Origins of low- and high-pressure discontinuities of $T_{c}$ in niobium

TL;DR

This paper investigates the origins of pressure-induced discontinuities in the critical temperature of niobium, revealing different mechanisms at low and high pressures related to phonon behavior and band structure changes.

Contribution

It provides a detailed theoretical analysis of the distinct mechanisms causing $T_{c}$ discontinuities in niobium under pressure using density-functional perturbation theory.

Findings

Low-pressure discontinuity linked to decreased nesting factor without band structure change.

High-pressure discontinuity associated with significant band structure modifications.

Both phenomena originate from Kohn anomalies and low-frequency phonons.

Abstract

The discontinuities of $T_{c}$ in Niobium under pressure are examined by means of the pseudopotential plane-wave implementation of the electron-phonon coupling calculated from density-functional perturbation theory. Both low- and high-pressure discontinuities of $T_{c}$ have their origin in the Kohn anomalies and are caused by the low-frequency phonons, but the mechanism leading to the discontinuities is different in the two cases. The low-pressure anomaly is associated with a global decrease of the nesting factor in the whole Brillouin Zone and not to a visible change in the band structure. The high-pressure anomaly is instead connected with a well-pronounced change in the band structure.