T. Nojima

Transport critical current density and dimensional crossover in superconducting Nb/NbZr multilayers

Abstract In order to understand the nature of the mixed state, the critical current density J c of Nb/NbZr multilayers was measured as a function of magnetic field H and of the angle between H and the film plane at the temperatures where the superconducting order parameter is localized in the Nb layers. In the parallel fields, which are perpendicular to the current J , the J c - H curves show a peak at H = H max . The observed angular dependence of J c below and above H max can be explained by the 3D stepwise vortex model and the 2D pancake vortex model, respectively. These results indicate that a dimensional crossover in the mixed state occurs at H max . This crossover may be the cause of the field dependence of J c which is an increasing function of H below H max .

Angular dependence of glass scaling behavior in current-voltage characteristics of CVD YBa2Cu3O7-δ films

Abstract In order to investigate the effect of intrinsic pinning on the glassy state, we have measured current-voltage ( I–V ) curves of CVD YBa 2 Cu 3 O 7- δ near the glass transition temperature T g as a function of the angle θ between magnetic field and c -axis. In the scaling analysis of I–V curves based on the vortex-glass model, it is found that the glass scaling function and the critical exponents are independent of θ, and do not show a remarkable change even at θ =90° ( H ⊥ c ). Our result indicates that the periodic plane-like intrinsic pinning is not so effective near T g and vortices are pinned in random point-like disorder for all field directions.

Current-voltage characteristics of YBa2Cu3O7−δ films at low magnetic fields

Abstract We have examined the scaling behavior and the functional form of the I − V characteristics of YBa 2 Cu 3 O 7− δ films at fields H from 0.001 to 6 T applied parallel to the c -axis. In all cases, the I − V curves obey the scaling law based on the second-order phase transition, and are fitted by V / I = R 0 exp[− I 0 / I ) μ ]. At high fields, the exponents ν , z , and μ , are independent of H and are consistent with the prediction of the vortex-glass (VG) model. However, below ∼ 0.05 T, these exponents become H -dependent, which is indicative of the existence of another phase at low fields.

Preferential position for nucleation of order parameter and angular dependence of upper critical fields in Nb/NbZr multilayers

Abstract The Nb/NbZr multilayer is a typical one composed of two materials with the same bulk transition temperature and different electron diffusion constants and exhibits an anomalous upturn at T+ in the temperature dependence of parallel upper critical field Hc2‖(T). For all samples which show the upturn behavior in Hc2‖(T), we have found the anomal minimum and maximum at a tilt of angle θ = θ ∗ and a perpendicular direction θ = 0, respectively. Of particular interest is the existence of a different tendency in the variation of θ ∗ and Hc2⊥-peak as a function of T below and above T+. From our experimental results, we suggest the dimensional crossover of the localization of Δ(r) for a tilt of angle. The angular dependence of Hc2(T) has been in a good agreement with the numerical calculations.

Current-voltage characteristics and layer coupling in amorphous W/Si multilayers

Abstract Current-voltage ( I - V ) characteristics have been measured on the amorphous W monolayers and W/Si multilayers in the absence of magnetic field. For the monolayers and the multilayers with thick Si layers, the I - V curves show straight lines on a log-log scale and the Kosterlitz-Thouless transition. On the other hand, for the multilayers with thin Si layers, they show negative curvatures on such a scale at low temperatures. This is attributed to the effect of interlayer coupling giving rise to the dimensional crossover of vortex configuration. Our results are discussed with the theories which take the coupling effect into consideration.

Preferential position of order parameter and angular dependence of upper critical fields in Nb/NbZr multilayers

The Nb/NbZr multilayer, which is composed of two superconductors with the same critical temperature and the different electron diffusion constant, is a typical system to exhibit the anomalous upturn inT-dependence of upper critical fieldHc2II parallcl to the film surface. For the angular dependence ofHc2, we have observed a minimum on the tilt of θ (as θ=0o forH⊥ film). This is interpreted as the dimensional crossover of the localized order parameter at (as θ=0o forH ⊥ film). This mechanism has been also confirmed in the variation ofI-V characteristics withH-direction to the film, where the samples were located at the specific configuration in the field.

Finite-size effect on vortex loop excitation in amorphous-W/Si multilayers

Abstract A finite size effect on vortex configuration is investigated by the measurements of current–voltage ( I – V ) characteristics on amorphous-W/Si multilayers as a function of the number of the bilayers N in the absence of field. Critical scaling analysis of the data for the samples with N ≥10 shows that the vortex loops are the favored excitation with the dynamic critical exponent z ∼3.32, while for 10> N ≥3 it reveals to deviate from the scaling function corresponding with a finite size effect on vortex loops. At a small current density, the vortex loop is probed at a length scale larger than the total sample thickness. A remarkable change in the I – V characteristics is observed between N =3 and 2. This indicates the vortex configuration crossover to vortex–antivortex string pairs piercing all layers at N =2. In sample with N =1, the typical KT transition is observed.

Scaling behavior ofI-V characteristics and vortex correlations in YBa2Cu3O7 films

The glass scaling behavior inI-V characteristics of YBCO(123) films has been studied as a function of magnetic fieldH (0.001–6 T) andH direction toc axis. In all cases, theI-V curves obey the scaling law based on the secondorder phase transition. At highH>0.1 T, the static and dynamic exponents show the universal values. However, these exponents deviate from the constants below a direction-independent fieldH*∼0.05. Our results indicate the existence of another low-field phase, which is divided from the vortex glass phase by the average intervortex spacing.

Superconducting properties and preferred orientations of epitaxial growth in V/Cr multilayers

Abstract We investigated the superconducting properties of V/Cr multilayers with the different preferred orientations of (100) and (110). These samples were grown epitaxially on MgO (100) and Al 2 O 3 (11 2 0) single-crystal substrates. The Cr layer thickness was varied from d cr =2 to 140A at a constant V layer thickness of 300 A. In (100) V/Cr, the critical temperature T c abruptly drops with increasing d Cr from 5 to 40 A and shows a plateau above 40 A. In (110) V/Cr, T c gradually decreases up to d Cr =80 A and keeps a higher value than in (100). For the temperature dependence of parallel upper critical fields, H c2∥ ( T ), the 3D-to-2D dimensional crossover phenomenon takes place in d Cr =10∼25 A for (100) V/Cr and 10∼70 A for (110) V/Cr. 2D behavior of H c2∥ ( T ), which implies the decoupling of superconducting V layers by the Cr layer, is achieved in the range of d Cr above 40 A for (100) and 80 A for (110). The ratio of H c2⊥ to H c2⊥ for (100) V/Cr is enhanced in comparison with that for (110). Our results indicate that the strong pair-breaking effect operates at the interfaces of (100) V/Cr.

Vortex nucleation to non-vortex state crossover in V/Cr multilayers

Abstract Vortex and non-vortex nucleation states are investigated in the superconducting V-layers isolated by Cr layers(d cr =8nm) for the applied magnetic field parallel to the V/Cr multilayers. In the samples with d v >90nm, the parallel upper critical field H c2| (T) exhibit the crossover from non-vortex to vortex state with decreasing a temperature. At the crossover point T ∗ , we obtained d v ξ( T ∗ )=2.3±0.2 . Enhancement of H c2| (0) was observed in the range of 35nm v