Giant magnetic-field dependence of the coupling between spin   Tomonaga-Luttinger liquids in BaCo2V2O8
  We use nuclear magnetic resonance to map the complete low-temperature phase diagram of the antiferromagnetic Ising-like spin-chain system BaCo2V2O8 as a function of the magnetic field applied along the chains. In contrast to the predicted crossover from the longitudinal incommensurate phase to the transverse antiferromagnetic phase, we find a sequence of three magnetically ordered phases between the critical fields 3.8 T and 22.8 T. Their origin is traced to the giant magnetic-field dependence of the total effective coupling between spin chains, extracted to vary by a factor of 24. We explain this novel phenomenon as emerging from the combination of nontrivially coupled spin chains and incommensurate spin fluctuations in the chains treated as Tomonaga-Luttinger liquids. 
M. Klanjsek, M. Horvatic, S. Kramer, S. Mukhopadhyay, H. Mayaffre, C.   Berthier, E. Canevet, B. Grenier, P. Lejay, E. Orignac
cond-mat.str-el