arXiv subjects

Ning Wang

Publications and source records attributed to Ning Wang.

At least 343 records · Page 19Linked to original sources

arXiv2017

Gate-tunable strong-weak localization transition in few-layer black phosphorus

Atomically thin black phosphorus (BP) field-effect transistors show strong-weak localization transition which is tunable through gate voltages. Hopping transports through charge impurity induced localized states are measured at low-carrier density regime. Variable-range hopping model is applied to simulate the charge carrier scattering behavior. In the high-carrier concentration regime, a negative magnetoresistance signals the weak localization effect. The extracted phase coherence length is power-law temperature dependent ($\sim T^{-0.48\pm0.03}$) and demonstrates electron-electron interactions in few-layer BP. The competition between the Strong localization length and phase coherence length is proposed and discussed based on the observed gate tunable strong-weak localization transition in few-layer BP.

cond-mat.mes-hall↗

arXiv2017

Beamforming and Power Splitting Designs for AN-aided Secure Multi-user MIMO SWIPT Systems

In this paper, an energy harvesting scheme for a multi-user multiple-input-multiple-output (MIMO) secrecy channel with artificial noise (AN) transmission is investigated. Joint optimization of the transmit beamforming matrix, the AN covariance matrix, and the power splitting ratio is conducted to minimize the transmit power under the target secrecy rate, the total transmit power, and the harvested energy constraints. The original problem is shown to be non-convex, which is tackled by a two-layer decomposition approach. The inner layer problem is solved through semi-definite relaxation, and the outer problem, on the other hand, is shown to be a single- variable optimization that can be solved by one-dimensional (1- D) line search. To reduce computational complexity, a sequential parametric convex approximation (SPCA) method is proposed to find a near-optimal solution. The work is then extended to the imperfect channel state information case with norm-bounded channel errors. Furthermore, tightness of the relaxation for the proposed schemes are validated by showing that the optimal solution of the relaxed problem is rank-one. Simulation results demonstrate that the proposed SPCA method achieves the same performance as the scheme based on 1-D but with much lower complexity.

cs.IT↗

arXiv2017

Microscopic dynamics simulations of multi-nucleon transfer in $^{86}$Kr+$^{64}$Ni at 25 MeV/nucleon

Multi-nucleon transfer in $^{86}$Kr+$^{64}$Ni at an incident energy of 25 MeV/nucleon is for the first time investigated with a microscopic dynamics model: improved quantum molecular dynamics (ImQMD) model. The measured isotope distributions are reasonably well reproduced by using the ImQMD model together with a statistical code (GEMINI) for describing the secondary decay of fragments. The reaction mechanism is explored with the microscopic dynamics simulations from central to peripheral collisions. In central collisions there exists a strong competition among fusion, deep-inelastic scattering and multi-fragmentation at such an incident energy. In semi-peripheral collisions, binary scattering together with nucleon transfer is dominant, and the probability of elastic+inelastic scattering events increases rapidly with impact parameter in peripheral collisions and approaches to one when $b>14$ fm. The mass-TKE distribution in central collisions due to the competition is quite different from those in peripheral collisions and the distribution of total kinetic energy loss (TKEL) for binary events with nucleon transfer is much more broader than those without transfer.

nucl-th↗

arXiv2017

Odd-integer quantum Hall states and giant spin susceptibility in p-type few-layer WSe2

We fabricate high-mobility p-type few-layer WSe2 field-effect transistors and surprisingly observe a series of quantum Hall (QH) states following an unconventional sequence predominated by odd-integer states under a moderate strength magnetic field. By tilting the magnetic field, we discover Landau level (LL) crossing effects at ultra-low coincident angles, revealing that the Zeeman energy is about three times as large as the cyclotron energy near the valence band top at Γ valley. This result implies the significant roles played by the exchange interactions in p-type few-layer WSe2, in which itinerant or QH ferromagnetism likely occurs. Evidently, the Γ valley of few-layer WSe2 offers a unique platform with unusually heavy hole-carriers and a substantially enhanced g-factor for exploring strongly correlated phenomena.

cond-mat.mes-hall↗

arXiv2016

Induced robust topological order on an ordinary insulator hetero-structured with a strong topological insulator

Topological states of matter originate from distinct topological electronic structures of materials. As for strong topological insulators (STIs), the topological surface (interface) is a direct consequence of electronic structure transition between materials categorized to different topological genus. Therefore, it is fundamentally interesting if such topological character can be manipulated. Besides tuning the crystal field and the strength of spin-orbital coupling (e.g., by external strain, or chemical doping), there is currently rare report on topological state induced in ordinary insulators (OIs) by the heterostructure of OI/STI. Here we report the observation of a Dirac cone topological surface state (TSS) induced on the Sb2Se3 layer up to 15 nm thick in the OI/STI heterostructure, in sharp contrast with the OI/OI heterostructure where no sign of TSS can be observed. This is evident for an induced topological state in an OI by heterostructure.

cond-mat.mtrl-sci↗

arXiv2016

Charge Density Wave Phase Transition on the Surface of Electrostatically Doped Multilayer Graphene

We demonstrate that charge density wave (CDW) phase transition occurs on the surface of electronically doped multilayer graphene when the Fermi level approaches the M points (also known as van Hove singularities where the density of states diverge) in the Brillouin zone of graphene band structure. The occurrence of such CDW phase transitions are supported by both the electrical transport measurement and optical measurements in electrostatically doped multilayer graphene. The CDW transition is accompanied with the sudden change of graphene channel resistance at T$_m$= 100K, as well as the splitting of Raman G peak (1580 cm$^{-1}$). The splitting of Raman G peak indicats the lifting of in-plane optical phonon branch degeneracy and the non-degenerate phonon branches are correlated to the lattice reconstructions of graphene -- the CDW phase transition.

cond-mat.mes-hall↗

arXiv2016

Secure Massive MIMO Systems with Limited RF Chains

In future practical deployments of massive multi-input multi-output (MIMO) systems, the number of radio frequency (RF) chains at the base stations (BSs) may be much smaller than the number of BS antennas to reduce the overall expenditure. In this paper, we propose a novel design framework for joint data and artificial noise (AN) precoding in a multiuser massive MIMO system with limited number of RF chains, which improves the wireless security performance. With imperfect channel state information (CSI), we analytically derive an achievable lower bound on the ergodic secrecy rate of any mobile terminal (MT), for both analog and hybrid precoding schemes. The closed-form lower bound is used to determine optimal power splitting between data and AN that maximizes the secrecy rate through simple one-dimensional search. Analytical and numerical results together reveal that the proposed hybrid precoder, although suffers from reduced secrecy rate compared with theoretical full-dimensional precoder, is free of the high computational complexity of large-scale matrix inversion and null-space calculations, and largely reduces the hardware cost.

cs.IT↗

arXiv2016

An Online Universal Classifier for Binary, Multi-class and Multi-label Classification

Classification involves the learning of the mapping function that associates input samples to corresponding target label. There are two major categories of classification problems: Single-label classification and Multi-label classification. Traditional binary and multi-class classifications are sub-categories of single-label classification. Several classifiers are developed for binary, multi-class and multi-label classification problems, but there are no classifiers available in the literature capable of performing all three types of classification. In this paper, a novel online universal classifier capable of performing all the three types of classification is proposed. Being a high speed online classifier, the proposed technique can be applied to streaming data applications. The performance of the developed classifier is evaluated using datasets from binary, multi-class and multi-label problems. The results obtained are compared with state-of-the-art techniques from each of the classification types.

cs.LG↗

arXiv2016

A High Speed Multi-label Classifier based on Extreme Learning Machines

In this paper a high speed neural network classifier based on extreme learning machines for multi-label classification problem is proposed and dis-cussed. Multi-label classification is a superset of traditional binary and multi-class classification problems. The proposed work extends the extreme learning machine technique to adapt to the multi-label problems. As opposed to the single-label problem, both the number of labels the sample belongs to, and each of those target labels are to be identified for multi-label classification resulting in in-creased complexity. The proposed high speed multi-label classifier is applied to six benchmark datasets comprising of different application areas such as multi-media, text and biology. The training time and testing time of the classifier are compared with those of the state-of-the-arts methods. Experimental studies show that for all the six datasets, our proposed technique have faster execution speed and better performance, thereby outperforming all the existing multi-label clas-sification methods.

cs.LG↗

arXiv2016

Design and Implementation of a TDD-Based 128-Antenna Massive MIMO Prototyping System

Spurred by the dramatic mobile IP growth and the emerging Internet of Things (IoT) and cloud-based applications, wireless networking is witnessing a paradigm shift. By fully exploiting the spatial degrees of freedom, the massive multipleinput- multiple-output (MIMO) technology promises significant gains in both data rates and link reliability. This paper presents a time-division duplex (TDD)-based 128-antenna massive MIMO prototyping system designed to operate on a 20 MHz bandwidth. Up to twelve single-antenna users can be served by the designed system at the same time. System model is provided and link-level simulation corresponding to our practical TDDbased massive MIMO prototyping system is conducted to validate our design and performance of the algorithms. Based on the system hardware design demonstrated in this paper, both uplink real-time video and downlink data transmissions are realized, and the experiment results show that 268.8 Mbps rate was achieved for eight single-antenna users using QPSK modulation. The maximum spectral efficiency of the designed system will be 80.64 bit/s/Hz by twelve single-antenna users with 256-QAM modulation.

cs.IT↗

arXiv2016

Analysis and Design of Secure Massive MIMO Systems in the Presence of Hardware Impairments

To keep the hardware costs of future communications systems manageable, the use of low-cost hardware components is desirable. This is particularly true for the emerging massive multiple-input multiple-output (MIMO) systems which equip base stations (BSs) with a large number of antenna elements. However, low-cost transceiver designs will further accentuate the hardware impairments which are present in any practical communication system. In this paper, we investigate the impact of hardware impairments on the secrecy performance of downlink massive MIMO systems in the presence of a passive multiple-antenna eavesdropper. Thereby, for the BS and the legitimate users, the joint effects of multiplicative phase noise, additive distortion noise, and amplified receiver noise are taken into account, whereas the eavesdropper is assumed to employ ideal hardware. We derive a lower bound for the ergodic secrecy rate of a given user when matched filter (MF) data precoding and artificial noise (AN) transmission are employed at the BS. Based on the derived analytical expression, we investigate the impact of the various system parameters on the secrecy rate and optimize both the pilot sets used for uplink training and the AN precoding. Our analytical and simulation results reveal that 1) the additive distortion noise at the BS may be beneficial for the secrecy performance, especially if the power assigned for AN emission is not sufficient; 2) all other hardware impairments have a negative impact on the secrecy performance; 3) {\textcolor{blue}{despite their susceptibility to pilot interference in the presence of phase noise}}, so-called spatially orthogonal pilot sequences are preferable unless the phase noise is very strong; 4) the proposed generalized null-space (NS) AN precoding method can efficiently mitigate the negative effects of phase noise.

cs.IT↗

arXiv2016

New neutron-rich isotope production in $^{154}$Sm+$^{160}$Gd

Deep inelastic scattering in $^{154}$Sm+$^{160}$Gd at energies above the Bass barrier is for the first time investigated with two different microscopic dynamics approaches: improved quantum molecular dynamics (ImQMD) model and time dependent Hartree-Fock (TDHF) theory. No fusion is observed from both models. The capture pocket disappears for this reaction due to strong Coulomb repulsion and the contact time of the di-nuclear system formed in head-on collisions is about 700 fm/c at an incident energy of 440 MeV. The isotope distribution of fragments in the deep inelastic scattering process is predicted with the simulations of the latest ImQMD-v2.2 model together with a statistical code (GEMINI) for describing the secondary decay of fragments. More than 40 extremely neutron-rich unmeasured nuclei with $58 \le Z\le 76$ are observed and the production cross sections are at the order of ${\rm μb}$ to mb. The multi-nucleon transfer reaction of Sm+Gd could be an alternative way to synthesize new neutron-rich lanthanides which are difficult to be produced with traditional fusion reactions or fission of actinides.

nucl-th↗

arXiv2016

Type-controlled Nanodevices Based on Encapsulated Few-layer Black Phosphorus for Quantum Transport

We demonstrate that encapsulation of atomically thin black phosphorus (BP) by hexagonal boron nitride (h-BN) sheets is very effective for minimizing the interface impurities induced during fabrication of BP channel material for quantum transport nanodevices. Highly stable BP nanodevices with ultrahigh mobility and controllable types are realized through depositing appropriate metal electrodes after conducting a selective etching to the BP encapsulation structure. Chromium and titanium are suitable metal electrodes for BP channels to control the transition from a p-type unipolar property to ambipolar characteristic because of different work functions. Record-high mobilities of 6000 $cm^2V^{-1}s^{-1}$ and 8400 $cm^2V^{-1}s^{-1}$ are respectively obtained for electrons and holes at cryogenic temperatures. High-mobility BP devices enable the investigation of quantum oscillations with an indistinguishable Zeeman effect in laboratory magnetic field.

cond-mat.mes-hall↗

arXiv2016

The production of unknown neutron-rich isotopes in $^{238}$U+$^{238}$U collisions at near-barrier energy

The production cross sections for primary and residual fragments with charge number from $Z$=70 to 120 produced in the collision of $^{238}$U+$^{238}$U at 7.0 MeV/nucleon are calculated by the improved quantum molecular dynamics (ImQMD) model incorporated with the statistical evaporation model (HIVAP code). The calculation results predict that about sixty unknown neutron-rich isotopes from element Ra ($Z$=88) to Db ($Z$=105) can be produced with the production cross sections above the lower bound of $10^{-8}$ mb in this reaction. And almost all of unknown neutron-rich isotopes are emitted at the laboratory angles $θ_{lab}\leq$ 60$^\circ$. Two cases, i.e. the production of the unknown uranium isotopes with $A\geq$ 244 and that of rutherfordium with $A\geq$ 269 are investigated for understanding the production mechanism of unknown neutron-rich isotopes. It is found that for the former case the collision time between two uranium nuclei is shorter and the primary fragments producing the residues have smaller excitation energies of $\leq$ 30 MeV and the outgoing angles of those residues cover a range of 30$^\circ$-60$^\circ$. For the later case, the longer collision time is needed for a large number of nucleons being transferred and thus it results in the higher excitation energies and smaller outgoing angles of primary fragments, and eventually results in a very small production cross section for the residues of Rf with $A\geq$ 269 which have a small interval of outgoing angles of $θ_{lab}$=40$^\circ$-50$^\circ$.

nucl-th↗

arXiv2016

Quantum Hall Effect in Ultrahigh Mobility Two-dimensional Hole Gas of Black Phosphorus

We demonstrate that a field effect transistor (FET) made of few layer black phosphorus (BP) encapsulated in hexagonal boron nitride (h-BN) in vacuum, exhibts the room temperature hole mobility of 5200 $cm^2/Vs$ being limited just by the phonon scattering. At cryogenic tempeature the FET mobility increases up to 45,000 $cm^2/Vs$, which is eight times higher compared with the mobility obtained in earlier reports. The unprecedentedly clean h-BN/BP/h-BN heterostructure exhibits Shubnikov-de Haas oscillations and quantum Hall effect with Landau level (LL) filling factors down to v=2 in conventional laboratory magnetic fields. Moreover, carrier density independent effective mass m=0.26 m_0 is measured, and Lande g-factor g=2.47 is reported. Furthermore, an indication for a distinct hole transport behavior with up and down spin orientation is found.

cond-mat.mes-hall↗

arXiv2016

An Electric Vehicle Charging Management Scheme Based on Publish/Subscribe Communication Framework

Motivated by alleviating CO$_2$ pollution, Electric Vehicle (EV) based applications have recently received wide interests from both commercial and research communities by using electric energy instead of traditional fuel energy. Although EVs are inherently with limited travelling distance, such limitation could be overcome by deploying public Charging Stations (CSs) to recharge EVs battery during their journeys. In this paper we propose a novel communication framework for on-the-move EV charging scenario, based on the Publish/Subscribe (P/S) mechanism for disseminating necessary CS information to EVs, in order for them to make optimized decisions on where to charge. A core part of our communication framework is the utilization of Road Side Units (RSUs) to bridge the information flow from CSs to EVs, which has been regarded as a type of cost-efficient communication infrastructure. Under this design, we introduce two complementary communication modes of signalling protocols, namely Push and Pull Modes, in order to enable the required information dissemination operation. Both analysis and simulation show the advantage of Pull Mode, in which the information is cached at RSUs to support asynchronous communication. We further propose a remote reservation service based on the Pull Mode, such that the CS-selection decision making can utilize the knowledge of EVs' charging reservation, as published from EVs through RSUs to CSs. Results show that both the performance at CS and EV sides are further improved based on using this anticipated information.

cs.NI↗

arXiv2016

Weak localization effect in topological insulator micro flakes grown on insulating ferrimagnet BaFe12O19

Many exotic physics anticipated in topological insulators require a gap to be opened for their topologica surface states by breaking time reversal symmetry. The gap opening has been achieved by doping magnetic impurities, which however inevitably create extra carriers and disorder that undermine the electronic transport. In contrast, the proximity to a ferromagnetic/ferrimagnetic insulator may improve the device quality, thus promises a better way to open the gap while minimizing the side-effects. Here, we grow thin single-crystal Sb1.9Bi0.1Te3 micro flakes on insulating ferrimagnet BaFe12O19 by using the van der Waals epitaxy technique. The micro flakes show a negative magnetoresistance in weak perpendicular fields below 50 K, which can be quenched by increasing temperature. The signature implies the weak localization effect as its origin, which is absent in intrinsic topological insulators, unless a surface state gap is opened. The surface state gap is estimated to be 10 meV by using the theory of the gap-induced weak localization effect. These results indicate that the magnetic proximity effect may open the gap for the topological surface attached to BaM insulating ferrimagnet. This heterostructure may pave the way for the realization of new physical effects as well as the potential applications of spintronics devices.

cond-mat.mes-hall↗

arXiv2016

Universal low-temperature Ohmic contacts for quantum transport in transition metal dichalcogenides

Low carrier mobility and high electrical contact resistance are two major obstacles prohibiting explorations of quantum transport in TMDCs. Here, we demonstrate an effective method to establish low-temperature Ohmic contacts in boron nitride encapsulated TMDC devices based on selective etching and conventional electron-beam evaporation of metal electrodes. This method works for most extensively studied TMDCs in recent years, including MoS2, MoSe2, WSe2, WS2, and 2H-MoTe2. Low electrical contact resistance is achieved at 2 K. All of the few-layer TMDC devices studied show excellent performance with remarkably improved field-effect mobilities ranging from 2300 cm2/V s to 16000 cm2/V s, as verified by the high carrier mobilities extracted from Hall effect measurements. Moreover, both high-mobility n-type and p-type TMDC channels can be realized by simply using appropriate contact metals. Prominent Shubnikov-de Haas oscillations have been observed and investigated in these high-quality TMDC devices.

cond-mat.mtrl-sci↗