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Communications in Theoretical Physics

DMRG Studies on Properties of Undoped and Doped Molecule-Based Ferromagnetic Chain

K.L. Yao, Yi Qin, Qingmei Liu, Zuli Liu

Published Dec 1, 2005• 0 citations
Publisher

Abstract

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.

DOI
10.1088/6102/44/6/1099

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By using density-matrix renormalization group(DMRG) method,the authors investigate a system of quasi-one-dimensional Heisenberg antiferromagnetic chain with symmetry breaking of sublattice.The ground state energy per unit cell,spin correlation function and staggered susceptibility are calculated.The calculations show that the system exhibits ferrimagnetic properties,although the spin-spin interactions are antiferromagnetic.The results are consistent qualitatively with the conclusion predicted by LIEB-MATTIS theorem and the results calculated by spin-wave theory.
The spin correlation (SC) function on the main chain of a theoretical model for organic ferromagnets is studied by using a density matrix renormalization group method with both periodic boundary conditions and open boundary conditions (OBCs). It is found that the chain–radical interaction, the on-site electron–electron repulsion, and the electron–phonon coupling affect the SC greatly and play important roles in the stability of the ferromagnetic state. There exist some types of 'weak' OBC, diminishing the alternation of the SC.
Using density-matrix renormalization group (DMRG) method, we studied the effect of magnetic frustrations due to next-nearest- neighbor bonds in a system of quasi-one-dimensional Heisenberg antiferromagnetic chain but with ferrimagnetic properties. In this paper, the ground state energy per unit cell, spin correlation function and spin gap are calculated. As a result of the frustrations, the quantum disordered phase with the spin gap appears at the spin order-disorder phase transition point αC≈0.412 to α＝1. And in the region of magnetic-order phase, the increasing magnetic frustrations reduce the magnetic order.
Different models for doping of two-orbital chains with mobile S=1/2S=1/2S=1/2 fermions and strong, ferromagnetic (FM) Hund's rule couplings stabilizing the S=1 spins are investigated by density matrix renormalization group (DMRG) methods. The competition between antiferromagnetic (AF) and FM order leads to a rich phase diagram with a narrow FM region for weak AF couplings and strongly enhanced triplet pairing correlations. Without a level difference between the orbitals, the spin gap persists upon doping, whereas gapless spin excitations are generated by interactions among itinerant polarons in the presence of a level difference. In the charge sector we find dominant singlet pairing correlations without a level difference, whereas upon the inclusion of a Coulomb repulsion between the orbitals or with a level difference, charge density wave (CDW) correlations decay slowest. The string correlation functions remain finite upon doping for all models.
By means of the density matrix renormalization group (DMRG) method, the magnetic properties of the J-J-J′^{\prime}′ quantum Heisenberg chains with spin S=1/2S=1/2S=1/2, 1, 3/2 and 2 in the ground states are investigated in the presence of a magnetic field. Two different cases are considered: (a) when JJJ is antiferromagnetic and J′J^{\prime}J′ is ferromagnetic (i.e. the AF-AF-F chain), the system is a ferrimagnet. The plateaus of the magnetization are observed. It is found that the width of the plateaus decreases with increasing the ferromagnetic coupling, and disappears when % J^{\prime}/J passes over a critical value. The saturated field is observed to be independent of the ferromagnetic coupling; (b) when JJJ is ferromagnetic and J′J^{\prime}J′ is antiferromagnetic (i.e. the F-F-AF chain), the system becomes an antiferromagnet. The plateaus of the magnetization are also seen. The width of the plateaus decreases with decreasing the antiferromagnetic coupling, and disappears when $J^{\prime}/J $ passes over a critical value. Though the ground state properties are quite different, the magnetization plateaus in both cases tend to disappear when the ferromagnetic coupling becomes more dominant. Besides, no fundamental difference between the systems with spin half-integer and integer has been found.