arXiv subjects

Liang Chen

Publications and source records attributed to Liang Chen.

At least 505 records · Page 28Linked to original sources

arXiv2012

Characterizing Short Necklace States in Logarithmic Transmission Spectrum of Strongly Localized Systems

High transmission plateaus exist widely in the logarithmic transmission spectra of localized systems. Their physical origins are short chains of coupled-localized-states embedded inside the localized system, which are dubbed as "short necklace states". In this work, we define the essential quantities and then, based on these quantities, we investigate the short necklace states' properties statistically and quantitatively. Two different approaches are utilized and the results from them agree with each other very well. In the first approach, the typical plateau-width and the typical order of short necklace states are obtained from the correlation function of logarithmic transmission. In the second approach, we investigate statistical distributions of the peak/plateau-width measured in logarithmic transmission spectra. A novel distribution is found, which can be exactly fitted by the summation of two Gaussian distributions. These two distributions are the results of sharp peaks of localized states and the high plateaus of short necklace states. The center of the second distribution also tells us the typical plateau-width of short necklace states. With increasing the system length, the scaling property of typical plateau-width is very special since it almost does not decrease. The methods and the quantities defined in this work can be widely used on Anderson localization studies.

cond-mat.dis-nn↗

arXiv2012

Short Necklace States, Logarithm Transmission Fluctuation and Localization Length

We investigate the widely-existing short necklace states in random systems. It is found that their peak width and relative height in lnT spectra keep almost constant when the system length increases, which is explained by the coupled-resonator theory with intrinsic parameters. This property makes them special in contribution of lnT fluctuation. Further, short necklace states can help us to deeply understand the physical meaning of localization length and the delocalized effectin localized regime.

cond-mat.dis-nn↗

arXiv2012

The central engines of two unusual radio-intermediate/quiet active galactic nuclei: III Zw 2 and PG 1407+265

We use the accretion disk/corona+jet model to fit the multi-band spectral energy distributions (SEDs) of two unusual radio-intermediate/quiet quasars. It is found that the optical/UV emission of III Zw 2 is probably dominated by the emission from the accretion disk. The X-ray emission should be dominated by the radiation from the jet, while the contribution of the disk corona is negligible. The optical/UV component in the SED of PG 1407+265 can be well modeled as the emission from the accretion disk, while the IR component is attributed to the thermal radiation from the dust torus with an opening angle ~ 50\circ. If the X-ray continuum emission is dominated by the synchrotron emission of the jet, the source should be a "high peak frequency blazar", which obviously deviates the normal blazar sequence. The observed SED can also be fitted quite well by the accretion disk/corona model with the viscosity parameter ? = 0:5. The spectrum of the accretion disk/corona in PG 1407+265 satisfies the weak line quasar criterion suggested in Laor & Davis.

astro-ph.HE↗

arXiv2011

Critical surface band gap of repulsive Casimir interaction between three dimensional topological insulators at finite temperature

We generalize the calculation of Casimir interaction between topological insulators with opposite topological magnetoelectric polarizabilities and finite surface band gaps to finite Temperature cases. We find that finite temperature quantitatively depress the repulsive peak and enlarge the critical surface gap $m_c$ for repulsive Casimir force. However the universal property $m_c a \sim 1/2$ is still valid for various oscillation strength, temperature region and topological magnetoelectric polarizabilities.

cond-mat.other↗

arXiv2011

A Uniform Correlation between Synchrotron Luminosity and Doppler Factor in Gamma-ray Bursts and Blazars: hint of similar intrinsic luminosities?

We compile 23 Gamma-ray Bursts (GRBs) and 21 blazars with estimated Doppler factors, and the Doppler factors of GRBs are estimated from their Lorentz factors by assuming their jet viewing angles θ->0 degree. Using the conventional assumption that the prompt emission of GRBs is dominated by the synchrotron radiation, we calculate the synchrotron luminosity of GRBs from their total isotropic energy and burst duration. Intriguingly, we discover a uniform correlation between the synchrotron luminosity and Doppler factor, L_syn \propto D^3.1, for GRBs and blazars, which suggests that they may share some similar jet physics. One possible reason is that GRBs and blazars have, more or less, similar intrinsic synchrotron luminosities and both of them are strongly enhanced by the beaming effect. After Doppler and redshift-correction, we find that the intrinsic peak energy of the GRBs ranges from 0.1 to 3 keV with a typical value of 1 keV. We further correct the beaming effect for the observed luminosity of GRBs and find that there exists a positive correlation between the intrinsic synchrotron luminosity and peak energy for GRBs, which is similar to that of blazars. Our results suggest that both the intrinsic positive correlation and the beaming effect may be responsible for the observed tight correlation between the isotropic energy and the peak energy in GRBs (so called "Amati" relation).

astro-ph.HE↗

arXiv2011

Casimir interaction between topological insulators with finite surface band gap

Casimir interaction between topological insulators with opposite topological magnetoelectric polarizabilities and finite surface band gaps has been investigated. For large surface band gap limit$(m\rightarrow\infty)$, we can obtain results given in [Phys. Rev. Lett. \textbf{106}, 020403 (2011)]. For small surface band gap limit$(m\rightarrow0)$, Casimir interaction between topological insulators is attractive and analogy to ideal mental in short separation limit. Generally, there is a critical value $m_c$ and when the surface band gap is greater than the critical value, the Casimir force is repulsive in an intermediate separation region. We estimate the critical surface band gap $m_c \sim 1/(2a)$, where $a$ is a critical separation where Casimir force vanishes.

cond-mat.other↗

arXiv2011

Evolutions of helical edge states in disordered HgTe/CdTe quantum wells

We study the evolutions of the nonmagnetic disorder-induced edge states with the disorder strength in the HgTe/CdTe quantum wells. From the supercell band structures and wave-functions, it is clearly shown that the conducting helical edge states, which are responsible for the reported quantized conductance plateau, appear above a critical disorder strength after a gap-closing phase transition. These edge states are then found to decline with the increase of disorder strength in a stepwise pattern due to the finite-width effect, where the opposite edges couple with each other through the localized states in the bulk. This is in sharp contrast with the localization of the edge states themselves if magnetic disorders are doped which breaks the time-reversal symmetry. The size-independent boundary of the topological phase is obtained by scaling analysis, and an Anderson transition to an Anderson insulator at even stronger disorder is identified, in-between of which, a metallic phase is found to separate the two topologically distinct phases.

cond-mat.mes-hall↗

arXiv2011

Implications on the blazar sequence and inverse Compton models from Fermi bright blazars

In this paper, we use the quasi-simultaneous spectra of Fermi bright blazars and Fermi detected narrow line Seyfert 1 (NLS1) to study the blazar sequence and inverse Compton (IC) models. I. The synchrotron peak luminosities (L_{s}) significantly inverse correlate with the synchrotron peak frequencies (ν_{s}), L_{s}\proptoν_{s}^{-0.44}, which is consistent with the blazar sequence. In addition to the correlation, there are some blazars showing low ν_{s} and low L_{s}. To study the relation between these low ν_{s} low L_{s} blazars and the blazar sequence, we present correlations of the parameter L_{s}ν_{s}^{1/4} with the ratio of Compton to synchrotron peak frequencies (r_{Cs}\equivν_{C}/ν_{s}) and with the ratio of Compton to synchrotron luminosities (CD\equiv L_{C}/L_{s}). The results indicate that both correlations are significant with a Pearson's probability for null correlation of p=0.0218 and p=0.0286 respectively. This does not support the idea that the low ν_{s} low L_{s} blazars are sources with less beaming. Another possibility, as suggested by Ghisellini & Tavecchio, is that these blazars have relative lower black hole masses. To test this, we collect the black hole masses of 30 blazars from archives, and find that the hole mass correlates with the parameter L_{s}ν_{s}^{0.44} (p=0.0344). Therefore, the black hole masses of low ν_{s} low L_{s} blazars are statistically small. The NLS1s are thought to have lower black hole masses. We find that the four NLS1s detected by Fermi have low ν_{s} and low L_{s}. This supports the above result. II. The ratio r_{Cs} correlates with CD significantly (p=0.00375). The external Compton (EC) model can naturally explain this correlation, while synchrotron self Compton (SSC) model can not. This agrees with the findings of many authors that the EC process dominates the gamma-ray emission of Flat Spectrum Radio Quasars.

astro-ph.CO↗

arXiv2010

Distributed and Optimal Reduced Primal-Dual Algorithm for Uplink OFDM Resource Allocation

Orthogonal Frequency Division Multiplexing (OFDM) is the key component of many emerging broadband wireless access standards. The resource allocation in OFDM uplink, however, is challenging due to heterogeneity of users' Quality of Service requirements, channel conditions, and individual resource constraints. We formulate the resource allocation problem as a non-strictly convex optimization problem, which typically has multiple global optimal solutions. We then propose a reduced primal-dual algorithm, which is distributed, low in computational complexity, and probably globally convergent to a global optimal solution. The performance of the algorithm is studied through a realistic OFDM simulator. Compared with the previously proposed centralized optimal algorithm, our algorithm not only significantly reduces the message overhead but also requires less iterations to converge.

cs.NI↗

arXiv2010

Possible gamma-ray emission of radio intermediate AGN III Zw 2 and its implication on the evolution of jets in AGNs

AGNs with hard $γ$-ray emission identified so far are radio-loud. III Zw 2 is a radio intermediate AGN with a relativistic jet. We study its spectral energy distribution (SED) and find that the broad band emissions are dominated by the non-thermal emissions from the jet. We model its SED through a synchrotron + inverse Compton (IC) model. The results show that the IC component of III Zw 2 peaks at a few MeV, and the flux density drops rapidly at higher energy with photon index $Γ\approx3.3$ above 0.1 GeV. The predicted flux is slightly over the sensitivity of $Fermi$/LAT, but it is not included in the first $Fermi$/LAT AGN catalog. The reason for this may be: 1) that the IC peak is low and the spectrum is very steep above 0.1 GeV, 2) that III Zw 2 is in the low state during the period of the $Fermi$/LAT operation. We also find that III Zw 2 follows similar jet processes as those in $γ$-ray AGNs, e.g., the relation between jet power and radiation power, called the blazar sequence. We suggest that III Zw 2 may be a young source at an earlier stage of jet activity.

astro-ph.HE↗

arXiv2010

A puzzle of radio-quiet quasar PG 1407+265: are optical and X-ray emissions produced from relativistic jet?

PG 1407+265 is a radio quiet quasar but has a relativistic jet. In this report, we show some peculiar properties of its optical and X-ray emissions, which indicate their possible non-thermal origins produced from jet. We use a simple synchrotron + synchrotron self Compton (SSC) model to fit the emissions with different ratios of energy densities between magnetic field and electrons (η=UB/Ue), which predict different γ-ray luminosity. The First LAT AGN Catalog (1LAC) did not include PG 1407+265, which indicates an upper limit of γ-ray luminosity. This upper limit constrains the ratio unreasonable large (η>10^4-10^5). This inversely indicates that the optical and X-ray emissions may be not produced from the beaming jet. We discuss the physical implications of these results.

astro-ph.HE↗

arXiv2010

Supply-demand diagrams and a new framework for analyzing the inhomogeneous Lighthill-Whitham-Richards model

Traditionally, the Lighthill-Whitham-Richards (LWR) models for homogeneous and inhomogeneous roads have been analyzed in flux-density space with the fundamental diagram of the flux-density relation. In this paper, we present a new framework for analyzing the LWR model, especially the Riemann problem at a linear boundary in which the upstream and downstream links are homogeneous and initially carry uniform traffic. We first review the definitions of local supply and demand functions and then introduce the so-called supply-demand diagram, on which a traffic state can be represented by its supply and demand, rather than as density and flux as on a fundamental diagram. It is well-known that the solutions to the Riemann problem at each link are self-similar with a stationary state, and that the wave on the link is determined by the stationary state and the initial state. In our new framework, there can also exist an interior state next to the linear boundary on each link, which takes infinitesimal space, and admissible conditions for the upstream and downstream stationary and interior states can be derived in supply-demand space. With an entropy condition consistent with a local supply-demand method in interior states, we show that the stationary states exist and are unique within the solution framework. We also develop a graphical scheme for solving the Riemann problem, and the results are shown to be consistent with those in the literature. We further discuss asymptotic stationary states on an inhomogeneous ring road with arbitrary initial conditions and demonstrate the existence of interior states with a numerical example. The framework developed in this study is simpler than existing ones and can be extended for analyzing the traffic dynamics in general road networks.

math.AP↗

arXiv2010

Minimal Conductivity of Topological Surface States with Magnetic Impurities

In this paper we use the semiclassical Boltzmann equation to investigate the transport properties of Dirac fermion on the surface of topological insulator with magnetic impurities. The results obtained show that there is also a minimal conductivity in this system as in graphene. We also argue the low temperature transport property, and find that there is no low temperature anomaly known as Kondo effect when the temperature is T>10^{-6}K.

cond-mat.str-el↗

arXiv2010

A fourth-order Runge-Kutta in the interaction picture method for coupled nonlinear Schrodinger equation

A fourth-order Runge-Kutta in the interaction picture (RK4IP) method is presented for solving the coupled nonlinear Schrodinger equation (CNLSE) that governs the light propagation in optical fibers with randomly varying birefringence. The computational error of RK4IP is caused by the fourth-order Runge-Kutta algorithm, better than the split-step approximation limited by the step size. As a result, the step size of RK4IP can have the same order of magnitude as the dispersion length and/or the nonlinear length of the fiber, provided the birefringence effect is small. For communication fibers with random birefringence, the step size of RK4IP can be orders of magnitude larger than the correlation length and the beating length of the fibers, depending on the interaction between linear and nonlinear effects. Our approach can be applied to the fibers having the general form of local birefringence and treat the Kerr nonlinearity without approximation. For the systems with realistic parameters, the RK4IP results are consistent with those using Manakov-PMD approximation. However, increased interaction between the linear and nonlinear terms in CNLSE leads to increased discrepancy between RK4IP and Manakov-PMD approximation.

physics.optics↗

arXiv2009

X-ray Radiation Mechanisms and Beaming Effect of Hot Spots and Knots in Active Galactic Nuclear Jets

The observed radio-optical-X-ray spectral energy distributions (SEDs) of 22 hot spots and 45 knots in the jets of 35 active galactic nuclei are complied from literature and modeled with single-zone lepton models. It is found that the observed luminosities at 5 GHz (L_5GHz) and at 1 keV (L_1keV) are tightly correlated, and the two kinds of sources can be roughly separated with a division of L_1keV=L_5GHz. Our SED fits show that the mechanisms of the X-rays are diverse. Considering the sources at rest, the synchrotron-self-Compton (SSC) scattering would dominate the IC process. This model can interpret the X-rays of some hot spots with a magnetic field strength (B_ssc^delta=1) being consistent with the equipartition magnetic field (B_eq^delta=1) in one order of magnitude, but an unreasonably low B_ssc^delta=1 is required to model the X-rays for all knots. The ratio R_B=B_eq^delta=1/B_ssc^delta=1 is greater than 1 and it is tightly anti-correlated with ratio R_L= L_1keV/L_5GHz for both the knots and the hot spots. We propose that the deviation may be due to the neglect of the relativistic bulk motion for these sources. Considering this effect, we show that the IC/CMB model well explains the X-ray emission for most sources. Both B_eq' and delta are tentatively correlated with R_L. Corrected by the beaming effect, the L'_5GHz-L'_1keV relations for the two kinds of sources are even tighter than the observed ones. These facts suggest that, under the equipartition condition, the observational differences of the X-rays from the knots and hot spots may be mainly due the differences on the Doppler boosting effect and the co-moving magnetic field of the two kinds of sources. Our IC scattering models predict a prominent GeV-TeV component in the SEDs for some sources, which are detectable with H.E.S.S. and Fermi/LAT.

astro-ph.HE↗

arXiv2009

Origin of the X-rays and Possible GeV-TeV Emission from the Western Hot Spot Of Pictor A

Pictor A is a nearby Fanaroff-Riley class II (FR II) radio galaxy with a bright hot spot, the western hot spot. Observation of high polarization in the optical emission of the hot spot indicates that the optical emission could be synchrotron radiation of relativistic electrons in the hot spot. These electrons may be able to produce high energy gamma-ray photons through inverse Compton (IC) scattering. We use single-zone and multi-zone synchrotron + synchrotron-self-Compton (SSC) models to fit the observed spectral energy distribution (SED) from the radio to the X-ray band of the hot spot. Our esults show that in the case of a much weaker magnetic field strength than the equipartition magnetic field, both the single-zone and multi-zone models can fit the SED, but the multi-zone model significantly improves the fit. The two models predict the hot spot as a GeV-TeV source, which might be marginally detectable with Fermi/LAT and HESS. The inverse Compton scattering of cosmic microwave background (IC/CMB) is also considered, but its contribution to GeV-TeV emission is negligible. Note that under the equipartition condition, the SED can also be fit with the multi-zone model, but the predicted flux at 10^22 Hz is too weak to be detectable. The detection of TeV gamma-rays from this FR II radio galaxy, if confirmed, would establish a new subclass of extragalactic source in this energy regime since most of the AGNs detected to date at TeV energies are high-energy-peaked BL Lac objects.

astro-ph.HE↗

arXiv2000

Robustness of Regional Matching Scheme over Global Matching Scheme

The paper has established and verified the theory prevailing widely among image and pattern recognition specialists that the bottom-up indirect regional matching process is the more stable and the more robust than the global matching process against concentrated types of noise represented by clutter, outlier or occlusion in the imagery. We have demonstrated this by analyzing the effect of concentrated noise on a typical decision making process of a simplified two candidate voting model where our theorem establishes the lower bounds to a critical breakdown point of election (or decision) result by the bottom-up matching process are greater than the exact bound of the global matching process implying that the former regional process is capable of accommodating a higher level of noise than the latter global process before the result of decision overturns. We present a convincing experimental verification supporting not only the theory by a white-black flag recognition problem in the presence of localized noise but also the validity of the conjecture by a facial recognition problem that the theorem remains valid for other decision making processes involving an important dimension-reducing transform such as principal component analysis or a Gabor transform.

cs.CV↗

arXiv1995

Local Moments Coupled to a Strongly Correlated Electron Chain

A 1D model hamiltonian that is motivated by the recent discovery of the heavy-fermion behavior in the cuprates of the $Nd_2CuO_4$ type is studied. It consists of $t-J$ interacting conduction electrons coupled to a lattice of localized spins through a Kondo exchange term $J_K$. Exact diagonalization and density matrix renormalization group methods are used. The latter method is generalized to arbitrary densities. At half-filling, a spin gap opens for all $J_K>0$. Away from half-filling$,$ it is shown that, at strong $J_K$% , the ground state is an unsaturated ferromagnet . At weak $J_K$ the system is in a paramagnetic phase with enhanced RKKY correlations. The importance of self-screening of the local moments in the depletion regime is discussed. We argue that these findings transcend the specifics of the model.

cond-mat↗