1,011 research outputs found
Photoluminescence Stokes shift and exciton fine structure in CdTe nanocrystals
The photoluminescence spectra of spherical CdTe nanocrystals with zincblende
structure are studied by size-selective spectroscopic techniques. We observe a
resonant Stokes shift of 15 meV when the excitation laser energy is tuned to
the red side of the absorption band at 2.236 eV. The experimental data are
analyzed within a symmetry-based tight-binding theory of the exciton spectrum,
which is first shown to account for the size dependence of the fundamental gap
reported previously in the literature. The theoretical Stokes shift presented
as a function of the gap shows a good agreement with the experimental data,
indicating that the measured Stokes shift indeed arises from the electron-hole
exchange interaction.Comment: 8 pages, 4 figures, LaTe
Electronic structure and optical properties of ZnS/CdS nanoheterostructures
The electronic and optical properties of spherical nanoheterostructures are
studied within the semi-empirical tight-binding model including
the spin-orbit interaction. We use a symmetry-based approach previously applied
to CdSe and CdTe quantum dots. The complete one-particle spectrum is obtained
by using group-theoretical methods. The excitonic eigenstates are then deduced
in the configuration-interaction approach by fully taking into account the
Coulomb direct and exchange interactions. Here we focus on ZnS/CdS, ZnS/CdS/ZnS
and CdS/ZnS nanocrystals with particular emphasis on recently reported
experimental data. The degree of carrier localization in the CdS well layer is
analyzed as a function of its thickness. We compute the excitonic fine
structure, i.e., the relative intensities of low-energy optical transitions.
The calculated values of the absorption gap show a good agreement with the
experimental ones. Enhanced resonant photoluminescence Stokes shifts are
predicted.Comment: 6 pages, 4 Figures, revtex
Exciton states and optical properties of CdSe nanocrystals
The optical spectra of CdSe nanocrystals up to 55 A in diameter are analyzed
in a wide range of energies from the fine structure of the low-energy
excitations to the so-called high-energy transitions. We apply a symmetry-based
method in two steps. First we take the tight-binding (TB) parameters from the
bulk sp^{3}s^{*} TB model, extended to include the spin-orbit interaction. The
full single-particle spectra are obtained from an exact diagonalization by
using a group-theoretical treatment. The electron-hole interaction is next
introduced: Both the Coulomb (direct) and exchange terms are considered. The
high-energy excitonic transitions are studied by computing the electric dipole
transition probabilities between single-particle states, while the transition
energies are obtained by taking into account the Coulomb interaction. The fine
structure of the lowest excitonic states is analyzed by including the
electron-hole exchange interaction and the wurtzite crystal-field terms in the
exciton Hamiltonian. The latter is diagonalized in the single electron-hole
pair excitation subspace of progressively increasing size until convergence.
The peaks in the theoretical transition spectra are then used to deduce the
resonant and nonresonant Stokes shifts, which are compared with their measured
values in photoluminescence experiments. We find that the final results depend
on the crystal-field term, the relative size of the surface and the degree of
saturation of the dangling bonds. The results show a satisfactory agreement
with the available experimental data.Comment: Revtex, 24 pages, 7 Postscript figure
A density matrix renormalisation group algorithm for quantum lattice systems with a large number of states per site
A variant of White's density matrix renormalisation group scheme which is
designed to compute low-lying energies of one-dimensional quantum lattice
models with a large number of degrees of freedom per site is described. The
method is tested on two exactly solvable models---the spin-1/2
antiferromagnetic Heisenberg chain and a dimerised XY spin chain. To illustrate
the potential of the method, it is applied to a model of spins interacting with
quantum phonons. It is shown that the method accurately resolves a number of
energy gaps on periodic rings which are sufficiently large to afford an
accurate investigation of critical properties via the use of finite-size
scaling theory.Comment: RevTeX, 8 pages, 2 figure
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Demonstration of the event identification capabilities of the NEXT-White detector
In experiments searching for neutrinoless double-beta decay, the possibility of identifying the two emitted electrons is a powerful tool in rejecting background events and therefore improving the overall sensitivity of the experiment. In this paper we present the first measurement of the efficiency of a cut based on the different event signatures of double and single electron tracks, using the data of the NEXT-White detector, the first detector of the NEXT experiment operating underground. Using a 228Th calibration source to produce signal-like and background-like events with energies near 1.6 MeV, a signal efficiency of 71.6 ± 1.5 stat± 0.3 sys% for a background acceptance of 20.6 ± 0.4 stat± 0.3 sys% is found, in good agreement with Monte Carlo simulations. An extrapolation to the energy region of the neutrinoless double beta decay by means of Monte Carlo simulations is also carried out, and the results obtained show an improvement in background rejection over those obtained at lower energies. [Figure not available: see fulltext.
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Radiogenic backgrounds in the NEXT double beta decay experiment
Natural radioactivity represents one of the main backgrounds in the search for neutrinoless double beta decay. Within the NEXT physics program, the radioactivity- induced backgrounds are measured with the NEXT-White detector. Data from 37.9 days of low-background operations at the Laboratorio SubterrĂĄneo de Canfranc with xenon depleted in 136Xe are analyzed to derive a total background rate of (0.84±0.02) mHz above 1000 keV. The comparison of data samples with and without the use of the radon abatement system demonstrates that the contribution of airborne-Rn is negligible. A radiogenic background model is built upon the extensive radiopurity screening campaign conducted by the NEXT collaboration. A spectral fit to this model yields the specific contributions of 60Co, 40K, 214Bi and 208Tl to the total background rate, as well as their location in the detector volumes. The results are used to evaluate the impact of the radiogenic backgrounds in the double beta decay analyses, after the application of topological cuts that reduce the total rate to (0.25±0.01) mHz. Based on the best-fit background model, the NEXT-White median sensitivity to the two-neutrino double beta decay is found to be 3.5Ï after 1 year of data taking. The background measurement in a QÎČÎȱ100 keV energy window validates the best-fit background model also for the neutrinoless double beta decay search with NEXT-100. Only one event is found, while the model expectation is (0.75±0.12) events. [Figure not available: see fulltext.]
Low-diffusion Xe-He gas mixtures for rare-event detection: electroluminescence yield
High pressure xenon Time Projection Chambers (TPC) based on secondary scintillation (electroluminescence) signal amplification are being proposed for rare event detection such as directional dark matter, double electron capture and double beta decay detection. The discrimination of the rare event through the topological signature of primary ionisation trails is a major asset for this type of TPC when compared to single liquid or double-phase TPCs, limited mainly by the high electron diffusion in pure xenon. Helium admixtures with xenon can be an attractive solution to reduce the electron diffu- sion significantly, improving the discrimination efficiency of these optical TPCs. We have measured the electroluminescence (EL) yield of XeâHe mixtures, in the range of 0 to 30% He and demonstrated the small impact on the EL yield of the addition of helium to pure xenon. For a typical reduced electric field of 2.5 kV/cm/bar in the EL region, the EL yield is lowered by ⌠2%, 3%, 6% and 10% for 10%, 15%, 20% and 30% of helium concentration, respectively. This decrease is less than what has been obtained from the most recent simulation framework in the literature. The impact of the addition of helium on EL statistical fluctuations is negligible, within the experimental uncertainties. The present results are an important benchmark for the simulation tools to be applied to future optical TPCs based on Xe-He mixtures. [Figure not available: see fulltext.]
Energy calibration of the NEXT-White detector with 1% resolution near Q ÎČÎČ of 136Xe
Excellent energy resolution is one of the primary advantages of electroluminescent high-pressure xenon TPCs. These detectors are promising tools in searching for rare physics events, such as neutrinoless double-beta decay (ÎČÎČ0Îœ), which require precise energy measurements. Using the NEXT-White detector, developed by the NEXT (Neutrino Experiment with a Xenon TPC) collaboration, we show for the first time that an energy resolution of 1% FWHM can be achieved at 2.6 MeV, establishing the present technology as the one with the best energy resolution of all xenon detectors for ÎČÎČ0Îœ searches. [Figure not available: see fulltext.
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