arXiv subjects

Z. Wang

Publications and source records attributed to Z. Wang.

At least 181 records · Page 10Linked to original sources

arXiv2018

Thermoelectric transport properties of silicon: Towards an ab initio approach

We have combined the Boltzmann transport equation with an {\it ab initio} approach to compute the thermoelectric coefficients of semiconductors. Electron-phonon, ionized impurity, and electron-plasmon scattering rates have been taken into account. The electronic band structure and average intervalley deformation potentials for the electron-phonon coupling are obtained from the density functional theory. The linearized Boltzmann equation has then been solved numerically beyond the relaxation time approximation. Our approach has been applied to crystalline silicon. We present results for the mobility, Seebeck coefficient, and electronic contribution to the thermal conductivity, as a function of the carrier concentration and temperature. The calculated coefficients are in good quantitative agreement with experimental results.

cond-mat.mtrl-sci↗

arXiv2018

Adsorption of Organic Molecules on Onion-Like-Carbons: Insights on the Formation of Interstellar Hydrocarbons

Using atomistic simulations, we characterize the adsorption process of organic molecules on carbon nanoparticles, both of which have been reported to be abundant in the interstellar medium (ISM). It is found that the aromatic organics are adsorbed more readily than the aliphatic ones. This selectivity would favor the formation of polycyclic aromatic hydrocarbons (PAHs) or fullerene-like structures in the ISM due to structural similarity. It is also observed in our simulations that the molecules form a monolayer over the nanoparticle surface before stacking up in aggregates. This suggests a possible layer-by-layer formation process of onion-like nanostructures in the ISM. These findings reveal the possible role of carbon nanoparticles as selective catalysts that could provide reaction substrates for the formation of interstellar PAHs, high-fullerenes and soots from gas-phase molecules.

astro-ph.GA↗

arXiv2018

Monte Carlo Simulations of Trapped Ultracold Neutrons in the UCNτ Experiment

In the UCNτ experiment, ultracold neutrons (UCN) are confined by magnetic fields and the Earth's gravitational field. Field-trapping mitigates the problem of UCN loss on material surfaces, which caused the largest correction in prior neutron experiments using material bottles. However, the neutron dynamics in field traps differ qualitatively from those in material bottles. In the latter case, neutrons bounce off material surfaces with significant diffusivity and the population quickly reaches a static spatial distribution with a density gradient induced by the gravitational potential. In contrast, the field-confined UCN -- whose dynamics can be described by Hamiltonian mechanics -- do not exhibit the stochastic behaviors typical of an ideal gas model as observed in material bottles. In this report, we will describe our efforts to simulate UCN trapping in the UCNτ magneto-gravitational trap. We compare the simulation output to the experimental results to determine the parameters of the neutron detector and the input neutron distribution. The tuned model is then used to understand the phase space evolution of neutrons observed in the UCNτ experiment. We will discuss the implications of chaotic dynamics on controlling the systematic effects, such as spectral cleaning and microphonic heating, for a successful UCN lifetime experiment to reach a 0.01% level of precision.

physics.ins-det↗

arXiv2018

12-photon entanglement and scalable scattershot boson sampling with optimal entangled-photon pairs from parametric down-conversion

Entangled photon sources with simultaneously near-unity heralding efficiency and indistinguishability are the fundamental elements for scalable photonic quantum technologies. We design and realize a degenerate entangled-photon source from an ultrafast pulsed laser pumped spontaneous parametric down-conversion (SPDC), which show simultaneously ~97% heralding efficiency and ~96% indistinguishability between independent single photons. Such a high-efficiency and frequency-uncorrelated SPDC source allows generation of the first 12-photon genuine entanglement with a state fidelity of 0.572(24). We further demonstrate a blueprint of scalable scattershot boson sampling using 12 SPDC sources and a 12*12-modes interferometer for three-, four-, and five-boson sampling, which yields count rates more than four orders of magnitudes higher than all previous SPDC experiments. Our work immediately enables high-efficiency implementations of multiplexing, scattershot boson sampling, and heralded creation of remotely entangled photons, opening up a promising pathway to scalable photonic quantum technologies.

quant-ph↗

arXiv2018

Excess resistivity in graphene superlattices caused by umklapp electron-electron scattering

Umklapp processes play a fundamental role as the only intrinsic mechanism that allows electrons to transfer momentum to the crystal lattice and, therefore, provide a finite electrical resistance in pure metals. However, umklapp scattering has proven to be elusive in experiment as it is easily obscured by other dissipation mechanisms. Here we show that electron-electron umklapp scattering dominates the transport properties of graphene-on-boron-nitride superlattices over a wide range of temperatures and carrier densities. The umklapp processes cause giant excess resistivity that rapidly increases with increasing the superlattice period and are responsible for deterioration of the room-temperature mobility by more than an order of magnitude as compared to standard, non-superlattice graphene devices. The umklapp scattering exhibits a quadratic temperature dependence accompanied by a pronounced electron-hole asymmetry with the effect being much stronger for holes rather than electrons. Aside from fundamental interest, our results have direct implications for design of possible electronic devices based on heterostructures featuring superlattices.

cond-mat.mes-hall↗

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

Cavity Attenuators for Superconducting Qubits

Dephasing induced by residual thermal photons in the readout resonator is a leading factor limiting the coherence times of qubits in the circuit QED architecture. This residual thermal population, of the order of $10^{-1}$--$10^{-3}$, is suspected to arise from noise impinging on the resonator from its input and output ports. To address this problem, we designed and tested a new type of band-pass microwave attenuator that consists of a dissipative cavity well thermalized to the mixing chamber stage of a dilution refrigerator. By adding such a cavity attenuator inline with a 3D superconducting cavity housing a transmon qubit, we have reproducibly measured increased qubit coherence times. At base temperature, through Hahn echo experiment, we measured $T_{2\mathrm{e}}/2T_1 = 1.0\,({+0.0}/{-0.1})$ for two qubits over multiple cooldowns. Through noise-induced dephasing measurement, we obtained an upper bound $2\times 10^{-4}$ on the residual photon population in the fundamental mode of the readout cavity, which to our knowledge is the lowest value reported so far. These results validate an effective method for protecting qubits against photon noise, which can be developed into a standard technology for quantum circuit experiments.

quant-ph↗

arXiv2018

T0 Fan-out for Back-n White Neutron Facility at CSNS

the main physics goal for Back-n white neutron facility at China Spallation Neutron Source (CSNS) is to measure nuclear data. The energy of neutrons is one of the most important parameters for measuring nuclear data. Method of time of flight (TOF) is used to obtain the energy of neutrons. The time when proton bunches hit the thick tungsten target is considered as the start point of TOF. T0 signal, generated from the CSNS accelerator, represents this start time. Besides, the T0 signal is also used as the gate control signal that triggers the readout electronics. Obviously, the timing precision of T0 directly affects the measurement precision of TOF and controls the running or readout electronics. In this paper, the T0 fan-out for Back-n white neutron facility at CSNS is proposed. The T0 signal travelling from the CSNS accelerator is fanned out to the two underground experiment stations respectively over long cables. To guarantee the timing precision, T0 signal is conditioned with good signal edge. Furthermore, techniques of signal pre-emphasizing and equalizing are used to improve signal quality after T0 being transmitted over long cables with about 100 m length. Experiments show that the T0 fan-out works well, the T0 signal transmitted over 100 m remains a good time resolution with a standard deviation of 25 ps. It absolutely meets the required accuracy of the measurement of TOF.

physics.ins-det↗

arXiv2018

Electronics of Time-of-flight Measurement for Back-n at CSNS

Back-n is a white neutron experimental facility at China Spallation Neutron Source (CSNS). The time structure of the primary proton beam make it fully applicable to use TOF (time-of-flight) method for neutron energy measuring. We implement the electronics of TOF measurement on the general-purpose readout electronics designed for all of the seven detectors in Back-n. The electronics is based on PXIe (Peripheral Component Interconnect Express eXtensions for Instrumentation) platform, which is composed of FDM (Field Digitizer Modules), TCM (Trigger and Clock Module), and SCM (Signal Conditioning Module). T0 signal synchronous to the CSNS accelerator represents the neutron emission from the target. It is the start of time stamp. The trigger and clock module (TCM) receives, synchronizes and distributes the T0 signal to each FDM based on the PXIe backplane bus. Meantime, detector signals after being conditioned are fed into FDMs for waveform digitizing. First sample point of the signal is the stop of time stamp. According to the start, stop time stamp and the time of signal over threshold, the total TOF can be obtained. FPGA-based (Field Programmable Gate Array) TDC is implemented on TCM to accurately acquire the time interval between the asynchronous T0 signal and the global synchronous clock phase. There is also an FPGA-based TDC on FDM to accurately acquire the time interval between T0 arriving at FDM and the first sample point of the detector signal, the over threshold time of signal is obtained offline. This method for TOF measurement is efficient and not needed for additional modules. Test result shows the accuracy of TOF is sub-nanosecond and can meet the requirement for Back-n at CSNS.

physics.ins-det↗

arXiv2018

Low-Complexity Linear Precoding for Secure Spatial Modulation

In this work, we investigate linear precoding for secure spatial modulation. With secure spatial modulation, the achievable secrecy rate does not have an easy-to-compute mathematical expression, and hence, has to be evaluated numerically, which leads to high complexity in the optimal precoder design. To address this issue, an accurate and analytical approximation of the secrecy rate is derived in this work. Using this approximation as the objective function, two low-complexity linear precoding methods based on gradient descend (GD) and successive convex approximation (SCA) are proposed. The GD-based method has much lower complexity but usually converges to a local optimum. On the other hand, the SCA-based method uses semi-definite relaxation to deal with the non-convexity in the precoder optimization problem and achieves near-optimal solution. Compared with the existing GD-based precoder design in the literature that directly uses the exact and numerically evaluated secrecy capacity as the objective function, the two proposed designs have significantly lower complexity. Our SCA-based design even achieves a higher secrecy rate than the existing GD-based design.

eess.SP↗

arXiv2018

Direct observation of intravalley spin relaxation in single-layer WS$_2$

In monolayer Transition Metal Dichalcogenides (TMDs) the valence and conduction bands are spin split because of the strong spin-orbit interaction. In tungsten-based TMDs the spin-ordering of the conduction band is such that the so-called dark exciton, consisting of an electron and a hole with opposite spin orientation, has lower energy than the A exciton. A possible mechanism leading to the transition from bright to dark excitons involves the scattering of the electrons from the upper to the lower conduction band state in K. Here we exploit the valley selective optical selection rules and use two-color helicity-resolved pump-probe spectroscopy to directly measure the intravalley spin-flip relaxation dynamics of electrons in the conduction band of single-layer WS$_2$. This process occurs on a sub-ps time scale and it is significantly dependent on the temperature, indicative of a phonon-assisted relaxation. These experimental results are supported by time-dependent ab-initio calculations which show that the intra-valley spin-flip scattering occurs on significantly longer time scales only exactly at the K point. In a realistic situation the occupation of states away from the minimum of the conduction band leads to a dramatic reduction of the scattering time.

cond-mat.mtrl-sci↗

arXiv2018

Broadband terahertz generation via the interface inverse Rashba-Edelstein effect

Novel mechanisms for electromagnetic wave emission in the terahertz (THz) frequency regime emerging at the nanometer scale have recently attracted intense attention for the purpose of searching next-generation broadband THz emitters. Here, we report a new mechanism for broadband THz emission, utilizing the interface inverse Rashba-Edelstein effect. By engineering the symmetry of the Ag/Bi Rashba interface, we demonstrate a controllable THz radiation (~0.1-5 THz) waveform emitted from metallic Fe/Ag/Bi heterostructures following photo-excitation. We further reveal that this type of THz radiation can be selectively superimposed on the emission discovered recently due to the inverse Spin Hall effect, yielding a unique film thickness dependent emission pattern. Our results thus offer new opportunities for versatile broadband THz radiation using the interface quantum effects.

cond-mat.mtrl-sci↗

arXiv2018

Intrinsic charge dynamics in high-Tc AFeAs(O,F) superconductors

We report the first determination of the in-plane complex optical conductivity of 1111 high-Tc superconducting iron oxypnictide single crystals PrFeAsO0.6F0.35 and thin films SmFeAsO1-xFx by means of bulk-sensitive conventional and micro-focused infrared spectroscopy, ellipsometry, and time-domain THz transmission spectroscopy. A strong itinerant contribution is found to exhibit a dramatic difference in coherence between the crystal and the film. Using extensive temperature-dependent measurements of THz transmission we identify a previously undetected 2.5-meV collective mode in the optical conductivity of SmFeAs(O,F), which is strongly suppressed at Tc and experiences an anomalous T-linear softening and narrowing below T*=110K>>Tc. The suppression of the infrared absorption in the superconducting state reveals a large optical superconducting gap with a similar gap ratio 2D=kBTc=7 in both materials, indicating strong pairing.

cond-mat.supr-con↗

arXiv2018

Search for dark matter decay of the free neutron from the UCNA experiment: n $\rightarrow χ+ e^+e^-$

It has been proposed recently that a previously unobserved neutron decay branch to a dark matter particle ($χ$) could account for the discrepancy in the neutron lifetime observed in experiments that use two different measurement techniques. One of the possible final states discussed includes a single $χ$ along with an $e^{+}e^{-}$ pair. We use data from the UCNA (Ultracold Neutron Asymmetry) experiment to set limits on this decay channel. Coincident electron-like events are detected with $\sim 4π$ acceptance using a pair of detectors that observe a volume of stored Ultracold Neutrons (UCNs). The summed kinetic energy ($E_{e^{+}e^{-}}$) from such events is used to set limits, as a function of the $χ$ mass, on the branching fraction for this decay channel. For $χ$ masses consistent with resolving the neutron lifetime discrepancy, we exclude this as the dominant dark matter decay channel at $\gg~5σ$ level for $100~\text{keV} < E_{e^{+}e^{-}} < 644~\text{keV}$. If the $χ+e^{+}e^{-}$ final state is not the only one, we set limits on its branching fraction of $< 10^{-4}$ for the above $E_{e^{+}e^{-}}$ range at $> 90\%$ confidence level.

nucl-ex↗

arXiv2018

PFA Oriented ECAL Optimization for the CEPC

The design and optimization of the Electromagnetic Calorimeter (ECAL) are crucial for the Circular Electron Positron Collider (CEPC) project, a proposed future Higgs/Z factory. Following the reference design of the International Large Detector (ILD), a set of silicon-tungsten sampling ECAL geometries are implemented into the Geant4 simulation, whose performance is then scanned using Arbor algorithm. At single particle level, the photon energy response at different ECAL longitudinal structures is analyzed. At bi-particle sample, the separation performance with different ECAL transverse cell sizes is investigated and parametrized. The overall performance is characterized by a set of physics benchmarks, including $ννH$ events where Higgs boson decays into a pair of photons (EM objects) or gluons (jets) and $Z\toτ^+τ^-$ events. Based on these results, we proposed an optimized ECAL geometry for the CEPC project.

physics.ins-det↗

arXiv2018

Measurement of the neutron lifetime using an asymmetric magneto- gravitational trap and in situ detection

The precise value of the mean neutron lifetime, $τ_n$, plays an important role in nuclear and particle physics and cosmology. It is a key input for predicting the ratio of protons to helium atoms in the primordial universe and is used to search for new physics beyond the Standard Model of particle physics. There is a 3.9 standard deviation discrepancy between $τ_n$ measured by counting the decay rate of free neutrons in a beam (887.7 $\pm$ 2.2 s) and by counting surviving ultracold neutrons stored for different storage times in a material trap (878.5$\pm$0.8 s). The experiment described here eliminates loss mechanisms present in previous trap experiments by levitating polarized ultracold neutrons above the surface of an asymmetric storage trap using a repulsive magnetic field gradient so that the stored neutrons do not interact with material trap walls and neutrons in quasi-stable orbits rapidly exit the trap. As a result of this approach and the use of a new in situ neutron detector, the lifetime reported here (877.7 $\pm$ 0.7 (stat) +0.4/-0.2 (sys) s) is the first modern measurement of $τ_n$ that does not require corrections larger than the quoted uncertainties.

nucl-ex↗

arXiv2018

Search for the Neutron Decay n$\rightarrow$ X+$γ$ where X is a dark matter particle

In a recent paper submitted to Physical Review Letters, Fornal and Grinstein have suggested that the discrepancy between two different methods of neutron lifetime measurements, the beam and bottle methods can be explained by a previously unobserved dark matter decay mode, n$\rightarrow$ X+$γ$ where X is a dark matter particle. We have performed a search for this decay mode over the allowed range of energies of the monoenergetic gamma ray for X to be a dark matter particle. We exclude the possibility of a sufficiently strong branch to explain the lifetime discrepancy with greater than 4 sigma confidence.

nucl-ex↗

arXiv2018

Development of a New Gas Puff Imaging Diagnostic on the HL-2A Tokamak

A new gas puff imaging (GPI) diagnostic has been developed on the HL-2A tokamak to study two-dimensional plasma edge turbulence in poloidal vs. radial plane. During a discharge, neutral helium or deuterium gas is puffed at the edge of the plasma through a rectangular multi-capillary nozzle to generate a gas cloud on the observing plane. Then a specially designed telescope and a high-speed camera are used to observe and photograph the emission from the neutral gas cloud. The brightness and contrast in the 2-D poloidal vs. radial frames reveal the structures and movements of the turbulence. The diagnostic was put into the first experiment during the latest campaign and successfully captured blob structures of different shapes and sizes in scrape-off layer (SOL).

physics.plasm-ph↗