Quadratic XY-anisotropy correction to the spontaneous staggered magnetization of the gapped XYZ spin-$1/2$ chain

The staggered order parameter of the gapped XYZ spin-$1/2$ chain follows from the eight-vertex solution, but in elliptic variables that fix the Hamiltonian couplings only implicitly. We derive a closed form for the coefficient governing the leading quadratic response of the spontaneous staggered mag