140 research outputs found
Pressure-Induced Simultaneous Metal-Insulator and Structural-Phase Transitions in LiH: a Quasiparticle Study
A pressure-induced simultaneous metal-insulator transition (MIT) and
structural-phase transformation in lithium hydride with about 1% volume
collapse has been predicted by means of the local density approximation (LDA)
in conjunction with an all-electron GW approximation method. The LDA wrongly
predicts that the MIT occurs before the structural phase transition. As a
byproduct, it is shown that only the use of the generalized-gradient
approximation together with the zero-point vibration produces an equilibrium
lattice parameter, bulk modulus, and an equation of state that are in excellent
agreement with experimental results.Comment: 7 pages, 4 figures, submitted to Europhysics Letter
Adsorption of Cu, Ag, and Au atoms on graphene including van der Waals interactions
We performed a systematic density functional study of the adsorption of
copper, silver, and gold adatoms on graphene, especially accounting for van der
Waals interactions by the vdW-DF and the PBE+D2 methods. In particular, we
analyze the preferred adsorption site (among top, bridge, and hollow positions)
together with the corresponding distortion of the graphene sheet and identify
diffusion paths. Both vdW schemes show that the coinage metal atoms do bind to
the graphene sheet and that in some cases the buckling of the graphene can be
significant. The results for silver are at variance with those obtained with
GGA, which gives no binding in this case. However, we observe some quantitative
differences between the vdW-DF and the PBE+D2 methods. For instance the
adsorption energies calculated with the PBE+D2 method are systematically higher
than the ones obtained with vdW-DF. Moreover, the equilibrium distances
computed with PBE+D2 are shorter than those calculated with the vdW-DF method
Systematic Study on Fluorine-doping Dependence of Superconducting and Normal State Properties in LaFePO1-xFx
We have investigated the fluorine-doping dependence of lattice constants,
transports and specific heat for polycrystalline LaFePO1-xFx. F doping slightly
and monotonically decreases the in-plane lattice parameter. In the normal
state, electrical resistivity at low temperature is proportional to the square
of temperature and the electronic specific heat coefficient has large value,
indicating the existence of moderate electron-electron correlation in this
system. Hall coefficient has large magnitude, and shows large temperature
dependence, indicating the low carrier density and multiple carriers in this
system. Temperature dependence of the upper critical field suggests that the
system is a two gap superconductor. The F-doping dependence of these properties
in this system are very weak, while in the FeAs system (LaFeAsO), the F doping
induces the large changes in electronic properties. This difference is probably
due to the different F-doping dependence of the lattice in these two systems.
It has been revealed that a pure effect of electron doping on electronic
properties is very weak in this Fe pnictide compound.Comment: 8 pages, 5 figures, accepted for publication in J. Phys. Soc. Jp
Assessing the Performance of Recent Density Functionals for Bulk Solids
We assess the performance of recent density functionals for the
exchange-correlation energy of a nonmolecular solid, by applying accurate
calculations with the GAUSSIAN, BAND, and VASP codes to a test set of 24 solid
metals and non-metals. The functionals tested are the modified
Perdew-Burke-Ernzerhof generalized gradient approximation (PBEsol GGA), the
second-order GGA (SOGGA), and the Armiento-Mattsson 2005 (AM05) GGA. For
completeness, we also test more-standard functionals: the local density
approximation, the original PBE GGA, and the Tao-Perdew-Staroverov-Scuseria
(TPSS) meta-GGA. We find that the recent density functionals for solids reach a
high accuracy for bulk properties (lattice constant and bulk modulus). For the
cohesive energy, PBE is better than PBEsol overall, as expected, but PBEsol is
actually better for the alkali metals and alkali halides. For fair comparison
of calculated and experimental results, we consider the zero-point phonon and
finite-temperature effects ignored by many workers. We show how Gaussian basis
sets and inaccurate experimental reference data may affect the rating of the
quality of the functionals. The results show that PBEsol and AM05 perform
somewhat differently from each other for alkali metal, alkaline earth metal and
alkali halide crystals (where the maximum value of the reduced density gradient
is about 2), but perform very similarly for most of the other solids (where it
is often about 1). Our explanation for this is consistent with the importance
of exchange-correlation nonlocality in regions of core-valence overlap.Comment: 32 pages, single pdf fil
Density functional investigations of defect induced mid-gap states in graphane
We have carried out ab initio electronic structure calculations on graphane
(hydrogenated graphene) with single and double vacancy defects. Our analysis of
the density of states reveal that such vacancies induce the mid gap states and
modify the band gap. The induced states are due to the unpaired electrons on
carbon atoms. Interestingly the placement and the number of such states is
found to be sensitive to the distance between the vacancies. Furthermore we
also found that in most of the cases the vacancies induce a local magnetic
moment.Comment: 15 page
Optical signature of symmetry variations and spin-valley coupling in atomically thin tungsten dichalcogenides
Motivated by the triumph and limitation of graphene for electronic
applications, atomically thin layers of group VI transition metal
dichalcogenides are attracting extensive interest as a class of graphene-like
semiconductors with a desired band-gap in the visible frequency range. The
monolayers feature a valence band spin splitting with opposite sign in the two
valleys located at corners of 1st Brillouin zone. This spin-valley coupling,
particularly pronounced in tungsten dichalcogenides, can benefit potential
spintronics and valleytronics with the important consequences of spin-valley
interplay and the suppression of spin and valley relaxations. Here we report
the first optical studies of WS2 and WSe2 monolayers and multilayers. The
efficiency of second harmonic generation shows a dramatic even-odd oscillation
with the number of layers, consistent with the presence (absence) of inversion
symmetry in even-layer (odd-layer). Photoluminescence (PL) measurements show
the crossover from an indirect band gap semiconductor at mutilayers to a
direct-gap one at monolayers. The PL spectra and first-principle calculations
consistently reveal a spin-valley coupling of 0.4 eV which suppresses
interlayer hopping and manifests as a thickness independent splitting pattern
at valence band edge near K points. This giant spin-valley coupling, together
with the valley dependent physical properties, may lead to rich possibilities
for manipulating spin and valley degrees of freedom in these atomically thin 2D
materials
Electronic properties of LaOFFeAs in the normal state probed by NMR/NQR
We report 139La, 57Fe and 75As nuclear magnetic resonance (NMR) and nuclear
quadrupole resonance (NQR) measurements on powders of the new LaO1-xFxFeAs
superconductor for x = 0 and x = 0.1 at temperatures up to 480 K, and compare
our measured NQR spectra with local density approximation (LDA) calculations.
For all three nuclei in the x = 0.1 material, it is found that the local Knight
shift increases monotonically with an increase in temperature, and scales with
the macroscopic susceptibility, suggesting a single magnetic degree of freedom.
Surprisingly, the spin lattice relaxation rates for all nuclei also scale with
one another, despite the fact that the form factors for each site sample
different regions of q-space. This result suggests a lack of any q-space
structure in the dynamical spin susceptibility that might be expected in the
presence of antiferromagnetic correlations. Rather, our results are more
compatible with simple quasi-particle scattering. Furthermore, we find that the
increase in the electric field gradient at the As cannot be accounted for by
LDA calculations, suggesting that structural changes, in particular the
position of the As in the unit cell, dominate the NQR response.Comment: 17 pages, 6 figure
To What Extent Iron-Pnictide New Superconductors Have Been Clarified: A Progress Report
In this review, the authors present a summary of experimental reports on
newly discovered iron-based superconductors as they were known at the end of
2008. At the same time, this paper is intended to be useful for experimenters
to know the current status of these superconductors. The authors introduce
experimental results that reveal basic physical properties in the normal and
superconducting states. The similarities and differences between iron-pnictide
superconductors and other unconventional superconductors are also discussed.Comment: 20 pages, 32 figures. Open selec
57Fe Mossbauer spectroscopy and magnetic measurements of oxygen deficient LaFeAsO
We report on the magnetic behavior of oxygen deficient LaFeAsO1-x (x-0.10)
compound, prepared by one-step synthesis, which crystallizes in the tetragonal
(S.G. P4/nmm) structure at room temperature. Resistivity measurements show a
strong anomaly near 150 K, which is ascribed to the spin density wave (SDW)
instability. On the other hand, dc magnetization data shows paramagnetic-like
features down to 5 K, with an effective moment of 0.83 mB/Fe. 57Fe Mossbauer
studies (MS) have been performed at 95 and 200 K. The spectra at both
temperatures are composed of two sub-spectra. At 200 K the major one (88%), is
almost a singlet, and corresponds to those Fe nuclei, which have two oxygen
ions in their close vicinity. The minor one, with a large quadrupole splitting,
corresponds to Fe nuclei, which have vacancies in their immediate neighborhood.
The spectrum at 95 K, exhibits a broadened magnetic split major (84%)
sub-spectrum and a very small magnetic splitting in the minor subspectrum. The
relative intensities of the subspectra facilitate in estimating the actual
amount of oxygen vacancies in the compound to be 7.0(5)%, instead of the
nominal LaFeAsO0.90. These results, when compared with reported 57Fe MS of
non-superconducting LaFeAsO and superconducting LaFeAsO0.9F0.1, confirm that
the studied LaFeAsO0.93 is a superconductivity-magnetism crossover compound of
the newly discovered Fe based superconducting family.Comment: 7 pages text + Figs : Comments/suggestions welcome
([email protected]
The effect of internal pressure on the tetragonal to monoclinic structural phase transition in ReOFeAs: the case of NdOFeAs
We report the temperature dependent x-ray powder diffraction of the
quaternary compound NdOFeAs (also called NdFeAsO) in the range between 300 K
and 95 K. We have detected the structural phase transition from the tetragonal
phase, with P4/nmm space group, to the orthorhombic or monoclinic phase, with
Cmma or P112/a1 (or P2/c) space group, over a broad temperature range from 150
K to 120 K, centered at T0 ~137 K. Therefore the temperature of this structural
phase transition is strongly reduced, by about ~30K, by increasing the internal
chemical pressure going from LaOFeAs to NdOFeAs. In contrast the
superconducting critical temperature increases from 27 K to 51 K going from
LaOFeAs to NdOFeAs doped samples. This result shows that the normal striped
orthorhombic Cmma phase competes with the superconducting tetragonal phase.
Therefore by controlling the internal chemical pressure in new materials it
should be possible to push toward zero the critical temperature T0 of the
structural phase transition, giving the striped phase, in order to get
superconductors with higher Tc.Comment: 9 pages, 3 figure
- …