17,420 research outputs found
New mechanism for impurity-induced step bunching
Codeposition of impurities during the growth of a vicinal surface leads to an
impurity concentration gradient on the terraces, which induces corresponding
gradients in the mobility and the chemical potential of the adatoms. Here it is
shown that the two types of gradients have opposing effects on the stability of
the surface: Step bunching can be caused by impurities which either lower the
adatom mobility, or increase the adatom chemical potential. In particular,
impurities acting as random barriers (without affecting the adatom binding)
cause step bunching, while for impurities acting as random traps the
combination of the two effects reduces to a modification of the attachment
boundary conditions at the steps. In this case attachment to descending steps,
and thus step bunching, is favored if the impurities bind adatoms more weakly
than the substrate.Comment: 7 pages, 3 figures. Substantial revisions and correction
Histogram comparison as a powerful tool for the search of new physics at LHC. Application to CMSSM
We propose a rigorous and effective way to compare experimental and
theoretical histograms, incorporating the different sources of statistical and
systematic uncertainties. This is a useful tool to extract as much information
as possible from the comparison between experimental data with theoretical
simulations, optimizing the chances of identifying New Physics at the LHC. We
illustrate this by showing how a search in the CMSSM parameter space, using
Bayesian techniques, can effectively find the correct values of the CMSSM
parameters by comparing histograms of events with multijets + missing
transverse momentum displayed in the effective-mass variable. The procedure is
in fact very efficient to identify the true supersymmetric model, in the case
supersymmetry is really there and accessible to the LHC
Energy weighted sum rules for mesons in hot and dense matter
We study energy weighted sum rules of the pion and kaon propagator in nuclear
matter at finite temperature. The sum rules are obtained from matching the
Dyson form of the meson propagator with its spectral Lehmann representation at
low and high energies. We calculate the sum rules for specific models of the
kaon and pion self-energy. The in-medium spectral densities of the K and anti-K
mesons are obtained from a chiral unitary approach in coupled channels which
incorporates the S- and P-waves of the kaon-nucleon interaction. The pion
self-energy is determined from the P-wave coupling to particle-hole and
Delta-hole excitations, modified by short range correlations. The sum rules for
the lower energy weights are fulfilled satisfactorily and reflect the
contributions from the different quasi-particle and collective modes of the
meson spectral function. We discuss the sensitivity of the sum rules to the
distribution of spectral strength and their usefulness as quality tests of
model calculations.Comment: 19 pages, 6 figures; one figure added, enhanced discussion, version
to appear in PR
Quasiparticle Breakdown and Spin Hamiltonian of the Frustrated Quantum Pyrochlore YbTiO in Magnetic Field
The frustrated pyrochlore magnet YbTiO has the remarkable
property that it orders magnetically, but has no propagating magnons over wide
regions of the Brillouin zone. Here we use inelastic neutron scattering to
follow how the spectrum evolves in cubic-axis magnetic fields. At high fields
we observe in addition to dispersive magnons also a two-magnon continuum, which
grows in intensity upon reducing the field and overlaps with the one-magnon
states at intermediate fields leading to strong renormalization of the
dispersion relations, and magnon decays. Using heat capacity measurements we
find that the low and high field regions are smoothly connected with no sharp
phase transition, with the spin gap increasing monotonically in field. Through
fits to an extensive data set we re-evaluate the spin Hamiltonian finding
dominant quantum exchange terms, which we propose are responsible for the
anomalously strong fluctuations and quasiparticle breakdown effects observed at
low fields.Comment: 5 pages main text + 19 pages supplemental materia
Nondispersive analytical solutions to the Dirac equation
This paper presents new analytic solutions to the Dirac equation employing a recently introduced method that is based on the formulation of spinorial fields and their driving electromagnetic fields in terms of geometric algebras. A first family of solutions describe the shape-preserving translation of a wavepacket along any desired trajectory in the x-y plane. In particular, we show that the dispersionless motion of a Gaussian wavepacket along both elliptical and circular paths can be achieved with rather simple electromagnetic field configurations. A second family of solutions involves a plane electromagnetic wave and a combination of generally inhomogeneous electric and magnetic fields. The novel analytical solutions of the Dirac equation given here provide important insights into the connection between the quantum relativistic dynamics of electrons and the underlying geometry of the Lorentz group
Excitations in the quantum paramagnetic phase of the quasi-one-dimensional Ising magnet CoNbO in a transverse field: Geometric frustration and quantum renormalization effects
The quasi-one-dimensional (1D) Ising ferromagnet CoNbO has recently
been driven via applied transverse magnetic fields through a continuous quantum
phase transition from spontaneous magnetic order to a quantum paramagnet, and
dramatic changes were observed in the spin dynamics, characteristic of weakly
perturbed 1D Ising quantum criticality. We report here extensive single-crystal
inelastic neutron scattering measurements of the magnetic excitations
throughout the three-dimensional (3D) Brillouin zone in the quantum
paramagnetic phase just above the critical field to characterize the effects of
the finite interchain couplings. In this phase, we observe that excitations
have a sharp, resolution-limited line shape at low energies and over most of
the dispersion bandwidth, as expected for spin-flip quasiparticles. We map the
full bandwidth along the strongly dispersive chain direction and resolve clear
modulations of the dispersions in the plane normal to the chains,
characteristic of frustrated interchain couplings in an antiferromagnetic
isosceles triangular lattice. The dispersions can be well parametrized using a
linear spin-wave model that includes interchain couplings and further neighbor
exchanges. The observed dispersion bandwidth along the chain direction is
smaller than that predicted by a linear spin-wave model using exchange values
determined at zero field, and this effect is attributed to quantum
renormalization of the dispersion beyond the spin-wave approximation in fields
slightly above the critical field, where quantum fluctuations are still
significant.Comment: 11 pages, 6 figures. Updated references. Minor changes to text and
figure
Strange and charm mesons at FAIR
We study the properties of strange and charm mesons in hot and dense matter
within a self-consistent coupled-channel approach for the experimental
conditions of density and temperature expected for the CBM experiment at
FAIR/GSI. The in-medium solution at finite temperature accounts for Pauli
blocking effects, mean-field binding of all the baryons involved, and meson
self-energies. We analyze the behaviour in this hot and dense environment of
dynamically-generated baryonic resonances together with the evolution with
density and temperature of the strange and open-charm meson spectral functions.
We test the spectral functions for strange mesons using energy-weighted sum
rules and finally discuss the implications of the properties of charm mesons on
the D_{s0}(2317) and the predicted X(3700) scalar resonances.Comment: 12 pages, 9 figures, invited talk at XXXI Mazurian Lakes Conference
on Physics: Nuclear Physics and the Road to FAIR, August 30-September 6,
2009, Piaski, Polan
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