NMR relaxation rate of (quasi-)one-dimensional spin-gapped systems in magnetic fields

The NMR relaxation rates of (quasi-)one-dimensional spin-gapped systems such as the S = 1/2 spin ladder with a diagonal interaction and the Haldane-gap system are investigated at low temperatures in magnetic fields. S;Ve show that in the regime where the gap is collapsed by magnetic fields, the NMR relaxation rate diverges with decreasing temperature and its exponent shows a behavior characteristic of the model as a function of the magnetic field. We discuss our results and compare them with recent experimental findings for the spin ladder system Cu-2(C5H12N2)(2)Cl-4 and for the Haldane-gap system (CH3)(4)NNi(NO2)(3).