NMR Evidence for a “Generalized Spin-Peierls Transition” in the High-Magnetic-Field Phase of the Spin Ladder ${\mathrm{Cu}}_2(C_5{H}_{12}{N}_2{)}_2{\mathrm{Cl}}_4$

The magnetic-field-induced 3D ordered phase of the two-leg spin ladder ${\mathrm{Cu}}_2({\mathrm{C}}_5{\mathrm{H}}_{12}{\mathrm{N}}_2{)}_2{\mathrm{Cl}}_4$ has been probed through measurements of ${}^1\mathrm{H}$ NMR spectra and $1/T_1$ in the temperature range 70 mK–1.2 K. The second order transition line $T_c\left(H\right)$ has been determined between $H_{c1\mathrm{}}=7.52\mathrm{}\mathrm{T}$ and $H_{c2\mathrm{}}=13.5\mathrm{}\mathrm{T}$ and varies as $({H-H}_{c1\mathrm{}}{)}^{2/3}$ close to $H_{c1\mathrm{}}$. From the observation of anomalous shifts and a crossover in $1/T_1$ above $T_c$, the mechanism of the 3D transition is argued to be magnetoelastic as in spin-Peierls chains, here involving a displacement of the protons along the longitudinal exchange ( $J_{\parallel }$) path.