217 research outputs found
Multibody Multipole Methods
A three-body potential function can account for interactions among triples of
particles which are uncaptured by pairwise interaction functions such as
Coulombic or Lennard-Jones potentials. Likewise, a multibody potential of order
can account for interactions among -tuples of particles uncaptured by
interaction functions of lower orders. To date, the computation of multibody
potential functions for a large number of particles has not been possible due
to its scaling cost. In this paper we describe a fast tree-code for
efficiently approximating multibody potentials that can be factorized as
products of functions of pairwise distances. For the first time, we show how to
derive a Barnes-Hut type algorithm for handling interactions among more than
two particles. Our algorithm uses two approximation schemes: 1) a deterministic
series expansion-based method; 2) a Monte Carlo-based approximation based on
the central limit theorem. Our approach guarantees a user-specified bound on
the absolute or relative error in the computed potential with an asymptotic
probability guarantee. We provide speedup results on a three-body dispersion
potential, the Axilrod-Teller potential.Comment: To appear in Journal of Computational Physic
Diagrammatic approach to orbital quantum impurities interacting with a many-particle environment
Recently it was shown that an impurity exchanging orbital angular momentum
with a surrounding bath can be described in terms of the angulon quasiparticle
[Phys. Rev. Lett. 118, 095301 (2017)]. The angulon consists of a quantum rotor
dressed by a many-particle field of boson excitations, and can be formed out
of, for example, a molecule or a nonspherical atom in superfluid helium, or out
of an electron coupled to lattice phonons or a Bose condensate. Here we develop
an approach to the angulon based on the path-integral formalism, which sets the
ground for a systematic, perturbative treatment of the angulon problem. The
resulting perturbation series can be interpreted in terms of Feynman diagrams,
from which, in turn, one can derive a set of diagrammatic rules. These rules
extend the machinery of the graphical theory of angular momentum - well known
from theoretical atomic spectroscopy - to the case where an environment with an
infinite number of degrees of freedom is present. In particular, we show that
each diagram can be interpreted as a 'skeleton', which enforces angular
momentum conservation, dressed by an additional many-body contribution. This
connection between the angulon theory and the graphical theory of angular
momentum is particularly important as it allows to systematically and
substantially simplify the analytical representation of each diagram. In order
to exemplify the technique, we calculate the 1- and 2-loop contributions to the
angulon self-energy, the spectral function, and the quasiparticle weight. The
diagrammatic theory we develop paves the way to investigate next-to-leading
order quantities in a more compact way compared to the variational approaches.Comment: 17 pages, 5 figure
Various damage mechanisms in carbon and silicon materials under femtosecond x-ray irradiation
We review the results of our research on damage mechanisms in materials
irradiated with femtosecond free-electron-laser (FEL) pulses. They were
obtained using our hybrid approach, XTANT (X-ray-induced Thermal And Nonthermal
Transitions). Various damage mechanisms are discussed with respect to the pulse
fluence and material properties on examples of diamond, amorphous carbon, C60
crystal, and silicon. We indicate conditions: producing thermal melting of
targets as a result of electron-ion energy exchange; nonthermal phase
transitions due to modification of the interatomic potential; Coulomb explosion
due to accumulated net charge in finite-size systems; spallation or ablation at
higher fluences due to detachment of sample fragments; and warm dense matter
formation. Transient optical coefficients are compared with experimental data
whenever available, proving the validity of our modeling approach. Predicted
diffraction patterns can be compared with the results of ongoing or future FEL
experiments. Limitations of our model and possible future directions of
development are outlined.Comment: This brief review is submitted for publicatio
Quantum spin liquid at finite temperature: proximate dynamics and persistent typicality
Quantum spin liquids are long-range entangled states of matter with emergent
gauge fields and fractionalized excitations. While candidate materials, such as
the Kitaev honeycomb ruthenate -RuCl, show magnetic order at low
temperatures , here we demonstrate numerically a dynamical crossover from
magnon-like behavior at low and frequencies to long-lived
fractionalized fermionic quasiparticles at higher and . This
crossover is akin to the presence of spinon continua in quasi-1D spin chains.
It is further shown to go hand in hand with persistent typicality down to very
low . This aspect, which has also been observed in the spin-1/2 kagome
Heisenberg antiferromagnet, is a signature of proximate spin liquidity and
emergent gauge degrees of freedom more generally, and can be the basis for the
numerical study of many finite- properties of putative spin liquids.Comment: 13 pages, 11 figures, accepted versio
Downfolding from Ab Initio to Interacting Model Hamiltonians: Comprehensive Analysis and Benchmarking
Model Hamiltonians are regularly derived from first-principles data to
describe correlated matter. However, the standard methods for this contain a
number of largely unexplored approximations. For a strongly correlated impurity
model system, here we carefully compare standard downfolding techniques with
the best-possible ground-truth estimates for charge-neutral excited state
energies and charge densities using state-of-the-art first-principles many-body
wave function approaches. To this end, we use the vanadocene molecule and
analyze all downfolding aspects, including the Hamiltonian form, target basis,
double counting correction, and Coulomb interaction screening models. We find
that the choice of target-space basis functions emerges as a key factor for the
quality of the downfolded results, while orbital-dependent double counting
correction diminishes the quality. Background screening to the Coulomb
interaction matrix elements primarily affects crystal-field excitations. Our
benchmark uncovers the relative importance of each downfolding step and offers
insights into the potential accuracy of minimal downfolded model Hamiltonians.Comment: 15 pages (+8 pages Supplemental Material), 8 figure
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