390 research outputs found
Absence of magnetic order for the spin-half Heisenberg antiferromagnet on the star lattice
We study the ground-state properties of the spin-half Heisenberg
antiferromagnet on the two-dimensional star lattice by spin-wave theory, exact
diagonalization and a variational mean-field approach. We find evidence that
the star lattice is (besides the \kagome lattice) a second candidate among the
11 uniform Archimedean lattices where quantum fluctuations in combination with
frustration lead to a quantum paramagnetic ground state. Although the classical
ground state of the Heisenberg antiferromagnet on the star exhibits a huge
non-trivial degeneracy like on the \kagome lattice, its quantum ground state is
most likely dimerized with a gap to all excitations. Finally, we find several
candidates for plateaux in the magnetization curve as well as a macroscopic
magnetization jump to saturation due to independent localized magnon states.Comment: new extended version (6 pages, 6 figures) as published in Physical
Review
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
A Comparative Study of the Magnetization Process of Two-Dimensional Antiferromagnets
Plateaux in the magnetization curves of the square, triangular and hexagonal
lattice spin-1/2 XXZ antiferromagnet are investigated. One finds a zero
magnetization plateau (corresponding to a spin-gap) on the square and hexagonal
lattice with Ising-like anisotropies, and a plateau with one third of the
saturation magnetization on the triangular lattice which survives a small
amount of easy-plane anisotropy. Here we start with transfer matrix
computations for the Ising limit and continue with series in the XXZ-anisotropy
for plateau-boundaries using the groundstates of the Ising limit. The main
focus is then a numerical computation of the magnetization curves with
anisotropies in the vicinity of the isotropic situation. Finally, we discuss
the universality class associated to the asymptotic behaviour of the
magnetization curve close to saturation, as observed numerically in two and
higher dimensions.Comment: 21 pages plain TeX (with macro package included), 7 PostScript
figures included using psfig.st
Enhanced magnetocaloric effect in frustrated magnetic molecules with icosahedral symmetry
We investigate the magnetocaloric properties of certain antiferromagnetic
spin systems that have already been or very likely can be synthesized as
magnetic molecules. It turns out that the special geometric frustration which
is present in antiferromagnets that consist of corner-sharing triangles leads
to an enhanced magnetocaloric effect with high cooling rates in the vicinity of
the saturation field. These findings are compared with the behavior of a simple
unfrustrated spin ring as well as with the properties of the icosahedron. To
our surprise, also for the icosahedron large cooling rates can be achieved but
due to a different kind of geometric frustration.Comment: 5 pages, 8 figures, more information at
http://obelix.physik.uni-osnabrueck.de/~schnack
Magnetism of Finite Graphene Samples: Mean-Field Theory compared with Exact Diagonalization and Quantum Monte Carlo Simulation
The magnetic properties of graphene on finite geometries are studied using a
self-consistent mean-field theory of the Hubbard model. This approach is known
to predict ferromagnetic edge states close to the zig-zag edges in single-layer
graphene quantum dots and nanoribbons. In order to assess the accuracy of this
method, we perform complementary exact diagonalization and quantum Monte Carlo
simulations. We observe good quantitative agreement for all quantities
investigated provided that the Coulomb interaction is not too strong.Comment: 5 pages including 3 figures; v3: error concerning middle panel of
Fig. 3 correcte
Phenomenological Issues in TeV scale Gravity with Light Neutrino Masses
The possible existence of bulk singlet neutrinos in the scenario with large
compactified dimensions and low string scale has important consequences
for low-energy observables. We demonstrate that intergenerational mass
splitting and mixing lead to the effective violation of the lepton universality
and flavor changing processes in charged lepton sector. Current experimental
constraints push to the scale of 10 TeV over most of the interesting
range for neutrino mass splitting.Comment: 12 pages. Standard Latex. 2 eps figures. Minor changes, to appear in
Physics Letters
Finite-temperature ordering in a two-dimensional highly frustrated spin model
We investigate the classical counterpart of an effective Hamiltonian for a
strongly trimerized kagome lattice. Although the Hamiltonian only has a
discrete symmetry, the classical groundstate manifold has a continuous global
rotational symmetry. Two cases should be distinguished for the sign of the
exchange constant. In one case, the groundstate has a 120^\circ spin structure.
To determine the transition temperature, we perform Monte-Carlo simulations and
measure specific heat, the order parameter as well as the associated Binder
cumulant. In the other case, the classical groundstates are macroscopically
degenerate. A thermal order-by-disorder mechanism is predicted to select
another 120^\circ spin-structure. A finite but very small transition
temperature is detected by Monte-Carlo simulations using the exchange method.Comment: 11 pages including 9 figures, uses IOP style files; to appear in J.
Phys.: Condensed Matter (proceedings of HFM2006
High-Order Coupled Cluster Method Study of Frustrated and Unfrustrated Quantum Magnets in External Magnetic Fields
We apply the coupled cluster method (CCM) in order to study the ground-state
properties of the (unfrustrated) square-lattice and (frustrated)
triangular-lattice spin-half Heisenberg antiferromagnets in the presence of
external magnetic fields. Here we determine and solve the basic CCM equations
by using the localised approximation scheme commonly referred to as the
`LSUB' approximation scheme and we carry out high-order calculations by
using intensive computational methods. We calculate the ground-state energy,
the uniform susceptibility, the total (lattice) magnetisation and the local
(sublattice) magnetisations as a function of the magnetic field strength. Our
results for the lattice magnetisation of the square-lattice case compare well
to those results of QMC for all values of the applied external magnetic field.
We find a value for magnetic susceptibility of for the
square-lattice antiferromagnet, which is also in agreement with the results of
other approximate methods (e.g., via QMC). Our estimate for the
range of the extent of the () magnetisation plateau for the
triangular-lattice antiferromagnet is , which is in good
agreement with results of spin-wave theory () and
exact diagonalisations (). The CCM value for the
in-plane magnetic susceptibility per site is , which is below the
result of the spin-wave theory (evaluated to order 1/S) of .Comment: 30 pages, 13 figures, 1 Tabl
Lanczos algorithm with Matrix Product States for dynamical correlation functions
The density-matrix renormalization group (DMRG) algorithm can be adapted to
the calculation of dynamical correlation functions in various ways which all
represent compromises between computational efficiency and physical accuracy.
In this paper we reconsider the oldest approach based on a suitable
Lanczos-generated approximate basis and implement it using matrix product
states (MPS) for the representation of the basis states. The direct use of
matrix product states combined with an ex-post reorthogonalization method
allows to avoid several shortcomings of the original approach, namely the
multi-targeting and the approximate representation of the Hamiltonian inherent
in earlier Lanczos-method implementations in the DMRG framework, and to deal
with the ghost problem of Lanczos methods, leading to a much better convergence
of the spectral weights and poles. We present results for the dynamic spin
structure factor of the spin-1/2 antiferromagnetic Heisenberg chain. A
comparison to Bethe ansatz results in the thermodynamic limit reveals that the
MPS-based Lanczos approach is much more accurate than earlier approaches at
minor additional numerical cost.Comment: final version 11 pages, 11 figure
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