14 research outputs found
Quantum Bose liquids with logarithmic nonlinearity: Self-sustainability and emergence of spatial extent
The Gross-Pitaevskii (GP) equation is a long-wavelength approach widely used
to describe the dilute Bose-Einstein condensates (BEC). However, in many
physical situations, such as higher densities, this approximation unlikely
suffices hence one might need models which would account for long-range
correlations and multi-body interactions. We show that the Bose liquid
described by the logarithmic wave equation has a number of drastic differences
from the GP one. It possesses the self-sustainability property: while the free
GP condensate tends to spill all over the available volume the logarithmic one
tends to form a Gaussian-type droplet - even in the absence of an external
trapping potential. The quasi-particle modes of the logarithmic BEC are shown
to acquire a finite size despite the bare particles being assumed point-like,
i.e., the spatial extent emerges here as a result of quantum many-body
correlations. Finally, we study the elementary excitations and demonstrate that
the background density changes the topological structure of their momentum
space which, in turn, affects their dispersion relations. Depending on the
density the latter can be of the massive relativistic, massless relativistic,
tachyonic and quaternionic type.Comment: 14 pages, 5 figures. Updates: v2: minor corrections (published
version
Dynamics of OH(2Pi)-He collisions in combined electric and magnetic fields
We use accurate quantum mechanical calculations to analyze the effects of
parallel electric and magnetic fields on collision dynamics of OH(2Pi)
molecules. It is demonstrated that spin relaxation in 3He-OH collisions at
temperatures below 0.01 K can be effectively suppressed by moderate electric
fields of order 10 kV/cm. We show that electric fields can be used to
manipulate Feshbach resonances in collisions of cold molecules. Our results can
be verified in experiments with OH molecules in Stark decelerated molecular
beams and electromagnetic traps.Comment: 20 pages, 5 figures, submitted to Faraday Discuss. 142: Cold and
Ultracold Molecule
Suppression of inelastic collisions of polar state molecules in an electrostatic field
Collisions of polar state molecules at ultralow energies are
considered, within a model that accounts for long-range dipole-dipole
interactions, plus rotation of the molecules. We predict a substantial
suppression of dipole-driven inelastic collisions at high values of the applied
electric field, namely, field values of several times . Here is
the rotational constant, and is the electric dipole moment of molecules.
The sudden large drop in the inelastic cross section is attributed to the
onset of degeneracy between molecular rotational levels, which dramatically
alters the scattering Hamiltonian. As a result of the large ratio of elastic to
inelastic collision rates, we predict that evaporative cooling may be feasible
for state molecules in weak-field-seeking states, provided a large
bias electric field is present
Field-linked States of Ultracold Polar Molecules
We explore the character of a novel set of ``field-linked'' states that were
predicted in [A. V. Avdeenkov and J. L. Bohn, Phys. Rev. Lett. 90, 043006
(2003)]. These states exist at ultralow temperatures in the presence of an
electrostatic field, and their properties are strongly dependent on the field's
strength. We clarify the nature of these quasi-bound states by constructing
their wave functions and determining their approximate quantum numbers. As the
properties of field-linked states are strongly defined by anisotropic dipolar
and Stark interactions, we construct adiabatic surfaces as functions of both
the intermolecular distance and the angle that the intermolecular axis makes
with the electric field. Within an adiabatic approximation we solve the 2-D
Schrodinger equation to find bound states, whose energies correlate well with
resonance features found in fully-converged multichannel scattering
calculations
Stability of fermionic Feshbach molecules in a Bose-Fermi mixture
In the wake of successful experiments in Fermi condensates, experimental
attention is broadening to study resonant interactions in degenerate Bose-Fermi
mixtures. Here we consider the properties and stability of the fermionic
molecules that can be created in such a mixture near a Feshbach resonance (FR).
To do this, we consider the two-body scattering matrix in the many-body
environment, and assess its complex poles. The stability properties of these
molecules strongly depend on their centre-of-mass motion, because they must
satisfy Fermi statistics. At low centre-of-mass momenta the molecules are more
stable than in the absence of the environment (due to Pauli-blocking effects),
while at high centre-of-mass momenta nontrivial many body effects render them
somewhat less stable
Hydrogen storage in aromatic carbon ring based molecular materials decorated with alkali or alkali-earth metals
On the basis of first-principles calculations of molecular electron structure, we discuss the strategy of modifying the carbon-based materials in order to increase their capacity to bind with molecular hydrogen. In particular, we have studied hydrogen adsorption on molecular complexes having anionic aromatic carbon-based rings stabilized by cations of alkali (Li+, Na+, K+) or alkali-earth metals (Be2+, Mg2+, Ca2+). The adsorption depends more on the properties of the cation than on the ring itself. The interaction of the H2 molecule with an electrostatic field leads to the binding of the hydrogen molecule with the strongly polarized ionic molecular complex. The number of the adsorbed molecules is driven by two factors acting in opposite directions: the binding energy, which should be larger than a 4–5 kJ/mol threshold needed to keep hydrogen molecules attached, and the area around the cation (coordination sphere), which is determined by its radius. As a compromise between these factors, we propose several promising candidates for building blocks of hydrogen storage materials, including diboratabenzene lithium, C4B2H6Li2, and diboratabenzene potassium, C4B2H6K2, which can adsorb 6 and 12 H2 molecules, correspondingly. We also discuss the possibility of linking these molecular complexes in three-dimensional structures.http://dx.doi.org/10.1021/jp305324phttp://pubs.acs.org/doi/pdf/10.1021/jp305324phttp://pubs.acs.org/doi/pdf/10.1021/jp305324
Modeling hydrogen storage in boron-substituted graphene decorated with potassium metal atoms
Boron-substituted graphene decorated with potassium metal atoms was considered as a novel material for hydrogen
storage. Density functional theory calculations were used to model key properties of the material, such as geometry,
hydrogen packing, and hydrogen adsorption energy. We found that the new material has extremely high hydrogen
storage capacity: 22.5wt%. It is explained by high-density packing of hydrogen molecules into hydrogen layers with
specific geometry. In turn, such geometry is determined by the composition and topology of the materialDepartment of
Science and Technology for funding through the Hydrogen
South Africa progra