J.H. Claassen

Growth and characterization of vanadium dioxide thin films prepared by reactive-biased target ion beam deposition

Using a novel growth technique called reactive bias target ion beam deposition, the authors have prepared highly oriented VO2 thin films on Al2O3 (0001) substrates at various growth temperatures ranging from 250to550°C. The influence of the growth parameters on the microstructure and transport properties of VO2 thin films was systematically investigated. A change in electrical conductivity of 103 was measured at 341K associated with the well known metal-insulator transition (MIT). It was observed that the MIT temperature can be tuned to higher temperatures by mixing VO2 and other vanadium oxide phases. In addition, a current/electric-field induced MIT was observed at room temperature with a drop in electrical conductivity by a factor of 8. The current densities required to induce the MIT in VO2 are about 3×104A∕cm2. The switching time of the MIT, as measured by voltage pulsed measurements, was determined to be no more than 10ns.

Epitaxial growth of superconducting niobium thin films by ultrahigh vacuum evaporation

A series of thin films of niobium have been grown on various orientations of single crystal sapphire by e‐beam evaporation in an ultrahigh vacuum molecular beam epitaxy system. The films were grown at temperatures between 600 and 950u2009°C and at growth rates from 0.1 to 10 A/s. The films were single‐crystal as determined by both in situ reflection high energy electron diffraction patterns and by Read photographs. The best films had rocking curve widths of the order of 1/10 of a degree. The superconducting transition temperature was measured as a function of film thickness and crystal orientation and ranged up to 9.35 K for a thick sample grown on [1102] sapphire.

Photoconductive response of granular superconducting films

The application of a DC magnetic field to disordered or granular films of Y-Ba-Cu-O is shown to lead an enhancement of a nonbolometric photoresponse at temperatures near and below T/sub c/. The disorder is evidenced by the broadened resistive transition to superconductivity and by the higher normal state resistance of the films. This observation is consistent with a nonequilibrium effect, which is described by the flux-flow model of the low-temperature photoresponse of granular superconductors. >

Inductive measurements of critical current density in superconducting thin films

A noncontacting method of probing the current-induced breakdown of superconductivity (i.e., J/sub c/) in thin films is described which makes use of a single pancake coil pressed against the film surface. The technique has a sensitivity that is approximately 100 times greater than direct transport measurements using room-temperature electronics, and it eliminates many of the attendant difficulties of the latter. Preliminary results on Nb and Y-Ba-Cu-O films at 4.2 K reveal an exponential voltage-current dependence, as expected from the activated flux creep model. It is noted that, this being the case, no unique critical-current density can be defined. In the case of the oxide superconductors the flux-pinning parameters are such that even a practical J/sub c/ definition is probably not useful. >

Multilayer Nb 3 Sn superconducting shields

Multilayer Nb 3 Sn shields have been designed, fabricated, and tested for use in low field, e.g. superconducting electronics, applications. Magnetic field profiles, shielding factors, and thermal effects are reported for these shields. The quality of the shields depends strongly on the reaction temperature used to form the Nb 3 Sn layers with lower reaction temperatures forming better quality shields. Nearly ideal behavior was observed with a reaction temperature of 750°C.

Relaxation dynamics of the metal-semiconductor transition in VO2 thin films

The temperature dependent metal-insulator transition in VO2 films displays substantial hysteresis. This is usually attributed to supercooling or superheating in the grains. In the hysteretic region at fixed temperature there is a relaxation of the resistance toward a presumed equilibrium value. We have made careful measurements of this relaxation, and find that it proceeds with a logarithmic dependence on time. If the transformation is thermally activated in individual grains, the log(t) behavior can be explained either by a wide distribution of activation energies U among grains or a dependence of U on the phase of adjacent grains.

A laser quenched superconducting switch for pulsed power applications

We have prepared micron thick films of NSN and NbCN by rf reactive sputtering. Sputtering conditions were varied in order to maximize the critical current density and the normal state resistivity. Both of these quantities are important for a normally closed switch in parallel with a load. The product of these two quantities is a figure of merit used to compare films prepared under different conditions. By varying substrate temperature, argon pressure and adding small amounts of oxygen to the reactive gas mixture, we have achieved a value of 900 V/cm. In addition we have fabricated some of these films into a laser activated switch which opens and recovers in times on the order of nanoseconds.

Thermal sensitivity of an rf SQUID

A well known limitation to the resolution of practical SQUID systems is set by the sensitivity of the SQUID output to temperature fluctuations. This is particularly serious in non-stationary systems where agitation of the helium bath inevitably results in temperature fluctuations of the SQUID. We have undertaken to characterize the thermal response of commercial RF SQUIDS and tried to understand the causes of this sensitivity. The measurements are done with the SQUID package mounted in a thermally isolated can with He exchange gas. The input coil is open-circuited, and the SQUID unit is placed inside a Nb shield, resulting in negligible sensitivity to external fields. The ambient magnetic field is reduced to less than 10/sup -2/ G with mu-metal shields. Our measurement procedure is to slowly cool the SQUID and Nb shield through their superconducting transition temperature in the presence of various externally applied magnetic fields, then observe the temperature dependence of the SQUID output ..delta.. /phi/ from 2K to the highest temperature at which it will operate. It is of course necessary to readjust the RF drive level to its optimum level at each temperature since the critical current of the weak link in the SQUID has a strongmorexa0» temperature dependence. We find that near the optimum setting the SQUID output depends only weakly on RF level, so this is not a crucial adjustment.«xa0less

NbN/BN granular films--A novel broadband bolometric detector for pulsed far infrared radiation

Thin films of NbN/BN are investigated as photodetectors for pulsed, far infrared radiation. It is found that there are two distinct modes of operation for these devices, depending on whether the device is biased in the superconducting or resistive regime. When the device is biased into the resistive region, the absorption of photons decreases its resistance. Conversely, when the device is superconducting, the absorption of photons causes the resistance to increase. The more sensitive mode occurs when the device is biased superconducting. Detector parameters are studied in this condition and are compared to other, more conventional detectors (ie. pyroelectric, point contact diodes, semiconductor photoconductors). It is found that the NbN/BN thin film detectors have response times faster than 1 ns, and sensitivities of about 0.2 V/W.

Investigation of noise sources in SQUID electronics

The performance of SQUID (superconducting quantum interference device)-based electronics may be degraded from that found in laboratory operation. Investigations on superconducting tubes, wires, and sheets have been conducted to identify contributions to such noise. Results have been obtained for bulk and thin-film samples utilizing both the conventional low-temperature materials and the new high temperature oxide materials. Experiments have been conducted to quantify flux redistribution and flux motion in superconducting samples subjected to temperature changes, temperature gradients, and magnetic field gradients. These investigations have been conducted for magnetic fields typical of many SQUID applications, with field intensities much smaller than the critical values H/sub c1/. Penetration-depth, flux-pinning, and flux-motion effects have been observed. The various types of experiments conducted along with specific results are described. >