Critical Temperature Superconductors Research Articles

Low-temperature proximity effect in clean metals with repulsive electron-electron interaction

Theories of proximity effect in layered superconductor--normal-metal (SN) structures usually deal with a hypothetic normal metal with no direct repulsive interaction between electrons and with finite temperatures often close to the superconductor critical temperature. We present an asymptotic solution of the Gor'kov equations in the opposite low-temperature limit for a clean normal metal with a repulsive interaction between electrons. The order parameter in the metal exhibits a power-law decay, $\ensuremath{\Delta}(x)\ensuremath{\propto}\ensuremath{\xi}/x$, as a function of the distance from the SN boundary, $x$, with a proximity length $\ensuremath{\xi}$ strongly depending on the repulsive interaction.

Superconducting niobium nitride films deposited by unbalanced magnetron sputtering

Niobium nitride (NbN) thin films were deposited under different configurations of the magnetic field using a magnetron sputtering system. The magnetic field configuration varied from balanced to unbalanced leading to different growth conditions and film properties. The aim of the paper was to identify correlations between deposition conditions, film properties and the electrical properties, specially the superconductive critical temperature ( T C). The results suggested that there is a critical deposition condition, having an optimum ion–atom arrival ratio that promotes a well ordered and textured nanocrystalline structure (cubic phase) with the minimum residual stress and only under this condition a high critical temperature (16K) was obtained. Lower T C values around 12K were obtained for the NbN samples having a lower degree of structural perfection and texture, and a larger fraction of intergranular voids. On the other hand, analysis of valence-band spectra showed that the contribution of the Nb 4d states remained essentially constant while the higher T C was correlated to a higher contribution of the N 2p states.

Phonon spectra of selenium and gallium dispersed in a porous glass

The Mossbauer investigation has revealed that the transition of selenium from the crystalline state to the vitreous state is accompanied by a shift of the phonon spectrum of 119Sn impurity atoms toward the low-frequency range. A similar shift of the phonon spectrum is observed for vitreous selenium dispersed in a porous glass with a pore diameter of ∼70 A. The “hardening” of the phonon spectrum is observed for 119Sn impurity atoms in gallium dispersed in the porous glass. The possible factors responsible for the increase in the critical temperature of superconductivity of gallium in the porous glass are discussed.

A model for the doping dependence of the superconducting transition temperature in cobaltates

Some properties of hydrated cobalt oxides Na x CoO 2⋅ yH 2O in the superconductor phase are studied using an extended Hubbard model on a triangular lattice. We derive the superconductive ordering temperatures for singlet and triplet symmetries by means of the Green’s functions of the system, calculated within a combined mean field and Hubbard I type approximation. It is found that both symmetries are possible. The coexistence of d + i d ′ and p + i p ′ pairing, in a certain doping region, is observed, but the s-extended pairing seems to be the more stable. The critical temperatures for superconductivity are calculated for both symmetries and compared to experimental results.

Investigation of non-magnetic impurity doping effect on theMgB2 superconductor critical temperature

We have investigated the effect of finite non-magnetic impurity doping concentration on the critical temperature(Tc) ofthe MgB2 superconductor by using the coherent potential approximation. We found, by choosing the chemical potentialμ = −0.47t and scatteringstrength δ = 4.5t, that Tc is reduced with impurity concentration similarly to the measured experimentalresults.

Borides of Ag and Au prepared by magnetron sputtering

Following the discovery of MgB 2 that has a superconductive critical temperature ( T c) of ∼39 K much research was initiated in the search for materials with higher T c. Theoretical studies predicted that diborides of metals having the AlB 2 type structure, among them AgB 2 and AuB 2, may be superconductive at values of T c even higher than 39 K. In our attempts to prepare AgB 2 and AuB 2 using films of the Ag–B and Au–B systems, no superconductivity was observed. Equilibrium phase diagrams of these systems do not support the possibility of forming AgB 2 or AuB 2 compounds, despite a report which indicates their existence. Films of the Ag–B and Au–B systems were obtained by magnetron cosputtering from two separate targets, that of B and Ag (or Au) on Si(1 0 0) substrates in Ar ambient. The specimens were subjected to X-ray diffraction (XRD), secondary electron microscopy (SEM), X-ray photo spectroscopy (XPS), Auger analysis and optical microscope (OM) investigations. No superconductivity was observed in the range 10–200 K on the basis of resistivity measurements. The films were not homogeneous, but rather the alloys had decomposed into two separate phases. One of the phases was in the form of dispersed particles. An XRD peak at 28° was observed which supposedly is characteristic of a noble metal boride. The possibility that the XRD peaks might belong to a boric acid phase also exists. The boric acid phase had the same diffraction peak as was observed by XRD in our films.

Superconductivity and structural characteristics of ceramic Pr2Ba4Cu7O15−δ prepared by ambient pressure synthesis using citrate pyrolysis method

Sintered material of Pr2Ba4Cu7O15−δ (Pr247) was synthesized at ambient pressure condition by using citrate pyrolysis precursor method, and the superconductivity caused by oxygen reduction treatment has been confirmingly observed and examined.By resistivity and magnetization experiments, it has been found that the critical temperature Tc of the reduced samples could reach about 10K higher than previously reported data for pure Pr247. Besides, from TEM observations, the present material was found to be heterogeneous system containing Pr247, Pr124 and novel stacking structure phases rich in CuO single chains. These results have suggested that oxygen–reduction at such region rich in the CuO single chains may affect the superconductivity of the adjacent crystal region containing CuO double chains, and might enhance the superconductive critical temperature.

Three-dimensional and two-dimensional high field vortex fluctuations in deoxygenated YBa 2Cu 3O 7− x depending on the vicinity of Tc( H)

We analyzed reversible magnetization data, M versus T curves, of three single crystals of YBa 2Cu 3O 7− x (Y123), with superconducting transition temperatures T c = 62.5 ( x = 0.35), 52 ( x = 0.5), and 41 K ( x = 0.6). M versus T curves of each sample exhibited a field independent crossing point, M( T ∗), occurring close to the superconductor critical temperature. These crossing points were shown to be due to fluctuations of vortices. Besides the reversible data of each sample were shown to obey a two-dimensional diamagnetic lowest-Landau-level (LLL) fluctuation theory, it is shown here that the data, within a temperature region where the crossing points occur for two samples (62.5 K and 52 K), are also explained by a three-dimensional version of this fluctuation theory. Since the crossing points for these two samples occur close to T c, these are interpreted as been due to three-dimensional vortex fluctuations instead two-dimensional ones. An expression for the field independent magnetization, M( T ∗), which is expected to occur at the crossing point of the various M versus T curves, is obtained for the case of three-dimensional vortex fluctuations, and compared to the experimental values of M( T ∗). This comparison produced consistent values for the coherence length along the c-axis of the samples with T c = 62.5 and 52 K, solving an inconsistent result previously published, when experimental values of M( T ∗) were compared with an expression obtained from two-dimensional vortex-fluctuations. The results of the present work show that, despite the fact that two-dimensional LLL fluctuations scaling is obeyed in a much wider temperature range for two studied samples ( T c = 52 ( x = 0.5), and 62.5 K ( x = 0.35)) when compared to the 3D-LLL scaling form, these systems behave as three-dimensional for temperatures close to T c( H).

Guidelines for LTS magnet design based on transient stability

Stabilities of low critical temperature superconducting (LTS) magnets and their designs are studied and discussed. There are two contradictory necessities; those are low cost and high performance, in the other words, high magnetic field and large current density. Especially, the maximum magnetic fields of the latest high performance Nb 3Sn magnets are around 20 T. Mentioned necessities result in the small stability margins. Needless to say, the superconducting magnet must produce its nominal field reliably. Therefore, maintaining adequate stability margin, the magnet design to draw out the high potential of the superconductor is required. The transient stability of the superconducting magnet is determined by the relationship between mechanical disturbance energy and stability margin. The minimum quench energy (MQE) is one of the index of stability margin and it is defined as the minimum energy to trigger quenching of a superconductor. MQE should be beyond any possible disturbance energy during the operation. It is difficult to identify the mechanical disturbance energy quantitatively. On the contrary, MQE had been evaluated precisely by means of our developed resistive carbon paste heater (CPH). At the same time, we can predict MQE by numerical simulations. Because the magnet comes to quench if the mechanical disturbance exceeds the MQE, the disturbance energies are suspected to be equivalent to MQEs during the magnet-training. When we achieved somewhat larger MQE, we may exclude numbers of training quenches. In this paper, we discuss the guidelines of LTS magnet design from the standpoint of MQE. We represent some case studies for various superconducting magnets and/or some different winding methods.

Ordering of a frustrated Josephson-coupled network system of sub-micron-scale grains

Ordering characteristics of a superconductive ceramic of YBa 2Cu 4O 8 are newly examined experimentally by analyzing the nonlinear magnetic responses, in order to discuss intergrain ordering of frustrated Josephson-coupled network system of sub-micron-scale grains below the superconductive critical temperature. The derived nonlinear magnetic susceptibility χ 2 shows a symmetric negative divergence at the temperature corresponding to the intergrain ordering point T c2. The result indicates that the ordering of a 3D XY-type spin glass is associated with the same singularity of nonlinear chiral susceptibility at the temperature.

Bulk metallic superconductive La 60Cu 20Ni 10Al 10 glass

A bulk metallic superconductive La 60Cu 20Ni 10Al 10 glass with highly glass forming ability and large supercooled liquid temperature region is developed. The superconductive critical temperature determined by the electrical resistance and the specific heat measurements is 2.5 K. The obtained electron–phonon interaction parameter demonstrates that the bulk metallic glass is an intermediate coupling superconductor. The new nonmagnetic rare-earth based glass offers an ideal material for studying the superconductivity and other low temperature properties in the metallic glasses.

Pressure dependence of superconducting and magnetic critical temperatures in the ruthenocuprates

We study the dependence on filling and pressure of the superconducting and ferromagnetic critical temperatures of the ruthenocuprates, within the two-band model. At zero pressure, we find separate regions of coexistence of superconductivity and ferromagnetism as a function of filling, with contiguous regions merging together as pressure increases. As a function of pressure, a stronger enhancement of the magnetic phase results in a reduced pressure effect on the superconducting critical temperature. Comparison with recent experiments on the determination of the critical temperatures as a function of the pressure is also discussed.

Phenomenological theory of superconductivity near domain walls in ferromagnets

We develop a phenomenological model of superconductivity near a domain wall in a ferromagnet. In addition to the electromagnetic interaction of the order parameter with the ferromagnetic magnetization, we take into account the possibility of a local enhancement or suppression of superconducting pairing in the vicinity of the wall and also a nonperfect transparency of the wall to electrons. It is found that the critical temperature of superconductivity near the domain wall might be substantially higher than in the bulk.

Current–voltage characteristics of aAg:Y Ba2Cu3O7−δ/Nb-dopedSrTiO3 bilayer structure

A Ag:Y Ba2Cu3O7−δ/SrNb0.01Ti0.99O3 bilayer structure was fabricated by pulsed-laser deposition. Current–voltage(I–V) dependence measurements of the bilayer were carried out in the temperature range5–250 K with variation of magnetic field from 0 to 50 kOe. Schottky junction behaviour wasobserved at temperatures higher than the critical temperature of superconductivity(Tc) due to the n-typesemiconductor SrNb0.01Ti0.99O3 and themetallic conductivity of Y Ba2Cu3O7−δ.At temperatures lower than Tc, the bilayer becomes a degenerate heterostructure p–n junction. TheI–V characteristic shows a sequence of current steps at forward voltage andan oscillation character at backward voltage. Energy diagrams of theAg:Y Ba2Cu3O7−δ/SrNb0.01Ti0.99O3 bilayer structure have been proposed to explain the peculiarI–V characteristics.

Effects of Nonmagnetic Impurities on Spin-Fluctuation Induced Superconductivity

We study the effects of nonmagnetic impurities on the phase diagram of a system of interacting electrons with a flat Fermi surface. The one-loop Wilsonian renormalization group flow of the angle dependent diffusion function D(theta;(1),theta;(2),theta;(3)) and interaction U(theta;(1),theta;(2),theta;(3)) determines the critical temperature and the nature of the low temperature state. As the imperfect nesting increases, the critical temperature decreases, and the low temperature phase changes from the spin-density wave (SDW) to the d-wave superconductivity (dSC) and finally, for bad nesting, to the charge-localized state. Both SDW and dSC phases are affected by disorder. The pair breaking depends on the imperfect nesting and is the most efficient when the critical temperature for superconductivity is maximal.

Annealing melt-processed Sm–Ba–Cu–O bulk superconductors under 10 atm oxygen

Annealing melt-processed Sm–Ba–Cu–O under high pressure oxygen was studied because a long time is necessary to oxygenate a large sample under atmospheric pressure while increasing the size of bulk superconductors is important for its performance. An increase in the critical temperature of superconductivity ( T c) was observed with increasing the oxygen pressure ( pO 2) up to 10.5 atm when the sample was annealed at 400 or 500 °C, while T c decreased with increasing pO 2 when annealed at 300 °C. The estimated diffusion length of oxygen at 400 °C increased by a factor of 2.5 with increasing pO 2 from 1 to 10.5 atm. It is demonstrated that the time necessary to fully oxygenate a 30 mm diameter sample can be reduced to less than half of the period that was required for annealing a similar sample under flowing oxygen at atmospheric pressure.

Order parameter symmetry in the superconducting ferromagnets UGe 2 and URhGe

In UGe 2, ZrZn 2 and URhGe the co-existence of superconductivity and ferromagnetism appears to arise as a co-operative phenomena rather than as the overlap of two mutually competing orders. In all three compounds the magnetism is in intimate contact with the electronic degrees of freedom while the Curie temperatures are more than an order of magnitude higher than their critical temperatures for superconductivity. The most direct indication that the two orders are conjugate however is that the superconductivity and ferromagnetism are suppressed at the same critical pressure in both UGe 2 and ZrZn 2. This has motivated the recent theoretical classification of the permitted superconducting order parameter symmetries for such states. In the following we will review the experimental evidence relevant to the identification of the actual symmetries. This is followed by a discussion of the possible states allowed theoretically. Finally we discuss briefly whether the magnetic order can indeed lead to an enhancement of the superconductivity.

Evolution from BCS superconductivity to Bose-Einstein condensation: Current correlation function in the broken-symmetry phase

We consider the current correlation function for a three-dimensional system of fermions embedded in a homogeneous background and mutually interacting via an attractive short-range potential, below the (superconducting) critical temperature. Diagrammatic contributions in the broken-symmetry phase are identified, that yield for the (wave-vector and frequency dependent) current correlation function the fermionic BCS approximation in the weak-coupling limit and the bosonic Bogoliubov approximation in the strong-coupling limit (whereby composite bosons form as bound-fermion pairs). The temperature dependence of the superfluid density (from the BCS exponential behavior at weak coupling to a power-law behavior at strong coupling) and the form of the Pippard-like kernel at zero temperature are explicitly obtained from weak to strong coupling. Quite generally, it is shown that the Pippard-like kernel is the sum of a local (London-like) term and of a nonlocal component, the local term being dominant in the strong-coupling limit and the nonlocal component in the BCS (weak-coupling) limit. It is also shown that the range of the nonlocal component is determined by the coherence length measuring the spatial correlations of the amplitude of the order parameter, namely, the correlations among different Cooper pairs (or composite bosons), rather than between the fermionic partners of a given pair. In addition, a prescription is provided for mapping the fermionic onto the bosonic diagrammatic theories in the broken-symmetry phase, thus complementing what already done in the normal phase.

Influence of disordering on the critical temperature of superconductors with a short coherence length

The influence of disordering on the critical temperature of s-wave superconductors with a short coherence length is studied by numerical simulation using the two-dimensional Hubbard model with attraction. The calculations are carried out within the Bogolyubov-De Gennes approach, which allows one to include the spatial inhomogeneity of the order parameter. The influence of the spatial inhomogeneity of physical quantities on the results obtained is studied. Disordering is shown to be able to cause a significant increase in the critical temperature.

Frequency dependence of the depinning and irreversibility lines in BSCCO

Penetration of AC magnetic field in Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub /spl delta// single crystals was studied using the magnetooptic technique. It was found that the apparent depinning line shifts toward higher (H, T) values with increasing field frequency. At higher frequencies irreversibility in magnetization is controlled by pinning but not by the geometrical or surface barrier. Comparison with the melting line has shown that the depinning line lies completely in the vortex liquid region. Moreover, at least in low DC fields, the vortex liquid does not become unpinned up to the critical temperature of superconductor.