95 research outputs found
Optimized Effective Potential for Quantum Electrodynamical Time-Dependent Density Functional Theory
We propose a practical approximation to the exchange-correlation functional
of (time-dependent) density functional theory for many-electron systems coupled
to photons. The (time non-local) optimized effective potential (OEP) equation
for the electron- photon system is derived. We test the new approximation in
the Rabi model from weak to strong coupling regimes. It is shown that the OEP
(i) improves the classical description, (ii) reproduces the quantitative
behavior of the exact ground-state properties and (iii) accurately captures the
dynamics entering the ultra-strong coupling regime. The present formalism opens
the path to a first-principles description of correlated electron-photon
systems, bridging the gap between electronic structure methods and quantum
optics for real material applications.Comment: 5 pages, 2 figure
Eliashberg theory with ab-initio Coulomb interactions: a minimal numerical scheme applied to layered superconductors
We present a minimal approach to include static Coulomb interactions in Eliashberg theory of superconductivity from first principles. The method can be easily implemented in any existing Eliashberg code (isotropic or anisotropic) to avoid the standard use of the semiempirical parameter , which adds unnecessary uncertainty to Tc predictions. We evaluate the prediction accuracy of the method by simulating the superconducting properties of a set of layered superconductors, which feature unconventional Coulomb effects: CaC6, MgB2, Li-doped β-ZrNCl and YNi2B2C. We find that the estimated critical temperatures are consistent with those from ab-initio density functional theory for superconductors, and in close agreement with the experimental values
Coulomb interactions and conventional superconductivity: when going beyond the random phase approximation is essential
In ab initio calculations of superconducting properties, the Coulomb
repulsion is accounted for at the GW level and is usually computed in RPA,
which amounts to neglecting vertex corrections both at the polarizability level
and in the self-energy. Although this approach is unjustified, the brute force
inclusion of higher order corrections to the self-energy is computationally
prohibitive. We propose to use a generalized GW self-energy, where vertex
corrections are incorporated into W by employing the Kukkonen and Overhauser
(KO) ansatz for the effective interaction between two electrons in the electron
gas. By computing the KO interaction in the adiabatic local density
approximation for a diverse set of conventional superconductors, and using it
in the Eliashberg equations, we find that vertex corrections lead to a
systematic decrease of the critical temperature (Tc), ranging from a few
percent in bulk lead to more than 40% in compressed lithium. We propose a set
of simple rules to identify those systems where large Tc corrections are to be
expected and hence the use of the KO interaction is recommended. Our approach
offers a rigorous extension of the RPA and GW methods for the prediction of
superconducting properties at a negligible extra computational cost.Comment: 8 pages, 4 figure
Quantum Electrodynamical time-dependent density functional theory
121 p.The last two decades have witnessed increasing experimental interest in the study and control of many-electron systems strongly interacting with quantum electromagnetic fields. This includes notable experiments in the areas of cavity and circuit quantum electrodynamics, quantum computing via photon mediated atom entanglement, electromagnetically induced transparency, quantum plasmonics, quantum simulators and chemistry. The description of realistic coupled matter-photon systems requires combining electronic structure methods from material science with quantum-optical methods. In this work, we propose a formally exact and numerically feasible approach, termed quantum electrodynamical time-dependent density functional theory (QED-TDDFT), that generalizes successful TDDFT for electronic structure calculations, to the electron-photon coupling. We develop a general framework for the description of coupled matter-photon systems in all possible realizations of interest, ranging from the fully relativistic case, to well-known quantum-optical model (e.g., the Rabi model). Of particular interest for condensed-matter applications is the density functionalisation of non-relativistic QED. The underlying theory in fact corresponds to standard non-relativistic quantum mechanics for the electrons, but with additional terms, which correct for relativity and electron-photon interactions. However, any application of density-functional theory requires approximations to the exchange-correlation (xc) functional. In this work we construct such an approximation for the description of electron-photon interaction effects in an optical cavity (the theory of reference is QED-TDDFT for many-electron systems coupled to cavity photons). The derived electron-photon optimized effective potential (OEP) reduces in the static limit to the Lamb shift of the ground state. The OEP is then tested on the Rabi model from weak to strong coupling regimes. The results are promising and open a path for describing complex strongly coupled matter-photon systems. Further, we consider the inclusion of relativistic spin-spin effects into the xc functional of standard spin density functional theory. In this way we provide quantum corrections that are essential for the description of magnetic inhomogeneities at the nanoscale. Such an approach improves over the current (classical) micromagnetic treatment of domain walls and skyrmions for spintronic applications
The Influence of Graphene Curvature on Hydrogen Adsorption: Towards Hydrogen Storage Devices
The ability of atomic hydrogen to chemisorb on graphene makes the latter a
promising material for hydrogen storage. Based on scanning tunneling microscopy
techniques, we report on site-selective adsorption of atomic hydrogen on
convexly curved regions of monolayer graphene grown on SiC(0001). This system
exhibits an intrinsic curvature owing to the interaction with the substrate. We
show that at low coverage hydrogen is found on convex areas of the graphene
lattice. No hydrogen is detected on concave regions. These findings are in
agreement with theoretical models which suggest that both binding energy and
adsorption barrier can be tuned by controlling the local curvature of the
graphene lattice. This curvature-dependence combined with the known graphene
flexibility may be exploited for storage and controlled release of hydrogen at
room temperature making it a valuable candidate for the implementation of
hydrogen-storage devices
A Meta-Analysis of Brain DNA Methylation Across Sex, Age, and Alzheimer\u27s Disease Points for Accelerated Epigenetic Aging in Neurodegeneration
Alzheimer\u27s disease (AD) is characterized by specific alterations of brain DNA methylation (DNAm) patterns. Age and sex, two major risk factors for AD, are also known to largely affect the epigenetic profiles in brain, but their contribution to AD-associated DNAm changes has been poorly investigated. In this study we considered publicly available DNAm datasets of four brain regions (temporal, frontal, entorhinal cortex, and cerebellum) from healthy adult subjects and AD patients, and performed a meta-analysis to identify sex-, age-, and AD-associated epigenetic profiles. In one of these datasets it was also possible to distinguish 5-methylcytosine (5mC) and 5-hydroxymethylcytosine (5hmC) profiles. We showed that DNAm differences between males and females tend to be shared between the four brain regions, while aging differently affects cortical regions compared to cerebellum. We found that the proportion of sex-dependent probes whose methylation is modified also during aging is higher than expected, but that differences between males and females tend to be maintained, with only a few probes showing age-by-sex interaction. We did not find significant overlaps between AD- and sex-associated probes, nor disease-by-sex interaction effects. On the contrary, we found that AD-related epigenetic modifications are significantly enriched in probes whose DNAm varies with age and that there is a high concordance between the direction of changes (hyper or hypo-methylation) in aging and AD, supporting accelerated epigenetic aging in the disease. In summary, our results suggest that age-associated DNAm patterns concur to the epigenetic deregulation observed in AD, providing new insights on how advanced age enables neurodegeneration
Evidence-Based Education. Ten points of dispute
<p>The approach called Evidence-Based Education (EBE) has had a very large expansion in recent years and has developed an international debate on potentialities and criticalities of scientific research in education. The paper presents a debate imagined in which ten criticisms that could be addressed to EBE by researchers and teachers are discussed. To these critics it is answered by trying to explain this approach and reflect on aspects that seem to affect its future progress.</p><div><h3>Evidence-Based Education. Dieci punti di controversia</h3><p>Come è noto, l’orientamento che va sotto il nome di Evidence-Based Education (EBE) ha avuto una fortissima espansione negli ultimi anni e ha sviluppato un forte dibattito internazionale sulle potenzialità e criticità della ricerca scientifica nell’ambito dell’educazione. Nell’intento di contribuire ad una maggiore chiarezza sugli elementi della controversia e sugli equivoci che si possono generare intorno al diffondersi di questo orientamento, questo lavoro ha cercato di sintetizzare in dieci punti i termini essenziali del dibattito in corso. Sono state individuate le critiche più note avanzate all’EBE e ad esse sono state contrapposte le argomentazioni dei suoi fautori.</p></div
Sigmund Tobias & Thomas M. Duffy, Constructivist Instruction: Success or Failure? New York: Routledge (2009).
Recensione del testo che vede la contrapposizione e il dialogo fra costruttivisti e istruttivisti
Revealing the atomic structure of the buffer layer between SiC(0001) and epitaxial graphene
On the SiC(0001) surface (the silicon face of SiC), epitaxial graphene is
obtained by sublimation of Si from the substrate. The graphene film is
separated from the bulk by a carbon-rich interface layer (hereafter called the
buffer layer) which in part covalently binds to the substrate. Its structural
and electronic properties are currently under debate. In the present work we
report scanning tunneling microscopy (STM) studies of the buffer layer and of
quasi-free-standing monolayer graphene (QFMLG) that is obtained by decoupling
the buffer layer from the SiC(0001) substrate by means of hydrogen
intercalation. Atomic resolution STM images of the buffer layer reveal that,
within the periodic structural corrugation of this interfacial layer, the
arrangement of atoms is topologically identical to that of graphene. After
hydrogen intercalation, we show that the resulting QFMLG is relieved from the
periodic corrugation and presents no detectable defect sites
Neuroprotective effects of thymoquinone by the modulation of ER stress and apoptotic pathway in in vitro model of excitotoxicity.
Experimental evidence indicates that the activation of ionotropic glutamate receptors plays an important role in neurological disorders’ models such as epilepsy, cerebral ischemia and trauma. The glutamate receptor agonist kainic acid (KA) induces seizures and excitotoxic cell death in the CA3 region of the hippocampus. Thymoquinone (TQ) is the most important component of the essential oil obtained from black cumin (Nigella sativa L.) seeds. It has many pharmacological actions including antioxidant, anti-inflammatory, and anti-apoptotic effects. TQ was used in an in vitro experimental model of primary cultures where excitotoxicity was induced. Briefly, rat organotypic hippocampal slices were exposed to 5 µM KA for 24 h. Cell death in the CA3 subregions of slices was quantified by measuring propidium iodide fluorescence. The cross-talk between TQ, ER stress and apoptotic pathways was investigated by Western blot. In untreated slices TQ (10 µM) induced a significant increase on the PSD95 levels and it decreased the excitotoxic injury induced by KA. Additionally, TQ was able to ameliorate the KA-induced increase in unfolded proteins GRP78 and GRP94 expression. Finally, TQ was able to partially rescue the reduction of the KA-induced apoptotic pathway activation. Our results suggest that TQ modulates the processes leading to post-kainate neuronal death in the CA3 hippocampal area
- …