Pure Metallic Bridge and Point-Contact Tunneling Into Single- And Polycrystalline YBa2Cu3O7-y
It is known theoretically and experimentally1 that when the resistance of a point-contact is decreased the current-voltage characteristic, I(V), changes from that of an insulating barrier to that of a pure metallic bridge. Such an I(V) still shows the energy gap but, because of its much lower resistance, it is less sensitive to extraneous conduction mechanisms. Values of between 25 and 35 meV are reported here for YBa2Cu3O7−y, and these are in agreement with our results for low-resistance point-contact tunneling in both single-and poly-crystal samples. Examples of I(V) and the conductance, dI/dV, will also be presented for pure metallic bridges as well as higher resistance point-contact tunneling. An electrically insulating layer is found on the surface of all samples, including single-crystals showing no evidence of impurities measured by Raman scattering2. Consequently, contact of the tunneling tip is necessary to mechanically scrape, and thus clean, the surface before a measurable current can be obtained. Although this represents a disadvantage, a more serious drawback of point-contact tunneling is the inability to measure the I(V) continuously through the transition temperature, Tc, due to thermal expansion of the mechanical apparatus. We will argue that thin film tunnel junctions are thus desirable both to unambiguously identify the energy gap with the 90 K bulk material, and to subtract the background conductance to evaluate the electron coupling mechanism in the high-Tc superconductors (HTS), that is analogous to electron-phonon coupling in traditional superconductors3.
KeywordsTunnel Junction Andreev Reflection Mechanical Apparatus Resonate Valence Bond Background Conductance
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