arXiv subjects

Xin Zhang

Publications and source records attributed to Xin Zhang.

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

arXiv2009

Brane inflation revisited after WMAP five-year results

In this paper, we revisit brane inflation models with the WMAP five-year results. The WMAP five-year data favor a red-tilted power spectrum of primordial fluctuations at the level of two standard deviations, which is the same as the WMAP three-year result qualitatively, but quantitatively the spectral index is slightly greater than the three-year value. This result can bring impacts on brane inflation models. According to the WMAP five-year data, we find that the KKLMMT model can survive at the level of one standard deviation, and the fine-tuning of the parameter $β$ can be alleviated to a certain extent at the level of two standard deviations.

astro-ph↗

arXiv2009

Chaplygin inflation in loop quantum cosmology

In this paper we discuss the inflationary universe in the context of a Chaplygin gas equation of state within the framework of the effective theory of loop quantum cosmology. Under the slow-roll approximation, we calculate the primordial perturbations for this model. We give the general expressions of the scalar spectral index, its running, and the tensor-to-scalar ratio, etc. For the chaotic inflation with a quadratic potential, using the WMAP 5-year results, we determine the parameters of the Chaplygin inflation model in loop quantum cosmology. The results are consistent with the WMAP observations.

gr-qc↗

arXiv2009

New agegraphic dark energy as a rolling tachyon

Combining the general relativity and the uncertainty relation in quantum mechanics, the energy density of quantum fluctuations of space-time can be viewed as dark energy. The so-called agegraphic dark energy model is just based on this viewpoint, in which the age of the universe is introduced as the length measure. Recently, the new agegraphic dark energy model was proposed, where the dynamical dark energy is measured by the conformal age of the universe. On the other hand, scalar-field dark energy models like tachyon are often regarded as an effective description of some underlying theory of dark energy. In this paper, we show that the new agegraphic dark energy can be described completely by a tachyon scalar-field. We thus reconstruct the potential and the dynamics of the tachyon scalar-field, according to the evolution of the new agegraphic dark energy.

astro-ph.CO↗

arXiv2009

Absolute free energies estimated by combining pre-calculated molecular fragment libraries

The absolute free energy -- or partition function, equivalently -- of a molecule can be estimated computationally using a suitable reference system. Here, we demonstrate a practical method for staging such calculations by growing a molecule based on a series of fragments. Significant computer time is saved by pre-calculating fragment configurations and interactions for re-use in a variety of molecules. We employ such fragment libraries and interaction tables for amino acids and capping groups to estimate free energies for small peptides. Equilibrium ensembles for the molecules are generated at no additional computational cost, and are used to check our results by comparison to standard dynamics simulation.

physics.bio-ph↗

arXiv2008

Comparison of Birefringent Metamaterials and Meanderline Structure as Quarter-Wave Plates at Terahertz Frequencies

We have fabricated a quarter-wave plate from a single layer birefringent metamaterial. For comparison, an appropriately scaled double layer meanderline structure was fabricated. At the design frequency of 639 GHz, the metamaterial structure achieves 99.9% circular polarization while the meanderline achieves 99.6%. The meanderline displays a larger bandwidth of operation, attaining over 99% circular polarization from 615 - 743 GHz, while the metamaterial achieves 99% from 626 - 660 GHz. However, both are broad enough for use with CW sources making metamaterials a more attractive choice due to the ease of fabrication. Both samples are free standing with a total thickness of 70um for the meanderline structure and a mere 20um for the metamaterial highlighting the large degree of birefringence exhibited with metamaterial structures.

physics.optics↗

arXiv2008

Planar wallpaper group metamaterial for novel terahertz applications

We present novel metamaterial structures based upon various planar wallpaper groups, in both hexagonal and square unit cells. An investigation of metamaterials consisting of one, two, and three unique sub-lattices with resonant frequencies in the terahertz (THz) was performed. We describe the theory, and perform simulations and experiments to characterize these multiple element metamaterials. A method for using these new structures as a means for bio / chemical hazard detection, as well as electromagnetic signature control is proposed.

cond-mat.mtrl-sci↗

arXiv2008

Comment on "Turnaround in Cyclic Cosmology"

We comment on a recent paper by L. Baum and P. H. Frampton [Phys. Rev. Lett. 98, 071301 (2007)] where it was argued that the entropy problem can be resolved in a peculiar cyclic universe model through a deflation mechanism (i.e., the universe is fragmented into many disconnected causal patches at the turnaround). We point out that in this cyclic model the Hubble length will become infinity at the turnaround, thus the deflation scenario is not valid.

hep-th↗

arXiv2008

A metamaterial absorber for the terahertz regime: Design, fabrication and characterization

We present a metamaterial that acts as a strongly resonant absorber at terahertz frequencies. Our design consists of a bilayer unit cell which allows for maximization of the absorption through independent tuning of the electrical permittivity and magnetic permeability. An experimental absorptivity of 70% at 1.3 terahertz is demonstrated. We utilize only a single unit cell in the propagation direction, thus achieving an absorption coefficient $α$ = 2000 cm$^{-1}$. These metamaterials are promising candidates as absorbing elements for thermally based THz imaging, due to their relatively low volume, low density, and narrow band response.

cond-mat.mtrl-sci↗

arXiv2008

Possible theoretical limits on holographic quintessence from weak gravity conjecture

The holographic dark energy model is one of the important ways for dealing with the dark energy problems in the quantum gravity framework. In this model, the dimensionless parameter $c$ plays an essential role in determining the evolution of the holographic dark energy. In particular, the holographic dark energy with $c\geq 1$ can be effectively described by a quintessence scalar-field. However, according to the requirement of the weak gravity conjecture the variation of the quintessence scalar-field should be less than the Planck mass, which would give theoretic constraints on the parameters $c$ and $Ω_{\mathrm{m0}}$. Therefore, we get the possible theoretical limits on the parameter $c$ for the holographic quintessence model.

astro-ph↗

arXiv2008

High Performance Cooperative Transmission Protocols Based on Multiuser Detection and Network Coding

Cooperative transmission is an emerging communication technique that takes advantage of the broadcast nature of wireless channels. However, due to low spectral efficiency and the requirement of orthogonal channels, its potential for use in future wireless networks is limited. In this paper, by making use of multiuser detection (MUD) and network coding, cooperative transmission protocols with high spectral efficiency, diversity order, and coding gain are developed. Compared with the traditional cooperative transmission protocols with single-user detection, in which the diversity gain is only for one source user, the proposed MUD cooperative transmission protocols have the merit that the improvement of one user's link can also benefit the other users. In addition, using MUD at the relay provides an environment in which network coding can be employed. The coding gain and high diversity order can be obtained by fully utilizing the link between the relay and the destination. From the analysis and simulation results, it is seen that the proposed protocols achieve higher diversity gain, better asymptotic efficiency, and lower bit error rate, compared to traditional MUD schemes and to existing cooperative transmission protocols. From the simulation results, the performance of the proposed scheme is near optimal as the performance gap is 0.12dB for average bit error rate (BER) 10^{-6} and 1.04dB for average BER 10^(-3), compared to two performance upper bounds.

cs.IT↗

arXiv2008

Agegraphic dark energy as a quintessence

Recently, a dark energy model characterized by the age of the universe, dubbed ``agegraphic dark energy'', was proposed by Cai. In this paper, a connection between the quintessence scalar-field and the agegraphic dark energy is established, and accordingly, the potential of the agegraphic quintessence field is constructed.

astro-ph↗

arXiv2007

Statefinder diagnosis for the interacting model of holographic dark energy

In this paper, we investigate the holographic dark energy model with interaction between dark energy and dark matter, from the statefinder viewpoint. We plot the trajectories of the interacting holographic dark energy model for different interaction cases as well as for different values of the parameter $c$ in the statefinder-plane. The statefinder diagrams characterize the properties of the holographic dark energy and show the discrimination between the two cases with and without interaction. As a result, we show the influence of the interaction to the evolution of the universe in the statefinder diagrams. Moreover, as a complement to the statefinder diagnosis, we study the interacting holographic dark energy model in the $w-w'$ plane, which can provide us with a dynamical diagnosis.

astro-ph↗

arXiv2007

Holographic dark energy in a cyclic universe

In this paper we study the cosmological evolution of the holographic dark energy in a cyclic universe, generalizing the model of holographic dark energy proposed by Li. The holographic dark energy with $c<1$ can realize a quintom behavior; namely, it evolves from a quintessence-like component to a phantom-like one. The holographic phantom energy density grows rapidly and dominates the late-time expanding phase, helping realize a cyclic universe scenario in which the high energy regime is modified by the effects of quantum gravity, causing a turnaround (and a bounce) of the universe. The dynamical evolution of holographic dark energy in the regimes of low energy and high energy is governed by two differential equations, respectively. It is of importance to link the two regimes for this scenario. We propose a link condition giving rise to a complete picture of holographic evolution of a cyclic universe.

hep-th↗

arXiv2007

Supersymmetric Hamiltonian Approach to Edge Excitations in $ν= 5/2$ Fractional Quantum Hall Effect

A supersymmetric Hamiltonian is constructed for the edge excitations of the Moore-Read (Pfaffian) like state, which is a realization of the N=2 supersymmetric CS model. Fermionic generators and their conjugates are introduced to deal with the fermion pairing, whose condensation form a BCS like state. After Bogoliubov transformation, a N=2 supersymmetric and nonrelativistic Hamiltonian is found to take a known form, which is integrable. The main difference between the Moore-Read state and our BCS like state is that the number of fermion pairs in our formalism is not fixed. However, we have also found that the excited states in our model looks similar but not exactly the same as Moore and Read's.

hep-th↗

arXiv2007

Inflationary universe in loop quantum cosmology

Loop quantum cosmology provides a nice solution of avoiding the big bang singularity through a big bounce mechanism in the high energy region. In loop quantum cosmology an inflationary universe is emergent after the big bounce, no matter what matter component is filled in the universe. A super-inflation phase without phantom matter will appear in a certain way in the initial stage after the bounce; then the universe will undergo a normal inflation stage. We discuss the condition of inflation in detail in this framework. Also, for slow-roll inflation, we expect the imprint from the effects of the loop quantum cosmology should be left in the primordial perturbation power spectrum. However, we show that this imprint is too weak to be observed.

gr-qc↗

arXiv2007

Optimal Power Allocation for Distributed Detection over MIMO Channels in Wireless Sensor Networks

In distributed detection systems with wireless sensor networks, the communication between sensors and a fusion center is not perfect due to interference and limited transmitter power at the sensors to combat noise at the fusion center's receiver. The problem of optimizing detection performance with such imperfect communication brings a new challenge to distributed detection. In this paper, sensors are assumed to have independent but nonidentically distributed observations, and a multi-input/multi-output (MIMO) channel model is included to account for imperfect communication between the sensors and the fusion center. The J-divergence between the distributions of the detection statistic under different hypotheses is used as a performance criterion in order to provide a tractable analysis. Optimizing the performance (in terms of the J-divergence) with individual and total transmitter power constraints on the sensors is studied, and the corresponding power allocation scheme is provided. It is interesting to see that the proposed power allocation is a tradeoff between two factors, the communication channel quality and the local decision quality. For the case with orthogonal channels under certain conditions, the power allocation can be solved by a weighted water-filling algorithm. Simulations show that, to achieve the same performance, the proposed power allocation in certain cases only consumes as little as 25 percent of the total power used by an equal power allocation scheme.

cs.IT↗

arXiv2007

Holographic tachyon model

We propose in this Letter a holographic model of tachyon dark energy. A connection between the tachyon scalar-field and the holographic dark energy is established, and accordingly, the potential of the holographic tachyon field is constructed. We show that the holographic evolution of the universe with $c\geqslant 1$ can be described completely by the resulting tachyon model in a certain way.

astro-ph↗

arXiv2007

Constraints on Holographic Dark Energy from Latest Supernovae, Galaxy Clustering, and Cosmic Microwave Background Anisotropy Observations

The holographic dark energy model is proposed by Li as an attempt for probing the nature of dark energy within the framework of quantum gravity. The main characteristic of holographic dark energy is governed by a numerical parameter $c$ in the model. The parameter $c$ can only be determined by observations. Thus, in order to characterize the evolving feature of dark energy and to predict the fate of the universe, it is of extraordinary importance to constrain the parameter $c$ by using the currently available observational data. In this paper, we derive constraints on the holographic dark energy model from the latest observational data including the gold sample of 182 Type Ia supernovae (SNIa), the shift parameter of the cosmic microwave background (CMB) given by the three-year {\it Wilkinson Microwave Anisotropy Probe} ({\it WMAP}) observations, and the baryon acoustic oscillation (BAO) measurement from the Sloan Digital Sky Survey (SDSS). The joint analysis gives the fit results in 1-$σ$: $c=0.91^{+0.26}_{-0.18}$ and $Ω_{\rm m0}=0.29\pm 0.03$. That is to say, though the possibility of $c<1$ is more favored, the possibility of $c>1$ can not be excluded in one-sigma error range, which is somewhat different from the result derived from previous investigations using earlier data. So, according to the new data, the evidence for the quintom feature in the holographic dark energy model is not as strong as before.

astro-ph↗