Two-gap and paramagnetic pair-breaking effects on upper critical field of SmFeAsO$_{0.85}$ and SmFeAsO$_{0.8}$F$_{0.2}$ single crystals

TL;DR

This study explores how two-gap superconductivity and paramagnetic effects influence the upper critical field in SmFeAsO-based single crystals, revealing deviations from traditional models and highlighting the complex pairing mechanisms.

Contribution

It demonstrates the roles of two-gap superconductivity and paramagnetic pair-breaking effects in shaping the upper critical field behavior of SmFeAsO crystals.

Findings

$H_{c2}(T)$ deviates from one-gap theory at low temperatures.

Two-gap nature affects $H_{c2}^c(T)$ behavior.

Paramagnetic effects influence $H_{c2}^{ab}(T)$.

Abstract

We investigated the temperature dependence of the upper critical field [$H_{c2}(T)$] of fluorine-free SmFeAsO$_{0.85}$ and fluorine-doped SmFeAsO$_{0.8}$F$_{0.2}$ single crystals by measuring the resistive transition in low static magnetic fields and in pulsed fields up to 60 T. Both crystals show that $H_{c2}(T)$'s along the c axis [$H_{c2}^c(T)$] and in an $ab$-planar direction [$H_{c2}^{ab}(T)$] exhibit a linear and a sublinear increase, respectively, with decreasing temperature below the superconducting transition. $H_{c2}(T)$'s in both directions deviate from the conventional one-gap Werthamer-Helfand-Hohenberg theoretical prediction at low temperatures. A two-gap nature and the paramagnetic pair-breaking effect are shown to be responsible for the temperature-dependent behavior of $H_{c2}^c$ and $H_{c2}^{ab}$, respectively.