Bosonic stimulation of cold 1s excitons into a harmonic potential minimum in Cu2O

The density distribution of cold exciton clouds generated into a strain-induced potential well by two-photon excitation in Cu₂O is studied at 2 K. We find that an anomalous spike, which can be interpreted as accumulation of the excitons into the ground state, emerges at the potential minimum. The accumulation can be due to stimulated scattering of cold excitons, mediated by acoustic phonon emission. The possibility of the formation of the thermodynamic Bose-Einstein condensate of paraexcitons is discussed.     

Study of the 1s orthoexciton luminescence in Cu2O under two-photon excitation

We have studied the luminescence of 1s orthoexcitons in Cu₂O under both resonant and non-resonant two-photon excitation. The intensity of the luminescence was enhanced while resonantly exciting at the 1s or 2s orthoexciton levels. The direct and phonon-assisted emissions were observed over a wide range of temperatures (1.8-294 K) under two-photon resonant excitation. The phonon-assisted spectra can be well fit with a Maxwellian distribution with a fitting temperature close to the lattice temperature. The integrated intensity shows a minimum at 20 K and a maximum around 180 K. The existence of the direct emission feature enables us to deduce the temperature dependent bandgap and the orthoexciton line width directly up to relatively high temperatures. The red-shift of the orthoexciton energy and the increase of its line width as the temperature increases are interpreted as a temperature dependence of the real and imaginary parts of the exciton self energy.     

Low-temperature X-ray and photoluminescence of lead halide crystals

The emission spectra of PbF₂, PbCl₂ and PbBr₂ monocrystals are measured under optical and X-ray excitation at liquid helium temperature. Certain emission bands are attributed to the radiative transitions in the self-trapped cation exciton (excited Pb²⁺ion). The increase of trapped exciton via electron-hole recombination is discussed.     

Phase transitions of the q-state Potts model on multiply-laced Sierpinski gaskets

We have investigated stress effects on yellow exciton states in a Cu₂O thin film sandwiched by MgO plates by measuring wavelength modulated (WM) absorption spectra and their thermal variations. In the WM absorption spectra, dispersive spectral structures owing to 2P~4P states in the yellow excitons are clearly resolved in the thin film sample. A stress due to the lattice mismatch between Cu₂O and MgO provides a large red shift of the band gap in the green excitonic system. However, in the yellow excitonic transitions, it is found that a red-shift of the 2P excitonic state is much smaller than that in the green excitonic system. This result suggests that a shallow potential minimum for the yellow excitons is built up in the Cu₂O thin film sandwiched by MgO plates.     

Features of the absorption spectra of thin films of M 2Ag1 − x CuxI3 solid solutions (M = Rb, Cs)

The absorption spectra of thin films of (MI)_{1 ? y} (Ag_{1 ? x} Cu_xI)_y solid solutions (M = Rb, Cs) with the initial molar concentration y = 0.33 have been investigated. It is established that, at low concentrations x, a local exciton band due to Cu⁺ ions is split off from the main long-wavelength exciton bands. In Rb₂Ag_{1 ? x} Cu_xI₃ solutions, the concentration shift of exciton bands indicates the formation of a persistent-type exciton spectrum. However, in Rb₂Ag_{1 ? x} Cu_xI₃ with x ≥ 0.5 and in Cs₂Ag_{1 ? x} Cu_xI₃ with x > 0.2, exciton spectra of amalgamation type are observed, which are related to the formation of more stable M ₃Ag_{2 ? 2x} Cu_2x I₅ solid solutions. The formation of these solutions leads to broadening of the exciton bands and to the concentration transition from persistent-to amalgamation-type exciton spectra.     

Phase transformations in CuO caused by bombardment by He<Superscript>+</Superscript> ions and by the action of spherical shock waves

The valence state of copper ions and the phase composition of copper monoxide CuO subjected to bombardment by He⁺ ions and explosive shock waves are studied by the methods of x-ray photoelectron spectroscopy (XPS) and x-ray emission spectroscopy (XES). Measurements of photoelectron Cu 2p and emission O K_α spectra revealed a decrease in the concentration of Cu²⁺ ions and partial reduction of CuO to Cu₂O as a result of both ion bombardment and shock-wave loading. The concentration of the Cu₂O phase attained values of 10–15%. The Cu₂O phase is revealed by the XPS and XES methods even at concentrations lower than its threshold concentration for detection by x-ray diffraction measurements. This points to the effectiveness of XPS and XES techniques in studying nanocrystalline materials and defect structures containing finely dispersed inclusions. A model for the emergence of Cu₂O due to the formation of charged clusters under the action of stress waves is proposed.     

X-ray Lα emission and LIII absorption spectra of copper compounds

The X-ray Lα _{1, 2} emission spectra and L_III absorption spectra in Cu, Cu₂O, CuCl, Cu₂S, CuO, CuCl₂, CuS, CuF₂ and Cu(en)₂Cl₂ are investigated. It is shown that the emission spectra of divalent copper compounds are considerably distorted due to a sharp absorption peak near the L_III edge. The nature of this peak is discussed and a relation is made with satellite peaks in X-ray photoelectron spectra.     

Characterization of an intermediate case exciton in the emission of Cd-doped and pure AgBr

Abstract

Pulsed emission data and ODMR spectra of the 580 nm band provide evidence for an intermediate case exciton in pure and Cd²⁺-doped AgBr. This interpretation is based on the observed g-factor and the absence of a wavelength shift of the emission after pulsed excitation in pure AgBr. It is supported by the response of the ODMR spectra as the microwave modulation frequency is changed. The two central ODMR lines observed in AgBr (g = 1. 78) are assigned to transitions from the m_s=±1 levels to the m_s = 0 level of an intermediate case triplet exciton, which has an exchange coupling (singlet-triplet splitting) of -1. 9 ± 0. 2 cm^?1.     

Effets de défauts cristallins sur le spectre d’absorption excitonique de Cu2O

A theory of the effects of lattice defects on the yellow series of the exciton absorption spectrum of Cu₂O is presented. In particular, the effects of lattice defects due to fast neutron irradiation is treated. It is shown that the modification of exciton absorption spectrum, the appearing of the exciton fine structure lines in particular, is due to the deformation potential associated with the large defect clusters which are produced by the fast neutron bombardment. An analogy is established between this effect and the Stark exciton effect observed under externally applied electric field. It is shown that the Stark effect of exciton may be considered as a particular example of the problem that we treat. Several models of deformation potentials are proposed to calculate the variation of the oscillator strength of the exciton absorption lines in Cu₂O as a function of flux of fast neutrons. The effects of point defects are also studied. It is shown that the point defects are not at the origin of the observed fine structure of exciton, but they may shift and broaden exciton obsorption lines. These theoretical considerations are compared qualitatively with experimental results.     

Localization of exciton excitation in planar structures Cd0.9Mn0.1Te/Cd0.7Mg0.3Te

The spectra of exciton emission in a series of periodic structures with ultrathin Cd_0.9Mn_0.1Te inclusions in the Cd_0.7Mg_0.3Te matrix have been studied. The complex structure of the luminescence spectra and their relation to the luminescence excitation spectra indicate large-scale fluctuations in thickness of narrow-band-gap layers. Two types of temperature dependences of the luminescence intensity of localized excitons are found: (1) the complete suppression of emission above 80 K and (2) the relative enhancement of temperature-insensitive low-energy band that corresponds to local bulges in planar Cd_0.9Mn_0.1Te layers.     

Excitation spectroscopy on the M-band of red HgI2

The luminescence of red HgI₂ is investigated as a function of excitation intensity and -wavelength. At low excitation, emission is due to free and bound exciton recombination, at high levels a “M-band” is dominating. This M-band is partially ascribed to biexciton decay, this assumption being supported by resonances found in the excitation spectra. The biexciton binding energy is determined to be 6±1 meV.     

Multiexciton transients in a single quantum dot

The relaxation dynamics of a multiple exciton complex (multiexciton) confined in a semiconductor quantum dot has been investigated. Emission signals from a single self-organized GaAs/Al_0.3Ga_0.7As quantum dot are temporally resolved with picosecond time resolution. The emission spectra consisting of the multiexciton structures are observed to depend on the delay time and the excitation intensity. Quantitative agreement is found between the experimental data and the calculation based on a model describing the successive relaxation of multiexcitons.     

Electron-lattice interaction of Ag− and Cu− centers in alkali halides

Absorption, emission, and excitation spectra of Ag^? centers in KCl, RbCl, CsCl, and CsBr are measured at low temperatures. The positions of theA emission bands are slightly different afterC andA band excitation, respectively. This is believed to be due to the existence of two different types of minima in the adiabatic potential energy surface of the³ T _1u state. The symmetry of the energy minima in the¹ T _1u state is trigonal for KCl∶Ag^? and Cu^?, but tetragonal for CsBr∶Ag^?. This becomes evident from the polarization properties of the emission. The energy and temperature dependence of the polarization is discussed. Uniaxial stress causes polarized emission of Ag^? and Cu^? centers measured from LHeT to 100 K. This is due to a splitting and mixing of the relaxed excited states by the stress. The effects are used to calculate the coupling constants between thep electron and theE _g andT _2g lattice modes. They are compared with predictions from the point-charge model for different lattice structures. A new assignment of the absorption bands of KCl∶Cu^? to the excited states of Cu^? is established on measurements of emission spectra and lifetimes.     

Excitonic nature of the dual fluorescence of 2,3-diazabicyclo[2.2.2]oct-2-ene and 1,4-dimethyl-(2,3-diazabicyclo[2.2.2]oct-2-ene)

Absorption, fluorescence excitation, and fluorescence spectra and the dependence of the degree of fluorescence polarization on the emission wavelength are measured for glass-like ethanol solutions of 2,3-diazabicyclo[2.2.2]oct-2-ene and 1,4-dimethyl-(2,3-diazabicyclo[2.2.2]oct-2-ene) at a temperature of 77 K. The analysis of the spectral polarization data shows that two excited electronic states S₁ and S₂ that contribute to the emission of the compounds are related to the exciton splitting and correspond to the symmetric (S₂) and antisymmetric (S₁) wave functions.     

Photoluminescence and excitation spectra of Cu(AlxGa1−x)S2 films grown by vapor phase epitaxy

Chalcopyrite Cu(Al_xGa_1−x)S₂ alloy films were successfully grown on GaP substrates by vapor phase epitaxy using metallic chlorides (CuCl, GaCl₃ and AlCl₃) and H₂S as source materials. Photoluminescence (PL) spectra of these films taken under a low excitation density using a super high pressure Hg lamp exhibited broad emissions in the orange region. Photoluminescence excitation (PLE) measurements revealed that these broad emissions are effectively excited at the photon energies of A- and the BC-exciton energies. Under the high excitation density using the pulsed XeCl laser, these alloy films showed the exciton related emissions composed of biexciton recombination, exciton-exciton and exciton-carrier scatterings. The influence of the compositional fluctuation was observed on the increase of the full-width at half maximum (FWHM) for the exciton related emission with increase in composition of x.     

Triplet exciton in a non-reactive bis-triphenyllead (or tin) diacetylene crystal. A spectroscopic study

The lowest triplet exciton state of crystals formed by (C₆H₅)₃?X?C ≡ C?C ≡ C?X?(C₆H₅)₃ where X is Pb or Sn and an equimolecular amount of CH₂Cl₂ solvent molecules, is studied through the analysis of absorption and of phosphorescence emission and excitation spectra at low temperatures. In the lead compound, the origins of absorption and emission are identical, showing the dominance of free exciton emission and the absence of crystal lattice relaxation in the excited state; the splitting is accounted for by symmetry considerations in the different crystalline phases; the triplet exciton band is several cm^-1 wide, although the nearest diacetylene-diacetylene distance is about 10 Å. There is evidence for triplet exciton motion, through the detection of trap (presumably X-trap) emission in the tin compound, and through the analysis of the excitation spectrum lineshape, showing the effect of exciton-exciton interaction in the lead compound.     

Photoluminescence from excitons in ZrS3

The photoluminescence spectrum of the layered ZrS₃ crystal shows several narrow emission lines under the excitation by an Ar⁺ laser in the wavelength range from 605–630 nm at 4.2 K. The excitation spectra for these emission lines and their temperature dependences suggest that the observed narrow emission lines originate from the radiative annihilation of indirect excitons bound to impurities. Some of these emission lines seem to be associated with the radiative annihilation of the indirect bound exciton with emission of phonons.     

Photoemission valence band spectra and electronic density of states in copper oxides and copper based ceramic super-conductors

Photoemission valence band spectra with three different photon energies (21.2 eV, 40.8 eV, 1253.6 eV) for Cu₂O and CuO and a number of copper oxide based superconducting ceramics are investigated, namely La₂CuO₄ (LCO), Y₁Ba₂Cu₃O₇ (YBCO), Bi₂Sr₂CaCu₂O₈ (BSCCO) and Tl₂Ba₂Ca₂Cu₃O₁₀ (TBCCO). From the Cu₂O data it can be infered, that the He-II (40.8 eV) spectra give a fair representation of the density of states (DOS). In addition the agreement between the calculated density of states (DOS) and the He-II photoemission spectrum is almost perfect for Cu₂O. This agreement is worse for CuO and the other materials which all have a Cu groundstate close to a 3d ⁹ configuration indicating a large contribution of thed-d correlation energy to the excitation spectra which is absent in Cu₂O because of the filled 3d-shell. In all cases the experimental DOS atE _F is very small and only the spectra of BSCCO show a well defined Fermi edge. The relevance of these findings with respect to the theoretical local functional density DOS calculations is discussed.     

Temperature properties of exciton luminescence from CdTe quantum wells with different thicknesses in the CdTe/CdMnTe structure

The optical spectra of the CdTe/Cd_0.7Mn_0.3Te structure containing three CdTe quantum wells with nominal thicknesses of 16, 8, and 4 monolayers have been investigated. The temperature dependences of parameters of the exciton luminescence spectra (integrated intensity, full-width at half-maximum, position of the maximum, Stokes shift) for quantum wells with different thicknesses differ substantially. These differences are explained by a strong thickness dependence of the energy of Coulomb coupling in the exciton, the energy of localization of the exciton on bulges of the quantum well, and the degree of penetration of the exciton wave function into the barrier. At high excitation power densities, the emission contours of the quantum wells with thicknesses of 8 and 16 monolayers contain short-wavelength tails that correspond to optical transitions between excited quantum-well levels.