Showing papers on "Energy (signal processing) published in 2015"

Experimental detection of a Majorana mode in the core of a magnetic vortex inside a topological insulator-superconductor Bi(2)Te(3)/NbSe(2) heterostructure.

Abstract: Majorana fermions have been intensively studied in recent years for their importance to both fundamental science and potential applications in topological quantum computing. They are predicted to exist in a vortex core of superconducting topological insulators. However, it is extremely difficult to distinguish them experimentally from other quasiparticle states for the tiny energy difference between Majorana fermions and these states, which is beyond the energy resolution of most available techniques. Here, we circumvent the problem by systematically investigating the spatial profile of the Majorana mode and the bound quasiparticle states within a vortex in ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ films grown on a superconductor ${\mathrm{NbSe}}_{2}$. While the zero bias peak in local conductance splits right off the vortex center in conventional superconductors, it splits off at a finite distance $\ensuremath{\sim}20\text{ }\text{ }\mathrm{nm}$ away from the vortex center in ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$. This unusual splitting behavior has never been observed before and could be possibly due to the Majorana fermion zero mode. While the Majorana mode is destroyed by the interaction between vortices, the zero bias peak splits as a conventional superconductor again. This work provides self-consistent evidences of Majorana fermions and also suggests a possible route to manipulating them.

Diagnosing Chaos Using Four-Point Functions in Two-Dimensional Conformal Field Theory

Abstract: We study chaotic dynamics in two-dimensional conformal field theory through out-of-time-order thermal correlators of the form ⟨W(t)VW(t)V⟩. We reproduce holographic calculations similar to those of Shenker and Stanford, by studying the large c Virasoro identity conformal block. The contribution of this block to the above correlation function begins to decrease exponentially after a delay of ~t_{*}-(β/2π)logβ^{2}E_{w}E_{v}, where t_{*} is the fast scrambling time (β/2π)logc and E_{w},E_{v} are the energy scales of the W,V operators.

Lattice dynamics and vibrational spectra of the orthorhombic, tetragonal, and cubic phases of methylammonium lead iodide

Abstract: The hybrid halide perovskite ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbI}}_{3}$ exhibits a complex structural behavior, with successive transitions between orthorhombic, tetragonal, and cubic polymorphs around 165 and 327 K. Herein we report first-principles lattice dynamics (phonon spectrum) for each phase of ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbI}}_{3}$. The equilibrium structures compare well to solutions of temperature-dependent powder neutron diffraction. By following the normal modes, we calculate infrared and Raman intensities of the vibrations, and compare them to the measurement of a single crystal where the Raman laser is controlled to avoid degradation of the sample. Despite a clear separation in energy between low-frequency modes associated with the inorganic (${\mathrm{PbI}}_{3}{}^{\ensuremath{-}}{)}_{n}$ network and high-frequency modes of the organic ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{}^{+}$ cation, significant coupling between them is found, which emphasizes the interplay between molecular orientation and the corner-sharing octahedral networks in the structural transformations. Soft modes are found at the boundary of the Brillouin zone of the cubic phase, consistent with displacive instabilities and anharmonicity involving tilting of the ${\mathrm{PbI}}_{6}$ octahedra around room temperature.

Discovering the QCD Axion with Black Holes and Gravitational Waves

Abstract: Advanced LIGO may be the first experiment to detect gravitational waves. Through superradiance of stellar black holes, it may also be the first experiment to discover the QCD axion with decay constant above the grand unification scale. When an axion's Compton wavelength is comparable to the size of a black hole, the axion binds to the black hole, forming a ``gravitational atom.'' Through the superradiance process, the number of axions occupying the bound levels grows exponentially, extracting energy and angular momentum from the black hole. Axions transitioning between levels of the gravitational atom and axions annihilating to gravitons can produce observable gravitational wave signals. The signals are long lasting, monochromatic, and can be distinguished from ordinary astrophysical sources. We estimate up to $\mathcal{O}(1)$ transition events at aLIGO for an axion between $1{0}^{\ensuremath{-}11}$ and $1{0}^{\ensuremath{-}10}\text{ }\text{ }\mathrm{eV}$ and up to $1{0}^{4}$ annihilation events for an axion between $1{0}^{\ensuremath{-}13}$ and $1{0}^{\ensuremath{-}11}\text{ }\text{ }\mathrm{eV}$. In the event of a null search, aLIGO can constrain the axion mass for a range of rapidly spinning black hole formation rates. Axion annihilations are also promising for much lighter masses at future lower-frequency gravitational wave observatories; the rates have large uncertainties, dominated by supermassive black hole spin distributions. Our projections for aLIGO are robust against perturbations from the black hole environment and account for our updated exclusion on the QCD axion of $6\ifmmode\times\else\texttimes\fi{}1{0}^{\ensuremath{-}13}\text{ }\text{ }\mathrm{eV}l{\ensuremath{\mu}}_{a}l2\ifmmode\times\else\texttimes\fi{}1{0}^{\ensuremath{-}11}\text{ }\text{ }\mathrm{eV}$ suggested by stellar black hole spin measurements.

A High-Efficiency Broadband Rectenna for Ambient Wireless Energy Harvesting

Abstract: This paper presents a novel broadband rectenna for ambient wireless energy harvesting over the frequency band from 1.8 to 2.5 GHz. First of all, the characteristics of the ambient radio-frequency energy are studied. The results are then used to aid the design of a new rectenna. A novel two-branch impedance matching circuit is introduced to enhance the performance and efficiency of the rectenna at a relatively low ambient input power level. A novel broadband dual-polarized cross-dipole antenna is proposed which has embedded harmonic rejection property and can reject the second and third harmonics to further improve the rectenna efficiency. The measured power sensitivity of this design is down to $- {\bf 35}\;{\bf dBm}$ and the conversion efficiency reaches 55% when the input power to the rectifier is $- {\bf 10}\;{\bf dBm}$ . It is demonstrated that the output power from the proposed rectenna is higher than the other published designs with a similar antenna size under the same ambient condition. The proposed broadband rectenna could be used to power many low-power electronic devices and sensors and found a range of potential applications.

Computational discovery of ferromagnetic semiconducting single-layer CrSnTe 3

Abstract: Despite many single-layer materials being reported in the past decade, few of them exhibit magnetism. Here we perform first-principles calculations using accurate hybrid density functional methods (HSE06) to predict that single-layer ${\mathrm{CrSnTe}}_{3}$ (CST) is a ferromagnetic semiconductor, with band gaps of 0.9 and 1.2 eV for the majority and minority spin channels, respectively. We determine the Curie temperature as 170 K, significantly higher than that of single-layer ${\mathrm{CrSiTe}}_{3}$ (90 K) and ${\mathrm{CrGeTe}}_{3}$ (130 K). This is due to the enhanced ionicity of the Sn-Te bond, which in turn increases the superexchange coupling between the magnetic Cr atoms. We further explore the mechanical and dynamical stability and strain response of this single-layer material for possible epitaxial growth. Our study provides an intuitive approach to understand and design single-layer magnetic semiconductors for a wide range of spintronics and energy applications.

On the Design of a Solar-Panel Receiver for Optical Wireless Communications With Simultaneous Energy Harvesting

Abstract: This paper proposes a novel design of an optical wireless communications (OWC) receiver using a solar panel as a photodetector. The proposed system is capable of simultaneous data transmission and energy harvesting. The solar panel can convert a modulated light signal into an electrical signal without any external power requirements. Furthermore, the direct current (DC) component of the modulated light can be harvested in the proposed receiver. The generated energy can potentially be used to power a user terminal or at least to prolong its operation time. The current work discusses the various parameters which need to be considered in the design of a system using a solar panel for simultaneous communication and energy harvesting. The presented theory is supported with an experimental implementation of orthogonal frequency division multiplexing (OFDM), thus, proving the validity of the analysis and demonstrating the feasibility of the proposed receiver. Using the propounded system, a communication link with a data rate of 11.84 Mbps is established for a received optical signal with a peak-to-peak amplitude of $\hbox{0.7}\times \hbox{10}^{-3}\ \hbox{W}/\hbox{cm}^{2}$ .

A 345 µW Multi-Sensor Biomedical SoC With Bio-Impedance, 3-Channel ECG, Motion Artifact Reduction, and Integrated DSP

Abstract: This paper presents a MUlti-SEnsor biomedical IC (MUSEIC). It features a high-performance, low-power analog front-end (AFE) and fully integrated DSP. The AFE has three biopotential readouts, one bio-impedance readout, and support for general-purpose analog sensors The biopotential readout channels can handle large differential electrode offsets ( ${\pm} $ 400 mV), achieve high input impedance ( ${>}$ 500 M $\Omega$ ), low noise ( ${ 620 nVrms in 150 Hz), and large CMRR ( ${>}$ 110 dB) without relying on trimming while consuming only 31 $\mu$ W/channel. In addition, fully integrated real-time motion artifact reduction, based on simultaneous electrode-tissue impedance measurement, with feedback to the analog domain is supported. The bio-impedance readout with pseudo-sine current generator achieves a resolution of 9.8 m $\Omega$ / $\surd$ Hz while consuming just 58 $\mu$ W/channel. The DSP has a general purpose ARM Cortex M0 processor and an HW accelerator optimized for energy-efficient execution of various biomedical signal processing algorithms achieving 10 $\times$ or more energy savings in vector multiply-accumulate executions.

RF-Based Energy Harvesting in Decode-and-Forward Relaying Systems: Ergodic and Outage Capacities

Abstract: Radio-frequency energy harvesting constitutes an effective way to prolong the lifetime of wireless networks, wean communication devices off the battery and power line, benefit the energy saving and lower the carbon footprint of wireless communications. In this paper, an interference aided energy harvesting scheme is proposed for cooperative relaying systems, where energy-constrained relays harvest energy from the received information signal and co-channel interference signals, and then use that harvested energy to forward the correctly decoded signal to the destination. The time-switching scheme (TS), in which the receiver switches between decoding information and harvesting energy, as well as the power-splitting scheme (PS), where a portion of the received power is used for energy harvesting and the remaining power is utilized for information processing, are adopted separately. Applying the proposed energy harvesting approach to a decode-and-forward relaying system with the three-terminal model, the analytical expressions of the ergodic capacity and the outage capacity are derived, and the corresponding achievable throughputs are determined. Comparative results are provided and show that PS is superior to TS at high signal-to-noise ratio (SNR) in terms of throughput, while at low SNR, TS outperforms PS. Furthermore, considering different interference power distributions with equal aggregate interference power at the relay, the corresponding system capacity relationship, i.e., the ordering of capacities, is obtained.

Multi-antenna Wireless Energy Transfer for Backscatter Communication Systems

Abstract: We study RF-enabled wireless energy transfer (WET) via energy beamforming, from a multi-antenna energy transmitter (ET) to multiple energy receivers (ERs) in a backscatter communication system such as RFID. The acquisition of the forward-channel (i.e., ET-to-ER) state information (F-CSI) at the ET (or RFID reader) is challenging, since the ERs (or RFID tags) are typically too energy-and-hardware-constrained to estimate or feedback the F-CSI. The ET leverages its observed backscatter signals to estimate the backscatter-channel (i.e., ET-to-ER-to-ET) state information (BS-CSI) directly. We first analyze the harvested energy obtained using the estimated BS-CSI. Furthermore, we optimize the resource allocation to maximize the total utility of harvested energy. For WET to single ER, we obtain the optimal channel-training energy in a semiclosed form. For WET to multiple ERs, we optimize the channel-training energy and the energy allocation weights for different energy beams. For the straightforward weighted-sum-energy (WSE) maximization, the optimal WET scheme is shown to use only one energy beam, which leads to unfairness among ERs and motivates us to consider the complicated proportional-fair-energy (PFE) maximization. For PFE maximization, we show that it is a biconvex problem, and propose a block-coordinate-descent-based algorithm to find the close-to-optimal solution. Numerical results show that with the optimized solutions, the harvested energy suffers slight reduction of less than 10%, compared to that obtained using the perfect F-CSI.

Systems and methods for wireless power charging

Abstract: Embodiments disclosed herein may generate and transmit power waves that, as result of their physical waveform characteristics (e.g., frequency, amplitude, phase, gain, direction), converge at a predetermined location in a transmission field to generate a pocket of energy. Receivers associated with an electronic device being powered by the wireless charging system, may extract energy from these pockets of energy and then convert that energy into usable electric power for the electronic device associated with a receiver. The pockets of energy may manifest as a three-dimensional field (e.g., transmission field) where energy may be harvested by a receiver positioned within or nearby the pocket of energy.

Analysis of the $P_c(4380)$ and $P_c(4450)$ as pentaquark states in the diquark model with QCD sum rules

Abstract: In this article, we construct the diquark-diquark-antiquark type interpolating currents, and study the masses and pole residues of the $J^P={\frac{3}{2}}^-$ and ${\frac{5}{2}}^+$ hidden-charmed pentaquark states in details with the QCD sum rules by calculating the contributions of the vacuum condensates up to dimension-10 in the operator product expansion. In calculations, we use the formula $\mu=\sqrt{M^2_{P_c}-(2{\mathbb{M}}_c)^2}$ to determine the energy scales of the QCD spectral densities. The present predictions favor assigning the $P_c(4380)$ and $P_c(4450)$ to be the ${\frac{3}{2}}^-$ and ${\frac{5}{2}}^+$ pentaquark states, respectively.

Identifying receivers in a wireless charging transmission field

Abstract: Embodiments disclosed herein may generate and transmit power waves that, as result of their physical waveform characteristics (e.g., frequency, amplitude, phase, gain, direction), converge at a predetermined location in a transmission field to generate a pocket of energy. Receivers associated with an electronic device being powered by the wireless charging system, may extract energy from these pockets of energy and then convert that energy into usable electric power for the electronic device associated with a receiver. The pockets of energy may manifest as a three-dimensional field (e.g., transmission field) where energy may be harvested by a receiver positioned within or nearby the pocket of energy.

Estimation of the shear viscosity at finite net-baryon density from A + A collision data at s NN = 7.7 – 200 GeV

Abstract: Hybrid approaches based on relativistic hydrodynamics and transport theory have been successfully applied for many years for the dynamical description of heavy-ion collisions at ultrarelativistic energies. In this work a new viscous hybrid model employing the hadron transport approach UrQMD for the early and late nonequilibrium stages of the reaction, and 3+1 dimensional viscous hydrodynamics for the hot and dense quark-gluon plasma stage, is introduced. This approach includes the equation of motion for finite baryon number and employs an equation of state with finite net-baryon density to allow for calculations in a large range of beam energies. The parameter space of the model is explored and constrained by comparison with the experimental data for bulk observables from Super Proton Synchrotron and the phase I beam energy scan at Relativistic Heavy Ion Collider. The favored parameter values depend on energy but allow extraction of the effective value of the shear viscosity coefficient over entropy density ratio $\ensuremath{\eta}/s$ in the fluid phase for the whole energy region under investigation. The estimated value of $\ensuremath{\eta}/s$ increases with decreasing collision energy, which may indicate that $\ensuremath{\eta}/s$ of the quark-gluon plasma depends on baryochemical potential ${\ensuremath{\mu}}_{B}$.

Nonlinear Dynamics of a Locally-Active Memristor

Abstract: This work elucidates some aspects of the nonlinear dynamics of a thermally-activated locally-active memristor based on a micro-structure consisting of a bi-layer of ${\rm Nb}_{2}{\rm O}_{5}$ and ${\rm Nb}_{2}{\rm O}_{x}$ materials. Through application of techniques from the theory of nonlinear dynamics to an accurate and simple mathematical model for the device, we gained a deep insight into the mechanisms at the origin of the emergence of local activity in the memristor. This theoretical study sets a general constraint on the biasing arrangement for the stabilization of the negative differential resistance effect in locally active memristors and provides a theoretical justification for an unexplained phenomenon observed at HP labs. As proof-of-principle, the constraint was used to enable a memristor to induce sustained oscillations in a one port cell. The capability of the oscillatory cell to amplify infinitesimal fluctuations of energy was theoretically and experimentally proved.

Calibrating transition-metal energy levels and oxygen bands in first-principles calculations: Accurate prediction of redox potentials and charge transfer in lithium transition-metal oxides

Abstract: Transition-metal (TM) oxides play an increasingly important role in technology today, including applications such as catalysis, solar energy harvesting, and energy storage. In many of these applications, the details of their electronic structure near the Fermi level are critically important for their properties. We propose a first-principles--based computational methodology for the accurate prediction of oxygen charge transfer in TM oxides and lithium TM (Li-TM) oxides. To obtain accurate electronic structures, the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional is adopted, and the amount of exact Hartree-Fock exchange (mixing parameter) is adjusted to reproduce reference band gaps. We show that the HSE06 functional with optimal mixing parameter yields not only improved electronic densities of states, but also better energetics (Li-intercalation voltages) for $\mathrm{LiCo}{\mathrm{O}}_{2}$ and $\mathrm{LiNi}{\mathrm{O}}_{2}$ as compared to the generalized gradient approximation (GGA), Hubbard $U$ corrected GGA ($\mathrm{GGA}+U$), and standard HSE06. We find that the optimal mixing parameters for TM oxides are system specific and correlate with the covalency (ionicity) of the TM species. The strong covalent (ionic) nature of TM-O bonding leads to lower (higher) optimal mixing parameters. We find that optimized HSE06 functionals predict stronger hybridization of the $\mathrm{Co}\phantom{\rule{0.28em}{0ex}}3d$ and $\mathrm{O}\phantom{\rule{0.28em}{0ex}}2p$ orbitals as compared to GGA, resulting in a greater contribution from oxygen states to charge compensation upon delithiation in $\mathrm{LiCo}{\mathrm{O}}_{2}$. We also find that the band gaps of Li-TM oxides increase linearly with the mixing parameter, enabling the straightforward determination of optimal mixing parameters based on GGA ($\ensuremath{\alpha}=0.0$) and HSE06 $(\ensuremath{\alpha}=0.25)$ calculations. Our results also show that ${G}_{0}{W}_{0}@\mathrm{GGA}+U$ band gaps of TM oxides ($M\mathrm{O},\phantom{\rule{0.28em}{0ex}}M=\mathrm{Mn},\phantom{\rule{0.28em}{0ex}}\mathrm{Co},\phantom{\rule{0.28em}{0ex}}\mathrm{Ni}$) and $\mathrm{LiCo}{\mathrm{O}}_{2}$ agree well with experimental references, suggesting that ${G}_{0}{W}_{0}$ calculations can be used as a reference for the calibration of the mixing parameter in cases when no experimental band gap has been reported.

Comparative Study of Advanced Signal Processing Techniques for Islanding Detection in a Hybrid Distributed Generation System

Abstract: In this paper, islanding detection in a hybrid distributed generation (DG) system is analyzed by the use of hyperbolic S-transform (HST), timetime transform, and mathematical morphology methods. The merits of these methods are thoroughly compared against commonly adopted wavelet transform (WT) and S-transform (ST) techniques, as a new contribution to earlier studies. The hybrid DG system consists of photovoltaic and wind energy systems connected to the grid within the IEEE 30-bus system. Negative sequence component of the voltage signal is extracted at the point of common coupling and passed through the above-mentioned techniques. The efficacy of the proposed methods is also compared by an energy-based technique with proper threshold selection to accurately detect the islanding phenomena. Further, to augment the accuracy of the result, the classification is done using support vector machine (SVM) to distinguish islanding from other power quality (PQ) disturbances. The results demonstrate effective performance and feasibility of the proposed techniques for islanding detection under both noise-free and noisy environments, and also in the presence of harmonics.

Strictly single-site DMRG algorithm with subspace expansion

Abstract: We introduce a strictly single-site DMRG algorithm based on the subspace expansion of the alternating minimal energy (AMEn) method. The proposed new MPS basis enrichment method is sufficient to avoid local minima during the optimization, similar to the density matrix perturbation method, but computationally cheaper. Each application of $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{H}$ to $|\ensuremath{\Psi}\ensuremath{\rangle}$ in the central eigensolver is reduced in cost for a speed-up of $\ensuremath{\approx}(d+1)/2$, with $d$ the physical site dimension. Further speed-ups result from cheaper auxiliary calculations and an often greatly improved convergence behavior. Runtime to convergence improves by up to a factor of 2.5 on the Fermi-Hubbard model compared to the previous single-site method and by up to a factor of 3.9 compared to two-site DMRG. The method is compatible with real-space parallelization and non-Abelian symmetries.

Collaborative Wireless Energy and Information Transfer in Interference Channel

Abstract: This paper studies the simultaneous wireless information and power transfer (SWIPT) in a multiuser wireless system, in which distributed transmitters send independent messages to their respective receivers, and at the same time cooperatively transmit wireless power to the receivers via energy beamforming. Accordingly, from the wireless information transmission (WIT) perspective, the system of interest can be modeled as the classic interference channel, while it also can be regarded as a distributed multiple-input multiple-output (MIMO) system for collaborative wireless energy transmission (WET). To enable both information decoding (ID) and energy harvesting (EH) in SWIPT, we adopt the low-complexity time switching operation at each receiver to switch between the ID and EH modes over scheduled time. For the hybrid system, we aim to characterize the achievable rate-energy (R–E) trade-offs by various transmitter-side collaboration schemes. Specifically, to facilitate the collaborative energy beamforming, we propose a new signal splitting scheme at the transmitters, where each transmit signal is generally split into an information signal and an energy signal for WIT and WET, respectively. With this new scheme, first, we study the two-user SWIPT system over the fading channel and derive the optimal mode switching rule at the receivers as well as the corresponding transmit signal optimization to achieve various R-E trade-offs. We also compare the R-E performance of our proposed scheme with transmit energy beamforming and signal splitting against two existing schemes with partial or no cooperation of the transmitters. Next, the general case of SWIPT systems with more than two users is studied, for which we propose a practical transmit collaboration scheme by extending the result for the two-user case: we group users into different pairs and apply the cooperation schemes obtained in the two-user case to each paired group. Furthermore, we present a benchmarking scheme based on joint cooperation of all the transmitters inspired by the principle of interference alignment , against which the performance of the proposed scheme is compared.

Effect of the magnon dispersion on the longitudinal spin Seebeck effect in yttrium iron garnets

Abstract: We study the temperature dependence of the longitudinal spin Seebeck effect (LSSE) in an yttrium iron garnet ${\mathrm{Y}}_{3}\mathrm{F}{\mathrm{e}}_{5}{\mathrm{O}}_{12}$ (YIG)/Pt system for samples of different thicknesses. In this system, the thermal spin torque is magnon driven. The LSSE signal peaks at a specific temperature that depends on the YIG sample thickness. We also observe freeze-out of the LSSE signal at high magnetic fields, which we attribute to the opening of an energy gap in the magnon dispersion. We observe partial freeze-out of the LSSE signal even at room temperature, where ${k}_{B}T$ is much larger than the gap. This suggests that a subset of the magnon population with an energy below ${k}_{B}{T}_{C} ({T}_{C}\ensuremath{\sim}40\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ contributes disproportionately to the LSSE; at temperatures above ${T}_{C}$, we label these magnons subthermal magnons. The $T$ dependence of the LSSE at temperatures below the maximum is interpreted in terms of an empirical model that ascribes most of the temperature dependence to that of the thermally driven magnon flux, which is related to the details of the magnon dispersion.

Quantum spin Hall effect and topological phase transition in two-dimensional square transition-metal dichalcogenides

Abstract: Two-dimensional (2D) topological insulators (TIs) hold promise for applications in spintronics based on the fact that the propagation direction of an edge electronic state of a 2D TI is locked to its spin orientation. Here, using first-principles calculations, we predict a family of robust 2D TIs in monolayer square transition-metal dichalcogenides $M{X}_{2}\phantom{\rule{0.28em}{0ex}}(M=\mathrm{Mo},\phantom{\rule{0.28em}{0ex}}\mathrm{W};\phantom{\rule{0.28em}{0ex}}X=\mathrm{S},\phantom{\rule{0.28em}{0ex}}\mathrm{Se},\phantom{\rule{0.28em}{0ex}}\mathrm{Te})$, which show sizeable intrinsic nontrivial band gaps ranged from 24 to 187 meV, thus ensuring the quantum spin Hall (QSH) effect at room temperature. Different from the most known 2D TIs with comparable band gaps, these sizeable energy gaps arise from the strong spin-orbit interaction related to $d$ electrons of the Mo/W atoms. A pair of topologically protected helical edge states emerges at the edge of these systems with a Dirac-type dispersion within the bulk band gap. The topologically nontrivial natures are confirmed by the nontrivial ${\mathrm{Z}}_{2}$-type topological invariant. More interestingly, with applied strain, a topological quantum phase transition between a QSH phase and a trivial insulating/metallic phase can be realized, and the corresponding topological phase diagram is well established.

Measurement of Scintillation and Ionization Yield and Scintillation Pulse Shape from Nuclear Recoils in Liquid Argon

Abstract: We have measured the scintillation and ionization yield of recoiling nuclei in liquid argon as a function of applied electric field by exposing a dual-phase liquid argon time projection chamber (LAr-TPC) to a low energy pulsed narrow band neutron beam produced at the Notre Dame Institute for Structure and Nuclear Astrophysics. Liquid scintillation counters were arranged to detect and identify neutrons scattered in the TPC and to select the energy of the recoiling nuclei. We report measurements of the scintillation yields for nuclear recoils with energies from 10.3 to 57.3 keV and for median applied electric fields from 0 to $970\text{ }\text{ }\mathrm{V}/\mathrm{cm}$. For the ionization yields, we report measurements from 16.9 to 57.3 keV and for electric fields from 96.4 to $486\text{ }\text{ }\mathrm{V}/\mathrm{cm}$. We also report the observation of an anticorrelation between scintillation and ionization from nuclear recoils, which is similar to the anticorrelation between scintillation and ionization from electron recoils. Assuming that the energy loss partitions into excitons and ion pairs from $^{83m}\mathrm{Kr}$ internal conversion electrons is comparable to that from $^{207}\mathrm{Bi}$ conversion electrons, we obtained the numbers of excitons (${N}_{\text{ex}}$) and ion pairs (${N}_{\mathrm{i}}$) and their ratio (${N}_{\text{ex}}/{N}_{\mathrm{i}}$) produced by nuclear recoils from 16.9 to 57.3 keV. Motivated by arguments suggesting direction sensitivity in LAr-TPC signals due to columnar recombination, a comparison of the light and charge yield of recoils parallel and perpendicular to the applied electric field is presented for the first time.

Analysis of $K$ -Tier Uplink Cellular Networks With Ambient RF Energy Harvesting

Abstract: We use stochastic geometry to develop a comprehensive modeling framework for $K$ -tier uplink cellular networks with RF energy harvesting from the concurrent cellular transmissions. In the considered system model, channel inversion power control is used and cellular users are equipped with energy storage units. We also use tools from queueing theory, namely, Markov chain analysis, to model the level of stored energy in each user's battery. A successful transmission is assumed only when the amount of energy stored in a user's battery is sufficient to perform channel inversion with a received signal-to-interference ratio $(\mbox{SIR})$ above a predefined threshold. The performance of the proposed system model is evaluated in terms of the transmission probability, the $\mbox{SIR}$ coverage probability, and the overall success probability. Using Poisson point processes (PPPs) enables us to derive simple expressions for these performance metrics in order to obtain insights for network design and optimization. We show the effect of varying the different network parameters such as the spatial density of BSs and the receiver sensitivity. In addition, we discuss several special cases and provide guidelines on the extensions of the proposed framework. We show that the gain of using RF energy harvesting can be highly improved by a proper choice of the network design parameters.

Atomistic analysis of the impact of alloy and well-width fluctuations on the electronic and optical properties of InGaN/GaN quantum wells

Abstract: We present an atomistic description of the electronic and optical properties of ${\text{In}}_{0.25}{\text{Ga}}_{0.75}\mathrm{N}/\mathrm{GaN}$ quantum wells. Our analysis accounts for fluctuations of well width, local alloy composition, strain and built-in field fluctuations as well as Coulomb effects. We find a strong hole and much weaker electron wave function localization in InGaN random alloy quantum wells. The presented calculations show that while the electron states are mainly localized by well-width fluctuations, the holes states are already localized by random alloy fluctuations. These localization effects affect significantly the quantum well optical properties, leading to strong inhomogeneous broadening of the lowest interband transition energy. Our results are compared with experimental literature data.

Subwavelength ultrasonic circulator based on spatiotemporal modulation

Abstract: Enabling efficient nonreciprocal acoustic devices is challenging, yet very desirable for a variety of applications, including acoustic imaging, underwater communications, energy concentration and harvesting, signal processing, and noise control. We discuss the theory and design of a fully linear compact acoustic circulator based on spatiotemporal modulation of the effective acoustic index, providing a compact and practical way to realize large sound circulation at any desired frequency. Our proposal enables tunable isolation levels of over 40 dB, with insertion losses as low as 0.3 dB, in a noise-free, integrable, frequency scalable device whose total size does not exceed $\ensuremath{\lambda}/6$.

Knee of the cosmic hydrogen and helium spectrum below 1 PeV measured by ARGO-YBJ and a Cherenkov telescope of LHAASO

Abstract: The measurement of the cosmic ray energy spectrum, in particular for individual species of nuclei, is an important tool to investigate cosmic ray production and propagation mechanisms. The determination of the ``knees'' in the spectra of different species remains one of the main challenges in cosmic ray physics. In fact, experimental results are still conflicting. In this paper we report a measurement of the mixed proton and helium energy spectrum, obtained with the combined data of the ARGO-YBJ experiment and a wide field of view Cherenkov telescope, a prototype of the future LHAASO experiment. By means of a multiparameter technique, we have selected a high-purity proton plus helium sample. The reconstructed energy resolution is found to be about 25% throughout the investigated energy range from 100 TeV to 3 PeV, with a systematic uncertainty in the absolute energy scale of 9.7%. The found energy spectrum can be fitted with a broken power-law function, with a break at the energy ${\mathrm{E}}_{k}=700\ifmmode\pm\else\textpm\fi{}230(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}70(\mathrm{sys})\text{ }\text{ }\mathrm{TeV}$, where the spectral index changes from $\ensuremath{-}2.56\ifmmode\pm\else\textpm\fi{}0.05$ to $\ensuremath{-}3.24\ifmmode\pm\else\textpm\fi{}0.36$. The statistical significance of the observed spectral break is 4.2 standard deviations.

A Real-Time Energy Management Architecture for Multisource Electric Vehicles

Abstract: This paper presents an energy management architecture for small urban electric vehicles based on hybrid energy sources and its real-time implementation. The energy management strategy uses an integrated rule-based metaheuristic approach to obtain solutions for sharing energy and power between two sources with different characteristics, namely, one with high specific energy and another with high specific power. A comprehensive real-time architecture for the energy management system is presented considering different management levels. The proposed approach determines an optimized real-time energy sharing between the sources without prior knowledge of the power demand profile. The multilevel energy management strategy has been validated using power-level reduced-scale hardware-in-the-loop simulations for a normalized urban driving cycle. The experimental results show the effectiveness of a real-time implementation based on particle swarm optimization supported by a set of rules restricting the search space. This strategy is effective in controlling the energy sources to work in their higher efficiency region and in satisfying the dynamic performance of the vehicle.