arXiv subjects

Gang Chen

Publications and source records attributed to Gang Chen.

738 records · Page 41Linked to original sources

arXiv2006

Artificial trapping of a stable high-density dipolar exciton fluid

We present compelling experimental evidence for a successful electrostatic trapping of two-dimensional dipolar excitons that results in stable formation of a well confined, high-density and spatially uniform dipolar exciton fluid. We show that, for at least half a microsecond, the exciton fluid sustains a density higher than the critical density for degeneracy if the exciton fluid temperature reaches the lattice temperature within that time. This method should allow for the study of strongly interacting bosons in two dimensions at low temperatures, and possibly lead towards the observation of quantum phase transitions of 2D interacting excitons, such as superfluidity and crystallization.

cond-mat.str-el↗

arXiv2006

Supernova Ia and Galaxy Cluster Gas Mass Fraction Constraints on Dark Energy

We use the Riess et al.(2004) supernova Ia apparent magnitude versus redshift data and the Allen et al.(2004) galaxy cluster gas mass fraction versus redshift data to constrain dark energy models. These data provide complementary constraints that when combined together ignificantly restrict model parameters and favor lowly-evolving dark energy density models, close to the Einstein cosmological constant limit of dark energy.

astro-ph↗

arXiv2006

Initial stage of the 2D-3D transition of a strained SiGe layer on a pit-patterned Si(001) template

We investigate the initial stage of the 2D-3D transition of strained Ge layers deposited on pit-patterned Si(001) templates. Within the pits, which assume the shape of inverted, truncated pyramids after optimized growth of a Si buffer layer, the Ge wetting layer develops a complex morphology consisting exclusively of {105} and (001) facets. These results are attributed to a strain-driven step-meandering instability on the facetted side-walls of the pits, and a step-bunching instability at the sharp concave intersections of these facets. Although both instabilities are strain-driven, their coexistence becomes mainly possible by the geometrical restrictions in the pits. It is shown that the morphological transformation of the pit surface into low-energy facets has strong influence on the preferential nucleation of Ge islands at the flat bottom of the pits.

cond-mat.mtrl-sci↗

arXiv2005

Measuring the Three Point Correlation Function of the Cosmic Microwave Background

We present a new method to estimate three-point correlations in Cosmic Microwave Background maps. Our Fast Fourier Transform based implementation estimates three-point functions using all possible configurations (triangles) at a controlled resolution. The speed of the technique depends both on the resolution and the total number of pixels $N$. The resulting $N\log N$ scaling is substantially faster than naive methods with prohibitive $N^3$ scaling. As an initial application, we measure three-point correlation functions in the First Year Data Release of the Wilkinson Anisotropy Probe. We estimate 336 cross-correlations of any triplet of maps from the 8 differential assemblies, scanning altogether 2.6 million triangular configurations. Estimating covariances from Gaussian signal plus realistic noise simulations, we perform a null-hypothesis testing with regards to the Gaussianity of the Cosmic Microwave Background. Our main result is that at the three-point level WMAP is fully consistent with Gaussianity. To quantify the level of possible deviations, we introduce false discovery rate analysis, a novel statistical technique to analyze for three-point measurements. This confirms that the data are consistent with Gaussianity at better than 1-$σ$ level when jointly considering all configurations. We constrain a specific non-Gaussian model using the quadratic approximation of weak non-Gaussianities in terms of the $f_{NLT}$ parameter, for which we construct an estimator from the the three-point function. We find that using the skewness alone is more constraining than a heuristic suboptimal combination of all our results; our best estimate is $f_{NLT} = -110\pm 150$ assuming a $Λ$CDM concordance model.

astro-ph↗

arXiv2005

Nonlinear dynamics of a dense two-dimensional dipolar exciton gas

We use a simple model to describe the nonlinear dynamics of a dense two dimensional dipolar exciton gas. The model predicts an initial fast expansion due to dipole-dipole pressure, followed by a much slower diffusion. The model is in very good agreement with recent experimental results. We show that the dipole pressure induced expansion strongly constrains the time available for achieving and observing Bose-Einstein quantum statistical effects, indicating a need for spatial exciton traps. We also suggest that nonlinear ballistic exciton transport due to the strong internal dipole pressure is readily achievable.

cond-mat.other↗

arXiv2005

Optical Gain from InAs Nanocrystal Quantum Dots in a Polymer Matrix

We report on the first observation of optical gain from InAs nanocrystal quantum dots emitting at 1.55 microns based on a three-beam, time resolved pump-probe technique. The nanocrystals were embedded into a transparent polymer matrix platform suitable for the fabrication of integrated photonic devices.

cond-mat.other↗

arXiv2005

Electrostatic traps for dipolar excitons

We consider the design of two-dimensional electrostatic traps for dipolar indirect excitons. We show that the excitons dipole-dipole interaction, combined with the in-plane electric fields that arise due to the trap geometry, constrain the maximal density and lifetime of trapped excitons. We derive an analytic estimate of these values and determine their dependence on the trap geometry, thus suggesting the optimal design for high density trapping as a route for observing excitonic Bose-Einstein condensation.

cond-mat.other↗

arXiv2004

Dynamics of in-plane charge separation front in 2D electron-hole gas

We show both experimentally and theoretically that the recently observed optically induced in-plane charge separation in quantum well (QW) structures and the exciton ring emission pattern at this charge separation boundary have an extremely long lifetime. The oppositely charged carriers remain separated and provide a reservoir of excitons at their boundary with a persistent emission which lasts hundreds of microseconds (orders of magnitude longer than their recombination time) after the external excitation is removed. This long lifetime is due to an interplay between the slow in-plane carrier diffusion and slow carrier tunnelling perpendicular to the QW plane.

cond-mat.other↗

arXiv2004

Constraints on Scalar-Field Dark Energy from Galaxy Cluster Gas Mass Fraction versus Redshift

We use the Allen et al. (2004) Chandra measurements of x-ray gas mass fraction of 26 rich clusters to place constraints on the scalar-field dark energy model with inverse power law potential energy density. The constraints are consistent with, and typically more constraining than, those from other cosmological tests, and mildly favor the Einstein cosmological constant limit of the dark energy model.

astro-ph↗

arXiv2004

Looking for Cosmological Alfven Waves in WMAP Data

A primordial cosmological magnetic field induces and supports vorticity or Alfven waves, which in turn generate cosmic microwave background (CMB) anisotropies. A homogeneous primordial magnetic field with fixed direction induces correlations between the $a_{l-1,m}$ and $a_{l+1,m}$ multipole coefficients of the CMB temperature anisotropy field. We discuss the constraints that can be placed on the strength of such a primordial magnetic field using CMB anisotropy data from the WMAP experiment. We place 3 $σ$ upper limits on the strength of the magnetic field of $B < 15$ nG for vector perturbation spectral index $n=-5$ and $B<1.7$ nG for $n=-7$.

astro-ph↗

arXiv2004

Constraints on Scalar-Field Dark Energy from the Cosmic Lens All-Sky Survey Gravitational Lens Statistics

We use the statistics of strong gravitational lensing based on the Cosmic Lens All-Sky Survey (CLASS) data to constrain cosmological parameters in a spatially-flat, inverse power-law potential energy density, scalar-field dark energy cosmological model. The lensing-based constraints are consistent with, but weaker than, those derived from Type Ia supernova redshift-magnitude data, and mildly favor the Einstein cosmological constant limit of this dark energy model.

astro-ph↗

arXiv2003

Mechanism of Luminescence Ring Pattern Formation in Quantum Well Structures: Optically-Induced In-Plane Charge Separation

About a year ago, two independent experiments [1,2], imaging indirect exciton luminescence from doped double quantum wells under applied bias and optical excitation, reported a very intriguing observation: under certain experimental conditions, the exciton luminescence exhibits a ring pattern with a dark region in between the center excitation spot and the luminescent ring that can extend more than a millimeter from the center spot. Initial speculations on the origin of this emission pattern included supersonic ballistic transport of excitons due to their dipole-dipole repulsion and Bose superfluidity of excitons. In this paper we show that the ring effect is also observed in single quantum well structures, where only direct excitons exist. More importantly, we find that these experimental results are quantitatively explained by a novel coupled 2D electron-hole plasma dynamics, namely, photoinduced in-plane charge separation. This charge separation explains extremely long luminescence times that may be more than a microsecond for the ring -- orders of magnitude longer than the emission lifetime of the excitons in the center spot. This method of continuously creating excitons may result in a highly dense exciton gas which is also well thermalized with the lattice (since the particles can cool over the very long luminescence time after their hot optical creation), thus opening up opportunities for a detailed study of quantum statistics. The in-plane separation of the charges into positive and negative regions, with a sharp interface between them is an interesting new example of nonequilibrium dynamics and pattern formation.

cond-mat↗

arXiv2003

Non-Gaussian Error Distribution of Hubble Constant Measurements

We construct the error distribution of Hubble constant ($H_0$) measurements from Huchra's compilation of 461 measurements of $H_0$ and the WMAP experiment central value $H_0$ = 71 km s$^{-1}$ Mpc$^{-1}$. This error distribution is non-Gaussian, with significantly larger probability in the tails of the distribution than predicted by a Gaussian distribution. The 95.4 % confidence limits are 7.0 $σ$ in terms of the quoted errors. It is remarkably well described by either a widened $n = 2$ Student's $t$ distribution or a widened double exponential distribution. These conclusions are unchanged if we use instead the central value $H_0$ = 67 km s$^{-1}$ Mpc$^{-1}$ found from a median statistics analysis of a major subset of $H_0$ measurements used here.

astro-ph↗

arXiv2003

Median Statistics and the Mass Density of the Universe

We use weighted mean and median statistics techniques to combine individual estimates of Omega_m0, the present mean mass density in non-relativistic matter, and determine the observed values and ranges of Omega_m0 from different combinations of data. The derived weighted mean Omega_m0 values are not good representatives of the individual measurements, under the assumptions of Gaussianity and negligible correlation between the individual measurements. This could mean that some observational error bars are under-estimated. Discarding the most discrepant about 5% of the measurements generally alleviates but does not completely resolve this problem. While the results derived from the different combinations of data are not identical, they are mostly consistent, and a reasonable summary of the median statistics analyses is 0.2 <= Omega_m0 <= 0.35 at two standard deviations.

astro-ph↗

arXiv2002

Cosmological Constraints from Compact Radio Source Angular Size versus Redshift Data

We use the Gurvits, Kellermann, & Frey compact radio source angular size versus redshift data to place constraints on cosmological model parameters in models with and without a constant or time-variable cosmological constant. The resulting constraints are consistent with but weaker than those determined using current supernova apparent magnitude versus redshift data.

astro-ph↗

arXiv2001

Measurement of the Scaling Property of Factorial Moments in Hadronic Z Decay

Both three- and one-dimensional studies of local multiplicity fluctuations in hadronic Z decay are performed using data of the L3 experiment at LEP. The normalized factorial moments in three dimensions exhibit power-law scaling, indicating that the fluctuations are isotropic, which correspends to a self-similar fractal. A detailed study of the corresponding one-dimensional moments confirms this conclusion. However, two-jet subsamples have anisotropic fluctuations, correspending to a self-affine fractal. These features are, at least qualitatively, reproduced by the Monte Carlo models \JETSET and \HERWIG.

hep-ph↗

arXiv2000

Anisotropy of Dynamical Fluctuations as a Probe for Soft and Hard Processes in High Energy Collisions

It is shown using Lund Monte Carlo that, unlike the average properties of the hadronic system inside jets, the anisotropy of dynamical fluctuations in these systems changes abruptly with the variation of the cut parameter $y_cut$. A transition point exists, where the dynamical fluctuations in the hadronic system inside jet behave like those in soft hadronic collisions. Thus the anisotropy property of the dynamical fluctuations can serve as a probe for the soft and hard processes in high energy collisions.

hep-ph↗

arXiv1998

Directional emission from asymmetric resonant cavities

Asymmetric resonant cavities (ARCs) with highly non-circular but convex cross-sections are predicted theoretically to have high-Q whispering gallery modes with very anisotropic emission. We develop a ray dynamics model for the emission pattern and present numerical and experimental confirmation of the theory.

physics.optics↗