Upper critical field measurements up to 60 T in arsenic-deficient LaO_(0.9)F_(0.1)FeAs_(1-delta): Pauli limiting behaviour at high fields vs improved superconductivity at low fields

TL;DR

This study measures the upper critical field in arsenic-deficient LaO_(0.9)F_(0.1)FeAs_(1-delta) up to 60 T, revealing Pauli limiting behavior at high fields and enhanced low-field superconductivity due to disorder.

Contribution

It provides the first detailed high-field data showing Pauli limiting behavior in disordered LaO_(0.9)F_(0.1)FeAs_(1-delta), linking disorder effects to superconducting properties.

Findings

Pauli limiting behavior observed at high fields (~63 T)

Enhanced superconductivity at low fields in disordered samples

Flattening of B_c2(T) above 23 K at high fields

Abstract

We report resistivity and upper critical field B_c2(T) data for As deficient LaO_(0.9)F_(0.1)FeAs_(1-delta) in a wide temperature and high field range up to 60 T. These disordered samples exhibit a slightly enhanced superconducting transition at T_c = 29 K and a significantly enlarged slope dB_(c2))/dT = -5.4 T/K near T_c which contrasts with a flattening of B_(c2)(T) starting near 23 K above 30 T. This flattening is interpreted as Pauli limiting behaviour (PLB) with B_(c2)(0) approx 63 T. We compare our results with B_(c2)(T)-data reported in the literature for clean and disordered samples. Whereas clean samples show no PLB for fields below 60 to 70 T, the hitherto unexplained flattening of B_(c2)(T) for applied fields H || ab observed for several disordered closely related systems is interpreted also as a manifestation of PLB. Consequences of our results are discussed in terms of disorder effects within the frame of conventional and unconventional superconductivity.