Anisotropic type-I superconductivity and anomalous superfluid density in OsB 2

TL;DR

This paper provides a microscopic analysis of OsB2, revealing it as a type-I superconductor with anisotropic single-gap superconductivity, explaining its superfluid density behavior through detailed electronic and phononic calculations.

Contribution

It is the first to combine first-principles calculations with Eliashberg theory to characterize OsB2's superconducting gap and anisotropy, and to identify its type-I nature.

Findings

OsB2 has three Fermi sheets consistent with experiments.

It is a type-I superconductor with a low Ginzburg-Landau parameter.

The superfluid density behavior is explained by anisotropic single-gap superconductivity.

Abstract

We present a microscopic study of superconductivity in OsB2 , and discuss the origin and characteristic length scales of the superconducting state. From first-principles we show that OsB2 is characterized by three different Fermi sheets, and we prove that this fermiology complies with recent quantum-oscillation experiments. Using the found microscopic properties, and experimental data from the literature, we employ Ginzburg-Landau relations to reveal that OsB2 is a distinctly type-I superconductor with very low Ginzburg-Landau parameter kappa - a rare property among compound materials. We show that the found coherence length and penetration depth corroborate the measured thermodynamic critical field. Moreover, our calculation of the superconducting gap structure using anisotropic Eliashberg theory and ab initio calculated electron-phonon interaction as input reveals a single but anisotropic gap. The calculated gap spectrum is shown to give an excellent account for the unconventional behavior of the superfluid density of OsB2 measured in experiments as a function of temperature. This reveals that gap anisotropy can explain such behavior, observed in several compounds, which was previously attributed solely to a two-gap nature of superconductivity.