| Bond-dilution effects on a ground state of the S=1/2 quantum antiferromagnetic (AF) Heisenberg model consisting of bond-alternating chains on a square lattice was investigated by means of the quantum Monte Carlo simulations with the continuous-imaginary-time loop algorithm. The magnitude of the stronger (weaker) intra-chain interaction is put unity (α) and that of the inter-chain interaction J'. The ground state of the pure system is the dimmer state with a finite spin gap for small α and J'. When spins are randomly removed from the system in the dimer state (site dilution), a spin which formed a singlet pair with the removed spin before dilution becomes nearly free, which we call effective spins. Between two of them, however, there exists the finite interaction Jmn mediated by a sea of singlet pairs. Since the effective interaction is AF (ferromagnetic) when the two effective spins are in the different (same) sublattices, an AF long-range order (LRO) is induced with an infinitesimal concentration of site dilution. When stronger bonds are randomly removed from the system in the dimer state (bond dilution), on the other hand, the effective spins are always induced in pairs at both ends of the removed bonds. Since the two spins are located on the different sublattices, a singlet pair is reformed through the short-range effective AF coupling Jaf of O(J'2). In contrast to the site-diluted case, there exist two effective interactions. For small concentration of dilution, if Jaf is sufficiently larger than Jmn, the system is in the disordered phase. The phase transition between the disordered and AF-LRO phases occurs when the magnitudes of Jaf and Jmn are equivalent. |
|
|
