Exact diagonalization study of charge order in the quarter-filled two-leg ladder system NaV2O5 - art. no. 144420

The charge ordering transition in the layer compound NaV2O5 is studied by m eans of exact diagonalization methods for finite systems. The two-leg ladde rs of the V-Trellis lattice are associated with one spin variable of the va nadium 3d electron in the rung and a pseudospin variable that describes its positional degree of freedom. The charge ordering (CO) due to intersite Co ulomb interactions is described by an effective Ising-like Hamiltonian for the pseudospins that are coupled to the spin fluctuations along the ladder. We employ a Lanczos algorithm on a two-dimensional lattice to compute char ge (pseudospin) and spin-correlation functions and the energies of the low- lying excited states. A CO phase diagram is constructed and the effect of i ntraladder exchange on the CO transition is studied. It is shown that a pha se with no long-range order exists between the in-line and zigzag ordered s tructures. We provide a finite-size scaling analysis for the spin excitatio n gap and also discuss the type of excitations. In addition we studied the effect of bond alternation of spin exchange and derived a scaling form for the spin gap in terms of the dimerization parameter.