2,970 research outputs found

### Magnetic ground state of coupled edge-sharing CuO_2 spin-chains

By means of density functional theory, we investigate the magnetic ground
state of edge-sharing CuO_2 spin-chains, as found in the
(La,Ca,Sr)_14Cu_24O_41system, for instance. Our data rely on spin-polarized
electronic structure calculations including onsite interaction (LDA+U) and an
effective model for the interchain coupling. Strong doping dependence of the
magnetic order is characteristic for edge-sharing CuO_2 spin-chains. We
determine the ground state magnetic structure as function of the spin-chain
filling and quantify the competing exchange interactions.Comment: 5 pages, 2 figures, 3 tables, accepted by Phys. Rev. Let

### Pneumatic boot for helicopter rotor deicing

Pneumatic deicer boots for helicopter rotor blades were tested. The tests were conducted in the 6 by 9 ft icing research tunnel on a stationary section of a UH-IH helicopter main rotor blade. The boots were effective in removing ice and in reducing aerodynamic drag due to ice

### Magnetic structure and orbital ordering in BaCoO3 from first-principles calculations

Ab initio calculations using the APW+lo method as implemented in the WIEN2k
code have been used to describe the electronic structure of the
quasi-one-dimensional system BaCoO3. Both, GGA and LDA+U approximations were
employed to study different orbital and magnetic orderings. GGA predicts a
metallic ground state whereas LDA+U calculations yield an insulating and
ferromagnetic ground state (in a low-spin state) with an alternating orbital
ordering along the Co-Co chains, consistent with the available experimental
data.Comment: 8 pages, 9 figure

### Role of Strain on Electronic and Mechanical Response of Semiconducting Transition-Metal Dichalcogenide Monolayers: an ab-initio study

We characterize the electronic structure and elasticity of monolayer
transition-metal dichalcogenides MX2 (M=Mo, W, Sn, Hf and X=S, Se, Te) with 2H
and 1T structures using fully relativistic first principles calculations based
on density functional theory. We focus on the role of strain on the band
structure and band alignment across the series 2D materials. We find that
strain has a significant effect on the band gap; a biaxial strain of 1%
decreases the band gap in the 2H structures, by as a much 0.2 eV in MoS2 and
WS2, while increasing it for the 1T materials. These results indicate that
strain is a powerful avenue to modulate their properties; for example, strain
enables the formation of, otherwise impossible, broken gap heterostructures
within the 2H class. These calculations provide insight and quantitative
information for the rational development of heterostructures based on these
class of materials accounting for the effect of strain.Comment: 16 pages, 4 figures, 1 table, supplementary materia

### Layered Kondo lattice model for quantum critical beta-YbAlB4

We analyze the magnetic and electronic properties of the quantum critical
heavy fermion superconductor beta-YbAlB4, calculating the Fermi surface and the
angular dependence of the extremal orbits relevant to the de Haas--van Alphen
measurements. Using a combination of the realistic materials modeling and
single-ion crystal field analysis, we are led to propose a layered Kondo
lattice model for this system, in which two dimensional boron layers are Kondo
coupled via interlayer Yb moments in a $J_{z}=\pm 5/2$ state. This model fits
the measured single ion magnetic susceptibility and predicts a substantial
change in the electronic anisotropy as the system is pressure-tuned through the
quantum critical point.Comment: Fig.3 and 4 have been updated, typos corrected in v2. Published at
http://link.aps.org/doi/10.1103/PhysRevLett.102.07720

### The effect of hydrogen on the magnetic properties of FeV superlattice

The electronic and magnetic structures of a hydrogenated and hydrogen free
superlattice of 3 iron monolayers and 9 vanadium monolayers are studied using
the first principle full-potential augmented-plane-wave method as implemented
in WIEN2k package. The volume, the total energy and the magnetic moments of the
system are studied versus the hydrogen positions at the octahedral sites within
the superlattice and also versus the filling of the vanadium octahedral
location by hydrogen atoms. It is found that the hydrogen locations at the
interior of vanadium layer are energetically more favourable. The local Fe
magnetic moment and the average magnetic moment per supercell are found to
increase as the H position moves towards the Fe-V interface. On the other hand,
the average magnetic moment per supercell is found to initially decrease up to
filling by 3 H atoms and then increases afterwards. To our knowledge, this is
the first reporting on the increase in the computed magnetic moment with
hydrogenation. These trends of magnetic moments are attributed to the volume
changes resulting from hydrogenation and not to electronic hydrogen-metal
interaction.Comment: 13 pages, 5 figures and 2 table

### Confinement-induced metal-to-insulator transition in strained LaNiO$_3$/LaAlO$_3$ superlattices

Using density functional theory calculations including a Hubbard $U$ term we
explore the effect of strain and confinement on the electronic ground state of
superlattices containing the band insulator LaAlO$_3$ and the correlated metal
LaNiO$_3$. Besides a suppression of holes at the apical oxygen, a central
feature is the asymmetric response to strain in single unit cell superlattices:
For tensile strain a band gap opens due to charge disproportionation at the Ni
sites with two distinct magnetic moments of 1.45$\mu_{\rm B}$ and 0.71$\mu_{\rm
B}$. Under compressive stain, charge disproportionation is nearly quenched and
the band gap collapses due to overlap of $d_{3z^2-r^2}$ bands through a
semimetallic state. This asymmetry in the electronic behavior is associated
with the difference in octahedral distortions and rotations under tensile and
compressive strain. The ligand hole density and the metallic state are quickly
restored with increasing thickness of the (LaAlO$_3$)$_n$/(LaNiO$_3$)$_n$
superlattice from $n=1$ to $n=3$.Comment: 7 pages, 10 Figure

### Iron-based layered superconductor LaO$_{1-x}$F$_x$FeAs: an antiferromagnetic semimetal

We have studied the newly found superconductor compound LaO$_{1-x}$F$_x$FeAs
through the first-principles density functional theory calculations. We find
that the parent compound LaOFeAs is a quasi-2-dimensional antiferromgnetic
semimetal with most carriers being electrons and with a magnetic moment of
$2.3\mu_B$ located around each Fe atom on the Fe-Fe square lattice. Furthermore
this is a commensurate antiferromagnetic spin density wave due to the Fermi
surface nesting, which is robust against the F-doping. The observed
superconduction happens on the Fe-Fe antiferromagnetic layer, suggesting a new
superconductivity mechanism, mediated by the spin fluctuations. An abrupt
change on the Hall measurement is further predicted for the parent compound
LaOFeAs.Comment: 4 pages, 7 figure

### Supercell studies of the Fermi surface changes in the electron-doped superconductor LaFeAsO$_{1-x}$F$_x$

We study the changes in the Fermi surface with electron doping in the
LaFeAsO$_{1-x}$F$_x$ 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 $\Gamma-Z$ shrink in size, and the third
hole pocket around $Z$ disappears for an electron doping concentration in
excess of about 7-8%, while the two elliptical electron cylinders along $M-A$
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

### On the calculation of the bandgap of periodic solids with MGGA functionals using the total energy

During the last few years, it has become more and more clear that functionals of the meta generalized gradient approximation (MGGA) are more accurate than GGA functionals for the geometry and energetics of electronic systems. However, MGGA functionals are also potentially more interesting for the electronic structure, in particular, when the potential is nonmultiplicative (i.e., when MGGAs are implemented in the generalized Kohn-Sham framework), which may help to get more accurate bandgaps. Here, we show that the calculation of bandgap of solids with MGGA functionals can also be done very accurately in a non-self-consistent manner. This scheme uses only the total energy and can, therefore, be very useful when the self-consistent implementation of a particular MGGA functional is not available. Since self-consistent MGGA calculations may be difficult to converge, the non-self-consistent scheme may also help to speed up the calculations. Furthermore, it can be applied to any other types of functionals, for which the implementation of the corresponding potential is not trivial

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