  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"
