January 4, 2002
We re-examine the tunneling data on A15 superconductors by performing a generalized McMillan-Rowell tunneling inversion that incorporates a nonconstant electronic density of states obtained from band-structure calculations. For Nb3Sn, we find that the fit to the experimental data can be slightly improved by taking into account the sharp structure in the density of states, but it is likely that such an analysis alone is not enough to completely explain the superconducting tunneling characteristics of this material. Nevertheless, the extracted Eliashberg function displays a number of features expected to be present for the highest quality Nb3Sn samples.
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September 20, 2003
Mathematical correct method for extraction of the complex superconducting energy gap parameter and electron- phonon interaction function from the tunnel density of states is presented. The computer program is shown to be robust against imperfections in experimental data and is widely used to investigate conventional (low Tc) as well as high- temperature superconductors.
December 24, 1996
We discuss the limits and the correct utilization of the standard program for the inversion of Eliashberg equations and the determination of the electron-phonon spectral function and the coulomb pseudopotential from tunneling measurements in high-Tc superconductors. In order to compare the calculated density of states with the experimental one, we introduce the results of the inversion procedure, applied to our recent tunneling data in Bi2Sr2CaCu2O{8+x} (BSCCO) single-crystal...
June 13, 2006
This article gives an overview of the available literature on simplified, well defined (quasi-)homogeneous laboratory samples. After more than 50 years of research on superconductivity in Nb3Sn, a significant amount of results are available, but these are scattered over a multitude of publications. Two reviews exist on the basic properties of A15 materials in general, but no specific review for Nb3Sn is available. This article is intended to provide such an overview. It start...
August 5, 1996
The McMillan-Rowell tunneling inversion program, which extracts the electron-phonon spectral function $\alpha^2F(\Omega)$ and the Coulomb pseudopotential $\mu^*$ from experimental tunneling data, is generalized to include the lowest-order vertex correction. We neglect the momentum dependence of the electron-phonon matrix elements, which is equivalent to using a local approximation. The perturbation theory is performed on the imaginary axis and then an exact analytic continuat...
December 27, 2022
An advanced theoretical framework is introduced and examined. Its main idea is to extract properties of the superconducting pairing gap function $\Delta(\omega)$ in the conventional, nearly localized superconductors. To test the approach, we present an experimentally relevant benchmark model with defined normal and superconducting sectors. The developed reverse engineering framework consists of two logic steps. First, dismantle the superconducting density of states into the e...
February 7, 2003
We have measured the tunneling density of states (DOS) in a superconductor carrying a supercurrent or exposed to an external magnetic field. The pair correlations are weakened by the supercurrent, leading to a modification of the DOS and to a reduction of the gap. As predicted by the theory of superconductivity in diffusive metals, we find that this effect is similar to that of an external magnetic field.
July 3, 2000
In the present work we calculate the theoretical tunneling conductance curves of SIN junctions involving high-Tc superconductors, for different possible symmetries of the order parameter (s, d, s+id, s+d, anisotropic s and extended s). To do so, we solve the real-axis Eliashberg equations in the case of an half-filled infinite band. We show that some of the peculiar characteristics of HTSC tunneling curves (dip and hump at eV > Delta, broadening of the gap peak, zero bias and...
December 4, 2003
We investigated the local electronic density of states in superconductor-normal metal (Nb-Au) bilayers using a very low temperature (60 mK) STM. High resolution tunneling spectra measured on the normal metal (Au) surface show a clear proximity effect with an energy gap of reduced amplitude compared to the bulk superconductor (Nb) gap. Within this mini-gap, the density of states does not reach zero and shows clear sub-gap features. We show that the experimental spectra cannot ...
October 31, 2009
Using density functional theory within the local density approximation we report a study of the electron-phonon coupling in NbC$_{1-x}$N$_x$ and NbN crystals in the rocksalt structure. The Fermi surface of these systems exhibits important nesting. The associated Kohn anomaly greatly increases the electron-phonon coupling and induces a structural instability when the electronic density of states reaches a critical value. Our results reproduce the observed rise in $T_{c}$ from ...
November 9, 2012
We perform the scanning tunneling spectroscopy based superconductor-vacuum-superconductor analogue to the seminal McMillan and Rowell superconductor-insulator-superconductor device study of phonons in the archetypal elemental superconductor Pb [W. L. McMillan and J. M. Rowell, Phys. Rev. Lett. 14, 108 (1965)]. We invert this spectroscopic data utilizing strong-coupling Eliashberg theory to obtain a local {\alpha}^2F({\omega}) and find broad underlying agreement with the pione...