858 research outputs found
Cosmological Solutions of Higher-Curvature String Effective Theories with Dilatons
We study the effect of higher-curvature terms in the string low-energy
effective actions on the cosmological solutions of the theory, up to
corrections quartic in the curvatures, for the bosonic and heterotic strings as
well as the type II superstring. We find that cosmological solutions exist for
all string types but they always disappear when the dilaton field is included,
a conclusion that can be avoided if string-loop effects are taken into account.Comment: 7 pages, plain Tex with panda.tex macro (included), no figure
Electron correlation and interference effects in strong-field processes
Several correlation and interference effects in strong-field physics are
investigated. We show that the interference of continuum wave packets can be
the dominant mechanism of high-harmonic generation (HHG) in the
over-the-barrier regime. Next, we combine HHG with resonant x-ray excitation to
force the recolliding continuum electron to recombine with a core hole rather
than the valence hole from that it was previously tunnel ionized. The scheme
opens up perspectives for nonlinear xuv physics, attosecond x-ray pulses, and
spectroscopy of core orbitals. Then, a method is proposed to generate
attochirp-free harmonic pulses by engineering the appropriate electron wave
packet. Finally, resonant photoionization mechanisms involving two atoms are
discussed which can dominate over the direct single-atom ionization channel at
interatomic distances in the nanometer range.Comment: to be published in Springer Proceedings "Multiphoton Processes and
Attosecond Physics
Symmetry Properties on Magnetization in the Hubbard Model at Finite Temperatures
By making use of some symmetry properties of the relevant Hamiltonian, two
fundamental relations between the ferromagnetic magnetization and a spin
correlation function are derived for the -dimensional Hubbard model
at finite temperatures. These can be viewed as a kind of Ward-Takahashi
identities. The properties of the magnetization as a function of the applied
field are discussed. The results thus obtained hold true for both repulsive and
attractive on-site Coulomb interactions, and for arbitrary electron fillings.Comment: Latex file, no figur
Noise-Driven Mechanism for Pattern Formation
We extend the mechanism for noise-induced phase transitions proposed by
Ibanes et al. [Phys. Rev. Lett. 87, 020601-1 (2001)] to pattern formation
phenomena. In contrast with known mechanisms for pure noise-induced pattern
formation, this mechanism is not driven by a short-time instability amplified
by collective effects. The phenomenon is analyzed by means of a modulated mean
field approximation and numerical simulations
Two-site dynamical mean-field theory
It is shown that a minimum realization of the dynamical mean-field theory
(DMFT) can be achieved by mapping a correlated lattice model onto an impurity
model in which the impurity is coupled to an uncorrelated bath that consists of
a single site only. The two-site impurity model can be solved exactly. The
mapping is approximate. The self-consistency conditions are constructed in a
way that the resulting ``two-site DMFT'' reduces to the previously discussed
linearized DMFT for the Mott transition. It is demonstrated that a reasonable
description of the mean-field physics is possible with a minimum computational
effort. This qualifies the simple two-site DMFT for a systematic study of more
complex lattice models which cannot be treated by the full DMFT in a feasible
way. To show the strengths and limitations of the new approach, the single-band
Hubbard model is investigated in detail. The predictions of the two-site DMFT
are compared with results of the full DMFT. Internal consistency checks are
performed which concern the Luttinger sum rule, other Fermi-liquid relations
and thermodynamic consistency.Comment: LaTeX, 14 pages, 8 eps figures included, Phys. Rev. B (in press
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