3,443 research outputs found
Hypernuclear Physics at PANDA
Hypernuclear research will be one of the main topics addressed by the PANDA
experiment at the planned Facility for Anti-proton and Ion Research FAIR at
Darmstadt, Germany. A copious production of Xi-hyperons at a dedicated internal
target in the stored anti-proton beam is expected, which will enable the
high-precision gamma-spectroscopy of double strange systems for the first time.
In addition to the general purpose PANDA setup, the hypernuclear experiments
require an active secondary target of silicon layers and absorber material as
well as high purity germanium (HPGe) crystals as gamma-detectors. The design of
the setup and the development of these detectors is progressing: a first HPGe
crystal with a new electromechanical cooling system was prepared and the
properties of a silicon strip detector as a prototype to be used in the
secondary target were studied. Simultaneously to the hardware projects,
detailed Monte Carlo simulations were performed to predict the yield of
particle stable hypernuclei. With the help of the Monte Carlo a procedure for
Lambda-Lambda-hypernuclei identification by the detection and correlation of
the weak decay pions was developed.Comment: prepared for the International Conference on Exotic Atoms and Related
Topics (EXA2011), Vienna, Sept. 5-9, 201
Size effects in surface reconstructed and silicon nanowires
The geometrical and electronic structure properties of
silicon nanowires in the absence of surface passivation are studied by means of
density-functional calculations. As we have shown in a recent publication [R.
Rurali and N. Lorente, Phys. Rev. Lett. {\bf 94}, 026805 (2005)] the
reconstruction of facets can give rise to surface metallic states. In this
work, we analyze the dependence of geometric and electronic structure features
on the size of the wire and on the growth direction
Representations of the discrete inhomogeneous Lorentz group and Dirac wave equation on the lattice
We propose the fundamental and two dimensional representation of the Lorentz
groups on a (3+1)-dimensional hypercubic lattice, from which representations of
higher dimensions can be constructed. For the unitary representation of the
discrete translation group we use the kernel of the Fourier transform. From the
Dirac representation of the Lorentz group (including reflections) we derive in
a natural way the wave equation on the lattice for spin 1/2 particles. Finally
the induced representation of the discrete inhomogeneous Lorentz group is
constructed by standard methods and its connection with the continuous case is
discussed.Comment: LaTeX, 20 pages, 1 eps figure, uses iopconf.sty (late submission
Raising and lowering operators and their factorization for generalized orthogonal polynomials of hypergeometric type on homogeneous and non-homogeneous lattice
We complete the construction of raising and lowering operators, given in a
previous work, for the orthogonal polynomials of hypergeometric type on
non-homogeneous lattice, and extend these operators to the generalized
orthogonal polynomials, namely, those difference of orthogonal polynomials that
satisfy a similar difference equation of hypergeometric type.Comment: LaTeX, 19 pages, (late submission to arXiv.org
Creating pseudo Kondo-resonances by field-induced diffusion of atomic hydrogen
In low temperature scanning tunneling microscopy (STM) experiments a cerium
adatom on Ag(100) possesses two discrete states with significantly different
apparent heights. These atomic switches also exhibit a Kondo-like feature in
spectroscopy experiments. By extensive theoretical simulations we find that
this behavior is due to diffusion of hydrogen from the surface onto the Ce
adatom in the presence of the STM tip field. The cerium adatom possesses
vibrational modes of very low energy (3-4meV) and very high efficiency (> 20%),
which are due to the large changes of Ce-states in the presence of hydrogen.
The atomic vibrations lead to a Kondo-like feature at very low bias voltages.
We predict that the same low-frequency/high-efficiency modes can also be
observed at lanthanum adatoms.Comment: five pages and four figure
The double torus as a 2D cosmos: groups, geometry and closed geodesics
The double torus provides a relativistic model for a closed 2D cosmos with
topology of genus 2 and constant negative curvature. Its unfolding into an
octagon extends to an octagonal tessellation of its universal covering, the
hyperbolic space H^2. The tessellation is analysed with tools from hyperbolic
crystallography. Actions on H^2 of groups/subgroups are identified for SU(1,
1), for a hyperbolic Coxeter group acting also on SU(1, 1), and for the
homotopy group \Phi_2 whose extension is normal in the Coxeter group. Closed
geodesics arise from links on H^2 between octagon centres. The direction and
length of the shortest closed geodesics is computed.Comment: Latex, 27 pages, 5 figures (late submission to arxiv.org
Many-body effects in magnetic inelastic electron tunneling spectroscopy
Magnetic inelastic electron tunneling spectroscopy (IETS) shows sharp
increases in conductance when a new conductance channel associated to a change
in magnetic structure is open. Typically, the magnetic moment carried by an
adsorbate can be changed by collision with a tunneling electron; in this
process the spin of the electron can flip or not. A previous one-electron
theory [Phys. Rev. Lett. {\bf 103}, 176601 (2009)] successfully explained both
the conductance thresholds and the magnitude of the conductance variation. The
elastic spin flip of conduction electrons by a magnetic impurity leads to the
well known Kondo effect. In the present work, we compare the theoretical
predictions for inelastic magnetic tunneling obtained with a one-electron
approach and with a many-body theory including Kondo-like phenomena. We apply
our theories to a singlet-triplet transition model system that contains most of
the characteristics revealed in magnetic IETS. We use two self-consistent
treatments (non-crossing approximation and self-consistent ladder
approximation). We show that, although the one-electron limit is properly
recovered, new intrinsic many-body features appear. In particular, sharp peaks
appear close to the inelastic thresholds; these are not localized exactly at
thresholds and could influence the determination of magnetic structures from
IETS experiments.Analysis of the evolution with temperature reveals that these
many-body features involve an energy scale different from that of the usual
Kondo peaks. Indeed, the many-body features perdure at temperatures much larger
than the one given by the Kondo energy scale of the system.Comment: 10 pages and 6 figure
Raising and lowering operators, factorization and differential/difference operators of hypergeometric type
Starting from Rodrigues formula we present a general construction of raising
and lowering operators for orthogonal polynomials of continuous and discrete
variable on uniform lattice. In order to have these operators mutually adjoint
we introduce orthonormal functions with respect to the scalar product of unit
weight. Using the Infeld-Hull factorization method, we generate from the
raising and lowering operators the second order self-adjoint
differential/difference operator of hypergeometric type.Comment: LaTeX, 24 pages, iopart style (late submission
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