1,587 research outputs found
Computing quantum phase transitions
This article first gives a concise introduction to quantum phase transitions,
emphasizing similarities with and differences to classical thermal transitions.
After pointing out the computational challenges posed by quantum phase
transitions, a number of successful computational approaches is discussed. The
focus is on classical and quantum Monte Carlo methods, with the former being
based on the quantum-to classical mapping while the latter directly attack the
quantum problem. These methods are illustrated by several examples of quantum
phase transitions in clean and disordered systems.Comment: 99 pages, 15 figures, submitted to Reviews in Computational Chemistr
Application of Local Information Entropy in Cluster Monte Carlo Algorithms
The chapter refers to a modification of the so-called adding probability used in cluster Monte Carlo algorithms. The modification is based on the fact that in real systems, different properties can influence its clusterization. Finally, an additional factor related to property disorder was introduced into the adding probability, which leads to more effective free energy minimization during MC iteration. As a measure of the disorder, we proposed to use a local information entropy. The proposed approach was tested and compared with the classical methods, showing its high efficiency in simulations of multiphase magnetic systems where magnetic anisotropy was used as the property influencing the system clusterization
Sign-problem-free Monte Carlo simulation of certain frustrated quantum magnets
We introduce a Quantum Monte Carlo (QMC) method which efficiently simulates
in a sign-problem-free way a broad class of frustrated models with
competing antiferromagnetic interactions. Our scheme uses the basis of total
spin eigenstates of clusters of spins to avoid the severe sign problem faced by
other QMC methods. We also flag important limitations of the new method, and
comment on possibilities for further progress.Comment: 6 pages + appendix with supplemental informatio
A simple analytical description of the non-stationary dynamics in Ising spin systems
The analytical description of the dynamics in models with discrete variables (e.g. Isingspins) is a notoriously difficult problem, that can be tackled only undersome approximation.Recently a novel variational approach to solve the stationary dynamical regime has beenintroduced by Pelizzola [Eur. Phys. J. B, 86 (2013) 120], where simpleclosed equations arederived under mean-field approximations based on the cluster variational method. Here wepropose to use the same approximation based on the cluster variational method also for thenon-stationary regime, which has not been considered up to now within this framework. Wecheck the validity of this approximation in describing the non-stationary dynamical regime ofseveral Ising models defined on Erdos-R Ìenyi random graphs: westudy ferromagnetic modelswith symmetric and partially asymmetric couplings, models with randomfields and also spinglass models. A comparison with the actual Glauber dynamics, solvednumerically, showsthat one of the two studied approximations (the so-called âdiamondâapproximation) providesvery accurate results in all the systems studied. Only for the spin glass models we find somesmall discrepancies in the very low temperature phase, probably due to the existence of alarge number of metastable states. Given the simplicity of the equations to be solved, webelieve the diamond approximation should be considered as the âminimalstandardâ in thedescription of the non-stationary regime of Ising-like models: any new method pretending toprovide a better approximate description to the dynamics of Ising-like models should performat least as good as the diamond approximation
Data clustering using a model granular magnet
We present a new approach to clustering, based on the physical properties of
an inhomogeneous ferromagnet. No assumption is made regarding the underlying
distribution of the data. We assign a Potts spin to each data point and
introduce an interaction between neighboring points, whose strength is a
decreasing function of the distance between the neighbors. This magnetic system
exhibits three phases. At very low temperatures it is completely ordered; all
spins are aligned. At very high temperatures the system does not exhibit any
ordering and in an intermediate regime clusters of relatively strongly coupled
spins become ordered, whereas different clusters remain uncorrelated. This
intermediate phase is identified by a jump in the order parameters. The
spin-spin correlation function is used to partition the spins and the
corresponding data points into clusters. We demonstrate on three synthetic and
three real data sets how the method works. Detailed comparison to the
performance of other techniques clearly indicates the relative success of our
method.Comment: 46 pages, postscript, 15 ps figures include
Magnetization processes of irregular dendrite structures - a Monte Carlo study
The paper refers to micromagnetic simulations of magnetization processes of dendrite-like object. The objects
were generated by the DLA fractal algorithm that allows obtaining fractals with different ratio of the spins
attributed to the surface to volume. The simulations were carried out using the cluster Monte Carlo algorithm
designed for spin continuous and multiphase magnetic systems. The presented researches include different
magnetic anisotropy of the surface and volume reflected magnetically soft, hard and ultra-high coercive phases.
As it was shown, the influence of microstructure on the coercivity mechanism is a complex phenomenon. In the
case of the fractals with magnetically soft volume the increasing surface contribution causes either increse or
decrease of the coercive field for relatively high or low magnetic anisotropy of the surface, respectively. For the
fractals with ultra-high coercive volume the occurrence of the surface anisotropy leads to the significant deterioration of their hard magnetic properties. The obtained spin configurations show that this effect is related to
non-colinear directions of the surface anisoropy and strong enough exchange coupling between the surface and
volume
Hybridization expansion impurity solver: General formulation and application to Kondo lattice and two-orbital models
A recently developed continuous time solver based on an expansion in
hybridization about an exactly solved local limit is reformulated in a manner
appropriate for general classes of quantum impurity models including spin
exchange and pair hopping terms. The utility of the approach is demonstrated
via applications to the dynamical mean field theory of the Kondo lattice and
two-orbital models. The algorithm can handle low temperatures and strong
couplings without encountering a sign problem.Comment: Published versio
Magnetic properties of polypyrrole - coated iron oxide nanoparticles
Iron oxide nanoparticles were prepared by sol -gel process. Insitu
polymerization of pyrrole monomer in the presence of oxygen in iron oxide
ethanol suspension resulted in a iron oxide - polypyrrole nanocomposite. The
structure and magnetic properties were investigated for varying pyrrole
concentrations. The presence of the gamma - iron oxide phase and polypyrrole
were confirmed by XRD and FTIR respectively. Agglomeration was found to be
comparatively much reduced for the coated samples, as shown by TEM. AC
susceptibility measurements confirmed the superparamagnetic behaviour.
Numerical simulations performed for an interacting model system are performed
to estimate the anisotropy and compare favourably with experimental results.Comment: 11 pages,8 figure
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