A thin film technology compatible with multilayer device fabrication is critical for exploring the potential of the 39-K superconductor magnesium diboride for superconducting electronics. Using a Hybrid Physical-Chemical Vapor Deposition (HPCVD) process, it is shown that the high Mg vapor pressure necessary to keep the MgB 2 phase thermodynamically stable can be achieved for the in situ growth of MgB 2 thin films. The films grow epitaxially on (0001) sapphire and (0001) 4H-SiC substrates and show a bulk-like T c of 39 K, a J c (4.2K) of 1.2 × 10 7 A/cm 2 in zero field, and a H c2 (0) of 29.2 T in parallel magnetic field. The surface is smooth with a root-mean-square roughness of 2.5 nm for MgB 2 films on SiC. This deposition method opens tremendous opportunities for superconducting electronics using MgB 2 .

We demonstrate that a high-intensity electron beam can be applied to create holes, gaps, and other patterns of atomic and nanometer dimensions on a single nanowire, to weld individual nanowires to form metal-metal or metal-semiconductor junctions, and to remove the oxide shell from a crystalline nanowire. In single-crystalline Si nanowires, the beam induces instant local vaporization and local amorphization. In metallic Au, Ag, Cu, and Sn nanowires, the beam induces rapid local surface melting and enhanced surface diffusion, in addition to local vaporization. These studies open up a novel approach for patterning and connecting nanomaterials in devices and circuits at the nanometer scale.

A simple method for penetrating the barrier layer of an anodic aluminum oxide (AAO) film and for detaching the AAO film from residual Al foil was developed by reversing the bias voltage in situ after the anodization process is completed. With this technique, we have been able to obtain large pieces of free-standing AAO membranes with regular pore sizes of sub-10 nm. By combining Ar ion milling and wetting enhancement processes, Au nanowires were grown in the sub-10 nm pores of the AAO films. Further scaling down of the pore size and extension to the deposition of nanowires and nanotubes of materials other than Au should be possible by further optimizing this procedure.

We report a systematic increase of the superconducting transition temperature T(c) with a biaxial tensile strain in MgB2 films to well beyond the bulk value. The tensile strain increases with the MgB2 film thickness, caused primarily by the coalescence of initially nucleated discrete islands (the Volmer-Weber growth mode.) The T(c) increase was observed in epitaxial films on SiC and sapphire substrates, although the T(c) values were different for the two substrates due to different lattice parameters and thermal expansion coefficients. We identified, by first-principles calculations, the underlying mechanism for the T(c) increase to be the softening of the bond-stretching E(2g) phonon mode, and we confirmed this conclusion by Raman scattering measurements. The result suggests that the E(2g) phonon softening is a possible avenue to achieve even higher T(c) in MgB2-related material systems.

We have studied the effect of deposition rate and layer thickness on the properties of epitaxial MgB 2 thin films grown by hybrid physical-chemical vapor deposition on 4H-SiC substrates. The MgB 2 film deposition rate depends linearly on the concentration of B 2 H 6 in the inlet gas mixture. We found that the superconducting and normal-state properties of the MgB 2 films are determined by the film thickness, not by the deposition rate. When the film thickness was increased, the transition temperature, T c , increased and the residual resistivity, ρ 0 , decreased. Above 300 nm, a T c of 41.8 K, a ρ 0 of 0.28 µΩ·cm, and a residual resistance ratio RRR of over 30 were obtained. These values represent the best MgB 2 properties reported thus far.

We have studied the effect of damage induced by 2 MeV alpha particles on the critical temperature, T c , and resistivity, , of MgB 2 thin films. This technique allows defects to be controllably introduced into MgB 2 in small successive steps. T c decreases linearly as the intragrain resistivity at 40 K increases, while the intergrain connectivity is not changed. T c is ultimately reduced to less than 7 K and we see no evidence for a saturation of T c at about 20 K, contrary to the predictions of the T c of MgB 2 in the dirty limit of interband scattering.

We have used two polytypes of silicon carbide single crystals, 4H-SiC and 6H-SiC, as the substrates for MgB 2 thin films grown by hybrid physical-chemical vapor deposition ͑HPCVD͒. The c-cut surface of both polytypes has a hexagonal lattice that matches closely with that of MgB 2. Thermodynamic calculations indicate that SiC is chemically stable under the in situ deposition conditions for MgB 2 using HPCVD. The MgB 2 films on both polytypes show high-quality epitaxy with a Rutherford backscattering channeling yield of 12%. They have T c above 40 K, low resistivities, high residual resistivity ratios, and high critical current densities. The results demonstrate that SiC is an ideal substrate for MgB 2 thin films.

Transport measurements were made on a system consisting of a zinc nanowire array sandwiched between two bulk superconducting electrodes (Sn or In). It was found that the superconductivity of Zn nanowires of 40 nm diameter is suppressed either completely or partially by the superconducting electrodes. When the electrodes are driven into their normal state by a magnetic field, the nanowires switch back to their superconducting state. This phenomenon is significantly weakened when one of the two superconducting electrodes is replaced by a normal metal. The phenomenon is not seen in wires with diameters equal to or thicker than 70 nm.