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Journal of Physics Condensed Matter

Density matrix renormalization group studies on -topology and spin alignment of an organic -conjugated spin system

Q M Liu, K.L. Yao, Z L Liu, Yi Qin

Published Mar 12, 2004• 3 citations
Publisher

Abstract

We carry out density matrix renormalization group (DMRG) calculations to study π-topology and spin alignment of an organic π-conjugated spin system in the Hubbard model. We discuss the spin correlation, the energy gap and the lattice configuration in half-filled and doped systems. It is confirmed that electronic doping can alter the spin state, and the spin alignment is influenced by the parity of the total site number N . The electron–electron interaction is in favour of the ferromagnetic stability of the system. For the hall-filled odd case and the hole-doped even case, there exist strong ferromagnetic correlation and large energy gap in π-conjugated spin systems.

DOI
10.1088/0953-8984/16/12/018

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We study a magnetic impurity embedded in a correlated electron system using the density-matrix renormalization group method. The correlated electron system is described by the one-dimensional Hubbard model. At half filling, we confirm that the binding energy of the singlet bound state increases exponentially in the weak-coupling regime and decreases inversely proportional to the correlation in the strong-coupling regime. The spin-spin correlation shows an exponential decay with distance from the impurity site. The correlation length becomes smaller with increasing the correlation strength. We find discontinuous reduction of the binding energy and of spin-spin correlations with hole doping. The binding energy is reduced by hole doping; however, it remains of the same order of magnitude as for the half-filled case.
By using density matrix renormalization group (DMRG) method a model for organic molecule-based ferromagnetic chain is proposed. It is found that the ground states of undoped and doped systems both exhibit ferrimagnetic ordering. The e - e repulsion plays an important role in the stability of the ferromagnetic state either in doped system or undoped system. For the undoped system, each unit cell contains half of the total spins, which is consistent with Lieb's theorem. It is convinced that when the system is doped with one electron, a charge density wave is excited, which decreases the amplitude of spin density wave, therefore acting against the stability of ferromagnetic state.
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The Density-matrix renormalization-group (DMRG) technique is used to study the one-dimensional (1D) dimerized Hubbard model at quarter filling. We carry out a precise calculation of the charge gap as a function of the interaction and dimerization strengths and evaluate the interaction strength of the effective 1D Hubbard model at half filling. We also examine the effect of nearest-neighbor repulsion on the charge gap, whereby we evaluate the realistic repulsive strengths for quasi-1D organic compounds.
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