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M. Klanjsek, H. Mayaffre, C. Berthier, M. Horvatic, T. Giamarchi
Luttinger liquid (LL) physics is a remarkable manifestation of strong correlations in 1D systems. Physical properties of a LL are determined by two Luttinger parameters characterizing the power law decay of most observables. In addition to the difficulty in realising LLs, no precise control over Luttinger parameters is usually possible. Notable exception are gapless phases of 1D antiferromagnets (AFs), such as spin chains and ladders, where the Luttinger parameters can be tuned by an external magnetic field. We show that CuBr4(C5H12N)2 (BPCB), a material containing weakly coupled spin-1/2 Heisenberg AF ladders, is a unique system for controlling and probing the LL physics. Using 14N nuclear magnetic resonance we quantitatively characterize the variation of Luttinger parameters over the gapless phase of BPCB. This allows us to fully account for the phase transition to a 3D ordered phase at temperatures below 110~mK, which takes place due to weak inter-ladder exchange coupling, in terms of weakly coupled LLs . Very close to the critical fields, BPCB recovers its 3D character and the LL description collapses. The 3D ordered phase in this quantum critical region can be described as a Bose-Einstein condensate of magnons. M. Klanjsek et al., arXiv :0804.2639 |
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