Flux-flow Voltage Research Articles

Development and performance of a 2.9 Tesla dipole magnet using high-temperature superconducting CORC® wires

Although the high-temperature superconducting (HTS) REBa2Cu3O x (REBCO, RE–rare earth elements) material has a strong potential to enable dipole magnetic fields above 20 T in future circular particle colliders, the magnet and conductor technology needs to be developed. As part of an ongoing development to address this need, here we report on our CORC® canted cosθ magnet called C2 with a target dipole field of 3 T in a 65 mm aperture. The magnet was wound with 70 m of 3.8 mm diameter CORC® wire on machined metal mandrels. The wire had 30 commercial REBCO tapes from SuperPower Inc. each 2 mm wide with a 30 µm thick substrate. The magnet generated a peak dipole field of 2.91 T at 6.290 kA, 4.2 K. The magnet could be consistently driven into the flux-flow regime with reproducible voltage rise at an engineering current density between 400–550 A mm−2, allowing reliable quench detection and magnet protection. The C2 magnet represents another successful step towards the development of high-field accelerator magnet and CORC® conductor technologies. The test results highlighted two development needs: continue improving the performance and flexibility of CORC® wires and develop the capability to identify locations of first onset of flux-flow voltage.

Three-Terminal Superconducting Digital Transistor

Discrete array of small Josephson junctions can exhibit flux-flow behavior, which is drastically different from that of long Josephson junctions. In particular, the flux-flow voltage does not monotonically increase with magnetic field; rather, it decreases after a threshold until it reaches zero. Here, using simulation and experimental results, we show that the flux-flow voltage switches abruptly to zero if a magnetic field larger than the threshold is applied to a part of the array. This provides for a digital NOT gate in which the flux-flow voltage corresponds to digital state 1 and zero voltage denotes digital state 0. For a device size of 230 μm, a switching frequency of 15 GHz is evaluated, which can be improved by downsizing the device. Since this device provides gain, its input and output are isolated and has unlimited fan-out, designing and implementing various digital systems is easily accomplished.

Second harmonic in the voltage of dc-carrying YBCO tape under a perpendicular alternating magnetic field

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.

Intrinsic Josephson junctions in the iron-based multi-band superconductor (V2Sr4O6)Fe2As2

In layered superconductors, Josephson junctions may be formed within the unit cell due to sufficiently low interlayer coupling. These intrinsic Josephson junction (iJJ) systems have attracted considerable interest for their application potential in quantum computing as well as efficient sources of THz radiation, closing the famous "THz gap". So far, iJJ have been demonstrated in single-band, copper-based high-Tc superconductors, mainly in Ba-Sr-Ca-Cu-O. Here we report clear experimental evidence for iJJ behavior in the iron-based superconductor (V2Sr4O6)Fe2As2. The intrinsic junctions are identified by periodic oscillations of the flux flow voltage upon increasing a well aligned in-plane magnetic field. The periodicity is well explained by commensurability effects between the Josephson vortex lattice and the crystal structure, which is a hallmark signature of Josephson vortices confined into iJJ stacks. This finding adds (V2Sr4O6)Fe2As2 as the first iron-based, multi-band superconductor to the copper-based iJJ materials of interest for Josephson junction applications, and in particular novel devices based on multi-band Josephson coupling may be realized.

A comparator based on soliton antisoliton pair generation in soliton diode

Modified sine-Gordon equation is solved for a soliton diode, which is a long Josephson junction in which the magnetic field of the control current reverses its direction at the center. Applying a properly directed bias current can start the spontaneous generation of soliton and antisolitons at the transition region of the diode and leads to a flux-flow voltage. The threshold bias current at which such a voltage generation starts is a function of the control current and the size of the transition region. Such phenomena can find applications in high-speed decision making electronic comparators.

Transition from slow Abrikosov to fast moving Josephson vortices in iron pnictide superconductors

Iron pnictides are layered high T(c) superconductors with moderate material anisotropy and thus Abrikosov vortices are expected in the mixed state. Yet, we have discovered a distinct change in the nature of the vortices from Abrikosov-like to Josephson-like in the pnictide superconductor SmFeAs(O,F) with T(c)~48-50 K on cooling below a temperature T*~41-42 K, despite its moderate electronic anisotropy γ~4-6. This transition is hallmarked by a sharp drop in the critical current and accordingly a jump in the flux-flow voltage in a magnetic field precisely aligned along the FeAs layers, indicative of highly mobile vortices. T* coincides well with the temperature where the coherence length ξ(c) perpendicular to the layers matches half of the FeAs-layer spacing. For fields slightly out-of-plane (> 0.1°- 0.15°) the vortices are completely immobilized as well-pinned Abrikosov segments are introduced when the vortex crosses the FeAs layers. We interpret these findings as a transition from well-pinned, slow moving Abrikosov vortices at high temperatures to weakly pinned, fast flowing Josephson vortices at low temperatures. This vortex dynamics could become technologically relevant as superconducting applications will always operate deep in the Josephson regime.

Controlling flux flow dissipation by changing flux pinning in superconducting films

We study the flux flow state in superconducting materials characterized by rather strong intrinsic pinning, such as Nb, NbN, and nanostructured Al thin films, in which we drag the superconducting dissipative state into the normal state by current biasing. We modify the vortex pinning strength either by ion irradiation, by tuning the measuring temperature or by including artificial pinning centers. We measure critical flux flow voltages for all materials and the same effect is observed: switching to low flux flow dissipations at low fields for an intermediate pinning regime. This mechanism offers a way to additionally promote the stability of the superconducting state.

Qualitative analysis on adiabatic and dynamic stability of LTS/HTS hybrid conductor with concentric configuration

The I–V characteristic of a superconductor is generally described by power-law, in which the superconductor with a high n value transfers quickly from superconducting state to the normal conducting state. With a high transport current, in the low n value area, flux flow voltage becomes lower than in the high n value area, so that the transient characteristics strongly affect its stability. Based on those properties, we propose a new hybrid conductor which is made of low temperature superconductor (LTS) and high temperature superconductor (HTS) with concentric configuration in which the HTS coat is located outside of the LTS core. According to their power-law models, the modified adiabatic and dynamic stability criteria are qualitatively obtained by taking account into not only their critical currents but also n values. As a result, the new hybrid conductors have potential applications with higher engineering current density and improved stability.

Nonlinear effects at high flux-flow electric fields

Ohm’s law with the linear relation between resistive voltage and electric current is strictlyvalid only in the limit of infinitesimally small voltages. On the other hand, at finite electricvoltages nonlinearities in the electric resistance can develop due to the energy picked up bythe charge carriers in the electric field. This can lead to important effects both inthe case of semiconductors and of superconductors, where the energy rise of thecharge carriers or the quasiparticles can become relatively large. In this paperwe limit our discussion to the flux-flow voltage in the mixed state of a type-IIsuperconductor. At sufficiently low temperatures the energy dependence of thequasiparticle density of states and, hence, of the quasiparticle scattering rate can causedistinct nonlinear effects in the flux-flow resistance. The recent advances in thin-filmsample preparation provided new opportunities for observing nonlinear effects of the latterkind.

Suppression of dissipation in Nb thin films with triangular antidot arrays by random removal of pinning sites

The depinning current ${I}_{c}$ versus applied magnetic field $B$ close to the transition temperature ${T}_{c}$ of Nb thin films with randomly diluted triangular arrays of antidots is investigated. Our experiments confirm essential features in ${I}_{c}(B)$ as predicted by Reichhardt and Olson Reichhardt [Phys. Rev. B 76, 094512 (2007)]. We show that, by introducing disorder into periodic pinning arrays, ${I}_{c}$ can be enhanced. In particular, for arrays with fixed density ${n}_{p}$ of antidots, an increase in dilution ${P}_{d}$ induces an increase in ${I}_{c}$ and decrease of the flux-flow voltage for $B>{B}_{p}={n}_{p}{\ensuremath{\Phi}}_{0}$.

Picovoltmeter for probing vortex dynamics in a single weak-pinning Corbino channel

We have developed a picovoltmeter using a Nb dc superconducting quantum interference device for measuring the flux-flow voltage from a small number of vortices moving through a submicron weak-pinning superconducting channel. We have applied this picovoltmeter to measure the vortex response in a single channel arranged in a circle on a Corbino disk geometry. The circular channel allows the vortices to follow closed orbits without encountering any sample edges, thus eliminating the influence of entry barriers.

Evaluation of Stability of NbTi Multi-Strand Conductor by Induced-Current Methods

NbTi cable-in-conduit-conductors (CICC's) are employed to fabricate plasma fusion experimental apparatuses. NbTi is a preferable material in terms of low cost and good manufacturability. In contrast, it shows lower stability compared to materials for high magnetic field applications, e. g. Nb3Sn, due to its small critical temperature. A characteristic feature in NbTi multi-strand cable is so called avalanche-like quench, which is specific instability in a parallel electric circuit consisting of superconducting lines. To evaluate stability of a NbTi cable, we employed induced-current methods. Current is increased and decreased by constant voltage source, i. e. magnetic flux change produced by the bias magnet. We evaluated the quenches and equilibrium condition between heat generation due to flux flow voltage and heat transfer to surroundings at critical current. A parameter of experiment was a ramp rate of the bias field that corresponds to electric field in the cable. Of course, the cable was stable with smaller electric field and/or critical current. To survey the influence of heat dissipation during this equilibrium, we changed the surrounding mediums; those are liquid helium, frozen ethanol and ice. Ice is the material with large thermal conductivity, therefore this results in good stability of the cable.

In-plane current-voltage characteristics and oscillatory Josephson-vortex flow resistance in La-freeBi2+xSr2−xCuO6+δsingle crystals in high magnetic fields

We have investigated the in-plane $I\text{\ensuremath{-}}V$ characteristics and the Josephson-vortex flow resistance in high-quality La-free ${\text{Bi}}_{2+x}{\text{Sr}}_{2\ensuremath{-}x}{\text{CuO}}_{6+\ensuremath{\delta}}$ (Bi-2201) single crystals in parallel and tilted magnetic fields at temperatures down to 40 mK. For parallel magnetic fields below the resistive upper critical field ${H}_{c2}^{\ensuremath{\ast}}$, the $I\text{\ensuremath{-}}V$ characteristics obey a power law with a smooth change with increasing magnetic field of the exponent from above 5 down to 1. In contrast to the double-layer cuprate Bi-2212, the observed smooth change suggests that there is no change in the mechanism of dissipation (no Kosterlitz-Thouless transition) over the range of temperatures investigated. At small angles between the applied field and the $ab$ plane, prominent current steps in the $I\text{\ensuremath{-}}V$ characteristics and periodic oscillations of Josephson-vortex flow resistance are observed. While the current steps are periodic in the voltage at constant fields, the voltage position of the steps, together with the flux-flow voltage, increases nonlinearly with magnetic field. The $ab$-flow resistance oscillates as a function of field with a constant period over a wide range of magnetic fields and temperatures. The current steps in the $I\text{\ensuremath{-}}V$ characteristics and the flow resistance oscillations can be linked to the motion of Josephson vortices across layers.

Vortex phase diagram and vortex dynamics at low temperature in a thick a-Mg xB 1− x film

We report on the equilibrium vortex phase diagram and vortex dynamics at low temperature T in a thick amorphous (a-)Mg x B 1− x film based on the measurements of the dc resistivity ρ and time ( t)-dependent component of the flux-flow voltage, δ V( t), respectively. Both ρ( T) in perpendicular fields and the vortex phase diagram are qualitatively similar to those for the a-Mo x Si 1− x films, in which evidence for the quantum-vortex-liquid (QVL) phase has been obtained. In either material system we observe anomalous vortex flow with the asymmetric distribution of δ V( t) in the QVL phase, suggesting that the anomalous flow is a universal phenomenon commonly observed for disordered amorphous films, independent of material.

Slowing down the Josephson vortex lattice inBi2Sr2CaCu2O8+δwith pancake vortices

We study theoretically and experimentally the influence of pancake vortices on motion of the Josephson vortex lattice in layered high-temperature superconductors. Mobility of the Josephson vortices in layered superconductors is strongly suppressed by a small amount of pancake-vortex stacks. Moving Josephson vortex lattice forces oscillating zigzag deformation of the pancake-vortex stacks contributing to damping. The salient feature of this contribution is its nonmonotonic dependence on the lattice velocity and the corresponding voltage. Maximum pancake effect is realized when the Josephson frequency matches the relaxation frequency of the stacks. The pancake-vortex damping is strongly suppressed by thermal fluctuations of the pancake vortices. This theoretical picture was qualitatively confirmed by experiments on two mesas prepared out of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ whiskers. We found that the Josephson-vortex flux-flow voltage is very sensitive to small $c$-axis magnetic field. The pancake-vortex contribution to the current indeed nonmonotonically depends on voltage and decreases with increasing temperature and in-plane magnetic field. We also found that irradiation with heavy ions has no noticeable direct influence on motion of the Josephson vortices but dramatically reduces the pancake-vortex contribution to the damping of the Josephson vortex lattice at low temperatures.

Electromagnetic Properties of Lap-Joint and Its Influences on the Conductor Test Data

A joint between superconducting cables is one of key elements in a large-scale magnet. Joint-resistance-induced nonuniform current distribution has been reported. The mechanism of this phenomenon is quite simple. The cable consists of the twisted superconducting strands. Therefore, in the DC condition, the current distribution is governed by the nonuniformity of individual sub-joint-resistances between twisted strands in different legs; those are mainly determined by the joint configuration. When one strand carries larger current and it saturates at the critical current, flux-flow resistance occurs. That nonlinear resistance functions as a current regulator. Once the current saturates in one strand, surplus current must go through other strands. Apparent critical current, n-value and current sharing temperature (Tcs) of the CICC are important parameters for magnet design. However these have been reported to be degraded from the expected values predicted from the strand performances. We analyzed the apparent evolution of flux-flow voltage along the cable considering the current nonuniformity due to the lap-joint quality

Vortex dynamics in two-dimensional amorphous films without edge effects

Through the comparative studies using the thin amorphous MoxSi1−x films with the Corbino-disk and strip-like contact geometries, we have demonstrated that edge pinning is not important on the static or dynamic vortex response over the broad temperature (T) and field ranges. Thus, both the field-driven superconductor–insulator transition and anomalous vortex motion observed so far in the low-T liquid phase of thin amorphous films are confirmed to be intrinsic properties. Also, we suggest that the real-time measurements of voltage fluctuations about the average flux-flow voltage may be very useful in detecting the onset of quantum-fluctuation effects.

Flux-flow noise driven by quantum fluctuations in a thick amorphous film

We measure the voltage-noise spectrum SV(f) (where f is a frequency) as well as the time (t)-dependent component δV(t) of the flux-flow voltage in the low temperature liquid phase of a thick amorphous MoxSi1−x film. In the quantum-liquid phase both the amplitude ∣δV∣ of voltage fluctuations and the asymmetry of the probability distribution of δV(t) show an anomalous increase; the spectral shape of SV(f) is of a Lorentzian type, suggesting the shot-noise-like vortex motion with a large “vortex-bundle size” and short characteristic time.

Anomalous vortex flow and noise in the quantum liquid phase of amorphous films

We measure the fluctuating component of the flux-flow voltage, δV(t), about the average voltage in the low-temperature (T) liquid phase of amorphous MoxSi1−x films. For the 100-nm-thick film δV(t) originating from the vortex motion is clearly visible in the quantum-vortex-liquid (QVL) phase, where the distribution of δV(t) is asymmetric, suggestive of the unusual vortex motion. For the thin (6 and 4nm) films, in which the QVL phase is not determined from the T dependence of the resistance, the similar unusual (avalanche-like) vortex motion is observed in nearly the same reduced-T regime. These results support the view that vortex dynamics in the low-T liquid phase of thick and thin films is dominated by common physical mechanisms related to quantum-fluctuation effects.

Unusual vortex dynamics in the quantum-liquid phase of a-MoxSi1−x films

We find the unusual vortex dynamics in the low-temperature liquid phase of amorphous MoxSi1−x films by measuring the fluctuating component of the flux-flow voltage δV(t) about the average voltage. For the thick film, in which the quantum vortex liquid (QVL) phase has been well-determined in the field-temperature plane, δV(t) originating from the vortex motion is clearly visible in the QVL phase, where the distribution of δV(t) is anomalously asymmetric, implying large velocity and/or number fluctuations of driven vortices. For the thin film, in which the QVL phase has not been determined from the static transport measurements, similar unusual vortex motion is observed in nearly the same reduced-temperature regime. We suggest that vortex dynamics in the low-temperature liquid phase of thick and thin films is dominated by common physical mechanisms related to quantum-fluctuation effects.