arXiv subjects

D. Wu

Publications and source records attributed to D. Wu.

At least 37 records · Page 2Linked to original sources

arXiv2024

Ion Kinetics and Neutron Generation Associated with Electromagnetic Turbulence in Laboratory-scale Counter-streaming Plasmas

Electromagnetic turbulence and ion kinetics in counter-streaming plasmas hold great significance in laboratory astrophysics, such as turbulence field amplification and particle energization. Here, we quantitatively demonstrate for the first time how electromagnetic turbulence affects ion kinetics under achievable laboratory conditions (millimeter-scale interpenetrating plasmas with initial velocity of $2000\ \mathrm{km/s}$, density of $4 \times 10^{19}\ \mathrm{cm}^{-3}$, and temperature of $100\ \mathrm{eV}$) utilizing a recently developed high-order implicit particle-in-cell code without scaling transformation. It is found that the electromagnetic turbulence is driven by ion two-stream and filamentation instabilities. For the magnetized scenarios where an applied magnetic field of tens of Tesla is perpendicular to plasma flows, the growth rates of instabilities increase with the strengthening of applied magnetic field, which therefore leads to a significant enhancement of turbulence fields. Under the competition between the stochastic acceleration due to electromagnetic turbulence and collisional thermalization, ion distribution function shows a distinct super-Gaussian shape, and the ion kinetics are manifested in neutron yields and spectra. Our results have well explained the recent unmagnetized experimental observations, and the findings of magnetized scenario can be verified by current astrophysical experiments.

physics.plasm-ph↗

arXiv2024

Extended Time-Dependent Density Functional Theory for Multi-Body Densities

Time-dependent density functional theory (TDDFT) is widely used for understanding and predicting properties and behaviors of matter. As one of the fundamental theorems in TDDFT, van Leeuwen's theorem [Phys. Rev. Lett. 82, 3863 (1999)] guarantees how to construct a unique potential with the same one-body density evolution. Here we extend van Leeuwen's theorem by exploring truncation criteria in BBGKY-hierarchy. Our generalized theorem demonstrates the existence of a unique non-local potential to accurately reconstruct the multi-body density evolution in binary interacting systems. Under non-stringent conditions, truncation of the BBGKY-hierarchy equations aligns with the behavior of multi-body density evolution, and maintains consistency in the reduced equations. As one of applications within the extended TDDFT supported by our theorem, multiple excitation energy can be typically solved as the eigenvalue of a generalized Casida's equation. The extended TDDFT provides an accurate and first-principle framework capable of describing the kinetic processes of correlated system, including strongly coupled particle transport, multiple excitation and ionization processes.

cond-mat.stat-mech↗

arXiv2023

$^{197}$Au($γ,\,xn;\,x\,=\,1\thicksim9$) Reaction Cross Section Measurements using Laser-Driven Ultra-Intense $γ$-Ray Source

We present a new method for the measurements of photonuclear reaction flux-weighted average cross sections and isomeric ratios using a laser-driven bremsstrahlung $γ$-ray source. An ultra-bright ultra-fast 60$\,\thicksim\,$250 MeV bremsstrahlung $γ$-ray source was established using the 200 TW laser facility in the Compact Laser Plasma Accelerator Laboratory, Peking University, which could cover the energy range from knocking out neutrons to producing pions. Stable quasi-monoenergetic electron beams were generated via laser wakefield acceleration with a charge of 300$\,\thicksim\,$600 pC per shot. The averaged $γ$-ray intensities ($\geqslant$8 MeV) were higher than 10$^{8}$ per shot and the instantaneous intensities can reach above 10$^{19}$ s$^{-1}$ with a duration time about 6.7 ps. $^{65}$Cu($γ,\,n$)$^{64}$Cu and $^{27}$Al($γ,\,x$)$^{24}$Na reactions were used as $γ$-ray flux monitors in the experiments. The flux-weighted average cross sections and isomeric ratios of $^{197}$Au($γ,\,xn;\,x\,=\,1\thicksim9$) reactions were analyzed through activation measurements. The results showed good agreement with previous works and proved this method to be accurate. The $^{197}$Au($γ,\,xn;\,x\,=\,7\thicksim\,9$) reaction cross sections were first achieved with the highest threshold energy of 71.410 MeV. Theoretical cross sections of TALYS 1.9 were calculated to compare with experiment results. This method offered a unique way of gaining insight into photonuclear reaction research, especially for short-lived isomers which extremely lack experimental data.

nucl-ex↗

arXiv2023

Colliding of two high Mach-number quantum degenerate plasma jets

Colliding of two high Mach-number quantum degenerate plasmas is one of the most essential components in the double-cone ignition (DCI) inertial confinement fusion scheme, in which two highly compressed plasma jets from the cone-tips collide along with rapid conversion from the colliding kinetic energies to the internal energy of a stagnated isochoric plasma. Due to the effects of high densities and high Mach-numbers of the colliding plasma jets, quantum degeneracy and kinetic physics might play important roles and challenge the predictions of traditional hydrodynamic models. In this work, the colliding process of two high Mach number quantum degenerate Deuterium-plasma jets with sizable scale ($\sim 1000\ \si{μm}$, $\sim 300\ \si{ps}$, $\sim 100\ \si{g/cc}$, $\sim 300\ \si{km/s}$) were investigated with first-principle kinetic simulations and theoretical analyses. In order to achieve high-density compression, the colliding kinetic pressure should be significantly higher than the pressure raised by the quantum degeneracy. This means high colliding Mach numbers are required. However, when the Mach number is further increased, we surprisingly found a decreasing trend of density compression, due to kinetic effects. It is therefore suggested that there is theoretically optimal colliding velocity to achieve the highest density compression. Our results would provide valuable suggestions for the base-line design of the DCI experiments and also might be of relevance in some violent astrophysical processes, such as the merger of two white dwarfs.

physics.plasm-ph↗

arXiv2023

Stopping power of high-density alpha-particle clusters in warm dense deuterium-tritium fuels

The state of burning plasma had been achieved in inertial confinement fusion (ICF), which was regarded as a great milestone for high-gain laser fusion energy. In the burning plasma, alpha particles incident on the cryogenic (warm dense) fuels cannot be simply regarded as single particles, and the new physics brought about by the density effects of alpha particles should be considered. In this work, the collective interaction between them has been considered, namely the effect of the superposition of wake waves. The stopping power of alpha-particle clusters, i.e. the rate of energy loss per unit distance traveled has been calculated using both analytical and simulation approaches. In theory, we obtain the stopping power of alpha clusters in cryogenic (warm dense) fuel by the dielectric function method, which illustrates the importance of the effective interaction between particles. Simulation results using the LAPINS code show that the collective stopping power of the alpha cluster is indeed increased via coherent superposition of excitation fields (the excitation of high-amplitude wake waves). However, the comparison between simulation and theoretical results also illustrates a coherent-decoherent transition of the stopping power of the cluster. The initial conditions with various sizes and densities of the alpha clusters have been considered to verify the condition of decoherence transition. Our work provides a theoretical description of the transport properties of high-density alpha particles in warm dense cryogenic fuel and might give some theoretical guidance for the design of actual fusion processes.

physics.plasm-ph↗

arXiv2023

Pinch effect of self-generated magnetic fields in the quantum degenerate plasmas on the heating process of the double-cone ignition scheme

In the double-cone ignition scheme, compressed fuels in two head-on cones are ejected to collide, forming a colliding plasma with an isochoric distribution for rapid heating by high flux fast electrons from picosecond petawatt laser beams in the perpendicular direction from the cone axis. In this work, we investigate the effects of quantum degeneracy on the transport of fast electrons in the colliding plasma, which rapidly evolves from the quantum degenerate in the outer region of the plasma to the classical state in the concentric core region heated by the colliding fronts of the plasma jets. With large scale particle-in-cell simulations, it is found that the self-generated magnetic field generated by the transport of fast electrons in the quantum degenerate state at the outer region is much stronger than in the corresponding classical state with the same fuel density in the core region. Theoretical analysis of the growth of the self-generated magnetic field is developed to explain the simulation results. Such strong self-generated magnetic fields in the quantum degenerate states can pinch the axially injected fast electrons to deposit their energy in the concentric core region, improving the heating efficiency for fast ignition.

physics.plasm-ph↗

arXiv2023

A high-efficiency proton-boron fusion scheme taking into account the effects of quantum degeneracy

The proton-boron (p-$^{11}$B) reaction is regarded as the holy grail of advanced fusion fuels, since the primary reaction produces three $α$ particles with few neutrons and induced radio-activities from second order reactions. Compared to the Deuterium-Tritium reaction a much higher reaction temperature is required. Moreover, bremsstrahlung energy losses due to the high nuclear charge of boron deem it seemingly apparent than a fusion reactor based on Deuterium-Tritium plasma in equilibrium is to say the least very difficult.It is becoming more appealing to collide intense laser beams or accelerated proton beams with a boron target to produce p-$^{11}$B reactions. The fusion yield of p-$^{11}$B reactions is closely related to proton beam parameters and boron target conditions such as density, temperature, and ingredients. Quantum degeneracy will increase fusion yields by reducing the stopping power of injected protons. In this work, we suggest a high-efficiency scheme for beam-target p-$^{11}$B fusions via injecting a MeV proton beam into a highly compressed quantum degenerated boron target. Such a boron target can be achieved via quasi-isentropic compression of solid boron by using precisely shaped laser pulses. Our results indicate that for densities ranging from $10^3$ to $10^4ρ_s$, where $ρ_s$ is the density of solid boron, contributions of bound and free electrons to the stopping of protons can be completely disregarded and dramatically reduced respectively. The result is an increase in fusion yield by orders of magnitude. Furthermore, in order to achieve multiplication factor $F$ greater than one, with $F$ defined as the ratio of output fusion energy to the energy of injected protons, it is found there exits a minimum possible density of boron target, which is $2.15 \times 10^4 ρ_s$ when the kinetic energy of injected protons is $0.8$ MeV.

physics.plasm-ph↗

arXiv2023

Head-on collision of large-scale high density plasmas jets: a first-principle kinetic simulation approach

In the double-cone ignition (DCI) inertial confinement fusion scheme, head-on collision of high density plasma jets is one of the most distinguished feature when compared with other schemes. However, the application of traditional hydrodynamic simulation methods become limited. To overcome such limitations, we propose a new simulation method for large-scale high density plasmas. This method takes advantages of modern particle-in-cell simulation techniques and binary Monte Carlo collisions, including both long-range collective electromagnetic fields and short-range particle-particle interactions. Especially, in this method, the restrictions of simulation grid size and time step, which usually appear in a fully kinetic description, are eliminated. In addition, collisional coupling and state-dependent coefficients are also removed in this method. The correctness and robustness of the new simulation method are verified, by comparing with fully kinetic simulations at small scales and purely hydrodynamic simulations at large scale. Following the conceptual design of the DCI scheme, the colliding process of two plasma jets with initial density of 100 g/cc, initial thermal temperature of 70 eV, and counter-propagating velocity at 300 km/s is investigated using this new method. Quantitative values, including density increment, increased plasma temperature, confinement time at stagnation and conversion efficiency from the colliding kinetic energy to thermal energy, are obtained. These values agree with the recent experimental measurements at a reasonable range.

physics.plasm-ph↗

arXiv2023

Flat optical conductivity in the topological kagome magnet TbMn$_6$Sn$_6$

Kagome magnet TbMn$_6$Sn$_6$ is a new type of topological material that is known to support exotic quantum magnetic states. Experimental work has identified that TbMn$_6$Sn$_6$ hosts Dirac electronic states that could lead to topological and Chern quantum phases, but the optical response of the Dirac fermions of TbMn$_6$Sn$_6$ and its properties remain to be explored. Here, we perform optical spectroscopy measurement combined with first-principles calculations on single-crystal sample of TbMn$_6$Sn$_6$ to investigate the associated exotic phenomena. TbMn$_6$Sn$_6$ exhibits frequency-independent optical conductivity spectra in a broad range from 1800 to 3000 cm$^{-1}$ (220-370 meV) in experiments. The theoretical band structures and optical conductivity spectra are calculated with several shifted Fermi energy to compare with the experiment. The theoretical spectra with 0.56 eV shift for Fermi energy are well consistent with our experimental results. Besides, the massive quasi-two-dimensional (quasi-2D) Dirac bands, which have linear band dispersion in $k_x$-$k_y$ plane and no band dispersion along the $k_z$ direction, exist close to the shifted Fermi energy. According to tight-binding model analysis, the quasi-2D Dirac bands give rise to a flat optical conductivity, while its value is smaller than, about one tenth of, that from the calculations and experiments. It indicates that the other trivial bands also contribute to the flat optical conductivity.

cond-mat.str-el↗

arXiv2022

Revealing strong coupling of collective modes between superconductivity and pseudogap in cuprate superconductor by terahertz third harmonic generation

The study of interaction between different degrees of freedom in solids is of fundamental importance to understand the functionalities of materials. One striking example of such interaction is the intertwined coupling or competition between superconductivity (SC), charge density wave (CDW), pseudogap state (PG), and other exotic phases in cuprate superconductors. Recent emergence of nonlinear Terahertz (THz) third harmonic generation (THG) spectroscopy provides a powerful tool for exploring the collective (Higgs) modes of superconductivity order parameters, and its interaction with intertwined/competing phases. In this study, we report on nonlinear THz THG spectroscopy of the YBa$_2$Cu$_3$O$_{6+x}$ (YBCO) thin films with different doping. We identify a characteristic temperature $T_{THG}$, below which third order suscepetility $χ^{(3)}$ emerges. Notably, the $T_{THG}$ is coincident with the crossover temperature $T^*$ of pseudogap in a wide range doping of phase diagram. Upon entering the superconducting state, THG increases sharply but exhibits an abnormal dip feature near $T_c$ which is more clearly seen in optimally doped sample. Strikingly, we observe a beating structure directly in the measured real time waveform of THG signal. Fourier transformation of the time domain waveform gives two separate modes below and above original THG frequency. The observation strongly indicates that an additional mode, presumably Higgs mode, appears at $T_c$ and couples to the mode already developed below $T^*$. The strong coupling effect offers new insight into the interplay between superconductivity and pseudogap. The result unambiguously suggests that the pseudogap phase is not a precursor of superconductivity but represents a distinct order.

cond-mat.supr-con↗

arXiv2022

Linear-in-Frequency Optical Conductivity over a broad range in the three-dimensional Dirac semimetal candidate Ir$_2$In$_8$Se

The optical conductivity of the new Dirac semimetal candidate Ir$_2$In$_8$Se is measured in a frequency range from 40 to 30000 cm$^{-1}$ at temperatures from 300 K down to 10 K. The measurement reveals that the compound is a low carrier density metal. We find that the real part of the conductivity $σ_1(ω)$ is linear in frequency over a broad range from 500 to 4000 cm$^{-1}$ at 300 K and varies slightly with cooling. This linearity strongly suggests the presence of three-dimensional linear electronic bands with band crossings near the Fermi level. Band structure calculations indicate the presence of type-II Dirac points. By comparing our data with the optical conductivity computed from the band structure, we conclude that the observed linear dependence mainly originates from the Dirac cones and the transition between the Dirac cones and the next lower bands. In addition, a weak energy gap feature is resolved below the charge density wave phase transition temperature in reflectivity spectra. An enhanced structure arising from the imperfect Fermi surface nesting is identified in the electronic susceptibility function, suggesting a Fermi surface nesting driven instability.

cond-mat.str-el↗

arXiv2022

Vehicle-road Cooperative Simulation and 3D Visualization System

The safety of single-vehicle autonomous driving technology is limited due to the limits of perception capability of on-board sensors. In contrast, vehicle-road collaboration technology can overcome those limits and improve the traffic safety and efficiency, by expanding the sensing range, improving the perception accuracy, and reducing the response time. However, such a technology is still under development; it requires rigorous testing and verification methods to ensure the reliability and trustworthiness of the technology. In this thesis, we focus on three major tasks: (1) analyze the functional characteristics related to the scenarios of vehicle-road cooperations, highlightening the differences between vehicle-road cooperative systems and traditional single-vehicle autonomous driving systems; (2) refine and classifiy the functional characteristics of vehicle-road cooperative systems; (3) design and develop a simulation system, and provide a visual interface to facilitate development and analysis. The efficiency and effectiveness the proposed method are verfied by experiments.

cs.RO↗

arXiv2022

Optical spectroscopy and ultrafast pump-probe study of a quasi-one-dimensional charge density wave in CuTe

CuTe is a two-dimensional (2D) layered material, yet forming a quasi-one-dimensional (quasi-1D) charge-density-wave (CDW) along the a-axis in the ab-plane at high transition temperature $T_{CDW}=335$ K. However, the anisotropic properties of CuTe remain to be explored. Here we performed combined transport, polarized infrared reflectivity, and ultrafast pump-probe spectroscopy to investigate the underlying CDW physics of CuTe. Polarized optical measurement clearly revealed that an energy gap gradually forms along the a-axis upon cooling, while optical evidence of gap signature is absent along the b-axis, suggesting pronounced electronic anisotropy in this quasi-2D material. Time-resolved optical reflectivity measurement revealed that the amplitude and relaxation time of photo-excited quasiparticles change dramatically across the CDW phase transition. Taking fast Fourier transformation of the oscillation signals arising from collective excitations, we identify the 1.65-THz mode as the CDW amplitude mode, whose energy softens gradually at elevated temperatures. Consequently, we provide further evidence for the formation of completely anisotropic CDW order in CuTe, which is quite rare in quasi-2D materials.

cond-mat.str-el↗

arXiv2021

Optical spectroscopy and ultrafast pump-probe study of structural phase transition in 1T'-TaTe2

1T'-TaTe2 exhibits an intriguing first-order structural phase transition at around 170 K. Understanding the electronic structural properties is a crucial way to comprehend the origin of the structural phase transition. We performed a combined optical and ultrafast pump-probe study on the compound across the transition temperature. The phase transition leads to abrupt changes of both optical spectra and ultrafast electronic relaxation dynamics. The measurements revealed a sudden reconstruction of band structure. We elaborate that the phase transition is of the first order and can not be attributed to the conventional density-wave type instability. Our work is illuminating for understanding the origin of the structural phase transition.

cond-mat.str-el↗

arXiv2021

Unconventional hysteretic transition in a charge density wave

Hysteresis underlies a large number of phase transitions in solids, giving rise to exotic metastable states that are otherwise inaccessible. Here, we report an unconventional hysteretic transition in a quasi-2D material, EuTe4. By combining transport, photoemission, diffraction, and x-ray absorption measurements, we observed that the hysteresis loop has a temperature width of more than 400 K, setting a record among crystalline solids. The transition has an origin distinct from known mechanisms, lying entirely within the incommensurate charge-density-wave (CDW) phase of EuTe4 with no change in the CDW modulation periodicity. We interpret the hysteresis as an unusual switching of the relative CDW phases in different layers, a phenomenon unique to quasi-2D compounds that is not present in either purely 2D or strongly-coupled 3D systems. Our findings challenge the established theories on metastable states in density wave systems, pushing the boundary of understanding hysteretic transitions in a broken-symmetry state.

cond-mat.mtrl-sci↗

arXiv2021

Unconventional charge density wave and photoinduced lattice symmetry change in Kagome Metal CsV$_3$Sb$_5$ probed by time-resolved spectroscopy

Recently, kagome lattice metal AV$_3$Sb$_5$ (A = K, Rb, Cs) family has received wide attention due to its presence of superconductivity, charge density wave (CDW) and peculiar properties from topological nontrivial electronic structure. With time-resolved pump-probe spectroscopy, we show that the excited quasiparticle relaxation dynamics can be explained by formation of energy gap below the phase transition being similar to a usual second-order CDW condensate, by contrast, the structure change is predominantly first order phase transition. Furthermore, no CDW amplitude mode is identified in the ordered phase. The results suggest that the CDW order is very different from the traditional CDW condensate. We also find that weak pump pulse can non-thermally melt the CDW order and drive the sample into its high temperature phase, revealing the fact that the difference in lattice potential between those phases is small.

cond-mat.supr-con↗

arXiv2021

Photoinduced multistage phase transitions in Ta2NiSe5

Utrafast control of material physical properties represents a rapid developing field in condensed matter physics. Yet, accessing to the long-lived photoinduced electronic states is still in its early stage, especially with respect to an insulator to metal phase transition. Here, by combing transport measurement with ultrashort photoexcitation and coherent phonon spectroscopy, we report on photoinduced multistage phase transitions in Ta2NiSe5. Upon excitation by weak pulse intensity, the system is triggered to a short-lived state accompanied by a structural change. Further increasing the excitation intensity beyond a threshold, a photoinduced steady new state is achieved where the resistivity drops by more than four orders at temperature 50 K. This new state is thermally stable up to at least 350 K and exhibits the lattice structure different from any of the thermally accessible equilibrium states. Transmission electron microscopy reveals an in-chain Ta atom displacement in the photoinduced new structure phase. We also found that nano-sheet samples with the thickness less than the optical penetration depth are required for attaining a complete transition.

cond-mat.str-el↗

arXiv2020

Revealing ultra-strong magnon-photon coupling in a polar antiferromagnet Fe2Mo3O8 by time domain terahertz spectroscopy

Strong coupling between magnon and electromagnetic wave can lead to the formation of a coupled spinphoton quasiparticle named as magnon-polariton. The phenomenon is well studied for ferromagnetic systems inside microwave cavities in recent years. However, formation of magnon-polariton is rarely seen for an antiferromagnet (AFM) because the strong coupling condition is not easily fulfilled. Here we present time-domain terahertz measurement on a multiferroic polar antiferromagnet Fe2Mo3O8. We find clearly beating between two modes at frequencies above and below the electric-active magnon frequency below TN, which we assign to the formation of AFM magnon-polariton. An ultra-strong spin-photon coupling effect is derived based on the energy level splitting. However, the AFM magnon-polariton is absent in the frequency domain measurement. Our work reveals that the coherent magnon formation driven by the ultrashort THz pulse provides a new way to detect polariton mode splitting.

cond-mat.str-el↗