50 research outputs found
Photoproduction of neutral pion pairs in the Coulomb field of the nucleus
The total cross section for Coulomb photoproduction of neutral pion pairs in
the reaction gamma A --> pi0 pi0 A is estimated within the effective chiral
lagrangian approach.
The amplitude of gamma gamma* --> pi0 pi0 with one off-shell photon is
calculated at O(p^6) in the momentum expansion; in addition, nuclear absorption
is taken into account.
Besides its experimental feasibility, the results of the calculation
demonstrate that the reaction gamma A --> pi0 pi0 A is a powerful source of
information on the process gamma gamma --> pi0 pi0 close to threshold.Comment: 18 pages, 6 figures, uses epsf, ioplppt.sty, iopl12.sty (included);
Added a discussion of main background from double pion photoproduction on
nucleon gamma N --> 2pi0 N via baryon resonance exchange mechanis
Pattern Formation in Semiconductors
In semiconductors, nonlinear generation and recombination processes of free carriers and nonlinear charge transport can give rise to non-equilibrium phase transitions. At low temperatures, the basic nonlinearity is due to the autocatalytic generation of free carriers by impact ionization of shallow impurities. The electric field accelerates free electrons, causing an abrupt increase in free carrier density at a critical electric field. In static electric fields, this nonlinearity is known to yield complex filamentary current patterns bound to electric contacts
Optical Spin Orientation under Inter- and Intra-Subband Transitions in QWs
It is shown that absorption of circularly polarized infrared radiation
achieved by inter-subband and intra-subband (Drude-like) transitions results in
a monopolar spin orientation of free carriers. The monopolar spin polarization
in zinc-blende-based quantum wells (QWs) is demonstrated by the observation of
the spin-galvanic and circular photogalvanic effects. It is shown that
monopolar spin orientation in n-type QWs becomes possible if an admixture of
valence band states to the conduction band wave function and the spin-orbit
splitting of the valence band are taken into account
Spin relaxation times of 2D holes from spin sensitive bleaching of inter-subband absorption
We present spin relaxation times of 2D holes obtained by means of spin
sensitive bleaching of the absorption of infrared radiation in p-type
GaAs/AlGaAs quantum wells (QWs). It is shown that the saturation of
inter-subband absorption of circularly polarized radiation is mainly controlled
by the spin relaxation time of the holes. The saturation behavior has been
determined for different QW widths and in a wide temperature range with the
result that the saturation intensity substantially decreases with narrowing of
the QWs. Spin relaxation times are derived from the measured saturation
intensities by making use of calculated (linear) absorption coefficients for
direct inter-subband transitions. It is shown that spin relaxation is due to
the D'yakonov-Perel' mechanism governed by hole-hole scattering. The problem of
selection rules is addressed.Comment: 14 pages, 5 figure
Quantum Oscillations of Photocurrents in HgTe Quantum Wells with Dirac and Parabolic Dispersions
We report on the observation of magneto-oscillations of terahertz radiation
induced photocurrent in HgTe/HgCdTe quantum wells (QWs) of different widths,
which are characterized by a Dirac-like, inverted and normal parabolic band
structure. The photocurrent data are accompanied by measurements of
photoresistance (photoconductivity), radiation transmission, as well as
magneto-transport. We develop a microscopic model of a cyclotron-resonance
assisted photogalvanic effect, which describes main experimental findings. We
demonstrate that the quantum oscillations of the photocurrent are caused by the
crossing of Fermi level by Landau levels resulting in the oscillations of spin
polarization and electron mobilities in spin subbands. Theory explains a
photocurrent direction reversal with the variation of magnetic field observed
in experiment. We describe the photoconductivity oscillations related with the
thermal suppression of the Shubnikov-de Haas effect.Comment: 16 pages, 13 figure
Photocurrents in bulk tellurium
We report a comprehensive study of polarized infrared/terahertz photocurrents
in bulk tellurium crystals. We observe different photocurrent contributions and
show that, depending on the experimental conditions, they are caused by the
trigonal photogalvanic effect, the transverse linear photon drag effect, and
the magnetic field induced linear and circular photogalvanic effects. All
observed photocurrents have not been reported before and are well explained by
the developed phenomenological and microscopic theory. We show that the effects
can be unambiguously distinguished by studying the polarization, magnetic
field, and radiation frequency dependence of the photocurrent. At frequencies
around 30 THz, the photocurrents are shown to be caused by the direct optical
transitions between subbands in the valence band. At lower frequencies of 1 to
3 THz, used in our experiment, these transitions become impossible and the
detected photocurrents are caused by the indirect optical transitions
(Drude-like radiation absorption).Comment: 21 pages, 7 figure
Room temperature high frequency transport of Dirac fermions in epitaxially grown Sb_2Te_3 based topological insulators
We report on the observation of photogalvanic effects in epitaxially grown
Sb_2Te_3 three-dimensional (3D) topological insulators (TI). We show that
asymmetric scattering of Dirac electrons driven back and forth by the terahertz
electric field results in a dc electric current. Due to the "symmetry
filtration" the dc current is generated in the surface electrons only and
provides an opto-electronic access to probe the electric transport in TI,
surface domains orientation and details of electron scattering even in 3D TI at
room temperature where conventional surface electron transport is usually
hindered by the high carrier density in the bulk
Magneto-gyrotropic effects in semiconductor quantum wells (review)
Magneto-gyrotropic photogalvanic effects in quantum wells are reviewed. We
discuss experimental data, results of phenomenological analysis and microscopic
models of these effects. The current flow is driven by spin-dependent
scattering in low-dimensional structures gyrotropic media resulted in asymmetry
of photoexcitation and relaxation processes. Several applications of the
effects are also considered.Comment: 28 pages, 13 figure