12 research outputs found
Steering Magnetic Skyrmions with Nonequilibrium Green's Functions
Magnetic skyrmions, topologically protected vortex-like configurations in
spin textures, are of wide conceptual and practical appeal for quantum
information technologies, notably in relation to the making of so-called
race-track memory devices. Skyrmions can be created, steered and destroyed with
magnetic fields and/or (spin) currents. Here we focus on the latter mechanism,
analyzed via a microscopic treatment of the skyrmion-current interaction. The
system we consider is an isolated skyrmion in a square-lattice cluster,
interacting with electrons spins in a current-carrying quantum wire. For the
theoretical description, we employ a quantum formulation of spin-dependent
currents via nonequilibrium Green's functions (NEGF) within the generalized
Kadanoff-Baym ansatz (GKBA). This is combined with a treatment of skyrmions
based on classical localized spins, with the skyrmion motion described via
Ehrenfest dynamics. With our mixed quantum-classical scheme, we assess how
time-dependent currents can affect the skyrmion dynamics, and how this in turn
depends on electron-electron and spin-orbit interactions in the wire. Our study
shows the usefulness of a quantum-classical treatment of skyrmion steering via
currents, as a way for example to validate/extract an effective,
classical-only, description of skyrmion dynamics from a microscopic quantum
modeling of the skyrmion-current interaction.Comment: 10 pages, 8 figures, contribution to the proceedings of "Progress in
Nonequilibrium Green's Functions VII
Nonequilibrium Green's functions and atom-surface dynamics: Simple views from a simple model system
We employ Non-equilibrium Green's functions (NEGF) to describe the real-time
dynamics of an adsorbate-surface model system exposed to ultrafast laser
pulses. For a finite number of electronic orbitals, the system is solved
exactly and within different levels of approximation. Specifically i) the full
exact quantum mechanical solution for electron and nuclear degrees of freedom
is used to benchmark ii) the Ehrenfest approximation (EA) for the nuclei, with
the electron dynamics still treated exactly. Then, using the EA, electronic
correlations are treated with NEGF within iii) 2nd Born and with iv) a recently
introduced hybrid scheme, which mixes 2nd Born self-energies with
non-perturbative, local exchange-correlation potentials of Density Functional
Theory (DFT). Finally, the effect of a semi-infinite substrate is considered:
we observe that a macroscopic number of de-excitation channels can hinder
desorption. While very preliminary in character and based on a simple and
rather specific model system, our results clearly illustrate the large
potential of NEGF to investigate atomic desorption, and more generally, the non
equilibrium dynamics of material surfaces subject to ultrafast laser fields.Comment: 10 pages, 5 figure
Nonequilibrium Kondo-vs-RKKY Scenarios in Nanoclusters
Ultrafast manipulations of magnetic phases are eliciting increasing attention
from the scientific community, because potentially relevant to the
understanding of nonequilibrium phase transitions and to novel technologies.
Here, we focus on manipulations applied to magnetic impurities in metallic
hosts. By considering small nanoring geometries, we show how currents can
induce a dynamical switching between different types of exchange interactions
in these systems. Our work thus opens a study window on nonequilibrium
Doniach's magnetic phase diagrams, and time-dependent Kondo-vs-RKKY scenarios.Comment: 6 pages, 5 figures, to appear in EP
Scaling properties of a spatial one-particle density-matrix entropy in many-body localized systems
We investigate a spatial subsystem entropy extracted from the one-particle density matrix (OPDM) of one-dimensional disordered interacting fermions that host a many-body localized (MBL) phase. Deep in the putative MBL regime, this OPDM entropy exhibits the salient scaling features of localization, even though it provides only an upper bound to the von Neumann entropy. First, we numerically show that the OPDM entropy of the eigenstates obeys an area law. Second, like the von Neumann entropy, the OPDM entropy grows logarithmically with time after a quantum quench, albeit with a different prefactor. Both these features survive at moderately large interactions and well toward the transition into the ergodic phase. We discuss prospects for calculating the OPDM entropy using approximate numerical methods and for its measurement in quantum gas experiments
Merging Features from Green's Functions and Time Dependent Density Functional Theory : A Route to the Description of Correlated Materials out of Equilibrium?
We propose a description of nonequilibrium systems via a simple protocol that combines exchangecorrelation
potentials from density functional theory with self-energies of many-body perturbation theory.
The approach, aimed to avoid double counting of interactions, is tested against exact results in Hubbardtype
systems, with respect to interaction strength, perturbation speed and inhomogeneity, and system
dimensionality and size. In many regimes, we find significant improvement over adiabatic time dependent
density functional theory or second Born nonequilibrium Green’s function approximations. We briefly
discuss the reasons for the residual discrepancies, and directions for future work.peerReviewe