507 research outputs found
Optical control of electron spin coherence in CdTe/(Cd,Mg)Te quantum wells
Optical control of the spin coherence of quantum well electrons by short
laser pulses with circular or linear polarization is studied experimentally and
theoretically. For that purpose the coherent electron spin dynamics in a
n-doped CdTe/(Cd,Mg)Te quantum well structure was measured by time-resolved
pump-probe Kerr rotation, using resonant excitation of the negatively charged
exciton (trion) state. The amplitude and phase shifts of the electron spin beat
signal in an external magnetic field, that are induced by laser control pulses,
depend on the pump-control delay and polarization of the control relative to
the pump pulse. Additive and non-additive contributions to pump-induced signal
due to the control are isolated experimentally. These contributions can be well
described in the framework of a two-level model for the optical excitation of
the resident electron to the trion.Comment: 15 pages, 18 figure
Linear polarization of the photoluminescence of quantum wells
The degree and orientation of the magnetic-field induced linear polarization
of the photoluminescence from a wide range of heterostructures containing
(Cd,Mn)Te quantum wells between (Cd,Mn,Mg)Te barriers has been studied as a
function of detection photon energy, applied magnetic field strength and
orientation in the quantum well plane. A theoretical description of this effect
in terms of an in-plane deformation acting on the valence band states is
presented and is verified by comparison with the experimental data. We
attempted to identify clues to the microscopic origin of the valence band spin
anisotropy and to the mechanisms which actually determine the linear
polarization of the PL in the quantum wells subject to the in-plane magnetic
field. The conclusions of the present paper apply in full measure to
non-magnetic QWs as well as ensembles of disk-like QDs with shape and/or strain
anisotropy.Comment: 21 pages, 10 figure
Spin coherence of a two-dimensional electron gas induced by resonant excitation of trions and excitons in CdTe/(Cd,Mg)Te quantum wells
The mechanisms for generation of long-lived spin coherence in a
two-dimensional electron gas (2DEG) have been studied experimentally by means
of a picosecond pump-probe Kerr rotation technique. CdTe/(Cd,Mg)Te quantum
wells with a diluted 2DEG were investigated. The strong Coulomb interaction
between electrons and holes, which results in large binding energies of neutral
excitons and negatively charged excitons (trions), allows one to address
selectively the exciton or trion states by resonant optical excitation.
Different scenarios of spin coherence generation were analyzed theoretically,
among them the direct trion photocreation, the formation of trions from
photogenerated excitons and the electron-exciton exchange scattering. Good
agreement between experiment and theory is found.Comment: 18 pages, 20 figure
Utilization of Guided Bone Regeneration Techniqes in Treatment of a Single Tooth Missing with Implant Supported Crown
Guided bone regeneration is developing very dynamically in dental surgery and in implantology. It relies on building up bone in places where it is lacking, utilizing a variety of grafting materials. Methods of guided bone regeneration utilize biological materials or synthetic specimens. The use of autogenous platelets rich plasma derived in the thromboforetic process (COBE spectra system) allows the employment of growth factors, which blood platelets contain in the formation of new bone tissues. Usage of BioOss together with platelet rich plasmas allows the creation of a resorbable carrier for growth factor (auto-xenogenic graft). The aim of the presentation is the analysis of clinical cases where usage of bone augmentation enabled the insertion of implants. Rebuilding the bone by means of guided bone regeneration facilitated the implant treatment and consequently the accomplishment of fixed prosthetics supported on implants
Ising Quantum Hall Ferromagnet in Magnetically Doped Quantum Wells
We report on the observation of the Ising quantum Hall ferromagnet with Curie
temperature as high as 2 K in a modulation-doped (Cd,Mn)Te
heterostructure. In this system field-induced crossing of Landau levels occurs
due to the giant spin-splitting effect. Magnetoresistance data, collected over
a wide range of temperatures, magnetic fields, tilt angles, and electron
densities, are discussed taking into account both Coulomb electron-electron
interactions and sd coupling to Mn spin fluctuations. The critical behavior
of the resistance ``spikes'' at corroborates theoretical
suggestions that the ferromagnet is destroyed by domain excitations.Comment: revised, 4 pages, 4 figure
Very large magnetoresistance in lateral ferromagnetic (Ga,Mn)As wires with nanoconstrictions
We have fabricated (Ga,Mn)As nanostructures in which domain walls can be
pinned by sub-10 nm constrictions. Controlled by shape anisotropy, we can
switch the regions on either side of the constriction to either parallel or
antiparallel magnetization. All samples exhibit a positive magnetoresistance,
consistent with domain-wall trapping. For metallic samples we find a
magnetoresistance up to 8%, which can be understood from spin accumulation. In
samples where, due to depletion at the constriction, a tunnel barrier is
formed, we observe a magnetoresistance of up to 2000 %.Comment: 4 pages, 3 figures, submited to Phys. Rev. Let
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Recent progress in the development of β-Ga2O3 scintillator crystals grown by the Czochralski method
A high-quality bulk single crystal of β-Ga2O3 has been grown by the Czochralski method and its basic scintillation characteristics (light yield, energy resolution, proportionality, and scintillation decay times) have been investigated. All the samples cut from the crystal show promising scintillation yields between 8400 and 8920 ph/MeV, which is a noticeable step forward compared to previous studies. The remaining parameters, i.e. the energy resolution slightly above 10% (at 662 keV) and the scintillation mean decay time just under 1 μs, are at the same level as we have formerly recognized for β-Ga2O3. The proportionality of yield seems not to deviate from standards determined by other commercial scintillators
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