209 research outputs found
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
Rigorous 2D Model for Study of Pulsed and Monochromatic Waves Propagation Near the Earth’s Surface
A model problem considered in the paper allows solving rather complex 2D problems of the electromagnetic wave propagation with a required accuracy using conventional personal computers. The problems are of great importance for the theory and practical applications. The association of FDTD schemes with exact absorbing conditions makes up the basis for constructing models of the kind. This approach reduces the original open initial boundary value problems to the equivalent closed problems which can be solved numerically using the standard grid methods
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
Electronic states and optical properties of GaAs/AlAs and GaAs/vacuum superlattices by the linear combination of bulk bands method
The linear combination of bulk bands method recently introduced by Wang,
Franceschetti and Zunger [Phys. Rev. Lett.78, 2819 (1997)] is applied to a
calculation of energy bands and optical constants of (GaAs)/(AlAs) and
(GaAs)/(vacuum) (001) superlattices with n ranging from 4 to 20.
Empirical pseudopotentials are used for the calculation of the bulk energy
bands. Quantum-confined induced shifts of critical point energies are
calculated and are found to be larger for the GaAs/vacuum system. The
peak in the absorption spectra has a blue shift and splits into two peaks for
decreasing superlattice period; the transition instead is found to be
split for large-period GaAs/AlAs superlattices. The band contribution to linear
birefringence of GaAs/AlAs superlattices is calculated and compared with recent
experimental results of Sirenko et al. [Phys. Rev. B 60, 8253 (1999)]. The
frequency-dependent part reproduces the observed increase with decreasing
superlattice period, while the calculated zero-frequency birefringence does not
account for the experimental results and points to the importance of
local-field effects.Comment: 10 pages, 11 .eps figures, 1 tabl
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]
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