Phase separation and vortex states in binary mixture of Bose-Einstein condensates in trapping potentials with displaced centers

A system of two simultaneously trapped condensates consisting of ⁸⁷Rb atoms in two different hyperfine states is investigated theoretically in the case where the minima of the trapping potentials are displaced with respect to each other. It is shown that the small shift of the minima of the trapping potentials leads to a considerable displacement of the centers of mass of the condensates, in agreement with the experiment. It is also shown that the critical angular velocities of the vortex states of the system drastically depend on the shift and the relative number of particles in the condensates, and there is a possibility to exchange the vortex states between condensates by shifting the centers of the trapping potentials.     

Two-equivalent and one-equivalent surface states

Two concepts are discussed for the control of semiconductor surface properties by chemical surface state additives. The first is that additives can be used which lead to both filled and empty states, and both are required to control the surface Fermi level. Chemically this is equivalent to adding the impurity in two different oxidation states (valences). It is shown that if one uses this approach, accurate control of the surface Fermi level should be possible for stable systems. The second concept is that it is important to distinguish between two-equivalent and one-equivalent species for use as surface states. The former are chemicals in which a relatively unstable oxidation state exists between the stable oxidation states of interest. The latter (one-equivalent) are chemicals with adjacent stable oxidation states, i.e. stable forms separated by only one electronic charge. Analysis shows that two-equivalent impurities often will behave like minority carrier trapping centers.     

Minimal excitation states of electrons in one-dimensional wires

A strategy is proposed to excite particles from a Fermi sea in a noise-free fashion by electromagnetic pulses with realistic parameters. We show that by using quantized pulses of simple form one can suppress the particle-hole pairs which are created by a generic excitation. The resulting many-body states are characterized by one or several particles excited above the Fermi surface accompanied by no disturbance below it. These excitations carry charge which is integer for noninteracting electron gas and fractional for Luttinger liquid. The operator algebra describing these excitations is derived, and a method of their detection which relies on noise measurement is proposed.     

On the formation of dissipative spatial patterns of charge carriers in biosystems

Summary It is shown that in systems like large aggregates of biological molecules, population inversion of charge carriers, for example as produced by photoexcitation processes, may have competitive advantage beyond critical levels of excitation to produce ordered spatial structures (morphological transitions). In our analysis electromagnetic radiation transfers electrons from bonding states into a continuum of itinerant antibonding states in ap-type doped sample. In this system, in which energy is pumped continuously by an external source, the interplay of collective and dissipative processes can be responsible for the condensation of a self-organized spatially ordered structure. The study we present here is carried out resorting to the powerful nonequilibrium statistical operator method, thus showing that it can be provide a mechano-statistical formalism at the microscopic level for the treatment of Prigogine's synergetic dissipative structures. The authors of this paper have agreed to not receive the proofs for correction     

Surface charge density,membrane potential and salt flux

The surface charge density of a passive membrane may be determined from the Gouy-Chapman double layer potentials at the membrane surfaces. Here, we show the formal aspect of a method which allows determination of the surface charge density from measurements of the membrane potential, diffusion potential and salt flux through the considered membranes.     

低强度飞秒激光激发下CdTe和CdTe/CdS核壳量子点的荧光上转换研究

利用400 nm和800 nm不同波长的低强度飞秒激光,对CdTe和CdTe/CdS核壳量子点溶胶进行激发,研究其稳态和时间分辨荧光性质.800 nm飞秒激光激发下,CdTe和CdTe/CdS核壳量子点产生上转换发光现象,上转换荧光峰与400 nm激发下的荧光峰相比蓝移最多达15 nm,而且蓝移值与荧光量子产率有关.变功率激发确认激发光功率与上转换荧光强度间满足二次方关系,时间分辨荧光的研究表明荧光动力学曲线服从双e指数衰减.提出表面态辅助的双光子吸收模型是低激发强度上转换发光的主要机理.CdTe和CdT 关键词： CdTe量子点 CdTe/CdS核壳量子点 时间分辨荧光 上转换荧光     

Time-resolved charge carrier generation from higher lying excited states in conjugated polymers

Sub-ps three-pulse transient differential transmission spectroscopy using two excitation pulses is used to directly investigate the generation of charge carriers in ladder-type poly(para)phenyl in bulk film. The role of higher excited singlet states of both even and odd symmetry is examined and the dynamics of the major processes involved is described quantitatively. The charge generation efficiency is found to depend strongly on the delay between the two excitation pulses. This is explained by the interplay between internal conversion, excitation energy migration, and on-site vibronic relaxation.     

Coherent control of the lattice dynamics of bismuth near the lindemann stability limit

The relative contributions of the anharmonicity of the lattice potential and the nonequilibrium concentration of charge carriers to the time dependence of the coherent A _1g phonon frequency in bismuth excited by high-energy ultrashort laser pulses are studied by the coherent control method. The contributions are separated by the pump-probe method in which excitation is performed by two pulses with a controlled time delay. It is shown that, depending on the relative delay between the pump pulses, both correlation and anticorrelation between the amplitude and the initial frequency of oscillations are observed while the chirp and the initial frequency of the coherent phonon are anticorrelated. In addition, it has been found that the contributions of the lattice and the electronic subsystem are always anticorrelated. Therefore, a certain phase can be assigned to an electronic excitation and it may be suggested that the time dependence of the phonon frequency is determined not only by instantaneous values of the lattice and electronic response but also by the phase relations between them.     

The influence of 3-photon absorption and pulse interactions on discrete X-waves in normally dispersive waveguide arrays

In this paper, we investigate the influences of 3-photon absorption on discrete X-waves in nonlinear normally dispersive waveguide arrays. It is found that 3-photon absorption can cause the decrease of the total power, which results in the appearances of the discrete diffraction for an intermediate input peak-power and the discrete X-wave for a higher input peak-power. Also, the interaction between pulses for different waveguide excitation are studied in detail. The results show that for the in-phase waveguide excitation of neighboring channels, the bound states can be formed by choosing properly the initial peak-power; for the in-phase waveguide excitation of distant channels, however, the bound states can not be formed. For the out-of-phase multiple waveguide excitation, due to interplay the repulsive force and nonlinearity, the interaction of two pulses can form the X-like wave or the double X-like wave as long as choosing the proper input peak-power.     

Influence of fixed electric charges on potential profile across the squid axon membrane

The potential profile for a model of squid axon membrane has been determined for two physiological states: resting and action states. The non-linear Poisson-Boltzmann equation has been solved by considering the volumetric charge densities due to charges dissolved in an electrolytic solution and fixed on both glycocalyx and cytoplasmatic proteins. Results showing the features of the potential profile along the outer electrolytic region are similar for both resting and action states. However, the potential fall along glycocalyx at action state is lower than at resting. A small variation in the Na⁺ concentration drastically affects the surface membrane potentials and vice versa. We conclude that effects on the potential profile due to surface lipidic bilayer charge and contiguous electric double layers are more relevant than those provoked by fixed charges distributed along the cell cytoplasm.     

Excitation of the spin density and current by coherent light pulses in quantum wells

We study the orbital and spin dynamics of charge carriers induced by non-overlapping linearly polarized light pulses in semiconductor quantum wells. It is shown that such an optical excitation with coherent pulses leads to a spin orientation of photocarriers and an electric current. The effects are caused by the interference of optical transitions driven by individual pulses. The distribution of carriers in the spin and momentum spaces depends on the crystallographic orientation of quantum wells and can be efficiently controlled by the pulse polarizations, time delay and phase shift between the pulses, as well as an external magnetic field.     

Polymers and molecular solids: New frontiers in surface science

A brief review is given of the present state of knowledge of the surface properties of polymers and molecular solids. These materials are shown to exhibit surface phenomena which are dramatically different from those characteristic of metals and covalent solids. The origin of these differences resides in the combined occurrence both of large electronic and atomic polarizabilities and of small probabilities for the transfer of an electronic excitation from one molecular site to another. The interplay of these two quantities leads to a diversity in the character of the resulting electronic excitations, ranging from localized molecular ion states in aromatic pendant-group polymers to quasi-one-dimensional metallic behavior in certain charge transfer salts and polymers. The primary role of the surface in such materials is the introduction of large, inhomogeneous fluctuations in the relaxation energies associated with the polarization of the solid by an excitation. These fluctuations produce a number of novel phenomena including localized surface states in the absence of dangling bonds, inhomogeneous broadening of photo-emission spectra, and alterations of the charge state of surface molecules. A simple, unified theoretical framework is developed for the interpretation of these phenomena.     

Coherent population trapping of single spins in diamond under optical excitation

Coherent population trapping is demonstrated in single nitrogen-vacancy centers in diamond under optical excitation. For sufficient excitation power, the fluorescence intensity drops almost to the background level when the laser modulation frequency matches the 2.88 GHz splitting of the ground states. The results are well described theoretically by a four-level model, allowing the relative transition strengths to be determined for individual centers. The results show that all-optical control of single spins is possible in diamond.     

Role of metastable states in circumventing laser pulse jitters in multi-step laser excitation experiments

The role of metastable states in circumventing laser pulse jitters in multi-step laser excitation of high-lying Rydberg and autoionizing states is discussed. A case study is presented where step-wise excitation of 3d4d ³G_3,4,5 states of Ca is achieved despite a large jitter between the laser pulses.     

Charging/discharging kinetics in LPCVD silicon nanocrystal MOS memory structures

The paper focuses on the peculiarities of charging/discharging kinetics and write/erase (W/E) window formation in nanocrystal metal-oxide semiconductor (MOS) non-volatile memory (NVM) structures prepared by low-pressure chemical vapor deposition (LPCVD) of amorphous silicon, followed by solid phase crystallization and thermal oxidation. It is generally known that the W/E window formation via pulse injection depends on the kinetics of carriers trapping (electrons and/or holes) in the nanocrystal NVM structure and consequently on the cumulative time of recharging bias application, i.e. pulse duration and number of applied pulses. In this work, we have shown that with the same cumulative time biasing but different charging pulse durations, the resulting W/E window width can be rather different, demonstrating a staircase window formation. This phenomenon is interpreted by a model of partial fast charge draining from the trapping sites in the vicinity of Si nanoclusters into the Si substrate. The detailed experimental investigation of charging/discharging kinetics of the considered structures in combination with computer simulations lead to the conclusion that there is a single process of negative charge trapping with a time constant of 235±35 ms and at least four processes of positive charge trapping with time constants distributed in the range from <10 ms to >10 s.     

Al掺杂对HfO2俘获层可靠性影响第一性原理研究

采用基于MS(Materials Studio)软件和密度泛函理论的第一性原理方法, 研究了HfO₂ 俘获层的电荷俘获式存储器(Charge Trapping Memory, CTM)中电荷的保持特性以及耐擦写性. 在对单斜晶HfO₂中四配位氧空位(V_O4) 缺陷和V_O4 与Al替位Hf掺杂的共存缺陷体(Al+V_O4)两种超晶胞模型进行优化之后, 分别计算了其相互作用能、形成能、Bader电荷、态密度以及缺陷俘获能. 相互作用能和形成能的计算结果表明共存缺陷体中当两种缺陷之间的距离为2.216 Å时, 结构最稳定、缺陷最容易形成; 俘获能计算结果表明, 共存缺陷体为双性俘获, 且与V_O4缺陷相比, 俘获能显著增大; Bader电荷分析表明共存缺陷体更有利于电荷保持; 态密度的结果说明共存缺陷体对空穴的局域能影响较强; 计算两种模型擦写电子前后的能量变化表明共存缺陷体的耐擦写性明显得到了改善. 因此在HfO₂俘获层中可以通过加入Al杂质来改善存储器的保持特性和耐擦写性. 本文的研究可为改善CTM数据保持特性和耐擦写性提供一定的理论指导.     

Potential of two-photon-excited fluorescence for distributed fiber sensing

The design and operating parameters of a new class of continuously distributed optical fiber sensor are described. By use of counterpropagating pulses, two-photon excitation of fluorescence from ions doped into the fiber enables any position to be monitored. By this means temperature and also strain may be sensed with high spatial and temporal resolution. As the doped fiber is transparent for single-photon absorption at the wavelength of the light pulses, attenuation does not set an upper limit to its length.     

Fast polariton relaxation in strongly coupled organic microcavities

We consider the regime of strong light-matter coupling in an organic microcavity, where large Rabi splitting can be achieved. As has been shown, the excitation spectrum of such a structure, besides coherent polaritonic states, contains a number of strongly spatially localized incoherent excited states. These states form the majority of the excited states of the microcavity and are supposed to play the decisive role in the relaxation dynamics of the excitations in the microcavity. We consider the non-radiative transition from an incoherent excited state into one of the coherent states of the lower polaritonic branch accompanied by emission of a high-energy intramolecular phonon. It is shown that this process may determine the lifetime of incoherent excited states in the microcavity. This observation may be important in the discussion of pump–probe experiments with short pulses. This process may also play an important role for the population of the lowest energy states in organic microcavities, and hence in the problem of condensation of cavity polaritons.     

Elliptic dark states: Explicit invariant form

An analysis is made of coherent population trapping as a result of resonant interaction of elliptically polarized light with atoms whose energy levels are degenerate with respect to the projection of the angular momentum and are coupled by a dipole transition. Explicit invariant expressions for dark states are obtained in tensor form for all transitions where population trapping occurs. A correspondence is established between the vector of the elliptic polarization and the pair of associated spinors. It is shown that all dark states can be constructed from these spinors by means of a multiple tensor product. For integer values of the angular momenta of the transitions these constructions reduce to spherical functions of a complex variable. As applications analytic expressions are obtained for the dark magneto-optic and geometric potentials, and the change in their profile with increasing angular momenta is analyzed.     

Diagnostics of many-particle electronic states: Non-stationary currents and residual charge dynamics

We propose the method for identifying many particle electronic states in the system of coupled quantum dots (impurities) with Coulomb correlations. We demonstrate that different electronic states can be distinguished by the complex analysis of localized charge dynamics and non-stationary characteristics. We show that localized charge time evolution strongly depends on the properties of initial state and analyze different time scales in charge kinetics for initially prepared singlet and triplet states. We reveal the conditions for existence of charge trapping effects governed by the selection rules for electron transitions between the states with different occupation numbers.