3,892 research outputs found
Halogenation of SiC for band-gap engineering and excitonic functionalization
The optical excitation spectra and excitonic resonances are investigated in
systematically functionalized SiC with Fluorine and/or Chlorine utilizing
density functional theory in combination with many-body perturbation theory.
The latter is required for a realistic description of the energy band-gaps as
well as for the theoretical realization of excitons. Structural, electronic and
optical properties are scrutinized and show the high stability of the predicted
two-dimensional materials. Their realization in laboratory is thus possible.
Huge band-gaps of the order of 4 eV are found in the so-called GW
approximation, with the occurrence of bright excitons, optically active in the
four investigated materials. Their binding energies vary from 0.9 eV to 1.75 eV
depending on the decoration choice and in one case, a dark exciton is foreseen
to exist in the fully chlorinated SiC. The wide variety of opto-electronic
properties suggest halogenated SiC as interesting materials with potential not
only for solar cell applications, anti-reflection coatings or high-reflective
systems but also for a possible realization of excitonic Bose-Einstein
condensation
Theory of real space imaging of Fermi surfaces
A scanning tunneling microscope can be used to visualize in real space Fermi
surfaces with buried impurities far below substrates acting as local probes. A
theory describing this feature is developed based on the stationary phase
approximation. It is demonstrated how a Fermi surface of a material acts as a
mirror focusing electrons that scatter at hidden impurities.Comment: 10 pages, 4 figure
Tuning paramagnetic spin-excitations of single adatoms
Around 50 years ago, Doniach [Proc. Phys. Soc. 91, 86 (1967)] predicted the
existence of paramagnons in nearly ferromagnetic materials, recently measured
in bulk Pd [Phys. Rev. Lett. 105, 027207 (2010)]. Here we predict the analogous
effect for single adatoms, namely paramagnetic spin-excitations (PSE). Based on
time-dependent density functional theory, we demonstrate that these overdamped
excitations acquire a well-defined peak structure in the meV energy region when
the adatom's Stoner criterion for magnetism is close to the critical point. In
addition, our calculations reveal a subtle tunability and enhancement of PSE by
external magnetic fields, exceeding by far the response of bulk paramagnons and
even featuring the atomic version of a quantum phase transition. We further
demonstrate how PSE can be detected as moving steps in the
signal of state-of-the-art inelastic scanning tunneling spectroscopy, opening a
potential route for experimentally accessing fundamental electronic properties
of non-magnetic adatoms, such as the Stoner parameter.Comment: 6 pages, 3 figure
Non-collinear Korringa-Kohn-Rostoker Green function method: Application to 3d nanostructures on Ni(001)
Magnetic nanostructures on non-magnetic or magnetic substrates have attracted
strong attention due to the development of new experimental methods with atomic
resolution. Motivated by this progress we have extended the full-potential
Korringa-Kohn-Rostoker (KKR) Green function method to treat non-collinear
magnetic nanostructures on surfaces. We focus on magnetic 3d impurity
nanoclusters, sitting as adatoms on or in the first surface layer on Ni(001),
and investigate the size and orientation of the local moments and moreover the
stabilization of non-collinear magnetic solutions. While clusters of Fe, Co, Ni
atoms are magnetically collinear, non-collinear magnetic coupling is expected
for Cr and Mn clusters on surfaces of elemental ferromagnets. The origin of
frustration is the competition of the antiferromagnetic exchange coupling among
the Cr or Mn atoms with the antiferromagnetic (for Cr) or ferromagnetic (for
Mn) exchange coupling between the impurities and the substrate. We find that Cr
and Mn first-neighbouring dimers and a Mn trimer on Ni(001) show non-collinear
behavior nearly degenerate with the most stable collinear configuration.
Increasing the distance between the dimer atoms leads to a collinear behavior,
similar to the one of the single impurities. Finally, we compare some of the
non-collinear {\it ab-initio} results to those obtained within a classical
Heisenberg model, where the exchange constants are fitted to total energies of
the collinear states; the agreement is surprisingly good.Comment: 11 page
Zero-point quantum swing of magnetic couples
Quantum fluctuations are ubiquitous in physics. Ranging from conventional
examples like the harmonic oscillator to intricate theories on the origin of
the universe, they alter virtually all aspects of matter -- including
superconductivity, phase transitions and nanoscale processes. As a rule of
thumb, the smaller the object, the larger their impact. This poses a serious
challenge to modern nanotechnology, which aims total control via atom-by-atom
engineered devices. In magnetic nanostructures, high stability of the magnetic
signal is crucial when targeting realistic applications in information
technology, e.g. miniaturized bits. Here, we demonstrate that zero-point
spin-fluctuations are paramount in determining the fundamental magnetic
exchange interactions that dictate the nature and stability of the magnetic
state. Hinging on the fluctuation-dissipation theorem, we establish that
quantum fluctuations correctly account for the large overestimation of the
interactions as obtained from conventional static first-principles frameworks,
filling in a crucial gap between theory and experiment [1,2]. Our analysis
further reveals that zero-point spin-fluctuations tend to promote the
non-collinearity and stability of chiral magnetic textures such as skyrmions --
a counter-intuitive quantum effect that inspires practical guidelines for
designing disruptive nanodevices
Theory of Local Dynamical Magnetic Susceptibilities from the Korringa-Kohn-Rostoker Green Function Method
Within the framework of time-dependent density functional theory combined
with the Korringa-Kohn-Rostoker Green function formalism, we present a real
space methodology to investigate dynamical magnetic excitations from
first-principles. We set forth a scheme which enables one to deduce the correct
effective Coulomb potential needed to preserve the spin-invariance signature in
the dynamical susceptibilities, i.e. the Goldstone mode. We use our approach to
explore the spin dynamics of 3d adatoms and different dimers deposited on a
Cu(001) with emphasis on their decay to particle-hole pairs.Comment: 32 pages (preprint), 6 figures, one tabl
Surface state scattering by adatoms on noble metals
When surface state electrons scatter at perturbations, such as magnetic or
nonmagnetic adatoms or clusters on surfaces, an electronic resonance, localized
at the adatom site, can develop below the bottom of the surface state band for
both spin channels. In the case of adatoms, these states have been found very
recently in scanning tunneling spectroscopy experiments\cite{limot,olsson} for
the Cu(111) and Ag(111) surfaces. Motivated by these experiments, we carried
out a systematic theoretical investigation of the electronic structure of these
surface states in the presence of magnetic and non-magnetic atoms on Cu(111).
We found that Ca and all 3 adatoms lead to a split-off state at the bottom
of the surface band which is, however, not seen for the elements Ga and
Ge. The situation is completely reversed if the impurities are embedded in the
surface: Ga and Ge are able to produce a split-off state whereas the 3
impurities do not. The resonance arises from the s-state of the impurities and
is explained in terms of strength and interaction nature (attraction or
repulsion) of the perturbing potential.Comment: 6 pages, 5 figure
Engineering elliptical spin-excitations by complex anisotropy fields in Fe adatoms and dimers on Cu(111)
We investigate the dynamics of Fe adatoms and dimers deposited on the Cu(111)
metallic surface in the presence of spin-orbit coupling, within time-dependent
density functional theory. The \textit{ab initio} results provide
material-dependent parameters that can be used in semiclassical approaches,
which are used for insightful interpretations of the excitation modes. By
manipulating the surroundings of the magnetic elements, we show that elliptical
precessional motion may be induced through the modification of the magnetic
anisotropy energy. We also demonstrate how different kinds of spin precession
are realized, considering the symmetry of the magnetic anisotropy energy, the
ferro- or antiferromagnetic nature of the exchange coupling between the
impurities, and the strength of the magnetic damping. In particular, the normal
modes of a dimer depend on the initial magnetic configuration, changing
drastically by going from a ferromagnetic metastable state to the
antiferromagnetic ground state. By taking into account the effect of the
damping into their resonant frequencies, we reveal that an important
contribution arises for strongly biaxial systems and specially for the
antiferromagnetic dimers with large exchange couplings. Counter intuitively,
our results indicate that the magnetic damping influences the quantum
fluctuations by decreasing the zero-point energy of the system
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