arXiv subjects

X. Chen

Publications and source records attributed to X. Chen.

At least 541 records · Page 30Linked to original sources

arXiv2011

First-principles study of high conductance DNA sequencing with carbon nanotube electrodes

Rapid and cost-effective DNA sequencing at the single nucleotide level might be achieved by measuring a transverse electronic current as single-stranded DNA is pulled through a nano-sized pore. In order to enhance the electronic coupling between the nucleotides and the electrodes and hence the current signals, we employ a pair of single-walled close-ended (6,6) carbon nanotubes (CNTs) as electrodes. We then investigate the electron transport properties of nucleotides sandwiched between such electrodes by using first-principles quantum transport theory. In particular we consider the extreme case where the separation between the electrodes is the smallest possible that still allows the DNA translocation. The benzene-like ring at the end cap of the CNT can strongly couple with the nucleobases and therefore both reduce conformational fluctuations and significantly improve the conductance. The optimal molecular configurations, at which the nucleotides strongly couple to the CNTs, and which yield the largest transmission, are first identified. Then the electronic structures and the electron transport of these optimal configurations are analyzed. The typical tunneling currents are of the order of 50 nA for voltages up to 1 V. At higher bias, where resonant transport through the molecular states is possible, the current is of the order of several $μ$A. Below 1 V the currents associated to the different nucleotides are consistently distinguishable, with adenine having the largest current, guanine the second-largest, cytosine the third and finally thymine the smallest. We further calculate the transmission coefficient profiles as the nucleotides are dragged along the DNA translocation path and investigate the effects of configurational variations. Based on these results we propose a DNA sequencing protocol combining three possible data analysis strategies.

cond-mat.mes-hall↗

arXiv2011

Planck Early Results. VII. The Early Release Compact Source Catalog

(Abridged) A brief description of the methodology of construction, contents and usage of the Planck Early Release Compact Source Catalogue (ERCSC), including the Early Cold Cores (ECC) and the Early Sunyaev-Zeldovich (ESZ) cluster catalogue is provided. The catalogue is based on data that consist of mapping the entire sky once and 60% of the sky a second time by Planck, thereby comprising the first high sensitivity radio/submillimetre observations of the entire sky. A Monte-Carlo algorithm based on the injection and extraction of artificial sources into the Planck maps was implemented to select reliable sources among all extracted candidates such that the cumulative reliability of the catalogue is >=90%. The 10sigma photometric flux density limit of the catalogue at |b|>30 deg is 0.49, 1.0, 0.67, 0.5, 0.33, 0.28, 0.25, 0.47 and 0.82 Jy at each of the nine frequencies between 30 and 857 GHz. Sources which are up to a factor of ~2 fainter than this limit, and which are present in "clean" regions of the Galaxy where the sky background due to emission from the interstellar medium is low, are included in the ERCSC if they meet the high reliability criterion. The Planck ERCSC sources have known associations to stars with dust shells, stellar cores, radio galaxies, blazars, infrared luminous galaxies and Galactic interstellar medium features. A significant fraction of unclassified sources are also present in the catalogs. In addition, two early release catalogs that contain 915 cold molecular cloud core candidates and 189 SZ cluster candidates that have been generated using multi-frequency algorithms are presented. The entire source list, with more than 15000 unique sources, is ripe for follow-up characterisation with Herschel, ATCA, VLA, SOFIA, ALMA and other ground-based observing facilities.

astro-ph.CO↗

arXiv2011

Relating field-induced shift in transition temperature to the kinetics of coexisting phases in magnetic shape memory alloys

In a magnetic shape memory alloy system, we vary composition following phenomenological arguments to tune macroscopic properties. We achieve significantly higher shift in austenite to martensitic phase transition temperature with magnetic field. This enhancement is accompanied by significant broadening of the transition and by field-induced arrest of kinetics, both of which are related to the dynamics of the coexisting phases. This reveals hitherto unknown interrelationship between different length-scales. This may serve as an effective route for comprehensive understanding of similar multicomponent systems which show considerable variation in physical properties by minor change in microscopic parameters.

cond-mat.str-el↗

arXiv2011

Conflict-Directed Backjumping Revisited

In recent years, many improvements to backtracking algorithms for solving constraint satisfaction problems have been proposed. The techniques for improving backtracking algorithms can be conveniently classified as look-ahead schemes and look-back schemes. Unfortunately, look-ahead and look-back schemes are not entirely orthogonal as it has been observed empirically that the enhancement of look-ahead techniques is sometimes counterproductive to the effects of look-back techniques. In this paper, we focus on the relationship between the two most important look-ahead techniques---using a variable ordering heuristic and maintaining a level of local consistency during the backtracking search---and the look-back technique of conflict-directed backjumping (CBJ). We show that there exists a "perfect" dynamic variable ordering such that CBJ becomes redundant. We also show theoretically that as the level of local consistency that is maintained in the backtracking search is increased, the less that backjumping will be an improvement. Our theoretical results partially explain why a backtracking algorithm doing more in the look-ahead phase cannot benefit more from the backjumping look-back scheme. Finally, we show empirically that adding CBJ to a backtracking algorithm that maintains generalized arc consistency (GAC), an algorithm that we refer to as GAC-CBJ, can still provide orders of magnitude speedups. Our empirical results contrast with Bessiere and Regin's conclusion (1996) that CBJ is useless to an algorithm that maintains arc consistency.

cs.AI↗

arXiv2011

Planck Early Results: Statistical properties of extragalactic radio sources in the Planck Early Release Compact Source Catalogue

The data reported in Planck's Early Release Compact Source Catalogue (ERCSC) are exploited to measure the number counts (dN/dS) of extragalactic radio sources at 30, 44, 70, 100, 143 and 217 GHz. Due to the full-sky nature of the catalogue, this measurement extends to the rarest and brightest sources in the sky. At lower frequencies (30, 44, and 70 GHz) our counts are in very good agreement with estimates based on WMAP data, being somewhat deeper at 30 and 70 GHz, and somewhat shallower at 44 GHz. Planck's source counts at 143 and 217 GHz join smoothly with the fainter ones provided by the SPT and ACT surveys over small fractions of the sky. An analysis of source spectra, exploiting Planck's uniquely broad spectral coverage, finds clear evidence of a steepening of the mean spectral index above about 70 GHz. This implies that, at these frequencies, the contamination of the CMB power spectrum by radio sources below the detection limit is significantly lower than previously estimated.

astro-ph.CO↗

arXiv2010

Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Galactic Foreground Emission

[Abridged] We present updated estimates of Galactic foreground emission using seven years of WMAP data. Using the power spectrum of differences between multi-frequency template-cleaned maps, we find no evidence for foreground contamination outside of the updated (KQ85y7) foreground mask. We place a 15 microKelvin upper bound on rms foreground contamination in the cleaned maps used for cosmological analysis. We find no indication in the polarization data of an extra "haze" of hard synchrotron emission from energetic electrons near the Galactic center. We provide an updated map of the cosmic microwave background (CMB) using the internal linear combination (ILC) method, updated foreground masks, and updates to point source catalogs with 62 newly detected sources. Also new are tests of the Markov chain Monte Carlo (MCMC) foreground fitting procedure against systematics in the time-stream data, and tests against the observed beam asymmetry. Within a few degrees of the Galactic plane, WMAP total intensity data show a rapidly steepening spectrum from 20-40 GHz, which may be due to emission from spinning dust grains, steepening synchrotron, or other effects. Comparisons are made to a 1-degree 408 MHz map (Haslam et al.) and the 11-degree ARCADE 2 data (Singal et al.). We find that spinning dust or steepening synchrotron models fit the combination of WMAP and 408 MHz data equally well. ARCADE data appear inconsistent with the steepening synchrotron model, and consistent with the spinning dust model, though some discrepancies remain regarding the relative strength of spinning dust emission. More high-resolution data in the 10-40 GHz range would shed much light on these issues.

astro-ph.GA↗

arXiv2010

Ultrafast carrier and phonon dynamics in Bi2Se3 crystals

Ultrafast time-resolved differential reflectivity of Bi2Se3 crystals is studied using optical pump-probe spectroscopy. Three distinct relaxation processes are found to contribute to the initial transient reflectivity changes. The deduced relaxation timescale and the sign of the reflectivity change suggest that electron-phonon interactions and defect-induced charge trapping are the underlying mechanisms for the three processes. After the crystal is exposed to air, the relative strength of these processes is altered and becomes strongly dependent on the excitation photon energy.

cond-mat.mtrl-sci↗

arXiv2010

Ab initio study of electron transport in dry poly(G)-poly(C) A-DNA strands

The bias-dependent transport properties of short poly(G)-poly(C) A-DNA strands attached to Au electrodes are investigated with first principles electronic transport methods. By using the non- equilibrium Green's function approach combined with self-interaction corrected density functional theory, we calculate the fully self-consistent coherent I-V curve of various double-strand polymeric DNA fragments. We show that electronic wave-function localization, induced either by the native electrical dipole and/or by the electrostatic disorder originating from the first few water solvation layers, drastically suppresses the magnitude of the elastic conductance of A-DNA oligonucleotides. We then argue that electron transport through DNA is the result of sequence-specific short-range tunneling across a few bases combined with general diffusive/inelastic processes.

cond-mat.mes-hall↗

arXiv2010

Finite-bias electronic transport of molecules in water solution

The effects of water wetting conditions on the transport properties of molecular nano-junctions are investigated theoretically by using a combination of classical molecular dynamics and first principles electronic transport calculations. These are at the level of the non-equilibrium Green's function method implemented for self-interaction corrected density functional theory. We find that water effectively produces electrostatic gating to the molecular junction, with a gating potential determined by the time-averaged water dipole field. Such a field is large for the polar benzene-dithiol molecule, resulting in a transmission spectrum shifted by about 0.6 eV with respect to that of the dry junction. The situation is drastically different for carbon nanotubes (CNTs). In fact, because of their hydro-phobic nature the gating is almost negligible, so that the average transmission spectrum of wet Au/CNT/Au junctions is essentially the same as that in dry conditions. This suggests that CNTs can be used as molecular interconnects also in water-wet situations, for instance as tips for scanning tunnel microscopy in solution or in biological sensors.

cond-mat.mes-hall↗

arXiv2009

Transitionless quantum drivings for the harmonic oscillator

Two methods to change a quantum harmonic oscillator frequency without transitions in a finite time are described and compared. The first method, a transitionless-tracking algorithm, makes use of a generalized harmonic oscillator and a non-local potential. The second method, based on engineering an invariant of motion, only modifies the harmonic frequency in time, keeping the potential local at all times.

quant-ph↗

arXiv2009

Primordial Binary Evolution and Blue Stragglers

Blue stragglers have been found in all populations. These objects are important in both stellar evolution and stellar population synthesis. Much evidence shows that blue stragglers are relevant to primordial binaries. Here, we summarize the links of binary evolution and blue stragglers, describe the characteristics of blue stragglers from different binary evolutionary channels, and show their consequences for binary population synthesis, such as for the integrated spectral energy distribution, the colour-magnitude diagram, the specific frequency, and the influences on colours etc..

astro-ph.SR↗

arXiv2009

Evolution of binary stars and its implications for evolutionary population synthesis

Most stars are members of binaries, and the evolution of a star in a close binary system differs from that of an ioslated star due to the proximity of its companion star. The components in a binary system interact in many ways and binary evolution leads to the formation of many peculiar stars, including blue stragglers and hot subdwarfs. We will discuss binary evolution and the formation of blue stragglers and hot subdwarfs, and show that those hot objects are important in the study of evolutionary population synthesis (EPS), and conclude that binary interactions should be included in the study of EPS. Indeed, binary interactions make a stellar population younger (hotter), and the far-ultraviolet (UV) excess in elliptical galaxies is shown to be most likely resulted from binary interactions. This has major implications for understanding the evolution of the far-UV excess and elliptical galaxies in general. In particular, it implies that the far-UV excess is not a sign of age, as had been postulated prviously and predicts that it should not be strongly dependent on the metallicity of the population, but exists universally from dwarf ellipticals to giant ellipticals.

astro-ph.SR↗

arXiv2009

The masses of hot subdwarfs

Masses are a fundamental parameter, but they are not well known for most hot subdwarfs. In general, the mass of a hot subdwarf is derived with asteroseismology or dynamical methods, for which it is often difficult to obtain the necessary data from observations. We intend to find an approach to deriving the masses of hot subdwarfs from observational data in the literature. We presented full evolutionary calculations for hot subdwarfs in a wide mass range (0.33 $M_\odot$ to 1.4 $M_\odot$) for a Population I metallicity of $Z$=0.02, and obtained a relation between $M_{\rm p}$ and $\log (\frac{T_{\rm eff}^4}{g})$, where $M_{\rm p}$, $T_{\rm eff}$, and $g$ are the most probable mass, effective temperature, and gravity. This relation is used to study the masses of some observed hot subdwarfs. We proposed a method of determining the masses of hot subdwarfs. Using this method, we studied the masses of hot subdwarfs from the ESO supernova Ia progenitor survey and Hamburg quasar survey. The study shows that most of subdwarf B stars have masses between 0.42 and 0.54 $M_\odot$, whilst most sdO stars are in the range 0.40 $\sim$ 0.55 $M_\odot$. Comparing our study to the theoretical mass distributions of Han et al. (2003), we found that sdO stars with mass less than $\sim$ 0.5 $M_\odot$ may evolve from sdB stars, whilst most high-mass($>$ 0.5 $M_\odot$) sdO stars result from mergers directly.

astro-ph.SR↗

arXiv2009

Optical conductivity of a metal-insulator transition for the Anderson-Hubbard model in 3 dimensions away from 1/2 filling

We have completed a numerical investigation of the Anderson-Hubbard model for three-dimensional simple cubic lattices using a real-space self-consistent Hartree-Fock decoupling approximation for the Hubbard interaction. In this formulation we treat the spatial disorder exactly, and therefore we account for effects arising from localization physics. We have examined the model for electronic densities well away 1/2 filling, thereby avoiding the physics of a Mott insulator. Several recent studies have made clear that the combined effects of electronic interactions and spatial disorder can give rise to a suppression of the electronic density of states, and a subsequent metal-insulator transition can occur. We augment such studies by calculating the ac conductivity for such systems. Our numerical results show that weak interactions enhance the density of states at the Fermi level and the low-frequency conductivity, there are no local magnetic moments, and the ac conductivity is Drude-like. However, with a large enough disorder strength and larger interactions the density of states at the Fermi level and the low-frequency conductivity are both suppressed, the conductivity becomes non-Drude-like, and these phenomena are accompanied by the presence of local magnetic moments. The low-frequency conductivity changes from a sigma-sigma_dc omega^{1/2} behaviour in the metallic phase, to a sigma omega^2 behaviour in the nonmetallic regime. Our numerical results show that the formation of magnetic moments is essential to the suppression of the density of states at the Fermi level, and therefore essential to the metal-insulator transition.

cond-mat.str-el↗

arXiv2009

Real-space variational Gutzwiller wave functions for the Anderson-Hubbard model

Partially-projected Gutzwiller variational wavefunctions are used to describe the ground state of disordered interacting systems of fermions. We compare several different variational ground states with the exact ground state for disordered one-dimensional chains, with the goal of determining a minimal set of variational parameters required to accurately describe the spatially-inhomogeneous charge densities and spin correlations. We find that, for weak and intermediate disorder, it is sufficient to include spatial variations of the charge densities in the product state alone, provided that screening of the disorder potential is accounted for. For strong disorder, this prescription is insufficient and it is necessary to include spatially inhomogeneous variational parameters as well.

cond-mat.str-el↗

arXiv2009

Enhanced tidal disruption rates from massive black hole binaries

"Hard" massive black hole (MBH) binaries embedded in steep stellar cusps can shrink via three-body slingshot interactions. We show that this process will inevitably be accompanied by a burst of stellar tidal disruptions, at a rate that can be several orders of magnitude larger than that appropriate for a single MBH. Our numerical scattering experiments reveal that: 1) a significant fraction of stars initially bound to the primary hole are scattered into its tidal disruption loss cone by gravitational interactions with the secondary hole, an enhancement effect that is more pronounced for very unequal-mass binaries; 2) about 25% (40%) of all strongly interacting stars are tidally disrupted by a MBH binary of mass ratio q=1/81 (q=1/243) and eccentricity 0.1; and 3) two mechanisms dominate the fueling of the tidal disruption loss cone, a Kozai non-resonant interaction that causes the secular evolution of the stellar angular momentum in the field of the binary, and the effect of close encounters with the secondary hole that change the stellar orbital parameters in a chaotic way. For a hard MBH binary of 10^7 solar masses and mass ratio 0.01, embedded in an isothermal stellar cusp of velocity dispersion sigma*=100 km/s, the tidal disruption rate can be as large as 1/yr. This is 4 orders of magnitude higher than estimated for a single MBH fed by two-body relaxation. When applied to the case of a putative intermediate-mass black hole inspiraling onto Sgr A*, our results predict tidal disruption rates ~0.05-0.1/yr.

astro-ph.GA↗

arXiv2009

Non-Gaussianity as a Probe of the Physics of the Primordial Universe and the Astrophysics of the Low Redshift Universe

A new and powerful probe of the origin and evolution of structures in the Universe has emerged and been actively developed over the last decade. In the coming decade, non-Gaussianity, i.e., the study of non-Gaussian contributions to the correlations of cosmological fluctuations, will become an important probe of both the early and the late Universe. Specifically, it will play a leading role in furthering our understanding of two fundamental aspects of cosmology and astrophysics: (i) the physics of the very early universe that created the primordial seeds for large-scale structures, and (ii) the subsequent growth of structures via gravitational instability and gas physics at later times. To date, observations of fluctuations in the Cosmic Microwave Background (CMB) and the Large-Scale Structure of the Universe (LSS) have focused largely on the Gaussian contribution as measured by the two-point correlations (or the power spectrum) of density fluctuations. However, an even greater amount of information is contained in non-Gaussianity and a large discovery space therefore still remains to be explored. Many observational probes can be used to measure non-Gaussianity, including CMB, LSS, gravitational lensing, Lyman-alpha forest, 21-cm fluctuations, and the abundance of rare objects such as clusters of galaxies and high-redshift galaxies. Not only does the study of non-Gaussianity maximize the science return from a plethora of present and future cosmological experiments and observations, but it also carries great potential for important discoveries in the coming decade.

astro-ph.CO↗

arXiv2009

The Origin of the Universe as Revealed Through the Polarization of the Cosmic Microwave Background

Modern cosmology has sharpened questions posed for millennia about the origin of our cosmic habitat. The age-old questions have been transformed into two pressing issues primed for attack in the coming decade: How did the Universe begin? and What physical laws govern the Universe at the highest energies? The clearest window onto these questions is the pattern of polarization in the Cosmic Microwave Background (CMB), which is uniquely sensitive to primordial gravity waves. A detection of the special pattern produced by gravity waves would be not only an unprecedented discovery, but also a direct probe of physics at the earliest observable instants of our Universe. Experiments which map CMB polarization over the coming decade will lead us on our first steps towards answering these age-old questions.

astro-ph.CO↗