NMR evidence for a "generalized spin-Peierls transition" in the high   magnetic field phase of the spin-ladder Cu2(C5N2H12)2Cl4
  The magnetic field-induced 3D ordered phase of the two-leg spin-ladder Cu2(C5N2H12)2Cl4 has been probed through measurements of 1H NMR spectra and 1/T1 in the temperature range 70~mK - 1.2 K. The second order transition line Tc(H) has been determined between Hc1 = 7.52 T and Hc2 = 13 T and varies as (H-Hc1)^(2/3) close to Hc1. From the observation of anomalous shifts and a crossover in 1/T1 above Tc, the mechanism of the 3D transition is argued to be magnetoelastic, involving a displacement of the protons along the longitudinal exchange (J//) path. 
H. Mayaffre, M. Horvatic, C. Berthier, M.-H. Julien, P. Segransan, L.   Levy, and O. Piovesana