Radiative recombination of excitons in semiconductors at high excitation intensities

The shape of the exciton luminescence band of the gaseous phase of free excitons in crystal Si is investigated under high level excitation. Only the exciton-exciton interaction is considered and the influence of collisions of excitons with phonons and electrons is not taken into account. The theoretical shape of the exciton luminescence band qualitatively agrees with experiment. The process of the radiative Auger-recombination of two indirect excitons without the participation of phonons is studied in Ge and Si. The expression for the frequency and temperature dependence of the probability of radiation is obtained. The band has the asymmetrical Gaussian form with the steep short-wave tail. The predicted luminescence band in Ge is shifted at the long-wavelength side from the well-known exciton luminescence bands with participation of the phonons. The selection rules for the probability amplitude of the process under consideration are obtained.     

Model calculation of the laser-semiconductor interaction in subpicosecond regime

We develop a model calculation to investigate the interaction of a subpicosecond laser pulse with a direct-gap semiconductor. In this problem, carrier distributions are extremely far from equilibrium and nonequilibrium LO phonons are generated at the center of the Brillouin zone. We solve numerically a system of three transport equations, namely one for the conduction band, one for the valence band and one for the LO phonons. Contrary to all previous work, we do not assume that carrier distributions are nondegenerate so that we can investigate the following problems: (a) conditions of internal thermalization in degenerate nonequilibrium plasma (thermalization time, influence of screening on the energy-loss rate, formation of nonequilibrium LO phonons) and (b) absorption saturation and transmission of the pumplaser pulse.     

Hg1-xCdxTe的低频吸收带

本文对Hg_1-xCd_xTe的低频吸收带,提出另一种解释。把Hg_1-xCd_xTe作为具有0—25％Cd(或Hg)替位杂质的HgTe(或CdTe)晶体,用格林函数计算含有一个Cd(或Hg)的HgTe(或CdTe)双原子链振动谱,进而讨论Hg_1-xCd_xTe混晶振动模的特性,结果表明Hg_1-xCd_xTe在k→0存在频率不趋于零的声学声子,它们有条件吸收远红外波段的光子,形成低频吸收带。它能说明已报道的实验上观察到的低频吸收带的主要特征,并表明对光学声子存在双模行为的其它混晶,在远红外吸收谱中很可能存在类似的低频吸收带。 关键词：     

Rotations as coherent states of SU(6) quadropole phonons in the SU(3) limit

Analytic expressions for the wavefunctions of the ground-state rotational band for even and odd nuclei are derived in terms of spherical quadropole phonons truncated at N_max phonons. For N_max → ∞ the Bohr-Mottelson rotational states are generated as an asymptotic gaussian distribution of quadropole phonons.     

Phonons localises dus a une monocouche de xenon en epitaxie sur la surface (0001) du graphite

We compute, by a simple method, the acoustical phonons due to the adsorption of a xenon monolayer on the (0001) surface of a graphite crystal. We show that the band of acoustical localized phonons is quasi-flat and very close to the Einstein's frequency associated to the vibrations polarized in the direction normal to the (0001) plane. The consequences of these features are discussed.     

Coupling of electrons and phonons in a doped antiferromagnet

It is well-known that low-energy electronic excitations in high-T _c superconductors have energies of the order of the exchange couplingJ, i.e. of the same order as the phonon energies. Therefore, low-energy electronic excitations and phonons should strongly influence each other. To investigate this problem we consider a coupled electron-phonon system. For the electronic degrees of freedom we start from the three band Hubbard or Emery model. In analogy to the transformation of the three band Hubbard model to thet?J model, studied by Zhang and Rice, we derive an effective electron-phonon interaction. Its electronic degrees of freedom are those of thet?J model which couple to the phonons of the original system. The coupling of electrons and phonons is discussed by means of the phonon Green function for a breathing-like mode.     

杂质中心吸收宽带的电声子耦合问题(Ⅰ)

将晶格振动的平面波线性组合得到一组充分反映杂质格位对称性的正则模(对称“驻波”)。任一对称类型的驻波的频谱分布只与整个格波的频谱分布差一个常数因子。使用对称分析得出电声子耦合导致杂质中心吸收或荧光带变宽的位差效应中的对称驻波选择定则,指出参与电声子耦合的声子的频率基本上遍布于全部格波的频谱,并且定性地估计了纵波声子的耦合强度对波长的函数关系的轮廓;从而对Williams位形坐标理论的本质作出了批判性的阐述。其次,讨论了在掺有铁族元素的刚玉晶体的宽带上出现先行线(或称系列线)的问题,指出了一般文献在这一问题上存在着的某些错误概念和格波频谱上临界点(k=0的除外)与先行线之间可能有的联系。文中也给出了有关对称驻波的数学分析,并以具体图象表明了对称驻波与孤立集团模型中的振动模之间的联系。     

Crystal structure analysis in (3+d) dimensions

Inelastic neutron scattering on in situ grown bcc single crystals of the group 4 metals Ti, Zr and Hf show a band of low energy and strongly damped phonons. Geometrical considerations show how these damped lattice vibrations achieve the displacements necessary for the two martensitic phase transitions from bcc to ω (under pressure) and from bcc to hcp (upon lowering the temperature). The low energy and temperature dependent phonons are precursor fluctuations of the hcp or ω phase within the bcc phase.     

Hot electron diffusivity in silicon

Hot electron diffusion coefficients in silicon at room temperature are theoretically studied by incorporating the band non-parabolicity and the effect of the diffusion current on the distribution function. Two models of intervalley scattering are considered: in one model, coupling with high-temperature intervalley phonons only is assumed; in the other, low-temperature intervalley phonons are also included. Both the models give practically identical results and the calculated values are found to agree closely with the experiment.     

Wave-vector dependence of electron-optical phonon coupling in GaAs

Induced non equilibrium distribution of optical phonons allows direct measurement of electron-phonon coupling as a function of phonon wave-vector K. Results indicate that near K=0(K<20, 000 cm^?1 coupling between TO phonons and electrons is independent of K whereas LO phonons show a K^-2 dependence. Results also suggest that electron relaxation in the conduction band by multiple phonon production is quite significant.     

Reciprocal space constraints create real-space anomalies in doped carbon nanotubes

When graphite is doped with electrons, carbon-carbon bonds lengthen and Raman-active phonons soften as antibonding states fill. However, in semiconducting carbon nanotubes, one Raman-active G-band mode increases in frequency at low doping levels. We show how phase constraints on the conduction-band wave function expose a latent bonding character in the conduction band of certain nanotubes. In these tubes, filling the lowest conduction band shortens the axial bonds even as it lengthens the circumferential bonds. The A{1}{LO} phonon, which preferentially stretches the axial bonds, then hardens even as the other phonons soften. Quantum confinement eliminates the angular averaging taken for granted in higher-dimensional systems and develops a new class of states, neither bonding nor antibonding, whose character depends on the angular orientation of the bonds in question.     

Resonance effects on coherent phonon generation in lead phthalocyanine crystalline films

We report the first observation of coherent phonons in crystalline lead phthalocyanine (PbPc) films grown on a (0001) sapphire substrate by using a pump–probe technique. Coherent phonon oscillations corresponding to intermolecular (lattice) vibrations in the PbPc film are observed in the frequency region from 1 to 5 THz. It is found that the intensities of coherent phonons are resonantly enhanced at the exciton energies of the Q band observed in the absorption spectrum. PACS 78.47.+p; 78.66.-w; 82.53.Xa; 78.40.Me     

Dynamics of exciton formation at the Si(100) c(4 x 2) surface

Carrier recombination at the Si(100) c(4 x 2) surface and the underlying surface electronic structure is unraveled by a combination of two-photon photoemission and many-body perturbation theory: An electron excited to the silicon conduction band by a femtosecond infrared laser pulse scatters within 220 ps to the unoccupied surface band, needs 1.5 ps to jump to the band bottom via emission of optical phonons, and finally relaxes within 5 ps with an excited hole in the occupied surface band to form an exciton living for nanoseconds.     

Electronic structure and kinetic properties of tight-binding metals at high temperature

The influence of the electron-phonon interaction on the electronic properties of tight-binding metals is considered with the one-band approximation. It is shown by using the previous results that the existence of the twofold effect is due to phonons. Firstly, phonons lead to a non-coherent scattering of electrons and as a result a finite lifetime for the electron states. Secondly, phonons lead to a renormalization of the periodic cristal potential. Latter was well known for the nearly free-electron crystals to be expressed as Debye-Waller corrections to the lattice potential. Such effect in tight-binding metals is shown here to result in the temperature dependence of overlap integrals. It gives rise to the increase of the band width and the electron velocity for the one-band approximation. The negative temperature coefficient of resistivity is shown to may arise in result at high temperatures.     

Effects of squeezing-antisqueezing in strongly coupled electron–phonon systems

The effects of squeezing-antisqueezing resulting from the motion and density fluctuation of the electrons on the properties of both electrons and phonons have been studied by using a new variational ansatz with correlated displacement and squeezing in strongly coupled electron–phonon systems. The effects results in (1) reduction of the ground state energy, and enhancement of stability of the systems, (2) increase of the binding energy of the polaron occurred and weakening of growing speed of polaron narrowing of electron band, (3) increase of the charge density wave order and (4) suppression of increased tendency of anomalous quantum fluctuation of the phonons in the systems. The antisqueezed effect plays an important role in determining the properties of the electrons and phonons in the strongly coupled electron–phonon systems.     

Polaronic quasiparticles in a strongly correlated electron band

We show that a strongly renormalized band of polaronic quasiparticle excitations is induced at the Fermi level of an interacting many-electron system on increasing the coupling of the electrons to local phonons. We give results for the local density of states at zero temperature both for the electrons and phonons. The polaronic quasiparticles satisfy Luttinger's theorem for all regimes considered, and their dispersion shows a kink similar to that observed experimentally in copper oxides. Our calculations are based on the dynamical mean field theory and the numerical renormalization group for the hole-doped Holstein-Hubbard model and large on-site repulsion.     

Indirect band gap investigation of orthorhombic single crystals of sulfur

Single crystals of pure and selenium doped orthorhombic sulfur crystal were prepared from carbon disulphide solution. They were investigated at two temperatures (6°C and 85°C) in an optical absorption apparatus. The optical absorption increased with temperature and it was found to be higher for Se doped crystals within the same range of temperatures. The indirect allowed band gap transitions and assisting phonons were observed. Their values were found to shift toward lower energy with increasing temperatures. They are also lower for doped samples. At 6°C, the indirect allowed band gaps are 2.61 eV and 2.56 eV for pure and selenium-doped crystals, respectively. At 85°C, they are 2.44 eV and 2.40 eV for pure and doped crystals, respectively. The phonons associated with the optical transitions could be attributed to the stretching band of S₈ rings.     

Evaluating the coupling strength of electron–hole pairs and phonons in a 0.9 μm-wavelength silicon light emitting diode using dressed-photon–phonons

We investigated the coupling strength between electron–hole pairs and phonons in a silicon light emitting diode (Si-LED) fabricated by dressed-photon-assisted annealing. This Si-LED emitted light in the 1.4 eV photon energy (0.9 μm wavelength) band, and phonon sidebands were observed in the emission spectrum. From a comparison with simulation results, these sidebands were found to be due to coupling of electron–hole pairs with LO-mode and TO-mode coherent phonons via dressed-photon–phonons. The value of the Huang–Rhys factor, $S$ , representing the coupling strength between the electron–hole pairs and the phonons was estimated to be $4.08 \pm 0.02$ .     

Picosecond time-resolved luminescence of localized excitons in CdS1-xSex

We present time-resolved luminescence results on CdS_0.36Se_0.64 which give a new insight on the kinetics of excitons localized by alloy potential fluctuations. By exciting in the localized exciton band with detection close to the exciting wavelength we obtain the lifetime across the band. Below the exciting laser energy two processes contribute to luminescence: transfer of localized excitons by tunnel effect assisted by acoustical phonons, and luminescence (assisted by acoustical phonons) of all the states excited at time t = 0 either directly or through their acoustical absorption wing. The time behavior of luminescence with respect to the detuning from the exciting energy helps to discriminate between those two contributions. Furthermore it shows that intermediate long-living states are involved in the exciton relaxation process.