103 research outputs found
Study of formation of high aspect GaAs structures based on the method of focused ion beams
This work was supported by the Russian Science Foundation Grant No. 15-19-10006. The results were obtained using the equipment of the Research and Education Center and Center for Collective Use "Nanotechnologies" of Southern Federal University
Photoluminescence of tetrahedral quantum-dot quantum wells
Taking into account the tetrahedral shape of a quantum dot quantum well
(QDQW) when describing excitonic states, phonon modes and the exciton-phonon
interaction in the structure, we obtain within a non-adiabatic approach a
quantitative interpretation of the photoluminescence spectrum of a single
CdS/HgS/CdS QDQW. We find that the exciton ground state in a tetrahedral QDQW
is bright, in contrast to the dark ground state for a spherical QDQW. The
position of the phonon peaks in the photoluminescence spectrum is attributed to
interface optical phonons. We also show that the experimental value of the
Huang-Rhys parameter can be obtained only within the nonadiabatic theory of
phonon-assisted transitions.Comment: 4 pages, 4 figures, E-mail addresses: [email protected],
[email protected], [email protected], [email protected],
to be published in Phys. Rev. Letter
Evidence for differentiation in the iron-helicoidal-chain in GdFe(BO)
We report on a single-crystal X-ray structure study of
at room temperature and at T=90 K. At room temperature
crystallizes in a trigonal space group R32 (No. 155), the same as found for
other members of iron-borate family . At 90 K the
structure of has transformed to the space group
(No. 152). The low-temperature structure determination gives new
insight into the weakly first-order structural phase transition at 156 K and
into the related Raman phonon anomalies. The discovery of two inequivalent iron
chains in the low temperature structure provide new point of view on the
low-temperature magnetic properties.Comment: Subm. to Acta Cryst.
Infrared study of lattice dynamics and spin-phonon and electron-phonon interactions in multiferroic TbFe3(BO3)4 and GdFe3(BO3)4
We present a comparative far-infrared reflection spectroscopy study of
phonons, phase transitions, spin-phonon and electron-phonon interactions in
isostructural multiferroic iron borates of gadolinium and terbium. The behavior
of phonon modes registered in a wide temperature range is consistent with a
weak first-order structural phase transition (Ts = 143 for GdFe3(BO3)4 and 200
K for TbFe3(BO3)4) from high-symmetry high-temperature R32 structure into
low-symmetry low-temperature P3121 one. The temperature dependences of
frequencies, oscillator strengths, and damping constants of some low-frequency
modes reveal an appreciable lattice anharmonicity. Peculiarities in the phonon
mode behavior in both compounds at the temperature of an antiferromagnetic
ordering (TN = 32 K for GdFe3(BO3)4 and 40 K for TbFe3(BO3)4) evidence the
spin-phonon interaction. In the energy range of phonons, GdFe3(BO3)4 has no
electronic levels but TbFe3(BO3)4 possesses several ones. We observe an onset
of new bands in the excitation spectrum of TbFe3(BO3)4, due to a resonance
interaction between a lattice phonon and 4f electronic crystal-field
excitations of Tb3+. This interaction causes delocalization of the CF
excitations, their Davydov splitting, and formation of coupled electron-phonon
modes.Comment: 26 pages, 4 tables, 8 firgure
Imbalanced d-wave superfluids in the BCS-BEC crossover regime at finite temperatures
Singlet pairing in a Fermi superfluid is frustrated when the amounts of
fermions of each pairing partner are unequal. The resulting `imbalanced
superfluid' has been realized experimentally for ultracold atomic gases with
s-wave interactions. Inspired by high-temperature superconductivity, we
investigate the case of d-wave interactions, and find marked differences from
the s-wave superfluid. Whereas s-wave imbalanced Fermi gases tend to phase
separate in real space, in a balanced condensate and an imbalanced normal halo,
we show that the d-wave gas can phase separate in reciprocal space so that
imbalance and superfluidity can coexist spatially. We show that the mechanism
explaining this property is the creation of polarized excitations in the nodes
of the gap. The Sarma mechanism, present only at nonzero temperatures for the
s-wave case, is still applicable in the temperature zero limit for the d-wave
case. As a result, the d-wave BCS superfluid is more robust with respect to
imbalance, and a region of the phase diagram can be identified where the s-wave
BCS superfluidity is suppressed whereas the d-wave superfluidity is not. When
these results are extended into the BEC limit of strongly bound molecules, the
symmetry of the order parameter matters less. The effects of fluctuations
beyond mean field is taken into account in the calculation of the structure
factor and the critical temperature. The poles of the structure factor
(corresponding to bound molecular states) are less damped in the d-wave case as
compared to s-wave. On the BCS side of the unitarity limit, the critical
temperature follows the temperature corresponding to the pair binding energy
and as such will also be more robust against imbalance. Possible routes for the
experimental observation of the d-wave superfluidity have been discussed.Comment: 22 pages, 7 figure
Infrared study of lattice and magnetic dynamics in a spin-chain compound Gd2 BaNiO5
We present infrared spectra of Gd2 BaNiO5, which is isostructural to a prototype S=1 Haldane compound Y2 BaNiO 5 containing Ni2⊃+ chains, in the spectral range 2 meV-0.55 eV. Unlike Y2 BaNiO5, the studied compound contains magnetic rare-earth sublattices and orders antiferromagnetically at TN =58 K. Detailed information on optical phonons is given. Temperature dependences of frequencies and half widths for the two lowest-frequency phonons polarized along the Ni-chain direction evidence the interaction of these lattice vibrations with magnetic excitations. With the help of lattice-dynamics calculations, we find relative displacement vectors of ions for all the phonon modes and use them to discuss the mechanism of phonon-magnon interaction. The optical spectra exhibit a broad absorption continuum for radiation polarized along the chains, probably of magnetic origin, gradually decreasing with lowering temperature. A new mode at about 30 cm⊃-1 polarized along the chains (a axis) emerges below ∼150 K. A midinfrared absorption peak at 1306 cm⊃-1 (0.16 eV) is observed and found to sharpen and shift significantly at TN. We argue that it can be attributed to a phonon-assisted magnetic absorption and discuss its nature in the framework of the Lorenzana-Sawatzky-Eder model. © 2010 The American Physical Society
Raman scattering from phonons and magnons in RFe3)BO3)4
Inelastic light scattering spectra of several members of the RFe3(BO3)4
family reveal a cascade of phase transitions as a function of temperature,
starting with a structural, weakly first order, phase transition followed by
two magnetic phase transitions. Those consist of the ordering of the Fe-spin
sublattice revealed by all the compound, and a subsequent spin-reorientational
transition for GdFe3(BO3)4. The Raman data evidence a strong coupling between
the lattice and magnetic degrees of freedom in these borates. The Fe-sublattice
ordering leads to a strong suppression of the low energy magnetic scattering,
and a multiple peaked two-magnon scattering continuum is observed. Evidence for
short-range correlations is found in the `paramagnetic' phase by the
observation of a broad magnetic continuum in the Raman data, which persists up
to surprisingly high temperatures.Comment: 17 pages, 13 figure
Many-body large polaron optical conductivity in SrTiNbO
Recent experimental data on the optical conductivity of niobium doped
SrTiO are interpreted in terms of a gas of large polarons with effective
coupling constant . The {theoretical approach takes into
account} many-body effects, the electron-phonon interaction with multiple
LO-phonon branches, and the degeneracy and the anisotropy of the Ti t
conduction band. {Based on the Fr\"{o}hlich interaction, the many-body
large-polaron theory} provides an interpretation for the essential
characteristics, except -- interestingly -- for the unexpectedly large
intensity of a peak at meV, of the observed optical conductivity
spectra of SrTiNbO \textit{without} any adjustment of
material parameters.Comment: to appear in Phys. Rev.
Polar optical phonons in wurtzite spheroidal quantum dots: Theory and application to ZnO and ZnO/MgZnO nanostructures
Polar optical-phonon modes are derived analytically for spheroidal quantum
dots with wurtzite crystal structure. The developed theory is applied to a
freestanding spheroidal ZnO quantum dot and to a spheroidal ZnO quantum dot
embedded into a MgZnO crystal. The wurtzite (anisotropic) quantum dots are
shown to have strongly different polar optical-phonon modes in comparison with
zincblende (isotropic) quantum dots. The obtained results allow one to explain
and accurately predict phonon peaks in the Raman spectra of wurtzite
nanocrystals, nanorods (prolate spheroids), and epitaxial quantum dots (oblate
spheroids).Comment: 11 page
Multi-phonon Raman scattering in semiconductor nanocrystals: importance of non-adiabatic transitions
Multi-phonon Raman scattering in semiconductor nanocrystals is treated taking
into account both adiabatic and non-adiabatic phonon-assisted optical
transitions. Because phonons of various symmetries are involved in scattering
processes, there is a considerable enhancement of intensities of multi-phonon
peaks in nanocrystal Raman spectra. Cases of strong and weak band mixing are
considered in detail. In the first case, fundamental scattering takes place via
internal electron-hole states and is participated by s- and d-phonons, while in
the second case, when the intensity of the one-phonon Raman peak is strongly
influenced by the interaction of an electron and of a hole with interface
imperfections (e. g., with trapped charge), p-phonons are most active.
Calculations of Raman scattering spectra for CdSe and PbS nanocrystals give a
good quantitative agreement with recent experimental results.Comment: 16 pages, 2 figures, E-mail addresses: [email protected],
[email protected], [email protected], accepted for publication in
Physical Review
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