Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session N39: Focus Session: Superconductivity-Thin Films and Junctions Magnesium Diboride and Related Compounds |
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Sponsoring Units: DMP Chair: John Rowell, Arizona State University Room: Baltimore Convention Center 342 |
Wednesday, March 15, 2006 8:00AM - 8:36AM |
N39.00001: Clean and Dirty MgB$_2$ Thin Films by Hybrid Physical-Chemical Vapor Deposition Invited Speaker: The interband and intraband scattering in MgB$_2$ have significant influences on various properties of this two-band superconductor. In order to explore new physical phenomena and realize the potential for applications in MgB$_2$, it is desirable to control the interband and intraband scattering arbitrarily and independently. Hybrid physical-chemical vapor deposition (HPCVD) is a promising technique towards this goal. It has produced very clean MgB$_2$ films with a residual resistivity ratio of 60. The tensile strain in the films due to crystallite coalescence results in a softening of the $E_{2g}$ phonon and higher-than-bulk $T_c$ values. The long mean free path of the films, which are the cleanest MgB$_2$ materials reported, allow magnetoresistance measurements that reveal rich features of the two-band Fermi surfaces. The HPCVD technique also allows doping of the clean films in a controlled manner, which modifies the interband and intraband scattering and results in record-high upper critical field $H_{c2}$ of over 60 T. The demonstrations of HPCVD for high quality polycrystalline coatings on coated conductor fibers, high deposition speed and thick films, and on various polycrystalline substrates make it a promising technology for high field applications of MgB$_2$. [Preview Abstract] |
Wednesday, March 15, 2006 8:36AM - 8:48AM |
N39.00002: Hybrid Physical-Chemical Vapor Deposition of MgB$_{2}$ Film on Flexible Dielectric and Metallic Substrates Eric Maertz, A. V. Pogrebnyakov, J. M. Redwing, X. X. Xi The need for flexible dielectric and metallic substrates arises when considering making wires or tapes for high field applications. To accomplish this, Cu wire and foil with a buffer layer, flexible yttrium stabilized zirconium (YSZ), as well as Nb, Ta, and stainless steel foil were used as substrates for polycrystalline MgB$_{2}$ film growth. The foil substrates used range from 1 to 3 mil thickness. The buffer layers deposited on Cu were Ni plating (on 28 BSG wire) as well as TiB$_{2}$ and Nb deposited by sputtering. These served as a barrier to prevent the chemical reaction between Cu and Mg that occurs during deposition of MbB$_{2}$. The resistance vs. temperature (R-T) dependences were recorded for the films successfully grown on these substrates. For the films on YSZ, R-T was recorded initially and then after bending of the film on the substrate over a diameter of 20mm. The T$_{C}$ of MgB$_{2}$ on stainless steel was 38K; on YSZ and Nb it was 38.5K. This is lower than epitaxial films on SiC substrate with T$_{c}$ up to 41.5K. The R-T curve for MgB$_{2}$ on YSZ remained almost completely unchanged after bending. These films hold promise for electromagnetic field generation applications. This work is supported by NSF and ONR. [Preview Abstract] |
Wednesday, March 15, 2006 8:48AM - 9:00AM |
N39.00003: Planar MgB$_{2}$ superconductor-normal metal-superconductor Josephson junctions Ke Chen, Y. Cui, Qi Li, X. X. Xi, Shane A. Cybart, R. C. Dynes, X. Wen, J. M. Redwing We have fabricated planar superconductor-normal metal-superconductor (SNS) MgB$_{2}$ Josephson junctions using MgB$_{2}$ films grown by hybrid physical chemical vapor deposition (HPCVD). The junctions exhibit resistively-shunted-junction-like (RSJ-like) current-voltage characteristics up to 31~K. Ac Josephson effect was observed and the behavior of the Shapiro steps are in good agreement with theoretical predictions. The magnetic field modulation of the critical current also agrees with the thin film planar junction behavior. The junction's behavior can be described by Likharev's proximity effect model with rigid boundary at dirty limit. A dc SQUID with modulation depth of 45~\textit{$\mu $}V at 29~K has been demonstrated. This work is supported by ONR, NSF, and AFOSR. [Preview Abstract] |
Wednesday, March 15, 2006 9:00AM - 9:12AM |
N39.00004: Fabrication and characterization of MgB$_{2}$/ thermal oxide barrier /Pb Josephson tunnel junctions Yi Cui, Ke Chen, Qi Li, Xiaoxing Xi, John M. Rowell Cross-bridge Josephson tunnel junctions were fabricated, using MgB$_{2}$ films grown by hybrid physical-chemical vapor deposition (HPCVD) and barriers made by thermal oxidation at different temperatures. The junctions showed clear Josephson tunneling characteristics with high supercurrents, high $I_{c}R_{N}$ products ($I_{c}R_{N}$ products $\sim $1.8 mV at 4.2 K), and small subgap current leakage. The external DC magnetic field dependence was also measured and showed clear Fraunhofer pattern. The properties of the thermal oxide barrier depend sensitively on the oxidation temperature. The potential height and barrier thickness of 0.7 eV and 1.8 nm, respectively, were inferred from conductance measurements at high bias voltage. Two superconducting gaps of 2.0 meV and 7.5 meV for MgB$_{2}$ were observed from these sandwich-type tunnel junctions. These results suggest the potential of using MgB$_{2}$ thermal oxide layers as a barrier for practical Josephson tunnel junction fabrication. This work is supported by ONR and NSF. [Preview Abstract] |
Wednesday, March 15, 2006 9:12AM - 9:24AM |
N39.00005: MgB2 Tunnel Junctions with Native or Thermal Oxide Barriers Rakesh Singh, Raghuram Gandikota, Jihhon Kim, Nathan Newman, John Rowell MgB$_{2}$ tunnel junctions (MgB$_{2}$/barrier/MgB$_{2})$ were fabricated using an oxide (or a mixture of oxides) grown on the first MgB$_{2}$ film as the tunnel barrier, by exposure to air at 20$^{o}$C (native oxide) or 160$^{o}$C (thermal oxide). Such barriers therefore survived the deposition of the second electrode at 300$^{o}$C, even over junction areas of $\sim $1mm$^{2}$. The sum of the superconducting gaps of the top and bottom electrodes, from conductance-voltage data, was as high as 4.3 mV and this sum gap remained non-zero for temperatures above 30 K. Conductance vs. voltage dependencies of all-MgB$_{2}$ junctions and those of the type MgB$_{2}$/Native or Thermal Oxide/Metal (Pb, Au, or Ag) were used to characterize the height and width of the barriers formed. Such barriers have surprisingly low barrier heights, with typical values for barrier height and width being 0.2 V and 4.5 nm respectively. These values are very different from those reported in the literature. These results show that tunnel barriers grown on MgB$_{2}$ can have different properties (barrier height and width), depending on the film growth, surface composition and oxidation conditions. [Preview Abstract] |
Wednesday, March 15, 2006 9:24AM - 9:36AM |
N39.00006: All MgB$_{2}$ tunnel junction with Al$_{2}$O$_{3}$ tunnel barrier Heejae Shim, Jagadeesh S. Moodera MgB$_{2}$ tunnel junctions are attractive not only from superconducting electronics application part of view but also from the fundamental physics to understand multi-gap superconductors. All MgB$_{2}$ planar junctions with Al$_{2}$O$_{3}$ tunnel barrier were fabricated in situ in an MBE system by coevaporation of Mg and B for MgB$_{2}$ and plasma oxidized Al for tunnel barrier on Si (111) substrate. The junctions exhibit the current-voltage characteristic for quasiparticle and Josephson tunneling including microwave induced Shapiro steps. From conductance spectrum at 1 K, we clearly observe features that correspond to different $\pi $ and $\sigma $ superconducting energy gaps for the two MgB$_{2}$ electrodes. The observed multi-gap structure will be discussed with the difference of crystallographic orientation of MgB$_{2}$ at the interface between tunnel barrier and both superconducting layers. [Preview Abstract] |
Wednesday, March 15, 2006 9:36AM - 9:48AM |
N39.00007: Scanning tunneling spectroscopy and high-frequency response of MgB2 films M.S. Rzchowski, D.M. Kim, J. Giencke, C.B. Eom, T.W. Heitmann The critical issues governing thin-film MgB2 applications can be traced to factors both microscopic, arising from the two-gap structure and scattering mechanisms, as well as mesoscopic, determined by connectivity and grain boundary characteristics. These are intertwined to the extent that substitutional doping to control gap characteristics and band scattering can strongly affect connectivity through grain boundary segregation and growth of second phases. These issues can be addressed with a combined approach of gap characterization by low-temperature scanning tunneling spectroscopy, and temperature-dependent microwave conductivity measurements. We discuss preliminary results characterizing gap properties with low-temperature superconducting-tip scanning tunneling microscopy, and 10 GHz cavity-based microwave conductivity measurements. [Preview Abstract] |
Wednesday, March 15, 2006 9:48AM - 10:00AM |
N39.00008: Thickness dependence of the properties of MgB$_{2}$ films grown by hybrid physical-chemical vapor deposition Alexej Pogrebnyakov, Arsen Soukiassian, Joan Redwing, Xiaoxing Xi Properties of pure MgB$_{2}$ films of different thicknesses (up to $\sim$1 $\mu$m) grown by hybrid-physical-chemical vapor deposition on sapphire substrates were studied. In accordance with the previous results for the films with thicknesses up to about 400 nm, {\it T}$_{c}$ of the films on Al$_{2}$O$_{3}$ levels off at a value of 40.0 - 40.5 K at thicknesses larger than 200 nm. The residual resistivity, {\it $\rho$}$_{0}$, monotonically decreases with thickness, which is caused by a reduction of the surface and interface scattering (size effect on resistivity). For films with thickness over $\sim$ 800 nm, {\it $\rho$}$_{0}$ is below 0.15 $\mu$$\Omega$.cm and {\it RRR} $>$ 60. X-ray studies of the films did not reveal any other phases besides MgB$_{2}$. In this talk, MgB$_{2}$ films of even larger thickness and the thickness dependence of critical current density will also be reported. [Preview Abstract] |
Wednesday, March 15, 2006 10:00AM - 10:12AM |
N39.00009: Far-infrared Studies of the Two-Gap Behavior in Epitaxial MgB$_{2}$ Films Jiufeng Tu, G.L. Carr, R.P.S.M. Lobo, P. Orgiani, X.X. Xi Far-infrared transmission and reflectivity measurements have been carried out for a series of pure and carbon-doped epitaxial MgB$_{2}$ films. While the carbon-doped film exhibits the typical characteristics for a dirty BCS superconductor in the T$_{S}$ /T$_{N}$ and R$_{S}$ /R$_{N}$ ratios, the pure MgB$_{2}$ films can only be understood knowing the multi-gap nature of the superconducting state in MgB$_{2} $. As a function of increasing T$_{c}$, the fraction of the Cooper pairs having the larger gap increases. Both gaps appear to follow the BCS temperature dependence. However, the two gaps exhibit different behavior when a magnetic field is applied along c-axis. While the smaller gap can be suppressed by a relatively small field, the larger gap can persist up to 10 T. These infrared measurements indicate that the two superfluids coexist quite independently in the superconducting state of MgB$_{2}$. [Preview Abstract] |
Wednesday, March 15, 2006 10:12AM - 10:24AM |
N39.00010: Penetration Depth Anisotropy in MgB$_{2}$ measured by Small-Angle Neutron Scattering D. Pal, L. DeBeer-Schmitt, T. Bera, M.R. Eskildsen, C.D. Dewhurst, R. Cubitt, J. Jun, N.D. Zhigadlo, J. Karpinski, V.G. Kogan Traditionally the anisotropy of a type-II superconductor is described either by $\gamma _{\lambda }=\lambda _{c}$/$\lambda _{ab}$ or $\gamma _{H}$~=~$H_{ab}$/$H_{c}$~=~$\xi _{ab}$/$\xi _{c}$. with the two considered to be identical. However, in materials with anisotropic gaps this is generally not the case. MgB$_{2}$ represents an extreme case in which $\gamma _{\lambda }$~$\ne $~$\gamma _{H}$. While there is consensus on the value of $\gamma _{H}(T)$, measurements of $\gamma _{\lambda }$ are still contradictory. Here we demonstrate a novel use of small-angle neutron scattering to determine $\gamma _{\lambda }$ in MgB$_{2}$, by measuring the misalignment between the applied field and the direction of the flux-line lattice as the field is rotated between the $c$ axis and the basal plane. Using a two-band/two-gap model we can fit the angular dependence of the misalignment, yielding $\gamma _{\lambda }$~=~1.1 $\pm $ 0.2 at 4.9 K and 0.4 T. [Preview Abstract] |
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