Effects of magnetic anisotropy and magnetic field on a frustrated two-leg spin-ladder system

Sugimoto, Takanori; Mori, Michiyasu  ; Toyama, Takami; Maekawa, Sadamichi

We numerically study magnetic excitations in a frustrated two-leg spin-ladder system, in which all magnetic exchange interactions, i.e., the nearest-, next-nearest-neighbor sites in the leg direction, and the nearest-neighbor sites in the rung direction, are antiferromagnetic. This is a minimal model describing a low-dimensional quantum spin compound, BiCuPO. Firstly, we examine the magnetic excitations by using the dynamical density-matrix renormalization-group method in possible magnetic phases, columnar dimer and rung singlet, without magnetic anisotropy and magnetic field. We can distinguish one phase from the other by distribution of the spectral weight. We examine frustration effect on the bonding mode, so-called bound triplon, with a perturbation theory from the strong rung-coupling limit. In addition, we show a splitting of the triplon mode, which is experimentally observed, with respect to magnetic anisotropy of the rung exchange interaction and magnetic field.