arXiv subjects

Jun Zhang

Publications and source records attributed to Jun Zhang.

At least 1,279 records · Page 71Linked to original sources

arXiv2012

Experimental study of pedestrian flow through a T-junction

In this study, series of experiments under laboratory conditions were carried out to investigate pedestrian flow through a T-junction, i.e., two branches merging into the main stream. The whole duration of the experiments was recorded by video cameras and the trajectories of each pedestrian were extracted using the software Petrack from these videos. The Voronoi method is used to resolve the fine structure of the fundamental diagram and spatial dependence of the measured quantities from trajectories. In our study, only the data in the stationary state are used by analyzing the time series of density and velocity. The density, velocity and specific flow profiles are obtained by refining the size of the measurement area (here 10 cm \times 10 cm are adopted). With such a high resolution, the spatial distribution of density, velocity and specific flow can be obtained separately and the regions with higher value can be observed intuitively. Finally, the fundamental diagrams of T-junction flow is compared in three different locations. It is shown that the fundamental diagrams of the two branches match well. However, the velocities in front of the merging are significantly lower than that in the main stream at the same densities. After the merging, the specific flow increases with the density ρtill 2.5 m-2. While in the branches, the specific flow is almost independent of the density between ρ= 1.5 m-2 and 3.5 m-2

physics.soc-ph↗

arXiv2012

Training Optimization for Energy Harvesting Communication Systems

Energy harvesting (EH) has recently emerged as an effective way to solve the lifetime challenge of wireless sensor networks, as it can continuously harvest energy from the environment. Unfortunately, it is challenging to guarantee a satisfactory short-term performance in EH communication systems because the harvested energy is sporadic. In this paper, we consider the channel training optimization problem in EH communication systems, i.e., how to obtain accurate channel state information to improve the communication performance. In contrast to conventional communication systems, the optimization of the training power and training period in EH communication systems is a coupled problem, which makes such optimization very challenging. We shall formulate the optimal training design problem for EH communication systems, and propose two solutions that adaptively adjust the training period and power based on either the instantaneous energy profile or the average energy harvesting rate. Numerical and simulation results will show that training optimization is important in EH communication systems. In particular, it will be shown that for short block lengths, training optimization is critical. In contrast, for long block lengths, the optimal training period is not too sensitive to the value of the block length nor to the energy profile. Therefore, a properly selected fixed training period value can be used.

cs.IT↗

arXiv2012

Recoverability Analysis for Modified Compressive Sensing with Partially Known Support

The recently proposed modified-compressive sensing (modified-CS), which utilizes the partially known support as prior knowledge, significantly improves the performance of recovering sparse signals. However, modified-CS depends heavily on the reliability of the known support. An important problem, which must be studied further, is the recoverability of modified-CS when the known support contains a number of errors. In this letter, we analyze the recoverability of modified-CS in a stochastic framework. A sufficient and necessary condition is established for exact recovery of a sparse signal. Utilizing this condition, the recovery probability that reflects the recoverability of modified-CS can be computed explicitly for a sparse signal with \ell nonzero entries, even though the known support exists some errors. Simulation experiments have been carried out to validate our theoretical results.

cs.IT↗

arXiv2012

Visibility of ultra-cold Bose system in triangular optical lattices

In this paper, by treating the hopping parameter in Bose-Hubbard model as a perturbation, with the help of the re-summed Green's function method and cumulants expansion, the momentum distribution function of the ultra-cold Bose system in triangular optical lattice is calculated analytically. By utilizing it, the time-of-flight absorption picture is plotted and the corresponding visibility is determined. The comparison between our analytical results and the experimental data from Ref.[4] exhibits a qualitative agreement.

cond-mat.quant-gas↗

arXiv2012

New Deep Holes of Generalized Reed-Solomon Codes

Deep holes play an important role in the decoding of generalized Reed-Solomon codes. Recently, Wu and Hong \cite{WH} found a new class of deep holes for standard Reed-Solomon codes. In the present paper, we give a concise method to obtain a new class of deep holes for generalized Reed-Solomon codes. In particular, for standard Reed-Solomon codes, we get the new class of deep holes given in \cite{WH}. Li and Wan \cite{L.W1} studied deep holes of generalized Reed-Solomon codes $GRS_{k}(\f,D)$ and characterized deep holes defined by polynomials of degree $k+1$. They showed that this problem is reduced to be a subset sum problem in finite fields. Using the method of Li and Wan, we obtain some new deep holes for special Reed-Solomon codes over finite fields with even characteristic. Furthermore, we study deep holes of the extended Reed-Solomon code, i.e., $D=\f$ and show polynomials of degree $k+2$ can not define deep holes.

cs.IT↗

arXiv2012

Effect of Cleaving Temperature on the Surface and Bulk Fermi Surface of Sr2RuO4 Investigated by High Resolution Angle-Resolved Photoemission

High resolution angle-resolved photoemission measurements are carried out to systematically investigate the effect of cleaving temperature on the electronic structure and Fermi surface of Sr$_2$RuO$_4$. Different from previous reports that high cleaving temperature can suppress surface Fermi surface, we find that the surface Fermi surface remains obvious and strong in Sr$_2$RuO$_4$ cleaved at high temperature, even at room temperature. This indicates that cleaving temperature is not a key effective factor in suppressing the surface bands. On the other hand, in the aged surface of Sr$_2$RuO$_4$ that is cleaved and held for a long time, the bulk bands can be enhanced. We have also carried out laser ARPES measurements on Sr$_2$RuO$_4$ by using vacuum ultra-violet laser (photon energy at 6.994 eV) and found an obvious enhancement of bulk bands even for samples cleaved at low temperature. These information are important in realizing an effective approach in manipulating and detecting the surface and bulk electronic structure of Sr$_2$RuO$_4$. In particular, the enhancement of bulk sensitivity, together with its super-high instrumental resolution of VUV laser ARPES, will be advantageous in investigating fine electronic structure and superconducting properties of Sr$_2$RuO$_4$ in the future.

cond-mat.supr-con↗

arXiv2012

Unusual Fermi Surface Sheet-Dependent Band Splitting in Sr2RuO4 Revealed by High Resolution Angle-Resolved Photoemission

High resolution angle-resolved photoemission measurements have been carried out on Sr2RuO4. We observe clearly two sets of Fermi surface sheets near the (π,0)-(0,π) line which are most likely attributed to the surface and bulk Fermi surface splitting of the βband. This is in strong contrast to the nearly null surface and bulk Fermi surface splitting of the αband although both have identical orbital components. Extensive band structure calculations are performed by considering various scenarios, including structural distortion, spin-orbit coupling and surface ferromagnetism. However, none of them can explain such a qualitative difference of the surface and bulk Fermi surface splitting between the αand βsheets. This unusual behavior points to an unknown order on the surface of Sr2RuO4 that remains to be uncovered. Its revelation will be important for studying and utilizing novel quantum phenomena associated with the surface of Sr2RuO4 as a result of its being a possible p-wave chiral superconductor and a topological superconductor.

cond-mat.supr-con↗

arXiv2012

Self-cancellation of ephemeral regions in the quiet Sun

With the observations from the Helioseismic and Magnetic Imager aboard the Solar Dynamics Observatory, we statistically investigate the ephemeral regions (ERs) in the quiet Sun. We find that there are two types of ERs: normal ERs (NERs) and self-cancelled ERs (SERs). Each NER emerges and grows with separation of its opposite polarity patches which will cancel or coalesce with other surrounding magnetic flux. Each SER also emerges and grows and its dipolar patches separate at first, but a part of magnetic flux of the SER will move together and cancel gradually, which is described with the term "self-cancellation" by us. We identify 2988 ERs among which there are 190 SERs, about 6.4% of the ERs. The mean value of self-cancellation fraction of SERs is 62.5%, and the total self-cancelled flux of SERs is 9.8% of the total ER flux. Our results also reveal that the higher the ER magnetic flux is, (i) the easier the performance of ER self-cancellation is, (ii) the smaller the self-cancellation fraction is, and (iii) the more the self-cancelled flux is. We think that the self-cancellation of SERs is caused by the submergence of magnetic loops connecting the dipolar patches, without magnetic energy release.

astro-ph.SR↗

arXiv2012

Direct Observation of Inner and Outer G' Band Double-resonance Raman Scattering in Free Standing Graphene

In contrary to the widely reported single and symmetric peak feature of G' (or 2D) band in Raman spectrum of graphene, we herein report the observation of splitting in G' band in free standing graphene. Our experimental findings provide a direct and strong support for the previous theoretical prediction that the coexistence of the outer and inner processes in the doubleresonance Raman scattering would cause the splitting of G' mode. Our investigation of the influence of trigonal warping effect on the spectral features of G' subbands further verified the theoretical interpretation established on the anisotropic electronic structure of graphene.

cond-mat.mtrl-sci↗

arXiv2012

Electronic Origin of High Temperature Superconductivity in Single-Layer FeSe Superconductor

The latest discovery of high temperature superconductivity signature in single-layer FeSe is significant because it is possible to break the superconducting critical temperature ceiling (maximum Tc~55 K) that has been stagnant since the discovery of Fe-based superconductivity in 2008. It also blows the superconductivity community by surprise because such a high Tc is unexpected in FeSe system with the bulk FeSe exhibiting a Tc at only 8 K at ambient pressure which can be enhanced to 38 K under high pressure. The Tc is still unusually high even considering the newly-discovered intercalated FeSe system A_xFe_{2-y}Se_2 (A=K, Cs, Rb and Tl) with a Tc at 32 K at ambient pressure and possible Tc near 48 K under high pressure. Particularly interesting is that such a high temperature superconductivity occurs in a single-layer FeSe system that is considered as a key building block of the Fe-based superconductors. Understanding the origin of high temperature superconductivity in such a strictly two-dimensional FeSe system is crucial to understanding the superconductivity mechanism in Fe-based superconductors in particular, and providing key insights on how to achieve high temperature superconductivity in general. Here we report distinct electronic structure associated with the single-layer FeSe superconductor. Its Fermi surface topology is different from other Fe-based superconductors; it consists only of electron pockets near the zone corner without indication of any Fermi surface around the zone center. Our observation of large and nearly isotropic superconducting gap in this strictly two-dimensional system rules out existence of node in the superconducting gap. These results have provided an unambiguous case that such a unique electronic structure is favorable for realizing high temperature superconductivity.

cond-mat.supr-con↗

arXiv2012

Vector-valued Reproducing Kernel Banach Spaces with Applications to Multi-task Learning

Motivated by multi-task machine learning with Banach spaces, we propose the notion of vector-valued reproducing kernel Banach spaces (RKBS). Basic properties of the spaces and the associated reproducing kernels are investigated. We also present feature map constructions and several concrete examples of vector-valued RKBS. The theory is then applied to multi-task machine learning. Especially, the representer theorem and characterization equations for the minimizer of regularized learning schemes in vector-valued RKBS are established.

math.FA↗

arXiv2012

Kinematics of an untwisting solar jet in polar coronal hole observed by SDO/AIA

Using the multi-wavelength data from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) spacecraft, we study a jet occurred in coronal hole near the northern pole of the Sun. The jet presented distinct helical upward motion during ejection. By tracking six identified moving features (MFs) in the jet, we found that the plasma moved at an approximately constant speed along the jet's axis, meanwhile, they made a circular motion in the plane transverse to the axis. Inferred from linear and trigonometric fittings to the axial and transverse heights of the six tracks, the mean values of axial velocities, transverse velocities, angular speeds, rotation periods, and rotation radiuses of the jet are 114 km s$^{-1}$, 136 km s$^{-1}$, 0.81\degr\ s$^{-1}$, 452 s, and 9.8 $\times$ 10$^{3}$ km respectively. As the MFs rose, the jet width at the corresponding height increased. For the first time, we derived the height variation of the longitudinal magnetic field strength in the jet from the assumption of magnetic flux conservation. Our results indicate that, at the heights of 1 $\times$ 10$^{4}$ $\sim$ 7 $\times$ 10$^{4}$ km from jet base, the flux density in the jet decreased from about 15 to 3 G as a function of B=0.5(R/R$_{\sun}$-1)$^{-0.84}$ (G). A comparison was made with the other results in previous studies.

astro-ph.SR↗

arXiv2012

Ordering in bidirectional pedestrian flows and its influence on the fundamental diagram

Experiments under laboratory conditions were carried out to study the ordering in bidirectional pedestrian streams and its influence on the fundamental diagram (density-speed-flow relation). The Voronoi method is used to resolve the fine structure of the resulting velocity-density relations and spatial dependence of the measurements. The data show that the specific flow concept is applicable also for bidirectional streams. For various forms of ordering in bidirectional streams, no large differences among density-flow relationships are found in the observed density range. The fundamental diagrams of bidirectional streams with different forms of ordering are compared with that of unidirectional streams. The result shows differences in the shape of the relation for ρ > 1.0 m-2. The maximum of the specific flow in unidirectional streams is significantly larger than that in all bidirectional streams examined.

physics.soc-ph↗

arXiv2011

Quantum phase transitions of ultra-cold Bose system in non-rectangular optical lattices

In this paper, we investigate systematically the Mott-insulator-Superfluid quantum phase transitions for ultracold scalar bosons in triangular, hexagonal, as well as Kagomé optical lattices. With the help of field-theoretical effective potential, by treating the hopping term in Bose-Hubbard model as perturbation, we calculate the phase boundaries analytically for different integer filling factors. Our analytical results are in good agreement with recent numerical results.

cond-mat.stat-mech↗

arXiv2011

Measuring the Reduced Shear

Neglecting the second order corrections in weak lensing measurements can lead to a few percent uncertainties on cosmic shears, and becomes more important for cluster lensing mass reconstructions. Existing methods which claim to measure the reduced shears are not necessarily accurate to the second order when a point spread function (PSF) is present. We show that the method of Zhang (2008) exactly measures the reduced shears at the second order level in the presence of PSF. A simple theorem is provided for further confirming our calculation, and for judging the accuracy of any shear measurement method at the second order based on its properties at the first order. The method of Zhang (2008) is well defined mathematically. It does not require assumptions on the morphologies of galaxies and the PSF. To reach a sub-percent level accuracy, the CCD pixel size is required to be not larger than 1/3 of the Full Width at Half Maximum (FWHM) of the PSF. Using a large ensemble (> 10^7) of mock galaxies of unrestricted morphologies, we find that contaminations to the shear signals from the noise of background photons can be removed in a well defined way because they are not correlated with the source shapes. The residual shear measurement errors due to background noise are consistent with zero at the sub-percent level even when the amplitude of such noise reaches about 1/10 of the source flux within the half-light radius of the source. This limit can in principle be extended further with a larger galaxy ensemble in our simulations. On the other hand, the source Poisson noise remains to be a cause of systematic errors. For a sub-percent level accuracy, our method requires the amplitude of the source Poisson noise to be less than 1/80 ~ 1/100 of the source flux within the half-light radius of the source, corresponding to collecting roughly 10^4 source photons.

astro-ph.CO↗

arXiv2011

SDO/AIA Observations of Secondary Waves Generated by Interaction of the 2011 June 7 Global EUV Wave With Solar Coronal Structures

We present SDO/AIA observations of the interaction of a global EUV wave on 2011 June 7 with active regions (ARs), coronal holes (CHs) and coronal bright structures. The primary global wave has a three-dimensional dome shape, with propagation speeds ranging from 430-780 km/s in different directions. The primary coronal wave runs in front of the expanding loops involved in the CME and its propagation speeds are approximately constant within 10-20 minutes. Upon arrival at an AR on its path, the primary EUV wave apparently disappears and a secondary wave rapidly reemerges 75 Mm within the AR boundary at a similar speed. When the EUV wave encounters a coronal bright structure, an additional wave front appears there and propagates in front of it at a velocity nearly a factor of 2 faster. Reflected waves from a polar CH and a coronal bright structure are observed and propagate fractionally slower than the primary waves. Some of these phenomena can be equally explained by either a wave or non-wave model alone. However, taken together, these observations provide new evidence for the multitudes of global EUV waves, in which a true MHD fast-mode wave or shock propagates in front of an expanding CME bubble.

astro-ph.SR↗

arXiv2011

Ubiquitous rotating network magnetic fields and EUV cyclones in the quiet Sun

We present the {\it Solar Dynamics Observatory} (SDO) Atmospheric Imaging Assembly (AIA) observations of EUV cyclones in the quiet Sun. These cyclones are rooted in the Rotating Network magnetic Fields (RNFs). Such cyclones can last several to more than ten hours, and, at the later phase, they are found to be associated with EUV brightenings (microflares) and even EUV waves. SDO Helioseismic and Magnetic Imager (HMI) observations show an ubiquitous presence of the RNFs. Using HMI line-of-sight magnetograms on 2010 July 8, we find 388 RNFs in an area of 800$\times$980 square arcseconds near the disk center where no active region is present. The sense of rotation shows a weak hemisphere preference. The unsigned magnetic flux of the RNFs is about 4.0$\times10^{21}$ Mx, or 78% of the total network flux. These observational phenomena at small scale reported in this letter are consistent with those at large scale in active regions. The ubiquitous RNFs and EUV cyclones over the quiet Sun may suggest an effective way to heat the corona.

astro-ph.SR↗

arXiv2011

A Galileon Design of Slow Expansion

We show a model of the slow expansion, in which the scale invariant spectrum of curvature perturbation is adiabatically induced by its increasing mode, by applying a generalized Galileon field. In this model, initially ε<< -1, which then is rapidly increasing, during this period the universe is slowly expanding. There is not the ghost instability, the perturbation theory is healthy. When ε\sim -1, the slow expansion phase ends, and the available energy of field can be released and the universe reheats. This scenario might be a viable design of the early universe.

astro-ph.CO↗