Quantum Kinetic Theory Research Articles

Fully quantum-kinetic theory of the steady-state cooperative photoluminescence from two near-identical emitters

We have developed a theory of cooperative photoluminescence from two closely spaced particles using the density matrix method. The theory provides simulations of the photoluminescence excitation and emission spectra by nonidentical quantum emitters entangled by the dipole–dipole interaction in presence of a cw laser beam. The excitation spectra are shown to be consistent with all known experiments to date with pairs of coherently coupled organic molecules in thin solid layers. The simulations were performed with the use of the Bogoliubov–Born–Green–Kirkwood–Yvon (BBGKY) hierarchies for reduced density matrices and correlation operators of the quantum emitters and photonic subsystems. This method gives a straightforward way to describe the dipole–dipole coupling and entanglement of the emitters. We have also calculated the emission spectra to demonstrate the expected spectral patterns.

Derivation of dirac exchange interaction potential from quantum plasma kinetic theory

The Dirac exchange interaction is derived from recent quantum kinetic theory for collisionless plasmas. For this purpose, the kinetic equation is written in the semiclassical and long wavelength approximations. The validity of the model for real systems is worked out, in terms of temperature and density parameters. Within the region of applicability, the correlation potential energy is shown to be always smaller than the exchange contribution. From the moments of the quantum kinetic equations, macroscopic, hydrodynamic equations are found, for an electron-ion plasma. The Dirac exchange term is explicitly derived, in the case of a completely degenerate electron gas. These results show, within quantum kinetic theory for charged particle systems, a new view of the Dirac exchange interaction frequently used in density functional theory parametrization. Finally, a simpler form of the quantum plasma exchange kinetic theory is also found.

Spin relaxation rate for baryons in a thermal pion gas

We study the relaxation dynamics of the spin polarization of baryons (nucleon and Λ baryon), in a thermal pion gas as a simple model of the hadronic phase of the QCD plasma produced in relativistic heavy-ion collisions. For this purpose, we formulate the quantum kinetic theory for the spin density matrix of baryons in the leading order of the gradient expansion. Considering the baryon-pion elastic scattering processes as the dominant interaction between baryons and thermal pions, we compute the spin relaxation rate of nucleons and Λ baryons in a pion gas up to temperature 200 MeV. In the case of nucleons, we evaluate the spin relaxation rate in the s-channel resonance approximation, based on the known experimental data on Δ resonances. We also estimate the spin relaxation rate for Λ baryons, based on experimental inputs and theoretical models for the low-energy Λπ scattering, including the chiral perturbation theory. Published by the American Physical Society 2024

Decoherence and Momentum Relaxation in Fermi-Polaron Rabi Dynamics: A Kinetic Equation Approach.

Despite the paradigmatic nature of the Fermi-polaron model, the theoretical description of its nonlinear dynamics poses challenges. Here, we apply a quantum kinetic theory of driven polarons to recent experiments with ultracold atoms, where Rabi oscillations between a Fermi-polaron state and a noninteracting level were reported. The resulting equations separate decoherence from momentum relaxation, with the corresponding rates showing a different dependence on microscopic scattering processes and quasiparticle properties. We describe both the polaron ground state and the excited repulsive-polaron state and we find a good quantitative agreement between our predictions and the available experimental data without any fitting parameter. Our approach not only takes into account collisional phenomena, but also it can be used to study the different roles played by decoherence and the collisional integral in the strongly interacting highly imbalanced mixture of Fermi gases.

Photon quantum kinetic equations and collective modes in an axion background

We develop a quantum kinetic theory for photons in the presence of an axion background and in the collisioness limit. In deriving the classical regime of our quantum kinetic equations, we observe that they capture well-known features of axion electrodynamics. By projecting the Wigner function onto a polarization basis, relating the Wigner matrix function with the Stokes parameters, we establish the dispersion relations and transport equations for each polarization space component. Additionally, we investigate how the axion background affects the dispersion relations of photon collective modes within an electron-positron plasma at equilibrium temperature T. While the plasmon remains unaffected, we find that the axion background breaks the degeneracy of transverse collective modes at order egaγT(∂a), where e represents the electron charge, gaγ denotes the photon-axion coupling, and ∂a represents the scale associated with variations in the axion field. Published by the American Physical Society 2024

Spin polarization and spin alignment from quantum kinetic theory with self-energy corrections

Spin polarization and spin alignment from quantum kinetic theory with self-energy corrections

Self-consistent quantum-kinetic theory for interacting drifting electrons and force-driven phonons in a 1D system

A self-consistent quantum-kinetic model is developed for studying strong-field nonlinear electron transport interacting with force-driven phonons within a nanowire system. For this model, phonons can be dragged into motion through strong electron–phonon scattering by fast-moving electrons along the opposite direction of the DC electric field. Meanwhile, the DC-field induced charge current of electrons can be either enhanced or reduced by the same electron–phonon scattering, depending on the relative direction of a DC field with respect to that of an applied temperature gradient for driving phonons. By making use of this quantum-kinetic model beyond the relaxation-time approximation, neither electron nor phonon temperature is required for describing ultrafast electron–phonon scattering and their correlated transports in this 1D electronic-lattice system.

Quantum kinetic theory with interactions for massive vector bosons

Quantum kinetic theory with interactions for massive vector bosons

Intrinsic nonlinear thermal Hall transport of magnons: A quantum kinetic theory approach

Intrinsic nonlinear thermal Hall transport of magnons: A quantum kinetic theory approach

Collective dynamics of polarized spin-half fermions in relativistic heavy-ion collisions

Standard relativistic hydrodynamics has been successful in describing the properties of the strongly interacting matter produced in the heavy-ion collision experiments. Recently, there has been a significant theoretical advancement in this field to explain spin polarization of hadrons emitted in these processes. Although current models have successfully explained some of the experimental data based on the coupling between spin polarization and vorticity of the medium, they still lack a clear understanding of the differential measurements. This is commonly interpreted as an indication that the spin needs to be treated as an independent degree of freedom whose dynamics is not entirely bound to flow circulation. In particular, if the spin is a macroscopic property of the system, in equilibrium its dynamics should follow hydrodynamic laws. Here, we develop a framework of relativistic hydrodynamics which includes spin degrees of freedom from the quantum kinetic theory for Dirac fermions and use it for modeling the dynamics of matter. Following experimental observations, we assume that the polarization effects are small and derive conservation laws for the net baryon current, the energy–momentum tensor and the spin tensor based on the de Groot–van Leeuwen–van Weert definitions of these currents. We present various properties of the spin polarization tensor and its components, analyze the propagation properties of the spin polarization components, and derive the spin-wave velocity for arbitrary statistics. We find that only the transverse spin components propagate, analogously to the electromagnetic waves. Finally, using our framework, we study the space–time evolution of the spin polarization for the systems respecting certain space–time symmetries and calculate the mean spin polarization per particle, which can be compared to the experimental data. We find that, for some observables, our spin polarization results agree qualitatively with the experimental findings and other model calculations.

Quantum kinetic theory of nonlinear thermal current

We investigate the second-order nonlinear electronic thermal transport induced by the temperature gradient. We develop the quantum kinetic theory framework to describe thermal transport in the presence of a temperature gradient. Using this, we predict an intrinsic scattering time-independent nonlinear thermal current in addition to the known extrinsic nonlinear Drude and Berry curvature dipole contributions. We show that the intrinsic thermal current is determined by the band geometric quantities and is nonzero only in systems in which both the space inversion and time-reversal symmetries are broken. We employ the developed theory to study the thermal response in tilted massive Dirac systems. We show that besides the different scattering time dependencies, the various current contributions have distinct temperature dependencies in the low-temperature limit. Our systematic and comprehensive theory for nonlinear thermal transport paves the way for future theoretical and experimental studies on intrinsic thermal responses.

Unravelling the nonlinear ideal density response of many-body systems

Nonlinear density response theory is revisited focusing on the harmonically perturbed finite temperature uniform electron gas. Within the non-interacting limit, brute force quantum kinetic theory calculations for the quadratic, cubic, quartic and quintic responses reveal a deep connection with the linear response. Careful analysis of the static long wavelength limit led us to conjecture a canonical non-interacting form that expresses arbitrary order nonlinear responses as the weighted sum of the linear responses evaluated at all multiple harmonics. This harmonic expansion is successfully validated against ab initio path integral Monte Carlo simulations.

Spin polarization and correlation of quarks from the glasma

We investigate the interaction of strong color fields in the glasma stage of high-energy nuclear collisions with the spins of quarks and antiquarks. We employ the perturbative solution of the quantum kinetic theory for the spin transport of (massive) quarks in a background color field governed by the linearized Yang-Mills equation and derive expressions for the quark-spin polarization and quark-antiquark spin correlation at small momentum in terms of field correlators. For the Golec-Biernat W\"usthoff dipole distribution the quark-spin polarization vanishes, but the out-of-plane spin correlation of quarks and antiquarks is nonzero. Our order-of-magnitude estimate of the correlation far exceeds that caused by vorticity effects, but does not fully explain the data for vector meson alignment. We identify possible mechanisms that could further increase the predicted spin correlation.

Quantum kinetic theory of nonlinear optical currents: Finite Fermi surface and Fermi sea contributions

The quantum kinetic framework provides a versatile method for investigating the dynamical optical and transport currents of crystalline solids. In this paper, starting from the density-matrix equations of motion, we present a general theoretical path to obtain the nonlinear optical response in an elegant and transparent manner. We devise an extensive kinetic theory that can be applied to materials with arbitrary band structures and captures intraband and interband coherence effects, finite Fermi surfaces, and disorder effects. We present a classification of the nonlinear optical currents arising from the interference of the interband and intraband components of the density matrix with distinct symmetry and quantum geometrical origin for each contribution. In this context, we report the following four primary findings: (i) The Fermi golden rule approach is insufficient to derive the correct expression for the injection current, a shortcoming that we remedy in our theory while associating the injection current with the intraband-interband contribution to the second-order density matrix. (ii) The interband-intraband contribution yields a resonant current that survives irrespective of any symmetry constraint in addition to the well-known anomalous nonlinear current (nonresonant), which requires time-reversal symmetry. (iii) Quite generally, the nonlinear current is significantly enhanced by contributions arising from the finite Fermi surface. (iv) The finite Fermi surface and Fermi sea additionally lead to sizable novel nonlinear effects via contributions we term double resonant and higher-order pole. We investigate such effects in sum frequency and difference frequency generation. As an illustration, we compute the nonlinear response of the topological antiferromagnet CuMnAs and thin film tilted Weyl semimetals as model systems dominated by interband coherence contributions. We find that the nonlinear response of CuMnAs is responsive to the direction of the finite magnetization field and the response of Weyl semimetal to the tilt. In addition, the choice of the polarization angle of the beam is crucial to have a nonlinear current in CuMnAs, while it is not the case for Weyl semimetals.

Generating tensor polarization from shear stress

We derive an expression for the tensor polarization of a system of massive spin-1 particles in a hydrodynamic framework. Starting from quantum kinetic theory based on the Wigner-function formalism, we employ a modified method of moments which also takes into account all spin degrees of freedom. It is shown that the tensor polarization of an uncharged fluid is determined by the shear-stress tensor. In order to quantify this novel polarization effect, we provide a formula which can be used for numerical calculations of vector-meson spin alignment in relativistic heavy-ion collisions.

Quantum kinetic theory of Jeans instability in non-minimal matter-curvature coupling gravity

We present a quantum treatment of the Jeans gravitational instability in the Newtonian limit of the non-minimal matter-curvature coupling gravity model. By relying on Wigner functions, allowing for the representation of quantum states in a classical phase space, we formulate a quantum kinetic treatment of this problem, generalizing the classical kinetic approach (Gomes in Eur Phys J C 80:633, 2020). This allows us to study the interplay between non-minimal matter-curvature coupling effects, quantum effects, and kinetic (finite-temperature) effects, on the Jeans criterion. We study in detail special cases of the model (general relativity, f(R) theories, pure non-minimal coupling, etc.) and confront the model with the observed stability of Bok globules.

Relativistic spin transport theory for spin-1/2 fermions

Global polarization effect is an important physical phenomenon reflecting spin-orbit couplings in heavy ion collisions. Since STAR’s observation of the global polarization of <inline-formula><tex-math id="M2">\begin{document}$\Lambda$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20222470_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20222470_M2.png"/></alternatives></inline-formula> hyperons in Au+Au collisions in 2017, this effect has attracted a lot of interests in the field. In the hot and dense matter produced in heavy ion collisions, the spin-orbit couplings come from non-local collisions between particles, in which the orbital angular momentum involves the space and momentum information of the colliding particles, so it is necessary to describe the particle collisions with spin-orbit couplings in phase space. In addition, the spin-orbit coupling is a quantum effect, which requires quantum theory. In combination of two aspects, the quantum kinetic theory based on covariant Wigner functions has become a powerful tool to describe the global polarization effect. In this paper, we introduce the quantum kinetic theory for spin-1/2 Fermion system based on Wigner functions as well as the spin transport theory developed on this basis. The recent research progress of spin transport theory provides a solid theoretical foundation for simulating the space-time evolution of spin polarization effects in heavy ion collisions.

Acoustic spin transport by superconducting quasiparticles

We theoretically study the generation of spin current due to a surface acoustic wave (SAW) in a superconductor. We model an $s$-wave superconductor as the mean-field Hamiltonian and calculate spin current generated via spin-vorticity coupling based on quantum kinetic theory. The results suggest that the spin current can be driven in a single-superconductor layer, and our estimation suggests that the detectable magnitude of the spin current can be generated in aluminum. Our proposal may contribute to the advancement of spin transport in superconductors from application and fundamental physics aspects.

Nonlinear chiral photocurrent in parity-violating magnetic Weyl semimetals

The strong correlation between the non-trivial band topology and the magnetic texture makes magnetic Weyl semimetals excellent candidates for the manipulation and detection of magnetization dynamics. The parity violation together with the Pauli blocking cause only one Weyl node to contribute to the photocurrent response, which in turn affects the magnetic texture due to the spin transfer torque. Utilizing the Landau-Lifshitz-Gilbert equation and the spin-transfer torque in non-centrosymmetric Weyl magnets, we show that the chiral photocurrent rotates the magnetization from the easy c axis to the a or b axis, which leads to an exotic current next to the photocurrent response. The chiral photocurrent is calculated in the context of quantum kinetic theory and it has a strong resonance on the order of mA/W near the Weyl nodes, the magnitude of which is controlled by the momentum relaxation time. Remarkably, we study the influence of magnetic texture dynamics on the topological nonlinear photocurrent response, including shift and injection currents along with the new chiral photocurrent, and show that both the magnitude and the in-plane orientation of the chiral photocurrent are strongly correlated with the direction of the magnetic moments.

Spin evolution of massive fermion in QED plasma

The dynamical evolution of spin of a massive probe fermion in a massless hot QED plasma at local equilibrium is investigated through the quantum kinetic theory. We consider the massive probe fermion undergoing 2-by-2 Coulomb scattering with the massless fermions in the medium. The axial kinetic equation is derived including the collision terms to the first order of gradients and leading logarithmic order of the coupling. The collision terms are vanishing at global equilibrium, around which the relaxation time can be extracted as an operator. We further decompose the axial kinetic equation into kinetic equations of axial-charge density as well as the transverse magnetic dipole moment, and illustrate the diffusion and polarization effect through preliminary numerical analysis.