115 research outputs found

### Manipulating scattering of ultracold atoms with light-induced dissipation

Recently it has been shown that pairs of atoms can form metastable bonds due
to non-conservative forces induced by dissipation [Lemeshko&Weimer, Nature
Comm. 4, 2230 (2013)]. Here we study the dynamics of interaction-induced
coherent population trapping - the process responsible for the formation of
dissipatively bound molecules. We derive the effective dissipative potentials
induced between ultracold atoms by laser light, and study the time evolution of
the scattering states. We demonstrate that binding occurs on short timescales
of ~10 microseconds, even if the initial kinetic energy of the atoms
significantly exceeds the depth of the dissipative potential.
Dissipatively-bound molecules with preordained bond lengths and vibrational
wavefunctions can be created and detected in current experiments with ultracold
atoms.Comment: Associate Editor's inaugural article for Frontiers in Physical
Chemistry and Chemical Physic

### Rotational structure of weakly bound molecular ions

Relying on the quantization rule of Raab and Friedrich [Phys. Rev. A (2009)
in press], we derive simple and accurate formulae for the number of rotational
states supported by a weakly bound vibrational level of a diatomic molecular
ion. We also provide analytic estimates of the rotational constants of any such
levels up to threshold for dissociation and obtain a criterion for determining
whether a given weakly bound vibrational level is rotationless. The results
depend solely on the long-range part of the molecular potential.Comment: 4 pages, 1 figur

### Anomalous screening of quantum impurities by a neutral environment

It is a common knowledge that an effective interaction of a quantum impurity
with an electromagnetic field can be screened by surrounding charge carriers,
whether mobile or static. Here we demonstrate that very strong, `anomalous'
screening can take place in the presence of a neutral, weakly-polarizable
environment, due to an exchange of orbital angular momentum between the
impurity and the bath. Furthermore, we show that it is possible to generalize
all phenomena related to isolated impurities in an external field to the case
when a many-body environment is present, by casting the problem in terms of the
angulon quasiparticle. As a result, the relevant observables such as the
effective Rabi frequency, geometric phase, and impurity spatial alignment are
straightforward to evaluate in terms of a single parameter: the
angular-momentum-dependent screening factor.Comment: 6 pages, 2 figures including appendi

### Anyonic statistics of quantum impurities in two dimensions

We demonstrate that identical impurities immersed in a two-dimensional
many-particle bath can be viewed as flux-tube-charged-particle composites
described by fractional statistics. In particular, we find that the bath
manifests itself as an external magnetic flux tube with respect to the
impurities, and hence the time-reversal symmetry is broken for the effective
Hamiltonian describing the impurities. The emerging flux tube acts as a
statistical gauge field after a certain critical coupling. This critical
coupling corresponds to the intersection point between the quasiparticle state
and the phonon wing, where the angular momentum is transferred from the
impurity to the bath. This amounts to a novel configuration with emerging
anyons. The proposed setup paves the way to realizing anyons using electrons
interacting with superfluid helium or lattice phonons, as well as using atomic
impurities in ultracold gases.Comment: 6 pages, 2 figur

### A model analysis of rotationally inelastic Ar + H$_2$O scattering in an electric field

We develop an analytic model of thermal state-to-state rotationally inelastic
collisions of asymmetric-top molecules with closed-shell atoms in electric
fields, and apply it to the Ar--H$_2$O collision system. The predicted cross
sections as well as the steric asymmetry of the collisions show at fields up to
150 kV/cm characteristic field-dependent features which can be experimentally
tested. Particularly suitable candidates for such tests are the $0_{00} \to
2_{20}$ and $1_{01} \to 2_{21}$ channels, arising from the relaxation of the
field-free selection rules due to the hybridization of $J$ states. Averaging
over the $M'$ product channels is found to largely obliterate the orientation
effects brought about by the field.Comment: 22 pages, 1 table, 7 figure

### Fine-tuning molecular energy levels by nonresonant laser pulses

We evaluate the shifts imparted to vibrational and rotational levels of a
linear molecule by a nonresonant laser field at intensities of up to 10^12
W/cm^2. Both types of shift are found to be either positive or negative,
depending on the initial rotational state acted upon by the field. An adiabatic
field-molecule interaction imparts a rotational energy shift which is negative
and exceeds the concomitant positive vibrational shift by a few orders of
magnitude. The rovibrational states are thus pushed downward in such a field. A
nonresonant pulsed laser field that interacts nonadiabatically with the
molecule is found to impart rotational and vibrational shifts of the same order
of magnitude. The nonadiabatic energy transfer occurs most readily at a pulse
duration which amounts to about a tenth of the molecule's rotational period,
and vanishes when the sudden regime is attained for shorter pulses. We applied
our treatment to the much studied 87Rb_2 molecule in the last bound vibrational
levels of its lowest singlet and triplet electronic states. Our calculations
indicate that 15 ns and 1.5 ns laser pulses of an intensity in excess of 5x10^9
W/cm^2 are capable of dissociating the molecule due to the vibrational shift.
Lesser shifts can be used to fine tune the rovibrational levels and thereby to
affect collisional resonances by the nonresonant light. The energy shifts may
be discernible spectroscopically, at a 10 MHz resolution.Comment: 9 pages, 9 figures, 4 table

### Deformation of a quantum many-particle system by a rotating impurity

During the last 70 years, the quantum theory of angular momentum has been
successfully applied to describing the properties of nuclei, atoms, and
molecules, their interactions with each other as well as with external fields.
Due to the properties of quantum rotations, the angular momentum algebra can be
of tremendous complexity even for a few interacting particles, such as valence
electrons of an atom, not to mention larger many-particle systems. In this
work, we study an example of the latter: a rotating quantum impurity coupled to
a many-body bosonic bath. In the regime of strong impurity-bath couplings the
problem involves addition of an infinite number of angular momenta which
renders it intractable using currently available techniques. Here, we introduce
a novel canonical transformation which allows to eliminate the complex angular
momentum algebra from such a class of many-body problems. In addition, the
transformation exposes the problem's constants of motion, and renders it
solvable exactly in the limit of a slowly-rotating impurity. We exemplify the
technique by showing that there exists a critical rotational speed at which the
impurity suddenly acquires one quantum of angular momentum from the
many-particle bath. Such an instability is accompanied by the deformation of
the phonon density in the frame rotating along with the impurity.Comment: 12 pages, 4 figures; revised version, section on experimental
implementation adde

### Effect of a magnetic field on molecule-solvent angular momentum transfer

Recently it was shown that a molecule rotating in a quantum solvent can be
described in terms of the `angulon' quasiparticle [Phys. Rev. Lett. 118, 095301
(2017)]. Here we extend the angulon theory to the case of molecules possessing
an additional spin-1/2 degree of freedom and study the behavior of the system
in the presence of a static magnetic field. We show that exchange of angular
momentum between the molecule and the solvent can be altered by the field, even
though the solvent itself is non-magnetic. In particular, we demonstrate a
possibility to control resonant emission of phonons with a given angular
momentum using a magnetic field.Comment: 8+5 pages; 4 figures; accepted versio

### Rotation of quantum impurities in the presence of a many-body environment

We develop a microscopic theory describing a quantum impurity whose
rotational degree of freedom is coupled to a many-particle bath. We approach
the problem by introducing the concept of an 'angulon' - a quantum rotor
dressed by a quantum field - and reveal its quasiparticle properties using a
combination of variational and diagrammatic techniques. Our theory predicts
renormalisation of the impurity rotational structure, such as observed in
experiments with molecules in superfluid helium droplets, in terms of a
rotational Lamb shift induced by the many-particle environment. Furthermore, we
discover a rich many-body-induced fine structure, emerging in rotational
spectra due to a redistribution of angular momentum within the quantum
many-body system.Comment: 5 pages, 2 figures; revised version, supplementary adde

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