563 research outputs found
Magnetization plateaux and jumps in a class of frustrated ladders: A simple route to a complex behaviour
We study the occurrence of plateaux and jumps in the magnetization curves of
a class of frustrated ladders for which the Hamiltonian can be written in terms
of the total spin of a rung. We argue on the basis of exact diagonalization of
finite clusters that the ground state energy as a function of magnetization can
be obtained as the minimum - with Maxwell constructions if necessary - of the
energies of a small set of spin chains with mixed spins. This allows us to
predict with very elementary methods the existence of plateaux and jumps in the
magnetization curves in a large parameter range, and to provide very accurate
estimates of these magnetization curves from exact or DMRG results for the
relevant spin chains.Comment: 14 pages REVTeX, 7 PostScript figures included using psfig.sty; this
is the final version to appear in Eur. Phys. J B; some references added and a
few other minor change
A Spin-1/2 Model for CsCuCl_3 in an External Magnetic Field
CsCuCl_3 is a ferromagnetically stacked triangular spin-1/2 antiferromagnet.
We discuss models for its zero-temperature magnetization process. The models
range from three antiferromagnetically coupled ferromagnetic chains to the full
three-dimensional situation. The situation with spin-1/2 is treated by
expansions around the Ising limit and exact diagonalization. Further,
weak-coupling perturbation theory is used mainly for three coupled chains which
are also investigated numerically using the density-matrix renormalization
group technique. We find that already the three-chain model gives rise to the
plateau-like feature at one third of the saturation magnetization which is
observed in magnetization experiments on CsCuCl_3 for a magnetic field
perpendicular to the crystal axis. For a magnetic field parallel to the crystal
axis, a jump is observed in the experimental magnetization curve in the region
of again about one third of the saturation magnetization. In contrast to
earlier spinwave computations, we do not find any evidence for such a jump with
the model in the appropriate parameter region.Comment: 13 pages LaTeX2e with EPJ macro package (included), 8 (e)ps figures
included using psfig.sty; this is the final version to appear in Eur. Phys. J
B; a few further explanations and one reference adde
Low-temperature properties of the Hubbard model on highly frustrated one-dimensional lattices
We consider the repulsive Hubbard model on three highly frustrated
one-dimensional lattices -- sawtooth chain and two kagom\'{e} chains -- with
completely dispersionless (flat) lowest single-electron bands. We construct the
complete manifold of {\em exact many-electron} ground states at low electron
fillings and calculate the degeneracy of these states. As a result, we obtain
closed-form expressions for low-temperature thermodynamic quantities around a
particular value of the chemical potential . We discuss specific
features of thermodynamic quantities of these ground-state ensembles such as
residual entropy, an extra low-temperature peak in the specific heat, and the
existence of ferromagnetism and paramagnetism. We confirm our analytical
results by comparison with exact diagonalization data for finite systems.Comment: 20 pages, 12 figures, 2 table
Flat-Band Ferromagnetism as a Pauli-Correlated Percolation Problem
We investigate the location and nature of the para-ferro transition of
interacting electrons in dispersionless bands using the example of the Hubbard
model on the Tasaki lattice. This case can be analyzed as a geometric
site-percolation problem where different configurations appear with nontrivial
weights. We provide a complete exact solution for the 1D case and develop a
numerical algorithm for the 2D case. In two dimensions the paramagnetic phase
persists beyond the uncorrelated percolation point, and the grand-canonical
transition is via a first-order jump to an unsaturated ferromagnetic phase.Comment: 6 pages, 5 figure
Numerical study of magnetization plateaux in the spin-1/2 kagome Heisenberg antiferromagnet
We clarify the existence of several magnetization plateaux for the kagome
antiferromagnetic Heisenberg model in a magnetic field. Using
approximate or exact localized magnon eigenstates, we are able to describe in a
similar manner the plateau states that occur for magnetization per site
, , and of the saturation value. These results are confirmed
using large-scale Exact Diagonalization on lattices up to 63 sites.Comment: 8 pages; minor changes; published versio
Exact results for one dimensional stochastic cellular automata for different types of updates
We study two common types of time-noncontinuous updates for one dimensional
stochastic cellular automata with arbitrary nearest neighbor interactions and
arbitrary open boundary conditions. We first construct the stationary states
using the matrix product formalism. This construction then allows to prove a
general connection between the stationary states which are produced by the two
different types of updates. Using this connection, we derive explicit relations
between the densities and correlation functions for these different stationary
states.Comment: 7 pages, Late
Exact eigenstates of highly frustrated spin lattices probed in high fields
Strongly frustrated antiferromagnets such as the magnetic molecule
{Mo72Fe30}, the kagome, or the pyrochlore lattice exhibit a variety of
fascinating properties like low-lying singlets, magnetization plateaus as well
as magnetization jumps. During recent years exact many-body eigenstates could
be constructed for several of these spin systems. These states become ground
states in high magnetic fields, and they also lead to exotic behavior. A key
concept to an understanding of these properties is provided by independent
localized magnons. The energy eigenvalue of these n-magnon states scales
linearly with the number n of independent magnons and thus with the total
magnetic quantum number M=Ns-n. In an applied field this results in a giant
magnetization jump which constitutes a new macroscopic quantum effect. It will
be demonstrated that this behavior is accompanied by a massive degeneracy, an
extensive (T=0)-entropy, and thus a large magnetocaloric effect at the
saturation field. The connection to flat band ferromagnetism will be outlined.Comment: 4 pages, submitted to the proceedings of the Yamada Conference LX on
Research in High Magnetic Fields, August 16-19, 2006 Sendai, Japa
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