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To achieve the goal of zero emission in aviation, it is critical to advance electric aircraft technologies, especially in short to medium range flights. Superconducting fault current limiters (SFCLs) have shown promising potential in electric aircraft system as a protection solution to limit the peak of fault current. Many research works focused on the SFCL performance under low impedance faults, where the fault current is much higher than the nominal current and the SFCL will transit to normal (metal) state quickly. However, there are limited reports on investigating the SFCL reacting to high impedance faults for electric aircraft system, where the SFCL does not abruptly transit to normal state. In this paper, we built an electric-thermal coupled model in MATLAB which simulates the behaviour of a resistive type SFCL both under low and high impedance faults. We investigated three mathematical relations of local electric field E and local current density J representing the transition from superconducting state to normal state, both under high and low impedance fault. The three mathematical relations are classic E-J power-law, modified E-J power-law and two- stage E-J power law. The electric-thermal model of the SFCL incorporating all these three types of mathematical relations, respectively, have been used to calculate the SFCL responses under high and low impedance faults, and compared with each other, as well as testing results. The limited fault current, voltage drop, resistance and temperature variations in the SFCL are observed and analysed. The results have shown that the proposed SFCL model is able to accurately characterise a resistive type SFCL under both high impedance and low impedance fault. [ABSTRACT FROM AUTHOR]

Vortex motion in ferromagnetic-superconducting bilayers constituted by thin manganite La 0.67 Sr 0.33 MnO 3 film deposited on the top of thin YBa 2 Cu 3 O 7 − δ film has been investigated. Both films were deposited on uniformly twinned single-crystalline substrates and patterned into strips directed at different angles to the twin lines pattern. These conditions lead to the guided vortex motion at different angles to the periodic pinning potential. It has been established that critical current and flux flow resistance in such an anisotropic pinning landscape-scale follow the elliptic dependence on the angle of the direction of the strip. • Anisotropic properties of YBaCuO/LaSrMnO3 bilayers. • Vortex motion properties of YBaCuO/LaSrMnO3 bilayers. • YBaCuO/LaSrMnO3 bilayers on twinned substrates. [ABSTRACT FROM AUTHOR]

When a type-II superconductor, exposed to a perpendicular alternating magnetic field, carries a direct current, a biased dc voltage response can occur. The response voltage can be classified into dynamic-resistance voltage and flux flow voltage according to its triggering mechanism. Experimental results for the phase differences of the two types of voltage have been provided in the paper. Harmonic analysis on the voltage response has been carried out to differentiate the two types of the response voltages. Based on analyzing the experimental data, a useful and flexible real-time method using the ratio of the dc component to the second harmonic component of the voltage response has been proposed to serve as a monitor indicating the state of dc-carrying HTS tapes subject to an ac magnetic field. The method is promising in the application of complex superconducting systems such as flux pumps, persistent current switches, superconducting cables and dc fault current limiters. [ABSTRACT FROM AUTHOR]

• Addition of LSMO, leads to increase the grain connectivity and grain growth. • The superconducting transition temperature decreases by increasing of LSMO concentration, and in contrast, decreases the residual electrical resistivity. • By the application of magnetic field, the broadening of resistive transition refers to thermal energy have been explained by Josephson's coherence. • Thermally activated flux lines and activation energy are well explained by fitting the experimental data with Ambegaokar-Halperin model. • The resistivity vs. temperature plot above 100 K has explained by the weak localization theory. Thick films of YBa 2 Cu 3 O 7-δ /La 0.67 Sr 0.33 MnO 3 have been prepared by diffusion reaction technique. The phase formation and surface morphology have been analyzed through XRD and SEM. From temperature dependent resistivity measurement, under the application of magnetic field, the broadening of resistive transition in the grain boundary region attributes to dissipative of flux pinning. The transition temperature (T c) and onset of global superconductivity (T C0) marked decreasing value and the resistive broadening is more prominent below the critical temperature in presence of magnetic field for La 0.67 Sr 0.33 MnO 3 incorporated into YBa 2 Cu 3 O 7-δ. From the slope of the Arrhenius plot, we have calculated the activation energy for thermally activated flux flow. Using the Ambegaokar and Halperin phase-slip model, both the magnetic and electric field dependence of activation energy has been interpreted. The above phase slip model followed a power law as (1-t)q with q = 1/2 and t = (T/Tc) has been discussed. The field independent metallic region above 100 K that follows a weak localization effect is being verified. [ABSTRACT FROM AUTHOR]

Abstract A cylindrical bulk superconductor that undergoes magnetization processes endures enormous mechanical stresses, which will crack the bulk, especially with the applied field changing at high speed. In this paper, the impact of the relative rate of applied field db a / dt on both radial and hoop stresses in a long cylindrical superconductor is discussed by solving the Kim model with viscous flux flow. Two magnetization processes are considered during the applied field decreasing: zero-field cooling (ZFC) and field cooling (FC). The radial and hoop stresses increase with increasing db a / dt in ZFC and the maximum radial and hoop stresses are equal and appear at a plateau region by same db a / dt in FC. In ZFC, two cases are considered: B a > B * and B a ≤ B *, where B a is the applied field and B* is the magnetic field where all the body forces point away. When B a > B *, the flux front r 0 is away from the center of the cylinder with increasing db a / dt. However, when B a ≤ B *, r 0 is always at the center with different db a / dt. By discussing the parameter of db a / dt , we find the influence of db a / dt on both radial and hoop stresses is appreciable, which can be distinguished from previous works that have discussed the effect of viscous flux flow velocity (υ 0) on the radial and hoop stresses by Bean model with viscous flux flow equation. In addition, we find that the Kim model has a wider applicability than the Bean model. [ABSTRACT FROM AUTHOR]

Highlights • The effect of viscous flux flow on the flux profiles within the superconductor was investigated by solving Bean model and viscous flux flow equation simultaneously. • The variation in the normalized stress intensity factor considering viscous flux flow was obtained based on the plane strain approach and J-integral theory. • The results indicate that the effect of viscous flux flow on crack propagation is more significant compared with the crack length. Abstract In this paper, an attempt was made to study how viscous flux flow affects crack propagation in a long cylindrical superconductor by determining the stress intensity factor during the decreasing field stage after a zero-field cooling process. Under the assumption that the crack forms a perfect barrier to the flow of the shielding current, the effect of viscous flux flow on the flux profiles within the superconductor was investigated by solving the Bean model and viscous flux flow equation simultaneously. Based on the plane strain approach and J-integral theory, the variation in the normalized stress intensity factor considering viscous flux flow was obtained for different viscous flux flow velocities and crack lengths. The results indicate that when with the crack lengths are compared, the effect of viscous flux flow on crack propagation is more significant. [ABSTRACT FROM AUTHOR]

We reveal that the temperature dependence of the basal-plane normal-state electrical resistance of optimally doped YBa 2 Cu 3 O 7 − δ single crystals can be with great accuracy approximated within the framework of the model of s-d electron-phonon scattering. This requires taking into account the fluctuation conductivity whose contribution exponentially increases with decreasing temperature and decreases with an increase of oxygen deficiency. Room-temperature annealing improves the sample and, thus, increases the superconducting transition temperature. The temperature of the 2D-3D crossover decreases during annealing. [ABSTRACT FROM AUTHOR]

The fastest effective fault limitation is one of the selection criterion of high T c superconductors (HTS) for resistive type fault current limiter (R-SFCLs) application. In the present work, we have given a comparative DC & AC performance study on HTS tapes with and without stabilizer. At first, we have investigated the temperature dependent DC resistivity of the SS stabilized Bi2223 based conductor, Cu (50 µm substrate) and SS (75 µm substrate) stabilized YBCO based coated conductors (CCs) and compared with the stabilizer free YBCO based CC (50 µm and 100 µm substrate). We have studied the DC transport properties ( E–I ) of the stabilized and stabilizer-free tapes to obtain flux-flow resistance before reaching normal state, critical current and ‘n’ factor at 1 µV/cm criterion. Further, the 1st peak limitation, response time and resistance attained during fault limitation have been compared for AC over-current pulses of 1.5 kA for 100 ms duration. The aforesaid measurements helped us to select a suitable high T c superconductor for fastest effective fault limitation to be used in R-SFCL application. [ABSTRACT FROM AUTHOR]

The hot-electron vortex flow instability in superconducting films in magnetic field B at substrate temperature T 0 ≪ T c is theoretically considered in the presence of pinning. The magnetic field dependences of the instability critical parameters (electric field E *, current density j *, resistivity ρ *, power density P * and vortex velocity v *) are derived for a cosine and a saw-tooth washboard pinning potential and compared with the results obtained earlier by M. Kunchur [Phys. Rev. Lett. 89 (2002) 137005] in absence of pinning. It is shown that the B -behavior of E *, j * and ρ * is monotonic , whereas the B -dependence of v * is quite different, namely dv */ dB may change its sign twice , as sometimes observed in experiments. The simplest heat balance equation for electrons in low- T c superconducting films is considered within the framework of the two-fluid model. A theoretical analysis reveals that the instability critical temperature T * ≈ 5 T c /6 at T 0 < T */2 with T * being independent of B . [ABSTRACT FROM AUTHOR]

Measurements of critical current in NbTiN as a function of applied magnetic field and temperature are reported for two samples: 700-nm-wide bridge and 100-nm-wide meander. In 700-nm-wide NbTiN bridge we pinpointed the limiting factors for the critical current density to be current-driven vortex de-pinning at low temperatures and thermally activated flux flow closer to the transition temperature. In 100-nm-wide NbTiN meander we found phase slips activation, accompanied by hotspots formation at all measured temperatures. These two types of structures demonstrate different dependence of the critical current on the applied magnetic field. Although our NbTiN meander structures has high de-pairing critical current densities ∼10 7 A/cm 2 at low temperatures, the real critical currents are smaller due to the presence of the local constrictions. [ABSTRACT FROM AUTHOR]

A conformal pinning array can be created by conformally transforming a uniform triangular pinning lattice to produce a new structure in which the six-fold ordering of the original lattice is conserved but where there is a spatial gradient in the density of pinning sites. Here we examine several aspects of vortices interacting with conformal pinning arrays and how they can be used to create a flux flow diode effect for driving vortices in different directions across the arrays. Under the application of an ac drive, a pronounced vortex ratchet effect occurs where the vortices flow in the easy direction of the array asymmetry. When the ac drive is applied perpendicular to the asymmetry direction of the array, it is possible to realize a transverse vortex ratchet effect where there is a generation of a dc flow of vortices perpendicular to the ac drive due to the creation of a noise correlation ratchet by the plastic motion of the vortices. We also examine vortex transport in experiments and compare the pinning effectiveness of conformal arrays to uniform triangular pinning arrays. We find that a triangular array generally pins the vortices more effectively at the first matching field and below, while the conformal array is more effective at higher fields where interstitial vortex flow occurs. [ABSTRACT FROM AUTHOR]

Vortices in superconductor give rise to a rich variety of phenomena because they interact with shielding currents, temperature gradients, sample defects, boundaries, and other neighboring vortices. It would be very important to understand particular features of vortex states in a downsized system. Our study focuses on vortex states in small star-shaped Mo 80 Ge 20 plates with and without an artificial pin at the plate center. Vortex states are greatly influenced by the sample geometry, the temperature and the magnetic field, and they can be occasionally exotic compared to the bulk case. We use the amorphous Mo 80 Ge 20 films due to the nature of weak pinning in studying vortex configurations. We applied scanning superconducting quantum interference device (SQUID) microscopy because it enables us to see vortex states directly and it is the most sensitive instrument for mapping tiny local current flows or magnetic moments without damaging the sample. We interpreted that vortex configurations had essentially the nature of mirror reflection symmetry in both cases with an artificial pin and without an artificial pin and pinned cases while the influence of disorder was seen in our observation on the specimen without an artificial pin. [ABSTRACT FROM AUTHOR]

The THz radiation due to the flux flow in a superconductor slab at non-laminar regime has been investigated and the radiated power spectrum has been calculated. The parameter ( τ ) is defined to show amount of non-laminarity. The results reveal that for small values of τ, the system radiated at the harmonics of famous washboard frequency. However, for large values of τ , the radiation spectrum will be changed and for extreme values of τ , the peaks will be flat. Therefore the washboard picture is not valid anymore. The results show that the radiation power is optimum for the special value of τ . Also, the results compared with other theoretical and experimental data. [ABSTRACT FROM AUTHOR]

Highlights • This manuscript presents investigation of the current-voltage characteristics of phase-slip-free NbN nanostructured superconducting single photon detector (SNSPD) as a function of temperature and applied magnetic field. • Presented results are compared with data received for phase-slip-free submicron-wide single bridge NbTiN structure. • Our data demonstrate essential physical difference in transport properties and in curvature of current-voltage curves for this structures. Abstract Measurements of the electrical transport properties of the phase-slip-free 100-nm-wide NbN superconducting nanostructured single photon detector (SNSPD) as a function of temperature and applied magnetic field are reported. The observed characteristics suggest Abrikosov vortices tunneling (flux creep) behavior at low temperatures and low voltages followed by flux flow regime at higher voltages and domination of this regime close to the critical temperature. It differs from the behavior of phase-slip-free 700-nm-wide NbTiN bridge structure of similar length where flux creep is followed by abrupt jump into highly resistive critical state. [ABSTRACT FROM AUTHOR]

In this paper, the effect of viscous flux flow on magnetization and ac susceptibility in type-II superconductors is studied. The magnetization curves and ac susceptibility was further investigated by numerically solving both the Bean model and the viscous flux flow equations. In the unified picture, the fundamental ac susceptibilities of the slab as a function of temperature under different viscous flux flow velocities is derived, and some unique characteristics are reproduced. The results indicate that the critical state model can be well described by the viscous flux flow equation, which has a significant impact on magnetization and ac susceptibility. [ABSTRACT FROM AUTHOR]

• Superconducting properties of polycrystalline FeSe 1 − x S x S x (x = 0.1 and 0.2) alloys are investigated. • Impact of external hydrostatic pressure and chemical pressure on the superconducting state. • Anomalous behavior of superconducting transition temperature. • Non-Fermi liquid type behavior of normal state resistivity. • Signature of flux flow resistance in the isothermal resistivity data as a function of external magnetic field. Here we report on the magnetic and magneto-transport behavior of Fe-based superconductor with nominal compositions FeSe 1 − x S x (x = 0.1 and 0.2). The impact of external hydrostatic pressure (P) and chemical pressure on the superconducting state of the FeSe system has been investigated in this work. Superconducting transition temperature (T c) is found to be shifted towards higher temperatures with 10% S doping at the Se site of the FeSe alloy. On further increase in the doping concentration (for x = 0.2), the superconducting T c decreases. We have also observed a significant effect of external P on the superconducting properties of these S-doped alloys. Different superconducting parameters calculated from our experimental data confirm the type-II superconducting nature of the studied compounds. The signature of flux flow resistance is observed in the isothermal resistivity data as a function of the external magnetic field. [ABSTRACT FROM AUTHOR]

We investigated the dissipative regime of the Bi 2 Te 3 /FeTe topological insulator-chalcogenide interface superconductor at temperatures well below the Berezinski-Kosterlitz-Thouless transition. We observe a transition in the current-resistance and temperature-resistance curves that quantitatively agrees with the Likharev vortex-explosion phenomenon. In the limit of low temperatures and high current densities, we were able to demonstrate the regime of complete vortex-antivortex dissociation arising from current driven vortex-antivortex pair breaking. [ABSTRACT FROM AUTHOR]

• A brief overview of models predicting guiding angles is given. • The guiding effect has been experimentally studied depending on the magnetic field magnitude and the excitation current on the Nb-Ti tape with rarefied α-Ti particles – strong pinning centers. • The experimental data and models are compared. The model that describes the experiment most accurately is chosen. • It is shown that the guiding experiments can serve as a tool to distinguish between plastic and elastic flux flow modes. This article provides a brief overview of existing models that describe the guided flux motion phenomenon. The first model was proposed by Niessen et al. This original model utilizes the single-vortex approximation and provides a qualitative explanation of the effect; however, in most cases it provides an overestimated guiding angle. A second stochastic model explains the experimentally observed guiding angle decreasing (so-called "slipping effect") by the influence of thermal fluctuations. However, the performed estimations show that thermal fluctuations should not be significant. Another model for predicting the guiding angle is the anisotropic pinning model. This model operates in the critical state approximation, and the slipping effect is the combined action of anisotropic pinning and vortex interaction. The predictions of all three models were experimentally tested on a wide superconducting Nb-Ti tape containing 6% by volume ratio of α-Ti particles that act as strong pinning centers. The number of samples was sliced at different angles to the rolling direction to control the driving force direction with respect to the principal material directions. The transverse and longitudinal components of the electrical field were measured simultaneously in a perpendicular magnetic field with increasing transport current. It was found that when the generated electric field was sufficiently small, the guiding angle varied significantly at different locations along the sample. In this case, the anisotropic pinning model predicted the guiding angle averaged over the sample length. With a further increase in the transport current (and driving force), the guiding angle became uniform along the sample. [ABSTRACT FROM AUTHOR]

We report the results from resistivity and magnetic measurements on polycrystalline Ce oxypnictide (CeFeAsO 1– x F x ) samples where x spans from 0.13 to 0.25. We find that the orbital limiting field is as high as 150 T and it systematically decreases with increasing doping. The Maki parameter is greater than one across the phase diagram and the large Maki parameter suggests that orbital and Pauli limiting effects contribute to the upper critical field. The broadening of the superconducting transition in the resistivity data was interpreted using the thermally activated flux flow (TAFF) model where we find that the TAFF activation energy, U 0 ( B ), is proportional to B − γ from 1 T to high fields, and γ does not significantly change with doping. However, U 0 and the superconducting critical current, J c , are peaked in the mid-doping region ( x = 0.15–0.20), and not in the low ( x < 0.15) or high doping ( x > 0.20) regions. Furthermore, U 0 is correlated with J c and follows the two fluid model for granular samples. [ABSTRACT FROM AUTHOR]

We report a detailed study of the S doping effect on the critical current density, J c , and vortex pinning in FeSe single crystal. The value of J c for FeSe is found to be enhanced from ∼4 × 10 4 A/cm 2 at 2 K under self-field to ∼6 × 10 4 A/cm 2 after S doping. Similar field decaying rates of J c and the absence of fishtail effect in both FeSe and FeSe 0.86 S 0.14 single crystals indicate the similar pinning mechanism for both crystals, i.e., the dominance of strong point-like pinning from nanometer-sized defects or imperfections. On the other hand, vortex dynamics study manifests that the S doping reduces the size of flux bundle and enhances the effective pinning energy, which may come from the lattice distortion caused by S doping. [ABSTRACT FROM AUTHOR]

We measured the surface impedance of BaFe 2 (As 0.55 P 0.45 ) 2 single crystals under finite magnetic fields by using a microwave technique. We found that the penetration depth increased as λ ( T ) = λ ( 0 ) + A × T n with an exponent of n ≈ 1.7 in the temperature region of 0.1 < T / T c < 0.2 . The observed fractional exponent can be understood by considering that this material has gaps with lines of nodes and nodeless gaps with large anisotropy. The flux-flow resistivity showed an almost H -linear dependence and a remarkably rapid enhancement in the low- H region; ρ f / ρ n ≈ α H / H c 2 with a gradient of α > 2.5 . Such a behavior is similar to that of SrFe 2 (As,P) 2 . By comparing these with results of 111-materials and SrFe 2 (As 0.7 P 0.3 ) 2 , it has been clarified that the gap anisotropy dominates the gradient α of iron-based materials. [ABSTRACT FROM AUTHOR]

Highlights: [•] We theoretically study an impurity scattering effect on the vortex core structure in a chiral p-wave superconductor. [•] A low-temperature vortex core shrinkage (or Kramer–Pesch effect) is investigated. [•] The robustness of the Kramer–Pesch effect against an impurity scattering in the Born limit is lost in the unitary limit. [ABSTRACT FROM AUTHOR]

Highlights: [•] We measure the microwave-surface impedance under finite magnetic fields. [•] The penetration depth indicates the realization of the gapless superconductivity. [•] NaFe0.97Co0.03As single crystals have the moderately clean ore. [•] The flux-flow resistivity of NaFe0.97Co0.03As shows Bardeen–Stephen like behavior. [ABSTRACT FROM AUTHOR]

Abstract: Recent enlargement of the size and the performance enhancement of bulk superconductor lead to the difficulty of magnetic flux trapping by a pulsed-field magnetization. This paper proposes the bulk material with small holes which makes the role of an artificial magnetic flux path to supply the flux in the bulk. The fundamental experiment of applying a single pulsed-field with varying amplitudes was carried out using a ∅60mm × 20mm GdBa2Cu3O7− x bulk material with four ∅2mm holes filled up with solder. Compared with a usual hole-less bulk, the total magnetic flux and the maximum trapped flux density were greatly increased in a low applied field. Moreover, those large reduction caused by flux flow was suppressed in a high applied field. [Copyright &y& Elsevier]

Abstract: We examine the expected performance of a brushless motor/generator that uses trapped-flux (TF) bulk high-temperature superconductors (HTSs) to provide magnetomotive force, where the stator windings are used to create the TF. A key feature is the use of dysprosium (Dy) for the stator and rotor cores. We also examine methods to energize TF in HTS for generators used in pulsed-power applications. [Copyright &y& Elsevier]

Abstract: We report on electrical transport measurements at high current densities on optimally doped YBa2Cu3O7−δ thin films grown on vicinal SrTiO3 substrates. Data were collected by using a pulsed-current technique in a four-probe arrangement, allowing to extend the current–voltage characteristics to high supercritical current densities (up to 24MAcm−2) and high electric fields (more than 20V/cm), in the superconducting state at temperatures between 30 and 80K. The electric measurements were performed on tracks perpendicular to the vicinal step direction, such that the current crossed between ab planes, under magnetic field rotated in the plane defined by the crystallographic c axis and the current density. At magnetic field orientation parallel to the cuprate layers, evidence for the sliding motion along the ab planes (vortex channeling) was found. The signature of vortex channeling appeared to get enhanced with increasing electric field, due to the peculiar depinning features in the kinked vortex range. They give rise to a current–voltage characteristics steeper than in the more off-plane rectilinear vortex orientations, in the electric field range below approximately 1V/cm. Roughly above this value, the high vortex channeling velocities (up to 8.6km/s) could be ascribed to the flux flow, although the signature of ohmic transport appeared to be altered by unavoidable macroscopic self-heating and hot-electron-like effects. [Copyright &y& Elsevier]

Abstract: A pulsed-filed magnetization (PFM) is an important technique for industrial applications of superconducting bulk magnets, and several advanced PFM methods are proposed to enhance the trapped field. In the well-known IMRA method, the channel through the magnetic flux is formed by the flux flow caused by heat generation when applying the strong pulsed-field, and the magnetic flux is made to penetrate into the bulk through the channel in the following pulse application. On the other hand, large applied field leads to large heat generation, and, therefore, the trapped field is decreased greatly. This paper proposes an effective magnetizing method in which the channel composed of magnetic field is artificially formed by field-cooling (FC) using a permanent magnet and the magnetic flux by PFM is induced to the channel. To confirm the validity of this method, the bulk was magnetized by FC using Nd–Fe–B magnets of the rectangular and the ring shapes, and thereafter, a pulsed-field of 6.2T was applied. As a result, the trapped field of the bulk magnetized by FC using the ring magnet was increased by about 20–25% as compared with that of the conventional PFM, and, moreover, it was observed that the channel was formed partially by measurement of the magnetic field distribution. [Copyright &y& Elsevier]

Abstract: We studied the YBa2Cu3O7 thin films with arrays of antidots by imaging the resistive photoresponse with a laser scanning microscope. The films were initially covered by a thin Au layer, which we removed in order to improve the spatial resolution and imaging contrast. This procedure did not change the superconducting properties of the samples. The improved spatial resolution allowed for visualizing the individual flow channels of vortices. The reported measurements with the laser scanning microscope feature direct measurements of the local critical currents of the samples. We show that minor variations of the bias current leave the spatial distribution of vortex flow channels unchanged. [Copyright &y& Elsevier]

Abstract: We report various RF induced effects on Josephson vortex flow of Bi2Sr2CaCu2O8+ y intrinsic Josephson junctions in vortex states being different with existence of pancake vortices. For pancake free state, RF induced zero-crossing steps, which had been found in ordinary Josephson flux flow oscillators, were first observed at voltage V = NΦ 0 f for N-stacked intrinsic Josephson junctions. In contrast the RF irradiations to the junctions in mixed state with pancakes those step responses changed features similar to Shapiro steps at Vn = nNΦ 0 f with order n. The former evidently demonstrates that electromagnetic waves propagate in the intrinsic Josephson junctions interacting with moving vortices. The later indicates that Josephson vortices move at the same speed coherently. [Copyright &y& Elsevier]

Abstract: We study the vortex phase diagram and dynamics in the low-temperature liquid phase of a thick amorphous a-Mo x Ge1−x film with weak pinning. The vortex solid state is ordered vortex lattice or Bragg glass, which is different from the disordered vortex glass of a-Mo x Si1−x films with stronger pinning. We find, however, that the vortex liquid state is similar to that for the a-Mo x Si1−x films, involving the quantum-vortex-liquid phase at low temperature, where we again detect the unusual vortex flow. [Copyright &y& Elsevier]

Abstract: Dissipation by the vortex motion in cuprate superconductors was investigated by the microwave surface impedance measurement. We discuss our experimental results, with particular interest in comparison with the quasiparticle scattering time in the vortex core with that in the Meissner state, and in the effect of impurity doping. It was found that the moderately clean core picture was rather generic in the high-T c cuprate. This means the importance of the Andreev reflection at the core boundary for the dissipation by the vortex motion. We argue that the large dissipation by the motion of vortex core is the manifestation of the novel mechanism characteristic of the moderately clean core. [Copyright &y& Elsevier]

Abstract: We report on the effects of perforating submicron holes on the vortex dynamics of amorphous Nb0.7Ge0.3 microbridges in the strongly nonequilibrium mixed state, when vortex properties change substantially. In contrast to the weak nonequilibrium—when the presence of holes may result in either an increase (close to T c) or a decrease (well below T c) of the dissipation, in the strong nonequilibrium an enhanced dissipation is observed irrespectively of the bath temperature. Close to T c this enhancement is similar to that in the weak nonequilibrium, but corresponds to vortices shrunk due to the Larkin–Ovchinnikov mechanism. At low temperatures the enhancement is a consequence of a weakening of the flux pinning by the holes in a regime where electron heating dominates the superconducting properties. [Copyright &y& Elsevier]

Abstract: We have studied the effect of preloading on the relaxation of the levitation force of bulk Y–Ba–Cu–O superconductors. The levitation force was measured by applying the external field with a superconducting solenoid. We set the standard magnetic induction (B st) to be 1.2T supposing that the bulk Y–Ba–Cu–O disk experiences this field in operation. Preloading was applied such that the external field was first ramped to 1.1B st and reduced to B st. We confirmed that the preloading was effective in reducing the relaxation of the levitation force, although the effective levitation force was largely depressed compared to that without preloading. The employment of an extra step to lower the field below B st before reaching B st was effective in improving the effective levitation force without sacrificing the ability to reduce the relaxation rate. This is qualitatively understood in terms of the field distribution inside the disk based on the critical state model. [Copyright &y& Elsevier]

Abstract: We have studied the vortex dynamics driven by the constant current I in the Meissner phase of a thick amorphous Mo x Si1−x film with the Corbino-disk contact geometry. In the certain I region we have observed unusual large voltage pulses that oscillate almost periodically with a period t 0 larger than ∼15 s and with the pulse width t w in the range 5–10s. With increasing I, t 0 initially decreases and then increases. Over the whole I region where the oscillation of V(t) is observed, both t 0 and t w are much longer than the characteristic time for free vortices to rotate in the circles. At high I a small peak(s) or cusp-like structure exceeding the normal-state voltage appears on the top (plateau) region of the pulses. These results indicate that the observed voltage pulses originate from large vortex-density fluctuations of moving vortices. [Copyright &y& Elsevier]

Abstract: We present measurements of the time(t)-dependent component of the flux-flow voltage δV(t) about the average voltage in the vortex-liquid phase of a thin amorphous Mo x Si1−x film. At low temperatures T, δV(t) due to the vortex motion is clearly visible and the distribution of δV(t) is asymmetric, indicative of large velocity and/or number fluctuations of driven vortices. This result is very similar to that which has been observed in the quantum-liquid phase of a thick film. The anomalous vortex motion for the thick and thin films is found to occur in nearly the same reduced-temperature regime. Also, we find that for the thin film the anomalous motion is visible only in the high-field regime. We suggest that vortex dynamics in the low-T liquid phase of thin and thick films is dominated by common physical mechanisms related to quantum effects. [Copyright &y& Elsevier]

We have studied vortex dynamics induced by dc current I in the Meissner phase of a thick amorphous MoxSi1-x film with the strip-like and Corbino-disk (CD) contact geometries. We have measured the time evolution of voltage V(t) induced by I. It is found that only in the CD unusual large voltage pulses appear almost periodically with a period of typically 40 s and with the pulse width of about 5 s. Both times are much longer than the characteristic time for free vortices to rotate (half) in the circles, indicating that the observed voltage pulses originate from collective motion of many vortices which leads to large density fluctuations of moving vortices. [Copyright &y& Elsevier]

To study the change in vortex dynamics associated with the change in equilibrium vortex states from the thermal to quantum-vortex-liquid (QVL) upon cooling, we have performed measurements of the fluctuating [time (t)-dependent] component of flux-flow voltage V(t), δV(t)≡V(t)-V0, about the average voltage V0 using a thick amorphous MoxSi1-x film. Only in the QVL phase, large δV(t) is observed. The distribution of δV(t) is asymmetric having a tail along the direction of flux motion [δV(t)>0], suggesting the presence of anomalous flux-flow motion in the QVL phase. [Copyright &y& Elsevier]

The reactive liquid infiltration (RLI) technique, applied to the MgB2 material, has allowed us to obtain either bulk or wire samples of good transport properties. The evaluation of the voltage–current characteristics, V–In, for typical samples, measured by transport at temperatures between 4.2 and 30 K, is presented. The behaviour of the n exponent is discussed in terms of a simple model and the differences between bulk and wire samples are outlined. [Copyright &y& Elsevier]

The attention of the fusion magnet specialists has been drawn in the last years to the volt–ampere characteristic (VAC) of the large cable-in-conduit conductors (CICC), as early voltage was observed in conductor and model coil tests, far away from the expected critical current. In this paper, the studies and models developed in the last decades for the VAC of bulk material and multifilament strands are shortly reviewed. The practical role of the index n of the VAC, both at the industry and in the magnet design is discussed for composite strands and for cabled conductors. Finally, few flashes of experimental results on the VAC of large CICC, all measured in the SULTAN test facility, are presented and discussed, with recommendation for test, QA and design for the fusion conductors. [Copyright &y& Elsevier]

It is pointed out theoretically that the electric field, E, vs. current density, J, characteristic of the vortex glass state is different from that predicted by M.P.A. Fisher; that is, the electric resistivity, ρ=E/J has a finite value even at J→0, while the Fisher theory predicts ρ→0 with J→0. It is also pointed out that the vortex glass–liquid transition is merely a kind of the bifurcation transition in the mixed state of type-2 superconductors containing pinning centers, which may be called the thermal depinning transition between the pinning state and the depinning state of fluxoids resulting from the thermal agitation on fluxoids. Furthermore, it is shown theoretically that only the flux flow resistivity obeys the scaling law near the thermal depinning transition temperature, Tdp, while the flux creep resistivity approaches a finite value as J is decreased to zero, even in the pinning state below Tdp. When ρ is measured down to a very small level of E, therefore, the noticeable deviation from the scaling law of ρ against J that was predicted by Fisher is expected to appear due to the above-mentioned behavior of the flux creep resistivity. The above theoretical conclusion that is contrary to the theoretical prediction by Fisher, however, seems to be supported by the recently observed data over a very wide range of the electric field, E provided by Kodama et al., because the present theoretical expression for the E vs. J, characteristics agrees quantitatively with these observed data. [Copyright &y& Elsevier]

The time evolution of the shielding current density in the high-Tc superconductor (HTS) is investigated by use of the numerical simulation and, on the basis of its result, the spectrum of the induced magnetic fields is analyzed. To simulate the crystallographic anisotropy in the shielding current density of the HTS plate, the multiple-thin-layer approximation is assumed. Under the assumption, the behavior of the shielding current density is expressed as an integro-differential equation. The numerical code to solve the initial-boundary-value problem of the equation has been developed by using the FEM and the θ-method. By use of the code, the shielding current analysis is performed for the coil-HTS system in which the disk-shaped HTS plate is exposed to the ac magnetic field generated by the one-turn coil. The magnetic field induced by the shielding current density is Fourier-transformed and its power spectrum is evaluated. [Copyright &y& Elsevier]

The vortex dynamics at high velocities in niobium and niobium/gold thin films is studied by taking notice of the role of the metallic layer. The proximity effect and its influence on superconducting properties is first identified. We report then in a flux flow regime an abrupt transition to the normal state which has been predicted theoretically by Larkin and Ovchinnikov. The influence of the metallic layer on the parameters of this instability is studied. It demonstrates that the gold film decreases the critical force and the vortex velocity driving the transition. [Copyright &y& Elsevier]

A 3-μm long intrinsic Josephson junction has been fabricated using a Bi2Sr2CaCu2Ox thin film. Current vs voltage measurement shows characteristics of a fivefold stacked Josephson junction. In the magnetic field (1–2 T) parallel to the junction planes, the current vs voltage curves at the low voltage region exhibit fine sub-branch structures. Numerical simulation results show fine structures as well, which are due to the flux-flow cavity resonance, and support that the in-phase motion may occur in the present experimental conditions. [Copyright &y& Elsevier]

The current carrying dissipative state well above the critical current it is known to be related to the pinning properties of the material and to the microscopic mechanisms of vortex dynamics. Moreover, it has been demonstrated that in low temperature superconducting films exhibiting moderately strong pinning the light ion irradiation has the effect of changing the distribution of the pinning centers without changing their pinning strength and this results into an increase of current stability in the flux flow state. Here we present the results of light ion irradiation on weak pinning superconducting films focusing on the influence of pinning properties of the material in the flux flow state. We realize that the possibility to switch to low dissipations by changing weak pinning is not straightforward. [ABSTRACT FROM AUTHOR]

In this work we solved the time dependent Ginzburg-Landau equations numerically finding profiles of the flux-flow resistivity for different widths of superconducting stripes. We found vortex pinning induced by the surface superconductivity. This pinning avoids the movement of the vortex lattice preventing the generation of a voltage. We also found the existence of a mesoscopic region where the flux-flow resistivity shows size effects and we observed a transition to a macroscopic regime as the width increases. [ABSTRACT FROM AUTHOR]

• A complex model is developed to estimate the effect of the electromagnetic force on fracture behavior in slab superconductor where the viscous flux flow and flux creep have been considered. • The equation for the flux profiles is modified in an infinite slab superconductor to consider the effect of flux creep and viscous flux flow. • The results indicate that both viscous flux flow and flux creep should be considered after ZFC process in order to avoid cracking within the superconductor slab. A relatively complex model is developed to estimate the effect of the electromagnetic force on fracture behavior in a slab superconductor with a center-situated crack, where the viscous flux flow and flux creep have been considered. The effects of the viscous flux flow and flux creep on the stress intensity factors and flux profiles are discussed for the descending field. The results indicate that both viscous flux flow and flux creep should be considered in zero-field cooling (ZFC) process in order to avoid cracking within the superconductor slab, which may be useful to researchers interested in cracking and mechanical failure of superconductors. [ABSTRACT FROM AUTHOR]

• From the flux flow instability in current-voltage characteristics of NbN films, the critical vortex velocity (ʋ *) and quasiparticle inelastic scattering time (τ E) of NbN films can be obtained within the framework of the Larkin-Ovchinnikov theory. • The critical vortex velocity ʋ *, at which the instability occurs, led to the transition from the power law ʋ *∝ B −1/2 dependent to field-independent behavior, while the nonequilibrium quasiparticles are uniformly distributed. • The quasiparticle inelastic scattering time obeys T −n temperature dependence, where the exponent is 2.5 and 2.3 for the 10 and 15-nm-thick NbN films, respectively, indicating a dominant electron-phonon scattering relaxation mechanism in disordered NbN films. • At 8 K, the value of τ E decreases from 88 to 12 ps, and film thickness decreased from 15 to 10 nm. Thinner films films with shorter τ E is a feature that could be desirable in the design of rapid photon detectors. A deep understanding of the quasiparticle inelastic scattering rate in superconducting materials is beneficial for the further development of ultrasensitive and rapid superconducting detectors. From the flux flow instability in current–voltage characteristics of NbN superconducting films, we have derived the critical vortex velocity (ʋ*) and quasiparticle inelastic scattering time (τ E) of the 10- and 15-nm-thick NbN films at different magnetic fields and temperatures within the framework of the Larkin–Ovchinnikov model. The critical vortex velocity drove the transition from the power law ʋ * ∝ B −1/2 dependent to field-independent behavior where the nonequilibrium quasiparticles are uniformly distributed. The inelastic scattering time of quasiparticles obeys T −n temperature dependence, where the exponents are 2.5 and 2.3 for the 10 and 15-nm-thick NbN films, respectively. As the thickness of the film decreases from 15 to 10 nm, a reduction of τ E from 88 to 12 ps is observed at 8 K. Moreover, the small value of τ E for ultra-thin films can be associated with the low-average value of the superconducting gap, which is precisely what the detector needs. [ABSTRACT FROM AUTHOR]

Abstract: We have fabricated Nb square superconducting networks and measured critical current density with current flowing along the direction of 0° and 45° tilted from the lattice of the networks. The temperature and magnetic field dependences of the critical current density for both directions are measured and compared with the flux penetration pattern in those samples. It is found that the anisotropy of the critical current density reflects the flux penetration pattern in the superconducting network samples. Near T c at zero field, anomalous enhancements of voltage for 0° are observed. [Copyright &y& Elsevier]

Abstract: Flux flow resistivity above critical temperature is analyzed based on the increased evidences that high temperature superconductors are intrinsically inhomogeneous, and local nonpercolating superconducting domains persist high above the critical temperature. It was found that above a certain field, the resistivity originated in the flow of flux motion starts to decrease with increasing field, due to the predominance of the suppression of the superconducting droplets over the increase of vortex core density. [Copyright &y& Elsevier]

Abstract: We discuss a flux flow resistivity in the system with two s-wave superconducting gaps. Our result agrees well with anomalous field dependence of the resistivity recently observed in the vortex state of MgB2. We comment on the possibility of the presence of vortex with irrational flux, which is represented as the unit flux times an irrational number. [Copyright &y& Elsevier]