arXiv subjects

Yu Zhang

Publications and source records attributed to Yu Zhang.

At least 1,639 records · Page 91Linked to original sources

arXiv2015

Distributed Stochastic Market Clearing with High-Penetration Wind Power

Integrating renewable energy into the modern power grid requires risk-cognizant dispatch of resources to account for the stochastic availability of renewables. Toward this goal, day-ahead stochastic market clearing with high-penetration wind energy is pursued in this paper based on the DC optimal power flow (OPF). The objective is to minimize the social cost which consists of conventional generation costs, end-user disutility, as well as a risk measure of the system re-dispatching cost. Capitalizing on the conditional value-at-risk (CVaR), the novel model is able to mitigate the potentially high risk of the recourse actions to compensate wind forecast errors. The resulting convex optimization task is tackled via a distribution-free sample average based approximation to bypass the prohibitively complex high-dimensional integration. Furthermore, to cope with possibly large-scale dispatchable loads, a fast distributed solver is developed with guaranteed convergence using the alternating direction method of multipliers (ADMM). Numerical results tested on a modified benchmark system are reported to corroborate the merits of the novel framework and proposed approaches.

math.OC↗

arXiv2015

Mode Selection in MU-MIMO Downlink Networks: A Physical Layer Security Perspective

In this paper, we consider a homogenous multi-antenna downlink network where a passive eavesdropper intends to intercept the communication between a base station (BS) and multiple secure users (SU) over Rayleigh fading channels. In order to guarantee the security of information transfer, physical layer security is employed accordingly. For such a multiple user (MU) secure network, the number of accessing SUs, namely transmission mode, has a great impact on the secrecy performance. Specifically, on the one hand, a large number of accessing SUs will arise high inter-user interference at SUs, resulting in a reduction of the capacity of the legitimate channel. On the other hand, high inter-user interference will interfere with the eavesdropper and thus degrades the performance of the eavesdropper channel. Generally speaking, the harmful inter-user interference may be transformed as a useful tool of anti-eavesdropping. The focus of this paper is on selecting the optimal transmission mode according to channel conditions and system parameters, so as to maximize the sum secrecy outage capacity. Moreover, through asymptotic analysis, we present several simple mode selection schemes in some extreme cases. Finally, simulation results validate the effectiveness of the proposed mode selection schemes in MU secure communications.

cs.IT↗

arXiv2015

The effects of binary interactions on parameter determinations for early-type galaxies

Based on stellar population models without (SSP) and with (BSP) binary interactions, we investigate the effects of binary interactions on parameter determinations for early-type galaxies (ETGs). We present photometric redshift (photo-z), age and spectral type for photometric data sample by fitting observed magnitudes with the SSP and BSP models. Our results show that binary interactions have no effect on photo-z estimation. Once we neglect binary interactions, the age of ETGs will be underestimated, by contrast, the effects on the age estimations can be negligible for other type of galaxies. For ETG sample, we derive their properties by fitting their spectra with the SSP and BSP models. When comparing these galaxy properties, we find no variation of the overall metallicities for ETGs among the SSP and BSP models. Moreover, the inclusion of binary interactions can affect age estimations. Our results show that the BSP-fitted ages in ~33.3% of ETG sample are around 0.5-1.0 Gyr larger than the SSP-fitted ages; ~44.2\% are only 0.1-0.5 Gyr larger; the rest ~22.5% are approximately equal. By comparisons, we find the difference of the star formation rate between the SSP and BSP models is large at the late evolution stage.

astro-ph.GA↗

arXiv2015

Dissipative time-dependent quantum transport theory: quantum interference and phonon induced decoherence dynamics

A time-dependent inelastic electron transport theory for strong electron-phonon interaction is established via the equations of motion method combined with the small polaron transformation. In this work, the dissipation via electron-phonon coupling is taken into account in the strong coupling regime, which validates the small polaron transformation. The corresponding equations of motion are developed, which are used to study the quantum interference effect and phonon-induced decoherence dynamics in molecular junctions. Numerical studies show clearly quantum interference effect of the transport electrons through two quasi-degenerate states with different coupling to the leads. We also found that the quantum interference can be suppressed by the electron-phonon interaction where the phase coherence is destroyed by phonon scattering. This indicates the importance of electron-phonon interaction in systems with prominent quantum interference effect.

cond-mat.mes-hall↗

arXiv2015

A bound system in the expanding universe with modified holographic Ricci dark energy and dark matter

The evolution of a bound system in the expanding background has been investigated in this paper. The background is described by a FRW universe with the modified holographic dark energy model, whose equation of state parameter changes with time and can cross the phantom boundary. To study the evolution of the bound system, an interpolating metric is considered, and on this basis the geodesics of a test particle are given. The equation of motion and the effective potential are also derived from the geodesics. By studying the the effective potential and the evolution of the radius of a test particle in the bound system of the Milky Way galaxy, we have found that the galaxy would go through three stages: expands from a singular point; stays in a discoid for a period of time; big rip in the future. With the help of analysing the critical angular momentum, we find that the test particle needs less angular momentum to escape from the center mass as time passes.

physics.gen-ph↗

arXiv2015

Dissipative quantum transport at arbitrary parameter regime: a variational method

Recent development of theoretical method for dissipative quantum transport have achieved notable progresses in the weak or strong electron-phonon coupling regime. However, a generalized theory for dissipative quantum transport at arbitrary parameter regime is not figured out until now. In this work, a variational method for dissipative quantum transport at arbitrary electron-phonon coupling regime is developed by employing variational polaron theory. The optimal polaron transformation is determined by the optimization of the Feynman-Bogoliubov upper bound of free energy. % which is variational in nature. The free energy minimization ends up with an optimal mean-field Hamiltonian and a minimal interaction Hamiltonian. Hence, second-order perturbation can be applied to the transformed system, resulting an accurate and efficient method for the treatment of dissipative quantum transport with arbitrary electron-phonon coupling strength. Numerical benchmark calculation on an single site model with coupling to one phonon mode is also presented.

cond-mat.mes-hall↗

arXiv2015

Etching-free transfer of wafer-scale MoS2 films

How to transfer MoS2 films from growth substrates onto target substrates is a critical issue for its practical applications. However, it remains a great challenge to avoid the sample degradation and substrate destruction, since current transfer method inevitably employs a wet chemical etching process. Herein, we develop an etching-free transfer method for transferring wafer-scale MoS2 films onto arbitrary substrates by using ultrasonication. Briefly, the collapse of ultrasonication-generated microbubbles at the interface between polymer-coated MoS2 film and substrates induce sufficient force to delaminate the MoS2 films. Using this method the MoS2 films can be transferred from all the substrates (silica, mica, strontium titanate, sapphire) and remains the original sample morphology and quality. This method guarantees a simple transfer process, allows the reuse of growth substrates, without the presence of any hazardous etchants. The etching-free transfer method may promote the broad applications of MoS2 in electronics, optoelectronics and catalysis.

cond-mat.mtrl-sci↗

arXiv2015

Outage Probability of Overhearing Amplify-and-Forward Cooperative Relaying

This paper investigates the outage performance of overhearing amplify-and-forward (AF) cooperative relaying, where a source transmits information to its destination through multiple helping overhearing AF relays with space-time network coding (STNC) employed. Firstly, the transmission protocol of such a relaying system, i.e., cooperative relaying with overhearing AF relays based on STNC (STNC-OHAF) is presented. Then, the instantaneous end-to-end SNR expression of STNC-OHAF is analysed. Based on this, an explicit expression of the outage probability for STNC-OHAF over independent but not necessarily identically distributed (i.n.i.d) Rayleigh fading channels is theoretically derived. Numerical results validate our theoretical analysis and show that by introducing overhearing among relays, the outage performance of the system can be greatly improved. It also shows that there is a trade-off between system sum outage capacity and the transmitted number of symbols.

cs.IT↗

arXiv2014

The emergent dynamical symmetry at the triple point of nuclear deformations

Based on the boson realization of the Euclidean algebras, it is shown that the five-dimensional Euclidean dynamical symmetry may emerge at the triple point of the shape phase diagram of the interacting boson model, which thus offers a symmetry-based understanding of this isolated point. It is further shown that the low-lying dynamics in $^{108}$Pd, $^{134}$Ba, $^{64}$Zn, and $^{114}$Cd may be dominated by the Euclidean dynamical symmetry.

nucl-th↗

arXiv2014

Outage Analysis and Optimization for Time Switching-based Two-Way Relaying with Energy Harvesting Relay Node

Energy harvesting (EH) and network coding (NC) have emerged as two promising technologies for future wireless networks. In this paper, we combine them together in a single system and then present a time switching-based network coding relaying (TSNCR) protocol for the two-way relay system, where an energy constrained relay harvests energy from the transmitted radio frequency (RF) signals from two sources, and then helps the two-way relay information exchange between the two sources with the consumption of the harvested energy. To evaluate the system performance, we derive an explicit expression of the outage probability for the proposed TSNCR protocol. In order to explore the system performance limit, we formulate an optimization problem to minimize the system outage probability. Since the problem is non-convex and cannot be directly solved, we design a genetic algorithm (GA)-based optimization algorithm for it. Numerical results validate our theoretical analysis and show that in such an EH two-way relay system, if NC is applied, the system outage probability can be greatly decreased. Moreover, it is shown that the relay position greatly affects the system performance of TSNCR, where relatively worse outage performance is achieved when the relay is placed in the middle of the two sources. This is the first time to observe such a phenomena in EH two-way relay systems.

cs.IT↗

arXiv2014

Modified holographic Ricci dark energy coupled to interacting relativistic and non-relativistic dark matter in the nonflat universe

The modified holographic Ricci dark energy coupled to interacting relativistic and non-relativistic dark matter is considered in the nonflat Friedmann-Robertson-Walker universe. Through examining the deceleration parameter, one can find that the transition time of the Universe from decelerating to accelerating phase in the interacting holographic Ricci dark energy model is close to that in the $Λ$ cold dark matter model. The evolution of modified holographic Ricci dark energy's state parameter and the evolution of dark matter and dark energy's densities shows that the dark energy holds the dominant position from the near past to the future. By studying the statefinder diagnostic and the evolution of the total pressure, one can find that this model could explain the Universe's transition from the radiation to accelerating expansion stage through the dust stage. According to the $Om$ diagnostic, it is easy to find that when the interaction is weak and the proportion of relativistic dark matter in total dark matter is small, this model is phantom-like. Through our studying, we find the interaction and the relativistic dark matter's proportion all have great influence on the evolution of the Universe.

gr-qc↗

arXiv2014

Plan or not: Remote Human-robot Teaming with Incomplete Task Information

Human-robot interaction can be divided into two categories based on the physical distance between the human and robot: remote and proximal. In proximal interaction, the human and robot often engage in close coordination; in remote interaction, the human and robot are less coupled due to communication constraints. As a result, providing automation for the robot in remote interaction becomes more important. Thus far, human factor studies on automation in remote human-robot interaction have been restricted to various forms of supervision, in which the robot is essentially being used as a smart mobile manipulation platform with sensing capabilities. In this paper, we investigate the incorporation of general planning capability into the robot to facilitate peer-to-peer human-robot teaming, in which the human and robot are viewed as teammates that are physically separated. The human and robot share the same global goal and collaborate to achieve it. Note that humans may feel uncomfortable at such robot autonomy, which can potentially reduce teaming performance. One important difference between peer-to-peer teaming and supervised teaming is that an autonomous robot in peer-to-peer teaming can achieve the goal alone when the task information is completely specified. However, incompleteness often exists, which implies information asymmetry. While information asymmetry can be desirable sometimes, it may also lead to the robot choosing improper actions that negatively influence the teaming performance. We aim to investigate the various trade-offs, e.g., mental workload and situation awareness, between these two types of remote human-robot teaming.

cs.AI↗

arXiv2014

Euclidean Dynamical Symmetry in Nuclear Shape Phase Transitions

The Euclidean dynamical symmetry hidden in the critical region of nuclear shape phase transitions is revealed by a novel algebraic F(5) description. With a nonlinear projection, it is shown that the dynamics in the critical region of the spherical--axial deformed and the spherical--$γ$ soft shape phase transitions can indeed be manifested by this description, which thus provides a unified symmetry--based interpretation of the critical phenomena in the region.

nucl-th↗

arXiv2014

Learning of Agent Capability Models with Applications in Multi-agent Planning

One important challenge for a set of agents to achieve more efficient collaboration is for these agents to maintain proper models of each other. An important aspect of these models of other agents is that they are often partial and incomplete. Thus far, there are two common representations of agent models: MDP based and action based, which are both based on action modeling. In many applications, agent models may not have been given, and hence must be learnt. While it may seem convenient to use either MDP based or action based models for learning, in this paper, we introduce a new representation based on capability models, which has several unique advantages. First, we show that learning capability models can be performed efficiently online via Bayesian learning, and the learning process is robust to high degrees of incompleteness in plan execution traces (e.g., with only start and end states). While high degrees of incompleteness in plan execution traces presents learning challenges for MDP based and action based models, capability models can still learn to {\em abstract} useful information out of these traces. As a result, capability models are useful in applications in which such incompleteness is common, e.g., robot learning human model from observations and interactions. Furthermore, when used in multi-agent planning (with each agent modeled separately), capability models provide flexible abstraction of actions. The limitation, however, is that the synthesized plan is incomplete and abstract.

cs.AI↗

arXiv2014

An Approximate Framework for Quantum Transport Calculation with Model Order Reduction

A new approximate computational framework is proposed for computing the non-equilibrium charge density in the context of the non-equilibrium Green's function (NEGF) method for quantum mechanical transport problems. The framework consists of a new formulation, called the X-formulation, for single-energy density calculation based on the solution of sparse linear systems, and a projection-based nonlinear model order reduction (MOR) approach to address the large number of energy points required for large applied biases. The advantages of the new methods are confirmed by numerical experiments.

physics.comp-ph↗

arXiv2014

An Efficient Primal-Dual Approach to Chance-Constrained Economic Dispatch

To effectively enhance the integration of distributed and renewable energy sources in future smart microgrids, economical energy management accounting for the principal challenge of the variable and non-dispatchable renewables is indispensable and of significant importance. Day-ahead economic generation dispatch with demand-side management for a microgrid in islanded mode is considered in this paper. With the goal of limiting the risk of the loss-of-load probability, a joint chance constrained optimization problem is formulated for the optimal multi-period energy scheduling with multiple wind farms. Bypassing the intractable spatio-temporal joint distribution of the wind power generation, a primal-dual approach is used to obtain a suboptimal solution efficiently. The method is based on first-order optimality conditions and successive approximation of the probabilistic constraint by generation of p-efficient points. Numerical results are reported to corroborate the merits of this approach.

math.OC↗

arXiv2014

Ultrafast Charge Transfer in Atomically Thin MoS2/WS2 Heterostructures

Van der Waals heterostructures have recently emerged as a new class of materials, where quantum coupling between stacked atomically thin two-dimensional (2D) layers, including graphene, hexagonal-boron nitride, and transition metal dichalcogenides (MX2), give rise to fascinating new phenomena. MX2 heterostructures are particularly exciting for novel optoelectronic and photovoltaic applications, because 2D MX2 monolayers can have an optical bandgap in the near-infrared to visible spectral range and exhibit extremely strong light-matter interactions. Theory predicts that many stacked MX2 heterostructures form type-II semiconductor heterojunctions that facilitate efficient electron-hole separation for light detection and harvesting. Here we report the first experimental observation of ultrafast charge transfer in photo-excited MoS2/WS2 heterostructures using both photoluminescence mapping and femtosecond (fs) pump-probe spectroscopy. We show that hole transfer from the MoS2 layer to the WS2 layer takes place within 50 fs after optical excitation, a remarkable rate for van der Waals coupled 2D layers. Such ultrafast charge transfer in van der Waals heterostructures can enable novel 2D devices for optoelectronics and light harvesting.

cond-mat.mes-hall↗

arXiv2014

Time-, Frequency-, and Wavevector-Resolved X-Ray Diffraction from Single Molecules

Using a quantum electrodynamic framework, we calculate the off-resonant scattering of a broad-band X-ray pulse from a sample initially prepared in an arbitrary superposition of electronic states. The signal consists of single-particle (incoherent) and two-particle (coherent) contributions that carry different particle form factors that involve different material transitions. Single-molecule experiments involving incoherent scattering are more influenced by inelastic processes compared to bulk measurements. The conditions under which the technique directly measures charge densities (and can be considered as diffraction) as opposed to correlation functions of the charge-density are specified. The results are illustrated with time- and wavevector-resolved signals from a single amino acid molecule (cysteine) following an impulsive excitation by a stimulated X-ray Raman process resonant with the sulfur K-edge. Our theory and simulations can guide future experimental studies on the structures of nano-particles and proteins.

physics.chem-ph↗