Correlation and Dimerization Effects on the Physical Behavior of the NR4[Ni(dmit)2]2NR_4 [Ni(dmit)_2]_2 Charge Transfer Salts : A DMRG Study of the Quarter-Filling t-J Model

The present work studies the quasi one-dimensional $Ni(dmit)_2$-based compounds within a correlated model. More specifically, we focus our attention on the composed influence of the electronic dimerization-factor and the repulsion, on the transport properties and the localization of the electronic density in the ground-state. Those properties are studied through the computation of the charge gaps (difference between the ionization potential and the electro-affinity: IP-EA) and the long- and short-bond orders of an infinite quarter-filled chain within a $t-J(t,U)$ model. The comparison between the computed gaps and the experimental activation energy of the semiconductor $NH_2Me_2 [Ni(dmit)_2]_2$ allows us to estimate the on-site electronic repulsion of the $Ni(dmit)_2$ molecule to $1.16eV$.Comment: 13 pages, 4 figures, RevTe

The infinite-range quantum random Heisenberg magnet

We study with exact diagonalization techniques the Heisenberg model for a system of SU(2) spins with S=1/2 and random infinite-range exchange interactions. We calculate the critical temperature T_g for the spin-glass to paramagnetic transition. We obtain T_g ~ 0.13, in good agreement with previous quantum Monte Carlo and analytical estimates. We provide a detailed picture for the different kind of excitations which intervene in the dynamical response chi''(w,T) at T=0 and analyze their evolution as T increases. We also calculate the specific heat Cv(T). We find that it displays a smooth maximum at TM ~ 0.25, in good qualitative agreement with experiments. We argue that the fact that TM>Tg is due to a quantum disorder effect.Comment: 17 pages, 14 figure