2,149 research outputs found
Hall-effect and resistivity measurements in CdTe and ZnTe at high pressure: Electronic structure of impurities in the zincblende phase and the semi-metallic or metallic character of the high-pressure phases
We carried out high-pressure resistivity and Hall-effect measurements in
single crystals of CdTe and ZnTe up to 12 GPa. Slight changes of transport
parameters in the zincblende phase of CdTe are consitent with the shallow
character of donor impurities. Drastic changes in all the transport parameters
of CdTe were found around 4 GPa, i.e. close to the onset of the cinnabar to
rock-salt transition. In particular, the carrier concentration increases by
more than five orders of magnitude. Additionally, an abrupt decrease of the
resistivity was detected around 10 GPa. These results are discussed in
comparison with optical, thermoelectric, and x-ray diffraction experiments. The
metallic character of the Cmcm phase of CdTe is confirmed and a semi-metallic
character is determined for the rock-salt phase. In zincblende ZnTe, the
increase of the hole concentration by more than two orders of magnitude is
proposed to be due to a deep-to-shallow transformation of the acceptor levels.
Between 9 and 11 GPa, transport parameters are consistent with the
semiconducting character of cinnabar ZnTe. A two orders of magnitude decrease
of the resistivity and a carrier-type inversion occurs at 11 GPa, in agreement
with the onset of the transition to the Cmcm phase of ZnTe. A metallic
character for this phase is deduced.Comment: 20 pages, 4 figure
Characteristics of the terrestrial field-aligned current system
We present the first ever comprehensive statistical study of the
spatiotemporal characteristics of field-aligned currents in the terrestrial
magnetosphere-ionosphere system using multi point measurements. We determine
how the FAC density, variability and scale size are coupled. The three ST 5
satellites were in a pearls-on-a-string formation making measurements of the
magnetic field with variable inter-spacecraft separations ranging from a few
seconds to about 10 min. More than 4700 sets of satellite passes are
analyzed using a robust correlation analysis aimed at determining the
variability of the FAC system as a function of scale size and satellite
spacing. We find significant differences between the FAC characteristics on
the dayside and on the nightside in terms of dynamics of the current
systems. On the dayside the FAC characteristics are found to be independent
of IMF <I>B</I><sub>z</sub> and geomagnetic activity while the nightside indicates increased
variability during disturbed conditions. The boundary separating highly and
poorly correlated FACs can be fitted by a linear line for satellite
separations shorter than 60 s (dayside) and 160 s (nightside). We interpret
this as the dayside and nightside magnetospheric reconfiguration times
respectively. For times exceeding this the FAC characteristics are suggested
to be controlled by the solar wind (dayside) and plasma sheet (nightside)
dynamics. Finally, the characteristics of FAC system with scale sizes larger
than ~200 km (at ionospheric altitude) appear to be stable and
repeatable on time scales of the order of a minute (i.e. comparable to the
low-altitude orbiting satellite's traverse time across the auroral belt). In
this sense, our results effectively validate the Iijima and Potemra (1978) assumption that on
average the large-scale currents with scale sizes of the Region1 and Region2
are quasi-persistently significant in the transport of energy and momentum
between the magnetosphere and the ionosphere
Lattice dynamics of MgSiO perovskite (bridgmanite) studied by inelastic x-ray scattering and ab initio calculations
We have determined the lattice dynamics of MgSiO perovskite (bridgmanite)
by a combination of single-crystal inelastic x-ray scattering and ab initio
calculations. We observe a remarkable agreement between experiment and theory,
and provide accurate results for phonon dispersion relations, phonon density of
states and the full elasticity tensor. The present work constitutes an
important milestone to extend this kind of combined studies to extreme
conditions of pressure and temperature, directly relevant for the physics and
the chemistry of Earth's lower mantle
Moments of generalized parton distributions and quark angular momentum of the nucleon
The internal structure of hadrons is important for a variety of topics,
including the hadron form factors, proton spin and spin asymmetry in polarized
proton scattering.
For a systematic study generalized parton distributions (GPDs) encode
important information on hadron structure in the entire impact parameter space.
We report on a computation of nucleon GPDs based on simulations with two
dynamical non-perturbatively improved Wilson quarks with pion masses down to
350MeV. We present results for the total angular momentum of quarks with chiral
extrapolation based on covariant baryon chiral perturbation theory.Comment: Presented at 25th International Symposium on Lattice Field Theory,
Regensburg, Germany, 30 Jul - 4 Aug 200
A novel procedure for fast surface structural analysis based on LEED intensity data
By evaluating LEED intensities from different diffraction beams taken only at discrete energy intervals (which may be as large as 15–20 eV) the same degree of reliability in surface structure determination can be reached as with the conventional techniques based on analysis of continuous I/V-spectra. The minimum of the corresponding R-factor can be found by a least-squares fit method, as will be exemplified with a system in which 8 structural parameters were subject to simultaneous refinement
Electrical control of spin coherence in ZnO
Electric field enhanced electron spin coherence is characterized using
time-resolved Faraday rotation spectroscopy in n-type ZnO epilayers grown by
molecular beam epitaxy. An in-plane dc electric field E almost doubles the
transverse spin lifetime at 20 K, without affecting the effective g-factor.
This effect persists till high temperatures, but decreases with increasing
carrier concentration. Comparisons of the variations in the spin lifetime, the
carrier recombination lifetime and photoluminescence lifetimes indicate that
the applied E enhances the radiative recombination rate. All observed effects
are independent of crystal directionality and are performed at low magnetic
fields (B < 0.2 T).Comment: 13 pages, 3 figure
- …