Quantum cascade phenomenon in Bi2Sr2CaCu2O(8+delta) single crystals

We study interlayer transport in Bi2Sr2CaCu2O(8+delta) cuprates, which represent stacks of atomic scale intrinsic Josephson junctions. A series of resonant dips in conductance is observed at condition when bremsstrahlung and recombination bands in nonequilibrium spectrum of Josephson junctions overlap. The phenomenon is explained in terms of self-detection of a new type of collective strongly nonequilibrium state in natural atomic superlattices, bearing certain resemblance with operation of a quantum cascade laser. Conclusions are supported by in situ generation-detection experiments and by numerical simulations.     

Narrow-band electrons in transition-metal oxides

Experiments on LaCoO₃ demonstrate that crystal-field theory and band theory describe two thermodynamically different electronic phases. For an integral number of electrons per atom, the phase transition is first-order. The critical parameter is an overlap integral, which may be either a cation-cation or a cation-anion-cation overlap integral. Intra-atomic exchange and electron-phonon interactions contribute significantly to electron localization. The characteristic feature of collective electrons is a Fermi surface. Those physical properties that depend upon the existence of a Fermi surface vary discontinuously through a localized-electron collective-electron transition; other physical properties, including electron mobility and paramagnetic susceptibility, apparently do not. It is argued that the spontaneous crystallographic distortions associated with semiconducting metallic phase changes manifest the existence of narrow, cation-sublattice bands if the cations are removed from the centers of symmetry of their interstices, narrow crystalline bands otherwise; ferroelectric and antiferroelectric transitions manifest the existence of a narrow valence band; the formation of a homologous series of shear structures in nonstoichiometric compounds manifests narrow conduction bands. These distortions all result from the creation, or enhancement, of an energy discontinuity at the Fermi surface. By contrast, conventional Jahn-Teller distortions, magnetostriction due to spin-orbit coupling, and the ordering of small polarons manifest localized electrons and the applicability of crystal-field theory. It is also shown that the critical overlap integral (or bandwidth) for spontaneous band magnetism is only a little larger than that for a localized-electron collective-electron transition. Preliminary data are compatible with two possibilities for bands that are more than half-filled: (1) saturation of orbitals of spin, which leads to localized electrons of spin and collective electrons of spin; (2) spontaneous magnetization (ferromagnetism) of only the antibonding electrons, which may lead to reduced atomic moments. Spontaneous band antiferromagnetism may be stabilized in bands that are half-filled or slightly less. It is represented by a spin-density wave with wavelength adjusted to create an energy discontinuity at the Fermi surface. Spin-density waves are also possible among collective-spin electrons that are coupled to localized-spin electrons.Operated with support from the U.S. Air Force.     

WELL-WIDTH DEPENDENCE OF THE EXCITON LIFETIME IN GaAs/AlGaAs QUANTUM WELLS

The dependence of the excitonic lifetime on the well width has been studied in conventional GaAs/AlGaAs quantum wells. Two clearly different variations of the measured excitonic lifetime have been observed. For wide well widths, we find a nearly linear decrease of the lifetime with decreasing well width. However, when the well is further decreased, a saturation and even increase of the lifetime with decreeing well width are observed. The experimental data are compared with the theory of radiative excitonic recombination, and show that well width dependence of the measured photoluminescence lifetime can be attributed mainly to the change of the excitonic effective volume and the overlap integral as well.     

Surface acoustic wave distribution and acoustooptic interactions in silica waveguide Bragg devices

The efficiency of acoustooptic interaction in single-mode strip silica waveguide is analyzed theoretically for the first time by determining the overlap integral between the optical and acoustic field distributions. The results show that there is a good overlap of the optical and SAW fields in the low SAW frequency range. At high acoustic frequencies, the overlap integral decreases with increasing acoustic frequency. At 216 MHz, the maximum of 0.8544 for the overlap integral is obtained provided that the H/Λ equals 0.02.     

Speech recognition with altered spectral distribution of envelope cues.

Recognition of consonants, vowels, and sentences was measured in conditions of reduced spectral resolution and distorted spectral distribution of temporal envelope cues. Speech materials were processed through four bandpass filters (analysis bands), half-wave rectified, and low-pass filtered to extract the temporal envelope from each band. The envelope from each speech band modulated a band-limited noise (carrier bands). Analysis and carrier bands were manipulated independently to alter the spectral distribution of envelope cues. Experiment I demonstrated that the location of the cutoff frequencies defining the bands was not a critical parameter for speech recognition, as long as the analysis and carrier bands were matched in frequency extent. Experiment II demonstrated a dramatic decrease in performance when the analysis and carrier bands did not match in frequency extent, which resulted in a warping of the spectral distribution of envelope cues. Experiment III demonstrated a large decrease in performance when the carrier bands were shifted in frequency, mimicking the basal position of electrodes in a cochlear implant. And experiment IV showed a relatively minor effect of the overlap in the noise carrier bands, simulating the overlap in neural populations responding to adjacent electrodes in a cochlear implant. Overall, these results show that, for four bands, the frequency alignment of the analysis bands and carrier bands is critical for good performance, while the exact frequency divisions and overlap in carrier bands are not as critical.     

Observation of bound exciton and distant pair resonances in amorphous phosphorus

For the first time in an amorphous semiconductor magnetic resonance of a bound exciton is reported. ODMR investigations of amorphous phosphorous show that the luminescence consists of two bands, a low energy emission which is consistent with triple exciton recombination at IVAPs and a high energy emission due to electron-hole recombination at distant IVAPs. The triplet exciton has g = 2.13 ± 0.05, and the distant pair resonance occurs at g = 2.0 ± 0.02.     

Electron impact excitation of higher energy states of molecular oxygen in the atmosphere of Europa

Recent measurements of integral cross sections for electron impact excitation of the Schumann-Runge continuum, longest band and second band of molecular oxygen are applied to calculations of emissions from the atmosphere of Europa. Molecules excited to these bands predissociate, producing O(¹D) (excited oxygen) atoms which subsequently decay to produce 630.0-nm radiation. Radiation of this wavelength is also produced by direct excitation of O atoms and by the recombination of O \hbox{₂⁺_2^+} + 2 with electrons, but these two processes also produce O(¹S) atoms which then emit at 557.7 nm. It is shown by modeling that the ratio of 630.0-nm to 557.7-nm is sensitive to the relative importance of the three processes, suggesting that the ratio would be a useful remote sensing probe in the atmosphere of Europa. In particular, the excitation of the Schumann-Runge continuum, longest band and second band is produced by magnetospheric electrons while the recombination is produced by secondary electrons produced in the atmosphere. This difference raises the possibility of determination of the secondary electron spectrum by measurement of light emissions.     

Photoluminescence enhancement in double Ge/Si quantum dot structures

The luminescence properties of double Ge/Si quantum dot structures are studied at liquid helium temperature depending on the Si spacer thickness d in QD molecules. A seven-fold increase in the integrated photoluminescence intensity is obtained for the structures with optimal thickness d = 2 nm. This enhancement is explained by increasing the overlap integral of electron and hole wavefunctions. Two main factors promote this increasing. The first one is that the electrons are localized at the QD base edges and their wavefunctions are the linear combinations of the states of in-plane Δ valleys, which are perpendicular in k-space to the growth direction [001]. This results in the increasing probability of electron penetration into Ge barriers. The second factor is the arrangement of Ge nanoclusters in closely spaced QD groups. The strong tunnel coupling of QDs within these groups increases the probability of hole finding at the QD base edge, that also promotes the increase in the radiative recombination probability.     

Efficiency droop alleviation in blue light emitting diodes using the InGaN/GaN triangular-shaped quantum well

The InGaN/GaN blue light emitting diode(LED) is numerically investigated using a triangular-shaped quantum well model,which involves analysis on its energy band,carrier concentration,overlap of electron and hole wave functions,radiative recombination rate,and internal quantum efficiency.The simulation results reveal that the InGaN/GaN blue light emitting diode with triangular quantum wells exhibits a higher radiative recombination rate than the conventional light emitting diode with rectangular quantum wells due to the enhanced overlap of electron and hole wave functions(above 90%) under the polarization field.Consequently,the efficiency droop is only 18% in the light emitting diode with triangular-shaped quantum wells,which is three times lower than that in a conventional LED.     

Temperature dependent energy bands and the metal-nonmetal transition in NiS

The LCMTO (linear combination of muffin-tin orbitals) technique has been used to calculate the first temperature dependent energy bands for a transtion metal compound. The particular compound studied was hexagonal NiS which undergoes a first-order metal-to-nonmetal transition as the temperature is lowered below 264K. We find the bands to agree well with XPS data and that the Sp bands overlap the bottom of the d bands as predicted by White and Mott. This p-d overlap, which reduces the correlation energy, is found to increase with temperature as a result of lattice vibrations whereas the lattice distortion accompanying the transition has very little effect on the energy bands. This suggests that the Mott-Hubbard transition in NiS is driven by the Debye-Waller modification of the potential and not the lattice distortion as suggested by White and Mott.     

Auger-limited carrier lifetime in HgZnTe ambient temperature 10.6μm photoresistors

Photoconductive lifetime measurements have been carried out on both undoped and Cu doped p-type Hg_0.885Zn_0.115Te material prepared by a modified quench-anneal technique. An analysis of lifetime vs doping variations suggests that the Auger recombination mechanism is dominant. The experimental determination of the overlap integral has led to the value of 0.15. This fact and additional advantages (hardness, stronger chemical bonds, lower diffusion coefficients) make Hg_1−xZn_xTe extremely interesting for ambient temperature IR photoresistors.     

Band-dependent quasiparticle dynamics in the hole-doped Ba-122 iron pnictides

We report on band-dependent quasiparticle dynamics in the hole-doped Ba-122 pnictides measured by ultrafast pump-probe spectroscopy. In the superconducting state of the optimal and over hole-doped samples, we observe two distinct relaxation processes: a fast component whose decay rate increases linearly with excitation density and a slow component whose relaxation is independent of excitation strength. We argue that these two components reflect the recombination of quasiparticles in the two hole bands through intraband and interband processes. We also find that the thermal recombination rate of quasiparticles increases quadratically with temperature in all samples. The temperature and excitation density dependence of the decays indicates fully gapped hole bands and nodal or very anisotropic electron bands.     

Electronic structure of transition metal monoantimonides: Analogy to the transition metals

A simple metallic band model is proposed for the transition metal monoantimonides, by analogy to the transition metals. It is assumed that the metal 3d electrons are itinerant in narrow bands which energetically overlap broader antimony 5p-derived bands, and the Fermi level lies in the overlap region. Consistency of the model with available data is assessed and found to be quite good.     

On the origin of photoluminescence in heavily-doped silicon

The origin of the photoluminescence in heavily-doped silicon is examined. Transient photoluminescence data for Si(P) are presented and used to identify the “Low Level” emission bands in terms of recombination of impurity band electrons with holes bound to acceptor sites. The “High Level” bands are attributed to recombination of impurity band electrons with free holes. The energies of the band gap and optical band gap in heavily-doped silicon are determined from the photoluminescence measurements.     

Radiative recombination of holes in delta-p-doped gallium arsenide

Photoluminescence spectra of δ-p-doped GaAs structures of different doping levels are studied experimentally. It is found that set of PL bands observed in δ-p-doped samples recently is regularly broadened with doping concentration increase due to appearance of additional low-energy bands and their subsequent red shift at higher doping levels. A red shift of the bands and a change of their relative intensities were caused also by excitation laser intensity decrease and/or temperature increase. These results confirm our previous assumption that the bands are due to radiative recombination of spatially separated photoelectrons with holes occupying size-quantization levels of δ-layer potential well.     

Photoluminescence of PbFCl and PbFBr single crystals

The luminescence properties of single crystals of PbFCl and PbFBr at 4.2 K under ultraviolet irradiation are presented for the first time. In PbFCl three and in PbFBr four emission bands have been observed. The red bands are ascribed to Pb⁺ centres. Direct exciton recombination is absent in both compounds.     

The photoelectron spectra of tricyclo[3.3.0.02,6]Octene and tricyclo[3.3.0.02,6]Octane

The photoelectron spectra of tricyclo[3.3.0.0^2,6]octene (1) and tricyclo[3.3.0.0^2,6]octane (2) have been recorded. The first bands in these spectra are correlated with orbitals which are linear combinations of the Walsh orbitals and of the olefinic π-orbital. This assignment is based on a zero differential overlap molecular orbital model as well as on the results of extended Hückel calculations. The resonance integral <π|H|W > for 1 is found to be ?1.34 eV.     

5,6,11,12-四苯基四苯并掺杂8-羟基喹啉铝薄膜的荧光谱及发光机理

用高荧光染料的5，6，11，12－四苯基四苯并对8－羟基喹啉铝进行掺杂，测量其光致发光和电致发光谱。结果表明：在低掺杂时，主发光体是Alq，掺入的Rubrene作为客发光体只是在Alq带隙中引入了分立能级；随着掺入的Rubrene浓度增加，Rubrene成了主发光体，Alq变成了客发光体，出现了发光体的互换现象。由于Rubrene的吸收光谱与Alq的发射谱重叠较大，在光致发光中存在从Alq向Rubrene的能量传递和电荷转移过程，而电致发光则是由于Rubrene导带中电子浓度远大于注入到Alq导带中电子浓度，造成Rubrene导带电子与价带空穴复合的几率比Alq中的复合几率大得多，其EL主要是Rubrene的发光。     

Short pulse physics of quantum well structures

This paper reports some recent results of time-resolved studies of the carrier dynamics in GaAs/GaAlAs quantum well structures with picosecond and subpicosecond time resolution. These experiments have provided insight into carrier trapping, energy relaxation, and carrier recombination processes. Carrier trapping into the quantum well layers is very efficient and determines the decay of the GaAlAs luminescence even for 1 μm thick cladding layers. Carrier recombination is enhanced particularly at low temperatures. This effect has been attributed to the increased overlap of electron and hole (exciton) wavefunctions in the quasi-two-dimensional carrier system.     

A simple model for the diffractive overlap integral

A model is introduced for the diffractive overlap integral in which the overlap is represented by a set of nonforward triple Regge diagrams. The formalism provides a clear framework within which to discuss questions concerning the peripherality of diffractive processes.