Damping of coherent oscillations in a quantum dot photodiode

In this letter, we develop a model to describe the Rabi oscillations observed in a quantum-dot photodiode. Using a multi-level density matrix formulation, which includes multi-exciton and single particle states, we show that the damping observed in recent experiments is the result of a non-resonant excitation from or to the continuum of the wetting layer states.     

Migration of Bosonic particles across a Mott insulator to a superfluid phase interface

We consider a boundary between a Mott insulator and a superfluid region of a Bose-Hubbard model at unit filling. Initially both regions are decoupled and cooled to their respective ground states. We show that, after switching on a small tunneling rate between both regions, all particles of the Mott region migrate to the superfluid area. This migration takes place whenever the difference between the chemical potentials of both regions is less than the maximal energy of any eigenmode of the superfluid. We verify our results numerically with density matrix renormalization group simulations and explain them analytically with a master equation approximation, finding good agreement between both approaches. Finally we carry out a feasibility study for the observation of the effect in coupled arrays of microcavities and optical lattices.     

How white noise generates power-law switching in bacterial flagellar motors

The clockwise (CW) or counterclockwise (CCW) spinning of a bacterial flagellar motor is controlled by the concentration [Y] of a phosphorylated protein, CheY-P. Representing the stochastic switching behavior of the motor by a dynamical two-state (CW and CCW) model, whose energy levels fluctuate in time (t) as [Y](t) fluctuates, we show that temporal fluctuations in [Y](t) can generate a power-law distribution for the durations of the CCW states, in agreement with recent experiments. Correlations between the duration times of nearby CCW (CW) intervals are predicted by our model, and shown to exist in the experimental data and to affect the power spectrum for motor switching.     

Microscopic simulation of the polarization switching at ferroelectric first-order phase transitions

Simulations of the kinetics of polarization switching at first-order ferroelectric phase transitions are carried out within the framework of Ishibashi dipole lattice model. We consider the influence of the dipole–dipole interaction between polar units on the polarization switching taking into account the existence of metastable states. We also examine the size effect on the switching kinetics. We calculate hysteresis loops and switching rates in the systems in which the continuum approximation is not valid. We show that in the case of hundreds of polar units our equations give the known analytical expressions for the switching rate.     

Magnetization switching driven by spin-transfer-torque in high-TMR magnetic tunnel junctions

This paper presents a numerical study of magnetization switching driven by spin-polarized current in high-TMR magnetic tunnel junctions (TMR>100%). The current density distribution throughout the free-layer is computed dynamically, by modeling the ferromagnet/insulator/ferromagnet trilayer as a series of parallel resistances. The validity of the main hypothesis, which states that the current flows perpendicular to the sample plane, has been verified by numerically solving the Poisson equation. Our results show that the nonuniform current density distribution is a source of asymmetry to the switching process. Furthermore, we observe that the reversal mechanisms are characterized by well-defined localized pre-switching oscillation modes.     

忆阻器单阻态下的记忆电容行为及多态特性

采用供体-受体类型的共聚物构建了Al/共聚物/ITO结构的有机记忆器件,并对其电流-电压(I-V)和电容-电压(C-V)特性进行了研究.结果表明:器件不仅表现出明显的记忆电阻特征,而且在单个电阻状态下还存在记忆电容行为,使器件呈现出两种电阻状态和与之对应的四种电容状态,具有电阻和电容的双参量记忆能力.在此基础上对器件的电容开关行为进行了电压幅值的调制,使器件出现了更多的电容状态,为多级存储的实现提供了一条有效途径.最后通过引入分子内部极化算符,建立了记忆电阻和记忆电容的关联性,给出了描述器件双参量多状态特征的矩阵模型.     

Traffic Flow States in a Freeway with Bottleneck

The system of mixture of single lane and double lane is studied by a cellular automata model, which is developed by us based on the Nagel and Schreckenberg's models. We justify that the model can reach a stable states quickly. The density distributions of the stable state is presented for several cases, which illustrate the manner of the congestion. The relationship between the outflow rate and the total number of vehicles and that between the outflow rate and the density just before the bottleneck are both given. Comparing with the relationship that occurring in the granular flow, we conclude that the transition from the free traffic flow to the congested traffic flow can also be attributed to the abrupt variation through unstable flow state, which can naturally explain the discontinuities and the complex time variation behavior observed in the traffic flow experiments.     

High-T(c) ferroelectricity emerging from magnetic degeneracy in cupric oxide

Cupric oxide is multiferroic at unusually high temperatures. From density functional calculations we find that the low-T magnetic phase is paraelectric, and the higher-T one is ferroelectric with a size and direction of polarization in good agreement with experiments. By mapping the ab?initio results on to an effective spin model, we show that the system has a manifold of almost degenerate ground states. In the high-T magnetic state noncollinearity and inversion symmetry breaking stabilize each other via the Dzyaloshinskii-Moriya interaction. This leads to an unconventional mechanism for multiferroicity, with the particular property that nonmagnetic impurities enhance the effect.     

Multiple mobility edges in a 1D Aubry chain with Hubbard interaction in presence of electric field: Controlled electron transport

Electronic behavior of a 1D Aubry chain with Hubbard interaction is critically analyzed in presence of electric field. Multiple energy bands are generated as a result of Hubbard correlation and Aubry potential, and, within these bands localized states are developed under the application of electric field. Within a tight-binding framework we compute electronic transmission probability and average density of states using Green's function approach where the interaction parameter is treated under Hartree–Fock mean field scheme. From our analysis we find that selective transmission can be obtained by tuning injecting electron energy, and thus, the present model can be utilized as a controlled switching device.     

基于分组交换的量子通信网络传输协议及性能分析

量子纠缠交换能够建立可靠的量子远程传输信道, 实现量子态的远程传输. 然而, 基于纠缠交换的量子信道要求网络高度稳定, 否则会浪费大量纠缠资源. 为节省纠缠资源, 本文根据隐形传态理论, 提出了一种基于分组交换的量子通信网络传输协议, 建立了发送量子态所需的纠缠数目与所经过的路由器数、链路错误率的定量关系, 并与纠缠交换传输协议进行了比较. 仿真结果表明, 在链路错误率为0.1% 时, 分组传输协议所使用的纠缠数目少于纠缠交换的数目, 另外, 随着错误率的升高, 分组传输协议所需的纠缠数比纠缠交换协议明显减少. 由此可见, 基于分组交换的量子通信网络传输协议在网络不稳定时, 能够节省大量纠缠资源, 适用于链路不稳定的量子通信网络.     

Regularity of the density of states in the Anderson model on a strip for potentials with singular continuous distributions

We derive regularity properties for the density of states in the Anderson model on a one-dimensional strip for potentials with singular continuous distributions. For example, if the characteristic function is infinitely differentiable with bounded derivatives and together with all its derivatives goes to zero at infinity, we show that the density of states is infinitely differentiable.     

Zur Struktur der Leitungsbänder der kubisch-raumzentrierten Übergangsmetalle

X-ray isochromats of Tantalum, Niobium and Tungsten for different temperatures are presented. They allow a localization of the Fermi edge in the conduction band and show, that the fine structure softens with increasing temperature. This is attributed to a change in the density of states in the conduction band. A modified rigid band model of the density of states for the bcc transition metals is constructed. It agrees well with our isochromats and with published calculations of the density of states for Tungsten. Other electronic properties also fit well into this model. In connection with the interpretation of our isochromats a short discussion of precision experiments for the evaluation ofh/e is added.     

Dynamical models of hadrons based on the string model and the behavior of strongly interacting matter at high density

We propose dynamical models of hadrons, the nucleation model and the free-decay model, in which results of the string model are used to represent interactions. The string model is examined by comparing its predictions with experimental data and parameters are fitted. The equilibrium properties of hadrons at high density are investigated in terms of the nucleation model; we find a singular behavior at energy density 3–5 GeV/fm³, where hadrons coalesce and create highly excited states. We argue that this singular behavior corresponds to the phase transition to quark-gluon plasma. The possibility of observing the production of high-density strongly interacting matter in collider experiments is discussed using the free-decay model, which produces pion distributions as decay products of resonances. We show that our free-decay model recovers features of hadron distributions obtained in hadron collison experiments. Finally, perspectives and extensions of the models are discussed.     

Order and disorder in coupled metronome systems

Metronomes placed on a smoothly rotating disk are used for exemplifying order-disorder type phase-transitions. The ordered phase corresponds to spontaneously synchronized beats, while the disordered state is when the metronomes swing in unsynchronized manner. Using a given metronome ensemble, we propose several methods for switching between ordered and disordered states. The system is studied by controlled experiments and a realistic model. The model reproduces the experimental results, and allows to study large ensembles with good statistics. Finite-size effects and the increased fluctuation in the vicinity of the phase-transition point are also successfully reproduced.     

基于Cu/SiO_x/Al结构的阻变存储器多值特性及机理的研究

采用氧化硅材料构建了Cu/SiOx/Al的三明治结构阻变存储器件.用半导体参数分析仪对其阻变特性进行测量,结果表明其具有明显的阻变特性,并且通过调节限制电流,得到了四个稳定的阻态,各相邻阻态的电阻比大于10,并且具有良好的数据保持能力.在不同温度条件下对各个阻态进行电学测试及拟合,明确了不同阻态的电子传输机理不尽相同:阻态1和阻态2为欧姆传导机制,阻态3为P-F(Pool-Frenkel)发射机制,阻态4为肖特基发射机制.根据电子传输机制,建立了铜细丝导电模型并对Cu/SiOx/Al阻变存储器件各个阻态的电致阻变机制进行解释.     

Decoherence of rabi oscillations in a single quantum dot

We develop a realistic model of Rabi oscillations in a quantum-dot photodiode. Based in a multiexciton density matrix formulation we show that for short pulses the two-level model fails and higher levels should be taken into account. This affects some of the experimental conclusions, such as the inferred efficiency of the state rotation (population inversion) and the deduced value of the dipole interaction. We also show that the damping observed cannot be explained using constant rates with fixed pulse duration. We demonstrate that the damping observed is in fact induced by an off-resonant excitation to or from the continuum of wetting layer states. Our model describes the nonlinear decoherence behavior observed in recent experiments.     

Surface reactivity and quantum-size effects on the electronic density decay length of ultrathin metal films

The origin of the correlation between surface reactivity and quantum-size effects, observed in recent experiments on the oxidation of ultrathin magnesium films, is addressed by means of ab initio calculations and model predictions. We show that the decay length in vacuum of the electronic local density of states at the Fermi energy exhibits systematic oscillations with film thickness, with local maxima induced when a quantum-well state at crosses the Fermi energy. The predicted changes in the decay length are expected to have a major impact on the electron transfer rate by tunneling, which has been proposed to control the initial sticking of in the oxidation process.     

Microscopic study of relaxation oscillations in quantum-dot VCSELs

We propose a theoretical model to investigate the switch-on dynamics of electrically pumped quantum dot vertical-cavity surface-emitting lasers. The model is based on the self-consistently combined quantum dot-wetting layer Maxwell–Bloch equations incorporating microscopically calculated Coulomb and phonon-assisted scattering processes between the quantum dot and the quantum dot-embedding wetting layer states. Our approach allows the calculation of the time delay before lasing as well as of the frequency and damping rate of the appearing relaxation oscillation. We study their dependence on the strength of the injection current density and on the number of QD-layers and DBR-layers by using the finite-difference time-domain method. The results show that switch-on delay time is in inversely proportion to the injection current density while the frequency and the damping rate of relaxation oscillation are proportional to the current density. If we increase the numbers of QD-layers and DBR-layers the delay time becomes shorter, and the frequency and the damping rate become larger.     

金属/C2H2/金属结构中的电子输运

在紧束缚近似下,利用转移矩阵法对“金属/C2H2/金属”结构的分子导线的电子输运性质进行了研究。通过转移矩阵法求出转移矩阵和散射矩阵,从而解析地得到体系的透射系数和态密度的变化,并详细分析了透射系数对C2H2与金属的耦合强度及两个(CH)间的耦合强度的依赖关系。我们的结果显示这种结构的分子导线有着较好的开关特性。     

Radiation-induced "zero-resistance state" and the photon-assisted transport

We demonstrate that the radiation-induced "zero-resistance state" observed in a two-dimensional electron gas is a result of the nontrivial structure of the density of states of the systems and the photon-assisted transport. A toy model of a quantum tunneling junction with oscillatory density of states in leads catches most of the important features of the experiments. We present a generalized Kubo-Greenwood conductivity formula for the photon-assisted transport in a general system and show essentially the same nature of the transport anomaly in a uniform system.