Abstract
We investigate the low-temperature phase diagram of the exactly solvedsu(4) two-leg spin ladder as a function of the rung coupling and magnetic field H by means of the thermodynamic Bethe ansatz (TBA). In the absence of a magneticfield the model exhibits three quantum phases, while in the presence of a strongmagnetic field there is no singlet ground state for ferromagnetic rung coupling.For antiferromagnetic rung coupling, there is a gapped phase in the regimeH < Hc1, a fully polarizedgapped phase for H > Hc2 and a Luttinger liquid magnetic phase in the regimeHc1 < H < Hc2.The critical behaviour derived using the TBA is consistent with the existingexperimental, numerical and perturbative results for the strong coupling laddercompounds. This includes the spin excitation gap and the critical fieldsHc1 andHc2, whichare in excellent agreement with the experimental values for the known strong coupling ladder compounds(5IAP)2CuBr4·2H2O,Cu2(C5H12N2)2Cl4 and(C5H12N)2CuBr4.In addition we predict the spin gap for the weak coupling compounds with , such as (VO)2P2O7,and also show that the gap opens for arbitrary .
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