Finite-size effect on the resistive state in a mesoscopic type-II superconducting stripe

This review is devoted to the studies of the resistive state in wide superconducting films induced by dc and ac currents in the absence of an external magnetic field. It is found experimentally that the current-voltage characteristics and their parameters are well described by the Aslamazov–Lempitskii theory of the resistive vortex state, which has long been lacking an adequate evidence to support it. There is experimental evidence that the vortex mechanism of resistivity is absent for currents exceeding the maximum current at which the resistive vortex state exists in wide films. The non-vortex nature of phase slip lines (PSL) in wide films is proved experimentally. It is shown that the phase-slip lines arise in a wide superconducting film subjected to an external microwave irradiation with the power exceeding some critical value. The resistance of these phase-slip lines depends on irradiation frequency. Results obtained in studies of PSL in wide films are similar to those obtained for the phase-slip centers in narrow channels. This suggests the identical nature of the phase-slip processes in these two phenomena. It is established that the current-carrying resistive state of a wide film arises due to two different mechanisms that occur one after the other: the penetration into the film and transverse motion of the Pearl vortices induced by the magnetic field of the transport current and the emergence of the phase-slip lines of the superconducting order parameter.

In type-II superconductors, where vortices and antivortices tend to annihilate, only a ‘ferromagnetic’ vortex lattice, with the same orientation of vortex magnetic moments, is usually formed in a homogeneous external magnetic field. Using the time-dependent Ginzburg–Landau formalism, we demonstrate that a checkerboard vortex–antivortex lattice (‘antiferromagnetic vortex lattice’), imprinted onto a superconducting film by a periodic array of underlying clockwise and counterclockwise microcoils generating spatially periodic positive and negative magnetic field pulses and then trapped by an array of artificial pinning centers, remains stable even after the imprinting magnetic field pulse is switched off.

Within the time-dependent Ginzburg–Landau (TDGL) theory, we theoretically investigated the dynamic properties of vortex–antivortex (V–Av) pairs in a current-carrying superconductor strip with one tilted slit or two transversely/longtitudinally arranged slits in the presence of a weak magnetic field. The effect of the rotation angle on the resistive state in the sample with one tilted slit was considered. The location of phase slippage in the sample can be predetermined by the rotation angle of the tilted slit. We found the coalescence of the phase-slip lines (PSLs) during a periodic multiharmonic voltage oscillation. This leads to two stages of voltage oscillation at zero applied field and three stages at weak magnetic field since the coalescence of left and right PSLs are staggered in the latter case. Moreover, the influence of relative position and size of the two slits on the transport properties of the sample are also studied. The dynamic behavior of V–Av pairs depends on not only the slit length but also their horizontal or vertical interval distance between the two slits. Since the geometry of sample has an important influence on the distribution of supercurrent across the sample, we demonstrate the possibility to efficiently guide the kinematic vortices by adjusting the relevant parameters.

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.

Nb3Sn superconductor is of significant interest for applications in constructing high-field magnets beyond the limit of NbTi. However, its critical current density decreases rapidly at high magnetic fields (12 T) and the state-of-the-art level of Nb3Sn superconductors still cannot meet the requirements of the planned future accelerator magnets. The primary flux pinning centers for Nb3Sn wire mainly arise from the grain boundaries (GBs). In the present paper, we theoretically investigate, through time-dependent Ginzburg-Landau theory and with graphics-processing unit parallel technique, the vortex pinning and the critical current density in large-scale polycrystalline Nb3Sn superconductor by varying the pinning potential of GB and grain size at various magnetic fields. Unlike the conventional dot-like pinning systems, it is found that the critical current is not a monotonous function by suppressing the superconductivity of the GBs. The optimal pinning potential of GB for maximum critical current density strongly depends on magnetic fields. Furthermore, we find that the critical current density can be significantly enhanced by reducing grain size at low magnetic fields, while increase of critical current density cannot always be observed at high magnetic fields. Actually, critical current density even decreases by reducing grain size, which depends on the superconductivity of GBs. The findings in the paper provide theoretical foundations to achieve further improvement of Nb3Sn with optimizing the flux pinning.

A phase diagram of the current states of superconducting films is produced on the basis of an experimental study of the resistive transitions induced by the transport current. It is found that a comparatively narrow film of width w<5λ⊥ (λ⊥ is the penetration depth of the magnetic field) never enters a vortex state, but undergoes a direct transition from the purely superconducting state into a resistive state with phase-slip centers as soon as the current exceeds the critical Ginzburg-Landau current IcGL. The Meissner current state of films with intermediate widths 5λ⊥(T)<w<10λ⊥(T) transforms, when I>0.8IcGL, into a vortex resistive state which exists at currents 0.8IcGL<I<Im, where the upper critical current is in good agreement with the theory. The vortex state of wide films with w>10λ⊥(T) exists for currents IcAL<U<Im, where IcAL is the current at the transition into the vortex state calculated for the limiting case w⪢λ⊥. When I>Im, films with widths w>5λ⊥(T) enter a vortex-free resistive state with phase-slip lines.

The transport properties in the mixed state of high-quality Ca0.8La0.2Fe0.98Co0.02As2 single crystal, a newly discovered 112-type iron pnictide superconductor, are comprehensively studied by magneto-resistivity measurement. The field-dependent activation energy, U0, is derived in the framework of thermally activated flux flow (TAFF) theory, yielding a power law dependence U0~Hα with a crossover at a magnetic field around 2 T in both H⊥ab and H//ab, which is ascribed to the different pinning mechanisms. Moreover, we have clearly observed the vortex phase transition from vortex-glass to vortex-liquid according to the vortex-glass model, and vortex phase diagrams are constructed for both H⊥ab and H//ab. Finally, the results of mixed-state Hall effect show that no sign reversal of transverse resistivity ρxy(H) is detected, indicating that the Hall component arising from the vortex flow is also negative based on the time dependent Ginzburg-Landau (TDGL) theory. Meanwhile, the transverse resistivity ρxy(H) and the longitudinal resistivity ρxx(H) follow the relation |ρxy(H)|=Aρxx(H)β well with an exponent β~2.0, which is in line with the results in theories or experiments previously reported on some high-Tc cuprates.

Noise spectral power and non-equilibrium dynamics of vortices in the mixed state of Bi 2Sr 2CaCu 2O y

Influence of an axial current density stepping on the critical currents and magnetic field of cylindrical magnets wound with Bi(2223)Ag anisotropic tapes—theoretical analysis

Two-dimensional (2D) and three-dimensional (3D) polycrystalline vanadium films grown on SiO2 substrates have been prepared by magnetron sputtering method. The superconductor-insulator transition (SIT) has been observed by adjusting the film thickness, average crystallite size and magnetic field respectively. The complete superconducting 3D vanadium film deposited at 523 K has an obvious two-step development of superconductivity when a magnetic field is applied. This behavior is attributed to non-uniform coupling due to variations in grain sizes and the intergranular distances across the sample. The metallic state induced by the magnetic field can be explained by a model that incorporates both thermally activated phase slippage and quantum phase slippage. Both the 2D vanadium film with thickness of 12 nm and the 3D vanadium film with average crystallite size of 11.9 nm exhibit a trend towards superconducting transition where the resistance decreases to a nonzero value. The SIT, tuned by adjusting the perpendicular magnetic field, has been observed in the above two films. The scaling behavior near the critical magnetic field B c distinguishes a superconducting phase from an insulating phase, indicating a quantum phase transition induced by a finite-size effect. The upper critical magnetic field H c 2 (0) increases as the grain size decreases, likely due to a reduction in the coherence length resulting from a decrease in the average free path with decreasing size.

The critical current (Jc) of highly twinned YBa2Cu3O7 films has been measured as a function of temperature, magnetic field, and angle. For much of the parameter space we observe a strong suppression of Jc for fields in the twin boundary (TB) directions; this is quantitatively modeled as flux-cutting-mediated vortex channeling. For certain temperatures and fields a crossover occurs to a regime in which channeling is blocked and the TBs act as planar pinning centers so that TB pinning enhances the overall Jc. In this regime, intrinsic pinning along the TBs is comparable to that between the twins.

Within the framework of the Ginzburg–Landau theory, we study the features of the localized nucleation of the order parameter in superconducting systems with inhomogeneous effective mass m of the Cooper pairs, which is due to the spatial modulation of the diffusion coefficient and/or fluctuations in the local anisotropy axis in the sample. In the asymptotics of the weak magnetic fields H, for which the magnetic length [Φ0/(2πH)]1/2, where Φ0 is the magnetic-flux quantum, is much shorter than the inhomogeneity scale, the spatial scale of the order parameter is determined by the sample-average coherence length and the regular lattice of the Abrikosov vortices is formed in the superconductor. In sufficiently strong magnetic fields H, the order parameter is localized near the minima of the coherence length ξ ∝ m −1/2, which results in an increase in the critical temperature and destruction of the regular lattice of the Abrikosov vortices. Therefore, competition between the two superconductivity-nucleation types is observed during a gradual increase in the magnetic field, which leads to the positive curvature of the phase-transition line. We have also studied the features of the temperature dependences of the upper critical magnetic field for some model spatial mass profiles of the Cooper pairs. The obtained results are in good agreement with direct numerical calculations.

Since the 1970’s the effect of the increase of the critical current of superconductors in the longitudinal external magnetic field has been known. The critical current density of a Nb-Ti wire at low longitudinal magnetic fields demonstrated several times the larger value compared with that in a transverse magnetic field. The same phenomenon has been observed for 2G HTS tapes. It is natural to try to use this phenomenon to increase the critical current in DC HTS power cables. In this paper, we present the analysis of feasibility to use the longitudinal magnetic field effect to increase critical currents in practical HTS power cables. The experimental model of the HTS DC cable has been developed, produced, and tested in liquid nitrogen. The increase of the critical current observed when the longitudinal magnetic field generated. This increase coincides with the calculations.

We show that a moving vortex lattice, as it comes to a crystal edge, radiates into a free space the harmonics of the washboard frequency, omega(0)=2pi v/a, up to a superconducting gap, Delta/variant Planck's over 2pi. Here v is the velocity of the vortex lattice and a is the intervortex spacing. We compute radiation power and show that this effect can be used for the generation of terahertz radiation and for characterization of moving vortex lattices.

Vortex lattice ordering in the flux flow state of Nb thin films