105 research outputs found
OCS in small para-hydrogen clusters: energetics and structure with N=1-8 complexed hydrogen molecules
We determine the structure and energetics of complexes of the linear OCS
molecule with small numbers of para-hydrogen molecules, N=1-8, using zero
temperature quantum Monte Carlo methods. Ground state calculations are carried
out with importance-sampled rigid body diffusion Monte Carlo (IS-RBDMC) and
excited state calculations with the projection operator imaginary time spectral
evolution (POITSE) methodology. The ground states are found to be highly
structured, with a gradual build up of two axial rings as N increases to 8.
Analysis of the azimuthal density correlations around the OCS molecule shows
that these rings are quite delocalized for small N values, but become strongly
localized for N \geq 5 . Excited state calculations are made for a range of
total cluster angular momentum values and the rotational energy levels fitted
to obtain effective rotational and distortion constants of the complexed OCS
molecule as a function of cluster size N. Detailed analysis of these
spectroscopic constants indicates that the complexes of OCS with para-hydrogen
have an unusually rich variation in dynamical behavior, with sizes N=1-2
showing near rigid behavior, sizes N=3-4 showing extremely floppy behavior, and
the larger sizes N=5-8 showing more rigid behavior again. The large values of
the distortion constant D obtained for N=3-4 are rationalized in terms of the
coupling between the OCS rotations and the "breathing" mode of the first,
partially filled ring of para-hydrogen molecules.Comment: 26 pages, 11 figures. accepted for publication in the Journal of
Chemical Physic
Microscopic description of anisotropic low-density dipolar Bose gases in two dimensions
A microscopic description of the zero energy two-body ground state and
many-body static properties of anisotropic homogeneous gases of bosonic dipoles
in two dimensions at low densities is presented and discussed. By changing the
polarization angle with respect to the plane, we study the impact of the
anisotropy, present in the dipole--dipole interaction, on the energy per
particle, comparing the results with mean field predictions. We restrict the
analysis to the regime where the interaction is always repulsive, although the
strength of the repulsion depends on the orientation with respect to the
polarization field. We present a series expansion of the solution of the zero
energy two-body problem which allows us to find the scattering length of the
interaction and to build a suitable Jastrow factor that we use as a trial wave
function for both a variational and diffusion Monte Carlo simulation of the
infinite system. We find that the anisotropy has an almost negligible impact on
the ground state properties of the many-body system in the universal regime
where the scattering length governs the physics of the system. We also show
that scaling in the gas parameter persists in the dipolar case up to values
where other isotropic interactions with the same scattering length yield
different predictions.Comment: 9 figures, 1 tabl
Quantum phases of dipolar rotors on two-dimensional lattices
The quantum phase transitions of dipoles confined to the vertices of two
dimensional (2D) lattices of square and triangular geometry is studied using
path integral ground state quantum Monte Carlo (PIGS). We analyze the phase
diagram as a function of the strength of both the dipolar interaction and a
transverse electric field. The study reveals the existence of a class of
orientational phases of quantum dipolar rotors whose properties are determined
by the ratios between the strength anisotropic dipole-dipole interaction, the
strength of the applied transverse field, and the rotational constant. For the
triangular lattice, the generic orientationally disordered phase found at zero
and weak values of both dipolar interaction strength and applied field, is
found to show a transition to a phase characterized by net polarization in the
lattice plane as the strength of the dipole-dipole interaction is increased,
independent of the strength of the applied transverse field, in addition to the
expected transition to a transverse polarized phase as the electric field
strength increases. The square lattice is also found to exhibit a transition
from a disordered phase to an ordered phase as the dipole-dipole interaction
strength is increased, as well as the expected transition to a transverse
polarized phase as the electric field strength increases. In contrast to the
situation with a triangular lattice, on square lattices the ordered phase at
high dipole-dipole interaction strength possesses a striped ordering. The
properties of these quantum dipolar rotor phases are dominated by the
anisotropy of the interaction and provide useful models for developing quantum
phases beyond the well-known paradigms of spin Hamiltonian models, realizing in
particular a novel physical realization of a quantum rotor-like Hamiltonian
that possesses an anisotropic long range interaction.Comment: Updated credit line and changed line spacin
Concentration Dependence of the Effective Mass of He-3 Atoms in He-3/He-4 Mixtures
Recent measurements by Yorozu et al. (S. Yorozu, H. Fukuyama, and H.
Ishimoto, Phys. Rev. B 48, 9660 (1993)) as well as by Simons and Mueller (R.
Simons and R. M. Mueller, Czhechoslowak Journal of Physics Suppl. 46, 201
(1976)) have determined the effective mass of He-3 atoms in a He-3/He-4 mixture
with great accuracy. We here report theoretical calculations for the dependence
of that effective mass on the He-3 concentration. Using correlated basis
functions perturbation theory to infinite order to compute effective
interactions in the appropriate channels, we obtain good agreement between
theory and experiment.Comment: 4 pages, 1 figur
Single Particle and Fermi Liquid Properties of He-3/--He-4 Mixtures: A Microscopic Analysis
We calculate microscopically the properties of the dilute He-3 component in a
He-3/--He-4 mixture. These depend on both, the dominant interaction between the
impurity atom and the background, and the Fermi liquid contribution due to the
interaction between the constituents of the He-3 component. We first calculate
the dynamic structure function of a He-3 impurity atom moving in He-3. From
that we obtain the excitation spectrum and the momentum dependent effective
mass. The pole strength of this excitation mode is strongly reduced from the
free particle value in agreement with experiments; part of the strength is
distributed over high frequency excitations. Above k > 1.7^{-1}$ the
motion of the impurity is damped due to the decay into a roton and a low energy
impurity mode. Next we determine the Fermi--Liquid interaction between He-4
atoms and calculate the pressure-- and concentration dependence of the
effective mass, magnetic susceptibility, and the He-3--He-3 scattering phase
shifts. The calculations are based on a dynamic theory that uses, as input,
effective interactions provided by the Fermi hypernetted--chain theory. The
relationship between both theories is discussed. Our theoretical effective
masses agree well with recent measurements by Yorozu et al. (Phys. Rev. B 48,
9660 (1993)) as well as those by R. Simons and R. M. Mueller (Czekoslowak
Journal of Physics Suppl. 46, 201 (1996)), but our analysis suggests a new
extrapolation to the zero-concentration limit. With that effective mass we also
find a good agreement with the measured Landau parameter F_0^a.Comment: 47 pages, 15 figure
Scattering of He-3 Atoms from He-4 Surfaces
We develop a first principles, microscopic theory of impurity atom scattering
from inhomogeneous quantum liquids such as adsorbed films, slabs, or clusters
of He-4. The theory is built upon a quantitative, microscopic description of
the ground state of both the host liquid as well as the impurity atom. Dynamic
effects are treated by allowing all ground-state correlation functions to be
time-dependent.
Our description includes both the elastic and inelastic coupling of impurity
motion to the excitations of the host liquid. As a specific example, we study
the scattering of He-3 atoms from adsorbed He-4 films. We examine the
dependence of ``quantum reflection'' on the substrate, and the consequences of
impurity bound states, resonances, and background excitations for scattering
properties.
A thorough analysis of the theoretical approach and the physical
circumstances point towards the essential role played by inelastic processes
which determine almost exclusively the reflection probabilities. The coupling
to impurity resonances within the film leads to a visible dependence of the
reflection coefficient on the direction of the impinging particle.Comment: 36 pages, 16 figure
The process of identifying, solving and preventing drug related problems in the LIMM-study
Quantum Rotation of HCN and DCN in Helium-4
We present calculations of rotational absorption spectra of the molecules HCN
and DCN in superfluid helium-4, using a combination of the Diffusion Monte
Carlo method for ground state properties and an analytic many-body method
(Correlated Basis Function theory) for the excited states. Our results agree
with the experimentally determined effective moment of inertia which has been
obtained from the spectral transition. The correlated basis function
analysis shows that, unlike heavy rotors such as OCS, the J=2 and higher
rotational excitations of HCN and DCN have high enough energy to strongly
couple to rotons, leading to large shifts of the lines and accordingly to
anomalous large spectroscopic distortion constants, to the emergence of
roton-maxon bands, and to secondary peaks in the absorption spectra for J=2 and
J=3.Comment: accepted by Phys. Rev. B; changes: included referee suggestions,
removed typos, added 10 ref
The STRANDS project: long-term autonomy in everyday environments
Thanks to the efforts of the robotics and autonomous systems community, the myriad applications and capacities of robots are ever increasing. There is increasing demand from end users for autonomous service robots that can operate in real environments for extended periods. In the Spatiotemporal Representations and Activities for Cognitive Control in Long-Term Scenarios (STRANDS) project (http://strandsproject.eu), we are tackling this demand head-on by integrating state-of-the-art artificial intelligence and robotics research into mobile service robots and deploying these systems for long-term installations in security and care environments. Our robots have been operational for a combined duration of 104 days over four deployments, autonomously performing end-user-defined tasks and traversing 116 km in the process. In this article, we describe the approach we used to enable long-term autonomous operation in everyday environments and how our robots are able to use their long run times to improve their own performance
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