Phase diagram of the S=1/2 two-leg spin ladder with staggered bond alternation

Miguel Angel Martin-Delgado, J. Dukelsky, Germán Sierra

Niko Avalishvili, Bachana Beradze, George I. Japaridze

Kiyomi Okamoto

The ground state of the $S=1/2$ two-leg ladder with the staggered bond alternations is investigated. We have determined the precise phase diagram on the $\ensuremath{\delta}\ensuremath{-}{J}^{\ensuremath{'}}$ plane $(\ensuremath{\delta}$ and ${J}^{\ensuremath{'}}$ are the magnitudes of the bond alternation and the rung interaction, respectively), employing the level spectroscopic analysis of the numerical diagonalization data obtained by the Lanczos algorithm with the twisted boundary condition. The phase boundary between the leg-dimer state and the rung-dimer state near $(\ensuremath{\delta}{,J}^{\ensuremath{'}})=(0,0)$ is of the form ${J}^{\ensuremath{'}}\ensuremath{\propto}{\ensuremath{\delta}}^{0.69},$ which is consistent with the prediction ${J}^{\ensuremath{'}}\ensuremath{\propto}{\ensuremath{\delta}}^{2/3}$ by Mart\'{\i}n-Delgado, Duleksky, and Sierra, and also by Wang and Nersesyan, but not with ${J}^{\ensuremath{'}}\ensuremath{\propto}{\ensuremath{\delta}}^{2}$ by Cabra and Grynberg.

Oleg A. Starykh, Takashi Hikihara

Masaaki Nakamura, Takahiro Yamamoto, Kôichirô Ide

We study phase transition between two topologically different gapped states in the spin-1/2 bond-alternating two-leg Heisenberg ladder by investigating string order parameters and the level-crossing method based on the exact diagonalization. We determine the phase diagram on the plane of the bond-alternation δ and the rung-leg coupling ratio J ⊥ / J , and conclude that the boundary is given as J ⊥ / J ∝δ 2/3 in the weak coupling region J ⊥ / J , δ≪1.

G. I. Japaridze, S. Mahdavifar