Specific-heat discontinuityΔCvs.Tcin annealed Ba(Fe1−xCox)2As2

Abstract

The low-temperature specific heat of annealed single-crystal samples of Ba(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ with compositions spanning the entire superconducting phase diagram was measured. Effort was made to discover the best annealing schedule to maximize ${T}_{c}$ and minimize transition width in these samples. Values of $\ensuremath{\Delta}C/{T}_{c}$ normalized to 100$%$ superconducting volume fractions varied proportionally to ${T}_{c}$${}^{\ensuremath{\alpha}}$. Within a rather narrow error bar of \ifmmode\pm\else\textpm\fi{}0.15, the exponent \ensuremath{\alpha} was the same (approximately 2) over a range of compositions (0.055 \ensuremath{\le} $x$ \ensuremath{\le} 0.15) around the optimal concentration $x$ = 0.08, where ${T}_{c}$ 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 (\ensuremath{\alpha} nearly 2 instead of \ensuremath{\approx}0.8--0.9 for BCS superconductors) of $\ensuremath{\Delta}C/{T}_{c}$ with ${T}_{c}$. The annealed samples in the present work, with increased $\ensuremath{\Delta}C/{T}_{c}$ and ${T}_{c}$ values compared to previous results for the Ba(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ alloy system, suggest that the intrinsic value of the exponent \ensuremath{\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.