Thermodynamic phase diagram of Fe(Se$_{0.5}$Te$_{0.5}$) single crystals up to 28 Tesla

TL;DR

This study maps the thermodynamic phase diagram of Fe(Se$_{0.5}$Te$_{0.5}$) single crystals up to 28 Tesla, revealing paramagnetic limiting effects and unconventional superconducting properties through various measurements.

Contribution

It provides the first thermodynamic $H_{c2}$ line up to 28 T, showing paramagnetic effects and small coherence lengths, highlighting strong mass renormalization and unconventional pairing.

Findings

Thermodynamic $H_{c2}$ exhibits strong downward curvature near $T_c$.

Paramagnetic effects limit $H_{c2}$ up to 0.99$T_c$, indicating small coherence lengths.

Superfluid density and specific heat suggest unconventional pairing mechanism.

Abstract

We report on specific heat ($C_p$), transport, Hall probe and penetration depth measurements performed on Fe(Se$_{0.5}$Te$_{0.5}$) single crystals ($T_c \sim 14$ K). The thermodynamic upper critical field $H_{c2}$ lines has been deduced from $C_p$ measurements up to 28 T for both $H\|c$ and $H\|ab$, and compared to the lines deduced from transport measurements (up to 55 T in pulsed magnetic fields). We show that this {\it thermodynamic} $H_{c2}$ line presents a very strong downward curvature for $T \rightarrow T_c$ which is not visible in transport measurements. This temperature dependence associated to an upward curvature of the field dependence of the Sommerfeld coefficient confirm that $H_{c2}$ is limited by paramagnetic effects. Surprisingly this paramagnetic limit is visible here up to $T/T_c \sim 0.99$ (for $H\|ab$) which is the consequence of a very small value of the coherence length $\xi_c(0) \sim 4 \AA$ (and $\xi_{ab}(0) \sim 15 \AA$), confirming the strong renormalisation of the effective mass (as compared to DMFT calculations) previously observed in ARPES measurements [Phys. Rev. Lett. 104, 097002 (2010)]. $H_{c1}$ measurements lead to $\lambda_{ab}(0) = 430 \pm 50$ nm and $\lambda_c(0) = 1600 \pm 200$ nm and the corresponding anisotropy is approximatively temperature independent ($\sim 4$), being close to the anisotropy of $H_{c2}$ for $T\rightarrow T_c$. The temperature dependence of both $\lambda$ ($\propto T^2$) and the electronic contribution to the specific heat confirm the non conventional coupling mechanism in this system.