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.
@article{arxiv.0809.0440,
title = {Field-Controlled Magnetic Order in the Quantum Spin-Ladder System (Hpip)2CuBr4},
author = {B. Thielemann and Ch. Rüegg and K. Kiefer and H. M. Rønnow and B. Normand and P. Bouillot and C. Kollath and E. Orignac and R. Citro and T. Giamarchi and A. M. Läuchli and D. Biner and K. Krämer and F. Wolff-Fabris and V. Zapf and M. Jaime and J. Stahn and N. B. Christensen and B. Grenier and D. F. McMorrow and J. Mesot},
journal= {arXiv preprint arXiv:0809.0440},
year = {2009}
}