110 research outputs found
Hidden by graphene -- towards effective screening of interface van der Waals interactions via monolayer coating
Recent atomic force microscopy (AFM) experiments~[ACS Nano {\bf 2014}, 8,
12410-12417] conducted on graphene-coated SiO demonstrated that monolayer
graphene (G) can effectively screen dispersion van der Waals (vdW) interactions
deriving from the underlying substrate: despite the single-atom thickness of G,
the AFM tip was almost insensitive to SiO, and the tip-substrate attraction
was essentially determined only by G. This G vdW {\it opacity} has far reaching
implications, encompassing stabilization of multilayer heterostructures,
micromechanical phenomena or even heterogeneous catalysis. Yet, detailed
experimental control and high-end applications of this phenomenon await sound
physical understanding of the underlying physical mechanism. By quantum
many-body analysis and ab-initio Density Functional Theory, here we address
this challenge providing theoretical rationalization of the observed G vdW {\it
opacity} for weakly interacting substrates. The non-local density response and
ultra slow decay of the G vdW interaction ensure compensation between standard
attractive terms and many-body repulsive contributions, enabling vdW {\it
opacity} over a broad range of adsorption distances. vdW {\it opacity} appears
most efficient in the low frequency limit and extends beyond London dispersion
including electrostatic Debye forces. By virtue of combined
theoretical/experimental validation, G hence emerges as a promising ultrathin
{\it shield} for modulation and switching of vdW interactions at interfaces and
complex nanoscale devices
Cohesive properties of noble metals by van der Waals-corrected Density Functional Theory
The cohesive energy, equilibrium lattice constant, and bulk modulus of noble
metals are computed by different van der Waals-corrected Density Functional
Theory methods, including vdW-DF, vdW-DF2, vdW-DF-cx, rVV10 and PBE-D. Two
specifically-designed methods are also developed in order to effectively
include dynamical screening effects: the DFT/vdW-WF2p method, based on the
generation of Maximally Localized Wannier Functions, and the RPAp scheme (in
two variants), based on a single-oscillator model of the localized electron
response. Comparison with results obtained without explicit inclusion of van
der Waals effects, such as with the LDA, PBE, PBEsol, or the hybrid PBE0
functional, elucidates the importance of a suitable description of screened van
der Waals interactions even in the case of strong metal bonding. Many-body
effects are also quantitatively evaluated within the RPAp approach.Comment: 3 figure
Van der Waals Interactions in DFT using Wannier Functions without empirical parameters
A new implementation is proposed for including van der Waals (vdW)
interactions in Density Functional Theory (DFT) using the Maximally-Localized
Wannier functions (MLWFs), which is free from empirical parameters. With
respect to the previous DFT/vdW-WF2 method, in the present DFT/vdW-WF2-x
approach, the empirical, short-range, damping function is replaced by an
estimate of the Pauli exchange repulsion, also obtained by the MLWFs
properties. Applications to systems contained in the popular S22 molecular
database and to the case of an Ar atom interacting with graphite, and
comparison with reference data, indicate that the new method, besides being
more physically founded, also leads to a systematic improvement in the
description of vdW-bonded systems.Comment: 3 figures. arXiv admin note: text overlap with arXiv:1111.6737,
arXiv:1305.7035, arXiv:1603.0866
Dynamical spin properties of confined Fermi and Bose systems in presence of spin-orbit coupling
Due to the recent experimental progress, tunable spin-orbit (SO) interactions
represent ideal candidates for the control of polarization and dynamical spin
properties in both quantum wells and cold atomic systems. A detailed
understanding of spin properties in SO coupled systems is thus a compelling
prerequisite for possible novel applications or improvements in the context of
spintronics and quantum computers. Here we analyze the case of equal Rashba and
Dresselhaus couplings in both homogeneous and laterally confined
two-dimensional systems. Starting from the single-particle picture and
subsequently introducing two-body interactions we observe that periodic spin
fluctuations can be induced and maintained in the system. Through an analytical
derivation we show that the two-body interaction does not involve decoherence
effects in the bosonic dimer, and, in the repulsive homogeneous Fermi gas it
may be even exploited in combination with the SO coupling to induce and tune
standing currents. By further studying the effects of a harmonic lateral
confinement --a particularly interesting case for Bose condensates-- we
evidence the possible appearance of non-trivial {\it spin textures}, whereas
the further application of a small Zeeman-type interaction can be exploited to
fine-tune the system polarizability.Comment: 13 pages, 3 figure
Polarization of a quasi two-dimensional repulsive Fermi gas with Rashba spin-orbit coupling: a variational study
Motivated by the remarkable experimental control of synthetic gauge fields in
ultracold atomic systems, we investigate the effect of an artificial Rashba
spin-orbit coupling on the spin polarization of a two-dimensional repulsive
Fermi gas. By using a variational many-body wavefunction, based on a suitable
spinorial structure, we find that the polarization properties of the system are
indeed controlled by the interplay between spin-orbit coupling and repulsive
interaction. In particular, two main effects are found: 1) The Rashba coupling
determines a gradual increase of the degree of polarization beyond the critical
repulsive interaction strength, at variance with conventional 2D Stoner
instability. 2) The critical interaction strength, above which finite
polarization is developed, shows a dependence on the Rashba coupling, i.e. it
is enhanced in case the Rashba coupling exceeds a critical value. A simple
analytic expression for the critical interaction strength is further derived in
the context of our variational formulation, which allows for a straightforward
and insightful analysis of the present problem.Comment: 7 pages, 3 figure
Long-range correlation energy calculated from coupled atomic response functions
An accurate determination of the electron correlation energy is essential for
describing the structure, stability, and function in a wide variety of systems,
ranging from gas-phase molecular assemblies to condensed matter and
organic/inorganic interfaces. Even small errors in the correlation energy can
have a large impact on the description of chemical and physical properties in
the systems of interest. In this context, the development of efficient
approaches for the accurate calculation of the long-range correlation energy
(and hence dispersion) is the main challenge. In the last years a number of
methods have been developed to augment density functional approximations via
dispersion energy corrections, but most of these approaches ignore the
intrinsic many-body nature of correlation effects, leading to inconsistent and
sometimes even qualitatively incorrect predictions. Here we build upon the
recent many-body dispersion (MBD) framework, which is intimately linked to the
random-phase approximation for the correlation energy. We separate the
correlation energy into short-range contributions that are modeled by
semi-local functionals and long-range contributions that are calculated by
mapping the complex all-electron problem onto a set of atomic response
functions coupled in the dipole approximation. We propose an effective
range-separation of the coupling between the atomic response functions that
extends the already broad applicability of the MBD method to non-metallic
materials with highly anisotropic responses, such as layered nanostructures.
Application to a variety of high-quality benchmark datasets illustrates the
accuracy and applicability of the improved MBD approach, which offers the
prospect of first-principles modeling of large structurally complex systems
with an accurate description of the long-range correlation energy.Comment: 15 pages, 3 figure
Van der Waals corrected DFT simulation of adsorption processes on transition-metal surfaces: Xe and graphene on Ni(111)
The DFT/vdW-WF2s1 method, recently developed to include the van der Waals interactions in the density functional theory and describe adsorption processes on metal surfaces by taking metal-screening effects into account, is applied to the case of the interaction of Xe and graphene with a transition-metal surface, namely, Ni(111). In general, the adsorption of rare-gas atoms on metal surfaces is important because it is prototypical for physisorption processes. Moreover, the interaction of graphene with Ni(111) is of great interest for practical applications, for instance concerning the efficient and large-scale production of high-quality graphene; from a theoretical point of view, it is particularly challenging, since it can be described by a delicate interplay between chemisorption and physisorption processes. The first-principles simulation of transition metals requires particular care also because they can be viewed as intermediate systems between simple metals and insulating crystals. Even in these cases the method performs well as demonstrated by comparing our results with available experimental data and other theoretical investigations. We confirm that the rare-gas Xe atom is preferentially adsorbed on the top-site configuration on the Ni(111) surface too. Our approach, based on the use of the maximally localized Wannier functions, also allow us to well characterize the bonds between graphene and Ni(111)
Interatomic Methods for the Dispersion Energy Derived from the Adiabatic Connection Fluctuation-Dissipation Theorem
Interatomic pairwise methods are currently among the most popular and
accurate ways to include dispersion energy in density functional theory (DFT)
calculations. However, when applied to more than two atoms, these methods are
still frequently perceived to be based on \textit{ad hoc} assumptions, rather
than a rigorous derivation from quantum mechanics. Starting from the adiabatic
connection fluctuation-dissipation (ACFD) theorem, an exact expression for the
electronic exchange-correlation energy, we demonstrate that the pairwise
interatomic dispersion energy for an arbitrary collection of isotropic
polarizable dipoles emerges from the second-order expansion of the ACFD
formula. Moreover, for a system of quantum harmonic oscillators coupled through
a dipole--dipole potential, we prove the equivalence between the full
interaction energy obtained from the Hamiltonian diagonalization and the ACFD
correlation energy in the random-phase approximation. This property makes the
Hamiltonian diagonalization an efficient method for the calculation of the
many-body dispersion energy. In addition, we show that the switching function
used to damp the dispersion interaction at short distances arises from a
short-range screened Coulomb potential, whose role is to account for the
spatial spread of the individual atomic dipole moments. By using the ACFD
formula we gain a deeper understanding of the approximations made in the
interatomic pairwise approaches, providing a powerful formalism for further
development of accurate and efficient methods for the calculation of the
dispersion energy
Electronic Properties of Molecules and Surfaces with a Self\uad-Consistent Interatomic van der Waals Density Functional.
How strong is the effect of van der Waals (vdW) interactions on the electronic properties of molecules
and extended systems? To answer this question, we derived a fully self-consistent implementation of the
density-dependent interatomic vdW functional of Tkatchenko and Scheffler [Phys. Rev. Lett. 102, 073005
(2009)]. Not surprisingly, vdW self-consistency leads to tiny modifications of the structure, stability, and
electronic properties of molecular dimers and crystals. However, unexpectedly large effects were found in
the binding energies, distances, and electrostatic moments of highly polarizable alkali-metal dimers. Most
importantly, vdW interactions induced complex and sizable electronic charge redistribution in the vicinity
of metallic surfaces and at organic-metal interfaces. As a result, a substantial influence on the computed
work functions was found, revealing a nontrivial connection between electrostatics and long-range electron
correlation effects
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