arXiv subjects

J. Liu

Publications and source records attributed to J. Liu.

At least 217 records · Page 12Linked to original sources

arXiv2019

The on-orbit calibration of DArk Matter Particle Explorer

The DArk Matter Particle Explorer (DAMPE), a satellite-based cosmic ray and gamma-ray detector, was launched on December 17, 2015, and began its on-orbit operation on December 24, 2015. In this work we document the on-orbit calibration procedures used by DAMPE and report the calibration results of the Plastic Scintillator strip Detector (PSD), the Silicon-Tungsten tracKer-converter (STK), the BGO imaging calorimeter (BGO), and the Neutron Detector (NUD). The results are obtained using Galactic cosmic rays, bright known GeV gamma-ray sources, and charge injection into the front-end electronics of each sub-detector. The determination of the boundary of the South Atlantic Anomaly (SAA), the measurement of the live time, and the alignments of the detectors are also introduced. The calibration results demonstrate the stability of the detectors in almost two years of the on-orbit operation.

astro-ph.IM↗

arXiv2019

Coulomb-induced ionization time lag after electrons tunnel out of a barrier

After electrons tunnel out of a laser-Coulomb-formed barrier, %formed by the strong laser field and the atomic Coulomb potential, the movement of the tunneling electron can be affected by the Coulomb tail. We show that this Coulomb effect induces a large time difference (longer than a hundred attoseconds) between the exiting time at which the electron exits the barrier and the ionization time at which the electron is free. This large time difference has important influences on strong-field processes such as above-threshold ionization and high-harmonic generation, with remarkably changing time-frequency properties of electron trajectories. Some semi-quantitative evaluations on these influences are addressed, which provide new insight into strong-field physics and give important suggestions on attosecond measurements.

physics.atom-ph↗

arXiv2019

Segmented YSO scintillation detectors as a new ${\rm β}$-implant detection tool for decay spectroscopy in fragmentation facilities

A newly developed segmented YSO scintillator detector was implemented for the first time at the RI-beam Factory at RIKEN Nishina Center as an implantation-decay counter. The results from the experiment demonstrate that the detector is a viable alternative to conventional silicon-strip detectors with its good timing resolution and high detection efficiency for ${\rm β}$ particles. A Position-Sensitive Photo-Multiplier Tube (PSPMT) is coupled with a $48\times48$ segmented YSO crystal. To demonstrate its capabilities, a known short-lived isomer in $^{76}$Ni and the ${\rm β}$ decay of $^{74}$Co were measured by implanting those ions into the YSO detector. The half-lives and ${\rm γ}$-rays observed in this work are consistent with the known values. The ${\rm β}$-ray detection efficiency is more than 80~\% for the decay of $^{74}$Co.

physics.ins-det↗

arXiv2019

Final results for the neutron $β$-asymmetry parameter $A_0$ from the UCNA experiment

The UCNA experiment was designed to measure the neutron $β$-asymmetry parameter $A_0$ using polarized ultracold neutrons (UCN). UCN produced via downscattering in solid deuterium were polarized via transport through a 7 T magnetic field, and then directed to a 1 T solenoidal electron spectrometer, where the decay electrons were detected in electron detector packages located on the two ends of the spectrometer. A value for $A_0$ was then extracted from the asymmetry in the numbers of counts in the two detector packages. We summarize all of the results from the UCNA experiment, obtained during run periods in 2007, 2008--2009, 2010, and 2011--2013, which ultimately culminated in a 0.67\% precision result for $A_0$.

nucl-ex↗

arXiv2019

Earth wind as a possible source of lunar surface hydration

Understanding the sources of lunar water is crucial for studying the history of lunar evolution, and also the solar wind interaction with the Moon and other airless bodies. Recent observations revealed lunar hydration is very likely a surficial dynamic process driven by solar wind. Solar wind is shielded over a period of 3-5 days as the Moon passes through the Earth's magnetosphere, during which a significant loss of hydration is expected from previous works.Here we study lunar hydration inside the magnetosphere using orbital spectral data, which unexpectedly found that the polar surficial OH/H2O abundance remains at the same level when in the solar wind and in the magnetosphere. We suggest that particles from the magnetosphere (Earth wind, naturally different from solar wind) contribute to lunar hydration. From lunar orbital plasma observations, we find the existence of optimal energy ranges, other than 1 keV as previously thought, for surface hydration formation. These optimal energy ranges deduced from space observations may provide strong implications for laboratory experiments simulating lunar hydration processes.

astro-ph.EP↗

arXiv2019

Fabrication and Characterization of High-Purity Germanium Detectors with Amorphous Germanium Contacts

Large, high-purity, germanium (HPGe) detectors are needed for neutrinoless double-beta decay and dark matter experiments. Currently, large (> 4 inches in diameter) HPGe crystals can be grown at the University of South Dakota (USD). We verify that the quality of the grown crystals is sufficient for use in large detectors by fabricating and characterizing smaller HPGe detectors made from those crystals. We report the results from eight detectors fabricated over six months using crystals grown at USD. Amorphous germanium (a-Ge) contacts are used for blocking both electrons and holes. Two types of geometry were used to fabricate HPGe detectors. As a result, the fabrication process of small planar detectors at USD is discussed in great detail. The impact of the procedure and geometry on the detector performance was analyzed for eight detectors. We characterized the detectors by measuring the leakage current, capacitance, and energy resolution at 662 keV with a Cs-137 source. Four detectors show good performance, which indicates that crystals grown at USD are suitable for making HPGe detectors.

physics.ins-det↗

arXiv2019

Search for gamma-ray emission from the Sun during solar minimum with the ARGO-YBJ experiment

The hadronic interaction of cosmic rays with solar atmosphere can produce high energy gamma rays. The gamma-ray luminosity is correlated both with the flux of primary cosmic rays and the intensity of the solar magnetic field. The gamma rays below 200 GeV have been observed by $Fermi$ without any evident energy cutoff. The bright gamma-ray flux above 100 GeV has been detected only during solar minimum. The only available data in TeV range come from the HAWC observations, however outside the solar minimum. The ARGO-YBJ dataset has been used to search for sub-TeV/TeV gamma rays from the Sun during the solar minimum from 2008 to 2010, the same time period covered by the Fermi data. A suitable model containing the Sun shadow, solar disk emission and inverse-Compton emission has been developed, and the chi-square minimization method was used to quantitatively estimate the disk gamma-ray signal. The result shows that no significant gamma-ray signal is detected and upper limits to the gamma-ray flux at 0.3$-$7 TeV are set at 95\% confidence level. In the low energy range these limits are consistent with the extrapolation of the Fermi-LAT measurements taken during solar minimum and are compatible with a softening of the gamma-ray spectrum below 1 TeV. They provide also an experimental upper bound to any solar disk emission at TeV energies. Models of dark matter annihilation via long-lived mediators predicting gamma-ray fluxes > $10^{-7}$ GeV $cm^{-2}$ $s^{-1}$ below 1 TeV are ruled out by the ARGO-YBJ limits.

astro-ph.HE↗

arXiv2019

Precise Charged Particle Timing with the PICOSEC Detector

The experimental requirements in near future accelerators (e.g. High Luminosity-LHC) has stimulated intense interest in development of detectors with high precision timing capabilities. With this as a goal, a new detection concept called PICOSEC, which is based to a "two-stage" MicroMegas detector coupled to a Cherenkov radiator equipped with a photocathode has been developed. Results obtained with this new detector yield a time resolution of 24\,ps for 150\,GeV muons and 76\,ps for single photoelectrons. In this paper we will report on the performance of the PICOSEC in test beams, as well as simulation studies and modelling of its timing characteristics.

physics.ins-det↗

arXiv2019

Unconventional field-induced spin gap in an $S = 1/2$ chiral staggered chain

We investigate the low-temperature magnetic properties of the molecule-based chiral spin chain [Cu(pym)(H$_2$O)$_4$]SiF$_6\cdot$H$_2$O (pym = pyrimidine). Electron-spin resonance, magnetometry and heat capacity measurements reveal the presence of staggered $g$ tensors, a rich low-temperature excitation spectrum, a staggered susceptibility and a spin gap that opens on the application of a magnetic field. These phenomena are reminiscent of those previously observed in non-chiral staggered chains, which are explicable within the sine-Gordon quantum-field theory. In the present case, however, although the sine-Gordon model accounts well for the form of the temperature-dependence of the heat capacity, the size of the gap and its measured linear field dependence do not fit with the sine-Gordon theory as it stands. We propose that the differences arise due to additional terms in the Hamiltonian resulting from the chiral structure of [Cu(pym)(H$_2$O)$_4$]SiF$_6\cdot$H$_2$O, particularly a uniform Dzyaloshinskii-Moriya coupling and a four-fold periodic staggered field.

cond-mat.str-el↗

arXiv2018

Investigation of Amorphous Germanium Contact Properties with Planar Detectors Made from Home-Grown Germanium Crystals

The characterization of detectors fabricated from home-grown crystals is the most direct way to study crystal properties. We fabricated planar detectors from high-purity germanium (HPGe) crystals grown at the University of South Dakota (USD). In the fabrication process, a HPGe crystal slice cut from a USD-grown crystal was coated with a high resistivity thin film of amorphous Ge (a-Ge) followed by depositing a thin layer of aluminum on top of the a-Ge film to define the physical area of the contacts. We investigated the detector performance including the $I$-$V$ characteristics, $C$-$V$ characteristics and spectroscopy measurements for a few detectors. The results document the good quality of the USD-grown crystals and electrical contacts.

physics.ins-det↗

arXiv2018

Understanding doped perovskite ferroelectrics with defective dipole model

While doping is widely used for tuning physical properties of perovskites in experiments, it remains a challenge to exactly know how doping achieves the desired effects. Here, we propose an empirical and computationally tractable model to understand the effects of doping with Fe-doped BaTiO$_{3}$ as an example. This model assumes that the lattice sites occupied by Fe ion and its nearest six neighbors lose their ability to polarize, giving rise to a small cluster of defective dipoles. Employing this model in Monte-Carlo simulations, many important features like reduced polarization and the convergence of phase transition temperatures, which have been observed experimentally in acceptor doped systems, are successfully obtained. Based on microscopic information of dipole configurations, we provide insights into the driving forces behind doping effects and propose that active dipoles, which exist in proximity to the defective dipoles, can account for experimentally observed phenomena. Close attention to these dipoles are necessary to understand and predict doping effects.

cond-mat.mtrl-sci↗

arXiv2018

Control of fast electron propagation in foam target by high-Z doping

The influence of high-Z dopant (Bromine) in low-Z foam (polystyrene) target on laser-driven fast electron propagation is studied by the 3D hybrid particle-in-cell (PIC)/fluid code HEETS.It is found that the fast electrons are better confined in doped targets due to the increasing resistivity of the target, which induces a stronger resistive magnetic field which acts to collimate the fast electron propagation.The energy deposition of fast electrons into the background target is increased slightly in the doped target, which is beneficial for applications requiring long distance propagation of fast electrons, such as fast ignition.

physics.plasm-ph↗

arXiv2018

The role of the rare earth in the lattice and magnetic coupling in multiferroic h-HoMnO3

We used Raman scattering to study the lattice and magnetic excitations in the hexagonal HoMnO3 single crystals. The E2 phonon mode at 237 cm-1 is affected by the magnetic order. This mode is related to the displacement of Mn and O ions in a-b plane and modulates the Mn-O-Mn bond angles in a-b plane and the in-plane Mn-Mn superexchange interaction. The mode at 269 cm-1 associated to the displacement of the apical Ho3+ ions along the c direction presents an abrupt change of slope at TN showing that the role of the rare earth ions can not be neglected in the magnetic transition. We have identified magnon and crystal field excitations. The temperature dependence of the magnetic excitations has been compared to the Mn and Ho moment and indicates that the exchange interaction pattern between Mn and Ho atoms drives the uniaxial anisotropy gap above the Mn-spin-rotation transition.

cond-mat.str-el↗

arXiv2018

Quantum Interference of Glory Rescattering in Strong-Field Atomic Ionization

During the ionization of atoms irradiated by linearly polarized intense laser fields, we find for the first time that the transverse momentum distribution of photoelectrons can be well fitted by a squared zeroth-order Bessel function because of the quantum interference effect of Glory rescattering. The characteristic of the Bessel function is determined by the common angular momentum of a bunch of semiclassical paths termed as Glory trajectories, which are launched with different nonzero initial transverse momenta distributed on a specific circle in the momentum plane and finally deflected to the same asymptotic momentum, which is along the polarization direction, through post-tunneling rescattering. Glory rescattering theory (GRT) based on the semiclassical path-integral formalism is developed to address this effect quantitatively. Our theory can resolve the long-standing discrepancies between existing theories and experiments on the fringe location, predict the sudden transition of the fringe structure in holographic patterns, and shed light on the quantum interference aspects of low-energy structures in strong-field atomic ionization.

physics.atom-ph↗

arXiv2018

Data and Spectrum Trading Policies in a Trusted Cognitive Dynamic Network

Future wireless networks will progressively displace service provisioning towards the edge to accommodate increasing growth in traffic. This paradigm shift calls for smart policies to efficiently share network resources and ensure service delivery. In this paper, we consider a cognitive dynamic network architecture (CDNA) where primary users (PUs) are rewarded for sharing their connectivities and acting as access points for secondary users (SUs). CDNA creates opportunities for capacity increase by network-wide harvesting of unused data plans and spectrum from different operators. Different policies for data and spectrum trading are presented based on centralized, hybrid and distributed schemes involving primary operator (PO), secondary operator (SO) and their respective end users. In these schemes, PO and SO progressively delegate trading to their end users and adopt more flexible cooperation agreements to reduce computational time and track available resources dynamically. A novel matching-with-pricing algorithm is presented to enable self-organized SU-PU associations, channel allocation and pricing for data and spectrum with low computational complexity. Since connectivity is provided by the actual users, the success of the underlying collaborative market relies on the trustworthiness of the connections. A behavioral-based access control mechanism is developed to incentivize/penalize honest/dishonest behavior and create a trusted collaborative network. Numerical results show that the computational time of the hybrid scheme is one order of magnitude faster than the benchmark centralized scheme and that the matching algorithm reconfigures the network up to three orders of magnitude faster than in the centralized scheme.

cs.NI↗

arXiv2018

Epitaxially strained SnTiO$_{3}$ at finite temperatures

Combining effective Hamiltonian and direct $ab$ $initio$ computation, we obtain the phase diagram of SnTiO$_{3}$ with respect to epitaxial strain and temperature, demonstrating the complex features of the phase diagram and providing insight into the epitaxially strained SnTiO$_{3}$, a presumably simple perovskite. In the phase diagram, two triple points are found, which may be exploited to to achieve high-performance piezoelectric effects. On the other hand, despite the inclusion of the degree of freedoms related to oxygen octahedron tilting, it is found that ferroelectric displacements dominate the structural phases over the whole strain misfit range. Finally, it is shown that the SnTiO$_{3}$ can be converted from hard to soft ferroelectrics with epitaxial strain.

cond-mat.mtrl-sci↗

arXiv2018

New result for the neutron $β$-asymmetry parameter $A_0$ from UCNA

The neutron $β$-decay asymmetry parameter $A_0$ defines the correlation between the spin of the neutron and the momentum of the emitted electron, which determines $λ=\frac{g_{A}}{g_{V}}$, the ratio of the axial-vector to vector weak coupling constants. The UCNA Experiment, located at the Ultracold Neutron facility at the Los Alamos Neutron Science Center, is the first to measure such a correlation coefficient using ultracold neutrons (UCN). Following improvements to the systematic uncertainties and increased statistics, we report the new result $A_0 = -0.12054(44)_{\mathrm{stat}}(68)_{\mathrm{syst}}$ which yields $λ\equiv \frac{g_{A}}{g_{V}}=-1.2783(22)$. Combination with the previous UCNA result and accounting for correlated systematic uncertainties produces $A_0=-0.12015(34)_{\mathrm{stat}}(63)_{\mathrm{syst}}$ and $λ\equiv \frac{g_{A}}{g_{V}}=-1.2772(20)$.

nucl-ex↗

arXiv2018

Charged particle timing at sub-25 picosecond precision: the PICOSEC detection concept

The PICOSEC detection concept consists in a "two-stage" Micromegas detector coupled to a Cherenkov radiator and equipped with a photocathode. A proof of concept has already been tested: a single-photoelectron response of 76 ps has been measured with a femtosecond UV laser at CEA/IRAMIS, while a time resolution of 24 ps with a mean yield of 10.4 photoelectrons has been measured for 150 GeV muons at the CERN SPS H4 secondary line. This work will present the main results of this prototype and the performance of the different detector configurations tested in 2016-18 beam campaigns: readouts (bulk, resistive, multipad) and photocathodes (metallic+CsI, pure metallic, diamond). Finally, the prospects for building a demonstrator based on PICOSEC detection concept for future experiments will be discussed. In particular, the scaling strategies for a large area coverage with a multichannel readout plane, the R\&D on solid converters for building a robust photocathode and the different resistive configurations for a robust readout.

physics.ins-det↗