2 research outputs found
Magnetic Properties of J-J-J' Quantum Heisenberg Chains with Spin S=1/2, 1, 3/2 and 2 in a Magnetic Field
By means of the density matrix renormalization group (DMRG) method, the
magnetic properties of the J-J-J quantum Heisenberg chains with spin
, 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 is
antiferromagnetic and 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 passes over a
critical value. The saturated field is observed to be independent of the
ferromagnetic coupling; (b) when is ferromagnetic and 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 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.Comment: 8 pages, 9 figures, to be published in J. Phys.: Condens. Matte
Evaluation of low-energy effective Hamiltonian techniques for coupled spin triangles
Motivated by recent work on Heisenberg antiferromagnetic spin systems on
various lattices made up of triangles, we examine the low-energy properties of
a chain of antiferromagnetically coupled triangles of half-odd-integer spins.
We derive the low-energy effective Hamiltonian to second order in the ratio of
the coupling J_2 between triangles to the coupling J_1 within each triangle.
The effective Hamiltonian contains four states for each triangle which are
given by the products of spin-1/2 states with the states of a pseudospin-1/2.
We compare the results obtained by exact diagonalization of the effective
Hamiltonian with those obtained for the full Hamiltonian using exact
diagonalization and the density-matrix renormalization group method. It is
found that the effective Hamiltonian is accurate only for the ground state for
rather low values of the ratio J_2 / J_1 and that too for the spin-1/2 case
with linear topology. The chain of spin-1/2 triangles shows interesting
properties like spontaneous dimerization and several singlet and triplet
excited states lying close to the ground state.Comment: Revtex, 11 pages, 11 eps figure