic equilibrium is reached. With the use of a special treatment, so called method of weighted histograms, we process the distribution energy of the vortex system. The results obtained demonstrate that the thermodynamical equilibrium state is reached.     

Energy Absorption by a Single Abrikosov’s Vortex in NbTi and YBaCuO Superconductors

The calculations of the absorption energy and the Q-factor of vibration of a vortex line under the effect of an external uniform harmonic force have been presented. The line tension of the vortex line and its effective mass, Magnus force, pinning force, and viscosity force were included in the equation of the dynamics. Two peaks of absorption in accordance with two modes of the oscillation spectrum were observed. The first peak of absorption is a result of flux pinning, which coincides with the De Gennes–Matricon solution in some limits. The second (high-frequency) peak is a result of a novel mode related to the effective mass of a vortex line. Temperature dependencies of absorption peaks were calculated for the NbTi and YBaCuO superconductors for orientations H∥c and H⊥c. The resonant frequency of the resonance peaks of absorption decreases with an increase in temperature. Some other peculiarities of the oscillating dynamics of the vortex line were discussed.     

Influence of Diffusion-Annealing Temperature on?the?Physico-Mechanical Properties of Au-doped Bi-2223 Superconductors

In order to investigate the influence of Au doping and diffusion-annealing temperature on the mechanical and superconducting properties of Bi-2223, Bi_1.8Pb_0.35Sr_1.9Ca_2.1Cu₃O _y superconductors were prepared by standard solid-state reaction methods. Doping of Bi-2223 was carried out by means of gold diffusion during sintering from an evaporated gold film on pellets. The investigation consisted of scanning electron microscopy, dc resistivity and hardness measurements. Electrical-resistivity measurements indicated that the room-temperature resistivity value decreased with decreasing diffusion-annealing temperature from 830 to 500?°C and these samples (G830, G800, G750, G700, G600 and G500) show the resistive behavior above the onset critical transition temperature with the zero-resistivity transition temperatures of 104 K, 80 K, 98 K, 95 K, 102 K and 103 K, respectively. To investigate mechanical properties of the samples, we have measured the diagonal length as a function of test load in the range of 0.245?C2.940 N. Mechanical properties (microhardness, Young??s modulus, yield strength and fracture toughness) of the samples are found to be load and diffusion-annealing temperature dependent. In addition, we have calculated the load independent hardness, Young??s modulus, yield strength, and fracture toughness of the samples. The possible reasons for the observed changes in superconducting and mechanical properties due to Au diffusion and diffusion-annealing temperature were discussed.     

Vortex rings and the superfluid λ-transition

A vortex-ring formulation of the₄He -transition provides a clear physical picture of the superfluid transition. The thermally excited rings are dipoles which orient in an applied flow field, and their net backflow cancels part of the applied flow, reducing the superfluid density. At the -point rings of infinite size drive the superfluid density to zero as a power law of the reduced temperature, with an exponent of 0.672. By fitting to the experimental superfluid amplitude the core energy of the smallest rings is found to be 6.1K at T and their diameter is 2.3 Å. It is proposed that these can be identified as the roton excitations of the Landau model. The vortex theory also yields new insights into topics such as boundary effects at a wall, finite-size effects, and the dynamics of the transition.     

Transverse Magnetic Field Distribution in the Vortex State of Noncentrosymmetric Superconductor with O Symmetry

We investigate the magnetic field distribution inside a Type II superconductor which has point group symmetry O such as Li₂Pt₃B. The absence of inversion symmetry as a departure from perfect cubic group O _h causes a magnetization collinear with the phase gradient associated with the order parameter, and a component of current collinear with the local magnetic field. In the vortex state, we predict, by solving the Maxwell equation, a local magnetic field transverse to the vortex lines. The probability distribution of this transverse field is also obtained for the prospective muon-spin-rotation measurements.     

Paramagnetic Meissner Transitions in Pb Films and the?Vortex Compression Model

The magnetic flux exclusion known as Meissner effect resulting from a perfectly diamagnetic susceptibility ??=?1 is one of the defining features of type-I superconductivity. More recently some materials have been reported to undergo a transition to a paramagnetic state as they are cooled through their superconducting transition temperature (Svedlindh et?al. in Physica?C, pp.?162?C164, 1989; Li in Phys. Rep. 376:133?C223, 2003). This is known as the Paramagnetic Meissner Effect (PME). Here we report the observation of a transition from the Meissner state to a PME state in thin Pb films. We go on to show how simple modifications to the vortex compression model developed by Koshelev and Larkin (Phys. Rev.?B 52:13559?C13562, 1995) yield magnetization-vs-temperature curves in good qualitative agreement with the magnetization curves reported in the experiment.     

Effect of f Electron Excitations in Heavy Fermion and?Unconventional Superconductors

The low energy crystalline electric field (CEF) excitations of f electrons can play a twofold role in heavy fermion and unconventional superconductors. They may act as a glue for the formation of Cooper pairs and simultaneously as a probe to investigate the symmetry of the order parameter. The former has been found in the skutterudite heavy fermion compound PrOs₄Sb₁₂, where a singlet?Ctriplet CEF excitation ?? contributes to the pair formation and enhances T _c with respect to L aOs₄Sb₁₂. In substituted P r(Os_1?x Ru _x )₄Sb₁₂,?a continuous increase of ?? leads to a crossing with the local Pr rattling phonon mode and vibronic mode formation caused by magnetoelastic interaction. Furthermore, the T _c enhancement turns into a reduction. It is proposed that this signifies a crossover from mainly pair-forming aspherical Coulomb to pair-breaking dipolar exchange scattering with increasing Ru content. In the Ce-based Fe pnictides CEF transitions show anomalous temperature dependence of the line width due to a pronounced feedback effect, which gives direct evidence for the unconventional 3d superconductivity.     

On the Role of Polarization of Oxygen Ions and Mechanism of Superconductivity in Cuprate Superconductors

A new approach to the valence electron state in cuprate superconductors leading to a new mechanism of high-temperature superconductivity is considered. The approach takes into account specific features of the outer electron shells of the compound-forming elements and polarization of anions by cations. The polarization of anions by copper cations results in sharing of a valence electron by every two neighboring ions, which gives rise to a highly correlated state of the valence electrons and formation of a localized spin-ordered electron lattice responsible for dielectric and magnetic properties of an undoped material. It is also shown that asymmetric polarization of an anion by cations with inert gas shells transforms an unshared anion electron pair into a superconducting one by exciting it into a hybrid state. The superconducting state of the material itself results from formation and delocalization of one-dimensional Wigner crystals of the excited pairs.     

Study and Application of Controlled Vortex Dynamics in?Patterned YBCO Films

Quantitative imaging of the local magnetic field and of current density distribution in superconductors (with microscopic resolution over macroscopic length scales) is achieved by means of the Magneto-Optical Imaging technique with an indicator film. We exploit this technique to study the vortex arrangement and the corresponding supercurrent distribution in high temperature superconducting YBa₂Cu₃O_7?x films. Several patterned superconducting films were studied, either non-simply connected structures, which imply macroscopic flux quantization, and superconductors whose local properties were tailored by means of confined heavy-ion irradiation. Moreover, by means of electrical transport measurements coupled with the real-time imaging of the magnetic pattern, it is directly shown how the local current distribution in patterned superconductors is affected by the electrical transport both in the Meissner and in the vortex regimes. The relevance of a controlled and localized dissipation induced by the confined vortex motion in tailored superconducting films is demonstrated for direct applications of this phenomenology to superconducting devices, such as magnetic field and photon detectors.     

The Decay of a Quantized Vortex Ring and the Influence of Tracer Particles

We capture the decay of a quantized vortex ring in superfluid helium-4 by imaging particles trapped on the vortex core. The ring shrinks in time, providing direct evidence for the dissipation of energy in the superfluid. The ring with trapped particles collapses more slowly than predicted by an available theory, but the collapse rate can be predicted correctly if the trapping of the particles on the core is taken into account. We theoretically explore the conditions under which particles may be considered passive tracers of quantized vortices and estimate, in particular, that their dynamics on the large-scale is largely unaffected by the burden of trapped particles if the latter are spaced by more than ten particle diameters along the vortex core, at temperatures between 1.5 K and 2.1 K.