Thermodynamics and spin gap of the Heisenberg ladder calculated by the   look-ahead Lanczos algorithm
  We have developed an improved version of the quantum transfer matrix algorithm. The extreme eigenvalues and eigenvectors of the transfer matrix are calculated by the recently developed look-ahead Lanczos algorithm for non-Hermitian matrices with higher efficiency and accuracy than by the power method. We have applied this method to the Heisenberg ladder. The temperature dependence of the susceptibility, specific heat, correlation length and nuclear spin relaxation rate $1/T_1$ are calculated. Our results support the existence of a spin gap of about $0.5 J$. The preprint is submitted as a uuencoded compressed PostScript file. In case of printing problems the original TeX file (without figures) can be retrieved by anonymous ftp to 'maggia.ethz.ch' filename 'CSSP/preprints/hblad.tex'. Alternatively from June 1, 1994 by WWW: URL "http://www.ips.id.ethz.ch/CSSP/preprints.html" 
Matthias Troyer, Hirokazu Tsunetsugu and Diethelm Wuertz