179 research outputs found
Infrared-active optical phonons in LiFePO4 single crystals
Infrared-active optical phonons were studied in olivine LiFePO4 oriented single crystals by means of both rotating analyzer and rotating compensator spectroscopic ellipsometry in the spectral range between 50 and 1400 cm-1. The eigenfrequencies, oscillator strengths, and broadenings of the phonon modes were determined from fits of the anisotropic harmonic oscillator model to the data. Optical phonons in a heterosite FePO4 crystal were measured from the delithiated ab-surface of the LiFePO4 crystal and compared with the phonon modes of the latter. Good agreement was found between experimental data and the results of solid-state hybrid density functional theory calculations for the phonon modes in both LiFePO4 and FePO4
Epitaxial checkerboard arrangement of nanorods in ZnMnGaO4 films studied by x-ray diffraction
The intriguing nano-structural properties of a ZnMnGaO4 film epitaxially
grown on MgO (001) substrate have been investigated using synchrotron
radiation-based x-ray diffraction. The ZnMnGaO4 film consisted of a
self-assembled checkerboard (CB) structure with perfectly aligned and regularly
spaced vertical nanorods. The lattice parameters of the orthorhombic and
rotated tetragonal phases of the CB structure were analyzed using H-K, H-L, and
K-L cross sections of the reciprocal space maps measured around various
symmetric and asymmetric reflections of the spinel structure. We demonstrate
that the symmetry of atomic displacements at the phases boundaries provides the
means for coherent coexistence of two domains types within the volume of the
film
Electron cyclotron mass in undoped CdTe/CdMnTe quantum wells
Optically detected cyclotron resonance of two-dimensional electrons has been
studied in nominally undoped CdTe/(Cd,Mn)Te quantum wells. The enhancement of
carrier quantum confinement results in an increase of the electron cyclotron
mass from 0.099 to 0.112 with well width decreasing from 30 down to
3.6 nm. Model calculations of the electron effective mass have been performed
for this material system and good agreement with experimental data is achieved
for an electron-phonon coupling constant =0.32
High carrier mobility in transparent Ba1-xLaxSnO3 crystals with a wide band gap
We discovered that perovskite (Ba,La)SnO3 can have excellent carrier mobility
even though its band gap is large. The Hall mobility of Ba0.98La0.02SnO3
crystals with the n-type carrier concentration of \sim 8-10\times10 19 cm-3 is
found to be \sim 103 cm2 V-1s-1 at room temperature, and the precise
measurement of the band gap \Delta of a BaSnO3 crystal shows \Delta=4.05 eV,
which is significantly larger than those of other transparent conductive
oxides. The high mobility with a wide band gap indicates that (Ba,La)SnO3 is a
promising candidate for transparent conductor applications and also epitaxial
all-perovskite multilayer devices
Coupling between magnon and ligand-field excitations in magnetoelectric Tb3Fe5O12 garnet
The spectra of far-infrared transmission in Tb3Fe5O12 magnetoelectric single
crystals have been studied in the range between 15 and 100 cm-1, in magnetic
fields up to 10 T, and for temperatures between 5 and 150 K. We attribute some
of the observed infrared-active excitations to electric-dipole transitions
between ligand-field split states of Tb3+ ions. Anticrossing between the
magnetic exchange excitation and the ligand-field transition occurs at the
temperature between 60 and 80 K. The corresponding coupling energy for this
interaction is 6 cm-1. Temperature-induced softening of the hybrid IR
excitation correlates with the increase of the static dielectric constant. We
discuss the possibility for hybrid excitations of magnons and ligand-field
states and their possible connection to the magnetoelectric effect in
Tb3Fe5O12.Comment: submitted to Phys. Rev. B on May 15th, 201
Acceptor binding energies in GaN and AlN
We employ effective mass theory for degenerate hole-bands to calculate the
acceptor binding energies for Be, Mg, Zn, Ca, C and Si substitutional acceptors
in GaN and AlN. The calculations are performed through the 66
Rashba-Sheka-Pikus and the Luttinger-Kohn matrix Hamiltonians for wurtzite (WZ)
and zincblende (ZB) crystal phases, respectively. An analytic representation
for the acceptor pseudopotential is used to introduce the specific nature of
the impurity atoms. The energy shift due to polaron effects is also considered
in this approach. The ionization energy estimates are in very good agreement
with those reported experimentally in WZ-GaN. The binding energies for ZB-GaN
acceptors are all predicted to be shallower than the corresponding impurities
in the WZ phase. The binding energy dependence upon the crystal field splitting
in WZ-GaN is analyzed. Ionization levels in AlN are found to have similar
`shallow' values to those in GaN, but with some important differences, which
depend on the band structure parameterizations, especially the value of crystal
field splitting used.Comment: REVTEX file - 1 figur
Optical identification of hybrid magnetic and electric excitations in Dy3Fe5O12 garnet
Far-infrared spectra of magneto-dielectric Dy3Fe5O12 garnet were studied
between 13 and 100 cm-1 and at low temperatures between 5 and 80 K. A
combination of transmission, reflectivity, and rotating analyzer ellipsometry
was used to unambiguously identify the type of the dipole activity of the
infrared modes. In addition to purely dielectric and magnetic modes, we
observed several hybrid modes with a mixed magnetic and electric dipole
activity. These modes originate from the superexchange between magnetic moments
of Fe and Dy ions. Using 4x4 matrix formalism for materials with Mu=/=1, we
modeled the experimental optical spectra and determined the far-infrared
dielectric and magnetic permeability functions. The matching condition
Mu(Wh)*Se=Eps(Wh)*Sm for the oscillator strengths Se(m) explains the observed
vanishing of certain hybrid modes at Wh in reflectivity.Comment: paper and supplement appendi
Development of an eight-band theory for quantum-dot heterostructures
We derive a nonsymmetrized 8-band effective-mass Hamiltonian for quantum-dot
heterostructures (QDHs) in Burt's envelope-function representation. The 8x8
radial Hamiltonian and the boundary conditions for the Schroedinger equation
are obtained for spherical QDHs. Boundary conditions for symmetrized and
nonsymmetrized radial Hamiltonians are compared with each other and with
connection rules that are commonly used to match the wave functions found from
the bulk kp Hamiltonians of two adjacent materials. Electron and hole energy
spectra in three spherical QDHs: HgS/CdS, InAs/GaAs, and GaAs/AlAs are
calculated as a function of the quantum dot radius within the approximate
symmetrized and exact nonsymmetrized 8x8 models. The parameters of dissymmetry
are shown to influence the energy levels and the wave functions of an electron
and a hole and, consequently, the energies of both intraband and interband
transitions.Comment: 36 pages, 10 figures, E-mail addresses: [email protected],
[email protected]
The dynamics of the population and peculiarities of the morphometric structure of Melophagus ovinus (Diptera, Hippoboscidae) in Ukraine
The indices of the Melophagus ovinus (Linnaeus, 1758) population in Ukraine, and also the peculiarities of morphological and metric structure of the insectsâ body at all the stages of their development within the conditions of the surveyed region were investigated. New data on morphometric differential signs of sexually mature males and females of sheep bloodsuckers were obtained. We established that M. ovinus is significantly widespread in Poltava and Zaporizhzhia regions, and parasitizes 26.1% of the examined sheep stock with the infection intensity of 92.7 ± 1.4 specimens and abundance â 24.7 specimens on one animal. The dynamics of M. ovinus population at different stages of development was characterized by the highest abundance of sexually mature males (11.1 specimens on one animal) and females (8.9 specimens). The given index concerning pupae and larvae was considerably lower (4.2 and 0.5 specimens on one animal). It was found that post-embryonic and adult development stages of M. ovinus differ in their metric indices.The length and width of the pupae were 17.4% and 13.2% larger than those of the larvae. The sizes of males and females relative to the indices of body length, the length and width of head, thoracic, and abdominal segments, the length of maxillary palpus and the length and width of the proboscis in fact differ in their values. The differential morphological species signs of M. ovinus are the form and location of the oculi, antennae, the structure of the head segment of the body, and the mouthparts, and of sexual dimorphism â the distance from the caudal segment of the copulatory apparatus to the rear of the insectâs last abdominal segment. The indices of the Melophagus ovinus (Linnaeus, 1758) population in Ukraine, and also the peculiarities of morphological and metric structure of the insectsâ body at all the stages of their development within the conditions of the surveyed region were investigated. New data on morphometric differential signs of sexually mature males and females of sheep bloodsuckers were obtained. It was established that M. ovinus are significantly widespread in Poltava and Zaporizhzhia regions, and they parasitize 26.1% of the examined sheep stock with the infection intensity of 92.7 ± 1.4 specimens and abundance â 24.7 specimens on one animal. The dynamics of M. ovinus population at different stages of development was characterized by the highest abundance of sexually mature males (11.1 specimens) and females (8.9 specimens). The given index concerning pupae and larvae was considerably lower (4.2 and 0.5 specimens on one animal). It was found that post-embryonic and adult development stages of M. ovinus differ in their metric indices.The length and width of the pupae were 17.4% and 13.2% larger than those of the larvae. The sizes of males and females relative to the indices of body length, the length and width of head, thoracic, and abdominal segments, the length of maxillary palpus and the length and width of the proboscis in fact differ in their values. The differential morphological species signs of M. ovinus are the form and location of oculi, antennae, the structure of the head segment of the body, and the mouthparts, and of sexual dimorphism â the distance from the caudal segment of the copulatory apparatus to the rear of the insectâs last abdominal segment.
Electromagnons in multiferroic RMn2O5 compounds and their microscopic origin
We summarize the existing experimental data on electromagnons in multiferroic
RMn2O5 compounds, where R denotes a rare earth ion, Y or Bi, and discuss a
realistic microscopic model of these materials based on assumption that the
microscopic mechanism of magnetically-induced ferroelectricity and
electromagnon absorption relies entirely on the isotropic Heisenberg exchange
and magnetostrictive coupling of spins to a polar lattice mode and does not
involve relativistic effects. This model explains many magnetic and optical
properties of RMn2O5 manganites, such as the spin re-orientation transition,
magnetically-induced polarisation, appearance of the electromagnon peak in the
non-collinear spin state and the polarisation of light for which this peak is
observed. We compare experimental and theoretical results on electromagnons in
RMn2O5 and RMnO3 compounds.Comment: 20 pages, 9 figures, to be published in J. Phys.: Condens. Matter,
special issue on multiferroic
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