6,992 research outputs found
Spectrum and thermal fluctuations of a microcavity polariton Bose-Einstein condensate
The Hartree-Fock-Popov theory of interacting Bose particles is developed, for
modeling exciton-polaritons in semiconductor microcavities undergoing
Bose-Einstein condensation. A self-consistent treatment of the linear
exciton-photon coupling and of the exciton non-linearity provides a thermal
equilibrium description of the collective excitation spectrum, of the polariton
energy shifts and of the phase diagram. Quantitative predictions support recent
experimental findings
Partially suppressed long-range order in the Bose-Einstein condensation of polaritons
We adopt a kinetic theory of polariton non-equilibrium Bose-Einstein
condensation, to describe the formation of off-diagonal long-range order. The
theory accounts properly for the dominant role of quantum fluctuations in the
condensate. In realistic situations with optical excitation at high energy, it
predicts a significant depletion of the condensate caused by long-wavelength
fluctuations. As a consequence, the one-body density matrix in space displays a
partially suppressed long-range order and a pronounced dependence on the finite
size of the system
Generation of charged droplets by field ionization of liquid helium
Positively charged helium droplets were produced by ionization of liquid helium in an electrostatic spraying experiment, in which fluid emerging from a thin glass capillary was ionized by applying a high voltage to a needle inside the capillary. At 2.2 K, fine droplets (<10 mu m in diameter) were produced in pulsed sprays or showers with total currents as high as 0.4 mu A at relatively low voltages (2-4 kV). Ionization was accompanied by a visible glow at the needle and glass tips. Droplet formation was suppressed at 3.5 K. In contrast, liquid nitrogen formed a well-defined Taylor cone with droplets having diameters comparable to the jet (approximate to 100 mu m) at much lower currents (3 nA) and higher voltages (9 kV), in agreement with previous results. The mechanism for charging in these liquids was proposed to be field ionization, identical to the processes leading to conduction in cryogenic insulating liquids observed by Gomer. The high currents resulting from field ionization in helium, together with the intrinsically low surface tension of helium I, led to charge densities that greatly exceeded the Rayleigh limit, thus preventing formation of a Taylor cone and resulting in Coulomb explosion of the liquid
Infrared spectra of the cluster ions H7O<sup> + </sup><sub>3</sub>·H2 and H9O<sup> + </sup><sub>4</sub>·H2
Infrared spectra of hydrated hydronium ions weakly bound to an H2 molecule, specifically H7O + 3 ·H2 and H9O + 4 ·H2, have been observed. Mass-selected parent ions, trapped in a radio frequency ion trap, are excited by a tunable infrared laser; following absorption, the complex predissociates with loss of the H2, and the resulting fragment ions are detected. Spectra have been taken from 3000 to 4000 cm^−1, with a resolution of 1.2 cm^−1. They are compared to recent theoretical and experimental spectra of the hydronium ion hydrates alone. Binding an H2 molecule to these clusters should only weakly perturb their vibrations; if so, our spectra should be similar to spectra of the hydrated hydronium ions H7O + 3 and H9O + 4
Constrained probability distributions of correlation functions
Context: Two-point correlation functions are used throughout cosmology as a
measure for the statistics of random fields. When used in Bayesian parameter
estimation, their likelihood function is usually replaced by a Gaussian
approximation. However, this has been shown to be insufficient.
Aims: For the case of Gaussian random fields, we search for an exact
probability distribution of correlation functions, which could improve the
accuracy of future data analyses.
Methods: We use a fully analytic approach, first expanding the random field
in its Fourier modes, and then calculating the characteristic function.
Finally, we derive the probability distribution function using integration by
residues. We use a numerical implementation of the full analytic formula to
discuss the behaviour of this function.
Results: We derive the univariate and bivariate probability distribution
function of the correlation functions of a Gaussian random field, and outline
how higher joint distributions could be calculated. We give the results in the
form of mode expansions, but in one special case we also find a closed-form
expression. We calculate the moments of the distribution and, in the univariate
case, we discuss the Edgeworth expansion approximation. We also comment on the
difficulties in a fast and exact numerical implementation of our results, and
on possible future applications.Comment: 13 pages, 5 figures, updated to match version published in A&A
(slightly expanded Sects. 5.3 and 6
Laboratory measurements and theoretical calculations of O_2 A band electric quadrupole transitions
Frequency-stabilized cavity ring-down spectroscopy was utilized to measure electric quadrupole transitions within the ^(16)O_2 A band, b^1Σ^+_g ← X^3Σ^-_g(0,0). We report quantitative measurements (relative uncertainties in intensity measurements from 4.4% to 11%) of nine ultraweak transitions in the ^NO, ^PO, ^RS, and ^TS branches with line intensities ranging from 3×10^(−30) to 2×10^(−29) cm molec.^(−1). A thorough discussion of relevant noise sources and uncertainties in this experiment and other cw-cavity ring-down spectrometers is given. For short-term averaging (t<100 s), we estimate a noise-equivalent absorption of 2.5×10^(−10) cm^(−1) Hz^(−1/2). The detection limit was reduced further by co-adding up to 100 spectra to yield a minimum detectable absorption coefficient equal to 1.8×10^(−11) cm^(−1), corresponding to a line intensity of ~2.5×10^(−31) cm molec.^(−1). We discuss calculations of electric quadrupole line positions based on a simultaneous fit of the ground and upper electronic state energies which have uncertainties <3 MHz, and we present calculations of electric quadrupole matrix elements and line intensities. The electric quadrupole line intensity calculations and measurements agreed on average to 5%, which is comparable to our average experimental uncertainty. The calculated electric quadrupole band intensity was 1.8(1)×10^(−27) cm molec.−1 which is equal to only ~8×10^(−6) of the magnetic dipole band intensity
Electronic spectroscopy of the alkaline-earth halide cluster Ca_2Cl_3
A visible spectrum of the cluster Ca_2Cl_3 was observed from 651 to 630 nm by 1 + 1[prime] resonant multiphoton ionization spectroscopy. Spectra were obtained for each of the four isotopomers: Ca2 35Cl3, Ca2 35Cl2 37Cl, Ca2 35Cl 37Cl2, and Ca2 37Cl3. The spectra were composed of a strong origin band at 15 350.8 cm^(–1) and several very weak vibronic bands. All of the bands were sharp with partially resolved rotational band contours. Density functional calculations predicted three minimum energy isomers. The spectrum was assigned to the 2B2<--X-tilde 2A1 transition of the lowest energy isomer, a planar C2v structure having a ring of two Cl and two Ca atoms and a terminal Cl atom. The ring isomer of Ca_2Cl_3 has the unpaired electron localized on one Ca^(2+) ion to form a Ca^+ chromophore. The two other predicted isomers, a D3h trigonal bipyramid and a C2v planar V-shaped structure, were not consistent with the observations
Near-Infrared Kinetic Spectroscopy of the HO_2 and C_2H_5O_2 Self-Reactions and Cross Reactions
The self-reactions and cross reactions of the peroxy radicals HO_2 and C_2H_5O_2 and HO_2 were monitored using simultaneous independent spectroscopic probes to observe each radical species. Wavelength modulation (WM) near-infrared (NIR) spectroscopy was used to detect HO_2, and UV absorption monitored HO_2 and C_2H_5O_2. The temperature dependences of these reactions were investigated over a range of interest to tropospheric chemistry, 221−296 K. The Arrhenius expression determined for the cross reaction, k_2(T) = (6.01^(+1.95)_(−1.47)) × 10^(−13) exp((638 ± 73)/T) cm^3 molecules^(−1) s^(−1) is in agreement with other work from the literature. The measurements of the HO_2 self-reaction agreed with previous work from this lab and were not further refined.(1) The C_2H_5O_2 self-reaction is complicated by secondary production of HO_2. This experiment performed the first direct measurement of the self-reaction rate constant, as well as the branching fraction to the radical channel, in part by measurement of the secondary HO_2. The Arrhenius expression for the self-reaction rate constant is k_3(T) = (1.29^(+0.34)_(−0.27)) × 10^(−13)exp((−23 ± 61)/T) cm^3 molecules^(−1) s^(−1), and the branching fraction value is α = 0.28 ± 0.06, independent of temperature. These values are in disagreement with previous measurements based on end product studies of the branching fraction. The results suggest that better characterization of the products from RO_2 self-reactions are required
BRST invariant Lagrangian of spontaneously broken gauge theories in noncommutative geometry
The quantization of spontaneously broken gauge theories in noncommutative
geometry(NCG) has been sought for some time, because quantization is crucial
for making the NCG approach a reliable and physically acceptable theory. Lee,
Hwang and Ne'eman recently succeeded in realizing the BRST quantization of
gauge theories in NCG in the matrix derivative approach proposed by Coquereaux
et al. The present author has proposed a characteristic formulation to
reconstruct a gauge theory in NCG on the discrete space .
Since this formulation is a generalization of the differential geometry on the
ordinary manifold to that on the discrete manifold, it is more familiar than
other approaches. In this paper, we show that within our formulation we can
obtain the BRST invariant Lagrangian in the same way as Lee, Hwang and Ne'eman
and apply it to the SU(2)U(1) gauge theory.Comment: RevTeX, page
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