Absence of nematic order in the pressure-induced intermediate phase of the iron-based superconductor Ba0.85K0.15Fe2As2

The hole doped Fe-based superconductors Ba(1-x)A(x)Fe(2)As(2) (where A = 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. Bohmer et al., Nat. Commun. 6,7911 (2015)]. A similar intermediate phase was reported to appear if pressure is applied to underdoped Ba1-xKxFe2As2 [E. Hassinger et al., Phys. Rev. B 86,40502(R) (2012)]. Here we report data of the electric resistivity, Hall effect, specific heat, and the thermoelectric Nernst and Seebeck coefficients measured on a Ba0.85K0.15Fe2As2 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.