S ep 1 99 8 Critical properties of S = 12 Heisenberg ladders in magnetic fields

The critical properties of the S = 1/2 Heisenberg two-leg ladders are investigated in a magnetic field. Combining the exact diagonalization method and the finite-size-scaling analysis based on conformal field theory, we calculate the critical exponents of spin correlation functions numerically. For a strong interchain coupling, magnetization dependence of the critical exponents shows characteristic behavior depending on the sign of the interchain coupling. We also calculate the critical exponents for the S = 1/2 Heisenberg two-leg ladder with a diagonal interaction, which is thought as a model Hamiltonian of the organic spin ladder compound Cu2(1,4-diazacycloheptane)2Cl4. Numerical results are compared with experimental results of temperature dependence of the NMR relaxation rate 1/T1. Typeset using REVTEX