1,312 research outputs found
A single chain analysis of doped quasi one dimensional spin 1 compounds: paramagnetic versus spin 1/2 doping
We present a numerical study of single chain models of doped spin 1
compounds. We use low energy effective one-dimensional models for both the
cases of paramagnetic and spin-1/2 doping. In the case of paramagnetic doping,
the effective model is equivalent to the bond disordered spin-1/2 chain model
recently analyzed by means of real space renormalization group by Hyman and
Yang. By means of exact diagonalizations in the XX limit, we confirm the
stability of the Haldane phase for weak disorder. Above a critical amount of
disorder, the effective model flows to the so called random singlet fixed
point. In the case of spin-1/2 doping, we argue that the Haldane phase should
be destabilized even for weak disorder. This picture is not in contradiction
with existing experimental data. We also discuss the possible occurrence of
(unobserved) antiferromagnetically ordered phases.Comment: 13 pages, 7 included figure
The Burst-Like Behavior of Aseismic Slip on a Rough Fault: The Creeping Section of the Haiyuan Fault, China
Recent observations suggesting the influence of creep on earthquakes nucleation and arrest are strong incentives to investigate the physical mechanisms controlling how active faults slip. We focus here on deriving generic characteristics of shallow creep along the Haiyuan fault, a major strikeâslip fault in China, by investigating the relationship between fault slip and geometry. We use optical images and time series of Synthetic Aperture Radar data to map the surface fault trace and the spatiotemporal distribution of surface slip along the creeping section of the Haiyuan fault. The fault trace roughness shows a powerâlaw behavior similar to that of the aseismic slip distribution, with a 0.8 roughness exponent, typical of a selfâaffine regime. One possible interpretation is that fault geometry controls to some extent the distribution of aseismic slip, as it has been shown previously for coseismic slip along active faults. Creep is characterized by local fluctuations in rates that we define as creep bursts. The potency of creep bursts follows a powerâlaw behavior similar to the GutenbergâRichter earthquake distribution, whereas the distribution of bursts velocity is nonâGaussian, suggesting an avalancheâlike behavior of these slip events. Such similarities with earthquakes and lab experiments lead us to interpret the rich dynamics of creep bursts observed along the Haiyuan fault as resulting from longârange elastic interactions within the heterogeneous Earthâs crust
Understanding Far-Infrared Absorption in the S=1 Antiferromagnetic Chain Compound NENP
Infrared transmission measurements on the antiferromagnetic chain
compound NENP in applied magnetic fields show a sharp absorption line at the
field-shifted Haldane gap. This violates a wave-vector selection rule of the
Hamiltonian normally used for NENP, as the gap excitations occur at the
Brillouin zone boundary. We argue that the crystal structure admits terms which
can explain the absorption lines. In addition, in an applied field, staggered
orientations of the g-tensors produce a staggered magnetic field. This can
explain the observation of a finite gap at all applied fields.Comment: 12 pages, revtex, preprint HU-CMT-93H9
3117 Contralateral corneal modification after 193 nm excimer and 213 nm UV solid state laser photoablations
Magnetization profiles and NMR spectra of doped Haldane chains at finite temperatures
Open segments of S=1 antiferromagnetic spin chains are studied at finite
temperatures and fields using continuous time Quantum Monte Carlo techniques.
By calculating the resulting magnetization profiles for a large range of chain
lengths with fixed field and temperature we reconstruct the experimentally
measured NMR spectrum of impurity doped YBaNiMgO. For
temperatures above the gap the calculated NMR spectra are in excellent
agreement with the experimental results, confirming the existence of
excitations at the end of open S=1 chain segments. At temperatures below the
gap, neglecting inter chain couplings, we still find well defined peaks in the
calculated NMR spectra corresponding to the chain end excitations. At
low temperatures, inter chain couplings could be important, resulting in a more
complicated phase.Comment: 7 pages, 5 figures, minor correction
Haldane gap in the quasi one-dimensional nonlinear -model
This work studies the appearance of a Haldane gap in quasi one-dimensional
antiferromagnets in the long wavelength limit, via the nonlinear
-model. The mapping from the three-dimensional, integer spin Heisenberg
model to the nonlinear -model is explained, taking into account two
antiferromagnetic couplings: one along the chain axis () and one along the
perpendicular planes () of a cubic lattice. An implicit equation for
the Haldane gap is derived, as a function of temperature and coupling ratio
. Solutions to these equations show the existence of a critical
coupling ratio beyond which a gap exists only above a transition temperature
. The cut-off dependence of these results is discussed.Comment: 14 pages (RevTeX 3.0), 3 PostScript figures appended (printing
instructions included
Models of impurities in valence bond spin chains and ladders
We present the class of models of a nonmagnetic impurity in S=1/2 generalized
ladder with an AKLT-type valence bond ground state, and of a S=1/2 impurity in
the S=1 AKLT chain. The ground state in presence of impurity can be found
exactly. Recently studied phenomenon of local enhancement of antiferromagnetic
correlations around the impurity is absent for this family of models.Comment: 4 pages revtex, 3 figures embedde
Low-temperature properties of the spin-1 antiferromagnetic Heisenberg chain with bond-alternation
We investigate the low-temperature properties of the spin-1 antiferromagnetic
Heisenberg chain with bond-alternation by the quantum Monte Carlo method (loop
algorithm). The strength of bond-alternation at the gapless point is estimated
as . We confirm numerically that the
low-temperature properties at the gapless point are consistent with field
theoretical predictions. The numerical results are compared with those of the
spin-1/2 antiferromagnetic Heisenberg chain and recent experimental results for
[\{Ni(333-tet)(-N)\}](ClO) (333-tet=tetraamine
-bis(3-aminopropyl)-1,3-propanediamine).Comment: 18 pages, RevTex, 9 figures, Submitted to Phys.Rev.
On negative higher-order Kerr effect and filamentation
As a contribution to the ongoing controversy about the role of higher-order
Kerr effect (HOKE) in laser filamentation, we first provide thorough details
about the protocol that has been employed to infer the HOKE indices from the
experiment. Next, we discuss potential sources of artifact in the experimental
measurements of these terms and show that neither the value of the observed
birefringence, nor its inversion, nor the intensity at which it is observed,
appear to be flawed. Furthermore, we argue that, independently on our values,
the principle of including HOKE is straightforward. Due to the different
temporal and spectral dynamics, the respective efficiency of defocusing by the
plasma and by the HOKE is expected to depend substantially on both incident
wavelength and pulse duration. The discussion should therefore focus on
defining the conditions where each filamentation regime dominates.Comment: 22 pages, 11 figures. Submitted to Laser physics as proceedings of
the Laser Physics 2010 conferenc
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