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 coupling energy gap, the uniform spin susceptibility and the nuclear-magnetic-resonance spin-relaxation rate are calculated as a function of temperature and magnetic field. For weak interladder coupling and when intraladder couplings are of the same order of magnitude, the energy spectrum vanishes at incommensurate wave vectors giving rise to nodes. Our results for the single ladder successfully compare to experiments on SrCu2O3 and (VO)(2)P2O7 materials and predictions concerning the coupling to the magnetic field are made.