Absence of nematic order in the pressure-induced intermediate phase of the iron-based superconductorBa0.85K0.15Fe2As2

The hole doped Fe-based superconductors $\mathrm{B}{{\mathrm{a}}₁}_{\ensuremath{-}x}{A}_x\mathrm{F}{\mathrm{e}}₂\mathrm{A}{\mathrm{s}}₂$ (where $A=\mathrm{Na}$ or K) show a particularly rich phase diagram. It was observed that an intermediate reentrant tetragonal phase, in which the ${C}₄$ fourfold rotational symmetry is restored, forms within the orthorhombic antiferromagnetically ordered stripe-type spin density wave state above the superconducting transition [S. Avci et al., Nat. Commun. 5, 3845 (2014); A. E. B\"ohmer et al., Nat. Commun. 6, 7911 (2015)]. A similar intermediate phase was reported to appear if pressure is applied to underdoped $\mathrm{B}{{\mathrm{a}}₁}_{\ensuremath{-}x}{\mathrm{K}}_x\mathrm{F}{\mathrm{e}}₂\mathrm{A}{\mathrm{s}}₂$ [E. Hassinger et al., Phys. Rev. B 86, 140502(R) (2012)]. Here we report data of the electric resistivity, Hall effect, specific heat, and the thermoelectric Nernst and Seebeck coefficients measured on a $\mathrm{B}{\mathrm{a}}_0.85{\mathrm{K}}_0.15\mathrm{F}{\mathrm{e}}₂\mathrm{A}{\mathrm{s}}₂$ single crystal under pressure up to 5.5 GPa. The data reveal a coexistence of the intermediate phase with filamentary superconductivity. The Nernst coefficient shows a large signature of nematic order that coincides with the stripe-type spin density wave state up to optimal pressure. In the pressure-induced intermediate phase the nematic order is removed, thus confirming that its nature is a reentrant tetragonal phase.