Field-induced suppression of the C2-reentrant phase in Ba_1-xK_xFe₂As₂

Hole doped Fe-based 122 materials all reveal in a narrow doping concentration a low temperature transition from the usual orthorhombic C2-stripe magnetic phase into a magnetic double-Q phase, in which the C4 symmetry is restored.  Ba_1-xK_xFe₂As₂ is special, because here superconductivity induces a transition from this double-Q phase back into the C2-stripe phase [1]. By applying a magnetic field, we investigate how the delicate balance between these competing phases changes. In particular, we present specific heat and thermal expansion measurements with applied magnetic fields of up to 10T along the [100] and [001] direction. While a suppression of the orthorhombic phase is observed for both field directions, the orthorhombic distortion itself behaves very anisotropic. It is largely suppressed for fields along [001] while being slightly enhanced for in-plane fields. In addition to the measurements we present a phenomenological model that qualitatively reproduces the observed thermodynamic measurements and the phase diagram.