Stable Excitons Research Articles

X-Ray Spectroscopic Investigation of Ammonium Chloride

The K α emission and K β emission spectra in fluorescence of Cl - in NH 4 Cl are measured with a two-crystal vacuum spectrometer. The Cl - K β emission spectrum is presented along with the Cl - K absorption, Cl - L 2,3 absorption and vacuum ultraviolet (VUV) absorption spectra of NH 4 Cl, reported previously. By using the measured emission spectra and the VUV absorption spectrum, the energy position of the bottom of the conduction band is shown in the Cl - K absorption and Cl - L 2,3 absorption spectra. The first sharp peak of the Cl - L 2,3 absorption is identified as being due to a core exciton, the energy level of which is formed below the bottom of the conduction band. The binding energy of the exciton is estimated to be 1.1( ±0.1) eV. The second sharp peak is related to two types of excitons; a stable exciton and a metastable exciton.

Size quantization by faceting in (110)-oriented GaAs/AlAs heterostructures

On GaAs (110) substrates a distinct step structure exists on the surface during molecular beam epitaxy of GaAs/AlAs multilayer structures which is monitored in situ by reflection high-energy electron diffraction. This peculiar surface structure results in thicker GaAs (AlAs) channels associated with the step planes giving rise to pronounced redshifts of the luminescence. The observation of hot exciton luminescence indicates increased exciton stability and exciton phonon interaction due to strong lateral localization of the excitons in the GaAs channels. The measured optical anisotropy is in agreement with the lateral potential introduced by the faceting of the GaAs/AlAs interface.

Energy competition of self-trapped excitons in alkali halides

We report the results of the calculation on the stability of self-trapped excitons localized on lattice sites with different geometrical structures in twelve alkali halides; AH, A = Li, Na, K, Rb, and H = Cl, Br, I. Present model leads us to the conclusions that excitons and holes in lithium halides, NaBr and NaI are most stable in energy when they are self-trapped as the three-center type and that those in potassium halides, KCl and NaCl are most stable when they are self-trapped as the two-center type. The energies of the two types of excitons are almost same in KBr and KI. This result shows us an important possibility that the Ex emission comes from the recombination of the three-center type self-trapped exciton.

Quantum-confined Stark effects on a GaAs cluster embedded in AlxGa

Quantum-confined Stark effects on the electronic structure of a GaAs microcluster with 63.5-A\r{} (120-a.u.) radius embedded in the ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As bulk crystal were investigated, within the effective-mass approximation, based on the Weyl-Titchmarsh theory. This approach provides both point and continuous spectral functions for the problem. Numerical calculations were performed on a continuous energy spectrum under an electric field for single-particle states of electron and heavy hole, and for an exciton state, using a 57:43 split of the band-gap discontinuity between the conduction and valence bands. The spherical-heterojunction potential barrier effects on the exciton state are discussed with regard to both exciton binding energy and exciton stability. The exciton Coulomb binding energy for the confined exciton in a cluster was found to be much larger than that in a one-dimensional superlattice system. Also, the Coulomb interaction brings about still longer exciton lifetimes compared with the external electrostatic field. Field ionization of the exciton is inhibited by the strong confined effects caused by the existence of the spherical quantum well.

Exciton-plasma transition in GaAs

In this work, we study the stability of excitons at high density, i.e. we calculate the reduction of the exciton binding energy due to exciton-exciton interactions in a high-density exciton gas. We derive first the effective electron-hole interaction in the presence of free carriers and excitons. We use the static approximation. The exciton binding energy is calculated by the variational technique. The computations are specialized to GaAs. We investigate the critical density when the exciton binding disappears, which corresponds to the exciton plasma transition. We conclude that this transition occurs at higher density than the reverse plasma exciton transition, determined by the standard criteria a 0 q D =1.19 [Rogers F. J., Graboske H. C., Jr. and Harword D. J., Phys. Rev. A1, 1577 (1970)].

Extended Peierls-Hubbard Model for One-Dimensional N-Sites N-Electrons System. II. Effect of Fluctuation, Optical and Magnetic Excitations

A one-dimensional many-electron system with site-diagonal electron-phonon coupling, intra- and inter-site electron-electron (e-e) repulsions is studied within the RPA for electrons and in the adiabatic limit for phonons, so as to clarify effects of fluctuation on the phase diagram of the ground state within the mean field theory (MFT), and also on various excitations therefrom. Energies of bound electron-hole (e-h) pairs in the MFT energy-gap such as magnon and charge-transfer (CT) excitons are calculated together with the ground-state energy-gain due to their virtual excitations. The resultant new phase diagram is such that the SDW phase erodes the CDW region given by the MFT, because of the magnon excitation. It is shown that the inter-site e-e repulsion always gives a stable singlet CT exciton that dominates the optical property of the CDW.

Soft X-Ray Absorption Spectra of Alkali Halides. I. KCl and NaCl

Soft X-ray absorption spectra of typical alkali chlorides, KCl and NaCl, were observed by using as a continuous background the synchrotron radiation from the 750 MeV electron synchrotron in the Institute for Nuclear Study, Tokyo. The L 2,3 -absorption spectrum of Cl - in these chlorides has been obtained. The spectrum shows one or two sharp absorption maxima at the long wavelength limit and a rapid increase in absorption at about 9 eV above the limit. A shoulder-like structure is observed at the foot of the second absorption band. A tentative X-ray energy level diagram suggests that the shoulder corresponds to the onset of interband transition from L 3 to the conduction band. The first absorption maximum is accordingly interpreted as the excitation of a stable X-ray exciton; a metastable X-ray exciton seems to be involved in the second maximum. The spectral behavior of the L 2,3 -absorption near the long wavelength limit has some similarity to the optical absorption spectrum in the vacuum ultraviolet regi...

On the quantum efficiencies of twenty alkali halides in the 12–21 eV region

Absolute photoelectric yields of twenty alkali halides have been measured, using a rare gas ion chamber to determine the flux distribution of the He 2 continuum between 12.4 and 21.2 eV. In addition, semiquantitative absorption coefficients were obtained by evaporating AX (alkali halide) films on 300 Å thick Al 2O 3 windows. For most of these materials the photoelectric yield rises to values as high as 0.8 election/ incident photon in the fundamental band beyond the stable exciton region. At photon energies near twice the threshold (or approximately 2 E g the yield often reaches a distinct minimum of the order of 0.4 electron/incident photon. This region is likely characteristic of double exciton production. Beyond 2 E g the yield generally rises to very large values, even over 1.0, strongly implying double excitation processes. Many broad diffuse bands, sometimes well correlated with the absorption coefficients, appear in the electron emission spectrum.