B. Y. Jin

Superconducting properties of V/Fe superlattices

Measurements are reported on the superconducting properties of V/Fe superlatitices showing the interplay between ferromagnetism and superconductivity. When the V layer thickness is on the order of the BCS coherence length and the Fe layer is only a few atomic planes thick, a 2D–3D crossover is observed in the temperature dependence of the parallel upper critical field Hc2‖. This implies the coexistence of superconductivity and ferromagnetism in the Fe layers. Three-dimensional behavior for thinner Fe layers is observed (∼1 atomic plane) and 2D behavior for thicker Fe layers (>10 atomic planes).

V/Fe composition‐modulated structures

V/Fe composition‐modulated structures have been grown on the (0001) surface of a sapphire substrate in an UHV evaporator. X‐ray θ‐2θ scans showed that the samples had a bcc (110) texture with a strong composition modulation. Samples with three atomic planes of Fe separated by thick layers of V showed a linear temperature dependence of the saturation magnetization over the range 5–400 K. This behavior is expected for the Bloch spin wave contribution to the magnetization of a 2D ferromagnet. Hysteresis measurements also confirmed that the samples were ferromagnetic. Superconductivity has been seen in samples with very thick V layers.

Superconducting properties of layered Nb0.53Ti0.47/Ge structures prepared by dc sputtering

Sandwich and superlattice structures composed of the high critical field superconducting alloy Nb0.53Ti0.47 (where the compositions refer to weight percent) and amorphous Ge were sputter‐deposited on sapphire substrates using a novel multi‐sputter‐gun system. The thicknesses of both the superconducting Nb0.53Ti0.47 and the insulating Ge layers were varied over a wide range in order to study the two‐dimensional (2D) and three‐dimensional (3D) superconducting properties of these structures. The observed suppression of the measured resistive superconducting transition temperature Tc with increasing sheet resistance is tentatively interpreted using the Maekawa–Fukuyama theory which incorporates contributions from localization and Coulomb interaction effects. The upper critical fields, with the field both normal Hc2⊥ and parallel Hc2∥ to the film, were measured up to 50 kG. Hc2∥ exhibited 2D behavior when the thickness of the superconducting layer was less than 200 A. The zero temperature values of Hc2∥(0) wer...

Anisotropic upper critical fields of disordered Nb0.53Ti0.47-Ge multilayers

Studies are reported of the upper critical fields of Nb0.53Ti0.47-Ge multilayers consisting of thick Ge layers and varying-thickness Nb0.53Ti0.47 layers. Both the angular dependence and the temperature dependence of the upper critical fields indicate a dimensional crossover at a Nb0.53Ti0.47 layer thickness near 200 Å. All the 2D samples display a cusplike upper critical field angular dependence with a sharper cusp for thinner Nb0.53Ti0.47 layers. The parallel upper critical fields are tentatively fitted with an expression combining the 2D field dependence of Rickayzen, the paramagnetic limiting behavior of Maki, and the disorder-related Coulomb interaction effects of Maekawa and Fukuyama. The perpendicular fields are fitted with the Maekawa, Ebisawa, and Fukuyama theory; better agreement is obtained for thinner Nb0.53Ti0.47 sublayers when the paramagnetic limiting effect is included.

Surface waves in SnTe/Sb superlattices

The surface wave velocity in SnTe/Sb superlattices has been determined using Brillouin scattering. The velocity is independent of modulation wavelength over the range studied but is lower than would be expected if the layers of Sb and SnTe possessed their bulk properties. This is consistent with x‐ray data which can be interpreted as evidence of a cubic polymorph of Sb which is unstable in bulk form.

Computer-controlled four-gun multisubstrate sputtering system for the preparation of composition-modulated structures

We describe a four‐gun sputtering system which has been used successfully to prepare composition‐modulated structures consisting of amorphous semiconductors and superconducting alloys or binary compounds. The substrates are mounted in ovens which can be individually heated to over 1000 °C. The ovens are mounted on a wheel which is driven by a computer‐controlled stepping motor. With appropriate programming of the movement of the stepping motor, the deposition sequence is determined.

Preparation and structural analysis of SnTe/Sb composition modulated structures

A new composition modulated structure composed of semiconducting SnTe and semimetallic Sb has been grown epitaxially on mica substrates with differing SnTe to Sb ratios. The samples were characterized by x‐ray diffraction using the transmission Laue patterns and θ‐2θ diffractometer scans with scattering vectors both normal and in the plane of the film. We observed that the modulation satellites were 180° out of phase for films with short modulation wavelengths, but were in phase for long wavelengths. This change in the phase of the modulation can be understood by assuming that Sb adopts a quasicubic structure at short modulation wavelength, due to the coherency strain, but relaxes back to a distorted rhombohedral structure, with the loss of coherency, at long wavelengths. A simple 1‐D model was used to estimate the modulation wavelength dependence of the sublattice displacement of Sb.

Superconducting fluctuations, weak anti-localization and interaction effects in Nb0.53Ti0.47-Ge multilayers

Abstract The effect of weak anti-localization, electron interactions and superconducting fluctuations on the transport properties of disorder Nb 0.53 Ti 0.47 -Ge multilayers were studied. The temperature dependence of the inelastic scattering time was found to be τ in ∼ T −1±0.25 and τ in ∼ T −1.75±0.25 for temperatures lower and higher than ∼ 6K, repectively for a sample with thick Ge layers. The effect of the Ge thickness on the prefacter A in the expression R □ ∼ ln T may arise from an interlayer electron-phonon process.

Superconducting tunneling through Nb0.53Ti0.47Ge multilayers

Abstract We have made a preliminary study of vertical electrical transport in Nb 0.53 Ti 0.47 Ge multilayers having the structure Nb/Ge/Nb0.53Ti0.47/Ge … Nb0.53Ti0.47/Ge/Nb. The initial and final Nb layers serve as equipotential electrodes, and measurements are analyzed only in the temperature range below which the thick Nb electrodes are superconducting. In-plane (parallel) transport studies were made on identical Nb 0.53 Ti 0.47 Ge multilayers (deposited at the same time on a different part of the substrate using an appropriate mask). The layered structure is confirmed by low angle X-ray diffraction and SEM microscopy. Depending on the transition temperature of the Nb 0.53 Ti 0.47 Ge multilayers, the temperature-dependent junction resistance shows several interesting features. The I–V characteristics and the first derivative dI/dV were measured, yielding a sum gap of nearly 20meV for a 16 layer structure having a Tc of 7K. Possible interpretations of these results are presented.

Preparation of Large Area NbN/AlN/NbN Josephson Junctions

AlN has been used as a barrier material in large josephson junctions. The chemical and structural compatibility of AlN with NbN make it possible to fabricate NbN/AlN/NbN junctions by sequential reactive sputtering in a common Ar and N2 atmosphere. In a junction with an area of about 1.0×1.0 mm2, having a transition temperature of 14.5K. the measured I–V and first derivative curves yield a sum gap value of about 3.0 meV.