199 research outputs found
Time-dependent transport through a T-coupled quantum dot
We are considering the time-dependent transport through a discrete system,
consiting of a quantum dot T-coupled to an infinite tight-binding chain. The
periodic driving that is induced on the coupling between the dot and the chain,
leads to the emergence of a characteristic multiple Fano resonant profile in
the transmission spectrum. We focus on investigating the underlying physical
mechanisms that give rise to the quantum resonances. To this end, we use
Floquet theory for calculating the transmission spectrum and in addition employ
the Geometric Phase Propagator (GPP) approach [Ann. Phys. 375, 351 (2016)] to
calculate the transition amplitudes of the time-resolved virtual processes, in
terms of which we describe the resonant behavior. This two fold approach,
allows us to give a rigorous definition of a quantum resonance in the context
of driven systems and explains the emergence of the characteristic Fano profile
in the transmission spectrum.Comment: 9 pages, 4 figure
Non-perturbative quenched propagator beyond the infrared approximation
A new approach to the quenched propagator in QED beyond the IR limit is
proposed. The method is based on evolution equations in the proper time.Comment: 13 pages, 1 figure; Misprint on reference correcte
Geometric-phase-propagator approach to time-dependent quantum systems
A field-theoretical approach to the scattering off an oscillating quantum system is developed. As a key ingredient it employs the adiabatic eigenstate basis and consists of a perturbative scheme for the calculation of the geometric phases influencing the transmission through the time-dependent potential landscape. The main advantage is the identification of basic diagrams which allow for an immediate interpretation of the underlying elementary physical processes contributing to the scattering and transmission behavior. We apply our method to the simple, but prototypical, problem of transmission through an one-dimensional oscillating δ potential and demonstrate how it enables a deeper understanding of the relevant physical processes
Theoretical evidence for a tachyonic ghost state contribution to the gluon propagator in high energy, forward quark-quark `scattering'
Implications stemming from the inclusion of non-perturbative, confinining
effects, as contained in the Stochastic Vacuum Model of Dosch and Simonov, are
considered in the context of a, hypothetical, quark-quark `scattering process'
in the Regge kinematical region. In a computation wherein the non-perturbative
input enters as a correction to established perturbative results, a careful
treatment of infrared divergencies is shown to imply the presence of an
effective propagator associated with the existence of a linear term in the
static potential. An equivalent statement is to say that the modified gluonic
propagator receives contribution from a tachyonic ghost state, an occurence
which is fully consistent with earlier such suggestions made in the context of
low energy QCD phenomenology.Comment: 14 page
Polaron Variational Methods In The Particle Representation Of Field Theory : I. General Formalism
We apply nonperturbative variational techniques to a relativistic scalar
field theory in which heavy bosons (``nucleons'') interact with light scalar
mesons via a Yukawa coupling. Integrating out the meson field and neglecting
the nucleon vacuum polarization one obtains an effective action in terms of the
heavy particle coordinates which is nonlocal in the proper time. As in
Feynman's polaron approach we approximate this action by a retarded quadratic
action whose parameters are to be determined variationally on the pole of the
two-point function. Several ans\"atze for the retardation function are studied
and for the most general case we derive a system of coupled variational
equations. An approximate analytic solution displays the instability of the
system for coupling constants beyond a critical value.Comment: 33 pages standard LaTeX, 3 uuencoded gzipped postscript figures
embedded with psfig.st
The Isgur-Wise function in a relativistic model for system
We use the Dirac equation with a ``(asymptotically free) Coulomb + (Lorentz
scalar) linear '' potential to estimate the light quark wavefunction for mesons in the limit . We use these wavefunctions to
calculate the Isgur-Wise function for orbital and radial
ground states in the phenomenologically interesting range . We find a simple expression for the zero-recoil slope, , where is the energy eigenvalue
of the light quark, which can be identified with the parameter
of the Heavy Quark Effective Theory. This result implies an upper bound of
for the slope . Also, because for a very light quark the size of the meson is determined mainly by the
``confining'' term in the potential , the shape of
is seen to be mostly sensitive to the dimensionless
ratio . We present results for the ranges of
parameters , and
light quark masses and compare to existing
experimental data and other theoretical estimates. Fits to the data give:
,
and [ARGUS
'93]; , and
[CLEO '93]; ${\bar\Lambda_{u,d}}^2/Comment: 22 pages, Latex, 4 figures (not included) available by fax or via
email upon reques
Quasiparticle dynamics in ferromagnetic compounds of the Co-Fe and Ni-Fe systems
We report a theoretical study of the quasiparticle lifetime and the
quasiparticle mean free path caused by inelastic electron-electron scattering
in ferromagnetic compounds of the Co-Fe and Ni-Fe systems. The study is based
on spin-polarized calculations, which are performed within the
approximation for equiatomic and Co- and Ni-rich compounds, as well as for
their constituents. We mainly focus on the spin asymmetry of the quasiparticle
properties, which leads to the spin-filtering effect experimentally observed in
spin-dependent transport of hot electrons and holes in the systems under study.
By comparing with available experimental data on the attenuation length, we
estimate the contribution of the inelastic mean free path to the latter.Comment: 10 pages, 10 figure
Taming Landau singularities in QCD perturbation theory: The analytic approach 2.0
The aim of this topical article is to outline the fundamental ideas
underlying the recently developed Fractional Analytic Perturbation Theory
(FAPT) of QCD and present its main calculational tools together with key
applications. For this, it is first necessary to review previous methods to
apply QCD perturbation theory at low spacelike momentum scales, where the
influence of the Landau singularities becomes inevitable. Several concepts are
considered and their limitations are pointed out. The usefulness of FAPT is
discussed in terms of two characteristic hadronic quantities: the
perturbatively calculable part of the pion's electromagnetic form factor in the
spacelike region and the Higgs-boson decay into a pair in the
timelike region. In the first case, the focus is on the optimization of the
prediction with respect to the choice of the renormalization scheme and the
dependence on the renormalization and the factorization scales. The second case
serves to show that the application of FAPT to this reaction reaches already at
the four-loop level an accuracy of the order of 1%, avoiding difficulties
inherent in the standard perturbative expansion. The obtained results are
compared with estimates from fixed-order and contour-improved QCD perturbation
theory. Using the brand-new Higgs mass value of about 125 GeV, measured at the
Large Hadron Collider (CERN), a prediction for is extracted.Comment: v3: 23 pages, 7 figures, Invited topical article published in
Particles and Nuclei with update using the CERN Higgs discovery. Abridged
version presented as plenary talk at International Conference on
Renormalization Group and Related Topics (RG 2008), Dubna, Russia, September
1 - 5, 2008. v4 typo in Eq. (3) correcte
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