NMR evidence for a "Generalized spin-peierls transition" in the high-magnetic-field phase of the spin ladder Cu-2(C5H12N2)(2)Cl-4

The magnetic-field-induced 3D ordered phase of the two-leg spin ladder Cu-2 (C5H12N2)(2)Cl-4 has been probed through measurements of H-1 NMR spectra an d 1/T-1 in the temperature range 70 mK-1.2 K. The second order transition l ine T-c(H) has been determined between H-c1 = 7.52 T and H-c2 = 13.5 T and varies as (H - H-c1)(2/3) close to H-c1. From the observation of anomalous shifts and a crossover in 1/T1 above T-c, the mechanism of the 3D transitio n is argued to be magnetoelastic as in spin-Peierls chains, here involving a displacement of the protons along the longitudinal exchange (J(parallel t o)) path.