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J. Chen

Publications and source records attributed to J. Chen.

At least 289 records · Page 16Linked to original sources
arXiv2009

Observations of two-fold shell filling and Kondo effect in a graphene nano-ribbon quantum dot device

A graphene nanoribbon (GNR) with orientation along its principle axis was obtained through a mechanical tearing process, and a quantum dot device was fabricated from the GNR. We have studied the transport property of the GNR quantum dot device down to dilution refrigerator temperatures. Two-fold charging periodicity was observed in the Coulomb-blockade measurement, signaling a shell-filling process with broken valley degeneracy. In one of the smaller Coulomb diamonds, Kondo-like resonance were observed, with two conductance peaks displaced symmetrically from the zero bias voltage. The splitting of Kondo resonance at zero magnetic field suggests spin-polarization of the quantum dot, possibly due to the edge states of a zigzag GNR.

cond-mat.mes-hall↗
arXiv2009

Collective responses of Bi-2212 stacked junction to 100 GHz microwave radiation under magnetic field oriented along the c-axis

We studied a response of Bi-2212 mesa type structures to 100 GHz microwave radiation. We found that applying magnetic field of about 0.1 T across the layers enables to observe collective Shapiro step response corresponding to a synchronization of all 50 intrinsic Josephson junctions (IJJ) of the mesa. At high microwave power we observed up to 10th harmonics of the fundamental Shapiro step. Besides, we found microwave induced flux-flow step position of which is proportional to the square root of microwave power and that can exceed at high enough powers 1 THz operating frequency of IJJ oscillations.

cond-mat.supr-con↗
arXiv2009

Laser-assisted collision effect on nonsequential double ionization of helium in a few-cycle laser pulse

Nonsequential double ionization (NSDI) of helium in an intense few-cycle laser pulse is investigated by applying the three-dimensional semi-classical re-scattering method. It is found that the momentum distribution of He$^{2+}$ shows a single-double-single peak structure as the pulse intensity increases. According to the different mechanisms dominating the NSDI process, the laser intensity can be classified into three regimes where the momentum distribution of He$^{2+}$ exhibits different characteristics. In the relatively high intensity regime, an NSDI mechanism named the "laser-assisted collision ionization" is found to be dominating the NSDI process and causing the single peak structure. This result can shed light on the study of non-sequential ionization of a highly charged ion in a relatively intense laser pulse.

physics.atom-ph↗
arXiv2008

The shape of disorder broadened Landau subbands in graphene

Density of states (DOS) of graphene under a high uniform magnetic field and white-noise random potential is numerically calculated. The disorder broadened zero-energy Landau band has a Gaussian shape whose width is proportional to the random potential variance and the square root of magnetic field. Wegner-type calculation is used to justify the results.

cond-mat.mes-hall↗
arXiv2008

Electronic Structure in Gapped Graphene with Coulomb Potential

In this paper, we numerically study the bound electron states induced by long range Coulomb impurity in gapped graphene and the quasi-bound states in supercritical region based on the lattice model. We present a detailed comparison between our numerical simulations and the prediction of the continuum model which is described by the Dirac equation in (2+1)-dimensional Quantum Electrodynamics (QED). We also use the Fano's formalism to investigate the quasi-bound state development and design an accessible experiments to test the decay of the supercritical vacuum in the gapped graphene.

cond-mat.mes-hall↗
arXiv2008

Modulations to molecular high order harmonic generation by electron de Broglie wave

We present a new theory that the molecular high order harmonic generation in an intense laser field is determined by molecular internal symmetry and momentum distribution of the tunneling-ionized electron. The molecular internal symmetry determines the quantum interference form of the returning electron inside the molecule. The electron momentum distribution determines the relative interference strength of each individual electron de Broglie wave. All individual electron de Broglie wave interferences add together to collectively modulate the molecular high harmonic generation. We specifically discuss the suppression of the generation on adjacent harmonic orders and the dependence of molecular high harmonic generation on laser intensities and molecular axis alignment. Our theoretical results are in good consistency with the experimental observations.

physics.atom-ph↗
arXiv2007

Complex Dynamics of Correlated Electrons in Molecular Double Ionization by an Ultrashort Intense Laser Pulse

With a semiclassical quasi-static model we achieve an insight into the complex dynamics of two correlated electrons under the combined influence of a two-center Coulomb potential and an intense laser field. The model calculation is able to reproduce experimental data of nitrogen molecules for a wide range of laser intensities from tunnelling to over-the-barrier regime, and predicts a significant alignment effect on the ratio of double over single ion yield. The classical trajectory analysis allows to unveil sub-cycle molecular double ionization dynamics.

physics.atm-clus↗
arXiv2006

A "super" star cluster grown old: the most massive star cluster in the Local Group

We independently redetermine the reddening and age of the globular cluster 037-B327 in M31 by comparing independently obtained multicolour photometry with theoretical stellar population synthesis models. 037-B327 has long been known to have a very large reddening value, which we confirm to be E(B-V)=1.360+-0.013, in good agreement with the previous results. We redetermine its most likely age at 12.4+- 3.2 Gyr. 037-B327 is a prime example of an unusually bright early counterpart to the ubiquitous ``super'' star clusters presently observed in most high-intensity star-forming regions in the local Universe. In order to have survived for a Hubble time, we conclude that its stellar IMF cannot have been top-heavy. Using this constraint, and a variety of SSP models, we determine a photometric mass of M_{GC}=(3.0+-0.5)*10^7, somewhat depending on the SSP models used, the metallicity and age adopted and the IMF representation. This mass, and its relatively small uncertainties, make this object the most massive star cluster of any age in the Local Group. Assuming that the photometric mass estimate thus derived is fairly close to its dynamical mass, we predict that this GC has a (one-dimensional) velocity dispersion of order (72+-13)km/s. As a surviving "super" star cluster, this object is of prime importance for theories aimed at describing massive star cluster evolution.

astro-ph↗
arXiv2005

Statistics of Magnification Perturbations by Substructure in the Cold Dark Matter Cosmological Model

We study the statistical properties of magnification perturbations by substructures in strong lensed systems using linear perturbation theory and an analytical substructure model including tidal truncation and a continuous substructure mass spectrum. We demonstrate that magnification perturbations are dominated by perturbers found within a tidal radius of an image, and that sizable magnification perturbations may arise from small, coherent contributions from several substructures within the lens halo. We find that the root-mean-square (rms) fluctuation of the magnification perturbation is 10% to 20% and both the average and rms perturbations are sensitive to the mass spectrum and density profile of the perturbers. Interestingly, we find that relative to a smooth model of the same mass, the average magnification in clumpy models is lower (higher) than that in smooth models for positive (negative) parity images. This is opposite from what is observed if one assumes that the image magnification predicted by the best-fit smooth model of a lens is a good proxy for what the observed magnification would have been if substructures were absent. While it is possible for this discrepancy to be resolved via nonlinear perturbers, we argue that a more likely explanation is that the assumption that the best-fit lens model is a good proxy for the magnification in the absence of substructure is not correct. We conclude that a better theoretical understanding of the predicted statistical properties of magnification perturbations by CDM substructure is needed in order to affirm that CDM substructures have been unambiguously detected.

astro-ph↗
arXiv2005

Optical Monitoring of PKS 1510-089: A Binary Black Hole System?

Three deep flux minima were observed with nearly the same time-scales and intervals for the blazar PKS 1510-089 in the past few years. A binary black hole system was proposed to be at the nucleus of this object, and a new minimum was predicted to occur in 2002 March. We monitored this source with a 60/90 cm Schmidt telescope from 2002 February to April. In combination with the data obtained by Xie et al. (2004) in the same period, we presented for the 2002 minimum a nearly symmetric light curve, which would be required by an eclipsing model of a binary black hole system. We also constrained the time-scale of the minimum to be 35 min, which is more consistent with the time-scales ~42 min of the three previous minima than the 89 min time-scale given by the same authors. The wiggling miniarcsecond radio jet observed in this object is taken as a further evidence for the binary black hole system. The `coupling' of the periodicity in light curve and the helicity in radio jet is discussed in the framework of a binary black hole system.

astro-ph↗
arXiv2004

Properties of MgB2 thin films with carbon doping

We have studied structural and superconducting properties of MgB2 thin films doped with carbon during the hybrid physical-chemical vapor deposition process. A carbon-containing metalorganic precursor bis(cyclopentadienyl)magnesium was added to the carrier gas to achieve carbon doping. As the amount of carbon in the films increases, the resistivity increases, Tc decreases, and the upper critical field increases dramatically as compared to the clean films. The self-field Jc in the carbon-doped films is lower than that in the clean films, but Jc remains relatively high to much higher magnetic fields, indicating stronger pinning. Structurally, the doped films are textured with nano-grains and highly resistive amorphous areas at the grain boundaries. The carbon doping approach can be used to produce MgB2 materials for high magnetic field applications.

cond-mat.supr-con↗
arXiv2004

Suppression of the superconducting energy gap in intrinsic Josephson junctions of $\mathbf{Bi_2Sr_2CaCu_2O_{8+δ}}$ single crystals

We have observed back-bending structures at high bias current in the current-voltage curves of intrinsic Josephson junctions. These structures may be caused by nonequilibrium quasiparticle injection and/or Joule heating. The energy gap suppression varies considerably with temperature. Different levels of the suppression are observed when the same level of current passes through top electrodes of different sizes. Another effect which is seen and discussed, is a super-current ``reentrance'' of a single intrinsic Josephson junction with high bias current.

cond-mat.supr-con↗
arXiv2003

Noise induced entanglement

We discuss the generation of entangled states of two two-level atoms coupled simultaneously with a dissipated atom. The dissipation of the atom is supposed to come from its coupling to a noise with adjustable intensity. We describe how the entanglement between the atoms arise in such a situation, and wether a noise except the white one could help preparation of entanglement. Besides, we confirm that the entanglement is maximized for intermediate values of the noise intensity, while it is a monotonic function of the spontaneous rates. This resembles the phenomenon of stochastic resonance and sheds more light on the idea to exploit noise in quantum information processing.

quant-ph↗
arXiv2002

Measurement of the R$_{LT}$ response function for $π^0$ electroproduction at $Q^2$ = 0.070 (GeV/c)$^2$ in the $N\toΔ$ transition}

Quadrupole amplitudes in the $γ^{*}N\toΔ$ transition are associated with the issue of nucleon deformation. A search for these small amplitudes has been the focus of a series of measurements undertaken at Bates/MIT by the OOPS collaboration. We report on results from H$(e,e^\prime p)π^0$ data obtained at $Q^2= 0.070$ (GeV/c)$^2$ and invariant mass of W=1155 MeV using the out-of-plane detection technique with the OOPS spectrometers. The $σ_{LT}$ and $σ_{T}+ε\cdot$ $σ_{L}$ response functions were isolated. These results, along with those of previous measurements at $W$=1172 MeV and $Q^2= 0.127$ (GeV/c)$^2$, aim in elucidating the interplay between resonant and non resonant amplitudes.

nucl-ex↗
arXiv2001

Physics development of web-based tools for use in hardware clusters doing lattice physics

Jefferson Lab and MIT are developing a set of web-based tools within the Lattice Hadron Physics Collaboration to allow lattice QCD theorists to treat the computational facilities located at the two sites as a single meta-facility. The prototype Lattice Portal provides researchers the ability to submit jobs to the cluster, browse data caches, and transfer files between cache and off-line storage. The user can view the configuration of the PBS servers and to monitor both the status of all batch queues as well as the jobs in each queue. Work is starting on expanding the present system to include job submissions at the meta-facility level (shared queue), as well as multi-site file transfers and enhanced policy-based data management capabilities.

hep-lat↗
arXiv2001

MgB2 tunnel junctions and 19 K low-noise dc superconducting quantum interference devices

Point contact junctions made from two pieces of MgB2 can be adjusted to exhibit either superconductor-insulator-superconductor (SIS) or superconductor-normal metal-superconductor (SNS) current-voltage characteristics. The SIS characteristics are in good agreement with the standard tunneling model for s-wave superconductors, and yield an energy gap of (2.02 +/- 0.08) meV. The SNS characteristics are in good agreement with the predictions of the resistively-shunted junction model. DC Superconducting QUantum Interference Devices made from two SNS junctions yield magnetic field noise as low as 35 fT/Hz^{1/2} at 19 K.

cond-mat.supr-con↗