7,634 research outputs found
Elementary excitations, exchange interaction and spin-Peierls transition in CuGeO
The microscopic description of the spin-Peierls transition in pure and doped
CuGeO_3 is developed taking into account realistic details of crystal
structure. It it shown that the presence of side-groups (here Ge) strongly
influences superexchange along Cu-O-Cu path, making it antiferromagnetic.
Nearest-neighbour and next-nearest neighbour exchange constants and
are calculated. Si doping effectively segments the CuO_2-chains
leading to or even slightly ferromagnetic. Strong
sensitivity of the exchange constants to Cu-O-Cu and (Cu-O-Cu)-Ge angles may be
responsible for the spin-Peierls transition itself (``bond-bending mechanism''
of the transition). The nature of excitations in the isolated and coupled
spin-Peierls chains is studied and it is shown that topological excitations
(solitons) play crucial role. Such solitons appear in particular in doped
systems (Cu_{1-x}Zn_xGeO_3, CuGe_{1-x}Si_xO_3) which can explain the
phase diagram.Comment: 7 pages, revtex, 7 Postscript figure
Quasi-particle behavior of composite fermions in the half-filled Landau level
We calculate the effect of infrared fluctuations of the Chern-Simons gauge
field on the single-particle Green's function of composite fermions in the
half-filled Landau level via higher-dimensional bosonization on a curved Fermi
surface. We find that composite fermions remain well-defined quasi-particles,
with an effective mass given by the mean-field value, but with anomalously
large damping and a spectral function that contains considerable weight away
from the quasi-particle peak.Comment: reference added; accepted for publication in Phys. Rev. Let
Phase diagram of disordered spin-Peierls systems
We study the competition between the spin-Peierls and the antiferromagnetic
ordering in disordered quasi-one-dimensional spin systems. We obtain the
temperature vs disorder-strength phase diagram, which qualitatively agrees with
recent experiments on doped CuGeO_3.Comment: 4 pages, revtex, epsf, 2 Postscript figure
Excitations of the field-induced soliton lattice in CuGeO3
Here we report the first inelastic neutron scattering study of the magnetic
excitations in the incommensurate phase of a spin-Peierls material. The results
on CuGeO3 provide direct evidence of a finite excitation gap, two sharp
magnetic excitation branches and a very low-lying excitation which is
identified as a phason mode, the Goldstone mode of the incommensurate soliton
lattice.Comment: 5 pages, revtex, 4 figures (*.eps), win-zippe
On the stability problem in the O(N) nonlinear sigma model
The stability problem for the O(N) nonlinear sigma model in the 2+\epsilon
dimensions is considered. We present the results of the 1/N^{2} order
calculations of the critical exponents (in the 2<d<4 dimensions) of the
composite operators relevant for this problem. The arguments in the favor of
the scenario with the conventional fixed point are given.Comment: 9 pages, revtex, 1 Postscript figur
Interference of a first-order transition with the formation of a spin-Peierls state in alpha'-NaV2O5?
We present results of high-resolution thermal-expansion and specific-heat
measurements on single crystalline alpha'-NaV2O5. We find clear evidence for
two almost degenerate phase transitions associated with the formation of the
dimerized state around 33K: A sharp first-order transition at T1=(33+-0.1)K
slightly below the onset of a second-order transition at T2onset around
(34+-0.1)K. The latter is accompanied by pronounced spontaneous strains. Our
results are consistent with a structural transformation at T1 induced by the
incipient spin-Peierls (SP) order parameter above T2=TSP.Comment: 5 pages, 7 figure
Magnetic excitation spectrum of dimerized antiferromagnetic chains
Motivated by recent measurements on CuGeO the spectrum of magnetic
excitations of an antiferromagnetic chain with alternating
coupling strength is investigated. Wave vector dependent magnons and a
continuum with square root behavior at the band edges are found. The spectral
density of the continua is calculated. Spin rotation symmetry fixes the gap of
the continuum to be twice the elementary magnon gap. This is in excellent
agreement with experimental results. In addition, the existence of bound states
of two magnons is predicted: below the continuum a singlet and a triplet, above
the continuum an ``anti-bound'' quintuplet. The results are based on field
theoretic arguments, RPA calculations, and consideration of the limit of strong
alternation.Comment: 4 pages, 4 figures included, Revte
Peierls Dimerization with Non-Adiabatic Spin-Phonon Coupling
We study the magnetic properties of a frustrated Heisenberg spin chain with a
dynamic spin-phonon interaction. By Lanczos diagonalization, preserving the
full lattice dynamics, we explore the non-adiabatic regime with phonon
frequencies comparable to the exchange coupling energy which is e.g. the
relevant limit for the spin-Peierls compound . When compared to the
static limit of an alternating spin chain the magnetic properties are strongly
renormalized due to the coupled dynamics of spin and lattice degrees of
freedom. The magnitude of the spin triplet excitation gap changes from a strong
to a weak dimerization dependence with increasing phonon frequencies implying
the necessity to include dynamic effects in an attempt for a quantitative
description of the spin-Peierls state.Comment: 4 pages, 5 figure
Suppression of Dimer Correlations in the Two-Dimensional - Heisenberg Model: an Exact Diagonalization Study
We present an exact diagonalization study of the ground state of the
spin-half model. Dimer correlation functions and the susceptibility
associated to the breaking of the translational invariance are calculated for
the and the clusters. These results -- especially when
compared to the one dimensional case, where the occurrence of a dimerized phase
for large enough frustration is well established -- suggest either a
homogeneous spin liquid or, possibly, a dimerized state with a rather small
order parameter
Más allá del desarrollo capitalista en Argentina : Límites, posibilidades y alternativas
En los últimos años la modalidad neoliberal de desarrollo capitalista ha entrado en una fase de profunda crisis en todas las regiones del mundo. El neoliberalismo, entendido como un proceso de restructuración de las lógicas de la acumulación de capital a escala global iniciado a fines de los años setenta y tendiente a restaurar el poder y los ingresos de la clase dominante (Harvey, 2007), fue un proceso “exitoso” en términos de estos objetivos pero condujo –al mismo tiempo– a profundizar las contradicciones económicas, sociales y políticas preexistentes y a generar nuevas contradicciones, sobre todo ligadas a la mundialización/ transnacionalización del capital y la preeminencia del capital ficticio sobre el productivo. La agudización de estas tensiones y contradicciones permiten comprender su crisis y han sido objeto de estudio reciente de una variedad de autores (Chesnais, 2010).Facultad de Humanidades y Ciencias de la Educació
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