Nonconstant electronic density of states tunneling inversion for A15 superconductors: Nb3Sn

This paper introduces the Julia package IsoME, an easy-to-use yet accurate and robust computational tool designed to calculate superconducting properties. Multiple levels of approximation are supported, ranging from the basic McMillan-Allen-Dynes formula and its machine learning-enhanced variant to Eliashberg theory including static Coulomb interactions derived from $GW$ calculations, offering a fully ab initio approach to determine superconducting properties, such as the cri...

Theories and experiments on dirty superconductors are sophisticated but important for both fundamentals and applications. It becomes more challenging when magnetic fields are present, because the field distribution, the electron density of states, and the superconducting pairing potentials are nonuniform. Here we present tunneling microspectroscopic experiments on NbC single crystals and show that NbC is a homogeneous dirty superconductor. When applying magnetic fields to the...

We present an analysis of the Nb3Sn surface layers grown on a bulk niobium (Nb) coupon prepared at the same time and by the same vapor diffusion process used to make Nb3Sn coatings on 1.3 GHz cavities. Tunneling spectroscopy reveals a well-developed, homogeneous superconducting density of states at the surface with a gap value distribution centered around 2.7 meV and superconducting critical temperature (Tc) up to 16.3 K. Scanning Electron microscopy (STEM) performed on cross...

We present a first-principles approach to describe magnetic and superconducting systems and the phenomena of competition between these electronic effects. We develop a density functional theory: SpinSCDFT, by extending the Hohenberg-Kohn theorem and constructing the non-interacting Kohn- Sham system. An exchange-correlation functional for SpinSCDFT is derived from the Sham Schl\"uter connection between the SpinSCDFT Kohn-Sham and a self-energy in Eliashberg approximation. The...

We elucidate the origin of the phonon-mediated superconductivity in 2$H$-NbS$_2$ using the ab initio anisotropic Migdal-Eliashberg theory including Coulomb interactions. We demonstrate that superconductivity is associated with Fermi surface hot spots exhibiting an unusually strong electron-phonon interaction. The electron-lattice coupling is dominated by low-energy anharmonic phonons, which place the system on the verge of a charge density wave instability. We also provide de...

We report tunneling measurements of the electron-phonon (e-ph) interaction in superconducting MgB$_2$ using the MgB$_2$-I-Nb junctions, where I stands for insulator. The phonon structure in tunneling density of states in MgB$_2$ clearly indicates strong e-ph coupling for the E$_{2g}$ in-plane boron phonons in a narrow range around 60 meV. The Eliashberg spectral function $\alpha^2(\omega)F(\omega)$ reconstructed from the tunneling data, exhibits significant additional contrib...

The basic thermodynamic parameters of the superconducting state in $\rm{YNi_{2}B_{2}C}$ were calculated in the framework of the one-band Eliashberg model. The effective Eliashberg function, determined on the basis of the transport function (R.S. Gonnelli, {\it et al.}, Physica C \textbf{341}, 1957 (2000)), was used during calculations. It was shown that the dimensionless ratios are equal to: $R_{1}\equiv 2\Delta(0)/k_{B}T_{C}=3.87$, $R_{2}\equiv\Delta C(T_{C})/C^{N}(T_{C})=1....

Advances in scanning tunneling spectroscopy reveal the presence of superconducting nanoregions well past the bulk thermal transition in strongly disordered superconductors. We use a Monte Carlo tool to capture the spatially differentiated amplitude and phase fluctuations in such a material and establish spatial maps of the coherence peak as the superconductor is driven through the thermal transition. Analysis of the local density of states reveals that superconducting regions...

A theoretical model for tunneling spectroscopy employing tight-binding band structure, d-wave gap symmetry, group velocity, and tunneling directionality can exhibit a wide range of characteristics similar to the ones found in tunneling experiments on high-Tc superconductors. Using a band structure obtained for optimally-doped Bi_2Sr_2CaCu_2O_8, the model tunneling density of states produces a good comparison to the point-contact tunneling conductance on the same material, esp...

In this work, we present the results of precise measurements of the resistivity of YNi$_2$B$_2$C single crystals with $T_{\rm c}$ = 15.5 K as a function of the temperature, and analyze the experimental data in the framework of the Bloch-Gr\"{u}neisen theory and electron-phonon coupling. The transport electron-phonon spectral function that best fits the resistivity data is then inserted in the real-axis Eliashberg equations, which are directly solved to determine the normalize...