GROUND-STATE AND THERMODYNAMIC PROPERTIES OF THE QUANTUM MIXED SPIN-12-1/2-1-1 CHAIN/

We investigate both analytically and numerically the ground-slate and thermodynamic properties of the quantum mixed spin-1/2-1/2-1-1 chain d escribed by the Hamiltonian H = Sigma l=1N/4(J(1) (s) over right arrow (4l-3).(s) over right arrow(4l-2)+J(2) (s) over right arrow(4l-2).(S) over right arrow(4l-1).+J(3) (S) over right arrow(4l-1).(S) over right arrow(4l)+J(2) (S) over right arrow(4l).(S) over right arrow(4l+1)), where two S = 1/2 spins ((s) over right arrow(4l-3) and (s) over right arrow(4l-2)) and two S=1 spins ((S) over right arrow(4l-1) and (S) ov er right arrow(4l)) are arranged alternatively. In several limiting ca ses of J(1), J(2), and J(3) we apply the Wigner-Eckalt theorem and car ry out a perturbation calculation to examine the behavior of the massl ess lines where the energy gap vanishes. Performing a quantum Monte Ca rlo calculation without global flips at a sufficiently low temperature for the case where J(1) = J(3) = 1.0 and J(2) > 0, we find that the g round state of the present system in this case undergoes a second-orde r phase transition accompanying the vanishing of the energy gap at J(2 ) = J(2c) with J(2c) = 0.77 +/- 0.01. We also find that the ground sta tes for both J(2)<J(2c) and J(2)>J(2c) can be understood by means of t he valence-bond-solid picture. A quantum Monte Carlo calculation which takes the global flips along the Trotter direction into account is ca rried out to elucidate the temperature dependences of the specific hea t and the magnetic susceptibility. In particular, it is found that the susceptibility per unit cell for J(2) = 0.77 with J(1) = J(3) = 1.0 t akes a finite value at absolute zero temperature and that the specific heat per unit cell versus temperature curve for J(2) = 5.0 with J(1) = J(3) = 1.0 has a double peak.