Non-monotonic pressure evolution of the upper critical field in superconducting FeSe

TL;DR

This study investigates the non-monotonic pressure dependence of the upper critical field in FeSe, revealing Fermi surface changes as a key factor influencing superconducting properties under pressure.

Contribution

It provides the first detailed analysis of the pressure evolution of the upper critical field in FeSe, linking Fermi surface modifications to superconducting behavior.

Findings

Non-monotonic pressure dependence of $H_{c2,c}$ and $T_c$ in FeSe.

Fermi surface changes correlate with local minima in $T_c$ at 1.2 GPa.

Different models describe the upper critical field slope across pressure regimes.

Abstract

The pressure dependence of the upper critical field, $H_\textrm{c2,c}$, of single crystalline FeSe was studied using measurements of the inter-plane resistivity, $\rho_{\textrm{c}}$ in magnetic fields parallel to tetragonal $c$-axis. $H_\textrm{c2,c}(T)$ curves obtained under hydrostatic pressures up to $1.56$ GPa, the range over which the superconducting transition temperature, $T_\textrm{c}$, of FeSe exhibits a non-monotonic dependence with local maximum at $p_1\approx$ 0.8 GPa and local minimum at $p_2\approx$ 1.2 GPa. The slope of the upper critical field at $T_\textrm{c}$, $\left(\textrm{d}H_\text{c2,c}/\textrm{d}T\right)_{T_\textrm{c}}$, also exhibits a non-monotonic pressure dependence with distinct changes at $p_1$ and $p_2$. For $p<p_1$ the slope can be described within multi-band orbital model. For both $p_1<p <p_2$ and $p>p_2$ the slope is in good quantitative agreement with a single band, orbital Helfand-Werthamer theory with Fermi velocities determined from Shubnikov-de Haas measurements. This finding indicates that Fermi surface changes are responsible for the local minimum of $T_\textrm{c}(p)$ at $p_2\approx$ 1.2 GPa.