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Search Results: 1 - 10 of 542491 matches for " C. M. Dion "
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Molecular Alignment and Orientation: From Laser-Induced Mechanisms to Optimal Control
O. Atabek,C. M. Dion
Physics , 2002,
Abstract: Genetic algorithms, as implemented in optimal control strategies, are currently successfully exploited in a wide range of problems in molecular physics. In this context, laser control of molecular alignment and orientation remains a very promising issue with challenging applications extending from chemical reactivity to nanoscale design. We emphasize the complementarity between basic quantum mechanisms monitoring alignment/orientation processes and optimal control scenarios. More explicitly, if on one hand we can help the optimal control scheme to take advantage of such mechanisms by appropriately building the targets and delineating the parameter sampling space, on the other hand we expect to learn, from optimal control results, some robust and physically sound dynamical mechanisms. We present basic mechanisms for alignment and orientation, such as pendular states accommodated by the molecule-plus-field effective potential and the "kick" mechanism obtained by a sudden excitation. Very interestingly, an optimal control scheme for orientation, based on genetic algorithms, also leads to a sudden pulsed field bearing the characteristic features of the kick mechanism. Optimal pulse shaping for very efficient and long-lasting orientation, together with robustness with respect to temperature effects, are among our future prospects.
Rotational dynamics of a diatomic molecular ion in a Paul trap
A. Hashemloo,C. M. Dion
Physics , 2015, DOI: 10.1063/1.4936425
Abstract: We present models for a heteronuclear diatomic molecular ion in a linear Paul trap in a rigid-rotor approximation, one purely classical, the other where the center-of-mass motion is treated classically while rotational motion is quantized. We study the rotational dynamics and their influence on the motion of the center-of-mass, in the presence of the coupling between the permanent dipole moment of the ion and the trapping electric field. We show that the presence of the permanent dipole moment affects the trajectory of the ion, and that it departs from the Mathieu equation solution found for atomic ions. For the case of quantum rotations, we also evidence the effect of the above-mentioned coupling on the rotational states of the ion.
Breaking the symmetry of a Brownian motor with symmetric potentials
H. Hagman,M. Zelan,C. M. Dion
Physics , 2010, DOI: 10.1088/1751-8113/44/15/155002
Abstract: The directed transport of Brownian particles requires a system with an asymmetry and with non-equilibrium noise. We here investigate numerically alternative ways of fulfilling these requirements for a two-state Brownian motor, realised with Brownian particles alternating between two phase-shifted, symmetric potentials. We show that, besides the previously known spatio-temporal asymmetry based on unequal transfer rates between the potentials, inequalities in the potential depths, the frictions, or the equilibrium temperatures of the two potentials also generate the required asymmetry. We also show that the effects of the thermal noise and the noise of the transfer's randomness depend on the way the asymmetry is induced.
Simulations of Sisyphus cooling including multiple excited states
F. Svensson,S. Jonsell,C. M. Dion
Physics , 2008, DOI: 10.1140/epjd/e2008-00080-8
Abstract: We extend the theory for laser cooling in a near-resonant optical lattice to include multiple excited hyperfine states. Simulations are performed treating the external degrees of freedom of the atom, i.e., position and momentum, classically, while the internal atomic states are treated quantum mechanically, allowing for arbitrary superpositions. Whereas theoretical treatments including only a single excited hyperfine state predict that the temperature should be a function of lattice depth only, except close to resonance, experiments have shown that the minimum temperature achieved depends also on the detuning from resonance of the lattice light. Our results resolve this discrepancy.
Wave-packet dynamics of an atomic ion in a Paul trap: approximations and stability
A. Hashemloo,C. M. Dion,G. Rahali
Physics , 2013, DOI: 10.1142/S0129183116500145
Abstract: Using numerical simulations of the time-dependent Schr\"odinger equation, we study the full quantum dynamics of the motion of an atomic ion in a linear Paul trap. Such a trap is based on a time-varying, periodic electric field, and hence corresponds to a time-dependent potential for the ion, which we model exactly. We compare the center of mass motion with that obtained from classical equations of motion, as well as to results based on a time-independent effective potential. We also study the oscillations of the width of the ion's wave packet, including close to the border between stable (bounded) and unstable (unbounded) trajectories. Our results confirm that the center-of-mass motion always follow the classical trajectory, that the width of the wave packet is bounded for trapping within the stability region, and therefore that the classical trapping criterion are fully applicable to quantum motion.
Program for quantum wave-packet dynamics with time-dependent potentials
C. M. Dion,A. Hashemloo,G. Rahali
Physics , 2013, DOI: 10.1016/j.cpc.2013.09.012
Abstract: We present a program to simulate the dynamics of a wave packet interacting with a time-dependent potential. The time-dependent Schr\"odinger equation is solved on a one-, two-, or three-dimensional spatial grid using the split operator method. The program can be compiled for execution either on a single processor or on a distributed-memory parallel computer.
On the Mechanism of Townsend Avalanche for Negative Molecular Ions
M. P. Dion,C. J. Martoff,M. Hosack
Physics , 2009, DOI: 10.1016/j.astropartphys.2010.02.002
Abstract: Time projection chambers drifting negative ions (NITPC) instead of electrons have several advantages. A NITPC can operate at very high reduced drift fields without diffusion runaway, and the readout digitization sampling rate requirement is considerably relaxed due to the low drift speed of negative ions. The initiation of Townsend avalanches to allow gas gain in these devices has not been understood until now. It is shown here that the avalanche in low pressure CS$_2$ vapor is most likely initiated by collisional detachment of the electron from the negative molecular ion. In mixtures of Nitromethane vapor with CO$_2$ the mechanism appears to be more complex.
Multidimensional instability and dynamics of spin-avalanches in crystals of nanomagnets
O. Jukimenko,C. M. Dion,M. Marklund,V. Bychkov
Physics , 2014, DOI: 10.1103/PhysRevLett.113.217206
Abstract: We obtain a fundamental instability of the magnetization-switching fronts in super-paramagnetic and ferromagnetic materials such as crystals of nanomagnets, ferromagnetic nanowires, and systems of quantum dots with large spin. We develop the instability theory for both linear and nonlinear stages. By using numerical simulations we investigate the instability properties focusing on spin avalanches in crystals of nanomagnets. The instability distorts spontaneously the fronts and leads to a complex multidimensional front dynamics. We show that the instability has a universal physical nature, with a deep relationship to a wide variety of physical systems, such as the Darrieus-Landau instability of deflagration fronts in combustion, inertial confinement fusion and thermonuclear su- pernovae, and the instability of doping fronts in organic semiconductors.
Directed transport with real-time steering and drifts along pre-designed paths using a Brownian motor
H. Hagman,M. Zelan,C. M. Dion,A. Kastberg
Physics , 2011, DOI: 10.1103/PhysRevE.83.020101
Abstract: We have realized real-time steering of the directed transport in a Brownian motor based on cold atoms in optical lattices, and demonstrate drifts along pre-designed paths. The transport is induced by spatiotemporal asymmetries in the system, where we can control the spatial part, and we show that the response to changes in asymmetry is very fast. In addition to the directional steering, a real-time control of the magnitude of the average drift velocity and an on/off switching of the motor are also demonstrated. We use a non-invasive real-time detection of the transport, enabling a feedback control of the system.
Counterpart of the Darrieus-Landau instability at a magnetic deflagration front
O. Jukimenko,M. Modestov,C. M. Dion,M. Marklund,V. Bychkov
Physics , 2015,
Abstract: The magnetic instability at the front of the spin avalanche in a crystal of molecular magnets is considered. This phenomenon reveals similar features with the Darrieus-Landau instability, inherent to classical combustion flame fronts. The instability growth rate and the cut-off wavelength are investigated with respect to the strength of the external magnetic field, both analytically in the limit of an infinitely thin front and numerically for finite-width fronts. The presence of quantum tunneling resonances is shown to increase the growth rate significantly, which may lead to a possible transition from deflagration to detonation regimes. Different orientations of the crystal easy axis are shown to exhibit opposite stability properties. In addition, we suggest experimental conditions that could evidence the instability and its influence on the magnetic deflagration velocity.
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