Electron Tunneling in High T c Superconductors
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a signature of the detailed binding mechanism would go a long way toward revealing the similarities and différences of thèse remarkable materials with the more conventional superconductors. This article will discuss some of what has been learned from tunneling measurements in the high Tc oxides. We consider the BaBi03 materials are also high Tc superconductors and support that thesis with arguments based on tunneling results. We will concentra te on measurements in BaPb 1 . I Bi I 0 3 and YBa2 Cu 3 0 7 (YBCO), because it is thèse compounds which hâve been most extensively studied and for which the most reproducible results exist. We will compare results obtained on other oxide components, but in less détail. We cannot include ail tunneling studies of high Tc superconductors because the literature is simply too vast, and so we will présent a rather parochial review of the field. The article is divided into a discussion of the superconducting properties and the normal-state properties. We discuss
x100
the superconducting measurements for obvious reasons. We believe the normalstate measurements illustrate some unique features of thèse materials and suspect that a key to understanding the superconducting properties lies in understanding thèse normal-state measurements. Superconducting Tunneling Extensive tunneling studies hâve been performed on the compound BaPb^BijOa in the région 0 < x < 025 where the material is a superconductor.2 By far, the cleanest data is obtained at x=0.25.1 For measurements x7^0.25 it is difficult to convince oneself that the material is single phase because the superconducting résistive transition is broad1 and the tunneling measurements show either very broad gap structure or multiple gaps.3 At x =0.25 high-quality tunnel junctions are routinely formed by either condensing a thin film on a freshly cleaved surface or bringing a métal into contact with the surface (point contact, solder blob, pressed métal contact, etc.). The tunnel barrier that forms occurs naturally and may be either a natural Shottky barrier or a région of depleted oxygen, for example. At any rate the junctions are of extremely high quality, as is evidenced by the energy gap structure of both the BaPbo.25Bio.25O3 ar>d t n e counter-electrode (Pb, Sn, In, etc.). A typical I-V characteristic of a BaPbo.75Blo.25O3/In junction where the In is driven normal by a modest (1 kG) magnetic field is shown in Figure 1. The energy gap edge is clearly évident as the point where current begins to flow. Studies of over 60 junctions of this type hâve shown similar behavior with very little variation in the extracted value of the energy gap 2A. For finite températures, the I-V characteristic of a superconductor insulatornormal métal tunnel junction is given by I(V) = CN0j
Ns(E)[f(E,T)
-f(E + V,T)]dE
Figure 1. Current vs. voltage for a typical BaPb0.25Bi0J5O}lln junction. From this plot, we obtain à—1.65 meV.
(1)
where V is the applied voltage, N0 is the normal métal density of states (assumed constant), C is a constant which contains ai
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