Tunneling Spectroscopy of Conventional and Unconventional Superconductors
Tunneling spectroscopy of conventional superconductors [1] such as Pb [2] leads to a complete description of the superconducting state. From the tunneling conductance \(\frac{{dI}}{{dV}}\) vs. V (appropriately normalized), one can obtain the quasiparticle
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8.1
Introduction
Tunneling spectroscopy of conventional superconductors [1] such as Pb [2] leads to a complete description of the superconducting state. From the tunneling conductance, vs. V (appropriately normalized), one can obtain the quasiparticle density of states, N(E). This gives an implicit measure of the complex, superconducting gap parameter, Ll(E). Using Migdal-Eliashberg
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K. H. Bennemann et al. (eds.), The Physics of Superconductors © Springer-Verlag Berlin Heidelberg 2003
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J. Zasadzinski
theory [3] the gap parameter can be "inverted" by the iterative McMillanRowell procedure (MR) [2] to obtain the microscopic interactions responsible for superconductivity, namely, the electron-phonon spectral function, 0;2 F(w), and the renormalized coulomb pseudopotential, /1>*. These quantities can then be used to determine the transition temperature, T c , as well as the electron mass enhancement, 1+,\, which enters normal state thermodynamic properties such as the specific heat [4]. In some cases the 0;2 F(w) obtained from tunneling has been used to provide a very good fit of the temperature dependent electrical resistivity far above Tc [5]. The fact that both superconducting and normal state quantities can be determined demonstrates both the accuracy of strong-coupling superconductivity theory and the power of the tunneling method. The various manifestations of the electron-phonon interaction can be found in the review articles of Carbotte [3] and Allen [4]. Using a straightforward extension of the basic tunneling measurement to normal metal superconductor (N IS) bi-Iayers, the quantities 0;2 F(w) and /1>* have been obtained for some transition metal elements (Nb, V, Ta), alloys (NbZr) and A-15 compounds such as Nb 3 Sn as well as nonsuperconducting materials such as Mg [1]. Furthermore, such proximity effect junctions of the form Nbl All AI-oxide/Nb are useful for a variety of devices from sensitive photon detectors to fast switches and are the mainstay of a successful superconducting electronics industry. Considering the potential of tunneling spectroscopy for basic research and applications, it is not surprising that many groups around the world have attempted to fabricate junctions on exotic superconductors including the high-Tc cuprates and bismuthates, organics, heavy fermion materials and the recently discovered MgB 2. Successful inversion of tunneling spectra using a variation of the traditional MR procedure has been accomplished for the bis-muthate, Bal_xKxBi03, and the relatively lower T c , electron-doped cuprate Nd1.85Ceo.15Cu04 [6]. The resulting 0;2 F(w) spectra strongly suggest phonon mediated pairing, however there remain unresolved questions such as whether this interaction is compatible with d-wave symmetry [7]. For all of the other classes of exotic materials the current state-of-the-art tunneling spectroscopy does not include a quantitative determination of the pairing mechanism. This is mostly a consequence of their remarkable complexity. Tunneling in these materials has, neverthe
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