276 research outputs found
Relevance of nonadiabatic effects in TiOCl
We analyze the effect of the phonon dynamics on a recently proposed model for
the uniform-incommensurate transition seen in TiOX compounds. The study is
based on a recently developed formalism for nonadiabatic spin-Peierls systems
based on bosonization and a mean field RPA approximation for the interchain
coupling. To reproduce the measured low temperature spin gap, a spin-phonon
coupling quite bigger than the one predicted from an adiabatic approach is
required. This high value is compatible with the renormalization of the phonons
in the high temperature phase seen in inelastic x-ray experiments. Our theory
accounts for the temperature of the incommensurate transition and the value of
the incommensurate wave vector at the transition point.Comment: 4 pages, 2 figure
From spinons to magnons in explicit and spontaneously dimerized antiferromagnetic chains
We reconsider the excitation spectra of a dimerized and frustrated
antiferromagnetic Heisenberg chain. This model is taken as the simpler example
of compiting spontaneous and explicit dimerization relevant for Spin-Peierls
compounds. The bosonized theory is a two frequency Sine-Gordon field theory. We
analize the excitation spectrum by semiclassical methods. The elementary
triplet excitation corresponds to an extended magnon whose radius diverge for
vanishing dimerization. The internal oscilations of the magnon give rise to a
series of excited state until another magnon is emited and a two magnon
continuum is reached. We discuss, for weak dimerization, in which way the
magnon forms as a result of a spinon-spinon interaction potential.Comment: 5 pages, latex, 3 figures embedded in the tex
Excitations with fractional spin less than 1/2 in frustrated magnetoelastic chains
We study the magnetic excitations on top of the plateaux states recently
discovered in spin-Peierls systems in a magnetic field. We show by means of
extensive density matrix renormalization group (DMRG) computations and an
analytic approach that one single spin-flip on top of
() plateau decays into elementary excitations each carrying a
fraction of the spin. This fractionalization goes beyond the
well-known decay of one magnon into two spinons taking place on top of the M=0
plateau. Concentrating on the plateau (N=3) we unravel the
microscopic structure of the domain walls which carry fractional
spin-, both from theory and numerics. These excitations are shown to
be noninteracting and should be observable in x-ray and nuclear magnetic
resonance experiments.Comment: 6 pages, 5 figures. Accepted to be published in Phys. Rev.
Domain excitations in spin-Peierls systems
We study a model of a Spin-Peierls material consisting of a set of
antiferromagnetic Heisenberg chains coupled with phonons and interacting among
them via an inter-chain elastic coupling. The excitation spectrum is analyzed
by bosonization techniques and the self-harmonic approximation. The elementary
excitation is the creation of a localized domain structure where the dimerized
order is the opposite to the one of the surroundings. It is a triplet
excitation whose formation energy is smaller than the magnon gap. Magnetic
internal excitations of the domain are possible and give the further
excitations of the system. We discuss these results in the context of recent
experimental measurements on the inorganic Spin-Peierls compound CuGeOComment: 5 pages, 2 figures, corrected version to appear in Phys. Rev.
Microscopic theory for the incommensurate transition in TiOCl
We propose a microscopic mechanism for the incommensurate phase in TiOX
compounds. The model includes the antiferromagnetic chains of Ti ions immersed
in the phonon bath of the bilayer structure. Making use of the Cross-Fisher
theory, we show that the geometrically frustrated character of the lattice is
responsible for the structural instability which leads the chains to an
incommensurate phase without an applied magnetic field. In the case of TiOCl,
we show that our model is consistent with the measured phonon frequencies at
and the value of the incommensuration vector at the transition
temperature. Moreover, we find that the dynamical structure factor shows a
progressive softening of an incommensurate phonon near the zone boundary as the
temperature decreases. This softening is accompanied by a broadening of the
peak which gets asymmetrical as well when going towards the transition
temperature. These features are in agreement with the experimental inelastic
X-ray measurements.Comment: 6 pages, 5 figures. Published versio
Antiferromagnetism in doped anisotropic two-dimensional spin-Peierls systems
We study the formation of antiferromagnetic correlations induced by impurity
doping in anisotropic two-dimensional spin-Peierls systems. Using a mean-field
approximation to deal with the inter-chain magnetic coupling, the intra-chain
correlations are treated exactly by numerical techniques. The magnetic coupling
between impurities is computed for both adiabatic and dynamical lattices and is
shown to have an alternating sign as a function of the impurity-impurity
distance, hence suppressing magnetic frustration. An effective model based on
our numerical results supports the coexistence of antiferromagnetism and
dimerization in this system.Comment: 5 pages, 4 figures; final version to appear in Phys. Rev.
Coexistence of charge density wave and spin-Peierls orders in quarter-filled quasi-one dimensional correlated electron systems
Charge and spin-Peierls instabilities in quarter-filled (n=1/2) compounds
consisting of coupled ladders and/or zig-zag chains are investigated. Hubbard
and t-J models including local Holstein and/or Peierls couplings to the lattice
are studied by numerical techniques. Next nearest neighbor hopping and magnetic
exchange, and short-range Coulomb interactions are also considered. We show
that, generically, these systems undergo instabilities towards the formation of
Charge Density Waves, Bond Order Waves and (generalized) spin-Peierls modulated
structures. Moderate electron-electron and electron-lattice couplings can lead
to a coexistence of these three types of orders. In the ladder, a zig-zag
pattern is stabilized by the Holstein coupling and the nearest-neighbor Coulomb
repulsion. In the case of an isolated chain, bond-centered and site-centered
2k_F and 4k_F modulations are induced by the local Holstein coupling. In
addition, we show that, in contrast to the ladders, a small charge ordering in
the chains, strongly enhances the spin-Peierls instability. Our results are
applied to the NaV_2O_5 compound (trellis lattice) and various phases with
coexisting charge disproportionation and spin-Peierls order are proposed and
discussed in the context of recent experiments. The role of the long-range
Coulomb potential is also outlined.Comment: 10 pages, Revtex, 10 encapsulated figure
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