20 research outputs found
Dynamical Structure Factor in Cu Benzoate and other spin-1/2 antiferromagnetic chains
Recent experiments of the quasi-one-dimensional spin-1/2 antiferromagnet
Copper Benzoate established the existence of a magnetic field induced gap. The
observed neutron scattering intensity exhibits resolution limited peaks at both
the antiferromagnetic wave number and at incommensurate wave numbers related to
the applied magnetic field. We determine the ratio of spectral weights of these
peaks within the framework of a low-energy effective field theory description
of the problem.Comment: 5 pages, 3figure
Field-induced gap in the spin-1/2 antiferromagnetic Heisenberg chain: A density matrix renormalization group study
We study the spin-1/2 antiferromagnetic Heisenberg chain in both uniform and
(perpendicular) staggered magnetic fields using the density-matrix
renormalization-group method. This model has been shown earlier to describe the
physics of the copper benzoate materials in magnetic field. In the present
work, we extend the study to more general case for a systematic investigation
of the field-induced gap and related properties of the spin-1/2
antiferromagnetic Heisenberg chain. In particular, we explore the high magnetic
field regime where interesting behaviors in the field-induced gap,
magnetization, and spin correlation functions are found. Careful examination of
the low energy properties and magnetization reveals interesting competing
effects of the staggered and uniform fields. The incommensurate behavior in the
spin correlation functions is demonstrated and discussed in detail. The present
work reproduces earlier results in good agreement with experimental data on
copper benzoate and predicts new interesting field-induced features at very
high magnetic field.Comment: 8 pages, 6 figure
Spectral functions of the Falicov-Kimball model with electronic ferroelectricity
We calculate the angular resolved photoemission spectrum of the
Falicov-Kimball model with electronic ferroelectricity where - and
-electrons have different hoppings. In mix-valence regimes, the presence of
strong scattering processes between - excitons and a hole, created by
emission of an electron, leads to the formation of pseudospin polarons and
novel electronic structures with bandwidth scaling with that of -
excitons. Especially, in the two-dimensional case, we find that flat regions
exist near the bottom of the quasiparticle band in a wide range of the - and
-level energy difference.Comment: 5 pages, 5 figure
Correlation Amplitudes for the spin-1/2 XXZ chain in a magnetic field
We present accurate numerical estimates for the correlation amplitudes of
leading and main subleading terms of the two- and four-spin correlation
functions in the one-dimensional spin-1/2 XXZ model under a magnetic field.
These data are obtained by fitting the correlation functions, computed
numerically with the density-matrix renormalization-group method, to the
corresponding correlation functions in the low-energy effective theory. For
this purpose we have developed the Abelian bosonization approach to the spin
chain under the open boundary conditions. We use the numerical data of the
correlation amplitudes to quantitatively estimate spin gaps induced by a
transverse staggered field and by exchange anisotropy.Comment: 18 pages, 6 figures, 1 tabl
Coupled Heisenberg antiferromagnetic chains in an effective staggered field
We present a systematic study of coupled Heisenberg antiferromagnetic
chains in an effective staggered field. We investigate several effects of the
staggered field in the {\em higher} ({\em two or three}) {\em dimensional} spin
system analytically. In particular, in the case where the staggered field and
the inter-chain interaction compete with each other, we predict, using
mean-field theory, a characteristic phase transition. The spin-wave theory
predicts that the behavior of the gaps induced by the staggered field is
different between the competitive case and the non-competitive case. When the
inter-chain interactions are sufficiently weak, we can improve the mean-field
phase diagram by using chain mean-field theory and the analytical results of
field theories. The ordered phase region predicted by the chain mean-field
theory is substantially smaller than that by the mean-field theory.Comment: 13pages, 12figures, to be published in PR
Two-spinon dynamic structure factor of the one-dimensional S=1/2 Heisenberg antiferromagnet
The exact expression derived by Bougourzi, Couture, and Kacir for the
2-spinon contribution to the dynamic spin structure factor
of he one-dimensional =1/2 Heisenberg antiferromagnet at is evaluated
for direct comparison with finite-chain transition rates () and an
approximate analytical result previously inferred from finite- data, sum
rules, and Bethe-ansatz calculations. The 2-spinon excitations account for
72.89% of the total intensity in . The singularity structure
of the exact result is determined analytically and its spectral-weight
distribution evaluated numerically over the entire range of the 2-spinon
continuum. The leading singularities of the frequency-dependent spin
autocorrelation function, static spin structure factor, and -dependent
susceptibility are determined via sum rules.Comment: 6 pages (RevTex) and 5 figures (Postscript
High-field magnetization study of the S = 1/2 antiferromagnetic Heisenberg chain [PM Cu(NO)(HO)] with a field-induced gap
We present a high-field magnetization study of the = 1/2
antiferromagnetic Heisenberg chain [PM Cu(NO)(HO)]. For
this material, as result of the Dzyaloshinskii-Moriya interaction and a
staggered tensor, the ground state is characterized by an anisotropic
field-induced spin excitation gap and a staggered magnetization. Our data
reveal the qualitatively different behavior in the directions of maximum and
zero spin excitation gap. The data are analyzed via exact diagonalization of a
linear spin chain with up to 20 sites and on basis of the Bethe ansatz
equations, respectively. For both directions we find very good agreement
between experimental data and theoretical calculations. We extract the magnetic
coupling strength along the chain direction to 36.3(5) K and determine
the field dependence of the staggered magnetization component .Comment: 5 pages, 2 figures (minor changes to manuscript and figures
Sr2V3O9 and Ba2V3O9: quasi one-dimensional spin-systems with an anomalous low temperature susceptibility
The magnetic behaviour of the low-dimensional Vanadium-oxides Sr2V3O9 and
Ba2V3O9 was investigated by means of magnetic susceptibility and specific heat
measurements. In both compounds, the results can be very well described by an
S=1/2 Heisenberg antiferromagnetic chain with an intrachain exchange of J = 82
K and J = 94 K in Sr2V3O9 and Ba2V3O9, respectively. In Sr2V3O9,
antiferromagnetic ordering at T_N = 5.3 K indicate a weak interchain exchange
of the order of J_perp ~ 2 K. In contrast, no evidence for magnetic order was
found in Ba2V3O9 down to 0.5 K, pointing to an even smaller interchain
coupling. In both compounds, we observe a pronounced Curie-like increase of the
susceptibility below 30 K, which we tentatively attribute to a staggered field
effect induced by the applied magnetic field. Results of LDA calculations
support the quasi one-dimensional character and indicate that in Sr2V3O9, the
magnetic chain is perpendicular to the structural one with the magnetic
exchange being transferred through VO4 tetrahedra.Comment: Submitted to Phy. Rev.
Critical properties of 1-D spin 1/2 antiferromagnetic Heisenberg model
We discuss numerical results for the 1-D spin 1/2 antiferromagnetic
Heisenberg model with next-to-nearest neighbour coupling and in the presence of
an uniform magnetic field. The model develops zero frequency excitations at
field dependent soft mode momenta. We compute critical quantities from finite
size dependence of static structure factors.Comment: talk given by H. Kr{\"o}ger at Heraeus Seminar Theory of Spin
Lattices and Lattice Gauge Models, Bad Honnef (1996), 20 pages, LaTeX + 18
figures, P
Quantum criticalities in a two-leg antiferromagnetic S=1/2 ladder induced by a staggered magnetic field
We study a two-leg antiferromagnetic spin-1/2 ladder in the presence of a
staggered magnetic field. We consider two parameter regimes: strong (weak)
coupling along the legs and weak (strong) coupling along the rungs. In both
cases, the staggered field drives the Haldane spin-liquid phase of the ladder
towards a Gaussian quantum criticality. In a generalized spin ladder with a
non-Haldane, spontaneously dimerized phase, the staggered magnetic field
induces an Ising quantum critical regime. In the vicinity of the critical
lines, we derive low-energy effective field theories and use these descriptions
to determine the dynamical response functions, the staggered spin
susceptibility and the string order parameter.Comment: 29 pages of revtex, 10 figure