10,065 research outputs found
Computing spectral sequences
In this paper, a set of programs enhancing the Kenzo system is presented.
Kenzo is a Common Lisp program designed for computing in Algebraic Topology, in
particular it allows the user to calculate homology and homotopy groups of
complicated spaces. The new programs presented here entirely compute Serre and
Eilenberg-Moore spectral sequences, in particular the groups and differential
maps for arbitrary r. They also determine when the spectral sequence has
converged and describe the filtration of the target homology groups induced by
the spectral sequence
Time-Dependent Magnons from First Principles
We propose an efficient and non-perturbative scheme to compute magnetic excitations for extended systems employing the framework of time-dependent density functional theory. Within our approach, we drive the system out of equilibrium using an ultrashort magnetic kick perpendicular to the ground-state magnetization of the material. The dynamical properties of the system are obtained by propagating the time-dependent Kohn–Sham equations in real time, and the analysis of the time-dependent magnetization reveals the transverse magnetic excitation spectrum of the magnet. We illustrate the performance of the method by computing the magnetization dynamics, obtained from a real-time propagation, for iron, cobalt, and nickel and compare them to known results obtained using the linear-response formulation of time-dependent density functional theory. Moreover, we point out that our time-dependent approach is not limited to the linear-response regime, and we present the first results for nonlinear magnetic excitations from first principles in iron
Transient charge and energy flow in the wide-band limit
The wide-band limit is a commonly used approximation to analyze transport
through nanoscale devices. In this work we investigate its applicability to the
study of charge and heat transport through molecular break junctions exposed to
voltage biases and temperature gradients. We find that while this approximation
faithfully describes the long-time charge and heat transport, it fails to
characterize the short-time behavior of the junction. In particular, we find
that the charge current flowing through the device shows a discontinuity when a
temperature gradient is applied, while the energy flow is discontinuous when a
voltage bias is switched on and even diverges when the junction is exposed to
both a temperature gradient and a voltage bias. We provide an explanation for
this pathological behavior and propose two possible solutions to this problem.Comment: 11 pages, 9 figure
Benchmarking Nonequilibrium Green's Functions against Configuration Interaction for time-dependent Auger decay processes
We have recently proposed a Nonequilibrium Green's Function (NEGF) approach
to include Auger decay processes in the ultrafast charge dynamics of
photoionized molecules. Within the so called Generalized Kadanoff-Baym Ansatz
the fundamental unknowns of the NEGF equations are the reduced one-particle
density matrix of bound electrons and the occupations of the continuum states.
Both unknowns are one-time functions like the density in Time-Dependent
Functional Theory (TDDFT). In this work we assess the accuracy of the approach
against Configuration Interaction (CI) calculations in one-dimensional model
systems. Our results show that NEGF correctly captures qualitative and
quantitative features of the relaxation dynamics provided that the energy of
the Auger electron is much larger than the Coulomb repulsion between two holes
in the valence shells. For the accuracy of the results dynamical
electron-electron correlations or, equivalently, memory effects play a pivotal
role. The combination of our NEGF approach with the Sham-Schl\"uter equation
may provide useful insights for the development of TDDFT exchange-correlation
potentials with a history dependence.Comment: 7 pages, 3 figure
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Universal slow plasmons and giant field enhancement in atomically thin quasi-two-dimensional metals
Plasmons depend strongly on dimensionality: while plasmons in three-dimensional systems start with finite energy at wavevector q = 0, plasmons in traditional two-dimensional (2D) electron gas disperse as ωp∼q√. However, besides graphene, plasmons in real, atomically thin quasi-2D materials were heretofore not well understood. Here we show that the plasmons in real quasi-2D metals are qualitatively different, being virtually dispersionless for wavevectors of typical experimental interest. This stems from a broken continuous translational symmetry which leads to interband screening; so, dispersionless plasmons are a universal intrinsic phenomenon in quasi-2D metals. Moreover, our ab initio calculations reveal that plasmons of monolayer metallic transition metal dichalcogenides are tunable, long lived, able to sustain field intensity enhancement exceeding 107, and localizable in real space (within ~20 nm) with little spreading over practical measurement time. This opens the possibility of tracking plasmon wave packets in real time for novel imaging techniques in atomically thin materials
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