1,569 research outputs found
Comparative study of FeCr2S4 and FeSc2S4: Spinels with orbitally active A site
Using first-principles density functional calculations, we perform a
comparative study of two Fe based spinel compounds, FeCr2S4 and FeSc2S4. Though
both systems contain an orbitally active A site with an Fe2+ ion, their
properties are rather dissimilar. Our study unravels the microscopic origin of
their behavior driven by the differences in hybridization of Fe d states with
Cr/Sc d states and S p states in the two cases. This leads to important
differences in the nature of the magnetic exchanges as well as the nearest
versus next nearest neighbor exchange parameter ratios, resulting into
significant frustration effects in FeSc2S4 which are absent in FeCr2S4.Comment: 5 pages, 4 figures Phys Rev B (rapid commun) to appear (2010
Supercell studies of the Fermi surface changes in the electron-doped superconductor LaFeAsOF
We study the changes in the Fermi surface with electron doping in the
LaFeAsOF superconductors with density-functional supercell
calculations using the linearized augmented planewave (LAPW) method. The
supercell calculations with explicit F substitution are compared with those
obtained from the virtual crystal approximation (VCA) and from a simple rigid
band shift. We find significant differences between the supercell results and
those obtained from the rigid-band shift with electron doping, although quite
remarkably the supercell results are in good agreement with the virtual crystal
approximation (VCA) where the nuclear charges of the O atoms are slightly
increased to mimic the addition of the extra electrons. With electron doping,
the two cylindrical hole pockets along shrink in size, and the third
hole pocket around disappears for an electron doping concentration in
excess of about 7-8%, while the two elliptical electron cylinders along
expand in size. The spin-orbit coupling does not affect the Fermi surface much
except to somewhat reduce the size of the third hole pocket in the undoped
case. We find that with the addition of the electrons the antiferromagnetic
state becomes energetically less stable as compared to the nonmagnetic state,
indicating that the electron doping may provide an extra degree of stability to
the formation of the superconducting ground state.Comment: 7 pages, 8 figure
Accounting for spin fluctuations beyond LSDA in the density functional theory
We present a method to correct the magnetic properties of itinerant systems
in local spin density approximation (LSDA) and we apply it to the
ferromagnetic-paramagnetic transition under pressure in a typical itinerant
system, NiAl. We obtain a scaling of the critical fluctuations as a
function of pressure equivalent to the one obtained within Moryia's theory.
Moreover we show that in this material the role of the bandstructure is crucial
in driving the transition. Finally we calculate the magnetic moment as a
function of pressure, and find that it gives a scaling of the Curie temperature
that is in good agreement with the experiment. The method can be easily
extended to the antiferromagnetic case and applied, for instance, to the
Fe-pnictides in order to correct the LSDA magnetic moment.Comment: 7 pages, 4 figure
Importance of electronic correlations for structural and magnetic properties of the iron pnictide superconductor LaFeAsO
We present calculations of structural and magnetic properties of the
iron-pnictide superconductor LaFeAsO including electron-electron correlations.
For this purpose we apply a fully charge self-consistent combination of
Density-Functional Theory with the Dynamical Mean-Field theory, allowing for
the calculation of total energies. We find that the inclusion of correlation
effects gives a good agreement of the Arsenic z position with experimental data
even in the paramagnetic (high-temperature) phase. Going to low temperatures,
we study the formation of the ordered moment in the striped spin-density-wave
phase, yielding an ordered moment of about 0.60, again in good agreement with
experiments. This shows that the inclusion of correlation effects improves both
structural and magnetic properties of LaFeAsO at the same time.Comment: 7 pages, 5 figures, published versio
Electronic states and pairing symmetry in the two-dimensional 16 band d-p model for iron-based superconductor
The electronic states of the FeAs plane in iron-based superconductors are
investigated on the basis of the two-dimensional 16-band d-p model, where the
tight-binding parameters are determined so as to fit the band structure
obtained by the density functional calculation for LaFeAsO. The model includes
the Coulomb interaction on a Fe site: the intra- and inter-orbital direct terms
U and U', the exchange coupling J and the pair-transfer J'. Within the random
phase approximation (RPA), we discuss the pairing symmetry of possible
superconducting states including s-wave and d-wave pairing on the U'-J plane.Comment: 2 pages, 4 figures; Proceedings of the Int. Symposium on
Fe-Oxipnictide Superconductors (Tokyo, 28-29th June 2008
Laser-induced spin protection and switching in a specially designed magnetic dot: A theoretical investigation
Most laser-induced femtosecond magnetism investigations are done in magnetic
thin films. Nanostructured magnetic dots, with their reduced dimensionality,
present new opportunities for spin manipulation. Here we predict that if a
magnetic dot has a dipole-forbidden transition between the lowest occupied
molecular orbital (LUMO) and the highest unoccupied molecular orbital (HOMO),
but a dipole-allowed transition between LUMO+1 and HOMO, electromagnetically
inducedtransparency can be used to prevent ultrafast laser-induced spin
momentum reduction, or spin protection. This is realized through a strong dump
pulse to funnel the population into LUMO+1. If the time delay between the pump
and dump pulses is longer than 60 fs, a population inversion starts and spin
switching is achieved. Thesepredictions are detectable experimentally.Comment: 6 pages, three figur
Theoretical evidence for strong correlations and incoherent metallic state in FeSe
The role of electronic Coulomb correlations in iron-based superconductors is
an important open question. We provide theoretical evidence for strong
correlation effects in the FeSe compound, based on dynamical mean field
calculations. A lower Hubbard band is found in the spectral properties.
Moreover, together with significant orbital-dependent mass enhancements, we
find that the normal state is a bad metal over an extended temperature range,
implying a non-Fermi liquid. Predictions for angle-resolved photoemission
spectroscopy are made.Comment: 5 pages, 5 figures, published versio
Are the renormalized band widths in TTF-TCNQ of structural or electronic origin? - An angular dependent NEXAFS study
We have performed angle-dependent near-edge x-ray absorption fine structure
measurements in the Auger electron yield mode on the correlated
quasi-one-dimensional organic conductor TTF-TCNQ in order to determine the
orientation of the molecules in the topmost surface layer. We find that the
tilt angles of the molecules with respect to the one-dimensional axis are
essentially the same as in the bulk. Thus we can rule out surface relaxation as
the origin of the renormalized band widths which were inferred from the
analysis of photoemission data within the one-dimensional Hubbard model.
Thereby recent theoretical results are corroborated which invoke long-range
Coulomb repulsion as alternative explanation to understand the spectral
dispersions of TTF-TCNQ quantitatively within an extended Hubbard model.Comment: 6 pages, 5 figure
Magnetism and superconductivity at LAO/STO-interfaces: the role of Ti 3d interface electrons
Ferromagnetism and superconductivity are in most cases adverse. However,
recent experiments reveal that they coexist at interfaces of LaAlO3 and SrTiO3.
We analyze the magnetic state within density functional theory and provide
evidence that magnetism is not an intrinsic property of the two-dimensional
electron liquid at the interface. We demonstrate that the robust ferromagnetic
state is induced by the oxygen vacancies in SrTiO3- or in the LaAlO3-layer.
This allows for the notion that areas with increased density of oxygen
vacancies produce ferromagnetic puddles and account for the previous
observation of a superparamagnetic behavior in the superconducting state.Comment: 5 pages, 4 figures, to appear in Physical Review B (Rapid
Communications
Fermi Surface and Anisotropic Spin-Orbit Coupling of Sb(111) studied by Angle-Resolved Photoemission Spectroscopy
High-resolution angle-resolved photoemission spectroscopy has been performed
on Sb(111) to elucidate the origin of anomalous electronic properties in
group-V semimetal surfaces. The surface was found to be metallic despite the
semimetallic character of bulk. We clearly observed two surface-derived Fermi
surfaces which are likely spin split, demonstrating that the spin-orbit
interaction plays a dominant role in characterising the surface electronic
states of group-V semimetals. Universality/disimilarity of the electronic
structure in Bi and Sb is discussed in relation to the granular
superconductivity, electron-phonon coupling, and surface charge/spin density
wave.Comment: 4 pages, 3 figures. to be published in Phys. Rev. Let
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