arXiv subjects

Z. L. Li

Publications and source records attributed to Z. L. Li.

At least 19 recordsLinked to original sources

arXiv2026

Angular momentum transfer in multiphoton pair production

We propose a method to accurately calculate the momentum distributions and the phase distributions of the probability amplitude for both boson and fermion pair production in a spatially homogeneous and time-dependent electric field. Applying this method to multiphoton pair production in a circularly polarized electric field rotating around the $z$-axis, we clarify that the topological charge appearing in the phase distribution of the probability amplitude for pair production reflects the orbital angular momentum (OAM) of the produced pairs rather than that of individual particles. On this basis, we demonstrate that, within the semiclassical framework, the $z$-component of the total angular momentum of the field and the particles is conserved, whereas the conservation of total angular momentum cannot be verified. The results also reveal that the pair production is also constrained by $C$-parity conservation, and that pairs with smaller OAM are produced more favorably. These findings provide deeper insight into angular momentum transfer in pair production.

hep-ph↗

arXiv2025

Electron-positron pair creation in a supercritical static asymmetric potential well

The electron-positron pair creation in a supercritical static asymmetric potential well, which is composed of a subcritical and a supercritical potential separated by a fixed distance, is investigated using computational quantum field theory. To explain the discrete peaks in the positron energy spectrum, an analytical formula for determining the positions of bound states in a subcritical asymmetric potential well is derived and extended to the supercritical asymmetric potential well in two ways. One of the two methods can not only predict the positions of bound states, but also offer the pair creation rate. This study also reveals that the subcritical potential height can optimize the energy spread of created electrons, providing a new way to produce high-energy electron beams with concentrated energy in experiments. Moreover, it is found that the pair creation rate in a supercritical asymmetric potential well, composed of a subcritical symmetric potential well and a supercritical Sauter potential, exceeds the sum of the pair creation rates produced by each potential individually. This finding suggests a potential method for enhancing pair yield.

quant-ph↗

arXiv2025

Enhancing pair production with optimized chirped laser fields

The optimal chirped field for enhancing electron-positron (EP) pair production is explored using a quantum kinetic approach. First, the momentum spectrum and number density of EP pairs produced by Gaussian chirped fields are investigated. The results show that the momentum spectrum exhibits distinct interference patterns, while the number density grows monotonically with chirp parameters but oscillates with the carrier angular frequency. Moreover, the number density increases by four orders of magnitude compared to chirp-free fields. The results are further compared with those from four other chirped fields: frequency-modulated, linear, quadratic, and sinusoidal chirp. The analysis reveals that the maximum number density for sinusoidally chirped fields is the highest, followed by Gaussian, frequency-modulated, quadratically, and linearly chirped fields. This ranking also applies to the maximum enhancement factors for these chirped fields. Notably, the number density for sinusoidally chirped fields improves nine orders of magnitude compared to chirp-free fields. These results not only deepen our understanding of pair production in chirped fields but also provide significant optimization strategies for future vacuum pair production experiments.

hep-ph↗

arXiv2025

Study of spin states in vacuum pair production via the Dirac-Heisenberg-Wigner formalism

A general spin-resolved momentum distribution of electron-positron pairs produced in strong external fields is derived by combining the covariant spin projection operator and the Dirac-Heisenberg-Wigner (DHW) formalism. The result shows that the spin-resolved and helicity-resolved momentum distributions given in previous literature are actually two special cases of it. For any spin-direction unit vector, numerical investigations demonstrate that when the $z$-component of the unit vector vanishes, the number density of produced spin-up and spin-down particles is equal, while their momentum distributions have some asymmetry. For a nonzero $z$-component of the unit vector, there is a difference of $1-3$ orders of magnitude in the number density of spin-up and spin-down particles induced by angular momentum transfer in multiphoton absorption. Moreover, as the electric field strength increases and/or the field frequency decreases, the asymmetry between the spin-up and spin-down particle number density decreases rapidly. These results offer an approach to study general spin states in vacuum pair production, and enhance our understanding of angular momentum transfer from fields to matter in extreme environments.

hep-ph↗

arXiv2025

Nonadiabatic quantum kinetic equations and Dirac-Heisenberg-Wigner formalism for Schwinger pair production in time-varying electric fields with multiple components

The nonadiabatic quantum kinetic equations and Dirac-Heisenberg-Wigner formalism for Schwinger pair production in a spatially uniform and time-varying electric field with multiple components are derived and proven to be equivalent. The relation between nonadiabatic and adiabatic quantum kinetic equations is also established. By analyzing the time evolution of the distribution functions of particles created in a circularly polarized Gaussian pulse field with a subcycle structure, it is found that the nonadiabatic and adiabatic distribution functions are the same after the field, with a sufficient number of oscillation cycles, fades away. However, during the presence of the field, the two distribution functions typically differ. Nonetheless, the time evolution characteristics of the nonadiabatic and adiabatic momentum distributions are similar. For instance, the number of spirals is one less than the number of photons absorbed in both cases. Furthermore, for a rapidly oscillating electric field, the nonadiabatic quantum kinetic approaches may provide a more meaningful description of pair production at intermediate times. These findings deepen our understanding of the nonadiabatic quantum kinetic approaches and their application in pair production.

hep-th↗

arXiv2024

Electron-hole pair production in graphene for two arbitrarily polarized electric fields with a time delay

The momentum distributions of electron-hole (EH) pair production in graphene for two arbitrarily polarized electric fields with a time delay are investigated employing a massless quantum kinetic equation and compared with the results obtained in electron-positron (EP) pair production from vacuum. For a single elliptically polarized electric field, the momentum distributions of created EH and EP pairs are similar in multiphoton absorption region. However, for two co-directional linearly polarized electric fields with a time delay and no field frequency, the momentum distribution of created EH pairs exhibits ring patterns, which is not present in EP pair production. For two circularly polarized fields with identical or opposite handedness, the momentum distributions of created EH pairs also show Ramsey interference and spiral structures, respectively. Different from EP pair production, the spiral structures are insensitive to the number of oscillation cycles in electric field pulses. For two elliptically polarized fields with same-sign or opposite-sign ellipticity, the momentum distributions of EH pairs are much more insensitive to ellipticity than those in EP pair production. These results provide further theoretical reference for simulating the EP pair production from vacuum in solid-state systems.

cond-mat.mes-hall↗

arXiv2024

Nonequilibrium carrier and phonon dynamics in the ferrimagnetic semiconductor Mn$_3$Si$_2$Te$_6$

We investigate the ultrafast carrier and phonon dynamics in the ferrimagnetic semiconductor Mn$_3$Si$_2$Te$_6$ using time-resolved optical pump-probe spectroscopy. Our results reveal that the electron-phonon thermalization process with a subpicosecond timescale is prolonged by the hot-phonon bottleneck effect. We identify the subsequent relaxation processes associated with two non-radiative recombination mechanisms, i.e., phonon-assisted electron-hole recombination and defect-related Shockley-Read-Hall recombination. Temperature-dependent measurements indicate that all three relaxation components show large variation around 175 and 78 K, which is related to the initiation of spin fluctuation and ferrimagnetic order in Mn$_3$Si$_2$Te$_6$. In addition, two pronounced coherent optical phonons are observed, in which the phonon with a frequency of 3.7 THz is attributed to the $A_{1g}$ mode of Te precipitates. Applying the strain pulse propagation model to the coherent acoustic phonons yields a penetration depth of 506 nm and a sound speed of 2.42 km/s in Mn$_3$Si$_2$Te$_6$. Our results develop understanding of the nonequilibrium properties of the ferrimagnetic semiconductor Mn$_3$Si$_2$Te$_6$, and also shed light on its potential applications in optoelectronic and spintronic devices.

cond-mat.mtrl-sci↗

arXiv2024

Nonadiabatic quantum Vlasov equation in spinor QED

The nonadiabatic quantum Vlasov equation in spinor QED is derived, and its relation to the well-known adiabatic one is established by three methods. One is by an explicitly analytical expression, the second is by the Dirac equation in the V gauge, and the last is by introducing a turn-off electric field. Wherein what the first two of them are given is an instantaneous relation. Moreover, the time evolution of the distribution function for a specific momentum and the momentum distribution of created particle pairs after turning off the electric field are calculated and compared with those in scalar QED. It is found that both the oscillation periods of the distribution functions in spinor and scalar QED equal pi divided by the total energy of a particle after the electric field is turned off. The momentum distributions in spinor and scalar QED show a novel oscillation and out-of-phase behavior that cannot be explained by the Stokes phenomenon. These findings will further deepen our understanding of the quantum Vlasov equation and its application in vacuum pair production.

hep-th↗

arXiv2023

Backreaction effect and plasma oscillation in pair production for rapidly oscillating electric fields

The backreaction effect and plasma oscillation in pair production for rapidly oscillating electric fields are investigated by solving quantum Vlasov equation. Contrary to previously thought, it is found that the backreaction effect can be neglected in the pair production for a rapidly oscillating but weak electric field, particularly, for a subcritical external electric field with frequency chirp. In some cases the oscillation period of created electron-positron plasma can be described by a simple formula constructed based on the Langmiur oscillation frequency, but it is impossible in general case because the plasma oscillation period directly depend not only on the final number density of created particles, but also on the external electric field parameters. Moreover, it is found that the momentum spectrum presents complex interferences after considering the backreaction. These results give us the safety range of external electric fields for taking no account of the backreaction effect and deepen our understanding of the pair production with the backreaction effect.

hep-ph↗

arXiv2022

Effects of control fields on the pair creation and the vacuum information transmission

The effects of control fields on the energy spectra and the number of created pairs and the information transmission by the Dirac vacuum modes are investigated by employing computational quantum field theory approach. It is found that the oscillation structures on the energy spectra are sensitive to the direction, the width, and the oscillation frequency of control fields. The pair yield can have obvious changes for a small frequency and a very large frequency. Moreover, the information encoded in the control fields, such as the field direction, the laser frequency and the time interval between two laser pulses, can also embodied by the vacuum modes in the change of pair-creation rate with time. These results not only can deepen our understanding of the control of pair creation and the information transmission, but also can provide a theoretical reference to the related experiments in the future.

quant-ph↗

arXiv2021

Study of pair production in inhomogeneous two-color electric fields using the computational quantum field theory

We first demonstrate theoretically that the computational quantum field theory is equivalent to the quantum kinetic theory for pair creation in a spatially homogeneous and time-dependent electric field, then verify numerically their equivalence for pair creation in one-dimensional time-dependent electric fields, and finally investigate detailedly the effects of the field frequency, spatial width, pulse duration, and relative phase on dynamically assisted Schwinger pair production in an inhomogeneous two-color electric field. It is found that the enhancement effect of pair creation is very sensitive to the field frequency and generally very obvious for a shorter field width, a longer pulse duration, and a relative phase of maximizing the field strength. These results can provide a significant reference for the optimal control theory of pair creation which aims to maximize the created pair yield within a given scope of field parameters.

hep-ph↗

arXiv2019

Boson pair production in arbitrarily polarized electric fields

The momentum spectrum and number density of created bosons for two types of arbitrarily polarized electric fields are calculated and compared with those of created fermions, employing the equal-time Feshbach-Villars-Heisenberg-Wigner formalism which is confirmed that for an uniform and time-varying electric field it is completely equivalent to the quantum Vlasov equation in scalar QED. For an elliptically polarized field, it is found that the number density of created bosons is a square root of the number density of spin-up electrons times that of spin-down ones for a circularly polarized multicycle field. Moreover, the degree of spin polarization roughly grows as the Keldysh adiabaticity parameter increases for arbitrarily polarized multicycle fields. For a field constituted of two circularly polarized fields with a time delay, it is shown that momentum vortices also exist in boson pair creation and are induced only by the orbital angular momentum of particles. However, the vortices can reproduce the quantum statistic effect due to the effect of spin of particles. These results further deepen the understanding of some significant signatures in pair production.

quant-ph↗

arXiv2019

Electron-positron pair production in frequency modulated laser fields

The momentum spectrum and the number density of created electron-positron pairs in a frequency modulated laser field are investigated using quantum kinetic equation. It is found that the momentum spectrum presents obvious interference pattern. This is an imprint of the frequency modulated field on the momentum spectrum, because the momentum peaks correspond to the pair production process by absorbing different frequency component photons. Moreover, the interference effect can also be understood qualitatively by analyzing turning point structures. The study of the pair number density shows that the number density is very sensitive to modulation parameters and can be enhanced by over two orders of magnitude for certain modulation parameters, which may provide a new way to increase the number of created electron-positron pairs in future experiments.

quant-ph↗

arXiv2017

Momentum vortices on pairs production by two counter-rotating fields

Multiphoton pair production is investigated by focusing on the momentum structures of produced pairs in the polarization plane for the two circularly polarized fields. Upon the momentum spectra, different from the concentric rings with the familiar Ramsey interference fringes for the same handedness, however, the obvious vortex structures are found constituted by the Archimedean spirals for two opposite handedness fields. The underlying physical reasons are analyzed and discussed. It is also found that the vortex patterns are sensitive to the relative carrier envelope phase, the time delay, and the handedness of two fields, which can be used to detect the applied laser field characteristics as a probe way.

quant-ph↗

arXiv2016

Pair production in strong SU(2) background fields

The fermion particle pair production in strong SU(2) gauged chromoelectric fields is studied by using Boltzmann-Vlasov equation in a classical way. The existence of pre-production process in a classical description is shown with the distribution evolution of non-Abelian particle production. It is interesting to find that the distribution center of particle number density is on two islands and has a split on color charge sphere as it evolutes and reaches a steady state at last, which is related to the amplitude and the varying of the field.

hep-ph↗

arXiv2015

Effects of electric field polarizations on pair production

The effects of electric field polarizations on pair production from a vacuum are investigated numerically by employing the real-time Dirac-Heisenberg-Wigner formalism. For few-cycle fields, it is found that the interference pattern in momentum spectra is absent and the circular distortion of momentum distribution becomes more apparent with the increase of polarization. For multi-cycle fields, it is found that the interference effects in momentum spectra are obvious. And as the polarization increases, the momentum distribution is split into two parts in the momentum $q_y$ direction first and then two separated parts are connected into a ring. The effects of polarizations on the number density of created particles exhibits two different characteristics. For a small laser frequency, the particle number density deceases with the polarization, while for a large laser frequency, the relation between them is sensitive to the field frequency nonlinearly. Some interpretations for the obtained results in this study, which is expected to be valuable to deepen the understanding of pair production in complex fields and also helpful for the study associated to strong-field ionization.

quant-ph↗

arXiv2015

Nonperturbative signatures in pair production for general elliptic polarization fields

The momentum signatures in nonperturbative multiphoton pair production for general elliptic polarization electric fields are investigated by employing the real-time Dirac-Heisenberg-Wigner formalism. For a linearly polarized electric field we find that the positions of the nodes in momenta spectra of created pairs depend only on the electric field frequency. The polarization of external fields could not only change the node structures or even make the nodes disappear but also change the thresholds of pair production. The momentum signatures associated to the node positions in which the even-number-photon pair creation process is forbid could be used to distinguish the orbital angular momentum of created pairs on the momenta spectra. These distinguishable momentum signatures could be relevant for providing the output information of created particles and also the input information of ultrashort laser pulses.

quant-ph↗

arXiv2014

Mass shift effects in nonperturbative multiphoton pair production for arbitrary polarized electric fields

The mass shift effects in multiphoton pair production of a nonperturbative nature for arbitrary polarized electric fields are investigated numerically by employing the real-time Dirac-Heisenberg-Wigner formalism, and theoretically by proposing an effective energy concept. It is found that the theoretical results are agreement with the numerical ones very well. It is the first time to consider the roles of the momenta of created particles and the polarizations of external fields played in the mass shift effects. These results can deepen the understanding of pair production in the nonperturbative threshold regime. Moreover, the distinct mass shift effects are observable in the forthcoming experiments and can be used as a probe to distinguish the electron-positron pair production from other background events.

hep-ph↗