Showing papers in "Physica C-superconductivity and Its Applications in 2010"

Degradation of the performance of a YBCO-coated conductor double pancake coil due to epoxy impregnation

Abstract: Now that YBCO-coated conductors have been commercialized, a number of YBCO coils have been developed. However, their basic performances have not been systematically investigated so far. Here, we demonstrate that of a YBCO double pancake coil. The critical current of an epoxy impregnated YBCO double pancake coil was substantially degraded, i.e. the normal voltage appears above 8 A, only 18% of that for the dry coil. It was inferred that degradation occurs if the cumulative radial stress developed during cool down exceeds the critical transverse stress for the YBCO-coated conductor (typically 10 MPa). Under these conditions, the conductor was debonded at the interface between the buffer layer and YBCO layers, or fractured in the YBCO layer itself, causing cracks on the YBCO layer, resulting in a significant decline of the critical current. These negative effects are suppressed if the coils are dry wound or impregnated with paraffin, as the bonding strengths between turns are negligible and therefore turns are separated if the cumulative radial stress tends to be tensile. For non-circular coils in which epoxy impregnation is inevitable, degradation due to cumulative tensile transverse stress is still the major problem.

European roadmap on superconductive electronics – status and perspectives☆

Abstract: For four decades semiconductor electronics has followed Moore's law: with each generation of integration the circuit features became smaller, more complex and faster. This development is now reaching a wall so that smaller is no longer any faster. The clock rate has saturated at about 3-5 GHz and the parallel processor approach will soon reach its limit. The prime reason for the limitation the semiconductor electronics experiences is not the switching speed of the individual transistor, but its power dissipation and thus heat. Digital superconductive electronics is a circuit- and device-technology that is inherently faster at much less power dissipation than semiconductor electronics. It makes use of superconductors and Josephson junctions as circuit elements, which can provide extremely fast digital devices in a frequency range - dependent on the material - of hundreds of GHz: for example a flip-flop has been demonstrated that operated at 750 GHz. This digital technique is scalable and follows similar design rules as semiconductor devices. Its very low power dissipation of only 0.1 mu W per gate at 100 GHz opens the possibility of three-dimensional integration. Circuits like microprocessors and analogue-to-digital converters for commercial and military applications have been demonstrated. In contrast to semiconductor circuits, the operation of superconducting circuits is based on naturally standardized digital pulses the area of which is exactly the flux quantum Phi(0). The flux quantum is also the natural quantization unit for digital-to-analogue and analogue-to-digital converters. The latter application is so precise, that it is being used as voltage standard and that the physical unit 'Volt' is defined by means of this standard. Apart from its outstanding features for digital electronics, superconductive electronics provides also the most sensitive sensor for magnetic fields: the Superconducting Quantum Interference Device (SQUID). Amongst many other applications SQUIDs are used as sensors for magnetic heart and brain signals in medical applications, as sensor for geological surveying and food-processing and for non-destructive testing. As amplifiers of electrical signals. SQUIDs can nearly reach the theoretical limit given by Quantum Mechanics. A further important field of application is the detection of very weak signals by 'transition-edge' bolo-meters, superconducting nanowire single-photon detectors, and superconductive tunnel junctions. Their application as radiation detectors in a wide frequency range, from microwaves to X-rays is now standard. The very low losses of superconductors have led to commercial microwave filter designs that are now widely used in the USA in base stations for cellular phones and in military communication applications. The number of demonstrated applications is continuously increasing and there is no area in professional electronics, in which superconductive electronics cannot be applied and surpasses the performance of classical devices. Superconductive electronics has to be cooled to very low temperatures. Whereas this was a bottleneck in the past, cooling techniques have made a huge step forward in recent years: very compact systems with high reliability and a wide range of cooling power are available commercially, from microcoolers of match-box size with milli-Watt cooling power to high-reliability coolers of many Watts of cooling power for satellite applications. Superconductive electronics will not replace semiconductor electronics and similar room-temperature techniques in standard applications, but for those applications which require very high speed, low-power consumption, extreme sensitivity or extremely high precision, superconductive electronics is superior to all other available techniques. To strengthen the European competitiveness in superconductor electronics research projects have to be set-up in the following field: - Ultra-sensitive sensing and imaging. - Quantum measurement instrumentation. - Advanced analogue-to-digital converters. - Superconductive electronics technology.

The Ginzburg-Landau theory in application

Abstract: A numerical approach to Ginzburg–Landau (GL) theory is demonstrated and we review its applications to several examples of current interest in the research on superconductivity. This analysis also shows the applicability of the two-dimensional approach to thin superconductors and the re-defined effective GL parameter κ . For two-gap superconductors, the conveniently written GL equations directly show that the magnetic behavior of the sample depends not just on the GL parameter of two bands, but also on the ratio of respective coherence lengths.

Large transport critical currents of powder-in-tube Sr0.6K0.4Fe2As2/Ag superconducting wires and tapes

Abstract: We report the achievement of transport critical currents in Sr 0.6 K 0.4 Fe 2 As 2 wires and tapes with a T c = 34 K. The wires and tapes were fabricated through an in situ powder-in-tube process. Silver was used as a chemical addition as well as a sheath material. All the wire and tape samples have shown the ability to transport superconducting current. Critical current density J c was enhanced upon silver addition, and at 4.2 K, a largest J c of ∼1200 A/cm 2 ( I c = 9 A) was achieved for 20% silver added tapes, which is the highest in iron-based wires and tapes so far. The J c is almost field independent between 1 T and 10 T, exhibiting a strong vortex pinning. Such a high transport critical current density is attributed to the weak reaction between the silver sheath and the superconducting core, as well as an improved connectivity between grains. We also identify a weak-link behavior from the apparent drop of J c at low fields and a hysteretic phenomenon. Finally, we found that compared to Fe, Ta and Nb tubes, Ag was the best sheath material for the fabrication of high-performance 122 type pnictide wires and tapes.

Batch-processed GdBCO–Ag bulk superconductors fabricated using generic seeds with high trapped fields

Abstract: Large, single grains of Y–Ba–Cu–O (YBCO) have been batch-processed to date by the top seeded melt growth (TSMG) process using NdBCO or SmBCO seed crystals. It has proved difficult, however, to economically batch-process light rare earth (LRE) LRE–Ba–Cu–O bulk high temperature superconductors, which have higher critical current densities and irreversibility fields than YBCO, and therefore greater potential for high field engineering applications. In this paper, we report a novel batch-process based on a cheap, readily available generic seed crystal, developed recently at Cambridge, and a TSMG melt processing technique based on cold seeding in air for the batch fabrication of Gd–Ba–Cu–O–Ag single grains. The superconducting properties of the (LRE)BCO single grains fabricated by this process are, in all respects, equivalent to those processed more conventionally in a reduced oxygen atmosphere.

Development of superconducting power devices in Europe

Abstract: Europe celebrated last year (2008) the 100-year anniversary of the first liquefaction of helium by H. Kammerling Onnes in Leiden. It led to the discovery of superconductivity in 1911. Europe is still active in the development of superconducting (SC) devices. The discovery of high critical temperature materials in 1986, again in Europe, has opened a lot of opportunities for SC devices by broking the 4 K cryogenic bottleneck. Electric networks experience deep changes due to the emergence of dispersed generation (renewable among other) and to the advances in ICT (Information Communication Technologies). The networks of the future will be “smart grids”. Superconductivity will offer “smart” devices for these grids like FCL (Fault Current Limiter) or VLI (Very Low Inductance) cable and would certainly play an important part. Superconductivity also will participate to the required sustainable development by lowering the losses and enhancing the mass specific powers. Different SC projects in Europe will be presented (Cable, FCL, SMES, Flywheel and Electrical Machine) but the description is not exhaustive. Nexans has commercialized the first two FCLs without public funds in the European grid (UK and Germany). The Amsterdam HTS cable is an exciting challenge in term of losses for long SC cables. European companies (Nexans, Air Liquide, Siemens, Converteam, …) are also very active for projects outside Europe (LIPA, DOE FCL, …).

Numerical analysis and finite element modelling of an HTS synchronous motor

Abstract: This paper investigates the electromagnetic properties of high-temperature superconductors with a particular focus on the AC loss in coils made from YBCO superconductors. The numerical analysis and finite element modelling of the YBCO superconductors used in Cambridge's superconducting permanent magnet synchronous motor currently in development is described. The stack of tapes in the superconducting coils is modelled using the direct H formulation, a B-dependent critical current density and a bulk approximation. Magnetic boundary conditions for this model are derived from a 2D finite element method (FEM) motor model. The combination of these models allows the total AC loss (combined transport and magnetisation losses) in the HTS coils used in an all-superconducting machine design to be estimated. The raw AC loss figures are compared to the output power of the motor for two test cases, and it is found that the AC loss contributes significantly to the total loss and therefore efficiency. An experimental rig is also described, which has been built in order to test the electromagnetic properties and performance of the motor. It is explained how this rig will be used to investigate the magnetisation of the rotor and carry out AC loss measurements on the stator coils.

Effects of Ni and Co doping on the physical properties of tetragonal FeSe0.5Te0.5 superconductor

Abstract: Isoelectronic Tellurium (Te) substitution for Selenium (Se) in the tetragonal phase of FeSe (β-FeSe) increases the superconducting transition temperature ( T c ) by applying a negative pressure on the lattice. However, the normal state resistivity increases and shows semi-metallic behavior for samples with higher Te concentration. With increasing Te concentration, the T c increases and reaches a maximum for FeSe 0.5 Te 0.5 and then decreases with further increase of Te. We have investigated the effect of Cobalt (Co) and Nickel (Ni) doping in FeSe 0.5 Te 0.5 in the nominal composition range Fe 1− x TM x Se 0.5 Te 0.5 (TM = Co ( x = 0.05, 0.1, 0.15, 0.2) and Ni ( x = 0.05, 0.1)). Both Co and Ni doping suppress T c and drives the system to metal–insulator transition. The in-plane (‘ a ’) and out-of-plane (‘ c ’) lattice constants decrease with increasing dopant concentration.

Development of YBCO power devices in Japan

Abstract: A new Japanese national project, called M-PACC, to develop high temperature superconducting electric power devices started in June last year (FY2008–FY2012). This project aims to develop three different types of electric power devices that are expected to provide stable power supplies with large capacity and small size by using YBCO coated conductors. The first program is the development of a 2 GJ class superconducting magnetic energy storage system to control stable electric power systems. It is planned to develop several sets of element coils for a 20 MJ class system as a technological feasibility study for a 2 GJ class coil. The second program is the development of two different types of power cables with higher performance than existing power cables; one is a three-core 66 kV–5 kA class large current cable and the other is a single-phase 275 kV–3 kA class high voltage cable. These cable were required several technological developments, namely, large current and low AC loss, high voltage insulation and low dielectric loss, and power and heat balance for both cables. The third program is the development of a 20 MVA class power transformer with 66 kV/6.9 kV as a distribution transformer. In this project, power transformer systematization technology including 2 kA class large current coil technology, anti short-circuit wire winding technology, AC loss reduction technology, and winding technology will be developed.

Generalized control and synchronization of chaos in RCL-shunted Josephson junction using backstepping design

Abstract: This paper generalizes the control and synchronization of chaotic dynamics in resistive–capacitive–inductive-shunted Josephson junction (RCLSJ) models via the backstepping nonlinear control theory. The method, which consists in a recursive approach that interlaces the choice of a Lyapunov function with the control, is used to design a generalized control function that is capable of controlling the chaotic dynamics exhibited by RCLSJ model to track desired dynamics. The result suggests that the generalized controller could be used as a device for tuning the junction signal into any desired form. The active backstepping technique is used to design a single control function that achieves generalized projective synchronization between two RCLSJ systems evolving from different initial conditions. This result suggests that the controller for generalized projective synchronization could be used to amplify the junction signal. Numerical simulation results show that the generalized control functions are effective in both the tracking control and generalized projective synchronization of RCLSJ models.

Fluctuation induced magneto-conductivity studies in YBa2Cu3O7−δ + xBaZrO3 composite high-Tc superconductors

Abstract: Fluctuations on the electrical conductivity of polycrystalline YBa 2 Cu 3 O 7−δ + x BaZrO 3 ( x = 10, 25, 50 and 100 wt%) superconductors were investigated from the resistivity vs temperature data for zero field and 8 T (Tesla) external magnetic fields Attempts have been made to identify the optimum inclusion of BaZrO 3 (BZO) in YBa 2 Cu 3 O 7−δ (YBCO) superconductors The phase formation, texture and grain alignments were analyzed by XRD and SEM techniques Then the effects of superconducting fluctuations on the electrical conductivity of granular composite superconductors were studied for zero field and 8 T external magnetic fields Though inclusions of BZO sub-micron particles are not expected to influence superconducting order-parameter fluctuation (SCOPF) much, the transition from 2D to 3D of the order parameter in the mean-field region depends on the BZO content in the composites It has been observed that BZO residing at the grain boundary of YBCO matrix influences the tailing region without having significant change in the mean-field critical temperature In the present work, attention has been focused mostly in the experimental domain relatively above the T c It reveals that, 1 wt% composite exhibits a better superconducting property in comparison with pure YBCO

New antiperovskite superconductor ZnNNi3, and related compounds CdNNi3 and InNNi3

Abstract: We briefly report the synthesis, electrical resistivity, magnetic susceptibility, and specific heat for a new antiperovskite superconductor ZnNNi 3 with T c ∼ 3 K, and related compounds CdNNi 3 and InNNi 3 . In these materials, CdNNi 3 has been synthesized for the first time by our work. The crystal structures of these compounds have the same antiperovskite-type such as MgCNi 3 and CdCNi 3 .

Structural, elastic, electronic properties and Fermi surface for superconducting Mo2GaC in comparison with V2GaC and Nb2GaC from first principles

Abstract: First-principles calculations were performed to investigate structural, elastic and electronic properties of the first discovered superconducting nanolaminate Mo 2 GaC in comparison with isostructural Ga-containing phases V 2 GaC and Nb 2 GaC. As a result, the optimized lattice parameters, independent elastic constants, bulk moduli, compressibility, shear moduli, Young’s moduli and Poisson’s ratio were evaluated. Besides, electronic bands, densities of states (DOS), total and site-projected l -decomposed DOS at the Fermi level, as well as the shapes of the Fermi surfaces for these Ga-containing nanolaminates were obtained and analyzed in comparison with the available theoretical and experimental data.

MBE growth of FeSe and Sr1-xKxFe2As2

Abstract: We report molecular beam epitaxy growth of FeSe and Sr1−xKxFe2As2. In FeSe growth, the compositional window to obtain the superconducting phase (β-FeSe) is very limited. The use of Al2O3–R substrates instead lattice-matched LaAlO3 substrates slightly expands the compositional window, and we observed a substantial drop of resistivity but with no zero resistivity attained. In Sr1−xKxFe2As2 growth, the largest problem is the volatility of K. Films grown at substrate temperatures higher than 300 °C contain no K. Superconducting films of Sr1−xKxFe2As2 can be obtained at the growth temperature of 250 °C with very low As flux. The best film has T c on = 30.3 K and T c end = 22.9 K .

Intergrain effects in the AC susceptibility of polycrystalline LaFeAsO_{0.94}F_{0.06}: comparison with cuprate superconductors

Abstract: The AC susceptibility at zero DC magnetic field of a polycrystalline sample of LaFeAsO_{0.94}F_{0.06} (T_c = 24 K) has been investigated as a function of the temperature, the amplitude of the AC magnetic field (in the range Hac = 0.003 - 4 Oe) and the frequency (in the range f = 10 kHz - 100 kHz). The temperature dependence of the AC susceptibility exhibits the typical two-step transition arising from the combined response of superconduncting grains and intergranular weak-coupled medium. The intergranular part of the susceptibility strongly depends on both the amplitude and the frequency of the AC driving field, from few Kelvin below T_c down to T = 4.2 K. Our results show that, in the investigated sample, the intergrain critical current is not determined by pinning of Josephson vortices but by Josephson critical current across neighboring grains.

Mössbauer studies on FeSe and FeTe

Abstract: We carried out 57 Fe Mossbauer measurements for FeSe and Fe 1.08 Te to investigate the magnetic properties. There was no sign of magnetic ordering above 4.2 K for superconducting FeSe. The magnetic sextet corresponding to antiferromagnetic ordering of Fe in low-spin state was observed for non-superconducting Fe 1.08 Te.

Recent progress of high performance Ag-sheathed Bi2223 wire

Abstract: Recently, we have updated the highest J c and I c at 77 K, self-field, for our R&D short length tapes and I c × L at 77 K, self-field, for our kilometer-length manufacturing product. For our short length R&D tapes, we have updated the I c to 236 A. The highest J c has reached 663 A/mm 2 . Furthermore, we have broken our own world record performance of I c × L with the production of a 4.2 mm wide, 2084 m long Bi wire that carries an average current of 176 A. This has resulted in a new record—high wire performance of 367 k Am.

R&D of coated conductors for applications in Japan

Abstract: On the research and development of coated conductors in Japan, the I c characteristics and the length have been remarkably improved in the national project. Five hundred meter-long tapes with higher I c values than 300 A/cm-width were realized by the pulsed laser deposition (PLD) and the metal organic deposition using trifluoroacetates (TFA-MOD) processings for the superconducting layer on the IBAD-GZO buffered substrates. In order to realize the low cost by the increasing the production rate of the ion beam assisted deposition (IBAD) layer, the process of IBAD-MgO was developed and a 1000 m-long IBAD buffer tape was fabricated at an extremely high production rate of 1 km/h. On the other hand, the artificial pinning center has been introduced in both PLD and MOD processing. The BaZrO 3 nano-rods were aligned along the c -axis of GdBCO superconducting films by the PLD process. The J c value in the magnetic field parallel to the c -axis was remarkably improved. Additionally, the BaZrO 3 nano-particles were uniformly dispersed in YGdBCO films by the TFA-MOD process and the J c – B–θ property was confirmed to be isotropic. Based on the above-mentioned achievements, the new national project, whose main objective is the development of some electric power applications using a coated conductor, started in 2008. In this project, the improvement of the coated conductor includes the five sub-themes: (1) degradation of tapes, (2) high I c under magnetic field, (3) low AC loss, (4) high mechanical strength and high J e and (5) low cost and high yield.

A common thread

Abstract: The structures, the phase diagrams, and the appearance of a neutron resonance in the superconducting state provide phenomenological evidence which relate the heavy fermion, cuprate and Fe superconductors. Single- and multi-band Hubbard models have been found to describe a number of the observed properties of these materials so that it is reasonable to examine the origin of the pairing interaction in these models. Here based on the experimental phenomenology and studies of the momentum and frequency dependence of the pairing interaction for Hubbard-like models, we suggest that spin-fluctuation mediated pairing is the common thread linking this broad class of superconducting materials.

Electrochemical Li-intercalation into the Fe-based superconductor FeSe1−xTex

Abstract: We have successfully synthesized Li-intercalated Li y FeSe 1− x Te x ( x = 0, 0.25, 0.5, 0.75, 1; y ∼ 1) using the electrochemical technique. The powder X-ray diffraction measurements have revealed that the lattice parameters do not change through the Li-intercalation for all x values. The magnetic susceptibility measurements have revealed that the superconducting transition temperature, T c , does not change in spite of electron doping through the Li-intercalation for all x values. It has been concluded that the superconductivity in FeSe 1− x Te x is not sensitive to the carrier concentration.

Bulk superconductivity at 2.6 K in undoped RbFe2As2

Abstract: The iron arsenide RbFe_2As_2 with the ThCr_2Si_2-type structure is found to be a bulk superconductor with T_c=26 K The onset of diamagnetism was used to estimate the upper critical field H_c2(T), resulting in dH_c2/dT=-14 T/K and an extrapolated H_c2(0)=25 T As a new representative of iron pnictide superconductors, superconducting RbFe_2As_2 contrasts with BaFe_2As_2, where the Fermi level is higher and a magnetic instability is observed Thus, the solid solution series (Rb,Ba)Fe_2As_2 is a promising system to study the crossover from superconductivity to magnetism

Basic study of HTS magnet using 2G wires for maglev train

Abstract: There are several advantages by applying a high-temperature superconducting wire to an on-board superconducting magnet for the maglev train. At first, an increase of thermal capacity of superconducting coils contributes a stability of the superconducting state of the coils. In addition, a reliability of superconducting magnet improves by simplification of the magnet structure. And the weight of the superconducting magnet and the energy consumption of the on-board cryocooler will decrease. Therefore, we examined the possibility on application of the 2G wire with a high critical current density in a high magnetic field. We performed numerical analysis regarding the weight of a superconducting magnet and the energy consumption of an on-board cryocooler in consideration of the characteristics of the 2G wire. Furthermore, we have carried out the I c measurement for the commercial 2G wires under various experimental conditions such as temperature, magnetic field strength and angle. We also performed the trial manufacture and evaluation of I c characteristics for the small race track-shaped superconducting coil.

Correlation between critical current density and n-value in MgB2/Nb/Monel superconductor wires

Abstract: Field and temperature dependences of critical current density ( J c ) and n -value were evaluated from un-doped and carbon doped MgB 2 wires, supplied by the Hyper Tech Research, Inc. In order to explore the possibility of persistent current mode, the variation of n -values for the different MgB 2 conductors was studied in more detail. It was observed that the n -value decreased gradually with an increasing magnetic field and operating temperature. From the power-law relationship between the J c and the n -value, n ∝ J c m , we found that there was a strong relationship, m = 0.371, for both wires at 4.2 and 20 K. On the basis of our experiments, a larger J c led to a higher n -value, whether there was doping or not.

Progress in high-linearity multi-element Josephson structures

Abstract: This paper summarizes both theoretical and experimental studies aimed at synthesis of high-linearity multi-element Josephson structures. Both the dynamic range and the voltage response linearity are two conjugated characteristics that must be improved jointly. Increase in dynamic range is reasonably associated with increase of number of elements N in the Josephson-junction structures. To improve the voltage response linearity one can use special design of the array structures. The other way is based on use novel basic cell with is bi-SQUID capable of providing highly linear voltage response. Both the approaches were used in designs of the reported high-linearity multi-element Josephson structures.

Development of JT-60SA superconducting magnet system

Abstract: The upgrade of JT-60U magnet system to superconducting coils (JT-60SA) is progressing by both parties of Japanese government and European commission in the framework of the Broader Approach agreement. The magnet system for JT-60SA consists of 18 Toroidal Field (TF) coils, a Central Solenoid (CS) with four modules, six Equilibrium Field (EF) coils. The TF coil case encloses the winding pack and is the main structural component of the magnet system. The CS consists of four independent winding pack modules, which is support from the bottom of the TF coils. The six EF coils are attached to the TF coil cases through supports with flexible plates allowing radial displacements. The construction of CS and EF coils was started in 2008 by Japan Atomic Energy Agency (JAEA). The construction of TF coils will be started in 2009 by Fusion for Energy and European voluntary contributors. This paper introduces the design of the JT-60SA superconducting magnet system and activities for development of magnet components in Japan.

Pressure-induced superconductivity in non-centrosymmetric compound CeIrGe3

Abstract: We have carried out electrical resistivity experiments on the non-centrosymmetric compound CeIrGe3 under high pressures up to 24 GPa. CeIrGe3 is an antiferromagnet with the Neel temperature T N1 = 8.5 K and a successive magnetic transition at T N2 = 4.7 K at ambient pressure. With increasing pressure, T N1 and T N2 merge around 3 GPa, becomes unchanged up to 10 GPa, and indicates a step-like decrease up to 20 GPa. Further increasing pressure, we have found superconductivity above 20 GPa with a transition temperature of 1.6 K at 24 GPa. The upper critical field for the magnetic field along the tetragonal [0 0 1] direction is expected to be much larger than 15 T.

Synchronized operation of two serially connected Bi2212 THz emitters

Abstract: Synchronization phenomena of two rectangular THz emitting mesas separately located side by side at a distance of 200 μm are reported on a high- T c superconductor Bi 2 Sr 2 CaCu 2 O 8+ δ single crystal. With varying bias current, two emission peaks observed from two mesas at frequencies around 0.7 THz merge into one emission line and stay together until the emission stops by Joule heating. The frequency locking phenomenon accompanied by the increase of total radiation power suggests that a coherent operation is realized between two separate mesas.

Development of prototype DC superconducting cable for railway system

Abstract: High Temperature Superconducting (HTSC) wire has significant potential for railway system applications. HTSC wire is currently a promising candidate for various engineering applications such as transformers and motors for railway system. HTSC direct current (DC) cable is ideal for a feeder of the overhead contact line system between the substation and the electric train. We completed a prototype Bi-2223 tape based direct current cable for trial purposes of several meters length. In the energizing experiment the current of 1720 A successfully constantly flew.

Influence of thickness, width and temperature on critical current density of Nb thin film structures

Abstract: The density of critical currents jC in Nb thin films with thickness smaller than 15 nm and width between 100 nm and 10 μm has been measured in a wide temperature range. We have found that the temperature dependencies of jC in sub-micrometer wide bridges at 0.7TC < T < TC are well described by the Ginzburg–Landau de-pairing critical current. In wider bridges already at T < 0.9TC the jC value is significantly reduced due to the penetration and de-pinning of magnetic vortices.

New aspects of microwave properties of Nb in the mixed state

Abstract: We present a study of the frequency dependence of the vortex dynamics in a conventional superconductor. We have employed a swept frequency, Corbino disk technique to investigate the temperature (3.6 K -T c ) and high-field (from H c2 /2 to H c2 ) microwave complex resistivity in Nb thin (20-40 nm) films as a function of the frequency (1-20 GHz). We have found several previously unnoticed features: (i) a field-dependent depinning frequency in the GHz range; (ii) deviations from the accepted frequency dependence [1], than can be ascribed to some kind of vortex creep; and (iii) the presence of switching phenomena, reminiscent of vortex instabilities. We discuss the possible origin of the features here reported.