arXiv subjects

J. Liu

Publications and source records attributed to J. Liu.

At least 397 records · Page 22Linked to original sources

arXiv2010

Pulse shape simulation for segmented true-coaxial HPGe detectors

A new package to simulate the formation of electrical pulses in segmented true-coaxial high purity germanium detectors is presented. The computation of the electric field and weighting potentials inside the detector as well as of the trajectories of the charge carriers is described. In addition, the treatment of bandwidth limitations and noise are discussed. Comparison of simulated to measured pulses, obtained from an 18-fold segmented detector operated inside a cryogenic test facility, are presented.

physics.ins-det↗

arXiv2010

Gate-Voltage Control of Chemical Potential and Weak Anti-localization in Bismuth Selenide

We report that Bi$_2$Se$_3$ thin films can be epitaxially grown on SrTiO$_{3}$ substrates, which allow for very large tunablity in carrier density with a back-gate. The observed low field magnetoconductivity due to weak anti-localization (WAL) has a very weak gate-voltage dependence unless the electron density is reduced to very low values. Such a transition in WAL is correlated with unusual changes in longitudinal and Hall resistivities. Our results suggest much suppressed bulk conductivity at large negative gate-voltages and a possible role of surface states in the WAL phenomena. This work may pave a way for realizing three-dimensional topological insulators at ambient conditions.

cond-mat.mes-hall↗

arXiv2010

Wide spin resonance with an rf-bunched proton beam

We recently used an rf solenoid to study the widths of rf spin resonances with both unbunched and bunched beams of 2.1 GeV_c polarized protons stored in the COSY synchrotron. A map, with unbunched beam at different fixed rf-solenoid frequencies, showed a very shallow possible depolarization dip at the resonance. Next we made frequency sweeps of 400Hz, centered at similar frequencies, which greatly enhanced the dip. But, with a bunched proton beam, both the fixed-frequency and frequency-sweep techniques produced similar maps, and both bunched maps showed full beam depolarization over a wide region. Moreover, both were more than twice as wide as the unbunched dip. This widening of the proton resonance due to bunching is exactly opposite to the recently observed narrowing of deuteron resonances due to bunching.

physics.acc-ph↗

arXiv2009

Measuring the Spins of Stellar Black Holes: A Progress Report

We use the Novikov-Thorne thin disk model to fit the thermal continuum X-ray spectra of black hole X-ray binaries, and thereby extract the dimensionless spin parameter a* = a/M of the black hole as a parameter of the fit. We summarize the results obtained to date for six systems and describe work in progress on additional systems. We also describe recent methodological advances, our current efforts to make our analysis software fully available to others, and our theoretical efforts to validate the Novikov-Thorne model.

astro-ph.HE↗

arXiv2009

Berry Phase of Nonlinear Correction

We investigate the geometric phase or Berry phase of adiabatic quantum evolution in the Bose-Einstein condensate (BEC) systems governed by nonlinear Gross-Pitaevskii(GP) equations. We study how this phase is modified by the nonlinearity and find that the Bogoliubov fluctuations around the eigenstates are accumulated during the nonlinear adiabatic evolution and contribute a finite phase of geometric nature. A two-mode BEC model is used to illustrate our theory. Our theory is applicable to other nonlinear systems such as paraxial wave equation for nonlinear optics and Ginzburg-Landau equations for complex order parameters in condensed-matter physics.

cond-mat.quant-gas↗

arXiv2009

Effects of reabsorption and spatial trap distributions on the radiative quantum efficiencies of ZnO

Ultrafast time-resolved photoluminescence spectroscopy following one- and two-photon excitation of ZnO powder is used to gain unprecedented insight into the surprisingly high external quantum efficiency of its "green" defect emission band. The role of exciton diffusion, the effects of reabsorption, and the spatial distributions of radiative and nonradiative traps are comparatively elucidated for the ultraviolet excitonic and "green" defect emission bands in both unannealed, nanometer-sized ZnO powders and annealed, micrometer-sized ZnO:Zn powders. We find that the primary mechanism limiting quantum efficiency is surface recombination because of the high density of nonradiative surface traps in these powders. It is found that unannealed ZnO has a high density of bulk nonradiative traps as well, but the annealing process reduces the density of these bulk traps while simultaneously creating a high density of green-emitting defects near the particle surface. The data are discussed in the context of a simple rate equation model that accounts for the quantum efficiencies of both emission bands. The results indicate how defect engineering could improve the efficiency of ultraviolet-excited ZnO:Zn-based white light phosphors.

cond-mat.mtrl-sci↗

arXiv2009

Perspectives of a Midrapidity Dimuon Program at RHIC: A Novel and Compact Muon Telescope Detector

We propose a large-area, cost-effective Muon Telescope Detector (MTD) for the Solenoidal Tracker at RHIC (STAR) at mid-rapidity and for the next generation of detectors at a possible electron-ion collider. We utilize Multi-gap Resistive Plate Chambers with large modules and long readout strips (Long-MRPC) in the detector design. The results from cosmic ray and beam tests show the intrinsic timing and spatial resolution for a Long-MRPC are 60-70 ps and $\sim1$ cm, respectively. The prototype performance of such a novel muon telescope detector at STAR indicates that muon identification at the transverse momentum of a few GeV/$c$ can be achieved through the combined information of track matching with the MTD, ionization energy loss in the Time Projection Chamber, and time-of-flight measurements. A primary muon over secondary muon ratio of better than 1/3 can be achieved. This provides a promising device for future quarkonium programs and primordial dilepton measurements at RHIC. Simulations of the muon efficiency, the signal-to-background ratio of $J/ψ$, the separation of $Υ$ 1S from 2S+3S states, and the electron-muon correlation from charm pair production in the RHIC environment are presented.

nucl-ex↗

arXiv2009

Parquet approximation for the 4x4 Hubbard cluster

We present a numerical solution of the parquet approximation (PA), a conserving diagrammatic approach which is self-consistent at both the single-particle and the two-particle levels. The fully irreducible vertex is approximated by the bare interaction thus producing the simplest approximation that one can perform with the set of equations involved in the formalism. The method is applied to the Hubbard model on a half-filled 4x4 cluster. Results are compared to those obtained from Determinant Quantum Monte Carlo (DQMC), FLuctuation EXchange (FLEX), and self-consistent second-order approximation methods. This comparison shows a satisfactory agreement with DQMC and a significant improvement over the FLEX or the self-consistent second-order approximation.

cond-mat.str-el↗

arXiv2009

Scintillator counters with WLS fiber/MPPC readout for the side muon range detector (SMRD)of the T2K experiment

The T2K neutrino experiment at J-PARC uses a set of near detectors to measure the properties of an unoscillated neutrino beam and neutrino interaction cross-sections. One of the sub-detectors of the near-detector complex, the side muon range detector (SMRD), is described in the paper. The detector is designed to help measure the neutrino energy spectrum, to identify background and to calibrate the other detectors. The active elements of the SMRD consist of 0.7 cm thick extruded scintillator slabs inserted into air gaps of the UA1 magnet yokes. The readout of each scintillator slab is provided through a single WLS fiber embedded into a serpentine shaped groove. Two Hamamatsu multi-pixel avalanche photodiodes (MPPC's) are coupled to both ends of the WLS fiber. This design allows us to achieve a high MIP detection efficiency of greater than 99%. A light yield of 25-50 p.e./MIP, a time resolution of about 1 ns and a spatial resolution along the slab better than 10 cm were obtained for the SMRD counters.

physics.ins-det↗

arXiv2009

Depth-Resolved Subsurface Defects in Chemically Etched SrTiO3

Depth-resolved cathodoluminescence spectroscopy of atomically flat TiO2-terminated SrTiO3 single crystal surfaces reveals dramatic differences in native point defects produced by conventional etching with buffered HF (BHF) and an alternative procedure using HCl-HNO3 acidic solution (HCLNO), which produces three times fewer oxygen vacancies before and nearly an order of magnitude fewer after pure oxygen annealing. BHF-produced defect densities extend hundreds of nm below the surface, whereas the lower HCLNO-treated densities extend less than 50 nm. This "Arkansas" HCLNO etch and anneal method avoids HF handling and provides high quality SrTiO3 surfaces with low native defect density for complex oxide heterostructure growth

cond-mat.mtrl-sci↗

arXiv2008

High-order Harmonic Generation in a Time-integrated Quantum Transition Picture

We develop a numerical scheme to investigate the high-order harmonic generation (HHG) in intense laser-matter interactions. Tracing the time evolution of every electronic laser-field-free state, we "observe" the HHG in a time-integrated quantum transition picture. Our full-quantum simulations reveal that continuum electrons with a broad energy distribution contribute equally to one harmonic and the excited state also plays an important role in the molecular HHG. These results imply a laser-intensity-dependent picture of intramolecular interference in the HHG.

physics.atom-ph↗

arXiv2008

Spin-dephasing anisotropy for electrons in a diffusive quasi-1D GaAs wire

We present a numerical study of dephasing of electron spin ensembles in a diffusive quasi-one-dimensional GaAs wire due to the D'yakonov-Perel' spin-dephasing mechanism. For widths of the wire below the spin precession length and for equal strength of Rashba and linear Dresselhaus spin-orbit fields a strong suppression of spin-dephasing is found. This suppression of spin-dephasing shows a strong dependence on the wire orientation with respect to the crystal lattice. The relevance for realistic cases is evaluated by studying how this effect degrades for deviating strength of Rashba and linear Dresselhaus fields, and with the inclusion of the cubic Dresselhaus term.

cond-mat.mes-hall↗

arXiv2008

Compact cryogenic Kerr microscope for time-resolved studies of electron spin transport in microstructures

A compact cryogenic Kerr microscope for operation in the small volume of high-field magnets is described. It is suited for measurements both in Voigt and Faraday configuration. Coupled with a pulsed laser source, the microscope is used to measure the time-resolved Kerr rotation response of semiconductor microstructures with ~1 micron spatial resolution. The microscope was designed to study spin transport, a critical issue in the field of spintronics. It is thus possible to generate spin polarization at a given location on a microstructure and probe it at a different location. The operation of the microscope is demonstrated by time-resolved measurements of micrometer distance diffusion of spin polarized electrons in a GaAs/AlGaAs heterojunction quantum well at 4.2 K and 7 Tesla.

cond-mat.mes-hall↗

arXiv2008

Charged pion form factor between $Q^2$=0.60 and 2.45 GeV$^2$. I. Measurements of the cross section for the ${^1}$H($e,e'π^+$)$n$ reaction

Cross sections for the reaction ${^1}$H($e,e'π^+$)$n$ were measured in Hall C at Thomas Jefferson National Accelerator Facility (JLab) using the CEBAF high-intensity, continous electron beam in order to determine the charged pion form factor. Data were taken for central four-momentum transfers ranging from $Q^2$=0.60 to 2.45 GeV$^2$ at an invariant mass of the virtual photon-nucleon system of $W$=1.95 and 2.22 GeV. The measured cross sections were separated into the four structure functions $σ_L$, $σ_T$, $σ_{LT}$, and $σ_{TT}$. The various parts of the experimental setup and the analysis steps are described in detail, including the calibrations and systematic studies, which were needed to obtain high precision results. The different types of systematic uncertainties are also discussed. The results for the separated cross sections as a function of the Mandelstam variable $t$ at the different values of $Q^2$ are presented. Some global features of the data are discussed, and the data are compared with the results of some model calculations for the reaction ${^1}$H($e,e'π^+$)$n$.

nucl-ex↗

arXiv2008

Charged pion form factor between Q^2=0.60 and 2.45 GeV^2. II. Determination of, and results for, the pion form factor

The charged pion form factor, Fpi(Q^2), is an important quantity which can be used to advance our knowledge of hadronic structure. However, the extraction of Fpi from data requires a model of the 1H(e,e'pi+)n reaction, and thus is inherently model dependent. Therefore, a detailed description of the extraction of the charged pion form factor from electroproduction data obtained recently at Jefferson Lab is presented, with particular focus given to the dominant uncertainties in this procedure. Results for Fpi are presented for Q^2=0.60-2.45 GeV^2. Above Q^2=1.5 GeV^2, the Fpi values are systematically below the monopole parameterization that describes the low Q^2 data used to determine the pion charge radius. The pion form factor can be calculated in a wide variety of theoretical approaches, and the experimental results are compared to a number of calculations. This comparison is helpful in understanding the role of soft versus hard contributions to hadronic structure in the intermediate Q^2 regime.

nucl-ex↗

arXiv2008

Physics at BES-III

This physics book provides detailed discussions on important topics in $τ$-charm physics that will be explored during the next few years at \bes3 . Both theoretical and experimental issues are covered, including extensive reviews of recent theoretical developments and experimental techniques. Among the subjects covered are: innovations in Partial Wave Analysis (PWA), theoretical and experimental techniques for Dalitz-plot analyses, analysis tools to extract absolute branching fractions and measurements of decay constants, form factors, and CP-violation and \DzDzb-oscillation parameters. Programs of QCD studies and near-threshold tau-lepton physics measurements are also discussed.

hep-ex↗

arXiv2008

A solenoidal electron spectrometer for a precision measurement of the neutron $β$-asymmetry with ultracold neutrons

We describe an electron spectrometer designed for a precision measurement of the neutron $β$-asymmetry with spin-polarized ultracold neutrons. The spectrometer consists of a 1.0-Tesla solenoidal field with two identical multiwire proportional chamber and plastic scintillator electron detector packages situated within 0.6-Tesla field-expansion regions. Select results from performance studies of the spectrometer with calibration sources are reported.

nucl-ex↗

arXiv2008

New Prototype Multi-gap Resistive Plate Chambers with Long Strips

A new kind of Multi-gap Resistive Plate Chamber (MRPC) has been built for the large-area Muon Telescope Detector (MTD) for the STAR experiment at RHIC. These long read-out strip MRPCs (LMRPCs) have an active area of 87.0 x 17.0 cm2 and ten 250 um-thick gas gaps arranged as a double stack. Each read-out strip is 2.5 cm wide and 90 cm long. The signals are read-out at both ends of each strip. Cosmic ray tests indicate a time resolution of ~70 ps and a detection efficiency of greater than 95%. Beam tests performed at T963 at Fermilab indicate a time resolution of 60-70 ps and a spatial resolution of ~1 cm along the strip direction.

physics.ins-det↗