656 research outputs found
Nonintegrability of the two-body problem in constant curvature spaces
We consider the reduced two-body problem with the Newton and the oscillator
potentials on the sphere and the hyperbolic plane .
For both types of interaction we prove the nonexistence of an additional
meromorphic integral for the complexified dynamic systems.Comment: 20 pages, typos correcte
Quantum Monte Carlo scheme for frustrated Heisenberg antiferromagnets
When one tries to simulate quantum spin systems by the Monte Carlo method,
often the 'minus-sign problem' is encountered. In such a case, an application
of probabilistic methods is not possible. In this paper the method has been
proposed how to avoid the minus sign problem for certain class of frustrated
Heisenberg models. The systems where this method is applicable are, for
instance, the pyrochlore lattice and the Heisenberg model. The method
works in singlet sector. It relies on expression of wave functions in dimer
(pseudo)basis and writing down the Hamiltonian as a sum over plaquettes. In
such a formulation, matrix elements of the exponent of Hamiltonian are
positive.Comment: 19 LaTeX pages, 6 figures, 1 tabl
The harmonic oscillator on Riemannian and Lorentzian configuration spaces of constant curvature
The harmonic oscillator as a distinguished dynamical system can be defined
not only on the Euclidean plane but also on the sphere and on the hyperbolic
plane, and more generally on any configuration space with constant curvature
and with a metric of any signature, either Riemannian (definite positive) or
Lorentzian (indefinite). In this paper we study the main properties of these
`curved' harmonic oscillators simultaneously on any such configuration space,
using a Cayley-Klein (CK) type approach, with two free parameters \ki, \kii
which altogether correspond to the possible values for curvature and signature
type: the generic Riemannian and Lorentzian spaces of constant curvature
(sphere , hyperbolic plane , AntiDeSitter sphere {\bf
AdS}^{\unomasuno} and DeSitter sphere {\bf dS}^{\unomasuno}) appear in this
family, with the Euclidean and Minkowski spaces as flat limits.
We solve the equations of motion for the `curved' harmonic oscillator and
obtain explicit expressions for the orbits by using three different methods:
first by direct integration, second by obtaining the general CK version of the
Binet's equation and third, as a consequence of its superintegrable character.
The orbits are conics with centre at the potential origin in any CK space,
thereby extending this well known Euclidean property to any constant curvature
configuration space. The final part of the article, that has a more geometric
character, presents those results of the theory of conics on spaces of constant
curvature which are pertinent.Comment: 29 pages, 6 figure
Two-dimensional charge order in layered 2-1-4 perovskite oxides
Monte Carlo simulations are performed on the three-dimensional (3D) Ising
model with the 2-1-4 layered perovskite structure as a minimal model for
checkerboard charge ordering phenomena in layered perovskite oxides. Due to the
interlayer frustration, only 2D long-range order emerges with a finite
correlation length along the c axis. Critical exponents of the transition
change continuously as a function of the interlayer coupling constant. The
interlayer long-range Coulomb interaction decays exponentially and is
negligible even between the second-neighbor layers. Instead, monoclinic
distortion of a tetragonal unit cell lifts the macroscopic degeneracy to induce
a 3D charge ordering. The dimensionality of the charge order in
LaSrMnO is discussed from this viewpoint.Comment: 5 pages including 6 figures, with major changes including discussion
on charge ordering phenomena in layered perovskite oxide
Establishing a lesser kestrel colony in an urban environment for research purposes
[ES] Debido al declive reciente de las poblaciones de Falco naumanni, se han llevado a cabo numerosos proyectos de reintroducción y refuerzo de sus poblaciones en varios paÃses mediterráneos. Aprovechando la experiencia de proyectos previos, intentamos establecer una colonia de F. naumanni en la cubierta de la sede central de la Estación Biológica de Doñana, un edificio moderno y recientemente construido en Sevilla, sur de España. Nuestro objetivo es obtener un acceso fácil a una población urbana y silvestre de esta especie modelo con fines cientÃficos. Después de la liberación de los primeros individuos mediante el método de crianza campestre hace cinco años, nuestra colonia experimental parece haberse establecido. Tres, seis y tres parejas intentaron reproducirse en los años 2010, 2011 y 2012, respectivamente.Peer reviewe
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Factors Influencing Optical Coherence Tomography Peripapillary Choroidal Thickness: A Multicenter Study.
Purpose:To quantify peripapillary choroidal thickness (PCT) and the factors that influence it in healthy participants who represent the racial and ethnic composition of the U.S. population. Methods:A total of 362 healthy participants underwent optical coherence tomography (OCT) enhanced depth imaging of the optic nerve head with a 24 radial B-scan pattern aligned to the fovea to Bruch's membrane opening axis. Bruch's membrane, anterior scleral canal opening (ASCO), and the anterior scleral surface were manually segmented. PCT was measured at 100, 300, 500, 700, 900, and 1100 μm from the ASCO globally and within 12 clock-hour sectors. The effects of age, axial length, intraocular pressure, ethnicity, sex, sector, and ASCO area on PCT were assessed by ANOVA and univariable and multivariable regressions. Results:Globally, PCT was thicker further from the ASCO border and thinner with older age, longer axial length, larger ASCO area, European descent, and female sex. Among these effectors, age and axial length explained the greatest proportion of variance. The rate of age-related decline increased further from the ASCO border. Sectorally, the inferior-temporal sectors were thinnest (10.7%-20.0% thinner than the thickest sector) and demonstrated a higher rate of age-related loss (from 15.6% to 20.7% faster) at each ASCO distance. Conclusions:In healthy eyes, PCT was thinnest in the inferior temporal sectors and thinner PCT was associated with older age, European descent, longer axial length, larger ASCO area, and female sex. Among these associations, age had the strongest influence, and its effect was greatest within the inferior temporal sectors
Wave function mapping in graphene quantum dots with soft confinement
Using low-temperature scanning tunneling spectroscopy, we map the local
density of states (LDOS) of graphene quantum dots supported on Ir(111). Due to
a band gap in the projected Ir band structure around the graphene K point, the
electronic properties of the QDs are dominantly graphene-like. Indeed, we
compare the results favorably with tight binding calculations on the honeycomb
lattice based on parameters derived from density functional theory. We find
that the interaction with the substrate near the edge of the island gradually
opens a gap in the Dirac cone, which implies soft-wall confinement.
Interestingly, this confinement results in highly symmetric wave functions.
Further influences of the substrate are given by the known moir{\'e} potential
and a 10% penetration of an Ir surface resonanceComment: 7 pages, 11 figures, DFT calculations directly showing the origin of
soft confinment, correct identification of the state penetrating from Ir(111)
into graphen
Two-dimensional periodic frustrated Ising models in a transverse field
We investigate the interplay of classical degeneracy and quantum dynamics in
a range of periodic frustrated transverse field Ising systems at zero
temperature. We find that such dynamics can lead to unusual ordered phases and
phase transitions, or to a quantum spin liquid (cooperative paramagnetic) phase
as in the triangular and kagome lattice antiferromagnets, respectively. For the
latter, we further predict passage to a bond-ordered phase followed by a
critical phase as the field is tilted. These systems also provide exact
realizations of quantum dimer models introduced in studies of high temperature
superconductivity.Comment: Revised introduction; numerical error in hexagonal section correcte
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