168 research outputs found
Interaction between polar molecules subject to a far-off-resonant optical field: Entangled dipoles up- or down-holding each other
We show that the electric dipole-dipole interaction between a pair of polar
molecules undergoes an all-out transformation when superimposed by a far-off
resonant optical field. The combined interaction potential becomes tunable by
variation of wavelength, polarization and intensity of the optical field and
its dependence on the intermolecular separation exhibits a crossover from an
inverse-power to an oscillating behavior. The ability thereby offered to
control molecular interactions opens up avenues toward the creation and
manipulation of novel phases of ultracold polar gases among whose
characteristics is a long-range entanglement of the dipoles' mutual
orientation. We devised an accurate analytic model of such
optical-field-dressed dipole-dipole interaction potentials, which enables a
straightforward access to the optical-field parameters required for the design
of intermolecular interactions in the laboratory.Comment: 11 pages, 6 figures, 1 table. arXiv admin note: substantial text
overlap with arXiv:1104.104
Diagrammatic Monte Carlo approach to angular momentum in quantum many-particle systems
We introduce a Diagrammatic Monte Carlo (DiagMC) approach to angular momentum
properties of quantum many-particle systems possessing a macroscopic number of
degrees of freedom. The treatment is based on a diagrammatic expansion that
merges the usual Feynman diagrams with the angular momentum diagrams known from
atomic and nuclear structure theory, thereby incorporating the non-Abelian
algebra inherent to quantum rotations. Our approach is applicable at arbitrary
coupling, is free of systematic errors and of finite size effects, and
naturally provides access to the impurity Green function. We exemplify the
technique by obtaining an all-coupling solution of the angulon model, however,
the method is quite general and can be applied to a broad variety of systems in
which particles exchange quantum angular momentum with their many-body
environment.Comment: 6+5 pages, 2+2 figures, accepted for publication in Phys. Rev. Let
Strongly aligned molecules inside helium droplets in the near-adiabatic regime
Iodine (I) molecules embedded in He nanodroplets are aligned by a 160 ps
long laser pulse. The highest degree of alignment, occurring at the peak of the
pulse and quantified by , is measured as a
function of the laser intensity. The results are well described by calculated for a gas of isolated molecules each
with an effective rotational constant of 0.6 times the gas-phase value, and at
a temperature of 0.4 K. Theoretical analysis using the angulon quasiparticle to
describe rotating molecules in superfluid helium rationalizes why the alignment
mechanism is similar to that of isolated molecules with an effective rotational
constant. A major advantage of molecules in He droplets is that their 0.4 K
temperature leads to stronger alignment than what can generally be achieved for
gas phase molecules -- here demonstrated by a direct comparison of the droplet
results to measurements on a 1 K supersonic beam of isolated molecules.
This point is further illustrated for more complex system by measurements on
1,4-diiodobenzene and 1,4-dibromobenzene. For all three molecular species
studied the highest values of achieved in
He droplets exceed 0.96.Comment: 11 pages, 8 figure
Collisions of paramagnetic molecules in magnetic ļ¬elds: An analytic model based on Fraunhofer diļ¬raction of matter waves
We investigate the eļ¬ects of a magnetic ļ¬eld on the dynamics of rotationally inelastic collisions of open-shell molecules (Ā² Ī£, Ā³Ī£, and Ā²Ī ) with closed-shell atoms. Our treatment makes use of the Fraunhofer model of matter wave scattering and its recent extension to collisions in electric [M. Lemeshko and B. Friedrich, J. Chem. Phys. 129, 024301 (2008)] and radiative ļ¬elds [M. Lemeshko and B. Friedrich, Int. J. Mass. Spec. in press (2008)]. A magnetic ļ¬eld aligns the molecule in the space-ļ¬xed frame and thereby alters the eļ¬ective shape of the diļ¬raction target. This signiļ¬cantly aļ¬ects the diļ¬erential and integral scattering cross sections. We exemplify our treatment by evaluating the magnetic-ļ¬eld-dependent scattering characteristics of the He ā CaH (XĀ² Ī£āŗ), He ā Oā (XĀ³Ī£ā») and He ā OH (XĀ²Ī Ī© ) systems at thermal collision energies. Since the cross sections can be obtained fordiļ¬erent orientations of the magnetic ļ¬eld with respect to the relative velocity vector, the model also oļ¬ers predictions about the frontal-versus-lateral steric asymmetry of the collisions. The steric asymmetry is found to be almost negligible for the He ā OH system, weak for the He ā CaH collisions, and strong for the He ā Oā . While odd āM transitions dominate the He ā OH (J = 3/2, f ā Jā² , e/f ) integral cross sections in a magnetic ļ¬eld parallel to the relative velocity vector, even āM transitions prevail in the case of the He ā CaH (XĀ²Ī£āŗ) and He ā Oā (XĀ³Ī£ā») collision systems. For the latter system, the magnetic ļ¬eld opens inelastic channels that are closed in the absence of the ļ¬eld. These involve the transitions N = 1, J = 0 ā Nā², Jā² with Jā² = Nā²
An analytic model of the stereodynamics of rotationally inelastic molecular collisions
We develop an analytic model of vector correlations in rotationally inelastic atom-diatom collisions and test it against the much examined Ar--NO (XĀ²Ī ) system. Based on the Fraunhofer scattering of matter waves, the model furnishes complex scattering amplitudes needed to evaluate the polarization moments characterizing the quantum stereodynamics. The analytic polarization moments are found to be in an excellent agreement with experimental results and with close-coupling calculations available at thermal energies. The model reveals that the stereodynamics is governed by diffraction from the repulsive core of the Ar--NO potential, which can be characterized by a single Legendre moment
Quantum many-body dynamics of the Einstein-de Haas effect
In 1915, Einstein and de Haas and Barnett demonstrated that changing the
magnetization of a magnetic material results in mechanical rotation, and vice
versa. At the microscopic level, this effect governs the transfer between
electron spin and orbital angular momentum, and lattice degrees of freedom,
understanding which is key for molecular magnets, nano-magneto-mechanics,
spintronics, and ultrafast magnetism. Until now, the timescales of
electron-to-lattice angular momentum transfer remain unclear, since modeling
this process on a microscopic level requires addition of an infinite amount of
quantum angular momenta. We show that this problem can be solved by
reformulating it in terms of the recently discovered angulon quasiparticles,
which results in a rotationally invariant quantum many-body theory. In
particular, we demonstrate that non-perturbative effects take place even if the
electron--phonon coupling is weak and give rise to angular momentum transfer on
femtosecond timescales.Comment: 15 pages, 5 figure
Exotic roton excitations in quadrupolar Bose-Einstein condensates
We investigate the occurrence of rotons in a quadrupolar Bose-Einstein
condensate confined to two dimensions. Depending on the particle density, the
ratio of the contact and quadrupole-quadrupole interactions, and the alignment
of the quadrupole moments with respect to the confinement plane, the dispersion
relation features two or four point-like roton minima, or one ring-shaped
minimum. We map out the entire parameter space of the roton behavior and
identify the instability regions. We propose to observe the exotic rotons by
monitoring the characteristic density wave dynamics resulting from a short
local perturbation, and discuss the possibilities to detect the predicted
effects in state-of-the-art experiments with ultracold homonuclear molecules
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