Non-exponential London penetration depth in RFeAsO$_{0.9}$F$_{0.1}$ (R=La,Nd) single crystals

TL;DR

This study measures the London penetration depth in RFeAsO$_{0.9}$F$_{0.1}$ single crystals, revealing a non-exponential, power-law behavior indicative of unconventional two-gap superconductivity, with paramagnetic effects considered.

Contribution

It provides the first detailed analysis of the penetration depth in RFeAsO$_{0.9}$F$_{0.1}$ crystals, demonstrating two-gap superconductivity and the impact of paramagnetism on measurements.

Findings

Penetration depth follows a T^2 power law at low temperatures.

Paramagnetism affects the measured penetration depth in Nd-1111.

Evidence supports two-gap superconductivity in RFeAsO and BaFe$_2$As$_2$ families.

Abstract

The superconducting penetration depth, $\lambda(T)$, has been measured in RFeAsO$_{0.9}$F$_{0.1}$ (R=La,Nd) single crystals (R-1111). In Nd-1111, we find an upturn in $\lambda(T)$ upon cooling and attribute it to the paramagnetism of the Nd ions, similar to the case of the electron-doped cuprate Nd-Ce-Cu-O. After the correction for paramagnetism, the London penetration depth variation is found to follow a power-law behavior, $\Delta \lambda_L(T)\propto T^{2}$ at low temperatures. The same $T^2$ variation of $\lambda(T)$ was found in non-magnetic La-1111 crystals. Analysis of the superfluid density and of penetration depth anisotropy over the full temperature range is consistent with two-gap superconductivity. Based on this and on our previous work, we conclude that both the RFeAsO (1111) and BaFe$_2$As$_2$ (122) families of pnictide superconductors exhibit unconventional two-gap superconductivity.