With improvements in diode fabrication techniques, values of 2Δ/kT c obtained with La films via tunneling have steadily increased1–5 from 1.6 to the full bulk value (this work) of 3.7–3.8.6,7 Nevertheless, tunneling studies continue to indicate an absence of strong structure directly attributable to f-band effects,8,9 revealing instead only modest deviations from the BCS tunneling density of states (TDS) reminiscent of phonon-induced structure described by strong-coupling theory. These observations suggest that strong-coupling theory may provide an adequate description of La, permitting a phonon spectrum α2F(w) to be extracted by the method of McMillan and Rowell.10 Effects due to f levels8’9 would presumably be reflected in anomalous values of λ(electron—phonon coupling strength) or µ* (effective Coulomb pseudopotential) obtained in the course of computing α2F(ω). McMillan’s approximate solution of the Eliashberg equations11,12 can then be used to calculate T c from λ, ,<ω>, and µ*. Failure to obtain agreement with observed values of T c would indicate difficulties with either the tunneling data or the applicability of strong-coupling theory.
KeywordsPhonon Spectrum Strong Structure Derivative Data Compute Spectrum Full Bulk
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