461 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
First-principles study of thin magnetic transition-metal silicide films on Si(001)
In order to combine silicon technology with the functionality of magnetic
systems, a number of ferromagnetic (FM) materials have been suggested for the
fabrication of metal/semiconductor heterojunctions. In this work, we present a
systematic study of several candidate materials in contact with the Si surface.
We employ density-functional theory calculations to address the thermodynamic
stability and magnetism of both pseudomorphic CsCl-like Si (=Mn, Fe, Co,
Ni) thin films and Heusler alloy MnSi (=Fe, Co, Ni) films on Si(001).
Our calculations show that Si-termination of the Si films is energetically
preferable during epitaxy since it minimizes the energetic cost of broken bonds
at the surface. Moreover, we can explain the calculated trends in thermodynamic
stability of the Si thin films in terms of the -Si bond-strength and the
3d orbital occupation. From our calculations, we predict that ultrathin
MnSi films are FM with sizable spin magnetic moments at the Mn atoms, while
FeSi and NiSi films are nonmagnetic. However, CoSi films display itinerant
ferromagnetism. For the MnSi films with Heusler-type structure, the MnSi
termination is found to have the highest thermodynamic stability. In the FM
ground state, the calculated strength of the effective coupling between the
magnetic moments of Mn atoms within the same layer approximately scales with
the measured Curie temperatures of the bulk MnSi compounds. In particular,
the CoMnSi/Si(001) thin film has a robust FM ground state as in the bulk,
and is found to be stable against a phase separation into CoSi/Si(001) and
MnSi/Si(001) films. Hence this material is of possible use in FM-Si
heterojunctions and deserves further experimental investigations.Comment: 13 pages, 8 figure
Spectrophotometric determination of the deltamethrin
In the province of Corrientes is found highest production of indoor plants from all Argentina, specially in the northeast of this province. This is because the low frequency orthelack of frosts, allowing a lower cost in the production, since heating is not needed. In this type of cultive is commonly used thedeltamethrin, as the result of this, the validation of a simple method comparable with the gas chromatography for the determination of this Pesticide on both, irrigation and consumption water, was studied. In this work, a spectrophotometric method is proposed for the determination of deltamethrin inirrigation water from the area of flower crops located in the Department of ConcepciĂłn in the province of Corrientes. Deltamethrin solutions in a range from 0,025 to 1 mg/L on irrigation water were prepared. The absorbance spectrum was scanned between 200 and 400 nm. The maximum absorbance was found at 220 nm. A calibration curve in the range from 0.025 to 1 mg/L, responded to A = (0.3246± 0.0224) C + (0.0096 ± 0.0068) with R2 = 0.998. The % RSD was 0.961 indicating good repeatability for the analytical procedure. The accuracy in the recovery experience was 99.0 - 109.6%. The statistical comparison using the t-test and the F-test indicates that there are no significant differences between GC and spectrophotometric methods, with a confidence level of 95%. The specificity and intermediate accuracy tests were satisfactory.Fil: Gimenez, L. I.. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Departamento de QuĂmica; ArgentinaFil: Michellod, A. M. M.. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Departamento de QuĂmica; ArgentinaFil: Jorge, M. J.. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Departamento de QuĂmica; ArgentinaFil: Pila, Andrea Natalia. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - Nordeste. Instituto de Modelado e InnovaciĂłn TecnolĂłgica. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Instituto de Modelado e InnovaciĂłn TecnolĂłgica; ArgentinaFil: BordĂłn, Alexander Germán. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Departamento de QuĂmica; ArgentinaFil: Profeta, Mariela InĂ©s. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Departamento de QuĂmica; ArgentinaFil: Romero, Jorge Marcelo. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Departamento de QuĂmica; ArgentinaFil: Jorge, Nelly Lidia. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Departamento de QuĂmica; Argentin
Adiabatic and non-adiabatic phonon dispersion in a Wannier function approach
We develop a first-principles scheme to calculate adiabatic and non-adiabatic
phonon frequencies in the full Brillouin zone. The method relies on the
variational properties of a force-constants functional with respect to the
first-order perturbation of the electronic charge density and on the
localization of the deformation potential in the Wannier function basis. This
allows for calculation of phonon dispersion curves free from convergence issues
related to Brillouin zone sampling. In addition our approach justify the use of
the static screened potential in the calculation of the phonon linewidth due to
decay in electron-hole pairs. We apply the method to the calculation of the
phonon dispersion and electron-phonon coupling in MgB and CaC. In both
compounds we demonstrate the occurrence of several Kohn anomalies, absent in
previous calculations, that are manifest only after careful electron and phonon
momentum integration. In MgB, the presence of Kohn anomalies on the
E branches improves the agreement with measured phonon spectra and
affects the position of the main peak in the Eliashberg function. In CaC we
show that the non-adiabatic effects on in-plane carbon vibrations are not
localized at zone center but are sizable throughout the full Brillouin zone.
Our method opens new perspectives in large-scale first-principles calculations
of dynamical properties and electron-phonon interaction.Comment: 18 pages, 8 figure
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
Fermi-surface topological phase transition and horizontal order-parameter nodes in CaFeAs under pressure
Iron-based compounds (IBS) display a surprising variety of superconducting
properties that seems to arise from the strong sensitivity of these systems to
tiny details of the lattice structure. In this respect, systems that become
superconducting under pressure, like CaFeAs, are of particular
interest. Here we report on the first directional point-contact
Andreev-reflection spectroscopy (PCARS) measurements on CaFeAs crystals
under quasi-hydrostatic pressure, and on the interpretation of the results
using a 3D model for Andreev reflection combined with ab-initio calculations of
the Fermi surface (within the density functional theory) and of the order
parameter symmetry (within a random-phase-approximation approach in a
ten-orbital model). The almost perfect agreement between PCARS results at
different pressures and theoretical predictions highlights the intimate
connection between the changes in the lattice structure, a topological
transition in the hole-like Fermi surface sheet, and the emergence on the same
sheet of an order parameter with a horizontal node line.Comment: 13 pages, 8 color figures. This is an author-created, un-copyedited
version of an article published in Scientific Reports. The published version
is available online, together with Supplementary Information, at
http://www.nature.com/articles/srep2639
Common effect of chemical and external pressures on the magnetic properties of RECoPO (RE = La, Pr)
We report a detailed investigation of RECoPO (RE = La, Pr) and LaCoAsO
materials performed by means of muon spin spectroscopy. Zero-field measurements
show that the electrons localized on the Pr ions do not play any role in
the static magnetic properties of the compounds. Magnetism at the local level
is indeed fully dominated by the weakly-itinerant ferromagnetism from the Co
sublattice only. The increase of the chemical pressure triggered by the
different ionic radii of La and Pr, on the other hand, plays a
crucial role in enhancing the value of the magnetic critical temperature and
can be mimicked by the application of external hydrostatic pressure up to 24
kbar. A sharp discontinuity in the local magnetic field at the muon site in
LaCoPO at around 5 kbar suggests a sizeable modification in the band structure
of the material upon increasing pressure. This scenario is qualitatively
supported by \emph{ab-initio} density-functional theory calculations.Comment: 13 pages, 10 figure
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