arXiv subjects

L. Deng

Publications and source records attributed to L. Deng.

At least 37 records · Page 2Linked to original sources

arXiv2015

The binary mass transfer origin of the red blue straggler sequence in M30

Two separated sequences of blue straggler stars (BSSs) have been revealed by Ferraro et al. (2009) in the color-magnitude diagram (CMD) of the Milky Way globular cluster M30. Their presence has been suggested to be related to the two BSS formation channels (namely, collisions and mass-transfer in close binaries) operating within the same stellar system. The blue sequence was indeed found to be well reproduced by collisional BSS models. In contrast, no specific models for mass transfer BSSs were available for an old stellar system like M30. Here we present binary evolution models, including case-B mass transfer and binary merging, specifically calculated for this cluster. We discuss in detail the evolutionary track of a $0.9+0.5 M_\odot$ binary, which spends approximately 4 Gyr in the BSS region of the CMD of a 13 Gyr old cluster. We also run Monte-Carlo simulations to study the distribution of mass transfer BSSs in the CMD and to compare it with the observational data. Our results show that: (1) the color and magnitude distribution of synthetic mass transfer BSSs defines a strip in the CMD that nicely matches the observed red BSS sequence, thus providing strong support to the mass transfer origin for these stars; (2) the CMD distribution of synthetic BSSs never attains the observed location of the blue BSS sequence, thus reinforcing the hypothesis that the latter formed through a different channel (likely collisions); (3) most ($\sim 60\%$) of the synthetic BSSs are produced by mass-transfer models, while the remaining $< 40\%$ requires the contribution from merger models.

astro-ph.SR↗

arXiv2014

All-Optical, High-Fidelity Polarization Gate Using Room-Temperature Atomic Vapor

An all-optical atomic Controlled-NOT (CNOT)/polarization gate operation is demonstrated with low light intensities in a room-temperature atomic medium. Using a Polarization-Selective-Kerr-Phase-Shift (PSKPS) technique, a $π$ phase shift is written to only one of the two circularly-polarized components of a linearly-polarized input signal field by a weak phase-control field with "magic" detuning. At the exit of the medium, the signal field maintains its original strength but acquires a 90$^{\rm o}$ linear polarization rotation, demonstrating the first fast, high-fidelity CNOT/polarization gate operation in a room-temperature atomic medium. This development opens the realm of possibilities for potential future extremely low light level telecommunication and information processing systems.

physics.optics↗

arXiv2013

Strong optical self-focusing effect in coherent light scattering with condensates

We present a theoretical investigation of optical self-focusing effects in light scattering with condensates. Using long (>200 μs), red-detuned pulses we show numerically that a non-negligible self-focusing effect is present that causes rapid optical beam width reduction as the scattered field propagates through a medium with an inhomogeneous density distribution. The rapid growth of the scattered field intensity and significant local density feedback positively to further enhance the wave generation process and condensate compression, leading to highly efficient collective atomic recoil motion.

cond-mat.quant-gas↗

arXiv2013

Hydrogen lines in LAMOST low resolution spectra of RR Lyrae stars

The LAMOST spectroscopic survey for Galactic structure and evolution has been in operation since October 2012, following a one-year pilot survey. The pilot survey produced a data release containing over 600,000 stellar spectra. By cross-checking with a large time series photometric database of RR Lyrae stars in high Galactic latitude regions, we found a total number of 157 stars that have been observed with LAMOST. In this sample, we successfully captured three RR Lyrae stars in the fast expansion phase, all of them showing significant hypersonic shock wave features in the spectra. By fitting the Hα line shape, we determine that the emission features seen within the broader Hα absorption line suggest hypersonic relative motions in the atmospheres of these three objects. With a further LAMOST survey of millions of stars, we will capture a large sample of RR Lyrae stars in their hypersonic expansion phase, and therefore provide a large database for the study of the internal structure and pulsation mechanism of RR Lyrae stars.

astro-ph.SR↗

arXiv2013

Effects of electron-phonon coupling in the Kondo regime of a two-orbital molecule

We study the interplay between strong electron-electron and electron-phonon interactions within a two-orbital molecule coupled to metallic leads, taking into account Holstein-like coupling of a local phonon mode to the molecular charge as well as phonon-mediated interorbital tunneling. By combining canonical transformations with numerical renormalization-group calculations to address the interactions nonperturbatively and on equal footing, we obtain a comprehensive description of the system's many-body physics in the anti-adiabatic regime where the phonons adjust rapidly to changes in the orbital occupancies, and are thereby able to strongly affect the Kondo physics. The electron-phonon interactions strongly modify the bare orbital energies and the Coulomb repulsion between electrons in the molecule, and tend to inhibit tunneling of electrons between the molecule and the leads. The consequences of these effects are considerably more pronounced when both molecular orbitals lie near the Fermi energy of the leads than when only one orbital is active. In situations where a local moment forms on the molecule, there is a crossover with increasing electron-phonon coupling from a regime of collective Kondo screening of the moment to a limit of local phonon quenching. At low temperatures, this crossover is associated with a rapid increase in the electronic occupancy of the molecule as well as a marked drop in the linear electrical conductance through the single-molecule junction.

cond-mat.str-el↗

arXiv2012

Impact of photo-assisted collisions on superradiant light scattering with Bose condensates

We present experimental evidence supporting the postulation that the secondary effects of light-assisted collisions are the main reason that the superradiant light scattering efficiency in condensates is asymmetric with respect to the sign of the pump-laser detuning. Contrary to the recent experimental study, however, we observe severe and comparable heating with all three pump-laser polarizations. We also perform two-color, double-pulse measurements to directly study the degradation of condensate coherence and the resulting impact on the superradiant scattering efficiency.

cond-mat.quant-gas↗

arXiv2011

Evidence of an Internally Generated Optical-Dipole Potential in Matter-Wave Superradiance

We present the first experimental evidence supporting the postulation that an optical-dipole potential in a condensate undergoing superradiant scattering modifies the structure factor of the system and significantly impacts the scattering. Several consequences of this new detuning-dependent mechanism are discussed and verified experimentally. Our experiments indicate that whenever the generation and propagation growth of a new field are significant, the dynamic response of the condensate can have a profound impact on the scattering process.

quant-ph↗

arXiv2010

Reply to Comment by Wolfgang Ketterle on "Electromagnetic Wave Dynamics in Matter-Wave Superradiant Scattering" (see arXiv:1010.3915)

The Comment by Wolfgang Ketterle (Ref.[1]) purports to present a viable model of superradiance in condensates. However, Ref.[1] is not able to explain the red/blue pump detuning asymmetry that was first observed recently by us (Ref.[2]). It is clear from our original paper (Ref.[3]) that the rate-equation-based theories of Ref.[1] are incomplete since they only model the final growth stage of the process when a red-detuned pump is used. Our theoretical framework (Ref.[3]), on the other hand, also treats the initial growth stage of superradiance and is therefore also capable of explaining the genesis of the red/blue detuning asymmetry (Ref.[2]). This is the key message of our response, which we frame in terms of reference to the specific points raised in Ref. [1].

cond-mat.quant-gas↗

arXiv2010

Observation of a red-blue detuning asymmetry in matter-wave superradiance

We report the first experimental observations of strong suppression of matter-wave superradiance using blue-detuned pump light and demonstrate a pump-laser detuning asymmetry in the collective atomic recoil motion. In contrast to all previous theoretical frameworks, which predict that the process should be symmetric with respect to the sign of the pump-laser detuning, we find that for condensates the symmetry is broken. With high condensate densities and red-detuned light, the familiar distinctive multi-order, matter-wave scattering pattern is clearly visible, whereas with blue-detuned light superradiance is strongly suppressed. In the limit of a dilute atomic gas, however, symmetry is restored.

cond-mat.quant-gas↗

arXiv2010

Origin of nonlinear optical processes in matter-wave superradiance

We study a highly efficient, matter-wave amplification mechanism in a longitudinally-excited, Bose-Einstein condensate and reveal a very large enhancement due to nonlinear gain from a sixmatter- optical, wave-mixing process involving four photons. Under suitable conditions this opticallydegenerate, four-photon process can be stronger than the usual two-photon inelastic light scattering mechanism, leading to nonlinear growth of the observed matter-wave scattering independent of any enhancement from bosonic stimulation. Our theoretical framework can be extended to encompass even higher-order, nonlinear superradiant processes that result in higher-order momentum transfer.

cond-mat.quant-gas↗

arXiv2010

Collective atomic recoil motion in short-pulse multi-matter-optical wave mixing

An analytical perturbation theory of short-pulse, matter-wave superradiant scatterings is presented. We show that Bragg resonant enhancement is incapacitated and both positive and negative order scatterings contribute equally. We further show that propagation gain is small and scattering events primarily occur at the end of the condensate where the generated field has maximum strength, thereby explaining the apparent ``asymmetry" in the scattered components with respect to the condensate center. In addition, the generated field travels near the speed of light in a vacuum, resulting in significant spontaneous emission when the one-photon detuning is not sufficiently large. Finally, we show that when the excitation rate increases, the generated-field front-edge-steepening and peak forward-shifting effects are due to depletion of the ground state matter wave.

cond-mat.quant-gas↗

arXiv2010

Observation of collective atomic recoil motion in a momentum-squeezed, ultra-cold, degenerate fermion gas

We demonstrate clear collective atomic recoil motion in a dilute, momentum-squeezed, ultra-cold degenerate fermion gas by circumventing the effects of Pauli blocking. Although gain from bosonic stimulation is necessarily absent because the quantum gas obeys Fermi-Dirac statistics, collective atomic recoil motion from the underlying wave-mixing process is clearly visible. With a single pump pulse of the proper polarization, we observe two mutually-perpendicular wave-mixing processes occurring simultaneously. Our experiments also indicate that the red-blue pump detuning asymmetry observed with Bose-Einstein condensates does not occur with fermions.

cond-mat.quant-gas↗

arXiv2009

Generating correlated (2+1)-photon in an active Raman gain medium

A scheme of generating controllable (2+1) photons in a double-$% Λ$ atomic system based on active-Raman-gain is presented in this paper. Such (2+1) photons can be a potential candidate to generate a correlated photon pair as the photon `1' acts as a trigger. Compared to other schemes of generating correlated photon pairs, our scheme exhibits a number of advantages due to the exploit of the stimulated Raman process.

quant-ph↗

arXiv2008

A spectroscopic study of the blue stragglers in M67

Based on spectrophotometric observations from the Guillermo Haro Observatory (Cananea, Mexico), a study of the spectral properties of the complete sample of 24 blue straggler stars (BSs) in the old Galactic open cluster M67 (NGC 2682) is presented. All spectra, calibrated using spectral standards, were recalibrated by means of photometric magnitudes in the Beijing-Arizona-Taipei-Connecticut system, which includes fluxes in 11 bands covering ~3500-10000 A. The set of parameters was obtained using two complementary approaches that rely on a comparison of the spectra with (i) an empirical sample of stars with well-established spectral types and (ii) a theoretical grid of optical spectra computed at both low and high resolution. The overall results indicate that the BSs in M67 span a wide range in Teff(~ 5600 -12600 K) and surface gravities that are fully compatible with those expected for main-sequence objects (log g = 3.5 -5.0 dex).

astro-ph↗

arXiv2008

Abundance Analysis of Barium Stars

We obtain the chemical abundances of six barium stars and two CH subgiant stars based on the high signal-to-noise ratio and high resolution Echelle spectra. The neutron capture process elements Y, Zr, Ba, La, Eu show obvious overabundance relative to the Sun, for example, their [Ba/Fe] values are from 0.45 to 1.27. Other elements, including Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Ni, show comparable abundances to the Solar ones, and their [Fe/H] cover a range from $-$0.40 to 0.21, which means they belong to Galactic disk. The predicts of the theoretical model of wind accretion for binary systems can explain the observed abundance patterns of the neutron capture process elements in these stars, which means that their overabundant heavy-elements could be caused by accreting the ejecta of AGB stars, the progenitors of the present white dwarf companions in the binary systems.

astro-ph↗

arXiv2007

Simple Stellar Population Models as probed by the Large Magellanic Cloud Star Cluster ESO 121-SC03

The presence of blue straggler stars (BSs) in star clusters has proven a challenge to conventional simple stellar population (SSP) models. Conventional SSP models are based on the evolution theory of single stars. Meanwhile, the typical locations of BSs in the colour-magnitude diagram of a cluster are brighter and bluer than the main sequence turn-off point. Such loci cannot be predicted by single-star evolution theory. However, stars with such properties contribute significantly to the integrated light of the cluster. In this paper, we reconstruct the integrated properties of the Large Magellanic Cloud cluster ESO 121-SC03, based on a detailed exploration of the individual cluster stars, and with particular emphasis on the cluster's BSs. We find that the integrated light properties of ESO 121-SC03 are dramatically modified by its BS component. The integrated spectral energy distribution (ISED) flux level is significantly enhanced toward shorter wavelengths, and all broad-band colours become bluer. When fitting the fully integrated ISED of this cluster based on conventional SSP models, the best-fitting values of age and metallicity are significantly underestimated compared to the true cluster parameters. The age underestimate is $\sim40$ per cent if we only include the BSs within the cluster's half-light radius and $\sim60$ per cent if all BSs are included. The corresponding underestimates of the cluster's metallicity are $\sim30$ and $\sim60$ per cent, respectively. The populous star clusters in the Magellanic Clouds are ideal objects to explore the potential importance of BSs for the integrated light properties of more distant unresolved star clusters in a statistically robust manner, since they cover a large range in age and metallicity.

astro-ph↗

arXiv2007

How to define the boundaries of a convective zone and how extended is overshooting?

Under nonlocal convection theory, convection extends without limit therefore no apparent boundary can be defined clearly as in the local theory. From the requirement of a similar structure for both local and non-local models having the same depth of convection zone, and taking into account the driving mechanism of turbulent convection, we argue that a proper definition of the boundary of a convective zone should be the place where the convective energy flux (i.e. the correlation of turbulent velocity and temperature) changes its sign. Therefore, it is convectively unstable region when the flux is positive, and it is convective overshooting zone when the flux becomes negative. The physical picture of the overshooting zone drawn by the usual non-local mixing-length theory is not correct. In fact, convection is already sub-adiabatic ($\nabla<\nabla_{ad}$) far before reaching the unstable boundary; while in the overshooting zone below the convective zone, convection is sub-adiabatic and super-radiative ($\nabla_{rad}<\nabla<\nabla_{ad}$). The transition between the adiabatic temperature gradient and the radiative one is continuous and smooth instead of a sudden switch. In the unstable zone the temperature gradient is approaching radiative rather than going to adiabatic. We would like to claim again that, the overshooting distance is different for different physical quantities......

astro-ph↗

arXiv2007

Superluminal propagation of an optical pulse in a Doppler broadened three-state, single channel active Raman gain medium

Using a single channel active Raman gain medium we show a $(220\pm 20)$ns advance time for an optical pulse of $τ_{FWHM}=15.4 μ$s propagating through a 10 cm medium, a lead time that is comparable to what was reported previously. In addition, we have verified experimentally all the features associated with this single channel Raman gain system. Our results show that the reported gain-assisted superluminal propagation should not be attributed to the interference between the two frequencies of the pump field.

quant-ph↗