Field-Controlled Magnetic Order in the Quantum Spin-Ladder System   (Hpip)2CuBr4
  Neutron diffraction is used to investigate the field-induced, antiferromagnetically ordered state in the two-leg spin-ladder material (Hpip)2CuBr4. This "classical" phase, a consequence of weak interladder coupling, is nevertheless highly unconventional: its properties are influenced strongly by the spin Luttinger-liquid state of the ladder subunits. We determine directly the order parameter (transverse magnetization), the ordering temperature, the spin structure, and the critical exponents around the transition. We introduce a minimal, microscopic model for the interladder coupling and calculate the quantum fluctuation corrections to the mean-field interaction. 
B. Thielemann, Ch. R\"uegg, K. Kiefer, H.M. R{\o}nnow, B. Normand, P.   Bouillot, C. Kollath, E. Orignac, R. Citro, T. Giama...