135 research outputs found
Direct evaluation of the isotope effect within the framework of density functional theory for superconductors
Within recent developments of density functional theory, its numerical implementation and of the superconducting density functional theory is nowadays possible to predict the superconducting critical temperature, Tc, with sufficient accuracy to anticipate the experimental verification. In this paper we present an analytical derivation of the isotope coefficient within the superconducting density functional theory. We calculate the partial derivative of Tc with respect to atomic masses. We verified the final expression by means of numerical calculations of isotope coefficient in monatomic superconductors (Pb) as well as polyatomic superconductors (CaC6). The results confirm the validity of the analytical derivation with respect to the finite difference methods, with considerable improvement in terms of computational time and calculation accuracy. Once the critical temperature is calculated (at the reference mass(es)), various isotope exponents can be simply obtained in the same run. In addition, we provide the expression of interesting quantities like partial derivatives of the deformation potential, phonon frequencies and eigenvectors with respect to atomic masses, which can be useful for other derivations and applications
Magneto-optics in pure and defective Ga_{1-x}Mn_xAs from first-principles
The magneto-optical properties of GaMnAs including their most
common defects were investigated with precise first--principles
density-functional FLAPW calculations in order to: {\em i}) elucidate the
origin of the features in the Kerr spectra in terms of the underlying
electronic structure; {\em ii}) perform an accurate comparison with
experiments; and {\em iii}) understand the role of the Mn concentration and
occupied sites in shaping the spectra. In the substitutional case, our results
show that most of the features have an interband origin and are only slightly
affected by Drude--like contributions, even at low photon energies. While not
strongly affected by the Mn concentration for the intermediately diluted range
( 10%), the Kerr factor shows a marked minimum (up to 1.5) occurring
at a photon energy of 0.5 eV. For interstitial Mn, the calculated
results bear a striking resemblance to the experimental spectra, pointing to
the comparison between simulated and experimental Kerr angles as a valid tool
to distinguish different defects in the diluted magnetic semiconductors
framework.Comment: 10 pages including 2 figures, submitted to Phys. Rev.
Optical response of metallic and insulating VO2 calculated with the LDA approach
We calculated the optical response of metallic and insulating VO2 using the
LDA approach. The band structure calculation was based in the full-potential
linear-muffin-tin method. The imaginary part of the dielectric function e2(w)
is related to the different optical transitions. The Drude tail in the
calculation of the metallic phase corresponds to intra-band d-d transitions.
The calculation in the insulating phase is characterized by the transitions to
the d||* band. The low frequency features, 0.0-5.0 eV, correspond to V 3d-V 3d
transitions, whereas the high frequency structures, 5.0-12 eV, are related to O
2p-V 3d transitions. The calculation helps to explain the imaginary part of the
dielectric function e2(w), as well as the electron-energy-loss and reflectance
spectra. The results reproduce not only the energy position and relative
intensity of the features in the spectra, but also the main changes across the
metal-insulator transition and the polarization dependence. The main difference
is a shift of about 0.6 eV in the calculation of the insulating phase. This
discrepancy arises because the LDA calculation underestimates the value of the
band gap
Static and Dynamical Susceptibility of LaO1-xFxFeAs
The mechanism of superconductivity and magnetism and their possible interplay
have recently been under debate in pnictides. A likely pairing mechanism
includes an important role of spin fluctuations and can be expressed in terms
of the magnetic susceptibility chi. The latter is therefore a key quantity in
the determination of both the magnetic properties of the system in the normal
state, and of the contribution of spin fluctuations to the pairing potential. A
basic ingredient to obtain chi is the independent-electron susceptibility chi0.
Using LaO1-xFxFeAs as a prototype material, in this report we present a
detailed ab-initio study of chi0(q,omega), as a function of doping and of the
internal atomic positions. The resulting static chi0(q,0) is consistent with
both the observed M-point related magnetic stripe phase in the parent compound,
and with the existence of incommensurate magnetic structures predicted by
ab-initio calculations upon doping.Comment: 15 pages, 8 figure
Cone beam computed tomography investigation of the antral artery anastomosis in a population of Central Italy
Background: The arterial vascular supply of maxillary sinus has to be considered in all the surgical procedures where it is involved. In particular, the intraosseous anastomosis between the posterior superior alveolar artery and the infraorbital artery branches in the bony canal can be tricky to a not well aware clinician. The aim of this study is to investigate the arterial blood supply of the maxillary sinus to give clinicians the basis for a better understanding of vascular complications that can derive from surgical procedures at this level.
Materials and methods: One hundred cone beam computed tomography were analysed by the Imaging software for three-dimensional images, i-Dixel 2.0. The parameters considered have been the presence (i), the calibre (ii), the dorso-ventral length (iii) and the cranio-caudal distance (iv). The data have been processed with means, standard deviations and verified by T-Student test.
Results: The statistical outputs showed that the 38% of samples presented the intraosseous anastomosis. Those anastomosis resulted long in dorso-ventral way both on left (12.55 ± 4.3 mm) and right side (12.4 ± 4.3 mm). The mean cranio- -caudal distance resulted 15.71 ± 5.08 mm on the left and 14.73 ± 4.74 mm on right side. The calibre measurements resulted quite big as well: 1.68 ± 0.3 mm on the left and 1.54 ± 0.38 mm on the right. The differences between the right and left sides were found not statistically significant.
Conclusions: This in vivo investigation shows how a knowledge of the maxillary sinus vascularisation is essential during the programming surgical phase in order to prevent blood complications during the operations involving this region.
Electric fields and valence band offsets at strained [111] heterojunctions
[111] ordered common atom strained layer superlattices (in particular the
common anion GaSb/InSb system and the common cation InAs/InSb system) are
investigated using the ab initio full potential linearized augmented plane wave
(FLAPW) method. We have focused our attention on the potential line-up at the
two sides of the homopolar isovalent heterojunctions considered, and in
particular on its dependence on the strain conditions and on the strain induced
electric fields. We propose a procedure to locate the interface plane where the
band alignment could be evaluated; furthermore, we suggest that the
polarization charges, due to piezoelectric effects, are approximately confined
to a narrow region close to the interface and do not affect the potential
discontinuity. We find that the interface contribution to the valence band
offset is substantially unaffected by strain conditions, whereas the total band
line-up is highly tunable, as a function of the strain conditions. Finally, we
compare our results with those obtained for [001] heterojunctions.Comment: 18 pages, Latex-file, to appear in Phys.Rev.
Petals of Crocus sativus L. as a potential source of the antioxidants crocin and kaempferol.
Collaboration between Leicester School of Pharmacy - De Montfort Universit, Department of Life, Health and Environmental Sciences - University of L'Aquila, and Department of Biochemistry and Biotechnology - University of Thessaly
The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link.Saffron fromthe province of L'Aquila, in the Abruzzo region of Italy, is highly prized and has been awarded a formal recognition by the European Union with EU Protected Designation of Origin (PDO) status. Despite this, the saffron regions are abandoned by the younger generations because the traditional cultivation of saffron (Crocus sativus L.) is labour intensive and yields only one crop of valuable saffron stamens per year. Petals of
the saffron Crocus have had additional uses in traditional medicine and may add value to the crops for local farmers. This is especially important because the plant only flowers between October andNovember, and farmers will need to make the best use of the flowers harvested in this period. Recently, the petals of C. sativus L., which are considered a wastematerial in the production of saffron spice,were identified as a potential source of natural antioxidants. The antioxidants crocin and kaempferol were purified by flash column chromatography, and identified by thin layer chromatography (TLC), HPLC–DAD, infrared (IR), and nuclear magnetic resonance (1H & 13C NMR) spectroscopy. The antioxidant activity was determined with the ABTS and DPPH tests. The antioxidant activities are
mainly attributed to carotenoid and flavonoid compounds, notably glycosides of crocin and kaempferol. We found in dried petals 0.6% (w/w) and 12.6 (w/w) of crocin and kaempferol, respectively. Petals of C. sativus L. have commercial potential as a source for kaempferol and crocetin glycosides, natural compounds with antioxidant activity that are considered to be the active ingredients in saffron-based herbal medicine
Optical study of the electronic phase transition of strongly correlated YbInCu_4
Infrared, visible and near-UV reflectivity measurements are used to obtain
conductivity as a function of temperature and frequency in YbInCu_4, which
exhibits an isostructural phase-transition into a mixed-valent phase below
T_v=42 K. In addition to a gradual loss of spectral weight with decreasing
temperature extending up to 1.5 eV, a sharp resonance appears at 0.25 eV in the
mixed-valent phase. This feature can be described in terms of excitations into
the Kondo (Abrikosov-Suhl) resonance, and, like the sudden reduction of
resistivity, provides a direct reflection of the onset of coherence in this
strongly correlated electron system.Comment: 4 pages, 3 figures (to appear in Phys. Rev. B
Superconducting properties of MgB2 from first principles
The discovery of superconductivity in MgB2, with a rather high transition temperature, has triggered a large number of theoretical and experimental investigations on important issues such as, e.g., the role of gap anisotropy over the Fermi surface (multi-gap superconductivity). We report here the results obtained in this compound using the density functional theory for superconductors, recently proposed by the authors. Without invoking any adjustable parameters, such as [mu]*, we obtain the transition temperature, the gaps, and the specific heat in very good agreement with experiment. Moreover, our calculations allow for a detailed study of how the phonon-mediated attraction and Coulomb repulsion act differently on [sigma] and [pi] states, thereby stabilizing the observed superconducting phase.http://www.sciencedirect.com/science/article/B6TVJ-4N20714-2/1/b96566edc28b627f83c5f7d5284e030
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