42 research outputs found
Discovery of dumbbell-shaped Cs*He_n exciplexes in solid He 4
We have observed several new spectral features in the fluorescence of cesium
atoms implanted in the hcp phase of solid helium following laser excitation to
the 6P states. Based on calculations of the emission spectra using
semiempirical Cs-He pair potentials the newly discovered lines can be assigned
to the decay of specific Cs*He exciplexes: an apple-shaped CsHe and a dumbbell-shaped CsHe exciplex with
a well defined number of bound helium atoms. While the former has been
observed in other enviroments, it was commonly believed that exciplexes with
might not exist. The calculations suggest CsHe to be
the most probable candidate for that exciplex, in which the helium atoms are
arranged on a ring around the waist of the dumbbell shaped electronic density
distribution of the cesium atom.Comment: 4 pages, 4 figure
Probing vortices in 4He nanodroplets
We present static and dynamical properties of linear vortices in 4He droplets
obtained from Density Functional calculations. By comparing the adsorption
properties of different atomic impurities embedded in pure droplets and in
droplets where a quantized vortex has been created, we suggest that Ca atoms
should be the dopant of choice to detect vortices by means of spectroscopic
experiments.Comment: Typeset using Revtex4, 4 pages and 2 Postscript file
Static Response Function for Longitudinal and Transverse Excitations in Superfluid Helium
The sum rule formalism is used to evaluate rigorous bounds for the density
and current static response functions in superfluid helium at zero temperature.
Both lower and upper bounds are considered. The bounds are expressed in terms
of ground state properties (density and current correlation funtions) and of
the interatomic potential. The results for the density static response
significantly improve the Feynman approximation and turn out to be close to the
experimental (neutron scattering) data. A quantitative prediction for the
transverse current response is given. The role of one-phonon and multi-particle
excitations in the longitudinal and transverse channels is discussed.
(Phys.Rev.B, in press)Comment: 19 pages (plain TeX) and 3 Figures (postscript), UTF-26
Collective Modes in a Slab of Interacting Nuclear Matter: The effects of finite range interactions
We consider a slab of nuclear matter and investigate the collective
excitations, which develop in the response function of the system. We introduce
a finite-range realistic interaction among the nucleons, which reproduces the
full G-matrix by a linear combination of gaussian potentials in the various
spin-isospin channels. We then analyze the collective modes of the slab in the
S=T=1 channel: for moderate momenta hard and soft zero-sound modes are found,
which exhaust most of the excitation strength. At variance with the results
obtained with a zero range force, new "massive" excitations are found for the
vector-isovector channel .Comment: 14 pages, TeX, 5 figures (separate uuencoded and tar-compressed
postscript files), Torino preprint DFTT 6/9
Dispersion of Ripplons in Superfluid 4he
A detailed study of the dispersion law of surface excitations in liquid \hef
at zero temperature is presented, with special emphasis to the short wave
length region. The hybridization mechanism between surface and bulk modes is
discussed on a general basis, investigating the scattering of slow rotons from
the surface. An accurate density functional, accounting for backflow effects,
is then used to determine the dispersion of both bulk and surface excitations.
The numerical results are close to the experimental data obtained on thick
films and explicitly reveal the occurrence of important hybridization effects
between ripplons and rotons.Comment: 23 pages, REVTEX 3.0, 11 figures upon request, UTF-326/9
Observing the spin of a free electron
Long ago, Bohr, Pauli, and Mott argued that it is not, in principle, possible to measure the spin components of a free electron. One can try to use a Stern-Gerlach type of device, but the finite size of the beam results in an uncertainty of the splitting force that is comparable with the gradient force. The result is that no definite spin measurement can be made. Recently there has been a revival of interest in this problem, and we will present our own analysis and quantum-mechanical wave-packet calculations which suggest that a spin measurement is possible for a careful choice of initial conditions
A Single Charged Quantum Dot in a Strong Optical Field: Absorption, Gain, and the AC Stark Effect
We investigate a singly-charged quantum dot under a strong optical driving
field by probing the system with a weak optical field. When the driving field
is detuned from the trion transition, the probe absorption spectrum is shifted
from the trion resonance as a consequence of the dynamic Stark effect.
Simultaneously, a gain sideband is created, resulting from the coherent energy
transfer between the optical fields through the quantum dot nonlinearity. As
the pump detuning is moved from red to blue, we map out the anticrossing of
these two spectral lines. The optical Bloch equations for a stationary
two-level atom can be used to describe the numerous spectral features seen in
this nano solid state system
A superfluid hydrodynamic model for the enhanced moments of inertia of molecules in liquid 4He
We present a superfluid hydrodynamic model for the increase in moment of
inertia, , of molecules rotating in liquid He. The static
inhomogeneous He density around each molecule (calculated using the Orsay-Paris
liquid He density functional) is assumed to adiabatically follow the
rotation of the molecule. We find that the values created by the
viscousless and irrotational flow are in good agreement with the observed
increases for several molecules [ OCS, (HCN), HCCCN, and HCCCH ]. For
HCN and HCCH, our model substantially overestimates . This is likely
to result from a (partial) breakdown of the adiabatic following approximation.Comment: 4 pages, 1 eps figure, corrected version of published paper. Erratum
has been submitted for change
Pinning of quantized vortices in helium drop by dopant atoms and molecules
Using a density functional method, we investigate the properties of liquid
4He droplets doped with atoms (Ne and Xe) and molecules (SF_6 and HCN). We
consider the case of droplets having a quantized vortex pinned to the dopant. A
liquid drop formula is proposed that accurately describes the total energy of
the complex and allows one to extrapolate the density functional results to
large N. For a given impurity, we find that the formation of a
dopant+vortex+4He_N complex is energetically favored below a critical size
N_cr. Our result support the possibility to observe quantized vortices in
helium droplets by means of spectroscopic techniques.Comment: Typeset using Revtex, 3 pages and 5 figures (4 Postscript, 1 jpeg
Surface Region of Superfluid Helium as an Inhomogeneous Bose-Condensed Gas
We present arguments that the low density surface region of self-bounded
superfluid He systems is an inhomogeneous dilute Bose gas, with almost all
of the atoms occupying the same single-particle state at . Numerical
evidence for this complete Bose-Einstein condensation was first given by the
many-body variational calculations of He droplets by Lewart, Pandharipande
and Pieper in 1988. We show that the low density surface region can be treated
rigorously using a generalized Gross-Pitaevskii equation for the Bose order
parameter.Comment: 4 pages, 1 Postscript figur