510 research outputs found
Universal critical temperature for Kosterlitz-Thouless transitions in bilayer quantum magnets
Recent experiments show that double layer quantum Hall systems may have a
ground state with canted antiferromagnetic order. In the experimentally
accessible vicinity of a quantum critical point, the order vanishes at a
temperature T_{KT} = \kappa H, where H is the magnetic field and \kappa is a
universal number determined by the interactions and Berry phases of the thermal
excitations. We present quantum Monte Carlo simulations on a model spin system
which support the universality of \kappa and determine its numerical value.
This allows experimental tests of an intrinsically quantum-mechanical universal
quantity, which is not also a property of a higher dimensional classical
critical point.Comment: 5 pages, 4 figure
Theoretical analysis of the experiments on the double-spin-chain compound -- KCuCl
We have analyzed the experimental susceptibility data of KCuCl and found
that the data are well-explained by the double-spin-chain models with strong
antiferromagnetic dimerization. Large quantum Monte Carlo calculations were
performed for the first time in the spin systems with frustration. This was
made possible by removing the negative-sign problem with the use of the dimer
basis that has the spin-reversal symmetry. The numerical data agree with the
experimental data within 1% relative errors in the whole temperature region. We
also present a theoretical estimate for the dispersion relation and compare it
with the recent neutron-scattering experiment. Finally, the magnitude of each
interaction bond is predicted.Comment: 4 pages, REVTeX, 5 figures in eps-file
Innovative system identification methods for monitoring applications
Monitoring the modal parameters of civil and mechanical system received plenty of interest the last decades. Several approaches have been proposed and successfully applied in civil engineering for structural health monitoring of bridges (mainly based on the monitoring of the resonant frequencies and mode shapes). In applications such as the monitoring of offshore wind turbines and flight flutter testing the monitoring of the damping ratios are essential. For offshore wind turbine monitoring the presence of time-varying harmonic components, close to the modes of interest, can complicate the identification process. The difficulty related to flight flutter testing is that, in general, only short data records are available. The aim of this contribution is to introduce system identification methods and monitoring strategies that result in more reliable decisions and that can cope with complex monitoring applications. Basic concepts of system identification will be recapitulated with attention for monitoring aspects. The proposed monitoring methodology is based on the recently introduced Transmissibility-based Operational Modal Analysis (TOMA) approach
Diffusion in the Continuous-Imaginary-Time Quantum World-Line Monte Carlo Simulations with Extended Ensembles
The dynamics of samples in the continuous-imaginary-time quantum world-line
Monte Carlo simulations with extended ensembles are investigated. In the case
of a conventional flat ensemble on the one-dimensional quantum S=1 bi-quadratic
model, the asymmetric behavior of Monte Carlo samples appears in the diffusion
process in the space of the number of vertices. We prove that a local
diffusivity is asymptotically proportional to the number of vertices, and we
demonstrate the asymmetric behavior in the flat ensemble case. On the basis of
the asymptotic form, we propose the weight of an optimal ensemble as
, where denotes the number of vertices in a sample. It is shown
that the asymmetric behavior completely vanishes in the case of the proposed
ensemble on the one-dimensional quantum S=1 bi-quadratic model.Comment: 4 pages, 2 figures, update a referenc
Spin dynamics of SrCuO and the Heisenberg ladder
The Heisenberg antiferromagnet in the ladder geometry is studied as a
model for the spin degrees of freedom of SrCuO. The susceptibility and
the spin echo decay rate are calculated using a quantum Monte Carlo technique,
and the spin-lattice relaxation rate is obtained by maximum entropy analytic
continuation of imaginary time correlation functions. All calculated quantities
are in reasonable agreement with experimental results for SrCuO if the
exchange coupling K, i.e. significantly smaller than in
high-T cuprates.Comment: 11 pages (Revtex) + 3 uuencoded ps files. To appear in Phys. Rev. B,
Rapid Com
Quantum Monte Carlo Loop Algorithm for the t-J Model
We propose a generalization of the Quantum Monte Carlo loop algorithm to the
t-J model by a mapping to three coupled six-vertex models. The autocorrelation
times are reduced by orders of magnitude compared to the conventional local
algorithms. The method is completely ergodic and can be formulated directly in
continuous time. We introduce improved estimators for simulations with a local
sign problem. Some first results of finite temperature simulations are
presented for a t-J chain, a frustrated Heisenberg chain, and t-J ladder
models.Comment: 22 pages, including 12 figures. RevTex v3.0, uses psf.te
Nuclear Spin Relaxation in Hole Doped Two-Leg Ladders
The nuclear spin-lattice relaxation rate () has been measured in the
single crystals of hole doped two-leg ladder compounds
SrCaCuO and in the undoped parent material
LaCaCuO. Comparison of at the Cu and the two
distinct oxygen sites revealed that the major spectral weight of low frequency
spin fluctuations is located near for most of the
temperature and doping ranges investigated. Remarkable difference in the
temperature dependence of for the two oxygen sites in the heavily doped
=12 sample revealed reduction of singlet correlations between two legs in
place of growing antiferromagnetic correlations along the leg direction with
increasing temperature. Such behavior is most likely caused by the dissociation
of bound hole pairs.Comment: 4 pages. to appear in J. Phys. Soc. Jpn. Vol. 6
Singlet Stripe Phases in the planar t-J Model
The energies of singlet stripe phases in which a plane is broken up into spin
liquid ladders by lines of holes, is examined. If the holes were static then
patterns containing spin liquids with a finite spin gap are favored. The case
of dynamic holes is treated by assembling t-J ladders oriented perpendicular to
the stripes. For a wide region around the hole-hole
correlations in a single ladder are found to be predominantly charge density
wave type but an attraction between hole pairs on adjacent ladders leads to a
stripe phase. A quantum mechanical melting of the hole lines at smaller
values leads to a Bose condensate of hole pairs, i.e. a superconducting phase.Comment: 5 pages, uuencoded compressed PostScript file including 5 figures,
ETH-TH/942
Nuclear spin relaxation rates in two-leg spin ladders
Using the transfer-matrix DMRG method, we study the nuclear spin relaxation
rate 1/T_1 in the two-leg s=1/2 ladder as function of the inter-chain
(J_{\perp}) and intra-chain (J_{|}) couplings. In particular, we separate the
q_y=0 and \pi contributions and show that the later contribute significantly to
the copper relaxation rate ^{63}(1/T_1) in the experimentally relevant coupling
and temperature range. We compare our results to both theoretical predictions
and experimental measures on ladder materials.Comment: Few modifications from the previous version 4 pages, 5 figures,
accepted for publication in PR
Quantum spin ladders of non-Abelian anyons
Quantum ladder models, consisting of coupled chains, form intriguing systems
bridging one and two dimensions and have been well studied in the context of
quantum magnets and fermionic systems. Here we consider ladder systems made of
more exotic quantum mechanical degrees of freedom, so-called non-Abelian
anyons, which can be thought of as certain quantum deformations of ordinary
SU(2) spins. Such non-Abelian anyons occur as quasiparticle excitations in
topological quantum fluids, including p_x + i p_y superconductors, certain
fractional quantum Hall states, and rotating Bose-Einstein condensates. Here we
use a combination of exact diagonalization and conformal field theory to
determine the phase diagrams of ladders with up to four chains. We discuss how
phenomena familiar from ordinary SU(2) spin ladders are generalized in their
anyonic counterparts, such as gapless and gapped phases, odd/even effects with
the ladder width, and elementary `magnon' excitations. Other features are
entirely due to the topological nature of the anyonic degrees of freedom.Comment: 12 pages, 17 figures, 3 tables, 2 references adde
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