Vison crystal formation in quantum spin ice on the breathing pyrochlore lattice

The U(1) spin liquid phase of pyrochlore quantum spin ice has long fascinated theorists with its deep connections to compact lattice QED. Recent excitement has come from the experimental discovery of pseudospin-12 breathing pyrochlores, including certain chromium spinels and Ba₃Yb₂Zn₅O₁₁, in which the `up' and `down' tetrahedra are different physical sizes. We show here that the often-neglected Jz_+/- coupling between spin-1/2 pyrochlore pseudospins, in combination with a breathing lattice, can produce a staggered offset in the ground state's plaquette fluxes. Very strong breathing anisotropies realise an unconventional `U(1)_π/2 phase,' which in some sense interpolates between the known U(1)₀ and U(1)_π phases of standard quantum spin ice models.