48,065 research outputs found
Energy resolved STM mapping of C on metal surfaces: A theoretical study
We present a detailed theoretical study of scanning tunneling imaging and
spectroscopy of \Csixty on silver and gold surfaces, motivated by the recent
experiments and discussion by X. Lu et al. [PRL \textbf{90}, 096802 (2003) and
PRB \textbf{70}, 115418 (2004)]. The surface/sample/tip system is described
within a self--consistent DFT based tight--binding model. The topographic and
conductance images are computed at constant current from a full
self--consistent transport theory based on nonequilibrium Green's functions and
compared with those simulated from the local density of states. The molecular
orbitals of \Csixty are clearly identified in the energy resolved maps, in
close correspondence with the experimental results. We show how the tip
structure and orientation can affect the images. In particular, we consider the
effects of truncated tips on the energy resolved maps.Comment: 9 pages, 8 figure
A 300 GHz "Always-in-Focus" Focusing System for Target Detection
A focusing system for a 300 GHz radar with 5 m target distance and 10 mm diameter spot size resolution is proposed. The focusing system is based on a Gaussian telescope scheme and its main parameters have been de¬signed using Gaussian beam quasi-optical propagation theory with an in-house developed MATLAB® based analysis tool. Then, this approach has been applied to a real focusing system based on two elliptical mirrors in order to reduce the distortion and cross-polar level and a plane mirror to provide scanning capabilities. The over¬all system has been simulated with a full-wave electromag¬netic simulator and its behavior is presented. With this approach, the focusing system always works "in-focus" since the only mirror that is rotated when scanning is the output plane mirror, so the beam is almost not distorted. The design process, although based in the well-known Gaussian beam quasi-optical propagation theory, provides a fast and accurate method and minimizes the overall size of the mirrors. As a consequence, the size of the focusing system is also reduced
Topological Properties of the QCD Vacuum at T=0 and T ~ T_c
We study on the lattice the topology of SU(2) and SU(3) Yang-Mills theories
at zero temperature and of QCD at temperatures around the phase transition. To
smooth out dislocations and the UV noise we cool the configurations with an
action which has scale invariant instanton solutions for instanton size above
about 2.3 lattice spacings. The corresponding "improved" topological charge
stabilizes at an integer value after few cooling sweeps. At zero temperature
the susceptibility calculated from this charge (about (195MeV)^4 for SU(2) and
(185 MeV)^4 for SU(3)) agrees very well with the phenomenological expectation.
At the minimal amount of cooling necessary to resolve the structure in terms of
instantons and anti-instantons we observe a dense ensemble where the total
number of peaks is by a factor 5-10 larger than the net charge. The average
size observed for these peaks at zero temperature is about 0.4-0.45 fm for
SU(2) and 0.5-0.6 fm for SU(3). The size distribution changes very little with
further cooling, although in this process up to 90% of the peaks disappear by
pair annihilation. For QCD we observe below T_c a reduction of the topological
susceptibility as an effect of the dynamical fermions. Nevertheless also here
the instantons form a dense ensemble with general characteristics similar to
those of the quenched theory. A further drop in the susceptibility above T_c is
also in rough agreement with what has been observed for pure SU(3). We see no
clear signal for dominant formation of instanton - anti-instanton molecules.Comment: Latex, 7 pages, 4 figures (one colour). Contribution to the 31st
International Symposium Ahrenshoop on the Theory of Elementary Particles,
Buckow, September 2-6, 199
Theory for the optimal control of time-averaged quantities in open quantum systems
We present variational theory for optimal control over a finite time interval
in quantum systems with relaxation. The corresponding Euler-Lagrange equations
determining the optimal control field are derived. In our theory the optimal
control field fulfills a high order differential equation, which we solve
analytically for some limiting cases. We determine quantitatively how
relaxation effects limit the control of the system. The theory is applied to
open two level quantum systems. An approximate analytical solution for the
level occupations in terms of the applied fields is presented. Different other
applications are discussed
A theoretical description of energy spectra and two-neutron separation energies for neutron-rich zirconium isotopes
Very recently the atomic masses of neutron-rich Zr isotopes, from Zr
to Zr, have been measured with high precision. Using a schematic
Interacting Boson Model (IBM) Hamiltonian, the evolution from spherical to
deformed shapes along the chain of Zr isotopes, describing at the same time the
excitation energies as well as the two-neutron separation energies, can be
rather well reproduced. The interplay between phase transitions and
configuration mixing of intruder excitations in this mass region is succinctly
addressed.Comment: Accepted in European Journal of Physics
Local topological and chiral properties of QCD
To elucidate the role played by instantons in chiral symmetry breaking, we
explore their properties in full QCD, around the critical temperature. We study
in particular spatial correlations between low-lying Dirac eigenmodes and
instantons. Our measurements are compared with the predictions of
instanton-based models.Comment: LATTICE98(confine), 3 pgs, 4 figures (3 color
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