63 research outputs found
Anomalous electron trapping by localized magnetic fields
We consider an electron with an anomalous magnetic moment g>2 confined to a
plane and interacting with a nonzero magnetic field B perpendicular to the
plane. We show that if B has compact support and the magnetic flux in the
natural units is F\ge 0, the corresponding Pauli Hamiltonian has at least 1+[F]
bound states, without making any assumptions about the field profile.
Furthermore, in the zero-flux case there is a pair of bound states with
opposite spin orientations. Using a Birman-Schwinger technique, we extend the
last claim to a weak rotationally symmetric field with B(r) = O(r^{-2-\delta})
correcting thus a recent result. Finally, we show that under mild regularity
assumptions the existence can be proved for non-symmetric fields with tails as
well.Comment: A LaTeX file, 12 pages; to appear in J. Phys. A: Math. Ge
I look at you to learn: Effects of the owner's sex on social learning in domestic dogs
Dogs have been shown to be able to learn from a human demonstrator. However, to date, there have been no studies investigating the effect of the demonstrator?s sex on such learning. The aim of our study was to evaluate this effect by comparing an experimental condition in which dogs received a demonstration from their owner on how to manipulate one of two possible containers to obtain food and a control condition without any human demonstration. Each of these conditions was divided into two groups: male-owned and female-owned dogs.Overall, the dogs performed better in the experimental condition compared to the control condition. This was evidentbased ona higher frequency of correct choices and opening the correct container, as well as a higher frequency of contact and gaze towards the demonstration. The female-owned group benefited from the demonstration by choosing the correct container more frequentlyin the experimental condition compared to the control. Conversely, male-owned dogs chose the correct container more often and looked more frequently at the demonstration than female-owned dogs, without differences between conditions. This could indicate a higher capacity for problem-solvingin this group of dogs beyond the human demonstration, and therefore would not reflect a modulatory effect of the owner?s sex over social learning in particular. In conclusion, the sex of the demonstrator seems to have an effect on social learning in dogs when the demonstrator is a female owner. This might have an impact on several applied settings as well as sampling criteria in canine social cognition research.Fil: Dzik, Marina Victoria. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Investigaciones Médicas. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Investigaciones Médicas; ArgentinaFil: Gutierrez Torres, J. S.. Universidad Central; ColombiaFil: Berdugo Lattke, M. L.. Universidad Central; ColombiaFil: Bentosela, Mariana. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Investigaciones Médicas. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Investigaciones Médicas; Argentin
One-Dimensional Discrete Stark Hamiltonian and Resonance Scattering by Impurities
A one-dimensional discrete Stark Hamiltonian with a continuous electric field
is constructed by extension theory methods. In absence of the impurities the
model is proved to be exactly solvable, the spectrum is shown to be simple,
continuous, filling the real axis; the eigenfunctions, the resolvent and the
spectral measure are constructed explicitly. For this (unperturbed) system the
resonance spectrum is shown to be empty. The model considering impurity in a
single node is also constructed using the operator extension theory methods.
The spectral analysis is performed and the dispersion equation for the
resolvent singularities is obtained. The resonance spectrum is shown to contain
infinite discrete set of resonances. One-to-one correspondence of the
constructed Hamiltonian to some Lee-Friedrichs model is established.Comment: 20 pages, Latex, no figure
Spectral flow and level spacing of edge states for quantum Hall hamiltonians
We consider a non relativistic particle on the surface of a semi-infinite
cylinder of circumference submitted to a perpendicular magnetic field of
strength and to the potential of impurities of maximal amplitude . This
model is of importance in the context of the integer quantum Hall effect. In
the regime of strong magnetic field or weak disorder it is known that
there are chiral edge states, which are localised within a few magnetic lengths
close to, and extended along the boundary of the cylinder, and whose energy
levels lie in the gaps of the bulk system. These energy levels have a spectral
flow, uniform in , as a function of a magnetic flux which threads the
cylinder along its axis. Through a detailed study of this spectral flow we
prove that the spacing between two consecutive levels of edge states is bounded
below by with , independent of , and of the
configuration of impurities. This implies that the level repulsion of the
chiral edge states is much stronger than that of extended states in the usual
Anderson model and their statistics cannot obey one of the Gaussian ensembles.
Our analysis uses the notion of relative index between two projections and
indicates that the level repulsion is connected to topological aspects of
quantum Hall systems.Comment: 22 pages, no figure
Bound States in Mildly Curved Layers
It has been shown recently that a nonrelativistic quantum particle
constrained to a hard-wall layer of constant width built over a geodesically
complete simply connected noncompact curved surface can have bound states
provided the surface is not a plane. In this paper we study the weak-coupling
asymptotics of these bound states, i.e. the situation when the surface is a
mildly curved plane. Under suitable assumptions about regularity and decay of
surface curvatures we derive the leading order in the ground-state eigenvalue
expansion. The argument is based on Birman-Schwinger analysis of Schroedinger
operators in a planar hard-wall layer.Comment: LaTeX 2e, 23 page
Magnetic strip waveguides
We analyze the spectrum of the "local" Iwatsuka model, i.e. a two-dimensional
charged particle interacting with a magnetic field which is homogeneous outside
a finite strip and translationally invariant along it. We derive two new
sufficient conditions for absolute continuity of the spectrum. We also show
that in most cases the number of open spectral gaps of the model is finite. To
illustrate these results we investigate numerically the situation when the
field is zero in the strip being screened, e.g. by a superconducting mask.Comment: 22 pages, a LaTeX source file with three eps figure
Resonances Width in Crossed Electric and Magnetic Fields
We study the spectral properties of a charged particle confined to a
two-dimensional plane and submitted to homogeneous magnetic and electric fields
and an impurity potential. We use the method of complex translations to prove
that the life-times of resonances induced by the presence of electric field are
at least Gaussian long as the electric field tends to zero.Comment: 3 figure
Intermixture of extended edge and localized bulk energy levels in macroscopic Hall systems
We study the spectrum of a random Schroedinger operator for an electron
submitted to a magnetic field in a finite but macroscopic two dimensional
system of linear dimensions equal to L. The y direction is periodic and in the
x direction the electron is confined by two smooth increasing boundary
potentials. The eigenvalues of the Hamiltonian are classified according to
their associated quantum mechanical current in the y direction. Here we look at
an interval of energies inside the first Landau band of the random operator for
the infinite plane. In this energy interval, with large probability, there
exist O(L) eigenvalues with positive or negative currents of O(1). Between each
of these there exist O(L^2) eigenvalues with infinitesimal current
O(exp(-cB(log L)^2)). We explain what is the relevance of this analysis to the
integer quantum Hall effect.Comment: 29 pages, no figure
Band spectra of rectangular graph superlattices
We consider rectangular graph superlattices of sides l1, l2 with the
wavefunction coupling at the junctions either of the delta type, when they are
continuous and the sum of their derivatives is proportional to the common value
at the junction with a coupling constant alpha, or the "delta-prime-S" type
with the roles of functions and derivatives reversed; the latter corresponds to
the situations where the junctions are realized by complicated geometric
scatterers. We show that the band spectra have a hidden fractal structure with
respect to the ratio theta := l1/l2. If the latter is an irrational badly
approximable by rationals, delta lattices have no gaps in the weak-coupling
case. We show that there is a quantization for the asymptotic critical values
of alpha at which new gap series open, and explain it in terms of
number-theoretic properties of theta. We also show how the irregularity is
manifested in terms of Fermi-surface dependence on energy, and possible
localization properties under influence of an external electric field.
KEYWORDS: Schroedinger operators, graphs, band spectra, fractals,
quasiperiodic systems, number-theoretic properties, contact interactions, delta
coupling, delta-prime coupling.Comment: 16 pages, LaTe
La mirada de los perros como señal comunicativa: ¿existen diferencias de razas?
En el presente trabajo, se estudia si distintos grupos de razas, originariamente seleccionados para cumplir diversas funciones en la sociedad humana, difieren en su respuesta de mirada hacia el humano.Sección Pósters.Facultad de Psicologí
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