arXiv subjects

J. -P. Joost

Publications and source records attributed to J. -P. Joost.

7 recordsLinked to original sources

arXiv2026

Quantum effects in plasmas

The year 2025 had been designated by UNESCO as the International Year of Quantum Science and Technology. 125 years ago Max Planck's discovery of radiation quanta started the quantum era and 100 years ago quantum mechanics was discovered by Schroedinger, Heisenberg, Bohr, Pauli, Dirac, Born, Fermi and many others. By now, quantum mechanics is the theoretical foundation of most fields of physics and chemistry, and it is the basis for modern nanotechnology. How about plasma physics? How important are quantum effects in plasmas? In what experiments quantum effects are observed and where do they govern the behavior of plasmas? How can these effects be treated theoretically and via computer simulations? Starting with a brief historical overview we discuss the broad parameter range that is characteristic for plasmas and outline where quantum effects are relevant. This is the case primarily for warm dense matter and inertial fusion plasmas. We provide an overview on the theoretical quantum methods that are available for these dense plasmas and how their respective advantages can be combined in order to achieve predictive capability. The key is a downfolding approach that is based on first principles simulations.

physics.plasm-ph↗

arXiv2020

Löwdin's symmetry dilemma within Green functions theory for the one-dimensional Hubbard model

The energy gap of correlated Hubbard clusters is well studied for one-dimensional systems using analytical methods and density-matrix-renormalization-group (DMRG) simulations. Beyond 1D, however, exact results are available only for small systems by quantum Monte Carlo. For this reason and, due to the problems of DMRG in simulating 2D and 3D systems, alternative methods such as Green functions combined with many-body approximations (GFMBA), that do not have this restriction, are highly important. However, it has remained open whether the approximate character of GFMBA simulations prevents the computation of the Hubbard gap. Here we present new GFMBA results that demonstrate that GFMBA simulations are capable of producing reliable data for the gap which agrees well with the DMRG benchmarks in 1D. An interesting observation is that the accuracy of the gap can be significantly increased when the simulations give up certain symmetry restriction of the exact system, such as spin symmetry and spatial homogeneity. This is seen as manifestation and generalization of the "symmetry dilemma" introduced by Löwdin for Hartree--Fock wave function calculations.

cond-mat.str-el↗

arXiv2020

The G1--G2 Scheme: Dramatic Acceleration of Nonequilibrium Green Functions Simulations Within the Hartree--Fock-GKBA

The time evolution in quantum many-body systems after external excitations is attracting high interest in many fields. The theoretical modeling of these processes is challenging, and the only rigorous quantum-dynamics approach that can treat correlated fermions in two and three dimensions is nonequilibrium Green functions (NEGF). However, NEGF simulations are computationally expensive due to their $T^3$-scaling with the simulation duration $T$. Recently, $T^2$-scaling was achieved with the generalized Kadanoff--Baym ansatz (GKBA), for the second-order Born (SOA) selfenergy, which has substantially extended the scope of NEGF simulations. In a recent Letter [Schlünzen \textit{et al.}, Phys. Rev. Lett. \textbf{124}, 076601 (2020)] we demonstrated that GKBA-NEGF simulations can be efficiently mapped onto coupled time-local equations for the single-particle and two-particle Green functions on the time diagonal, hence the method has been called G1--G2 scheme. This allows one to perform the same simulations with order $T^1$-scaling, both for SOA and $GW$ selfenergies giving rise to a dramatic speedup. Here we present more details on the G1--G2 scheme, including derivations of the basic equations including results for a general basis, for Hubbard systems and for jellium. Also, we demonstrate how to incorporate initial correlations into the G1--G2 scheme. Further, the derivations are extended to a broader class of selfenergies, including the $T$ matrix in the particle--particle and particle--hole channels, and the dynamically screened-ladder approximation. Finally, we demonstrate that, for all selfenergies, the CPU time scaling of the G1--G2 scheme with the basis dimension, $N_b$, can be improved compared to our first report: the overhead compared to the original GKBA, is not more than an additional factor $N_b$.

cond-mat.str-el↗

arXiv2018

Ion Impact Induced Ultrafast Electron Dynamics in Correlated Materials and Finite Graphene Clusters

Strongly correlated systems of fermions have an interesting phase diagram arising from the Hubbard gap. Excitation across the gap leads to the formation of doubly occupied lattice sites (doublons). This state offers interesting electronic and optical properties. Moreover, when the system is driven out of equilibrium interesting collective dynamics may arise that are related to the spatial propagation of doublons. Here, a novel mechanism that was recently proposed by us [Balzer \textit{et al.}, submitted for publication] is verified by exact diagonalization and nonequilibrium Green functions (NEGF) simulations---fermionic doublon creation by the impact of energetic ions. We report the formation of a nonequilibrium steady state with homogeneous doublon distribution. A physically intuitive picture is given in terms of an analytical model for a two-site system where the doublon formation is explained in terms of a two-fold passage of an avoided crossing (Landau-Zener picture). The effect should be particularly important for strongly correlated finite systems, such as graphene nanoribbons, and directly observable with fermionic atoms in optical lattices. We demonstrate that doublon formation and propagation in correlated lattice systems can be accurately simulated with NEGF. In addition to two-time results we present single-time results within the generalized Kadanoff-Baym ansatz (GKBA) with Hartree-Fock propagators (HF-GKBA), and we present systematic improvements that use correlated propagators (correlated GKBA).

cond-mat.str-el↗

arXiv2017

Wakefields in streaming plasmas: Characteristics of the induced charge density distribution

Motivated by experiments on the generation of streaming plasmas in high energy density facilities, industrial setups, and fundamental dusty plasma research the plasma polarization around a test charge in streaming plasmas is considered. The induced charge density distribution of plasma constituents is discussed for the subsonic, sonic, and supersonic regime. Also, it is shown that the plasma polarization in the vicinity of the test charge shows different scaling in subsonic and supersonic regimes.

physics.plasm-ph↗

arXiv2017

Nonequilibrium dynamics in the one-dimensional Fermi-Hubbard model: A comparison of the nonequilibrium Green functions approach and the density matrix renormalization group method

The nonequilibrium dynamics of strongly-correlated fermions in lattice systems have attracted considerable interest in the condensed matter and ultracold atomic-gas communities. While experiments have made remarkable progress in recent years, there remains a need for the further development of theoretical tools that can account for both the nonequilibrium conditions and strong correlations. For instance, time-dependent theoretical quantum approaches based on the density matrix renormalization group (DMRG) methods have been primarily applied to one-dimensional setups. Recently, two-dimensional quantum simulations of the expansion of fermions based on nonequilibrium Green functions (NEGF) have been presented [Schluenzen et al., Phys. Rev. B 93, 035107 (2016)] that showed excellent agreement with the experiments. Here we present an extensive comparison of the NEGF approach to numerically accurate DMRG results. The results indicate that NEGF are a reliable theoretical tool for weak to intermediate coupling strengths in arbitrary dimensions and make long simulations possible. This is complementary to DMRG simulations which are particularly efficient at strong coupling.

cond-mat.str-el↗

arXiv2016

Comment on "On the unphysical solutions of the Kadanoff--Baym equations in linear response: Correlation-induced homogeneous density-distribution and attractors"

In a recent Rapid Communication [A. Stan, Phys. Rev. B \textbf{93}, 041103(R) (2016)], the reliability of the Keldysh--Kadanoff--Baym equations (KBE) using correlated selfenergy approximations applied to linear and nonlinear response has been questioned. In particular, the existence of a universal attractor has been predicted that would drive the dynamics of any correlated system towards an unphysical homogeneous density distribution regardless of the system type, the interaction and the many-body approximation. Moreover, it was conjectured that even the mean-field dynamics would be damped. Here, by performing accurate solutions of the KBE for situations studied in that paper, we prove these claims wrong being caused by numerical inaccuracies.

cond-mat.str-el↗