Upper critical fields and thermally-activated transport of Nd(O_0.7F_0.3)FeAs single crystal

TL;DR

This study measures the upper critical fields and resistivity of Nd(O_0.7F_0.3)FeAs single crystals, revealing intrinsic high-field behavior, anisotropy reduction with temperature, and the influence of paramagnetic effects on superconductivity.

Contribution

It provides detailed experimental data on H_c2 and resistivity in Nd-based oxypnictides, highlighting intrinsic properties and the role of multiband and paramagnetic effects.

Findings

H_c2 reaches ~100 T, comparable to high T_c cuprates.

H_c2(T) shows upward curvature, intrinsic to oxypnictides.

Anisotropy decreases from 9.2 to 5 as temperature drops.

Abstract

We present measurements of the resistivity and the upper critical field H_c2 of Nd(O_0.7F_0.3)FeAs single crystals in strong DC and pulsed magnetic fields up to 45 T and 60 T, respectively. We found that the field scale of H_c2 is comparable to ~100 T of high T_c cuprates. H_c2(T) parallel to the c-axis exhibits a pronounced upward curvature similar to what was extracted from earlier measurements on polycrystalline samples. Thus this behavior is indeed an intrinsic feature of oxypnictides, rather than manifestation of vortex lattice melting or granularity. The orientational dependence of H_c2 shows deviations from the one-band Ginzburg-Landau scaling. The mass anisotropy decreases as T decreases, from 9.2 at 44K to 5 at 34K. Spin dependent magnetoresistance and nonlinearities in the Hall coefficient suggest contribution to the conductivity from electron-electron interactions modified by disorder reminiscent that of diluted magnetic semiconductors. The Ohmic resistivity measured below T_c but above the irreversibility field exhibits a clear Arrhenius thermally activated behavior over 4-5 decades. The activation energy has very different field dependencies for H||ab and H\perp ab. We discuss to what extent different pairing scenarios can manifest themselves in the observed behavior of H_{c2}, using the two-band model of superconductivity. The results indicate the importance of paramagnetic effects on H_c2(T),which may significantly reduce H_c2(0) as compared toH_c2(0)~200-300 T based on extrapolations of H_c2(T) near T_c down to low temperatures.