1,209,841 research outputs found
Systematic renormalization scheme in light-front dynamics with Fock space truncation
Within the framework of the covariant formulation of light-front dynamics, we
develop a general non-perturbative renormalization scheme based on the Fock
decomposition of the state vector and its truncation. The counterterms and bare
parameters needed to renormalize the theory depend on the Fock sectors. We
present a general strategy in order to calculate these quantities, as well as
state vectors of physical systems, in a truncated Fock space. The explicit
dependence of our formalism on the orientation of the light front plane is
essential in order to analyze the structure of the counterterms. We apply our
formalism to the two-body (one fermion and one boson) truncation in the Yukawa
model and in QED, and to the three-body truncation in a scalar model. In QED,
we recover analytically, without any perturbative expansion, the
renormalization of the electric charge, according to the requirements of the
Ward identity.Comment: 32 pages, 14 figures, submitted in Phys. Rev.
Metastability and paramagnetism in superconducting mesoscopic disks
A projected order parameter is used to calculate, not only local minima of
the Ginzburg-Landau energy functional, but also saddle points or energy
barriers responsible for the metastabilities observed in superconducting
mesoscopic disks (Geim et al. Nature {\bf 396}, 144 (1998)). We calculate the
local minima magnetization and find the energetic instability points between
vortex configurations with different vorticity. We also find that, for any
vorticity, the supercurrent can reverse its flow direction on decreasing the
magnetic field before one vortex can escape.Comment: Modified version as to appear in Phys. Rev. Let
Highly charged ions with E1, M1, and E2 transitions within laser range
Level crossings in the ground state of ions occur when the nuclear charge Z
and ion charge Z_ion are varied along an isoelectronic sequence until the two
outermost shells are nearly degenerate. We examine all available level
crossings in the periodic table for both near neutral ions and highly charged
ions (HCIs). Normal E1 transitions in HCIs are in X-ray range, however level
crossings allow for optical electromagnetic transitions that could form the
reference transition for high accuracy atomic clocks. Optical E1 (due to
configuration mixing), M1 and E2 transitions are available in HCIs near level
crossings. We present scaling laws for energies and amplitudes that allow us to
make simple estimates of systematic effects of relevance to atomic clocks. HCI
clocks could have some advantages over existing optical clocks because certain
systematic effects are reduced, for example they can have much smaller thermal
shifts. Other effects such as fine-structure and hyperfine splitting are much
larger in HCIs, which can allow for richer spectra. HCIs are excellent
candidates for probing variations in the fine-structure constant, alpha, in
atomic systems as there are transitions with the highest sensitivity to
alpha-variation
Thermal broadening of the J-band in disordered linear molecular aggregates: A theoretical study
We theoretically study the temperature dependence of the J-band width in
disordered linear molecular aggregates, caused by dephasing of the exciton
states due to scattering on vibrations of the host matrix. In particular, we
consider inelastic one- and two-phonon scattering between different exciton
states (energy-relaxation-induced dephasing), as well as elastic two-phonon
scattering of the excitons (pure dephasing). The exciton states follow from
numerical diagonalization of a Frenkel Hamiltonian with diagonal disorder; the
scattering rates between them are obtained using the Fermi Golden Rule. A
Debye-like model for the one- and two-phonon spectral densities is used in the
calculations. We find that, owing to the disorder, the dephasing rates of the
individual exciton states are distributed over a wide range of values. We also
demonstrate that the dominant channel of two-phonon scattering is not the
elastic one, as is often tacitly assumed, but rather comes from a similar
two-phonon inelastic scattering process. In order to study the temperature
dependence of the J-band width, we simulate the absorption spectrum, accounting
for the dephasing induced broadening of the exciton states. We find a power-law
(T^p) temperature scaling of the effective homogeneous width, with an exponent
p that depends on the shape of the spectral density of host vibrations. In
particular, for a Debye model of vibrations, we find p ~ 4, which is in good
agreement with experimental data on J-aggregates of pseudoisocyanine [J. Phys.
Chem. A 101, 7977 (1997)].Comment: 14 pages, 7 figure
Ab initio nonperturbative calculation of physical observables in light-front dynamics. Application to the Yukawa model
We present a coherent and operational strategy to calculate, in a
nonperturbative way, physical observables in light-front dynamics. This
strategy is based on the decomposition of the state vector of any compound
system in Fock components, and on the covariant formulation of light-front
dynamics, together with the so-called Fock sector dependent renormalization
scheme. We apply our approach to the calculation of the electromagnetic form
factors of a fermion in the Yukawa model, in the nontrivial three-body Fock
space truncation, for rather large values of the coupling constant. We find
that, once the renormalization conditions are properly taken into account, the
form factors do not depend on the regularization scale, when the latter is much
larger than the physical masses. We then extend the Fock space by including
antifermion degrees of freedom.Comment: 22 pages, 16 figure
Exchange interaction and Fano resonances in diatomic molecular systems
We propose a mechanism to use scanning tunneling microscopy (STM) for direct
measurements of the two-electron singlet-triplet exchange splitting in
diatomic molecular systems, unsing the coupling between the molecule and the
substrate electrons. The different pathways for electrons lead to interference
effects and generate kinks in the differential conductance at the energies for
the singlet and triplet states. These features are related to Fano resonance
due to the branched electron wave functions. The ratio between the tunneling
amplitudes through the two atoms can be modulated by spatial movements of the
tip along the surface.Comment: 4 pages, 2 figures, submitted - Changes in Fig. 1 (panel c) added),
and minor modification in the main text - new version, as publishe
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