Pressure dependence of the upper critical field of MgB2 and of YNi2B2C

TL;DR

This study investigates how pressure influences the upper critical magnetic field in MgB$_2$ and YNi$_2$B$_2$C, revealing changes in electronic properties and electron-phonon interactions through experimental measurements.

Contribution

It provides new insights into pressure effects on the Fermi surface velocities and electron-phonon coupling in MgB$_2$ and YNi$_2$B$_2$C, supported by experimental data.

Findings

Pressure alters the shape of H$_{c2}(T)$ in both materials.

Electron-phonon coupling in MgB$_2$'s $\sigma$ band decreases with pressure.

Positive curvature of H$_{c2}(T)$ in YNi$_2$B$_2$C is reduced under pressure.

Abstract

We present measurements of H$_{c2}(T)$ under pressure in MgB$_2$ and in YNi$_2$B$_2$C. The changes in the shape of H$_{c2}(T)$ are interpreted within current models and show the evolution of the main Fermi surface velocities $v_F$ and electron-phonon coupling parameters $\lambda$ with pressure. In MgB$_2$ the electron-phonon coupling strength of the nearly two dimensional $\sigma$ band, responsible for the high critical temperature, is more affected by pressure than the $\pi$ band coupling, and the hole doping of the $\sigma$ band decreases. In YNi$_2$B$_2$C, the peculiar positive curvature of H$_{c2}(T)$ is weakened by pressure.