arXiv subjects

J. Ma

Publications and source records attributed to J. Ma.

At least 127 records · Page 7Linked to original sources

arXiv2012

Dynamics aspect of subbarrier fusion reaction in light heavy ion systems

Subbarrier fusion of the 7Li + 12C reaction is studied using an antisymmetrized molecular dynamics model (AMD) with an after burner, GEMINI. In AMD, 7Li shows an α+ t structure at its ground state and it is significantly deformed. Simulations are made near the Coulomb barrier energies, i.e., E_{cm} = 3 - 8 MeV. The total fusion cross section of the AMD + GEMINI calculations as a function of incident energy is compared to the experimental results and both are in good agreement at E_{cm} > 3 MeV. The cross section for the different residue channels of the AMD + GEMINI at E_{cm} = 5 MeV is also compared to the experimental results.

nucl-th↗

arXiv2012

Thermoelectric transport properties of CaMg2Bi2, EuMg2Bi2, and YbMg2Bi2

The thermoelectric transport properties of CaMg2Bi2, EuMg2Bi2, and YbMg2Bi2 were characterized between 2 and 650K. As synthesized, the polycrystalline samples are found to have lower p-type carrier concentrations than single-crystalline samples of the same empirical formula. These low carrier concentration samples possess the highest mobilities yet reported for materials with the CaAl2Si2 structure type, with a mobility of ~740cm$^2$/V/s observed in EuMg2Bi2 at 50K. Despite decreases in the Seebeck coefficient (α) and electrical resistivity (ρ) with increasing temperature, the power factor (α^2/ρ) increases for all temperatures examined. This behavior suggests a strong asymmetry in the conduction of electrons and holes. The highest figure of merit (zT) is observed in YbMg2Bi2, with zT approaching 0.4 at 600K for two samples with carrier densities of approximately 2x10^{18}cm^{-3} and 8x10^{18}cm^{-3} at room temperature. Refinements of neutron powder diffraction data yield similar behavior for the structures of CaMg2Bi2 and YbMg2Bi2, with smooth lattice expansion and relative expansion in $c$ being ~35% larger than relative expansion in $a$ at 973K. First principles calculations reveal an increasing band gap as Bi is replaced by Sb then As, and subsequent Boltzmann transport calculations predict an increase in αfor a given $n$ associated with an increased effective mass as the gap opens. The magnitude and temperature dependence of αsuggests higher zT is likely to be achieved at larger carrier concentrations, roughly an order of magnitude higher than those in the current polycrystalline samples, which is also expected from the detailed calculations.

cond-mat.mtrl-sci↗

arXiv2011

Study of the K0(L) --> pi0 pi0 nu nu-bar decay

The rare decay K0(L) --> pi0 pi0 nu nu-bar was studied with the E391a detector at the KEK 12-GeV proton synchrotron. Based on 9.4 x 10^9 K0L decays, an upper limit of 8.1 x 10^{-7} was obtained for the branching fraction at 90% confidence level. We also set a limit on the K0(L) --> pi0 pi0 X (X --> invisible particles) process; the limit on the branching fraction varied from 7.0 x 10^{-7} to 4.0 x 10^{-5} for the mass of X ranging from 50 MeV/c^2 to 200 MeV/c^2.

hep-ex↗

arXiv2011

Role of Magnetic Exchange Energy on Charge Ordering in RSFO (R=La, Pr, and Nd)

Inelastic neutron scattering is applied to study the role of magnetism in stabilizing the charge ordered state in \emph{R}$_{1/3}$Sr$_{2/3}$FeO$_{3}$ (\emph{R}SFO) (\emph{R} = La, Pr, and Nd). The ratio of the ferromagnetic exchange energy (J$_F$) and antiferromagnetic exchange energy (J$_{AF}$), |J$_F$ /J$_{AF}|, is a key indicator of the stability of the charge ordered and antiferromagnetic ordered state. This ratio is obtained from the spin wave spectrum by inelastic neutron scattering and is sufficiently large to suggest that the magnetic exchange energy alone can stabilize the charge ordered state in La$_{1/3}$Sr$_{2/3}$FeO$_{3}$ and Pr$_{1/3}$Sr$_{2/3}$FeO$_{3}$. The exchange ratio decreases from La$_{1/3}$Sr$_{2/3}$FeO$_{3}$ to Nd1$_{1/3}$Sr$_{2/3}$FeO$_{3}$ which indicates a gradual destabilization of the magnetic exchange mechanism for charge ordering in correspondence with the observed reduction in the ordering temperature.

cond-mat.str-el↗

arXiv2011

Giant Anharmonic Phonon Scattering in PbTe

Understanding the microscopic processes affecting the bulk thermal conductivity is crucial to develop more efficient thermoelectric materials. PbTe is currently one of the leading thermoelectric materials, largely thanks to its low thermal conductivity. However, the origin of this low thermal conductivity in a simple rocksalt structure has so far been elusive. Using a combination of inelastic neutron scattering measurements and first-principles computations of the phonons, we identify a strong anharmonic coupling between the ferroelectric transverse optic (TO) mode and the longitudinal acoustic (LA) modes in PbTe. This interaction extends over a large portion of reciprocal space, and directly affects the heat-carrying LA phonons. The LA-TO anharmonic coupling is likely to play a central role in explaining the low thermal conductivity of PbTe. The present results provide a microscopic picture of why many good thermoelectric materials are found near a lattice instability of the ferroelectric type.

cond-mat.mtrl-sci↗

arXiv2010

Search for the decay $K_L^0 \rightarrow 3γ$

We performed a search for the decay $K_L^0 \rightarrow 3γ$ with the E391a detector at KEK. In the data accumulated in 2005, no event was observed in the signal region. Based on the assumption of $K_L^0 \rightarrow 3γ$ proceeding via parity-violation, we obtained the single event sensitivity to be $(3.23\pm0.14)\times10^{-8}$, and set an upper limit on the branching ratio to be $7.4\times10^{-8}$ at the 90% confidence level. This is a factor of 3.2 improvement compared to the previous results. The results of $K_L^0 \rightarrow 3γ$ proceeding via parity-conservation were also presented in this paper.

hep-ex↗

arXiv2010

The Tidal Tails of Globular Cluster Palomar 5 Based on Neural Networks Method

The Sixth Data Release (DR6) in the Sloan Digital Sky Survey (SDSS) provides more photometric regions, new features and more accurate data around globular cluster Palomar 5. A new method, Back Propagation Neural Network (BPNN), is used to estimate the probability of cluster member to detect its tidal tails. Cluster and field stars, used for training the networks, are extracted over a $40\times20$ deg$^2$ field by color-magnitude diagrams (CMDs). The best BPNNs with two hidden layers and Levenberg-Marquardt (LM) training algorithm are determined by the chosen cluster and field samples. The membership probabilities of stars in the whole field are obtained with the BPNNs, and contour maps of the probability distribution show that a tail extends $5.42\dg$ to the north of the cluster and a tail extends $3.77\dg$ to the south. The whole tails are similar to those detected by \citet{od03}, but no longer debris of the cluster is found to the northeast of the sky. The radial density profiles are investigated both along the tails and near the cluster center. Quite a few substructures are discovered in the tails. The number density profile of the cluster is fitted with the King model and the tidal radius is determined as $14.28'$. However, the King model cannot fit the observed profile at the outer regions ($R > 8'$) because of the tidal tails generated by the tidal force. Luminosity functions of the cluster and the tidal tails are calculated, which confirm that the tails originate from Palomar 5.

astro-ph.IM↗

arXiv2010

A Sino-German 6 cm polarization survey of the Galactic plane II. The region from 129 degree to 230 degree longitude

Linearly polarized Galactic synchrotron emission provides valuable information about the properties of the Galactic magnetic field and the interstellar magneto-ionic medium, when Faraday rotation along the line of sight is properly taken into account. We aim to survey the Galactic plane at 6 cm including linear polarization. At such a short wavelength Faraday rotation effects are in general small and the Galactic magnetic field properties can be probed to larger distances than at long wavelengths. The Urumqi 25-m telescope is used for a sensitive 6 cm survey in total and polarized intensities. WMAP K-band (22.8 GHz) polarization data are used to restore the absolute zero-level of the Urumqi U and Q maps by extrapolation. Total intensity and polarization maps are presented for a Galactic plane region of 129 degree < l < 230 degree and |b| < 5 degree in the anti-centre with an angular resolution of 9'5 and an average sensitivity of 0.6 mK and 0.4 mK Tb in total and polarized intensity, respectively. We briefly discuss the properties of some extended Faraday Screens detected in the 6 cm polarization maps. The Sino-German 6 cm polarization survey provides new information about the properties of the magnetic ISM. The survey also adds valuable information for discrete Galactic objects and is in particular suited to detect extended Faraday Screens with large rotation measures hosting strong regular magnetic fields.

astro-ph.GA↗

arXiv2009

Multicolor Photometry of the Galaxy Cluster A98: Substructures and Star Formation Properties

An optical photometric observation with the Beijing-Arizona-Taiwan-Connecticut (BATC) multicolor system is carried out for A98 (z=0.104), a galaxy cluster with two large enhancements in X-ray surface brightness. The spectral energy distributions (SEDs) covering 15 intermediate bands are obtained for all sources detected down to V ~ 20 mag in a field of $58' \times 58'$. After the star-galaxy separation by the color-color diagrams, a photometric redshift technique is applied to the galaxy sample for further membership determination. The color-magnitude relation is taken as a further restriction of the early-type cluster galaxies. As a result, a list of 198 faint member galaxies is achieved. Based on newly generated sample of member galaxies, the dynamical substructures, A98N, A98S, and A98W, are investigated in detail. A separate galaxy group, A98X, is also found to the south of main concentration of A98, which is gravitationally unbound to A98. For 74 spectroscopically confirmed member galaxies, the environmental effect on the star formation histories is found. The bright galaxies in the core region are found to have shorter time scales of star formation, longer mean stellar ages, and higher metallicities of interstellar medium, which can be interpreted in the context of hierarchical cosmological scenario.

astro-ph.CO↗

arXiv2009

Multicolour Photometric Study of M31 Globular Clusters

We present the photometry of 30 globular clusters (GCs) and GC candidates in 15 intermediate-band filters covering from ~3000 to ~10000 Åusing the archival CCD images of M31 observed as part of the Beijing - Arizona - Taiwan - Connecticut (BATC) Multicolour Sky Survey. We transform these intermediate-band photometric data to the photometry in the standard UBVRI broad-bands. These M31 GC candidates are selected from the Revised Bologna Catalogue (RBC V.3.5), and most of these candidates do not have any photometric data. Therefore the present photometric data are supplement to RBC V.3.5. We find that 4 out of 61 GCs and GC candidates in RBC V.3.5 do not show any signal on the BATC images at their locations. By linear fit of the distribution in colour-magnitude diagram of blue GCs and GC candidates using the data from RBC V.3.5, and in this study we find the ``blue-tilt'' of blue M31 GCs with a high confidence at 99.95% or 3.47 sigma for the confirmed GCs, and >99.99% or 4.87 sigma for GCs and GC candidates.

astro-ph.CO↗

arXiv2009

Search for a light pseudoscalar particle in the decay $K^0_L \to π^0 π^0 X$

We performed a search for a light pseudoscalar particle $X$ in the decay $K_L^0->pi0pi0X$, $X->γγ$ with the E391a detector at KEK. Such a particle with a mass of 214.3 MeV/$c^2$ was suggested by the HyperCP experiment. We found no evidence for $X$ and set an upper limit on the product branching ratio for $K_L^0->pi0pi0X$, $X->γγ$ of $2.4 \times 10^{-7}$ at the 90% confidence level. Upper limits on the branching ratios in the mass region of $X$ from 194.3 to 219.3 MeV/$c^2$ are also presented.

hep-ex↗

arXiv2008

Determination of the exchange anisotropy in perovskite antiferromagnets using powder inelastic neutron scattering

A procedure is outlined for the determination of magnetic exchange constants in anisotropic perovskite anitferromagnets using powder inelastic neutron scattering. Spin wave densities-of-states are measured using time-of-flight inelastic neutron scattering for LaMnO$_{3}$ ($A$-type antiferromagnet), LaVO$_{3}$ ($C$-type), and LaFeO$_{3}$ ($G$-type) and compared to Heisenberg model calculations. The anisotropy of in-plane ($J_{ab}$) and out-of-plane ($J_{c}$) exchange constants can be obtained from the data. The procedure quickly determines the magnetic exchange interactions without the need for single-crystal dispersion measurements and allows for rapid systematic studies of the evolution of magnetism in perovskite systems.

cond-mat.str-el↗

arXiv2007

Reddening, Colour and Metallicity of the M31 Globular Cluster System

Using metallicities from the literature, combined with the Revised Bologna Catalogue of photometric data for M31 clusters and cluster candidates (the latter of which is the most comprehensive catalogue of M31 clusters currently available, including 337 confirmed globular clusters -- GCs -- and 688 GC candidates), we determine 443 reddening values and intrinsic colours, and 209 metallicities for individual clusters without spectroscopic observations. This, the largest sample of M31 GCs presently available, is then used to analyse the metallicity distribution of M31 GCs, which is bimodal with peaks at $\rm {[Fe/H]}\approx -1.7$ and -0.7 dex. An exploration of metallicities as a function of radius from the M31 centre shows a metallicity gradient for the metal-poor GCs, but no such gradient for the metal-rich GCs. Our results show that the metal-rich clusters appear as a centrally concentrated spatial distribution; however, the metal-poor clusters tend to be less spatially concentrated. There is no correlation between luminosity and metallicity among the M31 sample clusters, which indicates that self-enrichment is indeed unimportant for cluster formation in M31. The reddening distribution shows that slightly more than half of the GCs are affected by a reddening of $E(B-V) \la 0.2$ mag; the mean reddening value is $E(B-V) = 0.28_{-0.14}^{+0.23}$ mag. The spatial distribution of the reddening values indicates that the reddening on the northwestern side of the M31 disc is more significant than that on the southeastern side, which is consistent with the conclusion that the northwestern side in nearer to us.

astro-ph↗

arXiv2007

Blind background prediction using a bifurcated analysis scheme

A technique for background prediction using data, but maintaining a closed signal box is described. The result is extended to two background sources. Conditions on the applicability under correlated cuts are described. This technique is applied to both a toy model and an analysis of data from a rare neutral kaon decay experiment.

physics.data-an↗

arXiv2007

Relativistic Tennis with Photons: Demonstration of Frequency Upshifting by a Relativistic Flying Mirror through Two Colliding Laser Pulses

Since the advent of chirped pulse amplification1 the peak power of lasers has grown dramatically and opened the new branch of high field science, delivering the focused irradiance, electric fields of which drive electrons into the relativistic regime. In a plasma wake wave generated by such a laser, modulations of the electron density naturally and robustly take the shape of paraboloidal dense shells, separated by evacuated regions, moving almost at the speed of light. When we inject another counter-propagating laser pulse, it is partially reflected from the shells, acting as relativistic flying (semi-transparent) mirrors, producing an extremely time-compressed frequency-multiplied pulse which may be focused tightly to the diffraction limit. This is as if the counterstreaming laser pulse bounces off a relativistically swung tennis racket, turning the ball of the laser photons into another ball of coherent X-ray photons but with a form extremely relativistically compressed to attosecond and zeptosecond levels. Here we report the first demonstration of the frequency multiplication detected from the reflection of a weak laser pulse in the region of the wake wave generated by the driver pulse in helium plasma. This leads to the possibility of very strong pulse compression and extreme coherent light intensification. This Relativistic Tennis with photon beams is demonstrated leading to the possibility toward reaching enormous electromagnetic field intensification and finally approaching the Schwinger field, toward which the vacuum nonlinearly warps and eventually breaks, producing electron-positron pairs.

physics.plasm-ph↗

arXiv2007

Structural Parameters of Mayall II = G1 in M31

Mayall II = G1 is one of the most luminous globular clusters (GCs) known in M31. New deep, high-resolution observations with the Advanced Camera for Surveys on the {\sl Hubble Space Telescope} are used to provide accurate photometric data to the smallest radii yet. In particular, we present the precise variation of ellipticity and position angle, and of surface brightness for the core of the object. Based on these accurate photometric data, we redetermine the structural parameters of G1 by fitting a single-mass isotropic King model. We derive a core radius, $r_c=0.21\pm0.01\arcsec (=0.78\pm0.04 \rm{pc})$, a tidal radius, $r_t=21.8\pm1.1\arcsec (=80.7\pm3.9 \rm{pc})$, and a concentration index $c=\log (r_t/r_c)=2.01\pm0.02$. The central surface brightness is 13.510 mag arcsec$^{-2}$. We also calculate the half-light radius, at $r_h=1.73\pm0.07\arcsec(=6.5\pm0.3 \rm{pc})$. The results show that, within 10 core radii, a King model fits the surface brightness distribution well. We find that this object falls in the same region of the $M_V$ vs. $\log R_h$ diagram as $ω$ Centauri, M54 and NGC 2419 in the Milky Way. All three of these objects have been claimed to be the stripped cores of now defunct dwarf galaxies. We discuss in detail whether GCs, stripped cores of dwarf spheroidals and normal dwarf galaxies form a continuous distribution in the $M_V$ versus $\log R_h$ plane, or if GCs and dwarf spheroidals constitute distinct classes of objects; we present arguments in favour of this latter view.

astro-ph↗

arXiv2006

Stabilization of charge ordering in La_(1/3)Sr_(2/3)FeO_(3-d) by magnetic exchange

The magnetic exchange energies in charge ordered La_(1/3)Sr_(2/3)FeO_(3-d) (LSFO) and its parent compound LaFeO_(3) (LFO) have been determined by inelastic neutron scattering. In LSFO, the measured ratio of ferromagnetic exchange between Fe3+ - Fe5+ pairs (J_F) and antiferromagnetic exchange between Fe3+ - Fe3+ pairs (J_AF) fulfills the criterion for charge ordering driven by magnetic interactions (|J_F/J_AF| > 1). The 30% reduction of J_AF as compared to LFO indicates that doped holes are delocalized, and charge ordering occurs without a dominant influence from Coulomb interactions.

cond-mat.str-el↗

arXiv2006

New age estimates of M31 globular clusters from multi-colour photometry

The large majority of extragalactic star cluster studies performed to date essentially use multi-colour photometry, combined with theoretical stellar synthesis models, to derive ages, masses, extinction estimates, and metallicities. M31 offers a unique laboratory for studies of globular cluster (GC) systems. In this paper, we obtain new age estimates for 91 M31 globular clusters, based on improved photometric data, updated theoretical stellar synthesis models and sophisticated new fitting methods. In particular, we used photometric measurements from the Two Micron All Sky Survey (2MASS), which, in combination with optical photometry, can partially break the well-known age-metallicity degeneracy operating at ages in excess of a few Gyr. We show robustly that previous age determinations based on photometric data were affected significantly by this age-metallicity degeneracy. Except for one cluster, the ages of our other sample GCs are all older than 1 Gyr. Their age distribution shows populations of young and intermediate-age GCs, peaking at $\sim3$ and 8 Gyr respectively, as well as the "usual" complement of well-known old GCs, i.e., GCs of similar age as the majority of the Galactic GCs. Our results also show that although there is significant scatter in metallicity at any age, there is a noticeable lack of young metal-poor and old metal-rich GCs, which might be indicative of an underlying age-metallicity relationship among the M31 GC population.

astro-ph↗