NMR at high magnetic fields

Based on high-field ³¹P nuclear magnetic resonance experiments and accompanying numerical calculations, it is argued that in the frustrated S=1/2 ladder compound BiCu₂PO₆ a external page field-induced soliton lattice develops above a critical field of μ₀H_c1=20.96(7) T. Solitons result from the fractionalization of the S=1, bosonlike triplet excitations, which in other quantum antiferromagnets are commonly known to experience Bose-Einstein condensation or to crystallize in a superstructure. Unlike in spin-Peierls systems, these field-induced quantum domain walls do not arise from a state with broken translational symmetry and are triggered exclusively by magnetic frustration. Our model predicts yet another second-order phase transition at H_c2>H_c1, driven by soliton-soliton interactions.