445 research outputs found
Post-Prior discrepancies in CDW-EIS calculations for ion impact ionization fully differential cross sections
In this work we present fully differential cross sections (FDCSs)
calculations using post and prior version of CDW--EIS theory for helium single
ionization by 100 MeV C amu and 3.6 MeV amu Au and
Au ions. We performed our calculations for different momentum transfer
and ejected electron energies. The influence of internuclear potential on the
ejected electron spectra is taken into account in all cases. We compare our
calculations with absolute experimental measurements. It is shown that prior
version calculations give better agreement with experiments in almost all
studied cases.Comment: 9 pages, 7 figure
Inverse problem for the Landau-Zener effect
We consider the inverse Landau-Zener problem which consists in finding the
energy-sweep functions W(t)=E1(t)-E2(t) resulting in the required time
dependences of the level populations for a two-level system crossing the
resonance one or more times during the sweep. We find sweep functions of
particular forms that let manipulate the system in a required way, including
complete switching from the state 1 to the state 2 and preparing the system at
the exact ground and excited states at resonance.Comment: 7 EPL pages, 6 figure
Symmetric eikonal model for projectile-electron excitation and loss in relativistic ion-atom collisions
At impact energies GeV/u the projectile-electron
excitation and loss occurring in collisions between highly charged ions and
neutral atoms is already strongly influenced by the presence of atomic
electrons. In order to treat these processes in collisions with heavy atoms we
generalize the symmetric eikonal model, used earlier for considerations of
electron transitions in ion-atom collisions within the scope of a three-body
Coulomb problem. We show that at asymptotically high collision energies this
model leads to an exact transition amplitude and is very well suited to
describe the projectile-electron excitation and loss at energies above a few
GeV/u. In particular, by considering a number of examples we demonstrate
advantages of this model over the first Born approximation at impact energies
--30 GeV/u, which are of special interest for atomic physics
experiments at the future GSI facilities.Comment: 14 pages, 5 figure
Thermally activated escape rates of uniaxial spin systems with transverse field
Classical escape rates of uniaxial spin systems are characterized by a
prefactor differing from and much smaller than that of the particle problem,
since the maximum of the spin energy is attained everywhere on the line of
constant latitude: theta=const, 0 =< phi =< 2*pi. If a transverse field is
applied, a saddle point of the energy is formed, and high, moderate, and low
damping regimes (similar to those for particles) appear. Here we present the
first analytical and numerical study of crossovers between the uniaxial and
other regimes for spin systems. It is shown that there is one HD-Uniaxial
crossover, whereas at low damping the uniaxial and LD regimes are separated by
two crossovers.Comment: 4 PR pages, 3 figures, final published versio
Landau-Zener-Stueckelberg effect in a model of interacting tunneling systems
The Landau-Zener-Stueckelberg (LZS) effect in a model system of interacting
tunneling particles is studied numerically and analytically. Each of N
tunneling particles interacts with each of the others with the same coupling J.
This problem maps onto that of the LZS effect for a large spin S=N/2. The
mean-field limit N=>\infty corresponds to the classical limit S=>\infty for the
effective spin. It is shown that the ferromagnetic coupling J>0 tends to
suppress the LZS transitions. For N=>\infty there is a critical value of J
above which the staying probability P does not go to zero in the slow sweep
limit, unlike the standard LZS effect. In the same limit for J>0 LZS
transitions are boosted and P=0 for a set of finite values of the sweep rate.
Various limiting cases such as strong and weak interaction, slow and fast sweep
are considered analytically. It is shown that the mean-field approach works
well for arbitrary N if the interaction J is weak.Comment: 13 PR pages, 15 Fig
Statistical Mechanics for Unstable States in Gel'fand Triplets and Investigations of Parabolic Potential Barriers
Free energies and other thermodynamical quantities are investigated in
canonical and grand canonical ensembles of statistical mechanics involving
unstable states which are described by the generalized eigenstates with complex
energy eigenvalues in the conjugate space of Gel'fand triplet. The theory is
applied to the systems containing parabolic potential barriers (PPB's). The
entropy and energy productions from PPB systems are studied. An equilibrium for
a chemical process described by reactions is also
discussed.Comment: 14 pages, AmS-LaTeX, no figur
Field dependence of the temperature at the peak of the ZFC magnetization
The effect of an applied magnetic field on the temperature at the maximum of
the ZFC magnetization, , is studied using the recently obtained
analytic results of Coffey et al. (Phys. Rev. Lett. {\bf 80}(1998) 5655) for
the prefactor of the N\'{e}el relaxation time which allow one to precisely
calculate the prefactor in the N\'{e}el-Brown model and thus the blocking
temperature as a function of the coefficients of the Taylor series expansion of
the magnetocrystalline anisotropy. The present calculations indicate that even
a precise determination of the prefactor in the N\'{e}el-Brown theory, which
always predicts a monotonic decrease of the relaxation time with increasing
field, is insufficient to explain the effect of an applied magnetic field on
the temperature at the maximum of the ZFC magnetization. On the other hand, we
find that the non linear field-dependence of the magnetization along with the
magnetocrystalline anisotropy appears to be of crucial importance to the
existence of this maximum.Comment: 14 LaTex209 pages, 6 EPS figures. To appear in J. Phys.: Condensed
Matte
Bound-bound pair production in relativistic collisions
Electron-positron pair production is considered in the relativistic collision
of a nucleus and an anti-nucleus, in which both leptons are created in bound
states of the corresponding nucleus-lepton system. Compared to free and
bound-free pair production this process is shown to display a qualitatively
different dependency both on the impact energy and charged of the colliding
particles. Interestingly, at high impact energies the cross section for this
process is found to be larger than that for the analogous atomic process of
non-radiative electron capture although the latter does not involve the
creation of new particles.Comment: 4 pages, 3 figure
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