1,230 research outputs found
Boltzmann Collision Term
We derive the Boltzmann equation for scalar fields using the
Schwinger-Keldysh formalism. The focus lies on the derivation of the collision
term. We show that the relevant self-energy diagrams have a factorization
property. The collision term assumes the Boltzmann-like form of scattering
probability times statistical factors for those self-energy diagrams which
correspond to tree level scattering processes. Our proof covers scattering
processes with any number of external particles, which come from self-energy
diagrams with any number of loops.Comment: 17 pages, 4 figure
Separation of Equilibration Time-Scales in the Gradient Expansion
We study thermalization by applying gradient expansion to the Kadanoff-Baym
equations of the 2PI effective action to two-loop in a theory with Dirac
fermions coupled to scalars. In addition to those chemical potentials which
equilibrate in the on-shell limit, we identify modes which are conserved in
this approximation, but which relax when off-shell effects are taken into
account. This implies that chemical equilibration does not require higher loop
contributions to the effective action and is compatible with the gradient
expansion. We explicitly calculate the damping time-scales of both, on- and
off-shell, chemical equilibration rates. It is shown that off-shell
equilibration is suppressed by the thermal width of the particles in the
plasma, which explains the separation of on- and off-shell chemical
equilibration time-scales.Comment: 20 pages, 3 figures, published versio
Nonequilibrium dynamical mean-field calculations based on the non-crossing approximation and its generalizations
We solve the impurity problem which arises within nonequilibrium dynamical
mean-field theory for the Hubbard model by means of a self-consistent
perturbation expansion around the atomic limit. While the lowest order, known
as the non-crossing approximation (NCA), is reliable only when the interaction
U is much larger than the bandwidth, low-order corrections to the NCA turn out
to be sufficient to reproduce numerically exact Monte Carlo results in a wide
parameter range that covers the insulating phase and the metal-insulator
crossover regime at not too low temperatures. As an application of the
perturbative strong-coupling impurity solver we investigate the response of the
double occupancy in the Mott insulating phase of the Hubbard model to a
dynamical change of the interaction or the hopping, a technique which has been
used as a probe of the Mott insulating state in ultracold fermionic gases.Comment: 14 pages, 9 figure
Measuring correlated electron dynamics with time-resolved photoemission spectroscopy
Time-resolved photoemission experiments can reveal fascinating quantum
dynamics of correlated electrons. However, the thermalization of the electronic
system is typically so fast that very short probe pulses are necessary to
resolve the time evolution of the quantum state, and this leads to poor energy
resolution due to the energy-time uncertainty relation. Although the
photoemission intensity can be calculated from the nonequilibrium electronic
Green functions, the converse procedure is therefore difficult. We analyze a
hypothetical time-resolved photoemission experiment on a correlated electronic
system, described by the Falicov-Kimball model in dynamical mean-field theory,
which relaxes between metallic and insulating phases. We find that the
real-time Green function which describes the transient behavior during the
buildup of the metallic state cannot be determined directly from the
photoemission signal. On the other hand, the characteristic
collapse-and-revival oscillations of an excited Mott insulator can be observed
as oscillating weight in the center of the Mott gap in the time-dependent
photoemission spectrum.Comment: 12 pages, 5 figure
The convolution theorem for nonlinear optics
We have expressed the nonlinear optical absorption of a semiconductor in
terms of its linear spectrum. We determined that the two-photon absorption
coefficient in a strong DC-electric field of a direct gap semiconductor can be
expressed as the product of a differential operator times the convolution
integral of the linear absorption without a DC-electric field and an Airy
function. We have applied this formalism to calculate the two-photon absorption
coefficient and nonlinear refraction for GaAs and ZnSe using their linear
absorption and have found excellent agreement with available experimental data.Comment: 8 pages, 2 figures (6 sub fugures
Metal nanofilm in strong ultrafast optical fields
We predict that a metal nanofilm subjected to an ultrashort (single
oscillation) optical pulse of a high field amplitude at
normal incidence undergoes an ultrafast (at subcycle times ) transition to a state resembling semimetal. Its reflectivity is
greatly reduced, while the transmissivity and the optical field inside the
metal are greatly increased. The temporal profiles of the optical fields are
predicted to exhibit pronounced subcycle oscillations, which are attributed to
the Bloch oscillations and formation of the Wannier-Stark ladder of electronic
states. The reflected, transmitted, and inside-the-metal pulses have non-zero
areas approaching half-cycle pulses. The effects predicted are promising for
applications to nanoplasmonic modulators and field-effect transistors with
petahertz bandwidth
Time-dependent density-functional theory for ultrafast interband excitations
We formulate a time-dependent density functional theory (TDDFT) in terms of
the density matrix to study ultrafast phenomena in semiconductor structures. A
system of equations for the density matrix components, which is equivalent to
the time-dependent Kohn-Sham equation, is derived. From this we obtain a TDDFT
version of the semiconductor Bloch equations, where the electronic many-body
effects are taken into account in principle exactly. As an example, we study
the optical response of a three-dimensional two-band insulator to an external
short-time pulsed laser field. We show that the optical absorption spectrum
acquires excitonic features when the exchange-correlation potential contains a
Coulomb singularity. A qualitative comparison of the TDDFT optical
absorption spectra with the corresponding results obtained within the
Hartree-Fock approximation is made
Conventional character of the BCS-BEC cross-over in ultra-cold gases of 40K
We use the standard fermionic and boson-fermion Hamiltonians to study the
BCS-BEC cross-over near the 202 G resonance in a two-component mixture of
fermionic 40K atoms employed in the experiment of C.A. Regal et al., Phys. Rev.
Lett. 92, 040403 (2004). Our mean-field analysis of many-body equilibrium
quantities shows virtually no differences between the predictions of the two
approaches, provided they are both implemented in a manner that properly
includes the effect of the highest excited bound state of the background
scattering potential, rather than just the magnetic-field dependence of the
scattering length. Consequently, we rule out the macroscopic occupation of the
molecular field as a mechanism behind the fermionic pair condensation and show
that the BCS-BEC cross-over in ultra-cold 40K gases can be analysed and
understood on the same basis as in the conventional systems of solid state
physics.Comment: 16 pages, 10 eps figures; final versio
Forming and confining of dipolar excitons by quantizing magnetic fields
We show that a magnetic field perpendicular to an AlGaAs/GaAs coupled quantum
well efficiently traps dipolar excitons and leads to the stabilization of the
excitonic formation and confinement in the illumination area. Hereby, the
density of dipolar excitons is remarkably enhanced up to . By means of Landau level spectroscopy we study the density of excess
holes in the illuminated region. Depending on the excitation power and the
applied electric field, the hole density can be tuned over one order of
magnitude up to - a value comparable with typical
carrier densities in modulation-doped structures.Comment: 4.3 Pages, 4 Figure
Stochastic dynamics of magnetization in a ferromagnetic nanoparticle out of equilibrium
We consider a small metallic particle (quantum dot) where ferromagnetism
arises as a consequence of Stoner instability. When the particle is connected
to electrodes, exchange of electrons between the particle and the electrodes
leads to a temperature- and bias-driven Brownian motion of the direction of the
particle magnetization. Under certain conditions this Brownian motion is
described by the stochastic Landau-Lifshitz-Gilbert equation. As an example of
its application, we calculate the frequency-dependent magnetic susceptibility
of the particle in a constant external magnetic field, which is relevant for
ferromagnetic resonance measurements.Comment: 15 pages, 6 figure
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