Superconducting-normal Transition Research Articles

Fine structure of the superconducting-normal and normal-superconducting transition

A non-linear fine structure of the superconducting-normal and normal-superconducting transition has been observed, based on the investigation of the temperature dependence of the ac susceptibility of high- T c superconductors. To detect this, a new research method, the so-called thermometric mapping, has been elaborated. Besides the usual intergrain and intragrain peaks, numerous springs on the dispersion and absorption curves have been shown, demonstrating the existence of the fine details of the gradual S-N or N-S transition. The model presented shows that the non-linearity is an inseparable property of quantum-mechanically coherent coupled systems.

Critical-temperature inhomogeneities and resistivity rounding in copper oxide superconductors.

By using effective-medium approaches, we obtain the onset of the electrical-resistivity rounding, above the normal-superconducting transition, associated with inhomogeneities of the mean-field critical temperature {ital T}{sub {ital c}0} at scales larger than the superconducting correlation length. These results are compared with available data in single-crystal and single-phase (to within 4%) polycrystalline YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} samples. This comparison shows that the measured resistivity rounding cannot be explained by these types of local {ital T}{sub {ital c}0} inhomogeneities. Complementarily, our calculations allow us to check some proposals on {ital T}{sub {ital c}0} inhomogeneities associated with local sample strains or oxygen-content variations. The interplay between {ital T}{sub {ital c}0} inhomogeneities and superconducting order-parameter fluctuations (SCOPF) leads to the conclusion that in the mean-field-like region (MFR) above the superconducting transition, the {ital T}{sub {ital c}0} inhomogeneity contribution to the measured resistivity rounding in high-quality (single-phase) cuprate oxide superconductors is negligible. In contrast, our analysis confirms that in the MFR these effects may be explained quantitatively on the grounds of the Lawrence-Doniach theory for SCOPF.

Phase diagram of UPt3 from ultrasonic velocity measurements.

We present measurements of longitudinal ultrasonic velocity on single crystals of the heavy-fermion superconductor ${\mathrm{UPt}}_{3}$. The measurements show clear signatures of second-order phase transitions in the superconducting state, with the velocity anomalies well accounted for by Ginzburg-Landau theory. From these signatures we construct a phase diagram for ${\mathrm{UPt}}_{3}$ that reveals all the boundary lines that have been identified as possible phase transitions. We are able to track the phase transition lines to a tetracritical point, located on the upper-critical-field curve, to within the width of the normal-superconducting transition.

Scanning tunneling potentiometry studies of Y1Ba2Cu3O7−x and gold thin films

We have used a scanning tunneling microscope to study the local electrostatic potential distribution and topography of thin films of gold at room temperature and high-Tc superconducting thin films at low temperature in the presence of a transport current. On continuous gold thin films (85 Å thick), high resolution scans often show abrupt changes in the potential, which are clearly associated with topographic features. On a relatively large scale (1 μm), a more uniform potential drop is observed but locally these scans also reveal small steps in the potential. These steps may be a characteristic property of transport in these thin polycrystalline films or, alternatively, could be artifacts due to discontinuous changes in the tunneling position on the sample during scanning. On high-Tc superconducting thin films , a thin gold overlayer permits potentiometry studies at low temperature. The first low-temperature potentiometry studies show a clear change in behavior at the normal-superconductor transition.

Flux lattice melting in high-Tc superconductors.

We derive the wave-vector-dependent elastic moduli for a flux line lattice in compounds with underlying tetragonal crystalline symmetry. We find that it is essential to retain wave-vector dependence of the moduli when dealing with compounds where \ensuremath{\kappa} is large, as it is in the high-${T}_{c}$ materials. We use our results to establish a Lindemann criterion for flux lattice melting, which we then compare with experimental data on two materials, and find excellent agreement. The melting curves are suppressed well below the mean-field superconducting-normal transition line and are linear in temperature over a wide range of magnetic fields. The point H=0, T=${T}_{c}$ is approached as 1-T/${T}_{c}$\ensuremath{\sim}${H}^{1/2}$. The degree of suppression of the melting curves among the different compounds is accounted for in the main by differences in mass anisotropy.

Muon-spin relaxation studies of weak magnetic correlations in U1-xThxBe13.

Zero-field positive muon-spin relaxation rates in ${\mathrm{U}}_{0.965}$${\mathrm{Th}}_{0.035}$${\mathrm{Be}}_{13}$ provide definitive evidence for the onset of magnetic correlations at the lower phase transition at temperature ${T}_{c2}$ below the normal-superconducting transition at ${T}_{c1}$. The transition at ${T}_{c2}$ is found to be second order and mean field like, with a moment (${10}^{\mathrm{\ensuremath{-}}3}$\char21{}${10}^{\mathrm{\ensuremath{-}}2}$)${\ensuremath{\mu}}_{B}$/f atom. Candidates for the second phase, such as magnetic order and magnetic superconducting states, are discussed.

Non-Ohmic dissipative regime in the superconducting transition of polycrystalline Y1Ba2Cu3Ox.

We have measured the low-field magnetoresistance and current-voltage characteristics of polycrystalline ${\mathrm{Y}}_{1}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ near the superconducting-normal transition. The development of a fully superconducting state occurs via an intermediate phase about 2 K in width. Within this intermediate phase, sample dissipation is highly non-Ohmic and exhibits extreme sensitivity to small magnetic fields. The nonlinear I-V characteristics describe a new phase with zero resistance and zero critical current which seems to arise from the presence of strong junction disorder.

Vortices in superconductors

In type II superconductors where the London penetration depthλ is larger than the coherence lengthξ, there is a possibility of flux penetration inside the sample for magnetic field greater than\(H_{0_1 } \left( { = \frac{{\phi _0 }}{{4\pi \lambda ^2 }}ln \lambda /\xi , \phi _0 = \frac{{hc}}{{2e}}} \right).\) The flux penetrates in the form of vortices with core of sizeξ. However these vortices differ from those in superfluid He4 in variation of currentj(r) circulating around them. For superconductorsj(r) ∼ 1/r only up to a distanceλ and then it falls exponentially whilev(r) ∼ 1/r for all distances in superfluids. The reason is that in superconductors vortex carries a magnetic flux which is screened by conduction electrons. This coupling of order parameter field (the pairing wavefunction) with the gauge field has many interesting implications for superconductors and for non-Abelian gauge theories. Some examples are as follows: 1. The energy of the vortices is reduced. The energy of vortex of lengthL (ind = 3 sample) is of orderL lnL for a superfluid, is of orderL in a superconductor, and (in ad = 2 sample) the energy of a vortex point which diverges like lnR (whereR is the size of the sample) in a superfluid becomes finite in a superconductor. 2. The superconducting-normal transition in three dimension is very weak first order, because the fluctuations of the gauge field, when summed over, add to Ginzburg Landau free energy a term proportional to |ψ|3, whereψ is the order parameter. 3. Because of the lnr behaviour of interaction energy of vortices, a two-dimensional superfluid sample can exhibit a Kosterlitz-Thouless type transition whereas a strictlyd = 2 superconductors should not have any. However for dirty superconducting films whereλ is large vortex binding-unbinding transition can be observed with quite a rich phase diagram.

Flux quantization in periodic networks containing tiles with irrational ratio of areas.

We report measurements of the superconducting-normal transition boundary ${T}_{c}$(H) of two-dimensional periodic networks with two different space-group symmetries in a magnetic field. Each network is a mixture of squares and equilateral triangles. In both cases, we observe maxima in ${T}_{c}$(H) with one major period, which does not correspond to the area of either the square or the triangle. We interpret the results in terms of flux configurations whose energies are sensitive to the geometry of a given network.

Shear-modulus change belowTcin YBa2Cu3O7−x

The ultrasonic transverse-wave velocity in the high-Tcmetal-oxide superconductor YBa2Cu3O7−xbetween 275 and 4 K was measured. AboveTcthe velocity shows normal behavior: during cooling, it displays a monotonic increase with decreasing slope. During cooling belowTcthe shear modulus departs from normal behavior, increasing as ΔG /G=b[1 − (T/Tc)n, wheren= 3. These results depart dramatically from those expected for a simple second-order normal-superconducting phase transition.

DENSITY DEPENDENCE OF THE MAGNETIC AND TRANSPORT PROPERTIES OF YBa2Cu3O7 COMPOUNDS

The dependence of the normal state resistivity, resistive and magnetic superconducting transitions (Tc , Δ Tc ) and of the upper critical field slope (dHC2/dT|T=Tc) on density has been investigated for several YBa2Cu3O7 samples. Resistivity decreases rapidly with increasing density whereas Tc and resistive Δ Tc are insensitive to apparent density. However Δ T'c obtained from a.c. susceptibility decreases rapidly with increasing density. dHC2/dT|T=TC depends sensitively on the actual preparation condition. The lower critical field slope for the most dense sample is also deduced.

Anelastic relaxation due to the tunneling of trapped D in Ta.

Internal friction measurements in the kHz range have been performed on Ta containing D, O, and N at liquid-helium temperature. A relaxation process is observed below 60 K showing a sharp discontinuity in the slope at the normal-superconductor transition temperature. The process is interpreted as relaxation of a two-level system consisting of O (N)-D complexes interacting mainly with the conduction electrons.

Small-scale experiments on static devices using AC superconductors

This paper reports on the results obtained on small-scale electric devices using the normal superconducting state transition and/or the AC properties of the superconducting wires. The results agree with the predicted theory and allow the development of larger experimental units taking into account industrial requirements.

Tests of superheated superconducting colloids using an RF SQUID

We report two tests designed to study the practicality of a superheated superconducting colloid detector using a SQUID read-out system. In the first test, the individual 'flips' of ten 15 μm radius tin grains were observed as the temperature was swept through the superheated superconducting-normal phase transition. In the second test, we were able to observe transitions induced by 90 keV γ-rays in a colloid of 5μmm radius grains in epoxy.

Transport and Meissner Effect Studies for YBa2(Cuy1−xZnx)3O7

ABSTRACTSingle phase orthorhombic YBa2(Cuy1−xZnx)3O7 samples were formed for 0<x<0.16. The high T superconductivityXfor x=6 (T =90 K) is rapidly depressed with increasing x, and is quenched for x>0.08. Low field (<100 G) cooled magnetization studies show that the superconducting component decreases as x approaches the critical value for suppression of superconductivity, and this is supported by high resolution specific heat measurements in the vicinity of T. Temperature dependent electrical resistivity studies for x<0.08 show metallic behavior; for x>0.10 semiconducting behavior. The electrical resistance was studied at high quasihydrostatic pressures also, and for x=0.08 showed that T is depressed with increasing pressure: T → 0 K for P >10 GPa. This is in°contrast to YBa2(Cuy1−xZnx)3O7 where dT /dP>0. The data support evidence for the high sensitivity to chemical and ice perturbations of the physical properties of samples near the superconducting-normal transition region.

Effect of uniaxial stress on the transport properties of TaSe3.

We have measured the effect of elastic, uniaxial stress on the resistance R, the thermoelectric power (TEP), and the superconducting transition temperature ${T}_{c}$ of ${\mathrm{TaSe}}_{3}$. We find that there is a nearly discontinuous change in R and in the TEP at a stress \ensuremath{\sigma} of about 1.2 GPa at 20 K. In this change R increases by several orders of magnitude, while the TEP changes sign and increases by several orders of magnitude. At higher temperatures this change becomes less pronounced and occurs over a larger range of \ensuremath{\sigma}. We suggest that this change is due to a Fermi-surface topology change, a structural phase transition, or a charge-density-wave transition. We found no evidence for a metal-to-nonmetal transition just above the normal-superconducting transition at ${T}_{c}$.

Acoustic-property measurements of the oxide superconductor BaPb1-xBixO3 by the sphere-resonance method.

The acoustic resonance frequency and amplitude of spherical sintered polycrystalline specimens of ${\mathrm{BaPb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Bi}}_{\mathrm{x}}$${\mathrm{O}}_{3}$ (x=0.20 and 0.25) were measured as a function of temperature (300--4.2 K) and external magnetic field (0--5 T). The resonance amplitude was found to be dependent on the magnetic field and temperature; it changed drastically at the second critical magnetic field and at the critical temperature of the normal-superconducting transition. This effect is interpreted in terms of a model in which the electron-phonon coupling of the thermal phonons play an essential role in determining the amplitude of the resonance.

Effect of Cu4Ti Compound Formation on the Characteristics of NbTi Accelerator Magnet Wire

High critical current density, J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> > 2500 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2^{, and small filament diameter, d ∽ 3 μm, are required in multifilamentary NbTi wire used for superconducting accelerator magnets. Wires obtained from various commercial sources had J_c's in the range 1000 to 2800 A/mm² and d's in the range 1 to 23 μm. The filaments were examined by means of scanning electron microscopy in order to determine the reason for the variation in J_c. It was found that the filaments in high J_c wires had clean smooth surfaces and uniform cross section along-their lengths. Filaments in low J_c wires show formation of Cu_{4_{Ti compound particles on their surfaces and large variations in cross section. The lower critical current measured in these wires is believed to be largely due to this effect. The superconducting-normal state transition is relatively wide in these wires.}}}</sup>

Errata: Stabilization of superconductivity by Phase-Slip Centers in current-carrying tin whiskers

The superconducting-normal transition in quasi-one-dimensional superconductors, such as whiskers, near the critical temperature is characterized by a large region of nonequilibrium superconductivity. Localized phase-slip centers lead heV-T andV-I characteristics. The width of the transition can be understood even by assuming the whisker to consist of very homogeneous material, because phase-slip centers stimulate the superconductivity in the whole sample, leading to an enhancement of the critical currents of other phase-slip centers. The present work reports quantitative measurements of his phenomenon and discusses possible explanations.

Elastic Modulus Measurement of BaPb1-X Bi X O3 by Resonance Method

The mechanical resonance frequency and amplitude of a spherical BaPb1-X Bi X O3 specimen were measured as a function of temperature (300 K–4.2 K) and external magnetic field (0–5 T). The change in the conduction electron mean free path associated with the normal-superconducting transition was observed indirectly through the change of the mechanical resonance amplitude.