Laser field dependence of intersubband transition in inverse V‐shaped quantum wells

The electron energy levels and the envelope wave functions in inverse GaAs/AlGaAs V‐shaped quantum wells (QWs) are calculated using the transfer matrix method. The influence of applied electric and laser fields on the electronic distribution is investigated. In studied systems, a laser‐induced attenuation for anomalous electric polarization of the excited state is found. Also, an oscillator strength increasing under high‐frequency laser radiation is obtained. The results presented in this article can be useful for novel device applications based on the intersubband transitions of electrons. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Binding energies of on-axis hydrogenic and nonhydrogenic donors in GaAs/Ga1−x Al x As

In a previous work [Phys. Rev. B43(1991)9262], the binding energies of hydrogenic and nonhydrogenic on-axis donors in GaAs/Ga_1?x A1 _X As quantum-well wires of circular cross section have been calculated as functions of the radius of the quantum-well wire. In both the hydrogenic and nonhydrogenic cases, a variational trial wave function was chosen that could be written as the product of an “envelope function”, and a function containing the variational parameter. It was assumed in these calculations that the potential barrier that exists at the surface of the GaAs cylinder and the surrounding Ga_1?x A1 _X As matrix is infinite. For the envelope function, an ordinary Bessel function of the first kind and of order zero was chosen. This envelope function satisfies the boundary condition, the vanishing of the trial function at the surface of the quantum-well wire. The question arises: how sensitive are the calculated binding energies to the choice of the envelope function? In the present work, we attempt to provide a partial answer to this question by choosing another envelope function, a spherical Bessel function of order zero. This function also satisfies the boundary condition and makes the trial wave function vanish at the interface between the GaAs cylinder and the Ga_1?x A1 _X As matrix. Our calculations show that the binding energies of both the hydrogenic and the nonhydrogenic on-axis donors depend on the choice of the envelope function.     

Local control of the quantum dynamics in multiple potential wells

The driven wave-packet dynamics in potentials exhibiting several potential wells is investigated. Therefore, local-control strategies are employed where the control field is constructed from the system's dynamics at any instant of time. It is shown that particles can be moved successively between various potential minima. Furthermore, results presented indicate that the intuitive local-control scheme allows for the initiation of a clockwise or counterclockwise rotational motion of a model molecular motor.     

Three-dimensional electronic spectroscopy of excitons in asymmetric double quantum wells

We demonstrate three-dimensional (3D) electronic spectroscopy of excitons in a double quantum well system using a three-dimensional phase retrieval algorithm to obtain the phase information that is lost in the measurement of intensities. By extending the analysis of two-dimensional spectroscopy to three dimensions, contributions from different quantum mechanical pathways can be further separated allowing greater insight into the mechanisms responsible for the observed peaks. By examining different slices of the complete three-dimensional spectrum, not only can the relative amplitudes be determined, but the peak shapes can also be analysed to reveal further details of the interactions with the environment and inhomogeneous broadening. We apply this technique to study the coupling between two coupled quantum wells, 5.7 nm and 8 nm wide, separated by a 4 nm barrier. Coupling between the heavy-hole excitons of each well results in a circular cross-peak indicating no correlation of the inhomogeneous broadening. An additional cross-peak is isolated in the 3D spectrum which is elongated in the diagonal direction indicating correlated inhomogeneous broadening. This is attributed to coupling of the excitons involving the two delocalised light-hole states and the electron state localised on the wide well. The attribution of this peak and the analysis of the peak shapes is supported by numerical simulations of the electron and hole wavefunctions and the three-dimensional spectrum based on a density matrix approach. An additional benefit of extending the phase retrieval algorithm from two to three dimensions is that it becomes substantially more reliable and less susceptible to noise as a result of the more extensive use of a priori information.     

Approximate energy expression in the calculation of photoelectron transition energies

The possibilities of using approximate self-consistent field energy functionals to reduce computation required in calculating photoelectron transition energies is examined. Numerical results for the H₂O molecule indicate that this is possible without unacceptable deterioration in accuracy of results in favourable cases.     

Fingerprints of delocalized transition states in quantum dynamics

Reactions with delocalized transition states (plateau reactions) can be characterized statically by their energy profile along the reaction path, where they exhibit a broad, flat region instead of one or several well-defined saddle points on the potential energy surface. Employing our new, highly flexible quantum dynamics code to perform two-dimensional and effective four-dimensional quantum wave packet propagations on ab initio based model potentials, we show that plateau reactions can also be discerned from the other standard reaction types by their dynamics.     

Time-dependent density-functional calculations of S0-S1 transition energies of poly(p-phenylene vinylene)

We used time-dependent density-functional-theory (TDDFT) approaches to calculate absorption (S(0)-->S(1)) and emission (S(1)-->S(0)) transition energies of poly(p-phenylene vinylene) (PPV). The absorption and emission energies were estimated to be 2.44 and 2.16 eV, respectively, from the extrapolation of calculated results for oligomers. Comparisons with available experimental data demonstrated that TDDFT is a very reliable tool for investigating the electronic transitions of PPV.     

Multiple dot-in-rod PbS/CdS heterostructures with high photoluminescence quantum yield in the near-infrared

Pb cations in PbS quantum rods made from CdS quantum rods by successive complete cationic exchange reactions are partially re-exchanged for Cd cations. Using STEM-HAADF, we show that this leads to the formation of unique multiple dot-in-rod PbS/CdS heteronanostructures, with a photoluminescence quantum yield of 45-55%. We argue that the formation of multiple dot-in-rods is related to the initial polycrystallinity of the PbS quantum rods, where each PbS crystallite transforms in a separate PbS/CdS dot-in-dot. Effective mass modeling indicates that electronic coupling between the different PbS conduction band states is feasible for the multiple dot-in-rod geometries obtained, while the hole states remain largely uncoupled.     

Electronic excitations in narrow quantum wells via intersubband Raman scattering: Theoretical considerations

In this work a generalized self-consistent field theory was applied to investigate the elementary excitations of two-dimensional electron gas formed from narrow quantum wells via resonant intersubband Raman scattering. The developed model considers the existence of equally coupled and degenerated excitations of the electron gas and allows to observe that in extreme resonance regime the plasma oscillations splits into two contributions: a set of renormalized collective excitations (plasmons) and unrenormalized electronic transitions (single-particle excitations). Our results show that the asymmetries which appear in the Raman profile of doped narrow quantum wells can be interpreted as the entrance or exit of resonance of collective modes overlapped with single-particle transitions.     

Effect of the second coordination sphere on the electron transition energies in ZnS-Mn

The influence of the second coordination sphere on the energy of electron transitions in ZnS-Mn is investigated using the crystal field theory. The obtained perturbation effect is small and may be usually neglected.

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Einfluß der zweiten Koordinationssphäre auf die Energie der Elektronenübergänge in ZnS-Mn (Kurze Mitteilung)

Zusammenfassung Der Einfluß der zweiten Koordinationssphäre auf die Energie der Elektronenübergänge in ZnS-Mn wird mit Hilfe der Kristallfeldtheorie untersucht. Es ergibt sich, daß der Störeffekt klein ist und gewöhnlich vernachlässigt werden kann.

     

Sol - gel transition in biopolymers

Rheological measurements have been performed on pectin biopolymers close to the sol-gel transition. From these measurements scaling exponents were determined independently for the viscosity, s = 0.82(5), for the elastic modulus, t = 1.93(8), for the frequency-dependent modulus, Δ = 0.71(2), and for the relaxation times below and above the transition, vz = 2.67(12) and vz' = 2.65(9). The exponents satisfy the scaling relations predicted by the theory and their numerical values agree with those from scalar elasticity percolation.     

基于噻吩并吡嗪类的-(D-A_1)_m-(D-A_2)_n-型三元无规共聚物的合成及光电性能

选用苯并二噻吩(BDT)类衍生物作为给体单元D,并选用噻吩并吡咯二酮(TPD)类衍生物(A1)和噻吩并吡嗪(TP)类衍生物(A2)作为共同的受体单元,通过Stille偶联聚合制备了-(DA_1)_m-(D-A_2)_n-型三元无规共聚物,并同时合成了基于BDT和TP的二元共聚物。采用核磁共振氢谱(1 H-NMR)、凝胶液相色谱(GPC)和热重(TG)表征聚合物的结构与性能;采用紫外-可见光谱和循环伏安法测试聚合物的光电性能,研究了以这类聚合物为给体材料制备的太阳能器件的光伏性能。结果表明:三元无规共聚物具有较高的相对分子量和热稳定性,在太阳光范围具有强而宽的吸收,同时具有相对较低的最高电子占用轨道HOMO和最低电子占用轨道LUMO能级。基于三元无规共聚物P2的器件,其光电转换效率可达到1.22%,优于相应的二元共聚物P1的1.15%。     

Environmentally induced quantum dynamical phase transition in the spin swapping operation

Quantum information processing relies on coherent quantum dynamics for a precise control of its basic operations. A swapping gate in a two-spin system exchanges the degenerate states |(up arrow, down arrow)> and |(down arrow, up arrow)>. In NMR, this is achieved turning on and off the spin-spin interaction b=DeltaE that splits the energy levels and induces an oscillation with a natural frequency DeltaE/Planck's. Interaction of strength Planck's/tau(SE), with an environment of neighboring spins, degrades this oscillation within a decoherence time scale tau(phi). While the experimental frequency omega and decoherence time tau(phi) were expected to be roughly proportional to b/Planck's and tau(SE), respectively, we present here experiments that show drastic deviations in both omega and tau(phi). By solving the many spin dynamics, we prove that the swapping regime is restricted to DeltaEtau(SE) similar or greater than Planck's. Beyond a critical interaction with the environment the swapping freezes and the decoherence rate drops as 1/tau(phi) proportional to (b/Planck's)2tau(SE). The transition between quantum dynamical phases occurs when omega proportional to sqrt (b/Planck's)2-(k/tau(SE)2 becomes imaginary, resembling an overdamped classical oscillator. Here, 0< or =k2< or =1 depends only on the anisotropy of the system-environment interaction, being 0 for isotropic and 1 for XY interactions. This critical onset of a phase dominated by the quantum Zeno effect opens up new opportunities for controlling quantum dynamics.     

Binding energies of first row diatomics in the light of the interacting quantum atoms approach

Binding energies of first row diatomics are revisited within the interacting quantum atoms (IQA) approach. This is a formalism in chemical bonding theory based upon the quantum theory of atoms in molecules. It is characterized by the preservation of the energetic identity of atoms within molecules. Quantum mechanically computed binding energies are recovered in IQA as a sum of small atomic deformation energies and large pairwise interaction terms. We show how this partition responds faithfully to chemical intuition, and how the different evolution of deformations and interactions accounts in a unified manner for the subtle variations of the binding energy of these molecules.     

Rotational and vibrational energy transfer in vibrationally excited acetylene at energies near 6560 cm(-1)

Collisional energy transfer kinetics of vibrationally excited acetylene has been examined for states with internal energies near 6560 cm(-1). Total population removal rate constants were determined for selected rotational levels of the (1,0,1,0(0),0(0)) and (0,1,1,2(0),0(0)) states. Values in the range of (10-18) × 10(-10) cm(3) s(-1) were obtained. Measurements of state-to-state rotational energy transfer rate constants were also carried out for these states. The rotational energy transfer kinetics was found to be consistent with simple energy gap models for the transfer probabilities. Vibrational transfer out of the (0,1,1,2(0),0(0)) state accounted for no more than 16% of the total removal process. Transfer from (1,0,1,0(0),0(0)) to the u-symmetry (0,2,0,3(1),1(-1)), (0,1,1,2(0),0(0)), and (1,1,0,1(1),1(-1)) states was observed. Applying the principle of detailed balance to these data indicated that vibrational transfer to (1,0,1,0(0),0(0)) accounted for ~0.1% of the population loss from (0,2,0,3(1),1(-1)) or (0,1,1,2(0),0(0)), and 3% of the loss from (1,1,0,1(1),1(-1)). Relative rotational transfer probabilities were obtained for transfer to the g-symmetry (1,1,0,2(0),0(0))∕(0,0,2,0(0),0(0)) dyad. These results are related to recent studies of optically pumped acetylene lasers.     

InxGa1-xAs盖层InAs量子点的发光性能研究

用分子束外延系统在GaAs(001)衬底上生长InAs量子点,在InAs量子点上插入3 nm的In_0.4Ga_0.6As层,可将量子点发射波长调谐到1 300 nm附近.对样品进行氢等离子处理,研究处理前后样品的InAs量子点光致发光(PL)强度的变化.结果表明,在InAs量子点与相邻层的界面上以及GaAs层中存在界面缺陷,采用氢等离子处理可有效地抑制界面缺陷,大幅度地提高发光效率.