Energy Gap from Tunneling and Metallic Sharvin Contacts onto MgB2: Evidence for a Weakened Surface Layer

Point-contact tunnel junctions using a Au tip on sintered MgB2 pellets reveal a sharp superconducting energy gap that is confirmed by subsequent metallic Sharvin contacts made on the same sample. The peak in the tunneling conductance and the Sharvin contact conductance follow the BCS form, but the gap values of 4.3 meV are less than the weak-coupling BCS value of 5.9 meV for the bulk Tc of 39 K. The low value of Delta compared to the BCS value for the bulk Tc is possibly due to chemical reactions at the surface.Comment: 3 pages, 3 figure

The optical response of Ba_{1-x}K_xBiO_3: Evidence for an unusual coupling mechanism of superconductivity?

We have analysed optical reflectivity data for Ba_{1-x}K_xBiO_3 in the far-infrared region using Migdal-Eliashberg theory and found it inconsistent with standard electron-phonon coupling: Whereas the superconducting state data could be explained using moderate coupling, \lambda=0.7, the normal state properties indicate \lambda \le 0.2. We have found that such behaviour could be understood using a simple model consisting of weak standard electron-phonon coupling plus weak coupling to an unspecified high energy excitation near 0.4 eV. This model is found to be in general agreement with the reflectivity data, except for the predicted superconducting gap size. The additional high energy excitation suggests that the dominant coupling mechanism in Ba_{1-x}K_xBiO_3 is not standard electron-phonon.Comment: 5 pages REVTex, 5 figures, 32 refs, accepted for publication in Phys. Rev.

Tunneling in Cuprate and Bismuthate Superconductors

Tunneling measurements using a point-contact technique are reported for the following high temperature superconducting oxides: Ba{sub 1-x}K{sub x}BiO{sub 3}( BKBO ), Nd{sub 2-x}Ce{sub x}CuO{sub 4}( NCCO ), Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 7}( BSCCO ) and Tl{sub 2}Ba{sub 2}CaCu{sub 2}O{sub x} ( TBCCO ). For the bismuthate, BKBO, ideal, S-I-N tunneling characteristics are observed using a Au tip. The normalized conductance is fitted to a BCS density of states and thermal smearing only proving there is no fundamental limitation in BKBO for device applications. For the cuprates, the normalized conductance displays BCS-like characteristics, but with a broadening larger than from thermal smearing. Energy gap values are presented for each material. For BKBO and NCCO the Eliashberg functions, {alpha}{sup 2}F({omega}), obtained from the tunneling are shown to be in good agreement with neutron scattering results. Proximity effect tunneling studies are reported for Au/BSCCO bilayers and show that the energy gap of BSCCO can be observed through Au layers up to 600 {Angstrom} thick

Defect Detection in Wire Manufacturing

In the sequential drawing of metallic wires from an initial diameter of 196 mils (4.9 mm) to a final diameter just under 1 mil (25 μm), the wires sometimes break. Subsequent examination via TEM of broken Al wires [1] revealed that breaks occurred most often due to inclusions of Si almost as large as the wire diameter. The challenge is to devise a technique that can detect inclusions well before the wire is drawn to about the diameter of the inclusions. For example, if there is a larger diameter wire that has commercial use, and some inclusions are detected at that diameter, then the drawing-down process could be stopped at that point, and only the wires without inclusions will be drawn to smaller diameters. A different problem occurs in the manufacture of composite superconducting wires, most typically multifilament NbTi in a Cu matrix. Here the sausaging of NbTi filaments causes a reduced critical current. For this case the challenge is to devise a ]nondestructive method at room temperature for determining the integrity and quality of the NbTi filaments during or immediately after the fabrication process.</p

Tunneling in cuprate and bismuthate superconductors

Tunneling measurements using a point-contact technique are reported for the following high temperature superconducting oxides: Ba{sub 1-x}K{sub x}BiO{sub 3}(BKBO), Nd{sub 2-x}Ce{sub x}CuO{sub 4}(NCCO), Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 7}(BSCCO) and Tl{sub 2}Ba{sub 2}CaCu{sub 2}O{sub x} (TBCCO). For the bismuthate, BKBO, ideal, S-I-N tunneling characteristics are observed using a Au tip. The normalized conductance is fitted to a BCS density of states and thermal smearing only proving there is no fundamental limitation in BKBO for device applications. For the cuprates, the normalized conductance displays BCS-like characteristics, but with a broadening larger than from thermal smearing. Energy gap values are presented for each material. For BKBO and NCCO the Eliashberg functions, {alpha}{sup 2}F({omega}), obtained from the tunneling are shown to be in good agreement with neutron scattering results. Proximity effect tunneling studies are reported for Au/BSCCO bilayers and show that the energy gap of BSCCO can be observed through Au layers up to 600 {Angstrom} thick

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In the sequential drawing of metallic wires from an initial diameter of 196 mils (4.9 mm) to a final diameter just under 1 mil (25 μm), the wires sometimes break. Subsequent examination via TEM of broken Al wires [1] revealed that breaks occurred most often due to inclusions of Si almost as large as the wire diameter. The challenge is to devise a technique that can detect inclusions well before the wire is drawn to about the diameter of the inclusions. For example, if there is a larger diameter wire that has commercial use, and some inclusions are detected at that diameter, then the drawing-down process could be stopped at that point, and only the wires without inclusions will be drawn to smaller diameters. A different problem occurs in the manufacture of composite superconducting wires, most typically multifilament NbTi in a Cu matrix. Here the sausaging of NbTi filaments causes a reduced critical current. For this case the challenge is to devise a ]nondestructive method at room temperature for determining the integrity and quality of the NbTi filaments during or immediately after the fabrication process

Tunneling spectroscopy measurements on low-leakage junctions of Nd sub 1. 85 Ce sub 0. 15 CuO sub 4 minus y and Ba sub 1 minus x K sub x BiO sub 3

Tunneling measurements are reported for Nd{sub 1.85}Ce{sub 0.15}CuO{sub 4{minus}y} (NCCO) and Ba{sub 1{minus}x}K{sub x}BiO{sub 3} (BKBO) using the point-contact technique with a Au or Nb tip. The junctions display very low zero-bias conductance values of <1% for BKBO and {approximately}10% for NCCO, and show sharp conductance peaks at eV = {plus minus} {Delta}. In the case of BKBO, the normalized conductance can be fit with a BCS density of states with thermal smearing only. The energy gaps are in good agreement with optical measurements and lead to values of 2{Delta}k{Tc} in the range of 3.5--4.0 for both compounds implying moderate coupling strength. The tunneling conductance data at high bias voltages display reproducible structures which are characteristic of phonon effects as found in conventional superconductors. We have obtained the Eliashberg functions, {alpha}{sup 2}F({omega}), and the resulting values of {lambda} determined from this analysis are consistent with the measured values of the strong-coupling ratio, 2{Delta}/k{Tc}. We conclude that the pairing mechanism in both materials is principally phonon mediated. 12 refs., 6 figs., 1 tab