Density matrix renormalization group study of critical behavior of thespin−12alternating Heisenberg chain

We investigate the critical behavior of the S=½ alternating Heisenberg chain using the density matrix renormalization group. The ground-state energy per spin, e 0 , and singlet-triplet energy gap Δ are determined for a range of alternations δ. Our results for the approach of e 0 to the uniform chain limit are well described by c δ p , with p1.45. The singlet-triplet gap is also well described by a power law, with p0.73, half of the e 0 power. The renormalization group predictions of power laws with logarithmic corrections can also accurately describe our data provided that a surprisingly large-scale parameter δ 0 is present in the logarithms.