Heat Capacity of a Strongly One-Dimensional Antiferromagnet Cu(C₆H₅COO)₂·3H₂O —Field-Induced Crossover of Spin-Dimensionality—

Abstract

Strong one-dimensionality of the spin correlation in Cu(C₆H₅COO)₂·3H₂O has been revealed by the heat capacity measurements in the temperature region from 20 K down to 0.07 K. The ratio of the estimated Néel temperature T_N(0) (<0.1 K) under the zero-external magnetic field to an interaction energy in the chain, k_BT_N(0)⁄2|J|S(S+1), is less than 0.7×10⁻², where the exchange constant J has been determined as J⁄k_B=−9.5±0.5 K from the heat capacity measurements between 0.5 K and 2.5 K. This one-dimensionality is comparable with or better than that of TMMC. An anomalous increase of the heat capacity has been found below 1.5 K under the external field up to 12 kOe applied along the c-axis which is perpendicular to the spin preferential direction. This anomaly can be explained qualitatively by considering the crossover effects of spin-dimensionality from the isotropic Heisenberg to the XY symmetry which is induced by the external field.