277 research outputs found
Anharmonic effects in magnetoelastic chains
We describe a new mechanism leading to the formation of rational
magnetization plateau phases, which is mainly due to the anharmonic spin-phonon
coupling. This anharmonicity produces plateaux in the magnetization curve at
unexpected values of the magnetization without explicit magnetic frustration in
the Hamiltonian and without an explicit breaking of the translational symmetry.
These plateau phases are accompanied by magneto-elastic deformations which are
not present in the harmonic case.Comment: 5 pages, 3 figure
Influence of lattice distortions in classical spin systems
We investigate a simple model of a frustrated classical spin chain coupled to
adiabatic phonons under an external magnetic field. A thorough study of the
magnetization properties is carried out both numerically and analytically. We
show that already a moderate coupling with the lattice can stabilize a plateau
at 1/3 of the saturation and discuss the deformation of the underlying lattice
in this phase. We also study the transition to saturation where either a first
or second order transition can occur, depending on the couplings strength.Comment: Submitted to Phys. Rev.
Ground state and low-lying excitations of the spin-1/2 XXZ model on the kagome lattice at magnetization 1/3
We study the ground state and low-lying excitations of the S=1/2 XXZ
antiferromagnet on the kagome lattice at magnetization one third of the
saturation. An exponential number of non-magnetic states is found below a
magnetic gap. The non-magnetic excitations also have a gap above the ground
state, but it is much smaller than the magnetic gap. This ground state
corresponds to an ordered pattern with resonances in one third of the hexagons.
The spin-spin correlation function is short ranged, but there is long-range
order of valence-bond crystal type.Comment: 2 pages, 1 figure included, to appear in Physica B (proceedings of
SCES'04
Generalized Pomeranchuk instabilities in graphene
We study the presence of Pomeranchuk instabilities induced by interactions on
a Fermi liquid description of a graphene layer. Using a recently developed
generalization of Pomeranchuk method we present a phase diagram in the space of
fillings versus on-site and nearest neighbors interactions. Interestingly, we
find that for both interactions being repulsive an instability region exists
near the Van Hove filling, in agreement with earlier theoretical work. In
contrast, near half filling, the Fermi liquid behavior appears to be stable, in
agreement with theoretical results and experimental findings using ARPES. The
method allows for a description of the complete phase diagram for arbitrary
filling.Comment: 9 pages, 3 figure
Quasi-periodic spin chains in a magnetic field
We study the interplay between a (quasi) periodic coupling array and an
external magnetic field in a spin-1/2 XXZ chain. A new class of magnetization
plateaux are obtained by means of Abelian bosonization methods which give rise
to a sufficient quantization condition. The investigation of magnetic phase
diagrams via exact diagonalization of finite clusters finds a complete
agreement with the continuum treatment in a variety of situations.Comment: 4 pages RevTeX, 5 PostScript figures included. Final version to
appear in PR
Fermionic description of spin-gap states of antiferromagnetic Heisenberg ladders in a magnetic field
Employing the Jordan-Wigner transformation on a unique path and then making a
mean-field treatment of the fermionic Hamiltonian, we semiquantitatively
describe the spin-gap states of Heisenberg ladders in a field. The appearance
of magnetization plateaux is clarified as a function of the number of legs.Comment: 2 pages, 3 figures embedded, J. Phys. Soc. Jpn. Vol. 71, No. 6, 1607
(2002
Massive and Massless Behavior in Dimerized Spin Ladders
We investigate the conditions under which a gap vanishes in the spectrum of
dimerized coupled spin-1/2 chains by means of Abelian bosonization and Lanczos
diagonalization techniques. Although both interchain () and dimerization
() couplings favor a gapful phase, it is shown that a suitable choice
of these interactions yields massless spin excitations. We also discuss the
influence of different arrays of relative dimerization on the appearance of
non-trivial magnetization plateaus.Comment: 5 pages, RevTex, 5 Postscript figure
Mechanisms for Non-Trivial Magnetization Plateaux of an S=1 Frustrated Spin Ladder
We investigate the non-trivial magnetization plateau at 1/4 of the saturation
magnetization of S=1 spin ladder, especially with reference to recent
experimental results on a new organic tetraradical
3,3',5,5'-tetrakis(N-tert-butylaminoxyl)biphenyl, abbreviated as BIP-TENO. We
propose three mechanisms for the formation of the plateau; the Neel mechanism,
the dimer mechanism and the spin-Peierls mechanism. We also discuss the effect
of four-spin exchange interactions.Comment: 3 pages, 5 figures, Orbital2001 (International Conference on Strongly
Correlated Electrons with Orbital Degrees of Freedom) (September 11-14, 2001.
Sendai
Non-perturbative effective field theory for two-leg antiferromagnetic spin ladders
We study the long wavelength limit of a spin 1/2 Heisenberg antiferromagnetic
two-leg ladder, treating the interchain coupling in a non-perturbative way. We
perform a mean field analysis and then include exactly the fluctuations. This
allows for a discussion of the phase diagram of the system and provides an
effective field theory for the low energy excitations. The coset fermionic
Lagrangian obtained corresponds to a perturbed SU(4)_1/U(1) Conformal Field
Theory (CFT). This effective theory is naturally embedded in a SU(2)_2 x Z_2
CFT, where perturbations are easily identified in terms of conformal operators
in the two sectors. Crossed and zig-zag ladders are also discussed using the
same approach.Comment: 14 pages LaTeX, 5 PostScript figures included using epsfig.sty; minor
corrections and a few references adde
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