Specific Heat Discontinuity DeltaC vs Tc in Annealed Ba(Fe1-xCox)2As2

TL;DR

This study measures the specific heat discontinuity in annealed Ba(Fe1-xCox)2As2 superconductors, revealing a consistent non-BCS behavior of deltaC/Tc proportional to Tc^2 across various compositions.

Contribution

It demonstrates that annealing enhances deltaC/Tc and Tc, and shows the non-BCS power-law behavior is independent of magnetic coexistence, suggesting intrinsic properties of iron superconductors.

Findings

DeltaC/Tc scales as Tc^2 across compositions.

Annealing increases Tc and deltaC/Tc values.

Non-BCS behavior persists regardless of magnetic coexistence.

Abstract

The low temperature specific heat of annealed single crystal samples of Ba(Fe1-xCox)2As2 with compositions spanning the entire superconducting phase diagram was measured. Effort was made to discover the best annealing schedule to maximize Tc and minimize transition width in these samples. Values of deltaC/Tc normalized to 100% superconducting volume fractions varied proportionally to Tc^alpha. Within a rather narrow error bar of +-0.15, the exponent alpha was the same (approximately 2) over a range of compositions (0.055 < x < 0.15) around the optimal concentration, x=0.08, where Tc is the maximum in the phase diagram. Thus, whether the superconductivity was coexistent with magnetism (underdoped) or not (overdoped) did not affect the non-BCS variation (alpha nearly 2 instead of 0.8-0.9 for BCS superconductors) of deltaC/Tc with Tc. The annealed samples in the present work, with increased deltaC/Tc and Tc values compared to previous results for the Ba(Fe1-xCox)2As2 alloy system, suggest that the intrinsic value of the exponent alpha for the iron superconductors, when the samples are annealed, with fewer defects, may indeed be even further from the BCS value of 0.8-0.9 than previously thought.