MAGNETIC AND CRITICAL PROPERTIES OF ALTERNATING SPIN HEISENBERG CHAININ A MAGNETIC-FIELD

We study magnetic and critical properties of the alternating spin anti ferromagnetic Heisenberg chain with S = 1/2 and 1 in a magnetic field at T = 0, The numerical diagonalization is applied to the system up to 2N = 20 sites. Checking numerically that magnetic states with the mag netization per site m obey a conformal field theory with conformal ano maly c = 1 for 1/4 < m < 3/4, we use the finite-size scaling of the co nformal invariance to obtain a magnetization curve in the thermodynami c limit. In the magnetization curve a plateau appears at m = 1/4. We a lso calculate two critical exponents eta and eta(2) for 1/4 < m < 3/4; which control the asymptotic behavior of the transverse and parallel spin correlation functions. We check the relation eta eta(z) = 1; whic h universally holds for a c = 1 conformal field theory.