481 research outputs found
Accelerating the Fourier split operator method via graphics processing units
Current generations of graphics processing units have turned into highly
parallel devices with general computing capabilities. Thus, graphics processing
units may be utilized, for example, to solve time dependent partial
differential equations by the Fourier split operator method. In this
contribution, we demonstrate that graphics processing units are capable to
calculate fast Fourier transforms much more efficiently than traditional
central processing units. Thus, graphics processing units render efficient
implementations of the Fourier split operator method possible. Performance
gains of more than an order of magnitude as compared to implementations for
traditional central processing units are reached in the solution of the time
dependent Schr\"odinger equation and the time dependent Dirac equation
Is Transparency to No Avail? Committee Decision-Making, Pre-Meetings, and Credible Deals
Transparent decision-making processes are widely regarded as a prerequisite for the working of a representative democracy. It facilitates accountability, and citizens may suspect that decisions, if taken behind closed doors, do not promote their interests. Why else the secrecy? We provide a model of committee decision-making that explains the public.s demand for transparency, and committee members. aversion to it. In line with case study evidence, we show how pressures to become transparent induce committee members to organize pre-meetings away from the public eye. Outcomes of pre-meetings, deals, are less determined, more anarchic, than those of formal meetings, but within bounds. We characterize deals that are self-enforcing in the formal meeting.Committee decision-making, reputational concerns, transparency, pre-meetings, deliberation, self-enforcing deals, coalitions
Decision Making and Learning in a Globalizing World
Decision-makers can benefit from the experience of others with solutions to common problems. If a best practice exists, the challenge is to recognize it and to ensure its diffusion. Information about different solutions is often dispersed, and decision-makers may be reluctant to switch for reputational reasons. We study how (i) the assignment of decision rights (who decides on the solutions.implementation?) and (ii) globalization (who knows what about solutions adopted in other places?) in.uence both the quality of the information on locally adopted solutions that decision-makers exchange and the quality of the solutions that are actually being used next.centralization, decentralization, learning, cheap talk, reputational concerns, globalization, health care consensus panels, EU Open Method of Coordination
Electron-spin dynamics induced by photon spins
Strong rotating magnetic fields may cause a precession of the electron's spin
around the rotation axis of the magnetic field. The superposition of two
counterpropagating laser beams with circular polarization and opposite helicity
features such a rotating magnetic field component but also carries spin. The
laser's spin density, that can be expressed in terms of the lase's
electromagnetic fields and potentials, couples to the electron's spin via a
relativistic correction to the Pauli equation. We show that the quantum
mechanical interaction of the electron's spin with the laser's rotating
magnetic field and with the laser's spin density counteract each other in such
a way that a net spin rotation remains with a precession frequency that is much
smaller than the frequency one would expect from the rotating magnetic field
alone. In particular, the frequency scales differently with the laser's
electric field strength depending on if relativistic corrections are taken into
account or not. Thus, the relativistic coupling of the electron's spin to the
laser's spin density changes the dynamics not only quantitatively but also
qualitatively as compared to the nonrelativistic theory. The electron's spin
dynamics is a genuine quantum mechanical relativistic effect
Ionization Time and Exit Momentum in Strong-Field Tunnel Ionization
Tunnel ionization belongs to the fundamental processes of atomic physics. The
so-called two-step model, which describes the ionization as instantaneous
tunneling at the electric field maximum and classical motion afterwards with
zero exit momentum, is commonly employed to describe tunnel ionization in
adiabatic regimes. In this contribution, we show by solving numerically the
time-dependent Schr\"odinger equation in one dimension and employing a virtual
detector at the tunnel exit that there is a nonvanishing positive time delay
between the electric field maximum and the instant of ionization. Moreover, we
find a nonzero exit momentum in the direction of the electric field. To extract
proper tunneling times from asymptotic momentum distributions of ionized
electrons, it is essential to incorporate the electron's initial momentum in
the direction of the external electric field
Spin dynamics in relativistic light-matter interaction
Various spin effects are expected to become observable in light-matter
interaction at relativistic intensities. Relativistic quantum mechanics
equipped with a suitable relativistic spin operator forms the theoretical
foundation for describing these effects. Various proposals for relativistic
spin operators have been offered by different authors, which are presented in a
unified way. As a result of the operators' mathematical properties only the
Foldy-Wouthuysen operator and the Pryce operator qualify as possible proper
relativistic spin operators. The ground states of highly charged hydrogen-like
ions can be utilized to identify a legitimate relativistic spin operator
experimentally. Subsequently, the Foldy-Wothuysen spin operator is employed to
study electron-spin precession in high-intensity standing light waves with
elliptical polarization. For a correct theoretical description of the predicted
electron-spin precession relativistic effects due to the spin angular momentum
of the electromagnetic wave has to be taken into account even in the limit of
low intensities
Kapitza-Dirac effect in the relativistic regime
A relativistic description of the Kapitza-Dirac effect in the so-called Bragg
regime with two and three interacting photons is presented by investigating
both numerical and perturbative solutions of the Dirac equation in momentum
space. We demonstrate that spin-flips can be observed in the two-photon and the
three-photon Kapitza-Dirac effect for certain parameters. During the
interaction with the laser field the electron's spin is rotated, and we give
explicit expressions for the rotation axis and the rotation angle. The
off-resonant Kapitza-Dirac effect, that is, when the Bragg condition is not
exactly fulfilled, is described by a generalized Rabi theory. We also analyze
the in-field quantum dynamics as obtained from the numerical solution of the
Dirac equation.Comment: minor correction
Relativistic spin operators in various electromagnetic environments
Different operators have been suggested in the literature to describe the
electron's spin degree of freedom within the relativistic Dirac theory. We
compare concrete predictions of the various proposed relativistic spin
operators in different physical situations. In particular, we investigate the
so-called Pauli, Foldy-Wouthuysen, Czachor, Frenkel, Chakrabarti, Pryce, and
Fradkin-Good spin operators. We demonstrate that when a quantum system
interacts with electromagnetic potentials the various spin operators predict
different expectation values. This is explicitly illustrated for the scattering
dynamics at a potential step and in a standing laser field and also for energy
eigenstates of hydrogenic ions. Therefore, one may distinguish between the
proposed relativistic spin operators experimentally
Spin effects in strong-field laser-electron interactions
The electron spin degree of freedom can play a significant role in
relativistic scattering processes involving intense laser fields. In this
contribution we discuss the influence of the electron spin on (i) Kapitza-Dirac
scattering in an x-ray laser field of high intensity, (ii) photo-induced
electron-positron pair production in a strong laser wave and (iii) multiphoton
electron-positron pair production on an atomic nucleus. We show that in all
cases under consideration the electron spin can have a characteristic impact on
the process properties and their total probabilities. To this end,
spin-resolved calculations based on the Dirac equation in the presence of an
intense laser field are performed. The predictions from Dirac theory are also
compared with the corresponding results from the Klein-Gordon equation.Comment: 9 pages, 6 figure
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