arXiv subjects
Publications and source records attributed to S. Zhang.
Tests were performed to pass a 100 MeV, 430 kWatt c.w. electron beam from the energy-recovery linac at the Jefferson Laboratory's FEL facility through a set of small apertures in a 127 mm long aluminum block. Beam transmission losses of 3 p.p.m. through a 2 mm diameter aperture were maintained during a 7 hour continuous run.
We report measurements of photon and neutron radiation levels observed while transmitting a 0.43 MW electron beam through millimeter-sized apertures and during beam-off, but accelerating gradient RF-on, operation. These measurements were conducted at the Free-Electron Laser (FEL) facility of the Jefferson National Accelerator Laboratory (JLab) using a 100 MeV electron beam from an energy-recovery linear accelerator. The beam was directed successively through 6 mm, 4 mm, and 2 mm diameter apertures of length 127 mm in aluminum at a maximum current of 4.3 mA (430 kW beam power). This study was conducted to characterize radiation levels for experiments that need to operate in this environment, such as the proposed DarkLight Experiment. We find that sustained transmission of a 430 kW continuous-wave (CW) beam through a 2 mm aperture is feasible with manageable beam-related backgrounds. We also find that during beam-off, RF-on operation, multipactoring inside the niobium cavities of the accelerator cryomodules is the primary source of ambient radiation when the machine is tuned for 130 MeV operation.
High power, relativistic electron beams from energy recovery linacs have great potential to realize new experimental paradigms for pioneering innovation in fundamental and applied research. A major design consideration for this new generation of experimental capabilities is the understanding of the halo associated with these bright, intense beams. In this Letter, we report on measurements performed using the 100 MeV, 430 kWatt CW electron beam from the energy recovery linac at the Jefferson Laboratory's Free Electron Laser facility as it traversed a set of small apertures in a 127 mm long aluminum block. Thermal measurements of the block together with neutron measurements near the beam-target interaction point yielded a consistent understanding of the beam losses. These were determined to be 3 ppm through a 2 mm diameter aperture and were maintained during a 7 hour continuous run.
After 2010, three outbursts of H 1743-322 were detected by RXTE. We have carried out timing and spectral analysis of the data, emphasizing on the two with relatively complete evolution history presented in the RXTE/PCA observations. We then constitute an enlarged outburst sample for H 1743-322 which allows to investigate the spectral transitions in more details. We find that the spectral transitions to high-soft state constrain a region for four outbursts in hardness-intensity diagram. An extension of the region locates in the vicinity of the failed outburst in 2008, and excludes it from a successful group. We therefore suggest the failed outburst in 2008 may present the first almost successful outburst sample, which is important for modelling the outburst, especially upon the conditions required for transition to high-soft state.
A uniformly accelerated detector (Unruh detector) in the Minkowski vacuum is excited as if it is exposed to the thermal bath with temperature proportional to its acceleration. In the inertial frame, since both of an excitation and a deexcitation of the detector are accompanied by emission of radiation into the Minkowski vacuum, one may suspect that the Unruh detector emits radiation like the Larmor radiation from an accelerated charged particle. However, it is known that the radiation is miraculously cancelled by a quantum interference effect. In this paper, we investigate under what condition the radiation cancels out. We first show that the cancellation occurs if the Green function satisfies a relation similar to the Kubo-Martin-Schwinger (KMS) condition. We then study two examples, Unruh detectors in the 3+1 dimensional Minkowski spacetime and in the de Sitter spacetime. In both cases, the relation holds only in a restricted region of the spacetime, but the radiation is cancelled in the whole spacetime. Hence the KMS-like relation is necessary but not sufficient for the cancellation to occur.
Dihadron azimuthal correlations in 200 GeV/c Au+Au have been simulated by a multi-phase transport (AMPT) model. Contribution from jet-medium interaction to correlation function is obtained by subtracting the combined harmonic flow background from the raw dihadron correlation function. Signals in centralities of 0-10%, 20-40% and 50-80% are compared in three associated transverse momentum ($p_{T}^{assoc}$) bins: 0.2-0.8 GeV/c, 0.8-1.4 GeV/c and 1.4-2.0 GeV/c. An obvious medium modification impact can be seen from the signal shape change and relative jet contribution in the above events, which shows different behaviors between central and peripheral events, and among different $p_{T}^{assoc}$ ranges in central events. More detailed $p_{T}^{assoc}$ dependence of the derived nonflow contribution is studied in central 0-10% events, in which a strong $p_{T}^{assoc}$ dependence of RMS width is observed. We also calculated that relative jet contributions in peripheral and central collisions in the above mentioned cases.
We present the production of light (anti)nuclei, (anti)hypertriton and di-Lambda based on coalescence model in central Au+Au collisions at $\sqrt{s_{NN}}=200GeV$. The invariant yields of \He(\Hebar), \hypert(\hypertbar), \Hee(\Heebar) obtained is found to be consistent with the STAR measurements. The $p_{T}$ integrated yields for di-Lambda $dN_{ΛΛ}/dy \sim 2.23\times10^{-5}$, and is not strongly dependent on the parameter employed for coalescence process. Relative particle ratios of light anti(nuclei) and (anti)hypertriton are explored, and agree with experimental data and thermal model predictions quite well. An exponential reducion behavior is presented for the differential invariant yields with increased baryon number. The production rate reduces by a factor of 1692 (1285) for each additional antinucleon (nucleon) added to antinuclei (nuclei), and the production rate of \Libar is predicted to be $10^{-16}$ which is consistent with STAR result.
IGR J18179-1621 is a hard X-ray binary transient discovered recently by INTEGRAL. Here we report on detailed timing and spectral analysis on IGR J18179-1621 in X-rays based on available INTEGRAL and Swift data. From the INTEGRAL analysis, IGR J18179-1621 is detected with a significance of 21.6 sigma in the 18-40 keV band by ISGRI and 15.3 sigma in the 3-25 keV band by JEM-X, between 2012-02-29 and 2012-03-01. We analyze two quasisimultaneous Swift ToO observations. A clear 11.82 seconds pulsation is detected above the white noise at a confidence level larger than 99.99%. The pulse fraction is estimated as 22+/-8% in 0.2-10 keV. No sign of pulsation is detected by INTEGRAL/ISGRI in the 18-40 keV band. With Swift and INTEGRAL spectra combined in soft and hard X-rays, IGR J18179-1621 could be fitted by an absorbed power law with a high energy cutoff plus a Gaussian absorption line centered at 21.5 keV. An additional absorption intrinsic to the source is found, while the absorption line is evidence for most probably originated from cyclotron resonant scattering and suggests a magnetic field in the emitting region of \sim 2.4 \times 10^12 Gauss.
Structural features of ferroic domains are fundamentally important for the understanding of microstructure and physical properties of ferroic materials, and they are also of technological significance for information storage and electronic switches. There are three well-known forms of ferroic orders in correlation with the spontaneous magnetization, electric polarization, and spontaneous strain, respectively. Ferroelectricity often accompanies ferroelastic strain, so ferroelectric domains tend to form in simple elongated configurations. However, due to strong boundary conditions, ferroelectric domains in nano-materials or near defects can have unusual configurations. In addition, vortex domains where multiple ferroelectric domains merge at one point have been observed in YMnO3. Herein, we report the discovery of unique ferroelectric domains with a concentric cylindrical shape in hexagonal manganites. We have also observed remarkable domains with multiple annular patterns. Our findings could open a new avenue for comprehensive studies of the ferroic domains, and this novel pattern could also leads to possibilities for new technologic applications.
The Fermi Large Area Telescope (LAT) reported the first definitive GeV detections of the binaries LS I +61\degree 303 and LS 5039 in the first year after its launch in June, 2008. These detections were unambiguous as a consequence of the reduced positional uncertainty and the detection of modulated gamma-ray emission on the corresponding orbital periods. An analysis of new data from the LAT, comprising 30 months of observations, identifies a change in the gamma-ray behavior of LS I +61\degree 303. An increase in flux is detected in March 2009 and a steady decline in the orbital flux modulation is observed. Significant emission up to 30GeV is detected by the LAT; prior datasets led to upper limits only. Contemporaneous TeV observations no longer detected the source, or found it -in one orbit- close to periastron, far from the phases at which the source previously appeared at TeV energies. The detailed numerical simulations and models that exist within the literature do not predict or explain many of these features now observed at GeV and TeV energies. New ideas and models are needed to fully explain and understand this behavior. A detailed phase-resolved analysis of the spectral characterization of LS I +61\degree 303 in the GeV regime ascribes a power law with an exponential cutoff spectrum along each analyzed portion of the system's orbit. The on-source exposure of LS 5039 is also substantially increased with respect to our prior publication. In this case, whereas the general gamma-ray properties remain consistent, the increased statistics of the current dataset allows for a deeper investigation of its orbital and spectral evolution.
An apparatus was developed to obtain, for the first time, 2D maps of the surface resistance of the inner surface of an operating superconducting radio-frequency niobium cavity by a low-temperature laser scanning microscopy technique. This allows identifying non-uniformities of the surface resistance with a spatial resolution of about one order of magnitude better than with earlier methods and surface resistance resolution of ~ 1 micro-Ohm at 3.3 GHz. A signal-to-noise ratio of about 10 dB was obtained with 240 mW laser power and 1 Hz modulation frequency. The various components of the apparatus, the experimental procedure and results are discussed in detail in this contribution.
Assume that A is a purely infinite simple C*-algebra whose K_0 is a torsion group, namely, contains no free element. Then a positive element a in A can be written as a finite sum of projections in A if and only if either a is a projection or the norm ||a||>1.
We give an overview of the question: which positive elements in an operator algebra can be written as a linear combination of projections with positive coefficients. A special case of independent interest is the question of which positive elements can be written as a sum of finitely many projections. We focus on von Neumann algebras, on purely infinite simple C*-algebras, and on their associated multiplier algebras.
In this paper we present new structural information about the multiplier algebra Mult (A) of a sigma-unital purely infinite simple C*-algebra A, by characterizing the positive elements a in Mult(A) that are strict sums of projections belonging to A. If a is not in A and is not a projection, then the necessary and sufficient condition for a to be a strict sum of projections belonging to A is that the norm ||a||>1 and that the essential norm ||a||_ess >=1. Based on a generalization of the Perera-Rordam weak divisibility of separable simple C*-algebras of real rank zero to all sigma-unital simple C*-algebras of real rank zero, we show that every positive element of A with norm greater than 1 can be approximated by finite sums of projections. Based on block tri-diagonal approximations, we decompose any positive element a in Mult(A) with ||a||>1 and ||a||_ess >=1 into a strictly converging sum of positive elements in A with norm greater than 1.
We demonstrate trapping and quantum state control of single Cesium atoms in a 532 nm wavelength bottle beam trap. The three dimensional trap is formed by crossing two unit charge vortex beams. Single atoms are loaded with 50% probability directly from a magneto-optical trap. We achieve a trapping lifetime of up to 6 s, and demonstrate fast Rabi oscillations with a coherence time of $T_2\sim 43 \pm 9\rm ms$.
Laser-induced demagnetization is theoretically studied by explicitly taking into account interactions among electrons, spins and lattice. Assuming that the demagnetization processes take place during the thermalization of the sub-systems, the temperature dynamics is given by the energy transfer between the thermalized interacting baths. These energy transfers are accounted for explicitly through electron-magnons and electron-phonons interaction, which govern the demagnetization time scale. By properly treating the spin system in a self-consistent random phase approximation, we derive magnetization dynamic equations for a broad range of temperature. The dependence of demagnetization on the temperature and pumping laser intensity is calculated in detail. In particular, we show several salient features for understanding magnetization dynamics near the Curie temperature. While the critical slowdown in dynamics occurs, we find that an external magnetic field can restore the fast dynamics. We discuss the implication of the fast dynamics in the application of heat assisted magnetic recording.
Within the framework of a multi-phase transport model, harmonic flows $v_n$ (n = 2, 3 and 4) are investigated for Au + Au collisions at $\sqrt{s_{NN}}$ = 200 GeV and Pb + Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV. The event-by-event geometry fluctuations significantly contribute to harmonic flows. Triangular flow ($v_3$) originates from initial triangularity ($ε_3$) and is developed by partonic interactions. The conversion efficiency ($v_n/ε_n$) decreases with harmonic order and increases with partonic interaction cross section. A mass ordering in the low $p_T$ region and number of constitute quark scaling in the middle $p_T$ region seem to work roughly for $n$-th harmonic flows at both energies. All features of harmonic flows show similar qualitative behaviors at RHIC and LHC energies, which implies that the formed partonic matters are similar at the two energies.
The influence of structural asymmetries (barrier height and exchange splitting), as well as inelastic scattering (magnons and phonons) on the bias dependence of the spin transfer torque in a magnetic tunnel junction is studied theoretically using the free electron model. We show that they modify the "conventional" bias dependence of the spin transfer torque, together with the bias dependence of the conductance. In particular, both structural asymmetries and bulk (inelastic) scattering add {\em antisymmetric} terms to the perpendicular torque ($\propto V$ and $\propto j_e|V|$), while the interfacial inelastic scattering conserves the junction symmetry and only produces {\em symmetric} terms ($\propto |V|^n$, $n\in\mathbb{N}$). The analysis of spin torque and conductance measurements displays a signature revealing the origin (asymmetry or inelastic scattering) of the discrepancy.