120 research outputs found
The role of non-local exchange in the electronic structure of correlated oxides
We present a systematic study of the electronic structure of several
prototypical correlated transition-metal oxides: VO2, V2O3, Ti2O3, LaTiO3, and
YTiO3. In all these materials, in the low-temperature insulating phases the
local and semilocal density approximations (LDA and GGA) of density-functional
theory yield a metallic Kohn-Sham band structure. Here we show that, without
invoking strong-correlation effects, the role of non-local exchange is
essential to cure the LDA/GGA delocalization error and provide a band-structure
description of the electronic properties in qualitative agreement with the
experimental photoemission results. To this end, we make use of hybrid
functionals that mix a portion of non-local Fock exchange with the local LDA
exchange-correlation potential. Finally, we discuss the advantages and the
shortcomings of using hybrid functionals for correlated transition-metal
oxides.Comment: submitte
Low-energy excitations in strongly correlated materials: A theoretical and experimental study of the dynamic structure factor in V2O3
PACS number(s): 71.45.Gm, 71.15.−m, 71.30.+h.-- et al.This work contains an experimental and theoretical study of the dynamic structure factor at large momentum transfer |Q|∼4 Å−1 of the strongly correlated transition-metal oxide V2O3. We focus in particular on the transitions between d states that give rise to the spectra below 6 eV. We show that the main peak in this energy range is mainly due to t2g→egσ transitions, and that it carries a signature of the phase transition between the paramagnetic insulator and the paramagnetic metal that can already be understood from the joint density of states calculated at the level of the static local density approximation. Instead, in order to obtain theoretical spectra that are overall similar to the measured ones, we have to go beyond the static approximation and include at least crystal local field effects. The latter turn out to be crucial in order to eliminate a spurious peak and hence allow a safe comparison between theory and experiment, including an analysis of the strong anisotropy of the spectra.Use of the Advanced Photon Source at Argonne National Laboratory was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. We are grateful for support by ETSF-I3 Grant No. 211956. Computer time was granted by IDRIS (544). F.I. also acknowledges financial
support from the CEA program Transversal Nanosciences and M.G. from the European Research Council Advanced Grant DYNamo (ERC-2010-AdG Proposal No. 267374), Spanish
Grants No. FIS2011-65702-C02-01 and No. PIB2010US-00652, ACI-Promociona (ACI2009-1036), Grupos Consolidados
UPV/EHU del Gobierno Vasco (IT-319-07), Consolider nanoTHERM (Grant No. CSD2010-00044), and European Commission projects CRONOS (280879-2 CRONOS CPFP7)
and THEMA (FP7-NMP-2008-SMALL-2, 228539).Peer reviewe
Role of nonlocal exchange in the electronic structure of correlated oxides
We present a systematic study of the electronic structure of several prototypical correlated transition-metal oxides: VO2, V2O3, Ti2O3, LaTiO3, and YTiO3. In all these materials, in the low-temperature insulating phases the local and semilocal density approximations (LDA and GGA, respectively) of density-functional theory yield a metallic Kohn-Sham band structure. Here we show that, without invoking strong-correlation effects, the role of nonlocal exchange is essential to cure the LDA/GGA delocalization error and provide a band-structure description of the electronic properties in qualitative agreement with the experimental photoemission results. To this end, we make use of hybrid functionals that mix a portion of nonlocal Fock exchange with the local LDA exchange-correlation potential. Finally, we discuss the advantages and the shortcomings of using hybrid functionals for correlated transition-metal oxides.Financial support was provided by Spanish MEC (Grants No. FIS2011-65702-C02-01 and No. PIB2010US-00652), ACI-Promociona (Grant No. ACI2009-1036), Grupos Consolidados UPV/EHU del GobiernoVasco (GrantsNo. IT-319-07), and the European Research CouncilAdvanced GrantDYNamo (ERC-2010-AdG -Proposal No. 267374). Computational time was granted by i2basque and BSC “Red Española de Supercomputación.”Peer Reviewe
Engineering Silicon Nanocrystals: Theoretical study of the effect of Codoping with Boron and Phosphorus
We show that the optical and electronic properties of nanocrystalline silicon
can be efficiently tuned using impurity doping. In particular, we give
evidence, by means of ab-initio calculations, that by properly controlling the
doping with either one or two atomic species, a significant modification of
both the absorption and the emission of light can be achieved. We have
considered impurities, either boron or phosphorous (doping) or both (codoping),
located at different substitutional sites of silicon nanocrystals with size
ranging from 1.1 nm to 1.8 nm in diameter. We have found that the codoped
nanocrystals have the lowest impurity formation energies when the two
impurities occupy nearest neighbor sites near the surface. In addition, such
systems present band-edge states localized on the impurities giving rise to a
red-shift of the absorption thresholds with respect to that of undoped
nanocrystals. Our detailed theoretical analysis shows that the creation of an
electron-hole pair due to light absorption determines a geometry distortion
that in turn results in a Stokes shift between adsorption and emission spectra.
In order to give a deeper insight in this effect, in one case we have
calculated the absorption and emission spectra going beyond the single-particle
approach showing the important role played by many-body effects. The entire set
of results we have collected in this work give a strong indication that with
the doping it is possible to tune the optical properties of silicon
nanocrystals.Comment: 14 pages 19 figure
Lattice radiation therapy in clinical practice: a systematic review
Purpose: Lattice radiation therapy (LRT) is an innovative type of spatially fractionated radiation therapy. It aims to increase large tumors control probability by administering ablative doses without an increased toxicity. Considering the rising number of positive clinical experiences, the objective of this work is to evaluate LRT safety and efficacy. Method: Reports about LRT clinical experience were identified with a systematic review conducted on four different databases (namely, Medline, Embase, Scopus, and Cochrane Library) through the August 2022. Only LRT clinical reports published in English and with the access to the full manuscript text were considered as eligible. The 2020 update version PRISMA statement was followed. Results: Data extraction was performed from 12 eligible records encompassing 7 case reports, 1 case series, and 4 clinical studies. 81 patients (84 lesions) with a large lesion ranging from 63.2 cc to 3713.5 cc were subjected to exclusive, hybrid, and metabolism guided LRT. Excluding two very severe toxicity with a questionable relation with LRT, available clinical experience seem to confirm LRT safety. When a complete response was not achieved 3-6 months after LRT, a median lesion reduction approximately ≥50 % was registered. Conclusion: This systematic review appear to suggest LRT safety, especially for exclusive LRT. The very low level of evidence and the studies heterogeneity preclude drawing definitive conclusions on LRT efficacy, even though an interesting trend in terms of lesions reduction has been described
Bismuth iron garnet: ab initio study of electronic properties
Bismuth iron garnet (BIG), i.e. Bi3Fe5O12, is a strong ferrimagnet that also
possess outstanding magneto-optical properties such as the largest known
Faraday rotation. These properties are related with the distribution of
magnetic moments on octahedral and tetrahedral sites, the presence of spin gaps
in the density of state and a strong spin-orbit coupling. In this work,
first-principles ab initio calculations are performed to study the structural,
electronic and magnetic properties of BIG using Density Functional Theory with
Hubbard+U (DFT+U) correction including spin-orbit coupling and HSE06 hybrid
functional. We found that the presence of spin gaps in the electronic structure
results from the interplay between exchange and correlation effects and the
crystal field strengths for tetrahedral and octahedral iron sublattices. The
DFT+U treatment tends to close the spin-gaps for larger U due to
over-localization effects, notably in the octahedral site. On the other hand,
the hybrid functional confirms the occurrences of three spin gaps in the iron
states of the conduction band as expected from optical measurements. A strong
exchange splitting at the top of the valence bands associated with a lone-pair
type mixture of O p and Bi s,p states is also obtained. Similar exchange
splitting was not previously observed for other iron based garnets, such as for
yttrium iron garnet. It follows that hole doping, as obtained by Ca
substitution at Bi sites, results in a full spin polarized density at the Fermi
energy. This work helps to shed more light on the theoretical comprehension of
the properties of BIG and opens the route towards the use of advanced Many Body
calculations to predict the magneto-optical coupling effects in BIG in a direct
comparison with the experimental measurements
Assessment of a standardized algorithm for cardiac pacing in older patients affected by severe unpredictable reflex syncopes
Can Radiotherapy Empower the Host Immune System to Counterattack Neoplastic Cells? A Systematic Review on Tumor Microenvironment Radiomodulation
Despite the rising evidence in favor of immunotherapy (IT), the treatment of oncological patients affected by so-called “cold tumors” still represents an open issue. Cold tumors are characterized by an immunosuppressive (so-called cold) tumor microenvironment (TME), which favors host immune system suppression, cancer immune-escape, and a worse response to IT. However, the TME is not a static element, but dynamically mutates and can be changed. Radiotherapy (RT) can modulate a cold microenvironment, rendering it better at tumor killing by priming the quiescent host immune system, with a consequent increase in immunotherapy response. The combination of TME radiomodulation and IT could therefore be a strategy for those patients affected by cold tumors, with limited or no response to IT. Thus, this review aims to provide an easy, rapid, and practical overview of how RT could convert the cold TME and why cold tumor radiomodulation could represent an interesting strategy in combination with IT
Structural and electronic properties of Si1-xGex alloy nanowires
We present first-principles density-functional calculations of Si1-xGex alloy nanowires. We show that given the composition of the alloy, the structural properties of the nanowires can be predicted with great accuracy by means of Vegard\u2019s law, linearly interpolating the values of a pure Si and a pure Ge nanowire of the same diameter. The same holds, to some extent, also for electronic properties such as the band-gap. We also assess to what extend the band-gap varies as a function of disorder, i.e., how it changes for different random realization of a given concentration. These results make possible to tailor the desired properties of SiGe alloy nanowires starting directly from the
data relative to the pristine wires
Conductance fluctuations in Si nanowires studied from first-principles
We study how the variability of the conductance associated with single-dopant configurations affects the overall conductivity of long, realistic ultrathin Si nanowires (NW). We calculate the resistance associated with each single-dopant configuration from density-functional theory (DFT) calculations and we sum them up classically to obtain the resistance of the long wire. This allows to identify limiting factors for the performance of Si NWs based devices
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