6,212 research outputs found
A simple model illustrating the impossibility of measuring off-shell effects
We consider a simple model and make use of field transformations in
Lagrangian field theories to illustrate the impossibility of measuring
off-shell effects in nucleon-nucleon bremsstrahlung and related processes.Comment: 3 pages, Latex2e, uses espcrc1.sty, contribution presented at the
16th International Conference on Few-Body Problems in Physics, Taipei,
Taiwan, 6-10 March 200
Optimized time-dependent perturbation theory for pulse-driven quantum dynamics in atomic or molecular systems
We present a time-dependent perturbative approach adapted to the treatment of
intense pulsed interactions. We show there is a freedom in choosing secular
terms and use it to optimize the accuracy of the approximation. We apply this
formulation to a unitary superconvergent technique and improve the accuracy by
several orders of magnitude with respect to the Magnus expansion.Comment: 4 pages, 2 figure
Strong Coupling Theory of Two Level Atoms in Periodic Fields
We present a new convergent strong coupling expansion for two-level atoms in
external periodic fields, free of secular terms. As a first application, we
show that the coherent destruction of tunnelling is a third-order effect. We
also present an exact treatment of the high-frequency region, and compare it
with the theory of averaging. The qualitative frequency spectrum of the
transition probability amplitude contains an effective Rabi frequency.Comment: 4 pages with 3 figure
Antiferromagnetic behavior in CeCoGe
We investigate the novel intermetallic ternary compounds
\emph{R}CoGe with \emph{R} = La and Ce by means of -ray
diffraction, susceptibility and specific heat measurements. CeCoGe
crystallizes in the space group 4/ and is characterized by the
coexistence of two different magnetic sublattices. The Ce-based sublattice,
with an effective moment close to the expected value for a Ce-ion,
exhibits a magnetically ordered ground state with K. The
Co-based sublattice, however, exhibits magnetic moments due to itinerant 3
electrons. The magnetic specific heat contribution of the Ce-sublattice is
discussed in terms of a resonance-level model implying the interplay between an
antiferromagnetic phase transition and the Kondo-effect and an underlying
Schottky-anomaly indicating a crystal field level scheme splitting into three
twofold degenerated micro states ( K, K).Comment: 4 pages, 3 figures, conference SCES0
Statistical Mechanics of semi-classical colored Objects
A microscopic model of deconfined matter based on color interactions between
semi-classical quarks is studied. A hadronization mechanism is imposed to
examine the properties and the disassembly of a thermalized quark plasma and to
investigate the possible existence of a phase transition from quark matter to
hadron matter.Comment: Submitted to Phys. Lett.
Direct picosecond time resolution of unimolecular reactions initiated by local mode excitation
The concept of local mode (LM) states [1] in large molecules raises the possibilty of inducing chemical reactions from a well-defined initial state (bond-selective chemistry). The results of linewidth and energy measurements in gases, [2(a)] and low temperature solids, [2(b)] however, indicate that the relaxation times for such high energy (> 15000 cm^-1) states can be extremely short, < 1ps. Because of the lack of direct time-resolved measurements, the following fundamental questions have not been unequivocally answered: What are the homogeneous linewidths of LM states and what are the rates of energy relaxation or reaction out of these states? Over the past five years we have made several attempts to observe the picosecond dynamics of LM states. Due to the inherent difficulties associated with making these measurements, such as the very small oscillator strength (Ï < 10^-23 cm^2), an extremely sensitive probing technique becomes imperative
Crossover from Single-Ion to Coherent Non-Fermi Liquid Behavior in CeLaNiGe
We report specific heat and magneto-resistance studies on the compound
CeLaNiGe for various concentrations over the entire
stoichiometric range. Our data reveal single-ion scaling with Ce-concentration
between and 0.95. Furthermore, CeNiGe turns out to have
the largest ever recorded value of the electronic specific heat 5.5 J at K which was found in Cerium
f-electron lattice systems. In the doped samples increases
logarithmically in the temperature range between 3 K and 50 mK typical for
non-Fermi liquid (nFl) behavior, while exhibits a Kondo-like minimum
around 30 K, followed by a single-ion local nFl behavior. In contrast to this,
CeNiGe flattens out in below 300 mK and displays a
pronounced maximum in the resistivity curve at 1.5 K indicating a coherent
heavy fermion groundstate. These properties render the compound
CeLaNiGe a unique system on the borderline between
Fermi liquid and nFl physics.Comment: 2 pages, 3 figures, SCES0
Competing magnetic interactions in CeNi9-xCoxGe4
CeNi9Ge4 exhibits outstanding heavy fermion features with remarkable
non-Fermi- liquid behavior which is mainly driven by single-ion effects. The
substitution of Ni by Cu causes a reduction of both, the RKKY coupling and
Kondo interaction, coming along with a dramatic change of the crystal field
(CF) splitting. Thereby a quasi-quartet ground state observed in CeNi9Ge4
reduces to a two-fold degenerate one in CeNi8CuGe4. This leads to a
modiffcation of the effective spin degeneracy of the Kondo lattice ground state
and to the appearance of antiferromagnetic (AFM) order. To obtain a better
understanding of consequences resulting from a reduction of the effective spin
degeneracy, we stepwise replaced Ni by Co. Thereby an increase of the Kondo and
RKKY interactions through the reduction of the effective d-electron count is
expected. Accordingly, a paramagnetic Fermi liquid ground state should arise.
Our experimental studies, however, reveal AFM order already for small Co
concentrations, which becomes even more pronounced with increasing Co content
x. Thereby the modiffcation of the effective spin degeneracy seems to play a
crucial role in this system
Quantitative effects of abrupt deceleration on pulmonary diffusion in man Technical report,
Quantitative effects of abrupt deceleration on pulmonary diffusion in ma
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