1,097 research outputs found
Finite temperature ordering of dilute graphene antiferromagnets
We employ large-scale quantum Monte Carlo simulations to study the magnetic
ordering transition among dilute magnetic moments randomly localized on the
graphene honeycomb lattice, induced by long-ranged RKKY interactions at low
charge carrier concentration. In this regime the effective exchange
interactions are ferromagnetic within each sublattice, and antiferromagnetic
between opposite sublattices, with an overall cubic decay of the interaction
strength with the separation between the moments. We verify explicitly, that
this commensurability leads to antiferromagnetic order among the magnetic
moments below a finite transition temperature in this two-dimensional system.
Furthermore, the ordering temperature shows a crossover in its power-law
scaling with the moments' dilution from a low- to a high-concentration regime.Comment: 8 pages, 8 figure
Effective models for strong electronic correlations at graphene edges
We describe a method for deriving effective low-energy theories of electronic
interactions at graphene edges. Our method is applicable to general edges of
honeycomb lattices (zigzag, chiral, and even disordered) as long as localized
low-energy states (edge states) are present. The central characteristic of the
effective theories is a dramatically reduced number of degrees of freedom. As a
consequence, the solution of the effective theory by exact diagonalization is
feasible for reasonably large ribbon sizes. The quality of the involved
approximations is critically assessed by comparing the correlation functions
obtained from the effective theory with numerically exact quantum Monte-Carlo
calculations. We discuss effective theories of two levels: a relatively
complicated fermionic edge state theory and a further reduced Heisenberg spin
model. The latter theory paves the way to an efficient description of the
magnetic features in long and structurally disordered graphene edges beyond the
mean-field approximation.Comment: 13 pages, 9 figure
Z2 topological invariants in two dimensions from quantum Monte Carlo
We employ quantum Monte Carlo techniques to calculate the topological
invariant in a two-dimensional model of interacting electrons that exhibits a
quantum spin Hall topological insulator phase. In particular, we consider the
parity invariant for inversion-symmetric systems, which can be obtained from
the bulk's imaginary-time Green's function after an appropriate continuation to
zero frequency. This topological invariant is used here in order to study the
trivial-band to topological-insulator transitions in an interacting system with
spin-orbit coupling and an explicit bond dimerization. We discuss the
accessibility and behavior of this topological invariant within quantum Monte
Carlo simulations.Comment: 7 pages, 6 figure
Sleep apnea-hypopnea quantification by cardiovascular data analysis
Sleep apnea is the most common sleep disturbance and it is an important risk
factor for cardiovascular disorders. Its detection relies on a polysomnography,
a combination of diverse exams.
In order to detect changes due to sleep disturbances such as sleep apnea
occurrences, without the need of combined recordings, we mainly analyze
systolic blood pressure signals (maximal blood pressure value of each beat to
beat interval). Nonstationarities in the data are uncovered by a segmentation
procedure, which provides local quantities that are correlated to
apnea-hypopnea events. Those quantities are the average length and average
variance of stationary patches. By comparing them to an apnea score previously
obtained by polysomnographic exams, we propose an apnea quantifier based on
blood pressure signal.
This furnishes an alternative procedure for the detection of apnea based on a
single time series, with an accuracy of 82%
Dimerized Solids and Resonating Plaquette Order in SU(N)-Dirac Fermions
We study the quantum phases of fermions with an explicit SU(N)-symmetric,
Heisenberg-like nearest-neighbor flavor exchange interaction on the honeycomb
lattice at half-filling. Employing projective (zero temperature) quantum Monte
Carlo simulations for even values of N, we explore the evolution from a
weak-coupling semimetal into the strong-coupling, insulating regime.
Furthermore, we compare our numerical results to a saddle-point approximation
in the large-N limit. From the large-N regime down to the SU(6) case, the
insulating state is found to be a columnar valence bond crystal, with a direct
transition to the semimetal at weak, finite coupling, in agreement with the
mean-field result in the large-N limit. At SU(4) however, the insulator
exhibits a subtly different valence bond crystal structure, stabilized by
resonating valence bond plaquettes. In the SU(2) limit, our results support a
direct transition between the semimetal and an antiferromagnetic insulator.Comment: 5 pages, 6 figure
Finite-Temperature Dynamics and Thermal Intraband Magnon Scattering in Haldane Spin-One Chains
The antiferromagnetic spin-one chain is considerably one of the most
fundamental quantum many-body systems, with symmetry protected topological
order in the ground state. Here, we present results for its dynamical spin
structure factor at finite temperatures, based on a combination of exact
numerical diagonalization, matrix-product-state calculations and quantum Monte
Carlo simulations. Open finite chains exhibit a sub-gap band in the thermal
spectral functions, indicative of localized edge-states. Moreover, we observe
the thermal activation of a distinct low-energy continuum contribution to the
spin spectral function with an enhanced spectral weight at low momenta and its
upper threshold. This emerging thermal spectral feature of the Haldane spin-one
chain is shown to result from intra-band magnon scattering due to the thermal
population of the single-magnon branch, which features a large bandwidth-to-gap
ratio. These findings are discussed with respect to possible future studies on
spin-one chain compounds based on inelastic neutron scattering.Comment: 10 pages with 11 figures total (including Supplemental Material);
changes in v2: new Figs. S1 and S5, Fig. S3 expanded + related discussion +
many smaller modifications to match published versio
Dynamical Signatures of Edge-State Magnetism on Graphene Nanoribbons
We investigate the edge-state magnetism of graphene nanoribbons using
projective quantum Monte Carlo simulations and a self-consistent mean-field
approximation of the Hubbard model. The static magnetic correlations are found
to be short ranged. Nevertheless, the correlation length increases with the
width of the ribbon such that already for ribbons of moderate widths we observe
a strong trend towards mean-field-type ferromagnetic correlations at a zigzag
edge. These correlations are accompanied by a dominant low-energy peak in the
local spectral function and we propose that this can be used to detect
edge-state magnetism by scanning tunneling microscopy. The dynamic spin
structure factor at the edge of a ribbon exhibits an approximately linearly
dispersing collective magnonlike mode at low energies that decays into Stoner
modes beyond the energy scale where it merges into the particle-hole continuum.Comment: 4+ pages including 4 figure
Quantum Monte Carlo studies of edge magnetism in chiral graphene nanoribbons
We investigate chiral graphene nanoribbons using projective quantum Monte
Carlo simulations within the local Hubbard model description and study the
effects of electron-electron interactions on the electronic and magnetic
properties at the ribbon edges. Static and dynamical properties are analyzed
for nanoribbons of varying width and edge chirality, and compared to a
self-consistent Hartee-Fock mean-field approximation. Our results show that for
chiral ribbons of sufficient width, the spin correlations exhibit exceedingly
long correlation lengths, even between zigzag segments that are well separated
by periodic armchair regions. Characteristic enhancements in the magnetic
correlations for distinct ribbon widths and chiralities are associated with
energy gaps in the tight-binding limit of such ribbons. We identify specific
signatures in the local density of states and low- energy modes in the local
spectral function which directly relate to enhanced electronic correlations
along graphene nanoribbons and which can be accessed scanning tunneling
spectroscopy.Comment: 11 pages, 15 figure
Antiferromagnetism in the Hubbard Model on the Bernal-stacked Honeycomb Bilayer
Using a combination of quantum Monte Carlo simulations, functional
renormalization group calculations and mean-field theory, we study the Hubbard
model on the Bernal-stacked honeycomb bilayer at half-filling as a model system
for bilayer graphene. The free bands consisting of two Fermi points with
quadratic dispersions lead to a finite density of states at the Fermi level,
which triggers an antiferromagnetic instability that spontaneously breaks
sublattice and spin rotational symmetry once local Coulomb repulsions are
introduced. Our results reveal an inhomogeneous participation of the spin
moments in the ordered ground state, with enhanced moments at the three-fold
coordinated sites. Furthermore, we find the antiferromagnetic ground state to
be robust with respect to enhanced interlayer couplings and extended Coulomb
interactions.Comment: 4+ pages, 4 figures; final versio
Parallel Mining of Association Rules Using a Lattice Based Approach
The discovery of interesting patterns from database transactions is one of the major problems in knowledge discovery in database. One such interesting pattern is the association rules extracted from these transactions. Parallel algorithms are required for the mining of association rules due to the very large databases used to store the transactions. In this paper we present a parallel algorithm for the mining of association rules. We implemented a parallel algorithm that used a lattice approach for mining association rules. The Dynamic Distributed Rule Mining (DDRM) is a lattice-based algorithm that partitions the lattice into sublattices to be assigned to processors for processing and identification of frequent itemsets. Experimental results show that DDRM utilizes the processors efficiently and performed better than the prefix-based and partition algorithms that use a static approach to assign classes to the processors. The DDRM algorithm scales well and shows good speedup
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