48 research outputs found
Electronic correlation effects in the Cr2GeC Mn+1AXn phase
The magnetic properties, electronic band structure and Fermi surfaces of the
hexagonal Cr2GeC system have been studied by means of both generalized gradient
approximation (GGA) and the +U corrected method (GGA+U). The effective U value
has been computed within the augmented plane-wave theoretical scheme by
following the constrained density functional theory formalism of Anisimov et
al. [1991 Phys. Rev. B 45, 7570]. On the basis of our GGA+U calculations, a
compensated anti-ferromagnetic spin ordering of Cr atoms has been found to be
the ground state solution for this material, where a Ge-mediated super-exchange
coupling is responsible for an opposite spin distribution between the ABA
stacked in-plane Cr-C networks. Structural properties have also been tested and
found to be in good agreement with the available experimental data. Topological
analysis of Fermi surfaces have been used to qualitatively address the
electronic transport properties of Cr2GeC and found an important asymmetrical
carrier-type distribution within the hexagonal crystal lattice. We conclude
that an appropriate description of the strongly correlated Cr-d electrons is an
essential issue for interpreting the material properties of this unusual
Cr-based MAX-phase.Comment: 13 pages, 10 picture
Chemical bonding and electronic-structure in MAX phases as viewed by X-ray spectroscopy and density functional theory
This is a critical review of MAX-phase carbides and nitrides from an
electronic-structure and chemical bonding perspective. This large group of
nanolaminated materials is of great scientific and technological interest and
exhibit a combination of metallic and ceramic features. These properties are
related to the special crystal structure and bonding characteristics with
alternating strong M-C bonds in high-density MC slabs, and relatively weak M-A
bonds between the slabs. Here, we review the trend and relationship between the
chemical bonding, conductivity, elastic and magnetic properties of the MAX
phases in comparison to the parent binary MX compounds with the underlying
electronic structure probed by polarized X-ray spectroscopy. Spectroscopic
studies constitute important tests of the results of state-of-the-art
electronic structure density functional theory that is extensively discussed
and are generally consistent. By replacing the elements on the M, A, or X-sites
in the crystal structure, the corresponding changes in the conductivity,
elasticity, magnetism and other materials properties makes it possible to
tailor the characteristics of this class of materials by controlling the
strengths of their chemical bonds.Comment: 46 Pages, 23 Figures, 6 Table
Magnetic anisotropy in Cr_(2)GeC investigated by X-ray magnetic circular dichroism and ab initio calculations
The magnetism in the inherently nanolaminated ternary MAX-phase Cr_(2)GeC is investigated by element-selective, polarization and temperature-dependent, soft X-ray absorption spectroscopy and X-ray magnetic circular dichroism. The measurements indicate an antiferro-magnetic Cr-Cr coupling along the c-axis of the hexagonal structure modulated by a ferromagnetic ordering in the nanolaminated ab-basal planes. The weak chromium magnetic moments are an order of magnitude stronger in the nanolaminated planes than along the vertical axis. Theoretically, a small but notable, non-spin-collinear component explains the existence of a non-perfect spin compensation along the c-axis. As shown in this work, this spin distortion generates an overall residual spin moment inside the unit cell resembling that of a ferri-magnet. Due to the different competing magnetic interactions, electron correlations and temperature effects both need to be considered to achieve a correct theoretical description of the Cr_(2)GeC magnetic properties
The electronic-structure origin of the anisotropic thermopower of nanolaminated Ti3SiC2 determined by polarized x-ray spectroscopy and Seebeck measurements
Nanolaminated materials exhibit characteristic magnetic, mechanical, and
thermoelectric properties, with large contemporary scientific and technological
interest. Here, we report on the anisotropic Seebeck coefficient in
nanolaminated Ti3SiC2 single-crystal thin films and trace the origin to
anisotropies in element-specific electronic states. In bulk polycrystalline
form, Ti3SiC2 has a virtually zero Seebeck coefficient over a wide temperature
range. In contrast, we find that the in-plane (basal ab) Seebeck coefficient of
Ti3SiC2, measured on single-crystal films has a substantial and positive value
of 4-6 muV/K. Employing a combination of polarized angle-dependent x-ray
spectroscopy and density functional theory we directly show electronic
structure anisotropy in inherently nanolaminated Ti3SiC2 single-crystal thin
films as a model system. The density of Ti 3d and C 2p states at the Fermi
level in the basal ab-plane is about 40 % higher than along the c-axis. The
Seebeck coefficient is related to electron and hole-like bands close to the
Fermi level but in contrast to ground state density functional theory modeling,
the electronic structure is also influenced by phonons that need to be taken
into account. Positive contribution to the Seebeck coefficient of the
element-specific electronic occupations in the basal plane is compensated by 73
% enhanced Si 3d electronic states across the laminate plane that give rise to
a negative Seebeck coefficient in that direction. Strong phonon vibration modes
with three to four times higher frequency along the c-axis than along the basal
ab-plane also influence the electronic population and the measured spectra by
the asymmetric average displacements of the Si atoms. These results constitute
experimental evidence explaining why the average Seebeck coefficient of Ti3SiC2
in polycrystals is negligible over a wide temperature range.Comment: 13 pages, 6 figures; http://prb.aps.org/abstract/PRB/v85/i19/e19513
Electronic structure investigation of the cubic inverse perovskite Sc3AlN
The electronic structure and chemical bonding of the recently discovered
inverse perovskite Sc3AlN, in comparison to ScN and Sc metal have been
investigated by bulk-sensitive soft x-ray emission spectroscopy. The measured
Sc L, N K, Al L1, and Al L2,3 emission spectra are compared with calculated
spectra using first principle density-functional theory including dipole
transition matrix elements. The main Sc 3d - N 2p and Sc 3d - Al 3p chemical
bond regions are identified at -4 eV and -1.4 eV below the Fermi level,
respectively. A strongly modified spectral shape of 3s states in the Al L2,3
emission from Sc3AlN in comparison to pure Al metal is found, which reflects
the Sc 3d - Al 3p hybridization observed in the Al L1 emission. The differences
between the electronic structure of Sc3AlN, ScN, and Sc metal are discussed in
relation to the change of the conductivity and elastic properties.Comment: 11 pages, 5 picture
Four Years of Earthquake Early Warning in Southern Iberia: 2016-2019
The performance of an earthquake early warning system (EEWS) for southern Iberia during the period of 2016-2019 is analyzed. The software PRESTo (PRobabilistic and Evolutionary early warning SysTem; the University of Naples Federico II, Italy) operating at the Universidad Complutense de Madrid has detected 728 events (2 < M-w < 6.3), with 680 earthquakes occurring in southern Iberia. Differences between the EEWS origin time and epicenter and those of the Instituto Geografico Nacional (IGN) catalog are less than 2 s and 20 km, respectively, for 70% of the detected earthquakes. The main differences correspond to the EEWS magnitude that is underestimated for earthquakes that occurred at the west of the Gibraltar Strait (M-w differences larger than 0.3 for 70%). To solve this problem, several relationships have been tested, and a modification to those that currently use PRESTo is proposed. Other improvements, such as to densify the network or to use 3D Earth models, are proposed to decrease the time needed to issue the alert and avoid the false alerts (19 events over a total of 728 events). The EEWS has estimated the depth for 680 events and compared to those from the IGN (491 events). The performance of PRESTo during the 2020-2021 Granada swarm is analyzed. The hypocentral locations for the three largest earthquakes are close to those from the IGN (differences from 1 to 7 km for the epicenter and 0 s for the time origin), although there are some differences in their magnitude estimations that varies from 0.2 to 0.5. The PRESTo first times are 17, 25, and 41 s after the origin time. This study shows that the actual PRESTo EEWS configured for the southern Iberia may generate effective warnings despite the low seismicity rate in this region. To decrease the warning time, the geometry and density of the seismic network must be improved together with the use of 3D Earth models and on-site system approaches.</p
Low viscosity of the Earth’s inner core
The Earth’s solid inner core is a highly attenuating medium. It consists mainly of iron. The high attenuation of sound wave propagation in the inner core is at odds with the widely accepted paradigm of hexagonal close-packed phase stability under inner core conditions, because sound waves propagate through the hexagonal iron without energy dissipation. Here we show by first-principles molecular dynamics that the body-centered cubic phase of iron, recently demonstrated to be thermodynamically stable under the inner core conditions, is considerably less elastic than the hexagonal phase. Being a crystalline phase, the bodycentered cubic phase of iron possesses the viscosity close to that of a liquid iron. The high attenuation of sound in the inner core is due to the unique diffusion characteristic of the body-centered cubic phase. The low viscosity of iron in the inner core enables the convection and resolves a number of controversies
Towards a generalized vision of oxides: disclosing the role of cations and anions in determining unit-cell dimensions
Theoretical calculations of the electron-localization function show that, at the volumes of the two CaO phases (rocksalt and CsCl type), the parent Ca structures (fcc: face-centred cubic; sc: simple cubic) exhibit charge-concentration zones which coincide with the positions occupied by the O atoms in their oxides. For the first time, the structure type, dimension and topology of CaO and BaSnO3 are explained in univocal physical terms
EPOS (European Plate Observation System)
[EN] EPOS (European Plate Observation System) (https://www.epos-ip.org/) is now established as the only European
multidisciplinary and global research infrastructure in Earth Sciences. It integrates several hundred national
observatories for the observation and measurement of the internal and dynamic structure of the planet, and in particular
in Europe, distributed in 25 European countries, including Spain. The EPOS project, included in the 2008 ESFRI
Roadmap, has been recognized by ESFRI in 2016 as a priority project for its implementation, because of its strategic
relevance in the European Research Area.[ES] EPOS (European Plate Observation System) (https://www.epos-ip.org/) se constituye actualmente como la única
Infraestructura europea de Investigación multidisciplinar y global en Ciencias de la Tierra. Integra varios cientos de
observatorios nacionales para la observación y medición de la estructura interna y dinámica del planeta, y en particular
en Europa, distribuidos en 25 países de Europa, entre ellos España. El proyecto EPOS, incluido en la Hoja de Ruta
ESFRI de 2008, ha sido reconocido por ESFRI en 2016 como proyecto prioritario para su implementación, por su
relevancia estratégica en el ERA (European Research Area).Este trabajo ha sido sufragado por los proyectos EPOS Implementation Phase (EPOS IP) (Grant agreement no:676564-EPOS IP) del EU VII Framework Program, ESFRI y Red Temática “EPOS ESPAÑA” (CGL2016- 81965-REDT) y Clúster de cálculo y servicio en remoto del nodo español de EPOS Espacio (UCMA15-EE3294), del Ministerio de Economía, Industria y
Competitividad, España.Fernández, J.; Gallart, J.; Carbonell, R.; Díaz, J.; Villaseñor, A.; Azor, A.; Gonzalez-Matesanz, F.... (2017). EPOS (European Plate Observation System). En Primer Congreso en Ingeniería Geomática. Libro de actas. Editorial Universitat Politècnica de València. 127-134. https://doi.org/10.4995/CIGeo2017.2017.6615OCS12713