Haldane phase, impurity effects and spin ladders

Pimonpan Sompet

Makoto Kaburagi, Takashi Tonegawa

Xiaoqun Wang

Using the density matrix renormalization group method, we determined the phase diagram of a frustrated antiferromagnetic spin-[Formula: see text] ladder at zero temperature. Two spin-gapped phases, the Haldane phase and the singlet phase, are identified. In particular, impurity effects are experimentally detectable for the Haldane phase. A phase transition between the two phases occurs at any nonzero value of frustration coupling J×. On the phase boundary, the spin gap vanishes for sufficiently small J×. Crossing this line of gapless states, the transition is of second order, while it is of first order for larger J×.

浩志 渡辺

Shinji Watanabe, Hisatoshi Yokoyama

The bulk properties and impurity effect in the one-dimensional S=1/2 Heisenberg model with dimerization and next-nearest-neighbor exchange are studied by the density matrix renormalization group (DMRG) and exact diagonalization methods. This model widely covers the S=1 Heisenberg model, two-leg ladders, bond-alternating chains and the S=1/2 Heisenberg model. Almost the whole parameter space belongs to the Haldane phase. In the non-dimerizing limit a transition to a kind of spin liquid takes place, characterized by the delocalized spin polarization. This transition causes the distinct impurity effects between a spin-Peierls compound CuGeO_{3} and the Haldane systems. Experimental aspects of a bond-alternating compound CuNb_{2}O_{6} are also discussed. Furthermore, the incommensurate correlation arising in this model is extensively studied, and a phase diagram is constructed.

Hiroshi K. Watanabe