arXiv subjects

Z. Li

Publications and source records attributed to Z. Li.

At least 271 records · Page 15Linked to original sources

arXiv2007

Chandra detection of diffuse hot gas in and around the M31 bulge

We report the detection of diffuse hot gas in M31, using archival Chandra observations which allow us to map out a 30' by 30' field (covering a galactocentric radius up to 4.5 kpc) and to detect sources in the galaxy down to a 0.5-8 keV luminosity limit of ~10^35 ergs/s. We estimate the remaining stellar contribution from fainter X-ray sources (primarily cataclysmic variables and coronally active binaries), assuming that they spatially follow the stellar distribution. Indeed, the near-IR K-band light of the galaxy closely traces the 2-8 keV unresolved X-rays, indicating a collective stellar X-ray emissivity consistent with those determined for the Galactic ridge and M32, whereas the amount of the 0.5-2 keV unresolved emission is significantly greater than the expected stellar contribution, especially within a galactocentric radius of ~2 kpc. Morphologically, this soft X-ray excess appears substantially rounder than the bulge as seen in K-band and is elongated approximately along the minor-axis at large radii. The excess thus most likely represents the emission of diffuse hot gas in and around the galactic bulge. Furthermore, the near side of the M31 disk casts an apparent shadow against the soft X-ray excess, indicating that the hot gas extends to at least 2.5 kpc from the galactic plane. We briefly discuss the implications of these results on the energy balance in the M31 bulge and on understanding the large-scale soft X-ray enhancement observed toward the inner region of our own Galaxy.

astro-ph↗

arXiv2007

VLA and XMM-Newton observations of the SNR W41/TeV Gamma-ray source HESS J1834-087

The recently discovered extended TeV source HESS J1834-087 is associated with both a diffuse X-ray enhancement and a molecular cloud, projected at the center of an old radio supernova remnant G23.3-0.3 (SNR W41). New HI observations from the VLA Galactic Plane Survey (VGPS) show unambiguous structures associated with W41 in the radial velocity range of 53 to 63 km/s, so we obtain for W41 a distance of 4+-0.2 kpc. A new higher sensitivity VGPS continuum image of W41 at 1420 MHz shows faint emission in its eastern part not detected by previous observations, so we give a new angular size of 36'x30' in b-l direction (average radius of 19 pc). We estimate for W41 a Sedov age of ~10e5 yr. New XMM-Newton observation reveals diffuse X-ray emission within the HESS source and suggests an association between the X-ray and Gamma-ray emission. The high-resolution ^{13}CO images of W41 further reveal a giant molecular cloud (GMC) located at the center of W41, likely associated with W41 in the the radial velocity range of 61 to 66 km/s. Altogether, the new observations can be interpreted as providing the first evidence that an old SNR encounters a GMC to emit TeV gamma-rays in the cloud material.

astro-ph↗

arXiv2006

Evidence of s-wave pairing symmetry in layered superconductor Li$_{0.68}$NbO$_2$ from the specific heat measurement

A high quality superconducting Li$_{0.68}$NbO$_2$ polycrystalline sample was synthesized by deintercalation of Li ions from Li$_{0.93}$NbO$_2$. The field dependent resistivity and specific heat were measured down to 0.5 K. The upper critical field $H_{c2} (T)$ is deduced from the resistivity data and $H_{c2}(0)$ is estimated to be $\sim 2.98$ T. A notable specific heat jump is observed at the superconducting transition temperature $T_c \sim 5.0$ K at zero field. Below $T_c$, the electronic specific heat shows a thermal activated behavior and agrees well with the theoretical result of the BCS s-wave superconductors. It indicates that the superconducting pairing in Li$_{0.68}$NbO$_2$ has s-wave symmetry.

cond-mat.supr-con↗

arXiv2005

Effects of current on vortex and transverse domain walls

By using the spin torque model in ferromagnets, we compare the response of vortex and transverse walls to the electrical current. For a defect-free sample and a small applied current, the steady state wall mobility is independent of the wall structure. In the presence of defects, the minimum current required to overcome the wall pinning potential is much smaller for the vortex wall than for the transverse wall. During the wall motion, the vortex wall tends to transform to the transverse wall. We construct a phase diagram for the wall mobility and the wall transformation driven by the current.

cond-mat.mtrl-sci↗

arXiv2005

Effects of spin current on ferromagnets

When a spin-polarized current flows through a ferromagnet, the local magnetization receives a spin torque. Two consequences of this spin torque are studied. First, the uniformly magnetized ferromagnet becomes unstable if a sufficiently large current is applied. The characteristics of the instability include spin wave generation and magnetization chaos. Second, the spin torque has profound effects on the structure and dynamics of the magnetic domain wall. A detail analysis on the domain wall mass, kinetic energy and wall depinning threshold is given.

cond-mat.mtrl-sci↗

arXiv2005

Spectral lags and the energy dependence of pulse width in gamma-ray bursts: contributions from the relativistic curvature effect

We compute the temporal profiles of the gamma-ray burst pulse in the four Burst and Transient Source Experiment (BATSE) Large Area Detector (LAD) discriminator energy channels, with the relativistic curvature effect of a expanding fireball being explicitly investigated. Assuming an intrinsic 'Band' shape spectrum and an intrinsic energy-independent emission profile, we show that merely the curvature effect can produce detectable spectral lags if the intrinsic pulse profile has a gradually decaying phase. We examine the spectral lag's dependences on some physical parameters, such as the Lorentz factor Γ, the low-energy spectral index, α, of the intrinsic spectrum, the duration of the intrinsic radiation t_d' and the fireball radius R. It is shown that approximately the lag \propto Γ^{-1} and \propto t_d', and a spectrum with a more extruded shape (a larger α) causes a larger lag. We find no dependence of the lag on R. Quantitatively, the lags produced from the curvature effect are marginally close to the observed ones, while larger lags require extreme physical parameter values, e.g., Γ< 50, or α> -0.5. The curvature effect causes an energy-dependent pulse width distribution but the energy dependence of the pulse width we obtained is much weaker than the observed W \propto E^{-0.4} one. This indicates that some intrinsic mechanism(s), other than the curvature effect, dominates the pulse narrowing of GRBs.

astro-ph↗

arXiv2005

Measurement of the Branching Fractions for J/psi to ell^+ ell^-

We present measurements of the branching fractions for J/psi --> e+e- and mu+mu- using 3M psi(2S) decays collected with the CLEO detector operating at the CESR e+e- collider. We obtain B(J/psi --> e+e-) = (5.945 +- 0.067 +- 0.042 )% and B(J/psi --> mu+mu-) = (5.960 +- 0.065 +- 0.050)%, leading to an average of B(J/psi --> l+l-) = (5.953 +- 0.056 +- 0.042)% and a ratio of B(J/psi --> e+e-) / B(J/psi --> mu+mu-) = (99.7 +- 1.2 +- 0.6)%, all consistent with, but more precise than, previous measurements.

hep-ex↗

arXiv2005

Dynamical coupled-channel study of K+ Lambda photoproduction

Results for the reaction gamma p --> K+ Lambda, studied within a constituent quark model and a dynamical coupled-channel approach, are presented and compared with recent data. Issues related to the search for missing baryon resonances are briefly discussed and the role played by a third S_{11} resonance is underlined.

nucl-th↗

arXiv2004

Exclusive Multi-Body Hadronic Decays of the psi(2S)

Using data accumulated with the CLEO detector corresponding to an integrated luminosity of 5.46 1/pb on the peak of the psi(2S) and 20.46 1/pb at E_CM=3.67 GeV, we report preliminary branching fraction measurements for seven new decay modes of the psi(2S) (eta 3pi, eta' 3pi, 2(K+ K-), p pbar K+ K-, Lambda Lambdabar pi+ pi-, Lambda pbar K+, and Lambda pbar K+ pi+ pi-) and more precise measurements of nine previously measured modes (2(pi+pi-), 2(pi+pi-) pi0, omega pi+pi-, K+ K- pi+ pi-, phi pi+ pi-, omega K+ K-, phi K+ K-, p pbar pi+ pi-, and Lambda Lambdabar). We also include a study of omega p pbar and obtain an improved upper limit for phi p pbar. Results are compared, where possible, with the corresponding J/psi branching ratios to test the 12% rule.

hep-ex↗

arXiv2004

Domain wall dynamics driven by adiabatic spin transfer torques

In a first approximation, known as the adiabatic process, the direction of the spin polarization of currents is parallel to the local magnetization vector in a domain wall. Thus the spatial variation of the direction of the spin current inside the domain wall results in an adiabatical spin transfer torque on the magnetization. We show that domain wall motion driven by this spin torque has many unique features that do not exist in the conventional wall motion driven by a magnetic field. By analytically and numerically solving the Landau-Lifshitz-Gilbert equation along with the adiabatic spin torque in magnetic nanowires, we find the domain wall has its maximum velocity at the initial application of the current but the velocity decreases to zero as the domain wall begins to deform during its motion. We have computed domain wall displacement and domain wall deformation of nanowires, and concluded that the spin torque based on the adiabatic propagation of the spin current in the domain wall is unable to maintain wall movement. We also introduce a novel concept of domain wall inductance to characterize the capacity of the spin-torque induced magnetic energy stored in a domain wall. In the presence of domain wall pinning centers, we construct a phase diagram for the domain wall depinning by the combined action of the magnetic field and the spin current.

math-ph↗

arXiv2004

Roles of non-equilibrium conduction electrons on magnetization dynamics of ferromagnets

The mutual dependence of spin-dependent conduction and magnetization dynamics of ferromagnets provides the key mechanisms in various spin-dependent phenomena. We compute the response of the conduction electron spins to a spatial and time varying magnetization ${\bf M} ({\bf r},t)$ within the time-dependent semiclassical transport theory. We show that the induced non-equilibrium conduction spin density in turn generates four spin torques acting on the magnetization--with each torque playing different roles in magnetization dynamics. By comparing with recent theoretical models, we find that one of these torques that has not been previously identified is crucial to consistently interpret experimental data on domain wall motion.

cond-mat.mtrl-sci↗

arXiv2004

Thermally assisted magnetization reversal in the presence of a spin-transfer torque

We propose a generalized stochastic Landau-Lifshitz equation and its corresponding Fokker-Planck equation for the magnetization dynamics in the presence of spin transfer torques. Since the spin transfer torque can pump a magnetic energy into the magnetic system, the equilibrium temperature of the magnetic system is ill-defined. We introduce an effective temperature based on a stationary solution of the Fokker-Planck equation. In the limit of high energy barriers, the law of thermal agitation is derived. We find that the Néel-Brown relaxation formula remains valid as long as we replace the temperature by an effective one that is linearly dependent of the spin torque. We carry out the numerical integration of the stochastic Landau-Lifshitz equation to support our theory. Our results agree with existing experimental data.

cond-mat.mtrl-sci↗

arXiv2003

Neutron star composition in strong magnetic fields

We study the problem of neutron star composition in the presence of a strong magnetic field. The effects of the anomalous magnetic moments of both nucleons and electrons are investigated in relativistic mean field calculations for a $β$-equilibrium system. Since neutrons are fully spin polarized in a large field, generally speaking, the proton fraction can never exceed the field free case. An extremely strong magnetic field may lead to a pure neutron matter instead of a proton-rich matter.

astro-ph↗

arXiv2003

Beyond the known baryon resonances

eta photoproduction data on the proton up to E=2 GeV are interpreted within a chiral constituent quark formalism, which embodies all known three and four star resonances. This study confirms the need for a new S11 resonance, with M=1.780 GeV and Gamma=280 MeV, already introduced in investigating lower energy data.

nucl-th↗

arXiv2003

Magnetization dynamics with a spin-transfer torque

The magnetization reversal and dynamics of a spin valve pillar, whose lateral size is 64$\times$64 nm$^2$, are studied by using micromagnetic simulation in the presence of spin transfer torque. Spin torques display both characteristics of magnetic damping (or anti-damping) and of an effective magnetic field. For a steady-state current, both M-I and M-H hysteresis loops show unique features, including multiple jumps, unusual plateaus and precessional states. These states originate from the competition between the energy dissipation due to Gilbert damping and the energy accumulation due to the spin torque supplied by the spin current. The magnetic energy oscillates as a function of time even for a steady-state current. For a pulsed current, the minimum width and amplitude of the spin torque for achieving current-driven magnetization reversal are quantitatively determined. The spin torque also shows very interesting thermal activation that is fundamentally different from an ordinary damping effect.

cond-mat.mtrl-sci↗

arXiv2002

Crossover from coherent to incoherent electronic excitations in the normal state of Bi2Sr2CaCu2O8

Angle resolved photoemission spectroscopy (ARPES) and resistivity measurements are used to explore the overdoped region of the high temperature superconductor Bi2Sr2CaCu2O8+delta. We find evidence for a new crossover line in the phase diagram between a coherent metal phase for lower temperatures and higher doping, and an incoherent metal phase for higher temperatures and lower doping. The former is characterized by two well-defined spectral peaks in ARPES due to coherent bilayer splitting and superlinear behavior in the resistivity, whereas the latter is characterized by a single broad spectral feature in ARPES and a linear temperature dependence of the resistivity.

cond-mat.supr-con↗

arXiv2002

A Multi-Moded RF Delay Line Distribution System for the Next Linear Collider

The Delay Line Distribution System (DLDS) is an alternative to conventional pulse compression, which enhances the peak power of rf sources while matching the long pulse of those sources to the shorter filling time of accelerator structures. We present an implementation of this scheme that combines pairs of parallel delay lines of the system into single lines. The power of several sources is combined into a single waveguide delay line using a multi-mode launcher. The output mode of the launcher is determined by the phase coding of the input signals. The combined power is extracted from the delay line using mode-selective extractors, each of which extracts a single mode. Hence, the phase coding of the sources controls the output port of the combined power. The power is then fed to the local accelerator structures. We present a detailed design of such a system, including several implementation methods for the launchers, extractors, and ancillary high power rf components. The system is designed so that it can handle the 600 MW peak power required by the NLC design while maintaining high efficiency.

physics.acc-ph↗

arXiv2002

Long-term Neutrino Afterglows from Gamma-Ray Bursts

It is widely believed that multiwavelength afterglows of gamma-ray bursts (GRBs) originate from relativistic blast waves. We here show that in such blast waves, a significant fraction of the energy of shock-accelerated protons would be lost due to pion production by interactions with afterglow photons. This could lead to long-term production of $10^{16}$--$10^{18}$ eV neutrinos and sub-TeV $γ$-rays that accompany with usual afterglows, provided that the protons are accelerated to $10^{19}$ eV in the blast waves.

astro-ph↗