INCOMMENSURATE NODES IN THE ENERGY-SPECTRUM OF COUPLED ANTIFERROMAGNETIC HEISENBERG LADDERS

Heisenberg ladders are investigated using the bond-mean-field theory [ M. Azzouz, Phys. Rev. B 48, 6136 (1993)]. The zero interladder couplin g energy gap, the uniform spin susceptibility and the nuclear-magnetic -resonance spin-relaxation rate are calculated as a function of temper ature and magnetic field. For weak interladder coupling and when intra ladder couplings are of the same order of magnitude, the energy spectr um vanishes at incommensurate wave vectors giving rise to nodes. Our r esults for the single ladder successfully compare to experiments on Sr Cu2O3 and (VO)(2)P2O7 materials and predictions concerning the couplin g to the magnetic field are made.