几种稠环芳烃有机EL材料性能的量子化学理论研究

采用量子化学半经验方法（ＲＨＦ／ＰＭ３）对几种稠环芳烃化合物电致发光（ＥＬ）材料的性质进行了理论研究。利用能量梯度法优化构型,对各优化的构型作振动分析,均未出现虚频率。在此基础上,采用ＲＨＦ／ＣＩＳ方法计算其电子光谱。并给出了化合物ＥＬ１,ＥＬ２电子光谱的最大波长λ与ＣＩＳ组态之间的关系。所有计算结果与实验值基本吻合。     

The Spectra of Two-Electron Quantum Dot: Shifted 1/N Method

The spectra of two-electron quantum dot in a magnetic field of arbitrary strength is studied by using the shifted 1/N expansion method. The comparisons show that our results are in good agreement with the results of fixed-phase quantum Monte Carlo method and exact ones [Bolton, Phys. Rev. B54 (1996) 47801.     

Dissipation, decoherence and preparation effects in the spin-boson system

The dynamics of the reduced density matrix of the driven dissipative two-state system is studied for a general diagonal/off-diagonal initial state. We derive exact formal series expressions for the populations and coherences and show that they can be cast into the form of coupled nonconvolutive exact master equations and integral relations. We show that neither the asymptotic distributions, nor the transition temperature between coherent and incoherent motion, nor the dephasing rate and relaxation rate towards the equilibrium state depend on the particular initial state chosen. However, in the underdamped regime, effects of the particular initial preparation, e.g. in an off-diagonal state of the density matrix, strongly affect the transient dynamics. We find that an appropriately tuned external ac-field can slow down decoherence and thus allow preparation effects to persist for longer times than in the absence of driving. Received 23 October 1998 and Received in final form 26 February 1999     

High-field magnetic phases of a two-leg spin ladder: Cu2(C5H12N2)2Cl4

CHCupCl is a two-leg spin-ladder where the Cu⁺⁺ ions are coupled by superexchange. Its magnetic properties change dramatically as a function of a magnetic field. Below the critical field H_c1, there is a singlet ground state separated from the triplet excited state by an energy gap. Between H_c1 and H_c2, several magnetic phases have been observed. Above 1 K, dynamical studies reveal a disordered quantum critical phase. At lower temperatures, a 3-D ordered phase has been observed in specific heat and NMR studies. New NMR results on the 3D-ordered and quantum critical phases are presented in this short report.     

Nonlinear Optical Properties of Semiconductor-Doped Sol-Gel Waveguides

Thin silica-titania films doped with CdS and PbS nanocrystals have been prepared by the sol-gel route. Their nonlinear properties have been studied using the techniques of degenerated four-wave mixing and m-lines with picosecond (ps) and nanosecond laser pulses. Depending on wavelength, doping level, and laser pulse duration, high negative nonlinearity was found for CdS-doped (n₂ = -2 x 10^-8 cm² per kW) and PbS-doped films (-10^-10 to -2 x 10^-7 cm² per kW). The response time of the nonlinearity was below 35 ps. Saturation of the nonlinearity was observed. Straight, monomode channel waveguides have been fabricated on these films. The influence of MIE-scattering due to the nanoparticles is investigated.     

Optical properties of individual manganese-doped quantum dots

The magnetic state of a single magnetic ion (Mn²⁺) embedded in an individual quantum dot is optically probed using micro-spectroscopy. The fine structure of a confined exciton in the exchange field of a single Mn²⁺ ion (S=) is analyzed in detail. The exciton–Mn²⁺ exchange interaction shifts the energy of the exciton depending on the Mn²⁺ spin component and six emission lines are observed at zero magnetic field. The emission spectra of individual quantum dots containing a single magnetic Mn atom differ strongly from dot to dot. The differences are explained by the influence of the system geometry, specifically the in-plane asymmetry of the quantum dot and the position of the Mn atom. Depending on both these parameters, one has different characteristic emission features which either reveal or hide the spin state of the magnetic atom. The observed behavior in both zero field and under magnetic field can be explained quantitatively by the interplay between the exciton–Mn²⁺ exchange interaction (dependent on the Mn position) and the anisotropic part of the electron–hole exchange interaction (related to the asymmetry of the quantum dot).     

The characteristics for H<Subscript> 2</Subscript><Superscript> +</Superscript>-like impurities confined by spherical quantum dots

The energy spectra of H₂ ⁺-like impurities confined in finite spherical quantum dots have been calculated as a function of the distance between nuclear with different sizes on the basis of effective-mass approximation by linear variational method. B-splines have been used as basis functions, which can easily construct the trial wavefunctions with appropriate boundary and cusp conditions. The quantitative analyses of the partial wave weights for ground state and some low lying states have been done.     

A Representation of Quantum Measurement in Order-Unit Spaces

A certain generalization of the mathematical formalism of quantum mechanics beyond operator algebras is considered. The approach is based on the concept of conditional probability and the interpretation of the Lüders-von Neumann quantum measurement as a probability conditionalization rule. A major result shows that the operator algebras must be replaced by order-unit spaces with some specific properties in the generalized approach, and it is analyzed under which conditions these order-unit spaces become Jordan algebras. An application of this result provides a characterization of the projection lattices in operator algebras.     

Surface-Active Fluorinated Quantum Dots for Enhanced Cellular Uptake

Fluorescent nanoparticles, such as quantum dots, hold great potential for biomedical applications, mainly sensing and bioimaging. However, the inefficient cell uptake of some nanoparticles hampers their application in clinical practice. Here, the effect of the modification of the quantum dot surface with fluorinated ligands to increase their surface activity and, thus, enhance their cellular uptake was explored.     

Energy band modulation of GaAs/Al0.26Ga0.74As quantum well in 3D self-assembled nanomembranes

In this study, we investigate the modulation of energy band in 3D self-assembled nanomembranes containing GaAs/Al_0.26Ga_0.74As quantum wells (QWs). Photoluminescence (PL) characterizations demonstrate that the self-assembled structures have different optical transition properties and the modulation of the energy band is thus realized. Detailed spectral analyses disclose that the small strain change in structures with different curvatures cannot cause remarkable change in energy bands in Al_0.26Ga_0.74As layer. On the other hand, the optical transitions of GaAs QW layer is influenced by the strain evolution in term of light emission intensity. We also find the first order Stark effect in rolled-up nanomembrane with diameter of 150 μm, which is closely connected with the coupling effect between the deformation potential and the piezoelectric potential. Our work may pave a way for the fabrication of high performance rolled-QW infrared photo-detectors.