6,763 research outputs found
Constrained Dynamics for Quantum Mechanics I. Restricting a Particle to a Surface
We analyze constrained quantum systems where the dynamics do not preserve the
constraints. This is done in particular for the restriction of a quantum
particle in Euclidean n-space to a curved submanifold, and we propose a method
of constraining and dynamics adjustment which produces the right Hamiltonian on
the submanifold when tested on known examples. This method we hope will become
the germ of a full Dirac algorithm for quantum constraints. We take a first
step in generalising it to the situation where the constraint is a general
selfadjoint operator with some additional structures.Comment: 49 pages, TEX, input files amssym.def, amssym.te
Nanomechanics of a Hydrogen Molecule Suspended between Two Equally Charged Tips
Geometric configuration and energy of a hydrogen molecule centered between
two point-shaped tips of equal charge are calculated with the variational
quantum Monte-Carlo (QMC) method without the restriction of the
Born-Oppenheimer (BO) approximation. Ground state nuclear distribution,
stability, and low vibrational excitation are investigated. Ground state
results predict significant deviations from the BO treatment that is based on a
potential energy surface (PES) obtained with the same QMC accuracy. The quantum
mechanical distribution of molecular axis direction and bond length at a
sub-nanometer level is fundamental for understanding nanomechanical dynamics
with embedded hydrogen. Because of the tips' arrangement, cylindrical symmetry
yields a uniform azimuthal distribution of the molecular axis vector relative
to the tip-tip axis. With approaching tips towards each other, the QMC sampling
shows an increasing loss of spherical symmetry with the molecular axis still
uniformly distributed over the azimuthal angle but peaked at the tip-tip
direction for negative tip charge while peaked at the equatorial plane for
positive charge. This directional behavior can be switched between both stable
configurations by changing the sign of the tip charge and by controlling the
tip-tip distance. This suggests an application in the field of molecular
machines.Comment: 20 pages, 10 figure
Anisotropic Temperature Factors in the Calculation of Low-Energy Electron Diffraction Intensities
Effective calculation of LEED intensities using symmetry-adapted functions
The calculation of LEED intensities in a spherical-wave representation can be substantially simplified by symmetry relations. The wave field around each atom is expanded in symmetry-adapted functions where the local point symmetry of the atomic site applies. For overlayer systems with more than one atom per unit cell symmetry-adapted functions can be used when the division of the crystal into monoatomic subplanes is replaced by division into subplanes containing all symmetrically equivalent atomic positions
Intramolecular Torque, an Indicator of the Internal Rotation Direction of Rotor Molecules and Similar Systems
Torque is ubiquitous in many molecular systems, including collisions,
chemical reactions, vibrations, electronic excitations and especially rotor
molecules. We present a straightforward theoretical method based on forces
acting on atoms and obtained from atomistic quantum mechanics calculations, to
quickly and qualitatively determine whether a molecule or sub-unit thereof has
a tendency to rotation and, if so, around which axis and in which sense:
clockwise or counterclockwise. The method also indicates which atoms, if any,
are predominant in causing the rotation. Our computational approach can in
general efficiently provide insights into the rotational ability of many
molecules and help to theoretically screen or modify them in advance of
experiments or before analyzing their rotational behavior in more detail with
more extensive computations guided by the results from the torque approach. As
an example, we demonstrate the effectiveness of the approach using a specific
light-driven molecular rotary motor which was successfully synthesized and
analyzed in prior experiments and simulations.Comment: 11 pages, 4 figures, 1 SI fil
Effect of bonding of a CO molecule on the conductance of atomic metal wires
We have measured the effect of bonding of a CO molecule on the conductance of
Au, Cu, Pt, and Ni atomic contacts at 4.2 K. When CO gas is admitted to the
metal nano contacts, a conductance feature appears in the conductance histogram
near 0.5 of the quantum unit of conductance, for all metals. For Au, the
intensity of this fractional conductance feature can be tuned with the bias
voltage, and it disappears at high bias voltage (above 200 mV). The
bonding of CO to Au appears to be weakest, and associated with monotomic Au
wire formation.Comment: 6 figure
On Quantization of Polynomial Momentum Observables
The paper is devoted to quantization of polynomial momentum observables in
the cotangent bundle of a smooth manifold. A quantization procedure is proposed
allowing to quantize a wide class of functions which are polynomials of any
order in momenta. In the last part of the paper the quantum mechanics of scalar
particle in curved space-time is studied with the use of proposed approach.Comment: LaTeX 2.09, 8
Clan structure analysis and new physics signals in pp collisions at LHC
The study of possible new physics signals in global event properties in pp
collisions in full phase space and in rapidity intervals accessible at LHC is
presented. The main characteristic is the presence of an elbow structure in
final charged particle MD's in addition to the shoulder observed at lower c.m.
energies.Comment: 9 pages, talk given at Focus on Multiplicity (Bari, Italy, June 2004
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