电子关联对聚乙炔中激子类型的影响

在SSH哈密顿基础上引进弱电子关联,对反式聚乙炔链中光致激子的产生和演化过程实施分子动力学模拟.弱关联强度参数U取值0～1.250 eV,V=U/2取值0～0.625 eV.计算结果表明,关联强度的大小影响链中元激发类型,U＜0.555 eV时产生的元激发为孤子-反孤子对,U＞0.555 eV时产生的元激发为正负荷电极化子对.为进一步讨论该类型一维系统中不同类型激子的产生、输运、衰减等动态过程,关联强度参数U的选择提供参考.     

有机共轭高聚物的分子链间耦合局域态

有机共轭高分子中,孤子、极化子及激子都是基本的元激发,对解释有机聚合材料的导电发光特性起着主导作用.孤子、极化子以及激子等在晶格位形上都是各具特征的空间局域状态.本文将讨论在有机共轭高分子中存在着另一种局域态——链间耦合局域态,这种局域态是由于分子链间的相互作用所导致,在相互作用分子链端附近形成势阱,可有效束缚电子和空穴等带电粒子.     

红色Hgl_2中激子-电子和激子-空穴相互作用

在N_2激光的强激发下研究了纯的红色HgI_2的光致发光光谱。所用HgI_2是层状的半导体,具有2.4eV的直接能带隙。光谱呈现出激子-空穴的相互作用,也呈现出激子-电子的相互作用。 脉冲强激发引起两条发射带(在150°K温度下为2.280eV和2.226eV)。当温度超过60°K时,发射强度随激发强度按平方规律增长。当温度降到60K以下时,另一个位于     

电子关联和孤子

聚乙炔中的电子关联和孤子可用相关基函数方法(CBF)来研究,通过考虑完全的电子-电子相互作用,得到含有单个电荷的奇数原子链的稳定位形为孤子的结论,随着电子-电子互作用强度U和互作用程α/β的增加,孤子定域性加强,产生能降低,因此,即使计入电子-电子相互作用,孤子仍然是导电聚合物中的主要载流子。 关键词：     

反式聚乙炔的极化子静态性质研究

在Su-Schrieffer-Heeger（SSH）模型基础上,考虑长程电子关联的影响,研究反式聚乙炔分子链中空穴极化子与电子极化子的稳态性质。比较不同关联强度下两类极化子的晶格位形、能级、电荷密度波、以及吸收谱线的变化。加深对分子链中极化子的认识,该理论方法也可推广到高分子链中其它元激发的静态性质研究中,为不同类型元激发动力学性质研究奠定基础。     

聚乙炔激子单线态和三线态的稳态性质与零磁场判据

在Su-Schrieffer-Heeger(SSH)模型基础上,计入弱电子关联的影响,研究聚乙炔单线态和三线态两类激子的行为.与单线态相比,三线态激子能级自旋简并丧失,能隙增大,电荷密度振荡局域分布以及自旋密度波产生,三线态激子吸收谱的相应特征峰发生~0.1 eV蓝移并且强度增加~40%.这些差异为区分两类激子提供了磁共振实验之外的判据.     

正硅酸锌中能量传递的激子机理

在α型正硅酸锌中,用接近吸收边(Eg～5.5eV)的光能激发,发现了激子能量传递到发光中心的某些特征。在一个弛豫状态上,激子扩散的激活能估计为0.075eV,激子在自局域化前通过的扩散体积为0.5×10-19cm-3。实验确定的磷光体中能量传递的参数可实用于选择最佳颗粒尺寸分布。     

双分子链中非线性多激子态的动力学研究

在超快激光激发下分子聚集体内有多个激子产生.针对具有链间耦合的双分子链系统,应用密度矩阵理论,在偶极-偶极近似和算符算术平均值近似下通过求解量子主方程计算了不同双链构型下的多激子动力学过程.研究发现在光激发作用下的多激子离域在双链分子系统中,在能量表象下形成激子态能带.激子态能带的宽度和激子占据能级随双分子链构型的不同而不同.对于链间和链内均为H型排列的分子链,激子态能带变宽,激子态优先占据在高阶能级上.在不同能级下的激子具有不同的动力学演变特点.若链间耦合较强,激子态通过链间耦合传递,与分子是否直接受到激发没有直接关系,激发态在两个链内的转移周期与最近邻的链间耦合有直接关系.     

共轭高聚物双分子链中激子生成的动力学研究

通过对耦合共轭高分子双链中激子生成的动力学研究,讨论了受到光激发后,激子在分子内或分子间生成的制约因素.激子的生成除了受分子链间耦合强度的制约,还受到光激发模式的影响.结果表明,激子既可以在分子内生成,也可以在耦合分子间生成.增大分子之间的相互作用强度,可以增大在分子之间生成激子的概率.     

铜掺杂氧化锌纳米棒的非线性光学响应竞争特性

利用飞秒脉冲激光激发Cu掺杂ZnO纳米棒,研究其特有的非线性光学性质和激发机制。在激发波长为750 nm的荧光光谱中,二次谐波峰非常弱,几乎可以忽略,存在非常强的激子发光峰和Cu掺杂导致缺陷发光峰。激发强度的增大会导致这两个发光峰强度呈非线性增大,激子发光峰位产生明显红移,而缺陷发光峰位没有变化。进一步增大激发强度,缺陷发光峰强度会出现饱和甚至有所下降,而激子发光峰强度持续增大。当激发波长增加到760 nm时,从样品的荧光光谱可以清楚地识别到二次谐波峰和激子发光峰以及缺陷发光峰并存。随着激发波长的进一步增加,二次谐波强度不断增大,而激子发光峰和缺陷发光峰的强度却随之下降。当激发波长为790 nm和800 nm时,未发现激子发光峰和缺陷发光峰,非线性光谱以二次谐波为主导。研究结果表明,通过选择合适的激发波长和激发强度,可以实现发光颜色的转变,使得Cu掺杂ZnO纳米棒在全光显示方面具有潜在的发展前景。     

Thermal dissociation of excitons in a type-I GaAs/AlAs superlattice studied by time-resolved photoluminescence measurements

In order to investigate thermal properties of excitons, we have performed time-resolved photoluminescence (PL) measurements for a type-I (GaAs)₁₅/(AlAs)₁₅ superlattice. At low temperatures, PL signals show ordinal exponential time decay, whereas at high temperatures, the PL shows power decay. Such change of PL signals is understood by considering thermal dissociation of exciton into account. At low temperatures, recombination of bound excitons generates PL which shows exponential decay. At temperatures higher than the exciton binding energy, correlation between electrons and holes disappears. Recombination of free excitons causes PL which decays by a power function. By means of the least-squares fitting, we evaluate the portion of bound excitons, recombination time of bound and free excitons as functions of temperatures.     

Silicon dioxide and the chalcogenide semiconductors; similarities and differences

The purpose of this article is to examine current hypotheses about the optical and electrical properties of the amorphous chalcogenide semiconductors, and to examine whether they can be extended to explain some of the properties of amorphous silicon dioxide. For the selenides and tellurides we assume the validity of the model of ‘charged dangling bonds’ introduced by Street and Mott. Absorption of light by these defects leads to photoluminescence with a large Stokes shift; formation of an electron-hole pair far from a defect, on the other hand, leads to radiationless recombination. In enquiring why this is so, we examine the nature of excitons in non-crystalline systems, and the mechanism of self-trapping both of excitons and of free carriers in crystalline and non-crystalline materials. Self-trapping by the V _K-type mechanism, if it occurs, takes place after a certain delay for a free hole or exciton, but not for one bound at a defect, so that free-exciton absorption bands are narrower than those from bound excitons at defects. The radiationless recombination of a free or bound exciton normally occurs through the mechanism first considered by Dexter et al., if more energy than ～1 eV is emitted.

In SiO₂ electrons are exceptionally mobile, but on the basis of work by Hughes, we think that holes are self-trapped after a delay, and can then move by polaron-type hopping. We propose that the first absorption peak (at 10.2 eV) in the optical spectrum is due to an optically allowed exciton, which recombines by the Dexter mechanism, but only after a delay of a type proposed here. The delay allows a comparatively narrow absorption peak, of width given by that of the exciton band. The absorption peak due to a Na⁺ ion linked to a non-bridging oxygen atom is considerably broader because it is due to a bound exciton. We estimate the band-gap in SiO₂ to be about 10.6 eV and also the position of the centre of gravity of the level due to a Na⁺ ion with a non-bridging oxygen atom to be ～1.5 eV above the valence band of SiO₂, less than usually supposed.

Finally, we examine the nature of the positive and negative charges which occur in SiO₂ grown thermally on silicon; these are responsible for Anderson localization of electrons in the inversion layer, and for the ‘slow states’ of silicon technology. We tentatively conclude that they are non-bridging oxygen atoms and that these, like dangling bonds in chalcogenides, are normally positively and negatively charged. We examine their positions in the gap and the activation energies of charging and discharging.     

VARIATIONAL STUDY ON MISSING BOND-CHARGE REPULSION ON THE EXTENDED HUBBARD MODEL: EFFECTS ON POLYACETYLENE

The variational approach is applied to the study on the effect of Coulomb interaction between electrons on the dimerization in a one-dimensional commensurability-two Peierls system, such as trans-polyacetylene. For the case of omitting direct repulsion between electrons in the bonds between sites W, it is confirmed that the weak interactions tend to enhance the dimerization; but when W is included, this enhancement will be attenuated. If |W| is sufficiently large, the net effect of electron-electron interactions will reduce the magnitude of the electron-phonon-induced dimerization.     

Berry phase effect on the exciton transport and on the exciton Bose-Einstein condensate

With the exciton lifetime much extended in semiconductor quantum-well structures, the exciton transport and Bose-Einstein condensation have become a focus of research in recent years. We reveal a momentum-space gauge field in the exciton center-of-mass dynamics due to Berry phase effects. We predict a spin-dependent transport of the excitons analogous to the anomalous Hall and Nernst effects for electrons. We also predict spin-dependent circulation of a trapped exciton gas and instability in an exciton condensate in favor of vortex formation.     

Exciton interaction potential in polar crystals

The problem whether polaron effects can lead to a repulsive barrier in the exciton interaction potential is reinvestigated, because this effect is crucial for the possibility of a Bose-Einstein condensation of excitons (rather than of excitonic molecules). The energy for two excitons is calculated accurately as a function of their mean distance in the case of a heavy hole mass by using an integral expansion method of molecular physics. The resulting interaction potential shows for certain polaron radii a very weak repulsive barrier. Taking into account corrections to the Haken potential which have been introduced by Pollmann and Büttner, it is shown that the interaction potential between two excitons in the relative singlet state is always attractive, so that the exciton system is also in polar materials unstable against exciton molecule formation.This is a project of the Sonderforschungsbereich Frankfurt/Darmstadt, financed by special funds of the Deutsche Forschungsgemeinschaft     

耦合量子阱中激子凝聚的研究进展

近年来，半导体量子阱中激子的玻色一爱因斯坦凝聚研究取得了很大进展．实验上利用耦合量子阱间接激子中电子和空穴在空间上的分离，显著提高了激子的冷却速度和寿命，成功地把激子冷却到1K以下，观察到了激子的准凝聚状态，并且在强激光照射下，发现了随光照强度增强而增大的激子发光环和环上形成的有规则斑点图案，引起了广泛的兴趣和重视．理论研究表明，发光环的出现是电子和空穴在量子阱中的反常输运行为造成的，但环上形成规则斑点的物理机理目前尚不清楚．文章介绍了这方面的实验背景和形成激子环的物理图像，指出了理论研究中存在的问题，并对解决问题的方案进行了讨论．     

The optical properties of Be,Mg and Zn-diffused gallium phosphide

Interest in the Ga-site acceptors Be and Mg was stimulated by the possibility that they might produce efficient luminescence on association with O, analogous to the well-known red Zn-O luminescence in GaP but at higher transition energy. Attention was directed to diffusion doping by Be and Mg of GaP O-doped during growth because the reactivity of Be and Mg with O renders double doping during crystal growth very difficult. Structured green donor-acceptor pair spectra were observed at 1.6°K from many Be-diffused crystals, yielding an accurate measure of (E_A)_BE, 50 ± 1 meV. Moderately efficient orange-red luminescence was also observed below ∼ 100 °K from these crystals, but the intensity of this luminescence decreased rapidly to negligible levels by ∼ 200°K. This luminescence also contains sharp structure at 1.6°K, of a form characteristic of the decay of excitons bound to complex centres. Many sharp phonon replicas occur, involving local modes as well as characteristic GaP modes. One set of no-phonon lines, at least, near 2.19 eV, shows zero-field splitting, luminescence decay times and behaviour in magnetic and external strain fields characteristic of exciton decay at a centre with <100>; or <111>-type symmetry axes, containing no extra electronic particles. The exciton state is split by 2.4 meV by J-J coupling, and the axial field of the centre splits the hole states by ∼ 1.0 meV. These bound excitons are specifically characteristics of diffused GaP and appear analogous to bound excitons observed below 2.12 eV in Zn-diffused GaP. It is probable that the relevant centres contain diffusion components such as Be or Zn interstitials and improbable that O_P is involved. By contrast, weak orange bound exciton luminescence observed in Mg-diffused GaP does involve O, presumably as O_P. No analysis of the magneto-optical behaviour of this Mg-related bound exciton was possible in our crystals, so its symmetry axis was not established. It is possible that this is the Mg_Ga-O_P bound exciton. If so, the two-fold reductions in the exciton localisation energy from ∼ 0.32 eV to ∼ 0.15 eV and in the mass of the Ga-site substituent has produced dramatic changes in the form of the phonon cooperation between the Zn-O and “Mg-O” excitons. The “Mg-O” exciton luminescence is not dominant in our crystals, even at low temperature. The exciton state is again split by a local crystal field as well as by J-J coupling, but here the former splitting is predominant; 2_∈0 = 3.9 meV, Δ = 0.60 meV.     

Primary photoexcitations and the origin of the photocurrent in rubrene single crystals

By simultaneously measuring the excitation spectra of transient luminescence and transient photoconductivity after picosecond pulsed excitation in rubrene single crystals, we show that free excitons are photoexcited starting at photon energies above 2.0 eV. We observe a competition between photoexcitation of free excitons and photoexcitation into vibronic states that subsequently decays into free carriers, while molecular excitons are instead formed predominantly through the free exciton. At photon energies below 2.25 eV, free charge carriers are created only through a long-lived intermediate state with a lifetime of up to 0.1 ms and no free carriers appear during the exciton lifetime.     

Kinetic properties of a system of spatially separated excitons and electrons in the presence of a Bose condensate of excitons

A system of interacting, spatially separated excitons and electrons is examined in the presence of a Bose condensate of excitons. The kinetic properties of the system that are governed by the interaction of excitations in the exciton subsystem with electrons are investigated. It is shown that a nonequilibrium distribution of excitations in the exciton subsystem gives rise to an induced electron current. Experimental observation of the kinetic phenomena described can provide new information on the exciton phase state. Zh. éksp. Teor. Fiz. 116, 1440–1449 (October 1999)     

Optoelectronic characteristics of CdTe/HgTe/CdTe quantum-dot quantum-well nanoparticles

Optoelectronic characteristics of CdTe/HgTe/CdTe quantum-dot quantum-well (QDQW) nanoparticles synthesized by the colloidal method are investigated in this study. Strong exciton bands were observed in absorption and photoluminescence (PL) spectra taken for the CdTe/HgTe/CdTe QDQW nanoparticles. The energy difference between the exciton absorption and PL bands is larger than those obtained with CdTe and HgTe nanoparticles. Photocurrent-voltage curves and time-dependent photocurrent curves were obtained for the CdTe/HgTe/CdTe QDQW nanoparticles. With regard to the photocurrent mechanism of these QDQW nanoparticles, those charge carriers participating in the formation of excitons may not contribute to the photocurrent, because of the large binding energy of the excitons. Moreover, it is suggested in this paper that free holes in the HgTe quantum-well in the valance band, rather than free electrons, are the main contributors to the photocurrent.