Tunneling measurements of the electron-phonon interaction inBa1−xK

Abstract

The conductance curves of point-contact tunnel junctions between Ag and Ba1−xKxBiO3 (x 0.4) reveal a BCS behavior with low leakage current at zero voltage and some broadening of the superconducting-gap structure. In the energy range above the superconducting energy gap, the structure in the voltage dependence of the second derivative d 2 V /dI 2 of the voltage with respect to the current of the tunnel junction has been investigated in detail in magnetic fields up to 10 T . While part of this structure is rapidly changing in a magnetic field, three reproducible peaks inremain stable up to the transition temperature from the superconducting to the normal state with only additional broadening in the applied magnetic field. An analysis of this structure in terms of strong-coupling effects yields the spectral function α 2 F for the electron-phonon interaction. The obtained spectral weight in the energy region 20-70 meV points to the importance of the oxygen optical modes in the electron-phonon coupling for the superconductivity of Ba1−xKxBiO3.

Magnetic pair breaking in superconductingBa1−xKxBiOSzabó1, Samuely2, Jansen3 et al. 2000Phys. Rev. BSelf Cite17170View full textAdd to dashboardCite

“…In our case with l/ξ 0 about ∼ 0.7 also the strong-coupling plays an important role as shown in ref. [30].…”

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“…Whereas isotope effects are generally no direct measure of phonon involvement in superconductivity (see e.g. [19] ), the uniqueness of a (supposedly close) correspondence between the Eliashberg spectral function ~2F(to) and the phonon density-ofstates is experimentally difficult to demonstrate [25,26]. Ab initio calculations show little sign of such a relation [29,30].…”

“…Upon comparing the functions F(to) and ~2F(~o) it is found that their frequency dependence is entirely different, which is surprising in view of the tunneling results [25,26]. With decreasing (K,Pb) doping, ~2F(og) shifts markedly to lower frequencies and develops more intensity, especially in the range 20-40 meV, whereas the phonon density-of-states F(og) is much less sensitive.…”

“…We refer to that paper for more insight into the potential of tunneling experiments. Values of the gap, the coupling parameter 2 and the Coulomb parameter #* deduced from inversion analyses vary between (A) 3.8 5.0 meV, (2) 0.7-1.3, and (/~*) 0-0.11 [25,26] ; the reduced gap 2d/kBTc amounts to 3.8-4.1, indicating medium-strong coupling.…”

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“…The only spurious feature is a dip outside the gap at ±5 mV. Such dips could originate from a redistribution of the current path when a critical current is reached in a weak link or a crack nearby the contact [15]. This efect is hardly avoidable in the powder samples.…”