302 research outputs found
Coulomb correlation effects in zinc monochalcogenides
Electronic structure and band characteristics for zinc monochalcogenides with
zinc-blende- and wurtzite-type structures are studied by first-principles
density-functional-theory calculations with different approximations. It is
shown that the local-density approximation underestimates the band gap and
energy splitting between the states at the top of the valence band, misplaces
the energy levels of the Zn-3d states, and overestimates the
crystal-field-splitting energy. Regardless of the structure type considered,
the spin-orbit-coupling energy is found to be overestimated for ZnO and
underestimated for ZnS with wurtzite-type structure, and more or less correct
for ZnSe and ZnTe with zinc-blende-type structure. The order of the states at
the top of the valence band is found to be anomalous for ZnO in both
zinc-blende- and wurtzite-type structure, but is normal for the other zinc
monochalcogenides considered. It is shown that the Zn-3d electrons and their
interference with the O-2p electrons are responsible for the anomalous order.
The typical errors in the calculated band gaps and related parameters for ZnO
originate from strong Coulomb correlations, which are found to be highly
significant for this compound. The LDA+U approach is by and large found to
correct the strong correlation of the Zn-3d electrons, and thus to improve the
agreement with the experimentally established location of the Zn-3d levels
compared with that derived from pure LDA calculations
Tuning surface metallicity and ferromagnetism by hydrogen adsorption at the polar ZnO(0001) surface
The adsorption of hydrogen on the polar Zn-ended ZnO(0001) surface has been
investigated by density functional {\it ab-initio} calculations. An on top
H(1x1) ordered overlayer with genuine H-Zn chemical bonds is shown to be
energetically favorable. The H covered surface is metallic and spin-polarized,
with a noticeable magnetic moment at the surface region. Lower hydrogen
coverages lead to strengthening of the H-Zn bonds, corrugation of the surface
layer and to an insulating surface. Our results explain experimental
observations of hydrogen adsorption on this surface, and not only predict a
metal-insulator transition, but primarily provide a method to reversible switch
surface magnetism by varying the hydrogen density on the surface.Comment: 4 pages, 3 figure
Contrast Mechanisms for the Detection of Ferroelectric Domains with Scanning Force Microscopy
We present a full analysis of the contrast mechanisms for the detection of
ferroelectric domains on all faces of bulk single crystals using scanning force
microscopy exemplified on hexagonally poled lithium niobate. The domain
contrast can be attributed to three different mechanisms: i) the thickness
change of the sample due to an out-of-plane piezoelectric response (standard
piezoresponse force microscopy), ii) the lateral displacement of the sample
surface due to an in-plane piezoresponse, and iii) the electrostatic tip-sample
interaction at the domain boundaries caused by surface charges on the
crystallographic y- and z-faces. A careful analysis of the movement of the
cantilever with respect to its orientation relative to the crystallographic
axes of the sample allows a clear attribution of the observed domain contrast
to the driving forces respectively.Comment: 8 pages, 8 figure
Determination of Rayleigh and Lamb Wave Velocities in Diamond Films using an Acoustic Microscope
Acoustic microscopy is a powerful method of determining acoustic surface wave velocities with high spatial resolution. This paper describes the use of an acoustic microscope to measure these velocities in polycrystalline diamond films. Acoustic waves in diamond have a relatively high velocity and that affects the choice of the lens diameter and focal length. A general guideline will be given to determine the type of lens needed. The velocities measured in three diamond films were found to vary greatly depending on the film thickness. In two of the films it was found that Lamb modes rather than leaky Rayleigh waves were generated. After correcting for the associated dispersion, the measured velocities were found to deviate from the Lamb and Rayleigh velocities calculated from the single crystal elastic constants. The possibility for using these deviations to characterize the films will be discussed.</p
Effect of c(2x2)-CO overlayer on the phonons of Cu(001): a first principles study
We have examined the effect of a c(2x2) overlayer of CO on the surface
phonons of the substrate, Cu(001), by applying the density functional
perturbation theory with both the local (LDA) and the generalized-gradient
(GGA) density approximations, through the Hedin-Lundqvist and the
Perdew-Burke-Ernzerhof functionals, respectively. Our results (GGA) trace the
Rayleigh wave softening detected by helium atom scattering (HAS) experiments to
changes in the force constants between the substrate surface atoms brought
about by CO chemisorption, resolving an ongoing debate on the subject. The
calculated surface phonon dispersion curves document the changes in the
polarization of some modes and show those of the modes originally along the
direction of the clean surface Brillouin zone (SBZ) which are
back-folded along the direction of the chemisorbed SBZ, to be
particularly consequential. The vertical and shear horizontal section of
in the SBZ of the clean surface, for example, is back-folded as a
longitudinal-vertical mode, indicating thereby that predicted a long
time back along for the clean surface may be indirectly
assessed at upon CO adsorption by standard planar scattering
techniques. These findings further suggest that some of the energy losses
detected by HAS along , which were associated to multiphonon
excitations of the adlayer frustrated translation mode, may actually correspond
to the back-folded substrate surface modes
Large spin-phonon coupling and magnetically induced phonon anisotropy in SrMO3 perovskites (M = V,Cr,Mn,Fe,Co)
First-principles calculations reveal large zone-center spin-phonon coupling and magnetically-driven phonon anisotropy in cubic perovskites SrMO3 (M = V,Cr,Mn,Fe,Co). In particular, the frequency and splitting of the polar Slater mode is found to depend strongly upon magnetic ordering. The coupling is parameterized in a crystalstructure-dependent Heisenberg model, and its main features seen to arise from the Goodenough-Kanamori rules. This coupling can be expected to produce distinct low-energy alternative phases, resulting in a rich variety of coupled magnetic, structural, and electronic phase transitions driven by temperature, stress, electric field, and cation substitutionclose1
Nuclear spin driven quantum relaxation in LiY_0.998Ho_0.002F_4
Staircase hysteresis loops of the magnetization of a LiY_0.998Ho_0.002F_4
single crystal are observed at subkelvin temperatures and low field sweep
rates. This behavior results from quantum dynamics at avoided level crossings
of the energy spectrum of single Ho^{3+} ions in the presence of hyperfine
interactions. Enhanced quantum relaxation in constant transverse fields allows
the study of the relative magnitude of tunnel splittings. At faster sweep
rates, non-equilibrated spin-phonon and spin-spin transitions, mediated by weak
dipolar interactions, lead to magnetization oscillations and additional steps.Comment: 5 pages, 5 eps figures, using RevTe
On the Chemical Origin of the Gap Bowing in (GaAs)1âxGe2x Alloys: A Combined DFTâQSGW Study
Motivated by the research and analysis of new materials for photovoltaics and by the possibility of tailoring their optical properties for improved solar energy conversion, we have focused our attention on the (GaAs)1âxGe2x series of alloys. We have investigated the structural properties of some (GaAs)1âxGe2x compounds within the local-density approximation to density-functional theory, and their optical properties within the Quasiparticle Self-consistent GW approximation. The QSGW results confirm the experimental evidence of asymmetric bandgap bowing. It is explained in terms of violations of the octet rule, as well as in terms of the orderâdisorder phase transition
Discovery of rare variants associated with blood pressure regulation through meta-analysis of 1.3 million individuals
Correction: Volume53, Issue5 Page 762-762 DOI: 10.1038/s41588-021-00832-z Published MAY 2021Genetic studies of blood pressure (BP) to date have mainly analyzed common variants (minor allele frequency > 0.05). In a meta-analysis of up to similar to 1.3 million participants, we discovered 106 new BP-associated genomic regions and 87 rare (minor allele frequencyPeer reviewe
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