1,705 research outputs found
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
Field-Induced Magnetic Order in Quantum Spin Liquids
We study magnetic field-induced three-dimensional ordering transitions in
low-dimensional quantum spin liquids, such as weakly coupled, antiferromagnetic
spin-1/2 Heisenberg dimers and ladders. Using stochastic series expansion
quantum Monte Carlo simulations, thermodynamic response functions are obtained
down to ultra-low temperatures. We extract the critical scaling exponents which
dictate the power-law dependence of the transition temperature on the applied
magnetic field. These are compared with recent experiments on candidate
materials and with predictions for the Bose-Einstein condensation of magnons
obtained in mean-field theory.Comment: RevTex, 4 pages with 5 figure
Field-Induced Magnetic Ordering in the Quantum Spin System KCuCl
KCuCl is a three-dimensional coupled spin-dimer system and has a singlet
ground state with an excitation gap K. High-field
magnetization measurements for KCuCl have been performed in static magnetic
fields of up to 30 T and in pulsed magnetic fields of up to 60 T. The entire
magnetization curve including the saturation region was obtained at K.
From the analysis of the magnetization curve, it was found that the exchange
parameters determined from the dispersion relations of the magnetic excitations
should be reduced, which suggests the importance of the renormalization effect
in the magnetic excitations. The field-induced magnetic ordering accompanied by
the cusplike minimum of the magnetization was observed as in the isomorphous
compound TlCuCl. The phase boundary was almost independent of the field
direction, and is represented by the power law. These results are consistent
with the magnon Bose-Einstein condensation picture for field-induced magnetic
ordering.Comment: 9 pages, 7 figures, 9 eps files, revtex styl
Random Bond Effect in the Quantum Spin System (TlK)CuCl
The effect of exchange bond randomness on the ground state and the
field-induced magnetic ordering was investigated through magnetization
measurements in the spin-1/2 mixed quantum spin system
(TlK)CuCl for . Both parent compounds TlCuCl and
KCuCl are coupled spin dimer systems, which have the singlet ground state
with excitation gaps K and 31 K, respectively. Due to
bond randomness, the singlet ground state turns into the magnetic state with
finite susceptibility, nevertheless, the excitation gap remains. Field-induced
magnetic ordering, which can be described by the Bose condensation of excited
triplets, magnons, was observed as in the parent systems. The phase transition
temperature is suppressed by the bond randomness. This behavior may be
attributed to the localization effect.Comment: 19 pages, 7 figures, 12 eps files, revtex, will appear in PR
Dimer-Quadrupolar Quantum Phase Transition in the Quasi-One-Dimensional Heisenberg Model with Biquadratic Interaction
The quasi-one-dimensional S=1 Heisenberg antiferromagnet with a biquadratic
term is investigated at zero temperature by quantum Monte Carlo simulation. As
the magnitude of the inter-chain coupling is increased, the system undergoes a
phase transition from a spontaneously dimerized phase to a N\'eel ordered or
spin nematic phase. The numerical results suggest the possibility of an
unconventional second-order transition in which the symmetry group
characterizing one phase is not a subgroup of the other.Comment: 4 pages, 4 figure
Synchronization in a neuronal feedback loop through asymmetric temporal delays
We consider the effect of asymmetric temporal delays in a system of two
coupled Hopfield neurons. For couplings of opposite signs, a limit cycle
emerges via a supercritical Hopf bifurcation when the sum of the delays reaches
a critical value. We show that the angular frequency of the limit cycle is
independent of an asymmetry in the delays. However, the delay asymmetry
determines the phase difference between the periodic activities of the two
components. Specifically, when the connection with negative coupling has a
delay much larger than the delay for the positive coupling, the system
approaches in-phase synchrony between the two components. Employing variational
perturbation theory (VPT), we achieve an approximate analytical evaluation of
the phase shift, in good agreement with numerical results.Comment: 5 pages, 4 figure
Recurrence Plot Based Measures of Complexity and its Application to Heart Rate Variability Data
The knowledge of transitions between regular, laminar or chaotic behavior is
essential to understand the underlying mechanisms behind complex systems. While
several linear approaches are often insufficient to describe such processes,
there are several nonlinear methods which however require rather long time
observations. To overcome these difficulties, we propose measures of complexity
based on vertical structures in recurrence plots and apply them to the logistic
map as well as to heart rate variability data. For the logistic map these
measures enable us not only to detect transitions between chaotic and periodic
states, but also to identify laminar states, i.e. chaos-chaos transitions. The
traditional recurrence quantification analysis fails to detect the latter
transitions. Applying our new measures to the heart rate variability data, we
are able to detect and quantify the laminar phases before a life-threatening
cardiac arrhythmia occurs thereby facilitating a prediction of such an event.
Our findings could be of importance for the therapy of malignant cardiac
arrhythmias
Performance Limitations of Flat Histogram Methods and Optimality of Wang-Landau Sampling
We determine the optimal scaling of local-update flat-histogram methods with
system size by using a perfect flat-histogram scheme based on the exact density
of states of 2D Ising models.The typical tunneling time needed to sample the
entire bandwidth does not scale with the number of spins N as the minimal N^2
of an unbiased random walk in energy space. While the scaling is power law for
the ferromagnetic and fully frustrated Ising model, for the +/- J
nearest-neighbor spin glass the distribution of tunneling times is governed by
a fat-tailed Frechet extremal value distribution that obeys exponential
scaling. We find that the Wang-Landau algorithm shows the same scaling as the
perfect scheme and is thus optimal.Comment: 5 pages, 6 figure
Urgent aftershock observation of the 2004 off the Kii Peninsula earthquake using ocean bottom seismometers
Field- and pressure-induced magnetic quantum phase transitions in TlCuCl_3
Thallium copper chloride is a quantum spin liquid of S = 1/2 Cu^2+ dimers.
Interdimer superexchange interactions give a three-dimensional magnon
dispersion and a spin gap significantly smaller than the dimer coupling. This
gap is closed by an applied hydrostatic pressure of approximately 2kbar or by a
magnetic field of 5.6T, offering a unique opportunity to explore the both types
of quantum phase transition and their associated critical phenomena. We use a
bond-operator formulation to obtain a continuous description of all disordered
and ordered phases, and thus of the transitions separating these. Both
pressure- and field-induced transitions may be considered as the Bose-Einstein
condensation of triplet magnon excitations, and the respective phases of
staggered magnetic order as linear combinations of dimer singlet and triplet
modes. We focus on the evolution with applied pressure and field of the
magnetic excitations in each phase, and in particular on the gapless
(Goldstone) modes in the ordered regimes which correspond to phase fluctuations
of the ordered moment. The bond-operator description yields a good account of
the magnetization curves and of magnon dispersion relations observed by
inelastic neutron scattering under applied fields, and a variety of
experimental predictions for pressure-dependent measurements.Comment: 20 pages, 17 figure
- …