arXiv subjects

Q. Liu

Publications and source records attributed to Q. Liu.

At least 73 records · Page 4Linked to original sources

arXiv2017

Selective decay from a candidate of the $σ$-bond linear-chain state in $^{14}$C

A cluster-transfer experiment $^9$Be($^9$Be,$^{14}$C$^*\rightarrowα$+$^{10}$Be)$α$ was carried out using an incident beam energy of 45 MeV. This reaction channel has a large $Q$-value that favors populating the high-lying states in $^{14}$C and separating various reaction channels. A number of resonant states are reconstructed from the forward emitting $^{10}$Be + $α$ fragments with respect to three sets of well discriminated final states in $^{10}$Be, most of which agree with the previous observations. A state at 22.5(1) MeV in $^{14}$C is found to decay predominantly into the states around 6 MeV in $^{10}$Be daughter nucleus, in line with the unique property of the predicted band head of the $σ$-bond linear-chain molecular states. A new state at 23.5(1) MeV is identified which decays strongly into the first excited state of $^{10}$Be.

nucl-ex↗

arXiv2016

Three-dimensional multiple-relaxation-time lattice Boltzmann model for convection heat transfer in porous media at the REV scale

In this paper, a three-dimensional (3D) multiple-relaxation-time (MRT) lattice Boltzmann (LB) model is presented for convection heat transfer in porous media at the representative elementary volume (REV) scale. The model is developed in the framework of the double-distribution-function (DDF) approach: an MRT-LB model of the density distribution function with the D3Q19 lattice (or D3Q15 lattice) is proposed to simulate the flow field based on the generalized non-Darcy model, while an MRT-LB model of the temperature distribution function with the D3Q7 lattice is proposed to simulate the temperature filed. The present model is employed to simulate mixed convection flow in a porous channel and natural convection in a cubical porous cavity. The numerical results demonstrate the effectiveness and accuracy of the present model in solving 3D convection heat transfer problems in porous media. The numerical results also demonstrate that the present model is approximately second-order accuracy in space. In addition, an enthalpy-based DDF-MRT model for 3D solid-liquid phase change with convection heat transfer in porous media is also presented.

physics.comp-ph↗

arXiv2016

Multiple-relaxation-time lattice Boltzmann model for convection heat transfer in porous media under local thermal non-equilibrium condition

In this paper, a multiple-relaxation-time (MRT) lattice Boltzmann (LB) model is proposed for convection heat transfer in porous media under local thermal non-equilibrium (LTNE) condition. The model is constructed within the framework of the three-distribution-function approach: two temperature-based MRT-LB equations are proposed for the temperature fields of fluid and solid phases in addition to the MRT-LB equation of a density distribution function for the velocity field described by the generalized non-Darcy model. The thermal non-equilibrium effects are incorporated into the model by adding source terms into the temperature-based MRT-LB equations. Moreover, the discrete lattice effects are considered in the introduction of source terms into the temperature-based MRT-LB equations. The source terms accounting for the thermal non-equilibrium effects are simple and the model retains the inherent features of the standard LB method. Numerical results demonstrate that the proposed model can be served as an accurate and efficient numerical method for studying convection heat transfer in porous media under LTNE condition.

physics.comp-ph↗

arXiv2016

Cluster Decay of the High-lying excited states in $^{14}$C

A cluster-transfer experiment of $^9\rm{Be}(^9\rm{Be},^{14}\rm{C}\rightarrowα+^{10}\rm{Be})α$ at an incident energy of 45 MeV was carried out in order to investigate the molecular structure in high-lying resonant states in $^{14}$C. This reaction is of extremely large $Q$-value, making it an excellent case to select the reaction mechanism and the final states in outgoing nuclei. The high-lying resonances in $^{14}$C are reconstructed for three sets of well discriminated final states in $^{10}$Be. The results confirm the previous decay measurements with clearly improved decay-channel selections and show also a new state at 23.5(1) MeV. The resonant states at 22.4(3) and 24.0(3) MeV decay primarily into the typical molecular states at about 6 MeV in $^{10}$Be, indicating a well developed cluster structure in these high-lying states in $^{14}$C. Further measurements of more states of this kind are suggested.

nucl-ex↗

arXiv2016

Structural and Magnetic Characterization of Large Area, Free-Standing Thin Films of Magnetic Ion Intercalated Dichalcogenides Mn0.25TaS2 and Fe0.25TaS2

Free-standing thin films of magnetic ion intercalated transition metal dichalcogenides are produced using ultramicrotoming techniques. Films of thicknesses ranging from 30nm to 250nm were achieved and characterized using transmission electron diffraction and X-ray magnetic circular dichroism. Diffraction measurements visualize the long range crystallographic ordering of the intercalated ions, while the dichroism measurements directly assess the orbital contributions to the total magnetic moment. We thus verify the unquenched orbital moment in Fe0.25TaS2 and measure the fully quenched orbital contribution in Mn0.25TaS2. Such films can be used in a wide variety of ultrafast X-ray and electron techniques that benefit from transmission geometries, and allow measurements of ultrafast structural, electronic, and magnetization dynamics in space and time.

cond-mat.mtrl-sci↗

arXiv2016

Cyclotron maser emission from power-law electrons with strong pitch-angle anisotropy

Energetic electrons with power-law spectrum are most commonly observed in astrophysics. This paper investigates electron cyclotron maser emission (ECME) from the power-law electrons, in which strong pitch-angle anisotropy is emphasized. The electron distribution function proposed in this paper can describe various types of pitch-angle anisotropy. Results show that the emission properties of ECME, including radiation growth, propagation, and frequency properties, depend considerably on the types of electron pitch-angle anisotropy, and different wave modes show different dependences on the pitch angle of electrons. In particular, the maximum growth rate of X2 mode rapidly decreases with respect to the electron pitch-angle cosine $μ_0$ at which the electron distribution peaks, while the growth rates for other modes (X1, O1, O2) initially increase before decreasing as $μ_0$ increases. Moreover, the O mode as well as the X mode can be the fastest growth mode, in terms of not only the plasma parameter but also the type of electron pitch-angle distribution. This result presents a significant extension of the recent researches on ECME driven by the lower-energy cutoff of power-law electrons, in which the X mode is generally the fastest growth mode.

astro-ph.SR↗

arXiv2015

Lattice Boltzmann methods for multiphase flow and phase-change heat transfer

Over the past few decades, tremendous progress has been made in the development of particle-based discrete simulation methods versus the conventional continuum-based methods. In particular, the lattice Boltzmann (LB) method has evolved from a theoretical novelty to a ubiquitous, versatile and powerful computational methodology for both fundamental research and engineering applications. It is a kinetic-based mesoscopic approach that bridges the microscales and macroscales, which offers distinctive advantages in simulation fidelity and computational efficiency. Applications of the LB method have been found in a wide range of disciplines including physics, chemistry, materials, biomedicine and various branches of engineering. The present work provides a comprehensive review of the LB method for thermofluids and energy applications, focusing on multiphase flows, thermal flows and thermal multiphase flows with phase change. The review first covers the theoretical framework of the LB method, revealing the existing inconsistencies and defects as well as common features of multiphase and thermal LB models. Recent developments in improving the thermodynamic and hydrodynamic consistency, reducing the spurious currents, enhancing the numerical stability, etc., are highlighted. These efforts have put the LB method on a firmer theoretical foundation with enhanced LB models that can achieve larger liquid-gas density ratio, higher Reynolds number and flexible surface tension. Examples of applications are provided in fuel cells and batteries, droplet collision, boiling heat transfer and evaporation, and energy storage. Finally, further developments and future prospect of the LB method are outlined for thermofluids and energy applications.

physics.comp-ph↗

arXiv2015

Systematic calculations of $α$-decay half-lives with an improved empirical formula

Based on the recent data in NUBASE2012, an improved empirical formula for evaluating the $α$-decay half-lives is presented, in which the hindrance effect resulted from the change of the ground state spins and parities of parent and daughter nuclei is included, together with a new correction factor for nuclei near the shell closures. The calculated $α$-decay half-lives are found to be in better agreements with the experimental data, and the corresponding root-mean-square (rms) deviation is reduced to $0.433$ when the experimental $Q$-values are employed. Furthermore, the $Q$-values derived from different nuclear mass models are used to predict $α$-decay half-lives with this improved formula. It is found that the calculated half-lives are very sensitive to the $Q$-values. Remarkably, when mass predictions are improved with the radial basis function (RBF), the resulting rms deviations can be significantly reduced. With the mass prediction from the latest version of Weizsäcker-Skyrme (WS4) model, the rms deviation of $α$-decay half-lives with respect to the known data falls to $0.697$.

nucl-th↗

arXiv2015

Electronic and Spin States of SrRuO3 Thin Films: an X-ray Magnetic Circular Dichroism Study

We report a study of the local magnetism in thin films of SrRuO3 grown on (111) and (001) oriented SrTiO3 substrates using x-ray magnetic circular dichroism spectroscopy (XMCD) at the Ru-L2,3 edges. The application of the sum rules to the XMCD data gives an almost quenched orbital moment and a spin moment close to the value expected for the low spin state S = 1 . Full-multiplet cluster calculations indicate that the low spin state is quite stable and suggest that the occurrence of a transition to the high spin state S = 2 in strained thin films of SrRuO3 is unlikely as it would be too expensive in energy.

cond-mat.str-el↗

arXiv2014

Design of a capacitor-less low-dropout voltage regulator

A solution to the stability of capacitor-less low-dropout regulators with a 4pF Miller capacitor in Multi-level current amplifier is proposed. With the Miller compensation, a more than 50°phase margin is guaranteed in full load. An extra fast transient circuit is adopted to reduce stable time and peak voltage. When the load changes from light to heavy, the peak voltage is 40mV and chip quiescent current is only 45uA.

eess.SY↗

arXiv2014

The time series forecasting: from the aspect of network

Forecasting can estimate the statement of events according to the historical data and it is considerably important in many disciplines. At present, time series models have been utilized to solve forecasting problems in various domains. In general, researchers use curve fitting and parameter estimation methods (moment estimation, maximum likelihood estimation and least square method) to forecast. In this paper, a new sight is given to the forecasting and a completely different method is proposed to forecast time series. Inspired by the visibility graph and link prediction, this letter converts time series into network and then finds the nodes which are mostly likelihood to link with the predicted node. Finally, the predicted value will be obtained according to the state of the link. The TAIEX data set is used in the case study to illustrate that the proposed method is effectiveness. Compared with ARIMA model, the proposed shows a good forecasting performance when there is a small amount of data.

physics.data-an↗

arXiv2013

The CAPTAIN Detector and Physics Program

The Cryogenic Apparatus for Precision Tests of Argon Interactions with Neutrino (CAP- TAIN) program is designed to make measurements of scientific importance to long-baseline neutrino physics and physics topics that will be explored by large underground detectors. The CAPTAIN detector is a liquid argon TPC deployed in a portable and evacuable cryostat. Five tons of liquid argon are instrumented with a 2,000 channel liquid argon TPC and a photon detection system. Subsequent to the commissioning phase, the detector will collect data in a high-energy neutron beamline that is part of the Los Alamos Neutron Science Center to measure cross-sections of spallation products that are backgrounds to measurements of neutrinos from a supernova burst, cross-sections of events that mimic the electron neutrino appearance signal in long-baseline neutrino physics and neutron signatures to constrain neutrino energy reconstruction in LBNE's long-baseline program. Subsequent to the neutron running, the CAPTAIN detector will be moved to a neutrino source. Two possibilities are an on-axis run in the NuMI beamline at FNAL and a run in the neutrino source produced by the SNS. An on-axis run at NuMI produces more than one million events of interest in a two or three year run at neutrino energies between 1 and 10 GeV - complementary to the MicroBooNE experiment, which will measure similar interactions at a lower energy range - 0.5 to 2 GeV. At the SNS the neutrinos result from the decays stopped positively charged pions and muons yielding a broad spectrum up to 50 MeV. If located close to the spallation target, CAPTAIN can detect several thousand events per year in the same neutrino energy regime where neutrinos from a supernova burst are. Measurements at the SNS yield a first measurement of the cross- section of neutrinos on argon in this important energy regime.

physics.ins-det↗

arXiv2013

Nuclear effective charge factor originating from covariant density functional theory

Guiding by the relativistic local density approximation, we explore a phenomenological formula for the coupling strength of Coulomb field to take into account the Coulomb exchange term effectively in the relativistic Hartree approximation. Its validity in finite nuclei is examined by comparing with the exact treatment of the Coulomb exchange energies in the relativistic Hartree-Fock-Bogoliubov approach. It is found that the exact Coulomb exchange energies can be reproduced by employing the phenomenological formula with the relative deviations less than 1% for semi-magic Ca, Ni, Sn, and Pb isotopes. Furthermore, we check the applicability of the phenomenological formula for the effective interactions in the relativistic Hartree approach by investigating the binding energy differences of mirror nuclei.

nucl-th↗

arXiv2013

Nuclear $β^+$/EC decays in covariant density functional theory and the impact of isoscalar proton-neutron pairing

Self-consistent proton-neutron quasiparticle random phase approximation based on the spherical nonlinear point-coupling relativistic Hartree-Bogoliubov theory is established and used to investigate the $β^+$/EC-decay half-lives of neutron-deficient Ar, Ca, Ti, Fe, Ni, Zn, Cd, and Sn isotopes. The isoscalar proton-neutron pairing is found to play an important role in reducing the decay half-lives, which is consistent with the same mechanism in the $β$ decays of neutron-rich nuclei. The experimental $β^+$/EC-decay half-lives can be well reproduced by a universal isoscalar proton-neutron pairing strength.

nucl-th↗

arXiv2013

Emerging understanding of the ΔI = 1/2 Rule from Lattice QCD

There has been much speculation as to the origin of the ΔI = 1/2 rule (Re A_0/Re A_2 \simeq 22.5). We find that the two dominant contributions to the ΔI=3/2, K \to ππ correlation functions have opposite signs leading to a significant cancellation. This partial cancellation occurs in our computation of Re A_2 with physical quark masses and kinematics (where we reproduce the experimental value of A_2) and also for heavier pions at threshold. For Re A_0, although we do not have results at physical kinematics, we do have results for pions at zero-momentum with m_π \simeq 420 MeV (Re A_0/Re A_2=9.1(2.1)) and m_π \simeq 330 MeV (Re A_0/Re A_2=12.0(1.7)). The contributions which partially cancel in Re A_2 are also the largest ones in Re A_0, but now they have the same sign and so enhance this amplitude. The emerging explanation of the ΔI=1/2 rule is a combination of the perturbative running to scales of O(2 GeV), a relative suppression of Re A_2 through the cancellation of the two dominant contributions and the corresponding enhancement of Re A_0. QCD and EWP penguin operators make only very small contributions at such scales.

hep-lat↗

arXiv2012

Lattice determination of the $K \to (ππ)_{I=2}$ Decay Amplitude $A_2$

We describe the computation of the amplitude A_2 for a kaon to decay into two pions with isospin I=2. The results presented in the letter Phys.Rev.Lett. 108 (2012) 141601 from an analysis of 63 gluon configurations are updated to 146 configurations giving Re$A_2=1.381(46)_{\textrm{stat}}(258)_{\textrm{syst}} 10^{-8}$ GeV and Im$A_2=-6.54(46)_{\textrm{stat}}(120)_{\textrm{syst}}10^{-13}$ GeV. Re$A_2$ is in good agreement with the experimental result, whereas the value of Im$A_2$ was hitherto unknown. We are also working towards a direct computation of the $K\to(ππ)_{I=0}$ amplitude $A_0$ but, within the standard model, our result for Im$A_2$ can be combined with the experimental results for Re$A_0$, Re$A_2$ and $ε^\prime/ε$ to give Im$A_0/$Re$A_0= -1.61(28)\times 10^{-4}$ . Our result for Im\,$A_2$ implies that the electroweak penguin (EWP) contribution to $ε^\prime/ε$ is Re$(ε^\prime/ε)_{\mathrm{EWP}} = -(6.25 \pm 0.44_{\textrm{stat}} \pm 1.19_{\textrm{syst}}) \times 10^{-4}$.

hep-lat↗

arXiv2012

A study of thinner-THGEM, with some applications

THGEMs (THick Gas Electron Multiplier) of varying thickness, hole diameter and hole pitch have been studied. For a thinner-THGEM of thickness 0.2 mm, with hole diameter 0.2 mm, pitch 0.2 mm and narrow (5-10 μm) rim, the performance of gain versus high voltage with different gas mixtures have been studied using 5.9 keV X-rays. In general, a gain of around 3\times10^3 was obtained with a single board in Ar/iC4H10, and gains higher than 10^4 were obtained in Ne mixture at lower voltage. The dependence of the energy resolution on the drift and induction electric fields was measured and an energy resolution of 15.9% was obtained. A curved thinner-THGEM chamber with one-dimensional readout has been assembled for use in the diffraction studies at the Beijing Synchrotron Radiation Facility (BSRF). A cosmic-ray muon hodoscope based on thinner-THGEM was developed as a teaching experiment for the Graduate University of the Chinese Academy of Sciences (GUCAS).

hep-ex↗

arXiv2012

Elastic and electronic properties of fluorite RuO_2 from first principle

The elastic, thermodynamic, and electronic properties of fluorite RuO_2 under high pressure are investigated by plane-wave pseudopotential density functional theory. The optimized lattice parameters, elastic constants, bulk modulus, and shear modulus are consistent with other theoretical values. The phase transition from modified fluorite-type to fluorite is 88 GPa (by localized density approximation, LDA) or 115.5 GPa (by generalized gradient approximation, GGA). The Young's modulus and Lamé's coefficients are also studied under high pressure. The structure turned out to be stable for the pressure up to 120 GPa by calculating elastic constants. In addition, the thermodynamic properties, including the Debye temperature, heat capacity, thermal expansion coefficient, Grüneisen parameter, and Poisson's ratio, are investigated. A small band gap is found in the electronic structure of fluorite RuO_2 and the bandwidth increases with the pressure. Also, the present mechanical and electronic properties demonstrate that the bonding nature is a combination of covalent, ionic, and metallic contributions.

cond-mat.mtrl-sci↗