Flux-flow Regime Research Articles

Current–Voltage Measurements on a Bi2Sr2CaCu2O x Whisker and Investigation of Its Vortex Dynamics in Self Field

We report current–voltage measurements carried out on a high quality Bi2Sr2CaCu2O x whisker in a large temperature range below the critical temperature. When a perpendicular field is applied, the current–voltage curves are typical of the flux flow regime at low fields and resemble flux creep characteristics at high fields. This change, related to what is described as the critical current peak effect is associated to a transition between different vortex phases. In this contribution, we focus on the dynamics of the vortices in self field. We show that the critical current in the flux flow regime can be calculated straightforwardly taking into account inter-vortex forces only, supposing that free vortices run between pinned vortices.

Doping effect on the anomalous behavior of the Hall effect in electron-doped superconductor Nd2−xCexCuO4+δ

Transport properties of Nd2−xCexCuO4+δ/SrTiO3 single crystal films (B‖c,J‖ab) are investigated in magnetic fields B up to 9T at T=(0.4–4.2)K. An analysis of normal state (at B>Bc2) Hall coefficient RHn dependence on Ce doping takes us to a conclusion about the existence both of electron-like and hole-like contributions to transport in nominally electron-doped system. In accordance with RHn(x) analysis an anomalous sign reversal of Hall effect in mixed state at B<Bc2 may be ascribed to a flux-flow regime for two types of carriers with opposite charges.

Current–Voltage Measurements and Vortex Dynamics in a Bi2Sr2CaCu2O x Whisker: Observation of the Peak Effect and Calculation of the Critical Current in the Flux Flow Regime with and Without Applied Field

In this article, we report current–voltage measurements carried out on a Bi2Sr2CaCu2Ox whisker in a large temperature range below the critical temperature with and without applied magnetic field. We examine the critical current peak effect and the vortex dynamics at low field. The critical current peak effect consists of the initial increase of the critical current that subsequently decreases as the applied field is increased. For current–voltage measurements, this effect is associated with a change in the current–voltage curves that are typical of the flux flow regime at low fields and resemble flux creep characteristics for higher fields. As a general rule, our observations are consistent with the models that link the peak effect to vortex phase transitions. We calculate the critical current in self-field in the flux flow regime taking into account intervortex forces. We suppose that most vortices are pinned by defects while mobile vortices move through plastic channels between the strongly pinned vortex regions. When an external field is applied, we suggest that the increase in the critical current that is observed is linked to oscillations of the pinned vortices.

Nonreciprocal microwave transmission through a long Josephson junction

We have measured the bidirectional propagation of microwaves through a long Josephson junction in a flux-flow regime. We demonstrate that the transmitted microwave power depends on the direction of microwave propagation with respect to the direction of the flux flow. This nonreciprocal behavior is explained by the interaction of the microwave signal with the moving fluxon chain inside the junction. Thus a long junction may act as an on-chip isolator for external microwave signals, with its transmission properties being fully controlled by the bias current and in-plane magnetic field.

Magnetic Field Dependent Photoinduced Superconductivity and Its Oxygen-Deficient Dependency in Undoped and BZO-doped YBCO Thin Films

Abstract We present a systematic study of photoinduced effiect illuminating the undoped and BZO-doped YBCO thin films with laser diode (1.88 eV). The pulsed laser deposited films were post-annealed in partial oxygen pressure in order to decrease the oxygen content and therefore understand the role of structural changes, oxygen ordering and hole doping in photoconductivity phenomenon. The resistivities were measured in the thermally activated flux-flow regime with wide applied magnetic field range. We find that as in undoped and optimally oxygenated YBCO films, the structural defects and dislocations formed by conventional BZO-doping does not contribute to photoinduced enhancement of superconductivity. On the contrary, in our oxygen-deficient films, the effect of photoexcitation is strongly dependent on the oxygen ordering and thereby free-carrier concentration and their mobility in the CuO2 double layers, which again has strong external magnetic field dependence. The magnetic field dependence of the increased superconducting transition shows that the illumination improves the pinning properties in high magnetic field range, indicating the existence of photosensitive pinning centers.

The effect of BZO doping concentration and thickness dependent properties of YBCO films grown by PLD on buffered NiW substrates

The effect of BaZrO 3 (BZO) doping is systematically studied in YBa 2Cu 3O 6+ x (YBCO) thin films deposited by pulsed laser deposition (PLD) on buffered NiW substrates. Based on the structural and magnetic properties, the optimal BZO doping concentration is obtained to vary between 4 and 7.5 wt.%, depending mainly on applied magnetic field. This relatively high optimal concentration is linked to the nanograined target material and metal substrate that cause low-angle grain-boundaries and in-plane spread of YBCO crystals on NiW. Thickness dependent analysis of undoped and BZO-doped YBCO films predicts differences in growth mechanisms where early growth next to the substrate interface is 2D-type in BZO-doped films. This leads to the situation where crystallographic structure as well as superconducting properties are improved when the film develops and the thickness is increased. Therefore from the resistivity measurements a threshold thicknesses where reasonable properties occur are determined for both set of films. Measurements in thermally activated flux-flow regime (TAFF) indicate that above the threshold thickness relatively strong and isotropic vortex pinning is realized in BZO-doped YBCO films. Generally, this paper demonstrates that especially for thin film applications on NiW substrates even more compatible buffer layer structures should be utilized.

Magneto-transport properties of curved mesoscopic superconducting strips

Transport properties of curved mesoscopic superconducting strips are investigated in theframework of the time-dependent Ginzburg–Landau formalism. The geometries of thesuperconducting strips considered here are either a section of a cylindrical shell ora full cylindrical shell in which the magnetic field is applied perpendicular tothe axis. The cylindrical section can exhibit considerably asymmetric transportproperties, making it potentially interesting as a sub-micrometer scale superconductingcurrent rectifier. The full cylindrical surface exhibits well developed dissipativebranches in the voltage–current curves, that can be accounted for by kinematicvortex–antivortex phase slip lines. Such kinds of phase slip lines cause voltage oscillationsin a frequency range higher than the one associated with the familiar flux flowregime.

Pulse shear stress for anaerobic membrane bioreactor fouling control

Increase of shear stress at membrane surfaces is a generally applied strategy to minimize membrane fouling. It has been reported that a two-phase flow, better known as slug flow, is an effective way to increase shear stress. Hence, slug flow was introduced into an anaerobic membrane bioreactor for membrane fouling control. Anaerobic suspended sludge was cultured in an anaerobic membrane bioreactor (AMBR) operated with a side stream inside-out tubular membrane unit applying sustainable flux flow regimes. The averaged particle diameter decreased from 20 to 5 microm during operation of the AMBR. However, the COD removal efficiency did not show any significant deterioration, whereas the specific methanogenic activity (SMA) increased from 0.16 to 0.41 gCOD/g VSS/day. Nevertheless, the imposed gas slug appeared to be insufficient for adequate fouling control, resulting in rapidly increasing trans membrane pressures (TMP) operating at a flux exceeding 16 L/m2/h. Addition of powdered activated carbon (PAC) enhanced the effect of slug flow on membrane fouling. However, the combined effect was still considered as not being significant. The tubular membrane was subsequently equipped with inert inserts for creating a locally increased shear stress for enhanced fouling control. Results show an increase in the membrane flux from 16 L/m2/h to 34 L/m2/h after the inserts were mounted in the membrane tube.

Investigation of Power Dissipation Mechanisms in Coated Conductors at High Current Densities Based on Ultra-Fast Pulsed Current Measurements

In this contribution, we report and discuss the physical meaning of pulse current measurements carried out on coated conductors (CCs) consisting of a superconducting YBCO film deposited on a Hastelloy substrate and coated with a thin metallic layer. The high current (up to 1000 A) and short duration pulses (from 10 μs to 1 ms) have allowed us to determine the current-voltage characteristics of two different samples in a situation near that of zero injected energy, and therefore remove the bias resulting from the temperature rise during the measurement. The characteristics obtained show a flux creep region and two linear regimes. The first linear regime is the flux flow regime. In this regime, we show that the vortex velocity is a constant that depends on the metal film resistivity. The second linear regime is ohmic and his origin is less clear. We propose models describing both linear regimes, that are in agreement with the measurements. Finally, we discuss the consequences of these results for the applications of the coated conductors in devices for power systems, especially fault current limiters and power transmission cables.

Current sharing between the metallic and superconducting layers of high temperature superconductor coated conductors operated above their critical current

In this contribution, we report and discuss the physical meaning of pulse current measurements carried out on coated conductors (CCs) consisting of a superconducting YBCO film deposited on a Hastelloy substrate and coated with a thin metallic layer. The high current (up to 1000 A) and short duration pulses have allowed us to determine the current-voltage characteristics of two different samples in a situation near that of zero injected energy and therefore remove the bias resulting from the temperature rise during the measurement. The characteristics obtained show a flux creep region and two linear regimes. The first linear regime is the flux flow regime. In this regime, we show that there is a constant vortex velocity that depends on the metal film resistivity. The second linear regime is also a vortex regime, but its precise nature is less clear. We propose models describing both linear regimes, in agreement with the measurements. Finally, we discuss the consequences of these results for the applications of CCs in devices for power systems, especially fault current limiters and power transmission cables.

Magnetic Flux Diffusion Inside a Superconducting Rod of Square Cross Section Submitted to a Sinusoidal Magnetic Field

The flux diffusion into a superconducting long rod of square cross-section in the flux flow regime is investigated numerically for sinusoidal variations of the external magnetic field. The real and imaginary parts of first harmonic as well as its penetration depth are determined in function of the field frequency. This penetration depth, which decreases exponentially in function of the frequency, is influenced by the change of the flux front shape from square to circular at low frequencies and seems to be a scaling length for both components of the first harmonic.

Non-linear Flux Flow Resistance of Type-II Superconducting Films

We analyze the flux-flow (FF) regime in type-II superconducting films exhibiting quite strong pinning. By driving the vortex lattice (VL) up to high dissipative states, the moving VL undergoes an instability, leading to an abrupt change from the FF to the normal state, which is displayed in the current-voltage characteristics as a voltage jump at a critical vortex velocity v ∗. The temperature and magnetic field dependence of v ∗ is investigated in different materials, and an unpredicted low field behavior of v ∗(B) is found. Moreover, for velocities lower than v ∗, a non-linear FF resistance is observed, with a “peak” in the current dependence of the dynamic resistance. This is a remarkable feature of a dynamic transition from disordered to ordered VL occurring in the FF state. We suggest that both unusual effects observed in v ∗(B) and R FF(I) can be accounted for intrinsic pinning.

Vortex dynamics in superconducting channels with periodic constrictions

Vortices confined to superconducting easy flow channels with periodic constrictions exhibit reversible oscillations in the critical current at which vortices begin moving as the external magnetic field is varied. This commensurability scales with the channel shape and arrangement, although screening effects play an important role. For large magnetic fields, some of the vortices become pinned outside of the channels, leading to magnetic hysteresis in the critical current. Some channel configurations also exhibit a dynamical hysteresis in the flux-flow regime near the matching fields.

Reduced intrinsic and strengthened columnar pinning of undoped and 4 wt%BaZrO3-dopedGdBa2Cu3O7 − δ thin films: a comparative resistivity study nearTc

The resistivities of undoped and 4 wt%BaZrO3-doped(BZO) GdBa2Cu3O7 − δ (GdBCO) thin films were measured in a temperature activated flux-flow regime(TAFF). In addition, resistivity versus rotation angle of magnetic field,ρ(Θ), measurements were done near the critical temperature,Tc. The results of undoped and doped GdBCO were compared with ones ofYBCO, and they showed that YBCO has better intrinsic pinning of CuO-planesthan GdBCO. This is explained by the extra stacking faults distorting theab-planes in GdBCO. The BZO-doping increased activation energy,U0, in geometry at fields higher than 1 T in YBCO and 3 T in GdBCO, but reducedU0 in the direction in the whole measured magnetic field range. Also, the irreversibility field,Birr, was enhanced in BZO-doped GdBCO at fields higher than 3 T in , but was reduced in the direction. The reduction of U0 and Birr in the direction in BZO-doped films is explained by BZO nanorods distorting theab-planes, too. The distortion may explain the more isotropicρ(Θ) in GdBCO and BZO-doped material. The 4 wt% BZO-doping seems to be more effective onGdBCO, and we suggest that either cation substitution, excess strain caused by theinteraction of the extra stacking faults and BZO nanorods, or oxygen deficiencyresult in nanosized regions whose superconducting properties are reduced andwhich act as extra pinning sites. According to the resistivity measurements nearTc, the undoped YBCO should be used if high pinning of theab-planes is needed. Further, from these materials 4 wt% BZO-doped GdBCO is best suitedfor applications where high magnetic fields are needed at high temperatures.

Asymmetry, bistability, and vortex dynamics in a finite-geometry ferromagnet-superconductor bilayer structure

We have recently demonstrated that Py/Nb bilayers patterned in a strip geometry can exhibit asymmetric transport properties and bistability. Here, with the help of numerical simulations in the framework of time-dependent Ginzburg-Landau model for superconductor and Landau-Lifshitz-Gilbert model for ferromagnet, it is demonstrated that the asymmetric and bistable magnetotransport response of the bilayers can be accounted for by the stray fields from the patterned ferromagnetic layer. Numerical simulations on vortex dynamics show that in the dissipative branch of the bilayer a peculiar spontaneous channeled flux flow regime is realized, with alternating vortex and antivortex chains moving in the opposite directions in the superconducting layer.

Guiding of vortices and ratchet effect in superconducting films with asymmetric pinning potential

Two-dimensional vortex dynamics in a ratchet washboard planar pinning potential in the presence of thermal fluctuations is considered on the basis of a Fokker-Planck equation. Explicit expressions for two nonlinear anisotropic voltages (longitudinal and transverse with respect to the current direction) are derived and analyzed. The physical origin of these odd (with respect to magnetic field or transport current direction reversal) voltages is caused by the interplay between the even effect of vortex guiding and the ratchet asymmetry. Both voltages are going to zero in the linear regimes of the vortex motion [i.e., in the thermally activated flux flow (TAFF) and Ohmic flux flow (FF) regimes] and have a bumplike current or temperature dependence in the vicinity of the highly nonlinear resistive transition from the TAFF to the FF.

Magnetic field dependence of the optimalBaZrO3 concentration innanostructured YBa2Cu3O7−δ films

The resistivity of YBa2Cu3O7−δ (YBCO) films deposited by a pulsed laser from nanograined targets doped with various amounts ofBaZrO3 (BZO) is investigated in the thermally activated flux-flow (TAFF) regime, where the activation energy,U, has a lineartemperature dependence. It is found that the magnetic field dependence of the irreversibility temperature,Tirr(B), has a maximum at some optimal BZO concentration, which depends on the appliedmagnetic field. Our results show that the superconducting materials used in differentmagnetic fields need optimization of the dopant concentration to achieve the bestproperties for various applications.

Anisotropy of static and dynamic order-disorder transition in YBa2Cu3O7-δ single crystal

Transport measurements were performed on YBa2Cu3O7-δ single crystal at the temperature 86.7 K. Magnetic field was oriented at different angles to the ab-plane and the vortex velocities were v = 10-4-2 m/s. Obtained voltage-current characteristics show that the pinning force Fp depends non-monotonously on magnetic field both at thermal assisted creep and in the flux flow regime. These dependences can be described in context of static and dynamic order-disorder (OD) transitions. The static transition corresponds a to minimum of Fp (H) dependence and dynamic transition corresponds to the peak in the ρd-dependences. Angular variation shifts static OD transition to lower values of magnetic field with increasing angle while the value of magnetic field corresponding to the dynamic OD transition increased. These behaviors can be possibly explained by the formation of oxygen vacancies clusters.

Self-organized mode-locking effect in superconductor/ferromagnet hybrids

The vortex dynamics in a low-temperature superconductor deposited on top of a rectangular array of micrometer size permalloy triangles is investigated experimentally. The rectangular unit cell is such that neighboring triangles physically touch each other along one direction. This design stabilizes remanent states which differ from the magnetic vortex state typical of individual noninteracting triangles. Magnetic force microscopy images have revealed that the magnetic landscape of the template can be switched to an ordered configuration after magnetizing the sample with an in-plane field. The ordered phase exhibits a broad flux-flow regime with relatively low critical current and a highly anisotropic response. This behavior is caused by the spontaneous formation of two separated rows of vortices and antivortices along each line of connected triangles. The existence of a clear flux-flow regime at zero external field supports this interpretation. The density of induced vortex-antivortex pairs is directly obtained using a high-frequency measurement technique which allows us to resolve the discrete motion of vortices. Strikingly, the presence of vortex-antivortex rows gives rise to a self-organized synchronized motion of vortices which manifests itself as field independent Shapiro steps in the current-voltage characteristics.

Narrow-band noise due to the moving vortex lattice in superconducting niobium

We report measurements of voltage noise due to vortex motion in Niobium, a conventional low-${T}_{c}$ superconductor. A coherent oscillation leading to narrow-band noise (NBN) is evidenced. Its characteristic frequency is a linear function of the overcritical transport current in the flux-flow regime, and hence scales as the main velocity of the vortex flow. The associated length scale is not the intervortex distance but the width of the sample, indicating temporal coherence at a large scale. NBN is also observed in the nonlinear part of the $V(I)$ at the onset of depinning, in apparent disagreement with a stochastic creep motion of flux bundles. NBN exists in the peak effect region, showing that long-range temporal correlations are preserved in this regime.