Dimensional crossover in a spin-liquid-to-helimagnet quantum phase transition

Neutron scattering is used to study magnetic-field-induced ordering in the quasi-one-dimensional quantum spin-tube compound Sul-Cu2 Cl4 that in zero field has a nonmagnetic spin-liquid ground state. The experiments reveal an incommensurate chiral high-field phase stabilized by a geometric frustration of the magnetic interactions. The measured critical exponents β≈0.235 and ν≈0.34 at Hc ≈3.7? T point to an unusual subcritical scaling regime and may reflect the chiral nature of the quantum critical point.