4,257 research outputs found
Magnetization plateaux and jumps in a frustrated four-leg spin tube under a magnetic field
We study the ground state phase diagram of a frustrated spin-1/2 four-leg
spin tube in an external magnetic field. We explore the parameter space of this
model in the regime of all-antiferromagnetic exchange couplings by means of
three different approaches: analysis of low-energy effective Hamiltonian (LEH),
a Hartree variational approach (HVA) and density matrix renormalization group
(DMRG) for finite clusters. We find that in the limit of weakly interacting
plaquettes, low-energy singlet, triplet and quintuplet states play an important
role in the formation of fractional magnetization plateaux. We study the
transition regions numerically and analytically, and find that they are
described, at first order in a strong- coupling expansion, by an XXZ spin-1/2
chain in a magnetic field; the second-order terms give corrections to the XXZ
model. All techniques provide consistent results which allow us to predict the
existence of fractional plateaux in an important region in the space of
parameters of the model.Comment: 10 pages, 7 figures. Accepted for publication in Physical Review
Electron Confinement Induced by Diluted Hydrogen-like Ad-atoms in Graphene Ribbons
We report the electronic properties of two-dimensional systems made of
graphene nanoribbons which are patterned with ad-atoms in two separated
regions. Due to the extra electronic confinement induced by the presence of the
impurities, we find resonant levels, quasi-bound and impurity-induced localized
states, which determine the transport properties of the system. Regardless of
the ad-atom distribution in the system, we apply band-folding procedures to
simple models and predict the energies and the spatial distribution of those
impurity-induced states. We take into account two different scenarios: gapped
graphene and the presence of randomly distributed ad-atoms in a low dilution
regime. In both cases the defect-induced resonances are still detected. Our
findings would encourage experimentalist to synthesize these systems and
characterize their quasi-localized states employing, for instance, scanning
tunneling spectroscopy (STS). Additionally, the resonant transport features
could be used in electronic applications and molecular sensor devices.Comment: 12 pages, 11 figures, submitted (minor changes
Metastable and scaling regimes of a one-dimensional Kawasaki dynamics
We investigate the large-time scaling regimes arising from a variety of
metastable structures in a chain of Ising spins with both first- and
second-neighbor couplings while subject to a Kawasaki dynamics. Depending on
the ratio and sign of these former, different dynamic exponents are suggested
by finite-size scaling analyses of relaxation times. At low but
nonzero-temperatures these are calculated via exact diagonalizations of the
evolution operator in finite chains under several activation barriers. In the
absence of metastability the dynamics is always diffusive.Comment: 18 pages, 8 figures. Brief additions. To appear in Phys. Rev.
Affine convex body semigroups
In this paper we present a new kind of semigroups called convex body
semigroups which are generated by convex bodies of R^k. They generalize to
arbitrary dimension the concept of proportionally modular numerical semigroup
of [7]. Several properties of these semigroups are proven. Affine convex body
semigroups obtained from circles and polygons of R^2 are characterized. The
algorithms for computing minimal system of generators of these semigroups are
given. We provide the implementation of some of them
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