arXiv subjects

J. Wang

Publications and source records attributed to J. Wang.

At least 253 records · Page 14Linked to original sources

arXiv2018

Statistical analysis of experimental multifragmentation events in $^{64}$Zn + $^{112}$Sn at 40 MeV/nucleon

A statistical multifragmentation model (SMM) is applied to the experimentally observed multifragmentation events in an intermediate heavy ion reaction.Using the temperature and symmetry energy extracted from the isobaric yield ratio (IYR) method based on the Modified Fisher Model (MFM), SMM is applied to the reaction $^{64}$Zn + $^{112}$Sn at 40 MeV/nucleon. The experimental isotope distribution and mass distribution of the primary reconstructed fragments are compared without afterburner and they are well reproduced. The extracted temperature $T$ and symmetry energy coefficient $a_{sym}$ from SMM simulated events, using the IYR method, are also consistent with those from the experiment. These results strongly suggest that in the multifragmentation process there is a freezeout volume, in which the thermal and chemical equilibrium is established before or at the time of the intermediate-mass fragments emission.

nucl-ex↗

arXiv2018

Efimov Physics in Quenched Unitary Bose Gases

We study the impact of three-body physics in quenched unitary Bose gases, focusing on the role of the Efimov effect. Using a local density model, we solve the three-body problem and determine three-body decay rates at unitary, finding density-dependent, log-periodic Efimov oscillations, violating the expected continuous scale-invariance in the system. We find that the breakdown of continuous scale-invariance, due to Efimov physics, manifests also in the earliest stages of evolution after the interaction quench to unitarity, where we find the growth of a substantial population of Efimov states for densities in which the interparticle distance is comparable to the size of an Efimov state. This agrees with the early-time dynamical growth of three-body correlations at unitarity [Colussi {\em et al}., Phys. Rev. Lett. 120, 100401 (2018)]. By varying the sweep rate away from unitarity, we also find a departure from the usual Landau-Zener analysis for state transfer when the system is allowed to evolve at unitarity and develop correlations.

physics.atom-ph↗

arXiv2018

Modification of structural disorder by hydrostatic-pressure in superconducting YBa$_{2}$Cu$_{3}$O$_{6.73}$ cuprate

Compelling efforts to improve the critical temperature ($T_{c}$) of superconductors have been made through high-pressure application. Understanding the underlying mechanism behind such improvements is critically important, however, much remains unclear. Here we studied ortho-III YBa$_{2}$Cu$_{3}$O$_{6.73}$ (YBCO) using x-ray scattering under hydrostatic-pressure (HP) up to ~6.0 GPa. We found the reinforced oxygen order (OO) of YBCO under HP, revealing an oxygen rearrangement in the Cu-O layer, which evidently shows the charge transfer phenomenon between the CuO$_{2}$ plane and Cu-O layer. Concurrently, we also observed no disorder-pinned charge density wave (CDW) signature in CuO$_{2}$ plane under HP. This indicates that the oxygen rearrangement modifies the quenched disorder state in the CuO$_{2}$ plane. Using these results, we appropriately explain why pressure-condition can achieve higher $T_{c}$ compared with the optimal $T_{c}$ under ambient pressure in YBa$_{2}$Cu$_{3}$O$_{6+x}$. As an implication of these results, finally, we have discussed that the change in disorder could make it easier for YBa$_{2}$Cu$_{3}$O$_{6+x}$ to undergo a transition to the nematic order under an external magnetic field.

cond-mat.supr-con↗

arXiv2018

Ultrafast Terahertz Conductivity Probes of Topologically Enhanced Surface Transport Driven by Mid-Infrared Laser Pulses in Bi$_2$Se$_3$

The recent discovery of topology-protected charge transport of ultimate thinness on surfaces of three-dimensional topological insulators (TIs) are breaking new ground in fundamental quantum science and transformative technology. Yet a challenge remains on how to isolate and disentangle helical spin transport on the surface from bulk conduction. Here we show that selective midinfrared femtosecond photoexcitation of exclusive intraband electronic transitions at low temperature underpins topological enhancement of terahertz (THz) surface transport in doped Bi2Se3, with no complication from interband excitations or need for controlled doping. The unique, hot electron state is characterized by conserved populations of surface/bulk bands and by frequency-dependent hot carrier cooling times that directly distinguish the faster surface channel than the bulk. We determine the topological enhancement ratio between bulk and surface scattering rates, i.e., $γ_\text{BS}/γ_\text{SS}\sim$3.80 in equilibrium. These behaviors are absent at elevated lattice temperatures and for high pumpphoton frequencies and uences. The selective, mid-infrared-induced THz conductivity provides a new paradigm to characterize TIs and may apply to emerging topological semimetals in order to separate the transport connected with the Weyl nodes from other bulk bands.

cond-mat.mtrl-sci↗

arXiv2018

Hundredfold Enhancement of Light Emission via Defect Control in Monolayer Transition-Metal Dichalcogenides

Two dimensional (2D) transition-metal dichalcogenide (TMD) based semiconductors have generated intense recent interest due to their novel optical and electronic properties, and potential for applications. In this work, we characterize the atomic and electronic nature of intrinsic point defects found in single crystals of these materials synthesized by two different methods - chemical vapor transport and self-flux growth. Using a combination of scanning tunneling microscopy (STM) and scanning transmission electron microscopy (STEM), we show that the two major intrinsic defects in these materials are metal vacancies and chalcogen antisites. We show that by control of the synthetic conditions, we can reduce the defect concentration from above $10^{13} /cm^2$ to below $10^{11} /cm^2$. Because these point defects act as centers for non-radiative recombination of excitons, this improvement in material quality leads to a hundred-fold increase in the radiative recombination efficiency.

cond-mat.mtrl-sci↗

arXiv2018

Theory of Ultrafast Spin--Charge Quantum Dynamics in Strongly Correlated Systems Controlled by Femtosecond Photoexcitation: an Application to Insulating Antiferromagnetic Manganites

We use a non-equilibrium many-body theory that engages the elements of transient coherence, correlation, and nonlinearity to describe changes in the magnetic and electronic phases of strongly correlated systems induced by femtosecond nonlinear photoexcitation. Using a generalized tight--binding mean field approach based on Hubbard operators and including the coupling of the laser field, we describe a mechanism for simultaneous insulator--to--metal and anti-- to ferro--magnetic transition to a transient state triggered by non-thermal ultrafast spin and charge coupled excitations. We demontrate, in particular, that photoexcitation of composite fermion quasiparticles induces quasi-instantaneous spin canting that quenches the energy gap of the antiferromagnetic insulator and acts as a nonadiabatic "initial condition" that triggers non-thermal lattice dynamics leading to an insulator to metal and antiferromagnetic (AFM) to ferromagnetic (FM) transitions. Our theoretical predictions are consistent with recent ultrafast pump-probe spectroscopy experiments that revealed a magnetic phase transition during 100fs laser pulse photoexcitation of the CE--type AFM insulating phase of colossal magnetoresistive manganites. In particular, experiment observes two distinct charge relaxation components, fs and ps, with nonlinear threshold dependence at a pump fluence threshold that coincides with that for femtosecond magnetization photo excitation. Our theory attributes the correlation between femtosecond spin and charge nonlinearity leading to transition in the magnetic and electronic state to spin/charge/lattice coupling and laser-induced quantum spin canting that accompanies the driven population inversion between two quasi--particle bands with different properties: a mostly occupied polaronic band and a mostly empty metallic band, whose dispersion is determined by quantum spin canting.

cond-mat.str-el↗

arXiv2018

Non-equilibrium Pair Breaking in Ba(Fe1-xCox)2As2 Superconductors: Evidence for Formation of Photo-Induced Excitonic Spin-Density-Wave State

Ultrafast terahertz (THz) pump{probe spectroscopy reveals unusual out-of-equilibrium Cooper pair dynamics driven by femtosecond (fs) optical quench of superconductivity (SC) in iron pnictides. We observe a two{step quench of the SC gap, where an abnormally slow (many 100's of ps) quench process is clearly distinguished from the usual fast (sub-ps) hot{phonon{mediated scattering channel. This pair breaking dynamics depends strongly on doping, pump uence, and temperature. The above observations, together with quantum kinetic modeling of non-equilibrium SC and magnetic correlations, provide evidence for photogeneration of a transient state where SC competes with build{up of spin-density-wave (SDW) excitonic correlation between quasi-particles (QP).

cond-mat.supr-con↗

arXiv2018

The statistical characteristics of static friction

Friction is one of the fundamental issues in physics, mechanics and material science with lots of practical applications. However, the understanding of macroscopic friction phenomena from microscopic aspect is still on the way. In this work, molecular dynamic simulations are performed to investigate the static friction between two planar crystal surfaces. The friction force experienced by each atom is tracked and the statistical characteristics of atomic friction force are illuminated. More importantly, the influences of normal load and temperature on the statistical features are generalized. This study provides a new insight on the micro-states of friction.

cond-mat.stat-mech↗

arXiv2018

Identification of SDSSJ141324.27+530527.0 as A New "Changing-Look" Quasar with a "Turn-on" Transition

We here report an identification of SDSSJ141324+530527.0 (SBS1411+533) at $z=0.456344$ as a new "changing-look" quasar with a "turn-on" spectral type transition from Type-1.9/2 to Type-1 within a rest frame time scale of 1-10 yr by a comparison of our new spectroscopic observation and the Sloan Digital Sky Survey (SDSS) archive data base. The SDSS DR7 spectrum taken in 2003 is dominated by a starlight emission from host galaxies redward of the Balmer limit, and has non-detectable broad H$β$ line. The new spectrum taken by us on June 1st, 2017 and SDSS DR14 spectrum taken on May 29, 2017 indicate that the object is of a typical quasar spectrum with a blue continuum and strong Balmer broad emission lines. In addition, an intermediate spectral type can be identified in the SDSS DR13 spectrum taken in 2015. The invariability of the line wing of MgII$\lambda2800$ emission and time scale argument (The invariability of [OIII]$λ$5007 line blue asymmetry) suggests that a variation of obscuration (an accelerating outflow) is not a favorable scenario. The time scale argument allows us to believe the type transition is possibly caused by either a viscous radial inflow or a disk instability around a $\sim5-9\times10^{7}M_\odot$ black hole.

astro-ph.GA↗

arXiv2018

Quantized spin pump on helical edge states of a topological insulator

We report a theoretical study of the quantized spin pump in a traditional two-parameter quantum pump device that is based on the helical edge states of a quantum spin Hall insulator. By introducing two time-dependent magnetizations out of phase as the pumping parameters, we found that when the Fermi energy resides in the energy gap opened by magnetization, an integer number of charges or spins can be pumped out in a pumping cycle and ascribed to the possible topological interface state born in between the two pumping potentials. The quantized pump current can be fully spin-polarized, spin-unpolarized, or pure spin current while its direction can be abruptly reversed by some system parameters such as the pumping phase and local gate voltage. Our findings may shed light on generation of a quantized spin pump.

cond-mat.mes-hall↗

arXiv2018

A Global Photoionization Response to Prompt Emission and Outliers: Different Origin of Long Gamma-ray Bursts?

By using the line ratio \ion{C}{4}$\lambda1549$/\ion{C}{2}$\lambda1335$ as a tracer of ionization ratio of the interstellar medium (ISM) illuminated by a long gamma-ray burst (LGRB), we identify a global photoionization response of the ionization ratio to the photon luminosity of the prompt emission assessed by either $L_{\mathrm{iso}}/E_{\mathrm{peak}}$ or $L_{\mathrm{iso}}/E^2_{\mathrm{peak}}$. The ionization ratio increases with both $L_{\mathrm{iso}}/E_{\mathrm{peak}}$ and $L_{\mathrm{iso}}/E^2_{\mathrm{peak}}$ for a majority of the LGRBs in our sample, although there are a few outliers. The identified dependence of \ion{C}{4}/\ion{C}{2} on $L_{\mathrm{iso}}/E^2_{\mathrm{peak}}$ suggests that the scatter of the widely accepted Amati relation is related with the ionization ratio in ISM. The outliers tend to have relatively high \ion{C}{4}/\ion{C}{2} values as well as relatively high \ion{C}{4}$\lambda1549$/\ion{Si}{4}$\lambda1403$ ratios, which suggests an existence of Wolf-Rayet stars in the environment of these LGRBs. We finally argue that the outliers and the LGRBs following the identified \ion{C}{4}/\ion{C}{2}$-L_{\mathrm{iso}}/E_{\mathrm{peak}}$ ($L_{\mathrm{iso}}/E^2_{\mathrm{peak}}$) correlation might come from different progenitors with different local environments.

astro-ph.HE↗

arXiv2018

Searching for Exoplanets Using a Microresonator Astrocomb

Detection of weak radial velocity shifts of host stars induced by orbiting planets is an important technique for discovering and characterizing planets beyond our solar system. Optical frequency combs enable calibration of stellar radial velocity shifts at levels required for detection of Earth analogs. A new chip-based device, the Kerr soliton microcomb, has properties ideal for ubiquitous application outside the lab and even in future space-borne instruments. Moreover, microcomb spectra are ideally suited for astronomical spectrograph calibration and eliminate filtering steps required by conventional mode-locked-laser frequency combs. Here, for the calibration of astronomical spectrographs, we demonstrate an atomic/molecular line-referenced, near-infrared soliton microcomb. Efforts to search for the known exoplanet HD 187123b were conducted at the Keck-II telescope as a first in-the-field demonstration of microcombs.

physics.optics↗

arXiv2017

Optimal Orientations of Quartz Crystals for Bulk Acoustic Wave Resonators with the Consideration of Thermal Properties

Piezoelectric crystals are widely used for acoustic wave resonators of different functioning modes and types including BAW and SAW. It is well-known that only some special orientations of crystals will exhibit desirable properties such as mode couplings, thermal sensitivity, acceleration sensitivity, and others that are important in design and applications of resonators. With extensive studies on physical properties in last decades and increasing industrial needs of novel products, it is necessary to comb the known knowledge of quartz crystal material for novel orientations and better products as agendas in the industry. With known material properties like elastic, piezoelectric, dielectric, and thermal constants, we can establish the relationships between vibrations and bias fields such as temperature to ensure a resonator immunizing from excessive response to changes causing significant degradation of resonator properties and performances. Since the theoretical framework of wave propagation in piezoelectric solids is known, we need to use the existing data and results for the validation of current orientations in actual products. The agreement will give us needed confidence of the theory and analytical procedures. Through rotations, we calculated physical properties as functions of angles and bias fields, enabling the calculation of resonator properties for the identification of optimal cuts. Such a procedure can also be applied to similar crystals for a careful examination of possible orientations to maximize the potential use of materials in acoustic wave resonators.

cond-mat.mtrl-sci↗

arXiv2017

Toward a determination of the proton-electron mass ratio from the Lamb-dip measurement of HD

Precision spectroscopy of the hydrogen molecule is a test ground of quantum electrodynamics (QED), and may serve for determination of fundamental constants. Using a comb-locked cavity ring-down spectrometer, for the first time, we observed the Lamb-dip spectrum of the R(1) line in the overtone of HD. The line position was determined to be 217 105 182.79 $\pm0.03_{stat}\pm0.08_{syst}$ MHz ($δν/ν=4\times 10^{-10}$), which is the most accurate transition ever measured for the hydrogen molecule. Moreover, from calculations including QED effects up to the order $m_eα^6$, we obtained predictions for this R(1) line as well as for the HD dissociation energy, which are less accurate but signaling the importance of the complete treatment of nonadiabatic effects. Provided that the theoretical calculation reaches the same accuracy, the present measurement will lead to a determination of the proton-electron mass ratio with a precision of 1.3 parts per billion.

physics.atom-ph↗

arXiv2017

Interplay of chiral and helical states in a Quantum Spin Hall Insulator lateral junction

We study the electronic transport across an electrostatically-gated lateral junction in a HgTe quantum well, a canonical 2D topological insulator, with and without applied magnetic field. We control carrier density inside and outside a junction region independently and hence tune the number and nature of 1D edge modes propagating in each of those regions. Outside the 2D gap, magnetic field drives the system to the quantum Hall regime, and chiral states propagate at the edge. In this regime, we observe fractional plateaus which reflect the equilibration between 1D chiral modes across the junction. As carrier density approaches zero in the central region and at moderate fields, we observe oscillations in resistance that we attribute to Fabry-Perot interference in the helical states, enabled by the broken time reversal symmetry. At higher fields, those oscillations disappear, in agreement with the expected absence of helical states when band inversion is lifted.

cond-mat.mes-hall↗

arXiv2017

A Study of Outflows in Luminous Quasars at Redshift $\sim0.4-0.8$

We perform a systematic study of outflow in the narrow-line region (NLR) of active galactic nuclei (AGNs) at $z\sim0.4-0.8$ basing upon a large sample of $\sim900$ quasars at $z\sim 0.4-0.8$. The sample is extracted from the Sloan Digital Sky Survey by mainly requiring 1) the g-band magnitude is brighter than 19 magnitude; and 2) the [OIII]$\lambda5007$ emission line has a signal-to-noise ration larger than 30. Profiles of multiple emission lines are modeled by a sum of several Gaussian functions. The spectral analysis allows us to identify 1) a prevalence of both [OIII]$\lambda5007$ line blue asymmetry and bulk velocity blueshift of both [NeIII]$\lambda3869$ and [NeV]$\lambda3426$ lines, when the [\ion{O}{2}]$\lambda3727$ line is used as a reference. The velocity offset of [\ion{O}{3}]$\lambda5007$ line is, however, distributed around zero value, except for a few outliers. 2) not only the significant [OIII]$\lambda5007$ line asymmetry, but also the large bulk velocity offsets of [NeIII]$\lambda3869$ and [NeV]$\lambda3426$ emission lines tend to occur in the objects with high $L/L_{\mathrm{Edd}}$, which is considerably consistent with the conclusions based on local AGNs. With three $M_{\mathrm{BH}}$ estimation methods, the significance level of the trend is found to be better than $2.9σ$, $3.2σ$ and $1.8σ$ for [OIII], [NeIII] and [NeV], respectively. \rm After excluding the role of radio jets, the revealed dependence of NLR gas outflow on $L/L_{\mathrm{Edd}}$ allows us to argue that the pressure caused by the wind/radiation launched/emitted from central supermassive black hole is the most likely origin of the outflow in these distant quasars, which implies that the outflow in luminous AGNs up to $z\sim1$ have the same origin.

astro-ph.GA↗

arXiv2017

Determining the Elemental and Isotopic Composition of the preSolar Nebula from Genesis Data Analysis: The Case of Oxygen

We compare element and isotopic fractionations measured in solar wind samples collected by NASA's Genesis mission with those predicted from models incorporating both the ponderomotive force in the chromosphere and conservation of the first adiabatic invariant in the low corona. Generally good agreement is found, suggesting that these factors are consistent with the process of solar wind fractionation. Based on bulk wind measurements, we also consider in more detail the isotopic and elemental abundances of O. We find mild support for an O abundance in the range 8.75 - 8.83, with a value as low as 8.69 disfavored. A stronger conclusion must await solar wind regime specific measurements from the Genesis samples.

astro-ph.SR↗

arXiv2017

SDSS J090152.05+624342.6: A New "Overlapping-Trough" FeLoBAL Quasar At Z$\sim2$

We here report an identification of SDSS J090152.04+624342.6 as a new "overlapping-trough" iron low-ionization broad absorption line quasar at redshift of $z\sim2.1$. No strong variation of the broad absorption lines can be revealed through the two spectra taken by the Sloan Digital Sky Survey with a time interval of $\sim6$yr. Further optical and infrared spectroscopic study on this object is suggested.

astro-ph.GA↗