arXiv subjects

Jie Zhang

Publications and source records attributed to Jie Zhang.

At least 1,081 records · Page 60Linked to original sources

arXiv2017

The Origin of Solar Filament Plasma Inferred from in situ Observations of Elemental Abundances

Solar filaments/prominences are one of the most common features in the corona, which may lead to energetic coronal mass ejections (CMEs) and flares when they erupt. Filaments are about one hundred times cooler and denser than the coronal material, and physical understanding of their material origin remains controversial. Two types of scenarios have been proposed: one argues that the filament plasma is brought into the corona from photosphere or chromosphere through a siphon or evaporation/injection process, while the other suggests that the material condenses from the surrounding coronal plasma due to thermal instability. The elemental abundance analysis is a reasonable clue to constrain the models, as the siphon or evaporation/injection model would predict that the filament material abundances are close to the photospheric or chromospheric ones, while the condensation model should have coronal abundances. In this letter, we analyze the elemental abundances of a magnetic cloud that contains the ejected filament material. The corresponding filament eruption occurred on 1998 April 29, accompanying an M6.8 class soft X-ray flare located at the heliographic coordinates S18E20 (NOAA 08210) and a fast halo CME with the linear velocity of 1374 km s$^{-1}$ near the Sun. We find that the abundance ratios of elements with low and high First Ionization Potential such as Fe/O, Mg/O, and Si/O are 0.150, 0.050, and 0.070, respectively, approaching their corresponding photospheric values 0.065, 0.081, and 0.066, which does not support the coronal origin of the filament plasma.

astro-ph.SR↗

arXiv2017

Effects of biaxial strain on the electronic structures and band topologies of group-V elemental monolayers

Using first-principles calculations, we systematically investigate the electronic structures and band topologies of four kinds of group-V elemental (P, As, Sb and Bi) monolayers with buckled honeycomb structure. It is found that all these monolayers can change from semiconducting to semimetallic under compressive strain. If a tensile strain is however applied, the P, As and Sb monolayers undergo phase transition from topologically trivial to non-trivial regime, whereas the topological insulating nature of Bi monolayer remains unchanged. With tunability of the band gaps and band topologies, it can be expected that these elemental monolayers could be promising candidates for future optoelectronic and spintronic applications.

cond-mat.mes-hall↗

arXiv2016

Multi-Photon Transitions in Coupled Plasmon-Cyclotron Resonance Measured by Millimeter-Wave Reflection

We construct a low-temperature microwave waveguide interferometer for measuring high-frequency properties of two-dimensional electron gases (2DEGs). Coupled plasmon-cyclotron resonance (PCR) spectra are used to extract effective mass, bulk plasmon frequency, and carrier relaxation times. In contrast to traditional transmission spectroscopy, this method does not require sample preparation and is nondestructive. PCR signals can be resolved with a microwave power source as low as 10 nW. We observe PCR in the multi-photon transition regime, which has been proposed to be relevant to the microwave-induced resistance oscillations.

cond-mat.mes-hall↗

arXiv2016

Origin of Strong Magnetic Fields of Magnetars

Since there is 3P2 neutron superfluid in neutron star interior, it can be treated as a system of magnetic dipoles. Under the presence of background magnetic field, the magnetic dipoles tend to align in the same direction. When the temperature is lower than 10**7K, the strong magnetic fields of the magnetars may originate from the induced magnetic moment of the 3P2 neutron Cooper pairs in the anisotropic neutron superfluid. And this gives a convenient explanation of the strong magnetic field of magnetars.

astro-ph.HE↗

arXiv2016

The Physics of Magnetars II - The Electron Fermi Energy of and the Origin of High X-ray Luminosity of Magnetars

In this paper we discuss in detail the quantization of Landau energy levels of a strongly magnetized and completely degenerate relativistic electron gas in neutron stars. In particular, we focus on the Fermi energy dependence of the magnetic field for a relativistic electron gas in the superstrong magnetic field of magnetars. We would like to point out that some of the results concerning the microscopic number density of states of a strongly magnetized electron gas given by well-known statistical physics text books are incorrect. The repeated use of these results in the last five decades probably seriously affects the physics of neutron stars and magnetars. The quantization of Landau energy levels is accurately delineated in terms of the Dirac (delta) function. Relatively simple calculation shows that the Fermi energy of relativistic electron gas in magnetars with superstrong magnetic fields( B > Bcr , here Bcr is the Landau critical magnetic field) increases with magnetic fieldstrength as B**(1/4). On the basis of this simple but importantnew result we are leading naturally to propose a new mechanism for the production of high X-ray luminosity from magnetars.

astro-ph.HE↗

arXiv2016

Carrier emission of n-type Gallium Nitride illuminated by femtosecond laser pulses

The carrier emission efficiency of light emitting diodes is of fundamental importance for many technological applications, including the performance of GaN and other semiconductor photocathodes. We have measured the evolution of the emitted carriers and the associated transient electric field after femtosecond laser excitation of n-type GaN single crystals. These processes were studied using subpicosecond, ultrashort, electron pulses and explained by means of a three-layer analytical model. We find that for pump laser intensities on the order of 10^11 W/cm^2, the electrons that escaped from the crystal surface have a charge of about 2.7 pc and a velocity of about 1.8 um/ps. The associated transient electrical field evolves at intervals ranging from picoseconds to nanoseconds. These results provide a dynamic perspective on the photoemission properties of semiconductor photocathodes.

cond-mat.mtrl-sci↗

arXiv2016

Nuclear Fusion in Laser-Driven Counter-Streaming Collisionless Plasmas

Nuclear fusion reactions are the most important processes in nature to power stars and produce new elements, and lie at the center of the understanding of nucleosynthesis in the universe. It is critically important to study the reactions in full plasma environments that are close to true astrophysical conditions. By using laser-driven counter-streaming collisionless plasmas, we studied the fusion D$+$D$\rightarrow n +^3$He in a Gamow-like window around 27 keV. The results show that astrophysical nuclear reaction yield can be modulated significantly by the self-generated electromagnetic fields and the collective motion of the plasma. This plasma-version mini-collider may provide a novel tool for studies of astrophysics-interested nuclear reactions in plasma with tunable energies in earth-based laboratories.

nucl-ex↗

arXiv2016

Diversity Pulse Shaped Transmission in Ultra-Dense Small Cell Networks

In ultra-dense small cell networks, spatial multiplexing gain is a challenge because of the different propagation conditions. The channels associated with different transmitreceive pairs can be highly correlated due to the i) high probability of line-of-sight (LOS) communication between user equipment (UE) and base station (BS), and ii) insufficient spacing between antenna elements at both UE and BS. In this paper, we propose a novel transmission technique titled Diversity Pulse Shaped Transmission (DPST) to enhance the throughput over the correlated MIMO channels in an ultra-dense small cell network. The fundamental of DPST is to shape transmit signals at adjacent antennas with distinct interpolating filters, introducing pulse shaping diversity. In DPST, each antenna transmits its own data stream with a relative deterministic time offset-which must be a fraction of the symbol period-with respect to the adjacent antenna. The delay is interpolated with the pulse shaped signal generating a virtual MIMO channel that benefits from increased diversity from the receiver perspective. To extract the diversity, the receiver must operate in an over-sampled domain and hence a fractionally spaced equaliser (FSE) is proposed. The joint impact of DPST and FSE helps the receiver to sense a less correlated channel, eventually enhancing the UE's throughput. Moreover, in order to minimise the spatial correlation, we aim to optimise the deterministic fractional delay. Simulation results show that applying DPST to a correlated channel can approximately enhance the UE throughput by 1.93x and 3.76x in 2x2 and 4x4 MIMO systems, respectively.

cs.IT↗

arXiv2016

Performance Impact of LOS and NLOS Transmissions in Dense Cellular Networks under Rician Fading

In this paper, we analyse the performance of dense small cell network (SCNs). We derive analytical expressions for both their coverage probability and their area spectral efficiency (ASE) using a path loss model that considers both line-of-sight (LOS) and non-LOS (NLOS) components. Due to the close proximity of small cell base stations (BSs) and user equipments (UEs) in such dense SCNs, we also consider Rician fading as the multi-path fading channel model for both the LOS and NLOS fading transmissions. The Rayleigh fading used in most of existing works analysing dense SCNs is not accurate enough. Then, we compare the performance impact of LOS and NLOS transmissions in dense SCNs under Rician fading with that based on Rayleigh fading. The analysis and the simulation results show that in dense SCNs where LOS transmissions dominate the performance, the impact of Rician fading on the overall system performance is minor, and does not help to address the performance losses brought by the transition of many interfering signals from NLOS to LOS.

cs.IT↗

arXiv2016

The origin and nature of Boson Peak: the normal mode analysis of disordered granular crystals

Despite extensive theoretical \cite{GanterPRL1998, ElliotPRL2001,SchirmacherPRL2007, TanakaNatureM2008, MonacoPNAS2009, MarruzzoSCIRP2013} and experimental studies \cite{ChumakovPRL2011, ChumakovPRL2014, KayaScience2010,KChenPRL2010, LXuPRL2012, BonnPreprint2014}, a longstanding puzzle in condensed matter physics remains regarding the origin and nature of "Boson peak" (BP), where the vibrational density of states (DOS) in glasses possesses an excess of states compared with the crystalline counterpart. Here we show that BP is successfully observed in 2D hexagonal granular packing, where the disorder is due to the force network, i.e. the spatial heterogeneity of elasticity \cite{MarruzzoSCIRP2013}. Using photo-elastic techniques \cite{TrushNature2005}, the disordered particle interaction can be precisely measured to resolve the origin and nature of BP for the first time in a real scenario. We discover the structures of DOS of disordered crystals reminiscent of the corresponding perfect crystals, which is consistent with the recent studies in $SiO_2$\cite{ChumakovPRL2014} and in disordered gels\cite{LorenzoGelPaper2011}, notwithstanding the drastically different systems. Moreover, we propose a mechanism to clarify that BP is not merely a broadened and shifted van Hove singularity \cite{ElliotPRL2001, ChumakovPRL2011} but instead it is due to an interplay of the mesoscopic screening effect and the microscopic elasticity disorder -- causing, respectively, the broadening and the attenuation of the first and the second van Hove singularity. This may lead to an in-depth understanding of BP in structure glasses.

cond-mat.soft↗

arXiv2016

On the route to shear jamming, are fragile states real?

Starting from an unjammed initial state, applying shear to a granular material of a fixed packing fraction below $ϕ_J$, i.e. the isotropic jamming density of frictionless spheres can produce shear jamming states, as have been discovered recently. In addition, it has also been discovered that the system will first experience a bulk fragile state before evolving into a shear jammed state. Due to the existence of friction between the system and the third dimension in the previous studies, it is unclear whether such fragile state would still exist on the route to shear jamming if the friction with the third dimension were completely eliminated and the background noise level were greatly reduced. Using a novel apparatus, we have completely eliminated the friction between particles and the third dimension by floating the particles on the surface of a shallow water layer thus revealing more details of the route of shear-jamming. We are able to measure weak boundary pressure of three orders of magnitude below the resolution of the photo-elastic method through the combination force-gauges of high sensitivity and a cantilever-like simple-beam apparatus. In a system with weak cohesion, we have indeed observed the bulk fragile states before the shear jamming; in addition, we have also discovered the boundary fragile states and regimes of negligible shear modus compared to bulk modulus. Most importantly, we now have a much better understanding of the bulk fragile state: its existence requires an auxiliary in some form, e.g. the weak cohesion in this experiment. By constructing a way to freely tune down the cohesion to more than six orders of magnitude smaller, to our surprise, we have discovered the complete vanishing of the bulk fragile states, which thus allows us to determine the plausible origin of the bulk fragile state.

cond-mat.soft↗

arXiv2016

Navigation by anomalous random walks on complex networks

Anomalous random walks having long-range jumps are a critical branch of dynamical processes on networks, which can model a number of search and transport processes. However, traditional measurements based on mean first passage time are not useful as they fail to characterize the cost associated with each jump. Here we introduce a new concept of mean first traverse distance (MFTD) to characterize anomalous random walks that represents the expected traverse distance taken by walkers searching from source node to target node, and we provide a procedure for calculating the MFTD between two nodes. We use Levy walks on networks as an example, and demonstrate that the proposed approach can unravel the interplay between diffusion dynamics of Levy walks and the underlying network structure. Interestingly, applying our framework to the famous PageRank search, we can explain why its damping factor empirically chosen to be around 0.85. The framework for analyzing anomalous random walks on complex networks offers a new useful paradigm to understand the dynamics of anomalous diffusion processes, and provides a unified scheme to characterize search and transport processes on networks.

physics.soc-ph↗

arXiv2016

Seeing the Forest from the Trees in Two Looks: Matrix Sketching by Cascaded Bilateral Sampling

Matrix sketching is aimed at finding close approximations of a matrix by factors of much smaller dimensions, which has important applications in optimization and machine learning. Given a matrix A of size m by n, state-of-the-art randomized algorithms take O(m * n) time and space to obtain its low-rank decomposition. Although quite useful, the need to store or manipulate the entire matrix makes it a computational bottleneck for truly large and dense inputs. Can we sketch an m-by-n matrix in O(m + n) cost by accessing only a small fraction of its rows and columns, without knowing anything about the remaining data? In this paper, we propose the cascaded bilateral sampling (CABS) framework to solve this problem. We start from demonstrating how the approximation quality of bilateral matrix sketching depends on the encoding powers of sampling. In particular, the sampled rows and columns should correspond to the code-vectors in the ground truth decompositions. Motivated by this analysis, we propose to first generate a pilot-sketch using simple random sampling, and then pursue more advanced, "follow-up" sampling on the pilot-sketch factors seeking maximal encoding powers. In this cascading process, the rise of approximation quality is shown to be lower-bounded by the improvement of encoding powers in the follow-up sampling step, thus theoretically guarantees the algorithmic boosting property. Computationally, our framework only takes linear time and space, and at the same time its performance rivals the quality of state-of-the-art algorithms consuming a quadratic amount of resources. Empirical evaluations on benchmark data fully demonstrate the potential of our methods in large scale matrix sketching and related areas.

cs.LG↗

arXiv2016

On the Propagation of a Geoeffective Coronal Mass Ejection during March 15 -- 17, 2015

The largest geomagnetic storm so far in the solar cycle 24 was produced by a fast coronal mass ejection (CME) originating on 2015 March 15. It was an initially west-oriented CME and expected to only cause a weak geomagnetic disturbance. Why did this CME finally cause such a large geomagnetic storm? We try to find some clues by investigating its propagation from the Sun to 1 AU. First, we reconstruct the CME's kinematic properties in the corona from the SOHO and SDO imaging data with the aid of the graduated cylindrical shell (GCS) model. It is suggested that the CME propagated to the west $\sim$$33^\circ$$\pm$$10^\circ$ away from the Sun-Earth line with a speed of about 817 km s$^{-1}$ before leaving the field of view of the SOHO/LASCO C3 camera. A magnetic cloud (MC) corresponding to this CME was measured in-situ by the Wind spacecraft two days later. By applying two MC reconstruction methods, we infer the configuration of the MC as well as some kinematic information, which implies that the CME possibly experienced an eastward deflection on its way to 1 AU. However, due to the lack of observations from the STEREO spacecraft, the CME's kinematic evolution in interplanetary space is not clear. In order to fill this gap, we utilize numerical MHD simulation, drag-based CME propagation model (DBM) and the model for CME deflection in interplanetary space (DIPS) to recover the propagation process, especially the trajectory, of the CME from $30 R_S$ to 1 AU. It is suggested that the trajectory of the CME was deflected toward the Earth by about $12^\circ$, consistent with the implication from the MC reconstruction at 1 AU. This eastward deflection probably contributed to the CME's unexpected geoeffectiveness by pushing the center of the initially west-oriented CME closer to the Earth.

physics.space-ph↗

arXiv2016

Natural selection in the colloid world: Active chiral spirals

We present a model system in which to study natural selection in the colloid world. In the assembly of active Janus particles into rotating pinwheels when mixed with trace amounts of homogeneous colloids in the presence of an AC electric field, broken symmetry in the rotation direction produces spiral, chiral shapes. Locked into a central rotation point by the center particle, the spiral arms are found to trail rotation of the overall cluster. To achieve a steady state, the spiral arms undergo an evolutionary process to coordinate their motion. Because all the particles as segments of the pinwheel arms are self-propelled, asymmetric arm lengths are tolerated. Reconfiguration of these structures can happen in various ways and various mechanisms of this directed structural change are analyzed in detail. We introduce the concept of VIP (very important particles) to express that sustainability of active structures is most sensitive to only a few particles at strategic locations in the moving self-assembled structures.

cond-mat.soft↗

arXiv2016

Pulse Shaping Diversity to Enhance Throughput in Ultra-Dense Small Cell Networks

Spatial multiplexing (SM) gains in multiple input multiple output (MIMO) cellular networks are limited when used in combination with ultra-dense small cell networks. This limitation is due to large spatial correlation among channel pairs. More specifically, it is due to i) line-of-sight (LOS) communication between user equipment (UE) and base station (BS) and ii) in-sufficient spacing between antenna elements. We propose to shape transmit signals at adjacent antennas with distinct interpolating filters which introduces pulse shaping diversity eventually leading to improved SINR and throughput at the UEs. In this technique, each antenna transmits its own data stream with a relative offset with respect to adjacent antenna. The delay which must be a fraction of symbol period is interpolated with the pulse shaped signal and generates a virtual MIMO channel that leads to improved diversity and SINR at the receiver. Note that non-integral sampling periods with inter-symbol interference (ISI) should be mitigated at the receiver. For this, we propose to use a fractionally spaced equalizer (FSE) designed based on the minimum mean squared error (MMSE) criterion. Simulation results show that for a 2x2 MIMO and with inter-site-distance (ISD) of 50 m, the median received SINR and throughput at the UE improves by a factor of 11 dB and 2x, respectively, which verifies that pulse shaping can overcome poor SM gains in ultra-dense small cell networks.

cs.IT↗

arXiv2016

First results of the new endcap TOF commissioning at BESIII

The upgrade of the current BESIII Endcap TOF (ETOF) is carried out with the Multi-gap Resistive Plate Chamber (MRPC) technology. The installation of the new ETOF has been finished in October 2015. The first results of the MRPCs commissioning at BESIII are reported in this paper.

physics.ins-det↗

arXiv2016

A Statistical Study of the Average Iron Charge Distributions inside Magnetic Clouds for Solar Cycle 23

Magnetic clouds (MCs) are the interplanetary counterpart of coronal magnetic flux ropes. They can provide valuable information to reveal the flux rope characteristics at their eruption stage in the corona, which are unable to be explored in situ at present. In this paper, we make a comprehensive survey of the average iron charge state ( Fe) distributions inside 96 MCs for solar cycle 23 using ACE (Advanced Composition Explorer) data. As the Fe in the solar wind are typically around 9+ to 11+, the Fe charge state is defined as high when the Fe is larger than 12+, which implies the existence of a considerable amount of Fe ions with high charge states (e.g., \geq 16+). The statistical results show that the Fe distributions of 92 (~ 96%) MCs can be classified into four groups with different characteristics. In group A (11 MCs), the Fe shows a bimodal distribution with both peaks higher than 12+. Group B (4 MCs) presents a unimodal distribution of Fe with its peak higher than 12+. In groups C (29 MCs) and D (48 MCs), the Fe remains higher and lower than 12+ throughout ACE passage through the MC, respectively. Possible explanations to these distributions are discussed.

astro-ph.SR↗