460 research outputs found
Optical second harmonic generation from Wannier excitons
Excitonic effects in the linear optical response of semiconductors are
well-known and the subject of countless experimental and theoretical studies.
For the technologically important second order nonlinear response, however,
description of excitonic effects has proved to be difficult. In this work, a
simplified three-band Wannier exciton model of cubic semiconductors is applied
and a closed form expression for the complex second harmonic response function
including broadening is derived. Our calculated spectra are found to be in
excellent agreement with the measured response near the band edge. In addition,
a very substantial enhancement of the nonlinear response is predicted for the
transparency region
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
Transferable Pair Potentials for CdS and ZnS Crystals
A set of interatomic pair potentials is developed for CdS and ZnS crystals.
We show that a simple energy function, which has been used to describe the
properties of CdSe [J. Chem. Phys. 116, 258 (2002)], can be parametrized to
accurately describe the lattice and elastic constants, and phonon dispersion
relations of bulk CdS and ZnS in the wurtzite and rocksalt crystal structures.
The predicted coexistence pressure of the wurtzite and rocksalt structures, as
well as the equation of state are in good agreement with experimental
observations. These new pair potentials enable the study of a wide range of
processes in bulk and nanocrystalline II-VI semiconductor materials
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
First principles calculations of the electronic and geometric structure of nanoalloy
\emph{Ab initio} calculations of the structure and electronic density of
states (DOS) of the perfect core-shell nanoalloy attest to its
symmetry and confirm that it has only 6 non-equivalent (2 and 4
) atoms. Analysis of bond-length, average formation energy, heat of
formation of and alloys provide an explanation
for the relative stability of the former with respect to the other nanoalloys
in the same family. The HOMO-LUMO gap is found to be 0.77 eV, in agreement with
previous results. Analysis of the DOS of , alloys
and related systems provides insight into the effects of low coordination,
contraction/expansion and the presence of foreign atoms on the DOS of and
. While some characteristics of the DOS are reminiscent of those of the
phonon-stable alloys, the and states hybridize
significantly in , compensating the -band narrowing that each
atom undergoes and hindering the dip in the DOS found in the bulk alloys.
Charge density plots of provide further insights into the
relative strengths of the various interatomic bonds. Our results for the
electronic and geometric structure of this nanoalloy can be explained in terms
of length and strength hierarchies of the bonds, which may have implications
also for the stability of alloy in any phase or size.Comment: 16 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
Why is the electrocaloric effect so small in ferroelectrics?
Ferroelectrics are attractive candidate materials for environmentally friendly solid state refrigeration free of greenhouse gases. Their thermal response upon variations of external electric fields is largest in the vicinity of their phase transitions, which may occur near room temperature. The magnitude of the effect, however, is too small for useful cooling applications even when they are driven close to dielectric breakdown. Insight from microscopic theory is therefore needed to characterize materials and provide guiding principles to search for new ones with enhanced electrocaloric performance. Here, we derive from well-known microscopic models of ferroelectricity meaningful figures of merit for a wide class of ferroelectric materials. Such figures of merit provide insight into the relation between the strength of the effect and the characteristic interactions of ferroelectrics such as dipolar forces. We find that the long range nature of these interactions results in a small effect. A strategy is proposed to make it larger by shortening the correlation lengths of fluctuations of polarization. In addition, we bring into question other widely used but empirical figures of merit and facilitate understanding of the recently observed secondary broad peak in the electrocalorics of relaxor ferroelectrics.U.S. Department of Energy, Office of Basic Energy Sciences under Contract No. DE-AC02-06CH11357UCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Básicas::Centro de Investigación en Ciencia e Ingeniería de Materiales (CICIMA
Acousto-optical multiple interference switches
The authors introduce an alternative approach for acousto-optical light control based on the interference of light propagating through several waveguides, each subjected to a periodic refractive index modulation induced by a surface acoustic wave. The feasibility of the concept is demonstrated by the realization of an optical switch for arbitrary time intervals with an on/off contrast ratio of 20
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
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
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