H. Q. Yang

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).

Magnetic properties of V/Fe superlattices

We have measured the magnetic and superconducting properties of V/Fe superlattices. It is observed that ultrathin Fe layers behave in a 2‐D fashion when isolated by thick V layers; however, on thinning the V layers a magnetic coupling between the Fe layers has been observed. No dead Fe layers were seen. When the V‐layer thicknesses were on the order of the BCS coherence length, superconductivity was observed. A 2‐D–3‐D crossover was observed in the temperature dependence of the parallel critical field when the Fe layers were only a few atomic planes thick, implying the coexistence of ferromagnetism and superconductivity.

Dual electron beam evaporator for the preparation of composition‐modulated structures

We describe the construction of an UHV evaporator with two electron beam guns and a heated substrate turntable which is used for the preparation of composition‐modulated structures (CMS’s). Using a suitably controlled stepping motor, up to 20 different CMS’s can be prepared in the same run. The system has been used to deposit V/Fe CMS’s and preliminary structural measurements indicate the samples are of high quality.

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.

Critical-field measurements in Nb-Ti composition-modulated alloys

We present measurements of the transition temperature and upper parallel critical field, to 15 kG, of layered Nb-Ti alloys for layer wavelengths between 6 and 6250 Å.

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...

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.

Computer-controlled, three-electron-gun, multisubstrate evaporator for the preparation of multilayer thin films

We have developed a novel multisource electron‐beam evaporator to prepare high‐quality multicomponent, multilayer thin films. The essential parts of the evaporator consist of three electron guns, a shutter turntable, and a multisubstrate turntable; the latter can be loaded with up to 20 substrates. A very attractive feature of the system is the implementation of computer control over the movement of the stepping motors. The two turntables are driven by separate stepping motors with optical encoders, and are used to: (i) position a selected substrate over a specified electron gun, and (ii) gate the flux for a desired time. With a suitable programming of the movement of the stepping motors, up to 20 different thin films, with individually tailored layers deposited sequentially from the three sources, can be prepared in a single run. The deposition rate and the sublayer thickness of the three constituents of a multilayer film are individually monitored by three film deposition controllers which communicate directly to the computer and the electron‐gun power supply. Predictable and reproducible deposition of thin films is accomplished by accurate, digital control of the deposition process. High‐quality films of YBaCuO have been prepared with this system.

MINIATURE MULTITARGET SPUTTERING SYSTEM FOR THE IN SITU X-RAY STUDY OF HIGH TC MULTILAYER FILM GROWTH

A miniature sputtering system equipped with mylar windows allowing for normal and grazing incidence x‐ray studies to be performed in real time during film growth is described herein. The system incorporates multitarget sputtering guns which allow for the deposition of up to four different materials. In this way multilayer structures can be studied in situ without changing the substrate environment or alignment between layers.

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.