基态非简并聚合物中的极化子和双极化子动力学

研究了极化子和双极化子在基态非简并聚合物中的动力学.弱电场下，发现载流子为带电极 化子或双极化子，它们的晶格态与电荷态始终耦合在一起同步运动.极化子比双极化子的运 动速度快，存在饱和速度，且它们的饱和速度随聚合物非简并度的增加而降低；强电场下， 元激发的电荷态将脱离晶格态的束缚而快速运动，参与导电的不再是极化子或双极化子，而 是电子直接导电. 关键词： 极化子 双极化子 动力学模拟     

抛物量子点中强耦合磁双极化子内部激发态性质

基于Lee-Low-Pines幺正变换,采用Pekar类型变分法研究了抛物量子点中强耦合磁双极化子的内部激发态性质,当考虑自旋和外磁场影响时,推导出二维量子点中强耦合磁双极化子基态的能量E0,声子平均数ˉN0以及第一激发态的能量E1,声子平均数ˉN1随量子点受限强度ω0,介电常数比η,电子-声子耦合强度α和磁场的回旋共振频率ωC的变化规律.结果表明,磁双极化子的基态能量E0和第一激发态能量E1由两电子的单粒子能量E E,两电子间库仑相互作用能E C,电子自旋与磁场相互作用能E s和电子-声子相互作用能E e-ph四部分组成;单粒子"轨道"运动与磁场相互作用导致了第一激发态能级E1分裂为E(1+1)1,E(1-1)1两条,而电子自旋-磁场相互作用的效应又使基态和第一激发态的各能级均产生了三条"精细结构";ˉN0和ˉN1随ω0,α和ωc的增加而增大,E e-ph的取值总是小于零,其绝对值随α,ω0和ωc的增加而增大;电子-声子相互作用的效应是束缚态磁双极化子形成的有力因素,而限定势和电子之间的库仑排斥能的存在不利于束缚态磁双极化子的形成;能量为E(1-1)1的磁双极化子要比能量为E(1+1)1的磁双极化子更容易且更稳定地处于束缚态.     

二维和三维大双极化子的稳定性

结合相对运动的近似波函数和修正的Lee(李政道)-Low-Pines变分法，研究了二维和三维大双极化子的稳定性.得到了临界耦合参数α_c和临界介电常数比η_c.与近来用完全不同的变分方法求得的结果相一致.还标定了在1/α,η平面上双极化子稳定存在的区域.结果显示，低维材料更有利于双极化子的形成. 关键词：     

Rashba自旋-轨道耦合下二维双极化子的基态性质

在考虑Rashba自旋-轨道耦合效应下,基于Lee-Low-Pines变换,采用Pekar型变分法研究了量子点中双极化子的基态性质.数值结果表明,在电子-声子强耦合(耦合常数α6)条件下,量子点中形成稳定双极化子结构的条件(结合能E_b0)自然满足;双极化子的结合能E_b随量子点受限强度ω_0、介质的介电常数比η和电子-声子耦合强度α的增大而增加,随Rashba自旋-轨道耦合常数αR的增加表现为直线增加和减小两种截然相反的情形;Rashba效应使双极化子的基态能量分裂为E(↑↑),E(↓↓)和E(↑↓)三条能级,分别对应两电子的自旋取向为"向上"、"向下"和"反平行"三种情形;基态能量的绝对值|E|随η和α的增加而增大,随αR的增加表现为直线增加和减小两种截然相反的情形;在双极化子的基态能量E中,电子-声子耦合能所占据的比例明显大于Rashba自旋-轨道耦合能所占比例,但电子-声子耦合与Rashba自旋-轨道耦合间相互渗透、彼此影响显著.     

氧化物高温超导体的双极化子解释的可能性

通过分析双极化子形成的条件,指出文献[1]给出的三元金属氧化物高温超导体的双极化子解释是可疑的。 关键词：     

Al_xGa_(1-x)As:Si中DX中心的双极化子机制及统计分析

基于极化子和双极化子的物理图像,采用无拟合参数的巨正则统计方法计算了Si掺杂的AlxGa1-xAs的导带载流子浓度,计算得到的理论结果从高温到低温都与实验结果定量一致.计算证实了AlxGa1-xAs:Si中的DX中心的基态DX-是电子-晶格相互作用产生的负电双极化子.处于热平衡状态时,施主Si在AlxGa1-xAs中除了电离状态,处于不同晶格构型的单、双极化子态共存,低温时双极化子态被冻结;光照下发生持续光电导时,双极化子态变成单极化子态同时向导带释放一个电子,此过程伴随着进一步的晶格弛豫.理论与实验的对照说明单电子局域的DX0态在热平衡时并不能稳定存在,这和提出的双极化子机制是完全一致的.     

低维凝聚态物理的进展(Ⅱ)聚合物中的元激发

本文介绍八十年代以来在聚合物研究中提出的新物理概念——各种非线性元激发。静态的有孤子、极化子、双极化子和孤子晶格;动力学的有呼吸子和定域振动模。本文着重描绘这些元激发的物理图景以及与实验现象之间的关系,适当给出基本的理论公式。     

低维凝聚态物理的进展(Ⅱ)聚合物中的元激发

本文介绍八十年代以来在聚合物研究中提出的新物理概念——各种非线性元激发。静态的有孤子、极化子、双极化子和孤子晶格;动力学的有呼吸子和定域振动模。本文着重描绘这些元激发的物理图景以及与实验现象之间的关系,适当给出基本的理论公式。     

基态非简并聚合物中的极化子和双极化子的动力学(Ⅱ)--强电场下研究

本文模拟了基态非简并聚合物中的极化子和双极化子在强电场中的运动 ,研究了在不同简并参数的系统中极化子和双极化子的动力学稳定性,发现在强场下简并参数对极化子的稳定性影响不大,极化子晶格畸变的运动滞后于电子的运动,价带上的电子在强电场的激发下参与导电.双极化子在简并参数大的系统中较为稳定.     

关于双极化子模型的一个注记

本文对双极化子的一个简单模型作了重新考虑;证明由于电声子相互作用,为了保证模型系统的总能量为稳定的极小声子系统必然处在一种新的相干态。作为推论,我们论证了单态双极化子作为系统的稳定基态组态的可能性要比以往一些作者所认为的大得多。 关键词：     

Understanding the role of particle size on photophysical properties of CdS:Eu nanocrystals

Here, we report the role of particle size on the photoluminescence (PL) properties of CdS:Eu³⁺ nanocrystals by steady-state and time-resolved PL spectroscopy. It is found that the average decay time 〈τ〉 of undoped CdS nanocrystals increases with increasing the size. The fast component (nanosecond) is assigned due to trapping and slow component (above 10 ns) is due to defect-related emission. The decrease of fast component from 6.6 to 1.32 ns and the slow component from 20 to 14.6 ns of CdS (host) is observed in presence of Eu ions, indicating that the energy transfer occurs from CdS nanoparticles to Eu³⁺ ions. The decay time of Eu³⁺ in CdS shows two decay components (microsecond scale) and we believe that the fast component is attributed to surface-bound Eu³⁺ ions and slow component is due to lattice-bound Eu³⁺ ions. Analysis suggests that PL efficiency of Eu³⁺ ions depends on size of nanoparticles.     

Polaron crossover and bipolaronic metal-insulator transition in the half-filled Holstein model

The formation of a finite-density polaronic state is analyzed in the context of the Holstein model using the dynamical mean-field theory. The spinless and spinful fermion cases are compared to disentangle the polaron crossover from the bipolaron formation. The exact solution of dynamical mean-field theory is compared with weak-coupling perturbation theory, noncrossing (Migdal), and vertex correction approximations. We show that polaron formation is not associated with a metal-insulator transition, which is instead due to bipolaron formation.     

The effects of temperature on the formation and stability of bipolarons in conjugated polymers

Within the one-dimensional tight-binding Su-Schrieffer-Heeger (SSH) model modified to include thermal effects and electron-electron interactions, the transition from polaron to bipolaron and the formation of bipolaron induced by injecting charges are separately simulated using a nonadiabatic evolution method. For the transition process, it is found that with the temperature effects taken into account, two separate polarons with the same charge and antiparallel spins can recombine into a bipolaron. The results show that with the temperature increasing, the time taken for the recombination of two polarons decreases. The effects of different distances between the polarons are also numerically simulated. For the bipolaron formation, we investigate the evolution of two charges injected into a polymer chain with the thermal effects. We find that the bipolaron is always quickly formed and its dynamical stability is less sensitive to the temperature change. Thermal effects can only affect the degree of the charges localization.     

Bipolaron in a quasi-0D quantum dot

Bipolaron states in a quasi-0D quantum dot with a spherical parabolic confinement potential are investigated by applying the Feynman variational principle. The bipolaron coupling energy and self-action potential energy are found to increase with an increase in the Fröhlich electron–phonon-coupling constant. There is also a non-monotonic dependence of the bipolaron coupling energy on the quantum dot radius. With decreasing structure radius the bipolaron coupling energy increases. However, from a critical radius it starts decreasing as the radius decreases, due to the dominance of the coulomb-to-phonon mediated interaction. When electrons in the bipolaron are forcefully neighboured, the polarization of the structure is intensified and consequently there is Coulomb repulsion. The possibility of bipolaron formation depends on the strength of the direct Coulomb repulsion which, in turn, depends on the quantum dot radius. The main contribution to the bipolaron coupling energy comes from the self-action potential. This self-action potential energy influences the energy state of the bipolaron considerably. The ratio of optical-to-static dielectric constants significantly affects the bipolaron coupling energy.     

聚对苯撑[poly(p-phenylene)]的基态、极化子和双极化子激发态

本文在紧束缚模型基础上,对聚对苯撑类材料提出一简单的哈密顿量,并计算了系统的能带结构,给出基态、极化子、双极化子激发态,得到了双极化子比两个单极化子更容易激发这一重要结论。在能带中除发现能隙内的两个极化子定域能级外,还发现导带和价带都要发生二劈裂,带边上出现浅能级,它们对应的电子态均是定域的。 关键词：     

Velocity distribution of desorbing CO2 in CO oxidation on Pd(110) under steady-state conditions

The velocity and angular distributions of desorbing CO₂ in CO oxidation on Pd(110) were studied under the steady-state conditions with cross-correlation time-of-flight techniques. Sharp changes were found in both distributions at the switching point of the rate-determining step. The velocity curves involved fast and slow components. The fast one, which was suddenly suppressed at the switching point, showed a translational temperature from 2300 to 4000 K. The slow component was thermalized to the surface temperature. A rapid site switching for CO₂ formation is proposed, in which the fast component is formed on flat parts of the surface and the slow one is formed on structural defects.     

Electron-phonon interaction in cylindrical and planar quantum wires

Within the framework of the Feynman variational method the bipolaron binding energy and the bipolaron effective mass in cylindrical and planar quantum wires with ‘parabolic’ confinement are calculated. An analogy is found between the effects due to strong confinement and those due to the application of a strong magnetic field. Strengthening the confinement leads to an enlargement of the bipolaron stability region.     

Size-dependent optical properties and carriers dynamics in CdSe/ZnS quantum dots

Size-dependence of optical properties and energy relaxation in CdSe/ZnS quantum dots （QDs） were investigated by two-colour femtosecond （fs） pump-probe （400/800 nm） and picosecond time-resolved photoluminescence （ps TRPL） experiments. Pump-probe measurement results show that there are two components for the excited carriers relaxation, the fast one with a time constant of several ps arises from the Auger-type recombination, which shows almost particle sizeindependence. The slow relaxation component with a time constant of several decades of ns can be clearly determined with ps TRPL spectroscopy in which the slow relaxation process shows strong particle size-dependence. The decay time constants increase from 21 to 34 ns with the decrease of particle size from 3.2 to 2.1 nm. The room-temperature decay lifetime is due to the thermal mixing of bright and dark excitons, and the size-dependence of slow relaxation process can be explained very well in terms of simple three-level model.     

Ultrafast photoconductive response of semiconducting diamond

The photoconductive response of natural p-type semiconducting diamond has been investigated using nanosecond 1.06μm laser pulses. The photocurrent indicates the presence of fast (τ ? 1 nsec) and slow (τ ～ 10 msec) mechanisms which contribute to the free hole lifetime. The fast decay component is attributed to valence band-acceptor recombination. The slow component results from bound holes optically excited from donors to acceptors being thermally promoted to the valence band. These holes return to the donors via the acceptors at a rate determined by the long acceptor-donor recombination time.     

Stability of bipolarons, electron-electron correlations, the variational principle, and the virial theorem

For bipolaron states, a relation is established between the variational principle and the virial theorem optimizing the electronic wave functions. It is shown that a bipolaron one-center state is unstable under any conditions that do not violate basic physical principles. It is established that a stage-by-stage increase in the flexibility of the electronic wave function due to the electron-electron correlations does not stabilize a one-center bipolaron. It is argued that the results of calculations based on the one-center bipolaron model are incorrect.