Superconducting State of the Two-Dimensional Superconductor α-(ET)2NH4Hg(SCN)4 Under the Magnetic Field Parallel to the Conducting Plane

The resistive transition to the superconducting (SC) state in -(ET)₂NH₄Hg(SCN)₄ was studied under magnetic fields applied parallel to the conducting plane. The SC state is divided into three regions: (1) temperature-dependent resistive region, (2) zero-resistive region, and (3) temperature-independent resistive region. We observed twofold symmetry in the resistance under an in-plane magnetic field near the midpoint of the SC transition, which indicates that the upper critical field ₀ H _c2 has the same twofold symmetry. This in-plane anisotropy can be ascribed to the crystal structure.     

Superconducting to normal transition of type-II Ta by a rectangular current pulse

The superconducting to normal transition by a rectangular pulse current in type-II Ta has been investigated. The potential drop along the specimen is observed as a function of time. The voltage pattern and the magnitude of the potential drop are obtained as functions of current amplitude, temperature, externally applied magnetic field, the angle between magnetic field and specimen axis, and values. The transition mechanisms are discussed. It is also shown in this experiment that the flux flow or flux creep and flux jump occur. A support to the paramagnetic current flow in the presence of longitudinal magnetic field is given.     

Resistive transition of superconducting wire networks. Influence of pinning and fluctuations

We have studied the resistive transition of several 2-D superconducting wire networks of various coupling strengths, which we characterize in terms of the Kosterlitz-Thouless transition temperature and the ratio /a of the coherence length to the array period. In the extreme strong coupling limit where the mesh size is of the order of the zero-temperature coherence length, the superconducting behavior is well described by the mean-field properties of the superconducting wave function. Extending to 2-D array, the 1-D phase slippage model explains the dissipative regime observed above the Ginzburg-Landau depairing critical current. On the other hand, when the coupling is weak, phase fluctuations below the Ginzburg-Landau transition and vortex depinning dominate the resistive behavior. An activated dissipation is observed even below the depairing critical current. Results obtained in this regime for critical temperature, magnetoresistance or critical current versus temperature, and magnetic field are shown; their periodic oscillations are discussed in terms of depinning of vortices on the array. A simple periodic pinning potential for a vortex in a wire network is calculated, and compared with the case of pinning in Josephson junction arrays. We show that this model explains qualitatively the experimental results observed for small /a.     

Magnetic field effects on the electrical conductivity due to flucíuations of the superconducting order parameter aboveT c

The electrical conductivity above the transition temperatureT _c due to fluctuations of the superconducting order parameter has been measured in aluminum films in the presence of a magnetic field (parallel and perpendicular to the film surface), as well as in the absence of a magnetic field. It has been found, in particular from the results in a perpendicular magnetic field, that the Aslamazov-Larkin theory (AL) and its extension can be applied for temperatures close toT _c . A deviation from the theoretical linear relation (AL) _n/(T) ln(T/T _c ) in zero and parallel field might be explained in terms of the Maki-Thompson model (MT), which is consistent with previous experiments in Al films. However, the transition width due to fluctuations in a perpendicular magnetic field is not always enhanced, contrary to the prediction of the MT model. Even in the cases in which they are enhanced, the transition width is smaller than the value predicted by the MT model.On leave from Department of Physics, Faculty of Science, Kyushu University, Fukuoka, Japan.     

Flux Flow Resistance and Transmittivity in Overdoped Tl2Ba2CuO6+δ

We have measured the surface resistance and transmittivity of overdoped Tl ₂ Ba ₂ CuO ₆₊ in magnetic field up to 7 T in order to investigate the unusual dc resistive transition. Introduction of columnar defects largely shifts the irreversibility line to higher field, showing that the resistive transition is caused by the melting of the vortex lattice. It is found that flux flow resistance of this compound is unusual as compared with that expected from the dc resistivity.     

Magnetic Field Dependence of Intergrain Pinning Potential in Bulk Granular Composites YBCO + CuO Demonstrating Large Magneto-Resistive Effect

The broadening of the resistive transition in magnetic field and isotherms of magnetoresistance of bulk composites Y–Ba–Cu–O + CuO have been studied. These composites exhibit large magneto-resistive effect in a wide temperature range below T _C due to weakening of Josephson coupling in this system. The broadening of the resistive transition and magnetoresistance are explained well by the Ambegaokar–Halperin (AH) model for phase slip in Josephson junctions. The magnetic field dependence of pinning potential in the intergrain boundaries deduced from AH model found out to be similar to that of critical current of an array of Josephson junctions. The values of pinning energy point out that the large magneto-resistive effect observed in the composites results from flux flow-like processes at the intergrain boundaries.     

The Positive Magnetic Susceptibility for Rb3C60 Superconductor and the Differential Paramagnetic Effect

AC magnetic susceptibility of Rb₃C₆₀ superconductor as a function of temperature was measured in an applied static magnetic field perpendicular to the AC magnetic field. The peaks in real part of AC susceptibility curves located in the transitional temperature indicate that there may exist the differential paramagnetic effect (T) = dM/dH > 0 in the specimen. The amplitude of the peak and the temperature of diamagnetic onset are proportional to intensity of the applied field, and upon cooling the peaks occur before the transition temperature of zero field. Through discussing and comparing our experimental result with those reported previously, we have put forward a new opinion on producing condition of the differential paramagnetic effect (DPE).     

Patterns of magnetic flux penetration in superconducting films

Intermediate-state magnetic flux patterns have been observed in thin superconducting films of tin, lead, and indium in perpendicular applied fields of 0.1–100 Oe. The spatial variation in flux density was measured very close to the film with a high-resolution Hall probe which could be scanned in a plane parallel to the surface of the film. The patterns appear to contain both laminar and round normal regions, with the latter predominant. A model for the intermediate state has been developed in which the normal regions are macroscopic flux spots. Experimental results for the number density of these flux spots in 1.1–6.3--thick indium samples are in good agreement with the temperature, field, and thickness dependences of our model; indium samples thinner than 1 µ appear to enter the mixed state. In tin and lead, however, the flux patterns are strongly influenced by flux pinning. Appearance of the flux pattern at the critical field or temperature has been observed and correlated with distinctive features in the resistive transition. The extent of flux penetration into the sample was determined from measurement of the average field and compared for various field-temperature paths to the intermediate state. With the applied field reduced to zero, the magnetic field outside a current-carrying superconducting strip has been measured and indicates that the current distribution within the strip is consistent with the form predicted by Bowers.Supported in part by the NSF, ONR, and ARPA.     

Effect of magnetic field and temperature on the voltage-current characteristics of YBCO superconductor

Voltage-current characteristics of YBCO superconductor was studied under magnetic field up to 0.4 T at different temperatures below T_c. The critical temperature decreases and the transition width broadens under magnetic field. V-I data below T_c were fitted to a power law expression V I^(T,B) in which (T,B) is found to decrease with increase of magnetic field and temperature, gradually approaching unity as T approaches T_c, being independent of magnetic field. Similarly, (T,B) approaches unity as magnetic field increases being independent of temperature.     

Critical current distributions in superconducting composites

The transition from the flux pinning state to the full flux flow state occurs over a range of current in composite superconductors. The critical current, therefore, does not have a single, unique value, there being a distribution of critical currents throughout the composite. This Paper describes a technique for deriving the critical current distribution from the resistive critical current transition. The analysis has been applied to Nb---Ti and Nb₃Sn composites of different types. The technique has been used to analyse good quality conductors, where the l_c is largely determined by the intrinsic fluxoid-microstructure interactions, At the other extreme, composites with irregular filaments in which the transport critical current is significantly less than the intrinsic critical current, have also been examined. The analysis shows that good conductors have narrow resistive transitions with average l_c values within 5–10% of a high sensitivity measurement of l_c. This difference broadens to ≈ 35% for a badly sausaged composite. A relationship between the n value of the resistive transition and the relative width of the l_c distribution is presented. A simple but accurate method of deriving the average l_c from the resistive transition is also presented.     

Voltage-current characteristics of polycrystalline (Bi,Pb)2Sr2Ca2Cu3O10 superconductor at different magnetic fields and temperatures

The voltage-current characteristics of polycrystalline (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ superconductor were investigated at temperatures from 80 to 105 K under magnetic field up to 0.4 T. In magnetic field considerable broadening in the transition width occurs which depends on the processing conditions and microstructure of the sample. I–V data below T_c were fitted to a power law expression V I to obtain . We have found that decreases rather rapidly with increase of magnetic field and temperature, in low magnetic fields and at temperatures quite below T_c, respectively. Similar behavior was observed for the variation of the critical current as a function of temperature and magnetic field. The values are found to change linearly as a function of critical current at relatively low values of I_c.     

Thermodynamic Properties of a Double Ring-Shaped Quantum Dot at Low and High Temperatures

In this work, we study thermodynamic properties of a GaAs double ring-shaped quantum dot under external magnetic and electric fields. To this end, we first solve the Schrödinger equation and obtain the energy levels and wave functions, analytically. Then, we calculate the entropy, heat capacity, average energy and magnetic susceptibility of the quantum dot in the presence of a magnetic field using the canonical ensemble approach. According to the results, it is found that the entropy is an increasing function of temperature. At low temperatures, the entropy increases monotonically with raising the temperature for all values of the magnetic fields and it is independent of the magnetic field. But, the entropy depends on the magnetic field at high temperatures. The entropy also decreases with increasing the magnetic field. The heat capacity and magnetic susceptibility show a peak structure. The heat capacity reduces with increasing the magnetic field at low temperatures. The magnetic susceptibility shows a transition between diamagnetic and paramagnetic below for \(T<4\hbox { K}\). The transition temperature depends on the magnetic field.     

Discussion of nonbranching laminar intermediate-state structures in superconducting bulk lead

This paper continues the discussion of laminar intermediate-state structures in superconducting bulk lead samples (thickness d in the range 0.02 d 0.12 mm) which have been qualitatively described previously. Quantitative results are given for the period of the structures, the widths of the domains, and the broadening of the normal domains at the transition from superconducting to normal state [(SI)_T transition] and from normal to superconducting state [(NI)_T transition] as functions of the applied magnetic field strength and the sample thickness. Differences between the results of the transitions with respect to field strength have been found, whereas the behavior of the structures depends on the sample thickness in the same way at both transitions. The results are compared with the models of Landau and of Kuper. Applying these models, values of the surface-energy parameter are calculated and the effect of the magnetic field on these results is discussed.     

Effects of the Experimental Geometry on the Vortex Dynamics in High-T c, Superconductors

A brief overview is given on the macroscopic electrodynamics of type-II superconductors with and without vortex pinning, of various shapes and in an applied magnetic field, but also with applied transport current and with finite London penetration depth . The extension to >0 is important at low inductions B to describe the transition to the Meissner state, and for films with thickness not much larger than . The finite width of the surface layer with screening currents and the correct dc and ac responses in various geometries follow from an equation of motion for the current density, with a dependent integral kernel.     

Vortex Phase Diagram in Zn-Doped YBa2Cu3O y Crystals

Effects of Zn impurities on the vortex phase diagram of YBa₂(Cu_1–x Zn _x )₃O _y single crystals with 0x0.064 have been investigated in magnetic field up to 18 T. The first-order vortex lattice melting transition and the field-driven disordering transition are extremely suppressed by small amount of Zn impurities. As for Zn-doped YBCO with x>0.001, the resistive transition shows a broad temperature dependence due to the second-order vortex glass melting transition, while the field-driven disordering transition survives up to 0.003. These behaviors suggest that disorderes induced by Zn impurities strongly affect the vortex system in comparison with the oxygen deficiency in YBCO.     

Magnetoresistance,Irreversibility Fields,and Critical Currents of Superconducting 2G Tape

The magnetoresistance, irreversibility fields, and critical current density were studied for a commercial 2G tape at the two relative orientations of magnetic field and superconductor plane. The critical temperatures of this tape of T _{c50 %} = 91.5 K and T _c0 = 90 K and the width of superconducting transition of ΔT = 1.5 K were obtained. The widths of the transition from the normal to the superconducting state do not increase at the applied magnetic field up to 90 kOe and do not depend on the orientation of the magnetic field with respect to the tape plane. The irreversibility field values were obtained and successfully fitted as a function of temperature using the formula: \(H_{\text {irr}} =H_{\text {irr0}} \left ({1-\frac {T}{T_{\text {c0}} }} \right )^{n}\). The irreversibility fields show an anisotropy, and at the liquid nitrogen temperature, they reach H _irr = 430 kOe and H _irr = 106 kOe for the parallel and perpendicular directions, respectively. The anisotropy ratio amounts to γ = 4 at 77 K and is small in comparison with other high-temperature superconducting materials. The critical current density of this tape was found to be of the order of 10⁶ A cm^?2 at 77 K in the self-magnetic field.     

Resistive voltage in the mixed state of superconducting niobium in the presence of small oscillatory magnetic fields

The resistive voltage near the critical current in the mixed state of superconducting niobium foils has been measured in the presence of small oscillatory magnetic fields oriented parallel to the static magnetic field and perpendicular to the foil. The oscillatory field was found to cause an enhancement in the resistive voltage which depends on frequency and amplitude of the oscillatory field. The voltage increments show a maximum at a frequency * which varies with the static magnetic field and the specimen thickness and which depends very little on the lateral sample dimensions and the amplitude of the oscillatory field. The results are discussed in terms of eddy-current damping within the superconductor of an external magnetic field variation.Based on work performed under the auspices of the U.S. Atomic Energy Commission.     

Static Josephson effects in circularly symmetric annular junctions

Static Josephson effects in circularly symmetric annular junctions are discussed. When the annular width W of a junction is smaller than _J, analytic solutions can be obtained. For annular width W of a junction larger than _J, the self-field effects must be considered; the solutions can be obtained numerically. For these two cases, called "small junction" and "large junction," respectively, the magnetic field dependence of the critical current I _m of a circularly symmetric annular junction is obtained. Further, for a width W larger than _J, the current density distribution is also given for several magnetic fields. In addition, for a given external magnetic field, when the feeding current I is smaller than the critical current I _m of the circular symmetric annular junction, there are multisolutions under such physical conditions. Finally, the stability of these solutions is also discussed.     

Magnetic Ordering in the Mixed Ruthenium-Copper Oxide Ba2Pr(Ru1−x Cu x )O6 System

We have synthesized a series of mixed ruthenium-copper oxides of general formulae Ba₂PrRu_1–x Cu _x O₆ with x=0, 0.05, 0.1, 0.15 by high temperature solid-state reaction process. From the SQUID measurements, we observed a strong diamagnetic response on zero-field-cooling (ZFC), meanwhile a ferromagnetic-like transition under field cooling, both at about 11 K. In addition, we also observed a broad transition at about 125 K by SQUID measurements. Electrical resistivity measurements also show a sharp resistive drop at about 150 K, suggesting the presence of a magnetic transition. High intensity and high resolution neutron diffraction studies further confirm the magnetic phase transition at 100 K into a Type I antiferromagnetic structure.