794 research outputs found
Non-equilibrium Bethe-Salpeter equation for transient photo-absorption spectroscopy
In this work we propose an accurate first-principle approach to calculate the
transient photo--absorption spectrum measured in Pump\&\,Probe experiments. We
formulate a condition of {\em adiabaticity} and thoroughly analyze the
simplifications brought about by the fulfillment of this condition in the
non--equilibrium Green's function (NEGF) framework. Starting from the
Kadanoff-Baym equations we derive a non--equilibrium Bethe--Salpeter equation
(BSE) for the response function that can be implemented in most of the already
existing {\em ab--initio} codes. In addition, the {\em adiabatic} approximation
is benchmarked against full NEGF simulations in simple model hamiltonians, even
under extreme, nonadiabatic conditions where it is expected to fail. We find
that the non--equilibrium BSE is very robust and captures important spectral
features in a wide range of experimental configurations.Comment: 13 pages, 5 captioned figure
First-principles approach to excitons in time-resolved and angle-resolved photoemission spectra
We show that any {\em quasi-particle} or GW approximation to the self-energy
does not capture excitonic features in time-resolved (TR) photoemission
spectroscopy. In this work we put forward a first-principles approach and
propose a feasible diagrammatic approximation to solve this problem. We also
derive an alternative formula for the TR photocurrent which involves a single
time-integral of the lesser Green's function. The diagrammatic approximation
applies to the {\em relaxed} regime characterized by the presence of
quasi-stationary excitons and vanishing polarization. The main distinctive
feature of the theory is that the diagrams must be evaluated using {\em
excited} Green's functions. As this is not standard the analytic derivation is
presented in detail. The final result is an expression for the lesser Green's
function in terms of quantities that can all be calculated {\em ab initio}. The
validity of the proposed theory is illustrated in a one-dimensional model
system with a direct gap. We discuss possible scenarios and highlight some
universal features of the exciton peaks. Our results indicate that the exciton
dispersion can be observed in TR {\em and} angle-resolved photoemission.Comment: 15 pages, 8 figure
Optical properties of periodic systems within the current-current response framework: pitfalls and remedies
We compare the optical absorption of extended systems using the
density-density and current-current linear response functions calculated within
many-body perturbation theory. The two approaches are formally equivalent for a
finite momentum of the external perturbation. At
, however, the equivalence is maintained only if a small
expansion of the density-density response function is used. Moreover, in
practical calculations this equivalence can be lost if one naively extends the
strategies usually employed in the density-based approach to the current-based
approach. Specifically we discuss the use of a smearing parameter or of the
quasiparticle lifetimes to describe the finite width of the spectral peaks and
the inclusion of electron-hole interaction. In those instances we show that the
incorrect definition of the velocity operator and the violation of the
conductivity sum rule introduce unphysical features in the optical absorption
spectra of three paradigmatic systems: silicon (semiconductor), copper (metal)
and lithium fluoride (insulator). We then demonstrate how to correctly
introduce lifetime effects and electron-hole interactions within the
current-based approach.Comment: 17 pages, 6 figure
Stabilization of tetragonal/cubic phase in Fe doped Zirconia grown by atomic layer deposition
Achieving high temperature ferromagnetism by doping transition metals thin
films is seen as a viable approach to integrate spin-based elements in
innovative spintronic devices. In this work we investigated the effect of Fe
doping on structural properties of ZrO2 grown by atomic layer deposition (ALD)
using Zr(TMHD)4 for Zr and Fe(TMHD)3 for Fe precursors and ozone as oxygen
source. The temperature during the growth process was fixed at 350{\deg}C. The
ALD process was tuned to obtain Fe doped ZrO2 films with uniform chemical
composition, as seen by time of flight secondary ion mass spectrometry. The
control of Fe content was effectively reached, by controlling the ALD precursor
pulse ratio, as checked by X-ray photoemission spectroscopy (XPS) and
spectroscopic ellipsometry. From XPS, Fe was found in Fe3+ chemical state,
which maximizes the magnetization per atom. We also found, by grazing incidence
X-ray diffraction, that the inclusion of Fe impurities in ZrO2 induces
amorphization in thin ZrO2 films, while stabilizes the high temperature
crystalline tetragonal/cubic phase after rapid thermal annealing at 600{\deg}C.Comment: 11 pages, 7 figures, 1 Tabl
Spinorial formulation of the GW-BSE equations and spin properties of excitons in two-dimensional transition metal dichalcogenides
In many paradigmatic materials, such as transition metal dichalcogenides, the role played by the spin
degrees of freedom is as important as the one played by the electron-electron interaction. Thus an accurate
treatment of the two effects and of their interaction is necessary for an accurate and predictive study of the
optical and electronic properties of these materials. Despite the fact that the GW-BSE approach correctly
accounts for electronic correlations, the spin-orbit coupling effect is often neglected or treated perturbatively.
Recently, spinorial formulations of GW-BSE have become available in different flavors in material-science
codes. However, an accurate validation and comparison of different approaches is still missing. In this work,
we go through the derivation of the noncollinear GW-BSE approach. The scheme is applied to transition
metal dichalcogenides comparing the perturbative and full spinorial approaches. Our calculations reveal that
dark-bright exciton splittings are generally improved when the spin-orbit coupling is included nonperturbatively.
The exchange-driven intravalley mixing between the A and B excitons is found to play a role for Mo-based
systems, being especially strong in the case of MoSe 2 . We finally compute the excitonic spin and use it to
sharply analyze the spinorial properties of transition metal dichalcogenide excitonic states
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