S. Nakano

Observation of anomalous transition in the upper critical fields of Nb/Nb0.5Zr0.5 multilayers

Abstract An anomalous upturn in the parallel upper critical field is observed in the Nb/Nb0.5Zr0.5 multilayer with the period Λ=400 A and 500 A , as predicted by Takahashi and Tachiki for multilayer composed of two materials with different electronic diffusion constants and equal transition temperature. We have found the anomalous feature in a temperature dependence of transition width ΔHc2‖ in the region where the upturn occurs. In the angular dependence of the upper critical fields, the slope dHc2/dθ near the field direction parallel to the film surface also exhibits an abrupt change in the range of upturn.

Anisotropy of critical current and anomalous upturn feature of parallel upper critical fields in the superconducting Nb/Nb0.5Zr0.5 multilayer

Abstract Measurements of the critical current J c have been made for the Nb/Nb 0.5 Zr 0.5 multilayer which showed a sharp upturn behavior in a parallel upper critical field H c2∥ ( T ), caused by the shift of the order-parameter Δ( r ) from Nb- to NbZr-layers. In the region where Δ( r ) is localized in the Nb-layers, the peak was observed in the J c − H curves for parallel field orientation. With increasing temperature, the J c -peak value drastically decreased and its position was shifting to a lower field. In the angular dependence of the critical current density, J c (θ) had a maximum when the flux lines are exactly aligned in parallel to the layer and sharply decreased with tilting of angle. The feature is caused by an intrinsic flux pinning mechanism. In the upturn region, we found the same characteristics of J c .

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 .

Correlation of homoepitaxial growth of high-T c A15 Nb3Ge with characteristics of substrate surface

Metastable Nb3Ge films were prepared by coevaporation in an ultrahigh vacuum with homoepitaxial growth onto Nb-rich A15 Nb-Ge base layers having two types of surface characteristics: one was formed with a two-phase mixture of Nb-Ge and NbO, and the other had a clean Nb-Ge phase without any impurities. The homoepitaxial films grown on the former surface had more stable and reproducible higherTc features than did those made on the latter surface. Compositional analysis near and at the interface between the homoepitaxial film and the base layer was made by Auger electron spectroscopy for representative samples and the oxide formed on the base layer was investigated by x-ray photoelectron spectroscopy.

Superconducting upper critical fields of V/Cr-superlattices

Abstract In V/Cr-superlattices, the superconducting transition temperature and upper critical fields are investigated for the superlattices of various Cr-layer thickness ( d Cr =20–90 A ) under a constant V-layer thickness ( d v =300 A ). These samples are prepared by UHV-deposition at 300 and 500°C onto the V-buffer layer that is grown on the sapphire substrates heated upto 500°C. The dependence of critical temperature on the Cr-layer thickness obeys the fits calculated by the Werthamer model for the films deposited at 300°C. A dimensional crossover in the parallel upper critical fields was observed in the samples deposited at 300°C and was not found in the films done at 500°C. Such a crossover is shown in the superlattices with d Cr ≤50 A and occurs when the effective superconducting layer thickness becomes comparable to the perpendicular coherence length ξ ⊥ at the crossover points.

Superconductivity and crystalline orientation in V/Cr-multilayers

Abstract We investigated the superconductivity of V/Cr-multilayers grown epitaxially on MgO(001) and Al2O3(1120) plate. The preferred orientation of V/Cr is (001) and (110) for MgO and Al2O3, respectively. Thickness of V-layer is fixed at d v =300 A and that of Cr-layer is varied from 20 to 80A. Critical temperatures of (001)V/Cr are lower than those of (110)V/Cr. Dimensional crossover in the parallel critical fields Hc2|(T) is observed for d Cr =20–60 A V/Cr on Al2O3 and d Cr =20 A V/Cr on MgO. Stronger anisotropy of Hc2| in 2D region is shown in the samples grown on MgO.

Influence of crystalline orientation on the superconducting properties in V/Cr-multilayers

We investigated the superconducting properties of V/Cr-multilayers grown epitaxially on MgO(001) and Al2O2(1120) substrates. The samples with a preferred orientation of (001) and (110) were fabricated on MgO and Al2O2, respectively. The Cr-layer thickness was varied from dCr equals 2 to 100 angstroms under 300 angstroms thick V- layer. The critical temperature for (001)V/Cr abruptly drops in the range of dCr equals 5 to 30 angstroms, becoming a plateau above it and keeps lower than that for (110)V/Cr, in which Tc is gradually decreased up to dCr equals 80 angstroms. The dimensional crossover phenomenon on the parallel upper critical fields HC2 takes place in the range of dCr equals 20 approximately 70 angstroms and 5 approximately 25 angstroms for (110)V/Cr and (001)V/Cr, respectively. The superconducting layers are decoupled by dCr equals 30 angstroms for (001) and 80 angstroms for (110). The multilayers with (001) texture exhibit a little larger anisotropy for HC2 in such decoupling region compared with the case of (110). The strong pair breaking effect for (001) samples may be caused by the magnetic structure of Cr(001) surface.

New phase line in the mixed state of Nb/NbZr multilayers

The superconducting multilayer Nb/NbZr is known to show an upturn feature in the parallel upper critical field Hc2(T) at a characteristic temperature T+ with decreasing T. This phenomenon occurs due to the change of the position for nucleation of order parameter (Delta) (r) from Nb to NbZr layers. In order to understand the behavior of the localized (Delta) (r) in the mixed state, we have measured the critical current density Jc of Nb/NbZr as a function of H and of angle between H and film plane. In this system, the peak effect has been reported in Jc(H) above T+ by previous work. In this work, besides this peak, two kinds of kink structures in Jc(H) are observed at fields HL1 and HL2 only in the temperatures above T+.HL1(T) and HL2(T) are plotted around the extrapolated line from Hc2(T) above T+. From the field dependence of the angular derivative of Jc and the discussion about the free energy in the system, we will show the possibility of the phase transition at these characteristic fields.

Anisotropy of I–V characteristics in YBa2Cu3O7−δ films

Abstract Current voltage (itI–V) curves CVD−YBa2Cu3O7−δ films near the glass transition temperature Tg in the mixed state are studied as a function of the angle θ between magnetic field andc axis. In the vortex-glass scaling analysis of I–V curves, it is found that the glass critical exponentsare independent of θ, and do not show a remarkable change even at θ=90°(H⊥c). Our results indicates that periodic plane-like intrinsic pinning is not so effective near Tg and vortices are pinned at the random point-like disorder for all field directions.

Angular dependence of upper critical fields in Nb/Nb1-xZrx-multilayers

Abstract The Nb/Nb0.5Zr0.5-multilayer shows an upturn feature in the parallel upper critical field Hc2|(T), caused by the shift of order-parameter from Nb- to NbZr-layers. In this work, the temperature dependence of the slope dHc2/dΘ near the field direction parallel to film is investigated in the region where the upturn occurs. This result is nearly in agreement with theoretical calculation, assuming the discontinuous periodic distribution of electron diffusion constant. Similar feature is found in Nb/Nb0.7Zr0.3 multilayers.