LOW ENERGY PROPERTIES OF A FRUSTRATED ANTIFERROMAGNETIC SPIN-1/2 LADDER

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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×.