344 research outputs found
Anharmonic coherent dynamics of the soft phonon mode of a PbTe crystal
We investigate the ultrafast optical response of PbTe to an intense
single-cycle terahertz pulse, resonant with the soft transverse optical (TO)
phonon mode of the crystal. We detect multifrequency oscillations of the
reflectance anisotropy, which we associate with nonlinear motion of the TO
phonon oscillator excited directly by the terahertz pulse. Our observation of
monotonically decaying optical anisotropy together with second harmonic
oscillations of the TO mode is an evidence of a transient non-centrosymmetric
state of the crystal lattice that can be accompanied by the ferroelectric
order. We suppose that this state is induced in the PbTe crystal by the intense
terahertz pulse via alignment of the local polar nanodomains. This hypothesis
is partially supported by the observation of coherent phonons near the
satellite phonon mode frequency that are impulsively generated by a femtosecond
laser pulse and are considerably enhanced by a synchronous terahertz pulse.Comment: 7 pages, 5 figure
On Possible Measurement of Gravitational Interaction Parameters on Board a Satellite
The recently suggested SEE (Satellite Energy Exchange) method of measuring
the gravitational constant , possible equivalence principle violation
(measured by the E\"{o}tv\"{o}s parameter ) and the hypothetic 5th force
parameters and on board a drag-free Earth's satellite is
discussed and further developed. Various particle trajectories near a heavy
ball are numerically simulated. Some basic sources of error are analysed. The
measurement procedure is modelled by noise insertion to a ``true''
trajectory. It is concluded that the present knowledge of (for
m) and can be improved by at least two orders of
magnitude.Comment: (only two misprints on title page) 7 page
Influence of e-e scattering on the temperature dependence of the resistance of a classical ballistic point contact in a two-dimensional electron system
We experimentally investigate the temperature (T) dependence of the
resistance of a classical ballistic point contact (PC) in a two-dimensional
electron system (2DES). The split-gate PC is realized in a high-quality
AlGaAs/GaAs heterostructure. The PC resistance is found to drop by more than
10% as T is raised from 0.5 K to 4.2 K. In the absence of a magnetic field, the
T dependence is roughly linear below 2 K and tends to saturate at higher T.
Perpendicular magnetic fields on the order of a few 10 mT suppress the
T-dependent contribution dR. This effect is more pronounced at lower
temperatures, causing a crossover to a nearly parabolic T dependence in a
magnetic field. The normalized magnetic field dependencies dR(B) permit an
empiric single parameter scaling in a wide range of PC gate voltages. These
observations give strong evidence for the influence of electron-electron (e-e)
scattering on the resistance of ballistic PCs. Our results are in qualitative
agreement with a recent theory of the e-e scattering based T dependence of the
conductance of classical ballistic PCs [ Phys. Rev. Lett. 101 216807 (2008) and
Phys. Rev. B 81 125316 (2010)].Comment: as publishe
The effect of different AC current density on the magnetoimpedance of CoFeMoSiB amorphous ribbons in the presence of iron oxide nanoparticles water based ferrofluid
Selected measurements were performed at SGIker services of UPV-EHU. This work was supported by Spanish ACTIMAT grant. We thank A.P. Safronov, I.V. Beketov and Yu. P. Novoselova for special support
Lidar investigations of M-zone
The creation of pulse dye lasers tuned to resonant line of meteor produced admixtures of atmospheric constituents has made it possible to begin lidar investigations of the vertical distribution of mesospheric sodium concentration and its dynamics in the upper atmosphere. The observed morning increase of sodium concentration in the vertical column is probably caused by diurnal variations of sporadic meteors. The study of the dynamics of the sodium column concentration in the period of meteor streams activity confirms the suggestion of cosmic origin of these atoms. The short lived increase of sodium concentration brought about by a meteor stream, however, exceeds by one order the level of the sporadic background
Spin configurations in circular and rectangular vertical quantum dots in a magnetic field: Three-dimensional self-consistent simulation
The magnetic field dependence of the electronic properties of \textit{real}
single vertical quantum dots in circular and rectangular mesas is investigated
within a full three-dimensional multiscale self-consistent approach without any
{\it \'a priori} assumptions about the shape and strength of the confinement
potential. The calculated zero field electron addition energies are in good
agreement with available experimental data for both mesa geometries. Charging
diagrams in a magnetic field for number of electrons up to five are also
computed. Consistent with the experimental data, we found that the charging
curves for the rectangular mesa dot in a magnetic field are flatter and exhibit
less features than for a circular mesa dot. Evolution of the singlet-triplet
energy separation in the two electron system for both dot geometries in
magnetic field was also investigated. In the limit of large field, beyond the
singlet-triplet transition, the singlet-triplet energy difference continues to
become more negative in a circular mesa dot without any saturation within the
range of considered magnetic fields whilst it is predicted to asymptotically
approach zero for the rectangular mesa dot. This different behavior is
attributed to the symmetry "breaking" that occurs in the singlet wave-functions
in the rectangular mesa dot but not in the circular one.Comment: 12 pages, 8 gifure
Classical effects in the weak-field magnetoresistance of InGaAs/InAlAs quantum wells
We observe an unusual behavior of the low-temperature magnetoresistance of
the high-mobility two-dimensional electron gas in InGaAs/InAlAs quantum wells
in weak perpendicular magnetic fields. The observed magnetoresistance is
qualitatively similar to that expected for the weak localization and
anti-localization but its quantity exceeds significantly the scale of the
quantum corrections. The calculations show that the obtained data can be
explained by the classical effects in electron motion along the open orbits in
a quasiperiodic potential relief manifested by the presence of ridges on the
quantum well surface
Effect of carbon alloying on hydrogen embrittlement of a Cantor alloy
We have investigated the effect of hydrogen-charging on the mechanical properties and fracture mechanisms of high-entropy alloys Fe20Mn20Cr20Ni20Co20 and Fe20Mn20Cr20Ni20Co19C1 (at.%). Both alloys have a coarse-grained single-phase face-centered cubic (fcc) structure. It was found that doping with carbon decreases the content of hydrogen absorbed by the specimens during electrochemical hydrogen-charging (in a 3 % NaCl water solution, at j =10 mA / cm2 for 50 h): 134 wppm and 63 wppm for carbon-free and carbon-doped alloy, respectively. Hydrogen-charging contributes to an increase in the yield strength and a decrease in the ductility of the alloys. Despite the lower concentration of dissolved hydrogen, the hydrogen-associated solid-solution strengthening of the carbon-doped alloy is higher than that in the interstitial-free alloy. The hydrogen embrittlement index, IH =17 %, for carbon-alloyed specimens is lower than IH = 25 % for interstitial-free specimens. In both alloys, the hydrogen-affected surface layers of the specimens fracture in a similar brittle mode — intergranular fracture dominates
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