Large Anisotropic Uniaxial Pressure Dependencies of $T_c$ in Single Crystalline $\mathrm{Ba}({\mathrm{Fe}}_{0.92}{\mathrm{Co}}_{0.08}{)}_2{\mathrm{As}}_2$

Using high-resolution dilatometry, we study the thermodynamic response of the lattice parameters to superconducting order in a self-flux grown $\mathrm{Ba}({\mathrm{Fe}}_{0.92}{\mathrm{Co}}_{0.08}{)}_2{\mathrm{As}}_2$ single crystal. The uniaxial-pressure dependencies of the critical temperature $T_c$ calculated using our thermal-expansion and specific-heat data via the Ehrenfest relation, are found to be quite large and very anisotropic ($d{T}_c/d{p}_a=3.1(1)\mathrm{K}/\mathrm{GPa}$ and $d{T}_c/d{p}_c=-7.0(2)\mathrm{K}/\mathrm{GPa}$). Our results show that there is a strong coupling of the $c/a$ ratio to superconducting order, which demonstrates that $T_c$ is far from the optimal value.

Figure 1

Field cooled (FC) and zero-field cooled (ZFC) magnetization in a field of 5 mT.

Figure 2

Resistance versus temperature.

Figure 3

Linear thermal expansivity versus temperature (a) between 5 K and 275 K, and (b) around $T_c$.

Figure 4

Specific-heat anomaly at $T_c$ obtained by subtracting a phonon background (see text for details).