275 research outputs found
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
Spectral properties of the 2D Holstein t-J model
Employing the Lanczos algorithm in combination with a kernel polynomial
moment expansion (KPM) and the maximum entropy method (MEM), we show a way of
calculating charge and spin excitations in the Holstein t-J model, including
the full quantum nature of phonons. To analyze polaron band formation we
evaluate the hole spectral function for a wide range of electron-phonon
coupling strengths. For the first time, we present results for the optical
conductivity of the 2D Holstein t-J model.Comment: 2 pages, Latex. Submitted to Physica C, Proc. Int. Conf. on M2HTSC
Mixing of magnetic and phononic excitations in incommensurate Spin-Peierls systems
We analyze the excitation spectra of a spin-phonon coupled chain in the
presence of a soliton. This is taken as a microscopic model of a Spin-Peierls
material placed in a high magnetic field. We show, by using a semiclassical
approximation in the bosonized representation of the spins that a trapped
magnetic state obtained in the adiabatic approximation is destroyed by
dynamical phonons. Low energy states are phonons trapped by the soliton. When
the magnetic gap is smaller than the phonon frequencies the only low energy
state is a mixed magneto-phonon state with the energy of the gap. We emphasize
that our results are relevant for the Raman spectra of the inorganic
Spin-Peierls material CuGeO.Comment: 5 pages, latex, 2 figures embedded in the tex
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
Polaron Formation in the Three-Band Peierls-Hubbard Model for Cuprate Superconductors
Exact diagonalization calculations show a continuous transition from
delocalized to small polaron behavior as a function of intersite
electron-lattice coupling. A transition, found previously at Hartree-Fock level
[Yonemitsu et al., Phys. Rev. Lett. {\bf 69}, 965 (1992)], between a magnetic
and a non magnetic state does not subsist when fluctuations are included. Local
phonon modes become softer close to the polaron and by comparison with optical
measurements of doped cuprates we conclude that they are close to the
transition region between polaronic and non-polaronic behavior. The barrier to
adiabatically move a hole vanishes in that region suggesting large mobilities.Comment: 7 pages + 3 poscript figures, Revtex 3.0, MSC-199
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.
Density Matrix Renormalization Group Study of One-Dimensional Acoustic Phonons
We study the application of the density matrix renormalization group (DMRG)
to systems with one-dimensional acoustic phonons. We show how the use of a
local oscillator basis circumvents the difficulties with the long-range
interactions generated in real space using the normal phonon basis. When
applied to a harmonic atomic chain, we find excellent agreement with the exact
solution even when using a modest number of oscillator and block states (a few
times ten). We discuss the use of this algorithm in more complex cases and
point out its value when other techniques are deficient.Comment: 12 pages. To be published in PRB rapid co
On the soliton width in the incommensurate phase of spin-Peierls systems
We study using bosonization techniques the effects of frustration due to
competing interactions and of the interchain elastic couplings on the soliton
width and soliton structure in spin-Peierls systems. We compare the predictions
of this study with numerical results obtained by exact diagonalization of
finite chains. We conclude that frustration produces in general a reduction of
the soliton width while the interchain elastic coupling increases it. We
discuss these results in connection with recent measurements of the soliton
width in the incommensurate phase of CuGeO_3.Comment: 4 pages, latex, 2 figures embedded in the tex
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