65 research outputs found
Systematic study of Mn-doping trends in optical properties of (Ga,Mn)As
We report on a systematic study of optical properties of (Ga,Mn)As epilayers
spanning the wide range of accessible substitutional Mn_Ga dopings. The growth
and post-growth annealing procedures were optimized for each nominal Mn doping
in order to obtain films which are as close as possible to uniform
uncompensated (Ga,Mn)As mixed crystals. We observe a broad maximum in the
mid-infrared absorption spectra whose position exhibits a prevailing blue-shift
for increasing Mn-doping. In the visible range, a peak in the magnetic circular
dichroism blue shifts with increasing Mn-doping. These observed trends confirm
that disorder-broadened valence band states provide a better one-particle
representation for the electronic structure of high-doped (Ga,Mn)As with
metallic conduction than an energy spectrum assuming the Fermi level pinned in
a narrow impurity band.Comment: 22 pages, 14 figure
Terahertz and infrared spectroscopic evidence of phonon-paramagnon coupling in hexagonal piezomagnetic YMnO3
Terahertz and far-infrared electric and magnetic responses of hexagonal
piezomagnetic YMnO3 single crystals are investigated. Antiferromagnetic
resonance is observed in the spectra of magnetic permeability mu_a [H(omega)
oriented within the hexagonal plane] below the Neel temperature T_N. This
excitation softens from 41 to 32 cm-1 on heating and finally disappears above
T_N. An additional weak and heavily-damped excitation is seen in the spectra of
complex dielectric permittivity epsilon_c within the same frequency range. This
excitation contributes to the dielectric spectra in both antiferromagnetic and
paramagnetic phases. Its oscillator strength significantly increases on heating
towards room temperature thus providing evidence of piezomagnetic or
higher-order couplings to polar phonons. Other heavily-damped dielectric
excitations are detected near 100 cm-1 in the paramagnetic phase in both
epsilon_c and epsilon_a spectra and they exhibit similar temperature behavior.
These excitations appearing in the frequency range of magnon branches well
below polar phonons could remind electromagnons; however, their temperature
dependence is quite different. We have used density functional theory for
calculating phonon dispersion branches in the whole Brillouin zone. A detailed
analysis of these results and of previously published magnon dispersion
branches brought us to the conclusion that the observed absorption bands stem
from phonon-phonon and phonon- paramagnon differential absorption processes.
The latter is enabled by a strong short-range in-plane spin correlations in the
paramagnetic phase.Comment: subm. to PR
Defect studies of ZnO films prepared by pulsed laser deposition on various substrates
ZnO thin films deposited on various substrates were characterized by slow positron implantation spectroscopy (SPIS) combined with X-ray diffraction (XRD). All films studied exhibit wurtzite structure and crystallite size 20-100 nm. The mosaic spread of crystallites is relatively small for the films grown on single crystalline substrates while it is substantial for the film grown on amorphous substrate. SPIS investigations revealed that ZnO films deposited on single crystalline substrates exhibit significantly higher density of defects than the film deposited on amorphous substrate. This is most probably due to a higher density of misfit dislocations, which compensate for the lattice mismatch between the film and the substrate
Terahertz probing of anisotropic conductivity and morphology of CuMnAs epitaxial thin films
Antiferromagnetic CuMnAs thin films have attracted attention since the
discovery of the manipulation of their magnetic structure via electrical,
optical, and terahertz pulses of electric fields, enabling convenient
approaches to the switching between magnetoresistive states of the film for the
information storage. However, the magnetic structure and, thus, the efficiency
of the manipulation can be affected by the film morphology and growth defects.
In this study, we investigate the properties of CuMnAs thin films by probing
the defect-related uniaxial anisotropy of electric conductivity by contact-free
terahertz transmission spectroscopy. We show that the terahertz measurements
conveniently detect the conductivity anisotropy, that are consistent with
conventional DC Hall-bar measurements. Moreover, the terahertz technique allows
for considerably finer determination of anisotropy axes and it is less
sensitive to the local film degradation. Thanks to the averaging over a large
detection area, the THz probing also allows for an analysis of strongly
non-uniform thin films. Using scanning near-field terahertz and electron
microscopies, we relate the observed anisotropic conductivity of CuMnAs to the
elongation and orientation of growth defects, which influence the local
microscopic conductivity. We also demonstrate control over the morphology of
defects by using vicinal substrates.Comment: 33 pages, 16 figure
Band structure of CuMnAs probed by optical and photoemission spectroscopy
The tetragonal phase of CuMnAs progressively appears as one of the key materials for antiferromagnetic spintronics due to efficient current-induced spin-torques whose existence can be directly inferred from crystal symmetry. Theoretical understanding of spintronic phenomena in this material, however, relies on the detailed knowledge of electronic structure (band structure and corresponding wave functions) which has so far been tested only to a limited extent. We show that AC permittivity (obtained from ellipsometry) and UV photoelectron spectra agree with density functional calculations. Together with the x-ray diffraction and precession electron diffraction tomography, our analysis confirms recent theoretical claim [Phys. Rev. B 96, 094406 (2017)] that copper atoms occupy lattice positions in the basal plane of the tetragonal unit cell
Basal to Non-Basal Transition for In-Plane Deformation of AZ31 Magnesium Alloys
The increases in strain hardening observed for the in-plane tensile deformation of well-aged magnesium alloy AZ31 are related to a transition from basal to non-basal slip. Based on the results of texture measurements, the double prismatic slip is proposed as the dominant secondary mechanism. The necessary fast strengthening of the basal slip is modelled by an accommodation contribution to the grain boundary resistance. The transition may be consistently modelled by involving the accommodation rather than the production of new sessile dislocations into the evolution laws for dislocation populations
Systematic study of terahertz response of SrTiO based heterostructures: Influence of strain, temperature, and electric field
Epitaxial heterostructures consisting of a variable number of SrTiO3/DyScO3 bilayers deposited onDyScO3 substrates were investigated using time-domain terahertz spectroscopy down to helium temperatures.Interdigitated electrodes deposited on top of the structures allowed probing of the terahertz response upon anapplied electric field. The phase transition into a ferroelectric state is observed in SrTiO3 films in all samplesclose to room temperature (between 250 and 310 K) due to in-plane epitaxial strain induced by the substrateand intercalated layers of DyScO3. Evolution of the dielectric spectra with temperature and external electricfield is described by a general model which involves a damped harmonic oscillator (soft mode) coupled to aDebye relaxation (central mode). Both modes are connected with the soft mode eigenvector, as recently shownby molecular dynamics simulations, and they reflect a strong anharmonicity of the soft-mode potential. At hightemperatures the soft-mode frequency variation drives all the changes observed in the spectra with temperatureand applied field. At low temperatures, deep in the ferroelectric phase, the soft mode significantly hardens andloses its importance for the terahertz dynamics; the central mode becomes stronger and it almost completelydetermines the shape of the measured spectra. The observed variation of phase transition temperature and of thedielectric response among the structures is ascribed to a partial epitaxial strain relaxation confirmed also by x-raydiffraction
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