1,871 research outputs found
First-principles studies of water adsorption on graphene: The role of the substrate
We investigate the electronic properties of graphene upon water adsorption
and study the influence of the SiO2 substrate in this context using density
functional calculations. Perfect suspended graphene is rather insensitive to
H2O adsorbates, as doping requires highly oriented H2O clusters. For graphene
on a defective SiO2 substrate, we find a strongly different behavior: H2O
adsorbates can shift the substrate's impurity bands and change their
hybridization with the graphene bands. In this way, H2O can lead to doping of
graphene for much lower adsorbate concentrations than for free hanged graphene.
The effect depends strongly on the microscopic substrate properties.Comment: 4 pages, 3 figure
Decoupling method for dynamical mean field theory calculations
In this paper we explore the use of an equation of motion decoupling method
as an impurity solver to be used in conjunction with the dynamical mean field
self-consistency condition for the solution of lattice models. We benchmark the
impurity solver against exact diagonalization, and apply the method to study
the infinite Hubbard model, the periodic Anderson model and the model.
This simple and numerically efficient approach yields the spectra expected for
strongly correlated materials, with a quasiparticle peak and a Hubbard band. It
works in a large range of parameters, and therefore can be used for the
exploration of real materials using LDA+DMFT.Comment: 30 pages, 7 figure
Adhesion and electronic structure of graphene on hexagonal boron nitride substrates
We investigate the adsorption of graphene sheets on h-BN substrates by means
of first-principles calculations in the framework of adiabatic connection
fluctuation-dissipation theory in the random phase approximation. We obtain
adhesion energies for different crystallographic stacking configurations and
show that the interlayer bonding is due to long-range van der Waals forces. The
interplay of elastic and adhesion energies is shown to lead to stacking
disorder and moir\'e structures. Band structure calculations reveal substrate
induced mass terms in graphene which change their sign with the stacking
configuration. The dispersion, absolute band gaps and the real space shape of
the low energy electronic states in the moir\'e structures are discussed. We
find that the absolute band gaps in the moir\'e structures are at least an
order of magnitude smaller than the maximum local values of the mass term. Our
results are in agreement with recent STM experiments.Comment: 8 pages, 8 figures, revised and extended version, to appear in Phys.
Rev.
Probing of valley polarization in graphene via optical second-harmonic generation
Valley polarization in graphene breaks inversion symmetry and therefore leads
to second-harmonic generation. We present a complete theory of this effect
within a single-particle approximation. It is shown that this may be a
sensitive tool to measure the valley polarization created, e.g., by polarized
light and, thus, can be used for a development of ultrafast valleytronics in
graphene.Comment: 5 pages, 3 figure
Local impurity effects in superconducting graphene
We study the effect of impurities in superconducting graphene and discuss
their influence on the local electronic properties. In particular, we consider
the case of magnetic and non-magnetic impurities being either strongly
localized or acting as a potential averaged over one unit cell. The spin
dependent local density of states is calculated and possibilities for
visualizing impurities by means of scanning tunneling experiments is pointed
out. A possibility of identifying magnetic scatters even by non spin-polarized
scanning tunneling spectroscopy is explained.Comment: 4 pages, 4 figure
Correlation effects on the electronic structure of TiOCl: a NMTO+DMFT study
Using the recently developed N-th order muffin-tin orbital-based downfolding
technique in combination with the Dynamical Mean Field theory, we investigate
the electronic properties of the much discussed Mott insulator TiOCl in the
undimerized phase. Inclusion of correlation effects through this approach
provides a description of the spectral function into an upper and a lower
Hubbard band with broad valence states formed out of the orbitally polarized,
lower Hubbard band. We find that these results are in good agreement with
recent photo-emission spectra.Comment: 4 pages, 3 figure
Nature of the Mott transition in Ca2RuO4
We study the origin of the temperature-induced Mott transition in Ca2RuO4. As
a method we use the local-density approximation+dynamical mean-field theory. We
show the following. (i) The Mott transition is driven by the change in
structure from long to short c-axis layered perovskite (L-Pbca to S-Pbca); it
occurs together with orbital order, which follows, rather than produces, the
structural transition. (ii) In the metallic L-Pbca phase the orbital
polarization is ~0. (iii) In the insulating S-Pbca phase the lower energy
orbital, ~xy, is full. (iv) The spin-flip and pair-hopping Coulomb terms reduce
the effective masses in the metallic phase. Our results indicate that a similar
scenario applies to Ca_{2-x}Sr_xRuO_4 (x<0.2). In the metallic x< 0.5
structures electrons are progressively transferred to the xz/yz bands with
increasing x, however we find no orbital-selective Mott transition down to ~300
K.Comment: 4 pages, 3 figures; published versio
Double Counting in LDA+DMFT - The Example of NiO
An intrinsic issue of the LDA+DMFT approach is the so called double counting
of interaction terms. How to choose the double-counting potential in a manner
that is both physically sound and consistent is unknown. We have conducted an
extensive study of the charge transfer system NiO in the LDA+DMFT framework
using quantum Monte Carlo and exact diagonalization as impurity solvers. By
explicitly treating the double-counting correction as an adjustable parameter
we systematically investigated the effects of different choices for the double
counting on the spectral function. Different methods for fixing the double
counting can drive the result from Mott insulating to almost metallic. We
propose a reasonable scheme for the determination of double-counting
corrections for insulating systems.Comment: 7 pages, 6 figure
Formal Design of Asynchronous Fault Detection and Identification Components using Temporal Epistemic Logic
Autonomous critical systems, such as satellites and space rovers, must be
able to detect the occurrence of faults in order to ensure correct operation.
This task is carried out by Fault Detection and Identification (FDI)
components, that are embedded in those systems and are in charge of detecting
faults in an automated and timely manner by reading data from sensors and
triggering predefined alarms. The design of effective FDI components is an
extremely hard problem, also due to the lack of a complete theoretical
foundation, and of precise specification and validation techniques. In this
paper, we present the first formal approach to the design of FDI components for
discrete event systems, both in a synchronous and asynchronous setting. We
propose a logical language for the specification of FDI requirements that
accounts for a wide class of practical cases, and includes novel aspects such
as maximality and trace-diagnosability. The language is equipped with a clear
semantics based on temporal epistemic logic, and is proved to enjoy suitable
properties. We discuss how to validate the requirements and how to verify that
a given FDI component satisfies them. We propose an algorithm for the synthesis
of correct-by-construction FDI components, and report on the applicability of
the design approach on an industrial case-study coming from aerospace.Comment: 33 pages, 20 figure
- …