Antiferromagnetic zigzag spin chain in magnetic fields at finite temperatures

We study thermodynamic behaviors of the antiferromagnetic zigzag spill chai n in magnetic fields, using the density-matrix renormalization-group method for the quantum transfer matrix. We focus on the thermodynamics of the sys tem near the critical fields in the ground-state magnetization process (M-H curve): the saturation field, the lower critical field associated with exc itation gap, and the field at the middle-field cusp singularity. We calcula te magnetization. susceptibility, and specific heat of the zigzag chain in magnetic fields at finite temperatures. and then discuss how the calculated quantities reflect the low-lying excitations of the system related with th e critical behaviors in the M-H curve.