FIELD-DEPENDENT THERMODYNAMICS AND QUANTUM CRITICAL PHENOMENA IN THE DIMERIZED SPIN SYSTEM CU-2(C5H12N2)(2)CL-4

Experimental data for the uniform susceptibility, magnetization, and s pecific heat for the material Cu-3(C5H12N2)(2)Cl-4 (CuHpCl) as a funct ion of temperature and external field are compared with those of three different dimerized spin models: alternating spin chains, spin ladder s, and the bilayer Heisenberg model. It is shown that because this mat erial consists of weakly coupled spin dimers, much of the data are ins ensitive to how the dimers are coupled together and what the effective dimensionality of the system is. When such a system is tuned to the q uantum critical point by application of a field, dimensionality shows up in the power-law dependences of thermodynamic quantities on the tem perature. We discuss the temperature window for such a quantum critica l behavior in CuHpCl. [S0163-1829(98)05838-X].