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 interactio