Middle-field cusp singularities in the magnetization process of one-dimensional quantum antiferromagnets

We study the zero-temperature magnetization process (M-H curve) of one-dime nsional quantum antiferromagnets using a variant of the density-matrix reno rmalization group method. For both the S=1/2 zig-zag spin ladder and the S= 1 bilinear-biquadratic chain, we find clear cusp-type singularities in the middle-field region of the M-H curve. These singularities are successfully explained in terms of the double-minimum shape of the energy dispersion of the low-lying excitations. For the S=1/2 zig-zag spin ladder, we find that the cusp formation accompanies the Fermi-liquid to non-Fermi-liquid transit ion. [S0163-1829(99)50834-5].