  We study the magnetic-field-induced quantum phase transition from a gapped
quantum phase that has no magnetic long-range order into a gapless phase in the
spin-1/2 ladder compound bis(2,3-dimethylpyridinium) tetrabromocuprate (DIMPY).
At temperatures below about 1 K, the specific heat in the gapless phase attains
an asymptotic linear temperature dependence, characteristic of a
Tomonaga-Luttinger liquid. Inelastic neutron scattering and the specific heat
measurements in both phases are in good agreement with theoretical
calculations, demonstrating that DIMPY is the first model material for an S=1/2
two-leg spin ladder in the strong-leg regime.
