Ground-State and Thermodynamic Properties of the Quantum Mixed Spin-1/2-1/2-1-1 Chain

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We investigate both analytically and nutmerically the grotrnd-state and thermodynamicproperties of the quantum mixed spin-l/2-1/2-1-1 chain described by the Hamiltonianspins (s3( 3 and I,)( 2) and two .5:1 spins (Sqjz 1 and 5.)j) are arranged alternatively. Inseveral liuniting cases of .71, Jz, and J3 we apply the Wigner-Eckart theorena and carry otrt aperturbation calctulation to examine the behavior of the naassless lines wlaere the energy gapvanislaes. Perforuning a qtranttrm ]VIonte Carlo calctrlation without global flips at a sufficientlylow temperature for the case where ,71 : J3 : 1.0 and J? >0, we find that the ground state of thepresent systern in this case trndergoes a second-order phase transition accoraapanying the vanishing of the energy gap at .72 : J2. with J3. m O.TT i 0.01. We also find that the ground statesfor botlt .72< Jz. and ,72 >J3. can be understood by rneans of the valence-bond-solid picttnre.A qtranttrm lVIonte Carlo calctrlation xvhich takes the global flips along the Trotter directioninto account is carried out to elucidate the temperature dependences of the specific heat and theraaagnetic susceptibility. In partictrlar, it is f'ound that the susceptibility per trnit cell for J7 : 0.77xvith J) = J3 = 1.0 takes a finite value at absolute zero temperature and tlaat the specific heatper trnit cell verstrs tenaperature curve for J2 : 5.0 with J) : J3 :::: 1.0 has a dotrble peak.
社団法人日本物理学会の論文
1998-03-15