3,741 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 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
Thermally activated magnetization reversal in monoatomic magnetic chains on surfaces studied by classical atomistic spin-dynamics simulations
We analyze the spontaneous magnetization reversal of supported monoatomic
chains of finite length due to thermal fluctuations via atomistic spin-dynamics
simulations. Our approach is based on the integration of the Landau-Lifshitz
equation of motion of a classical spin Hamiltonian at the presence of
stochastic forces. The associated magnetization lifetime is found to obey an
Arrhenius law with an activation barrier equal to the domain wall energy in the
chain. For chains longer than one domain-wall width, the reversal is initiated
by nucleation of a reversed magnetization domain primarily at the chain edge
followed by a subsequent propagation of the domain wall to the other edge in a
random-walk fashion. This results in a linear dependence of the lifetime on the
chain length, if the magnetization correlation length is not exceeded. We
studied chains of uniaxial and tri-axial anisotropy and found that a tri-axial
anisotropy leads to a reduction of the magnetization lifetime due to a higher
reversal attempt rate, even though the activation barrier is not changed.Comment: 2nd version contains some improvements and new Appendi
Spin Orbit Coupling and Spin Waves in Ultrathin Ferromagnets: The Spin Wave Rashba Effect
We present theoretical studies of the influence of spin orbit coupling on the
spin wave excitations of the Fe monolayer and bilayer on the W(110) surface.
The Dzyaloshinskii-Moriya interaction is active in such films, by virtue of the
absence of reflection symmetry in the plane of the film. When the magnetization
is in plane, this leads to a linear term in the spin wave dispersion relation
for propagation across the magnetization. The dispersion relation thus assumes
a form similar to that of an energy band of an electron trapped on a
semiconductor surfaces with Rashba coupling active. We also show SPEELS
response functions that illustrate the role of spin orbit coupling in such
measurements. In addition to the modifications of the dispersion relations for
spin waves, the presence of spin orbit coupling in the W substrate leads to a
substantial increase in the linewidth of the spin wave modes. The formalism we
have developed applies to a wide range of systems, and the particular system
explored in the numerical calculations provides us with an illustration of
phenomena which will be present in other ultrathin ferromagnet/substrate
combinations
Lifetime reduction of surface states at Cu, Ag and Au(111) caused by impurity scattering
We present density-functional results on the lifetime of the (111) surface
state of the noble metals. We consider scattering on the Fermi surface caused
by impurity atoms belonging to the 3d and 4sp series. The results are analyzed
with respect to film thickness and with respect to separation of scattering
into bulk or into surface states. While for impurities in the surface layer the
overall trends are similar to the long-known bulk-state scattering, for
adatom-induced scattering we find a surprising behavior with respect to the
adatom atomic number. A plateau emerges in the scattering rate of the 3d
adatoms, instead of a peak characteristic of the d resonance. Additionally, the
scattering rate of 4sp adatoms changes in a zig-zag pattern, contrary to a
smooth parabolic increase following Linde's rule that is observed in bulk. We
interpret these results in terms of the weaker charge-screening and of
interference effects induced by the lowering of symmetry at the surface
Tuning Nanocrystal Surface Depletion by Controlling Dopant Distribution as a Route Toward Enhanced Film Conductivity
Electron conduction through bare metal oxide nanocrystal (NC) films is
hindered by surface depletion regions resulting from the presence of surface
states. We control the radial dopant distribution in tin-doped indium oxide
(ITO) NCs as a means to manipulate the NC depletion width. We find in films of
ITO NCs of equal overall dopant concentration that those with dopant-enriched
surfaces show decreased depletion width and increased conductivity. Variable
temperature conductivity data shows electron localization length increases and
associated depletion width decreases monotonically with increased density of
dopants near the NC surface. We calculate band profiles for NCs of differing
radial dopant distributions and, in agreement with variable temperature
conductivity fits, find NCs with dopant-enriched surfaces have narrower
depletion widths and longer localization lengths than those with
dopant-enriched cores. Following amelioration of NC surface depletion by atomic
layer deposition of alumina, all films of equal overall dopant concentration
have similar conductivity. Variable temperature conductivity measurements on
alumina-capped films indicate all films behave as granular metals. Herein, we
conclude that dopant-enriched surfaces decrease the near-surface depletion
region, which directly increases the electron localization length and
conductivity of NC films
Itinerant Nature of Atom-Magnetization Excitation by Tunneling Electrons
We have performed single-atom magnetization curve (SAMC) measurements and
inelastic scanning tunneling spectroscopy (ISTS) on individual Fe atoms on a
Cu(111) surface. The SAMCs show a broad distribution of magnetic moments with
\unit[3.5]{\mu_{\rm B}} being the mean value. ISTS reveals a magnetization
excitation with a lifetime of \unit[200]{fsec} which decreases by a factor of
two upon application of a magnetic field of \unit[12]{T}. The experimental
observations are quantitatively explained by the decay of the magnetization
excitation into Stoner modes of the itinerant electron system as shown by newly
developed theoretical modeling.Comment: 3 Figures, Supplement not included, updated version after revisio
Unoccupied surface and interface states in Pd thin films deposited on Fe/Ir(111) surface
We present a systematic first-principles study of the electronic surface
states and resonances occuring in thin films of Pd of various thicknesses
deposited on a single ferromagnetic monolayer of Fe on top of Ir(111)
substrate. This system is of interest since one Pd layer deposited on
Fe/Ir(111) hosts small magnetic skyrmions. The latter are topological magnetic
objects with swirling spin-textures with possible implications in the context
of spintronic devices since they have the potential to be used as magnetic bits
for information technology. The stabilization, detection and manipulation of
such non-collinear magnetic entities require a quantitative investigation and a
fundamental understanding of their electronic structure. Here we investigate
the nature of the unoccupied electronic states in Pd/Fe/Ir(111), which are
essential in the large spin-mixing magnetoresistance (XMR) signature captured
using non spin-polarized scanning tunnelling microscopy [Crum et al., Nat.
Commun. {\bf 6} 8541 (2015); Hanneken et al., Nat. Nanotech. {\bf 10}, 1039
(2015)]. To provide a complete analysis, we investigate bare Fe/Ir(111) and
Pd/Fe/Ir(111) surfaces. Our results demonstrate the emergence of
surface and interface states after deposition of Pd monolayers, which are
strongly impacted by the large spin-orbit coupling of Ir surface.Comment: 16 pages, 11 figure
Double-lambda microscopic model for entangled light generation by four-wave-mixing
Motivated by recent experiments, we study four-wave-mixing in an atomic
double-{\Lambda} system driven by a far-detuned pump. Using the
Heisenberg-Langevin formalism, and based on the microscopic properties of the
medium, we calculate the classical and quantum properties of seed and conju-
gate beams beyond the linear amplifier approximation. A continuous variable
approach gives us access to relative-intensity noise spectra that can be
directly compared to experiments. Restricting ourselves to the cold-atom
regime, we predict the generation of quantum-correlated beams with a
relative-intensity noise spectrum well below the standard quantum limit (down
to -6 dB). Moreover entanglement between seed and conjugate beams measured by
an inseparability down to 0.25 is expected. This work opens the way to the
generation of entangled beams by four-wave mixing in a cold atomic sample.Comment: 11 pages, 6 figures, submitted to PR
Control of atomic decay rates via manipulation of reservoir mode frequencies
We analyse the problem of a two-level atom interacting with a time-dependent
dissipative environment modelled by a bath of reservoir modes. In the model of
this paper the principal features of the reservoir structure remain constant in
time, but the microscopic structure does not. In the context of an atom in a
leaky cavity this corresponds to a fixed cavity and a time-dependent external
bath. In this situation we show that by chirping the reservoir modes
sufficiently fast it is possible to inhibit, or dramatically enhance the decay
of the atomic system, even though the gross reservoir structure is fixed. Thus
it is possible to extract energy from a cavity-atom system faster than the
empty cavity rate. Similar, but less dramatic effects are possible for moderate
chirps where partial trapping of atomic population is also possible.Comment: 12 pages, 9 figure
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