16,499 research outputs found
Projected entangled-pair states can describe chiral topological states
We show that Projected Entangled-Pair States (PEPS) in two spatial dimensions
can describe chiral topological states by explicitly constructing a family of
such states with a non-trivial Chern number. They are ground states of two
different kinds of free-fermion Hamiltonians: (i) local and gapless; (ii)
gapped, but with hopping amplitudes that decay according to a power law. We
derive general conditions on topological free fermionic PEPS which show that
they cannot correspond to exact ground states of gapped, local parent
Hamiltonians, and provide numerical evidence demonstrating that they can
nevertheless approximate well the physical properties of topological insulators
with local Hamiltonians at arbitrary temperatures.Comment: v2: minor changes, references added. v3: accepted version,
Journal-Ref adde
Answer Sets for Logic Programs with Arbitrary Abstract Constraint Atoms
In this paper, we present two alternative approaches to defining answer sets
for logic programs with arbitrary types of abstract constraint atoms (c-atoms).
These approaches generalize the fixpoint-based and the level mapping based
answer set semantics of normal logic programs to the case of logic programs
with arbitrary types of c-atoms. The results are four different answer set
definitions which are equivalent when applied to normal logic programs. The
standard fixpoint-based semantics of logic programs is generalized in two
directions, called answer set by reduct and answer set by complement. These
definitions, which differ from each other in the treatment of
negation-as-failure (naf) atoms, make use of an immediate consequence operator
to perform answer set checking, whose definition relies on the notion of
conditional satisfaction of c-atoms w.r.t. a pair of interpretations. The other
two definitions, called strongly and weakly well-supported models, are
generalizations of the notion of well-supported models of normal logic programs
to the case of programs with c-atoms. As for the case of fixpoint-based
semantics, the difference between these two definitions is rooted in the
treatment of naf atoms. We prove that answer sets by reduct (resp. by
complement) are equivalent to weakly (resp. strongly) well-supported models of
a program, thus generalizing the theorem on the correspondence between stable
models and well-supported models of a normal logic program to the class of
programs with c-atoms. We show that the newly defined semantics coincide with
previously introduced semantics for logic programs with monotone c-atoms, and
they extend the original answer set semantics of normal logic programs. We also
study some properties of answer sets of programs with c-atoms, and relate our
definitions to several semantics for logic programs with aggregates presented
in the literature
Individual differences in the modulation of visual search performance by auditory concurrent stimulation in very young children
Purpose To investigate whether visual search performance could be enhanced by rhythmic auditory stimulations and to assess the influence of individual external and internal factors. Methods Eye movement data were obtained from 29 young children from 16 to 48 months of age (17 male; mean = 36.9; sd = 9.89) during visual search tasks. In the within-subjects design, the order of visual (V) and audiovisual (AV) conditions was counterbalanced. Each condition contained 10 practice and experimental 64 trials. Search target located on 8 different spots with equal distance from the center while distractors were randomly distributed. Each set size (4, 6, 8, and 10) was presented 16 times. Fixation dot was shown for 1000 ms followed by a trial of 1000 ms Auditory stimulation was an isochronous and metrical sequence with 8 tones. The first tone that was accompanied by the appearance of visual stimuli, had a frequency of 880 Hz. The other seven tones had a frequency of 440 Hz. Each tone lasted 100 ms with an inter-onset-interval of 250 ms. Besides, all caregivers received questionnaires that assessed external (home music environment, and music use of caregiver) and internal (temperament types, and sensory processing patterns) factors. Results The major findings were (1) the number of search items per second was positively correlated with age and it was significantly higher (p = 0.046) in AV (2.07/sec) than V (1.84/sec); (2) in the lower set sizes (4 and 6), the number of responses on the target was higher in AV than V. (3) Analysis of a serial of questionnaires using the ordinary least squares regression with corrected cluster robust standard errors, we observed multiple casual associations between search performance and internal factors (sensory profile and temperament types) but not with external factors. More interestingly, the associations between internal factors search performance differed from AV and V. Conclusion In sum, our results showed concurrent rhythmic auditory stimulations might influence visual search speed. Moreover, internal individual differences in the early years play a significant role in visual search. We suggest that future studies further focus on investigating the effect of varying AV regularity and/or synchrony, and the dynamic interaction between individual differences and search performance
String order and hidden topological symmetry in the SO(2n+1) symmetric matrix product states
We have introduced a class of exactly soluble Hamiltonian with either
SO(2n+1) or SU(2) symmetry, whose ground states are the SO(2n+1) symmetric
matrix product states. The hidden topological order in these states can be
fully identified and characterized by a set of nonlocal string order
parameters. The Hamiltonian possesses a hidden
topological symmetry. The breaking of this hidden symmetry leads to
degenerate ground states with disentangled edge states in an open chain system.
Such matrix product states can be regarded as cluster states, applicable to
measurement-based quantum computation.Comment: 5 pages, 1 figur
Frustration Effects in Antiferromagnetic FCC Heisenberg Films
We study the effects of frustration in an antiferromagnetic film of FCC
lattice with Heisenberg spin model including an Ising-like anisotropy. Monte
Carlo (MC) simulations have been used to study thermodynamic properties of the
film. We show that the presence of the surface reduces the ground state (GS)
degeneracy found in the bulk. The GS is shown to depend on the surface in-plane
interaction with a critical value at which ordering of type I coexists
with ordering of type II. Near this value a reentrant phase is found. Various
physical quantities such as layer magnetizations and layer susceptibilities are
shown and discussed. The nature of the phase transition is also studied by
histogram technique. We have also used the Green's function (GF) method for the
quantum counterpart model. The results at low- show interesting effects of
quantum fluctuations. Results obtained by the GF method at high are
compared to those of MC simulations. A good agreement is observed.Comment: 11 pages, 19 figures, submitted to J. Phys.: Condensed Matte
Non-Hermitian coherent coupling of nanomagnets by exchange spin waves
Non-Hermitian physics has recently attracted much attention in optics and
photonics. Less explored is non-Hermitian magnonics that provides opportunities
to take advantage of the inevitable dissipation of magnons or spin waves in
magnetic systems. Here we demonstrate non-Hermitian coherent coupling of two
distant nanomagnets by fast spin waves with sub-50 nm wavelengths. Magnons in
two nanomagnets are unidirectionally phase-locked with phase shifts controlled
by magnon spin torque and spin-wave propagation. Our results are attractive for
analog neuromorphic computing that requires unidirectional information
transmission
Probing Quantum Hall Pseudospin Ferromagnet by Resistively Detected NMR
Resistively Detected Nuclear Magnetic Resonance (RD-NMR) has been used to
investigate a two-subband electron system in a regime where quantum Hall
pseudo-spin ferromagnetic (QHPF) states are prominently developed. It reveals
that the easy-axis QHPF state around the total filling factor can be
detected by the RD-NMR measurement. Approaching one of the Landau level (LL)
crossing points, the RD-NMR signal strength and the nuclear spin relaxation
rate enhance significantly, a signature of low energy spin
excitations. However, the RD-NMR signal at another identical LL crossing point
is surprisingly missing which presents a puzzle
Comment on: Role of Intermittency in Urban Development: A Model of Large-Scale City Formation
Comment to D.H. Zanette and S.C. Manrubia, Phys. Rev. Lett. 79, 523 (1997).Comment: 1 page no figure
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