Superconducting gap evolution in overdoped BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$ single crystals through nanocalorimetry

Abstract

We report on specific heat measurements on clean overdoped $\mathrm{BaF}{\mathrm{e}}_2(\mathrm{A}{\mathrm{s}}_{1-\mathrm{x}}{\mathrm{P}}_{\mathrm{x}}{)}_2$ single crystals performed with a high resolution membrane-based nanocalorimeter. A nonzero residual electronic specific heat coefficient at zero temperature ${\gamma }_{\mathrm{r}}=C/T{|}_{T\to 0}$ is seen for all doping compositions, indicating a considerable fraction of the Fermi surface ungapped or having very deep minima. The remaining superconducting electronic specific heat is analyzed through a two-band s-wave $\alpha$ model in order to investigate the gap structure. Close to optimal doping we detect a single zero-temperature gap of ${\Delta }_0\sim 5.3\mathrm{meV}$, corresponding to ${\Delta }_0/k_{\mathrm{B}}{T}_{\mathrm{c}}\sim 2.2$. Increasing the phosphorus concentration $x$, the main gap reduces till a value of ${\Delta }_0\sim 1.9\mathrm{meV}$ for $x=0.55$ and a second weaker gap becomes evident. From the magnetic field effect on ${\gamma }_{\mathrm{r}}$, all samples however show similar behavior [${\gamma }_{\mathrm{r}}(H)-{\gamma }_{\mathrm{r}}(H=0)\propto H^n$, with $n$ between 0.6 and 0.7]. This indicates that, despite a considerable redistribution of the gap weights, the total degree of gap anisotropy does not change drastically with doping.

5 pages, 3 figures

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