2,154 research outputs found
Spin dynamics and magnetic-field-induced polarization of excitons in ultrathin GaAs/AlAs quantum wells with indirect band gap and type-II band alignment
The exciton spin dynamics are investigated both experimentally and
theoretically in two-monolayer-thick GaAs/AlAs quantum wells with an indirect
band gap and a type-II band alignment. The magnetic-field-induced circular
polarization of photoluminescence, , is studied as function of the
magnetic field strength and direction as well as sample temperature. The
observed nonmonotonic behaviour of these functions is provided by the interplay
of bright and dark exciton states contributing to the emission. To interpret
the experiment, we have developed a kinetic master equation model which
accounts for the dynamics of the spin states in this exciton quartet, radiative
and nonradiative recombination processes, and redistribution of excitons
between these states as result of spin relaxation. The model offers
quantitative agreement with experiment and allows us to evaluate, for the
studied structure, the heavy-hole factor, , and the spin
relaxation times of electron, s, and hole, s, bound in the exciton.Comment: 17 pages, 16 figure
Helicity-dependent photocurrents in graphene layers excited by mid-infrared radiation of a CO-laser
We report the study of the helicity driven photocurrents in graphene excited
by mid-infrared light of a CO-laser. Illuminating an unbiased monolayer
sheet of graphene with circularly polarized radiation generates -- under
oblique incidence -- an electric current perpendicular to the plane of
incidence, whose sign is reversed by switching the radiation helicity. We show
that the current is caused by the interplay of the circular Hall effect
and the circular photogalvanic effect. Studying the frequency dependence of the
current in graphene layers grown on the SiC substrate we observe that the
current exhibits a resonance at frequencies matching the longitudinal optical
phonon in SiC
Spin relaxation of two-dimensional electrons with a hierarchy of spin-orbit couplings
The density matrix formalism is applied to calculate the spin-relaxation time
for two-dimensional systems with a hierarchy of spin-orbit couplings, such as
Rashba-type, Dresselhaus-type and so on. It is found that the spin-relaxation
time can be infinite if those coupling strengths , ,
and satisfy either condition (i) or (ii)
, which correspond to the vanishing Yang-Mills
"magnetic" field. The effect caused by the application of an external magnetic
field is also discussed. It is found that the longitudinal and in-plane spin
components can possess infinite life time when the spin components, the Larmor
precession frequency and the external magnetic field satisfy certain relations.Comment: Revtex, 7 pages, 3 figure
New insights into electron spin dynamics in the presence of correlated noise
The changes of the spin depolarization length in zinc-blende semiconductors
when an external component of correlated noise is added to a static driving
electric field are analyzed for different values of field strength, noise
amplitude and correlation time. Electron dynamics is simulated by a Monte Carlo
procedure which keeps into account all the possible scattering phenomena of the
hot electrons in the medium and includes the evolution of spin polarization.
Spin depolarization is studied by examinating the decay of the initial spin
polarization of the conduction electrons through the D'yakonov-Perel process,
the only relevant relaxation mechanism in III-V crystals. Our results show
that, for electric field amplitude lower than the Gunn field, the dephasing
length shortens with the increasing of the noise intensity. Moreover, a
nonmonotonic behavior of spin depolarization length with the noise correlation
time is found, characterized by a maximum variation for values of noise
correlation time comparable with the dephasing time. Instead, in high field
conditions, we find that, critically depending on the noise correlation time,
external fluctuations can positively affect the relaxation length. The
influence of the inclusion of the electron-electron scattering mechanism is
also shown and discussed.Comment: Published on "Journal of Physics: Condensed Matter" as "Fast Track
Communications", 11 pages, 9 figure
Spin- and energy relaxation of hot electrons at GaAs surfaces
The mechanisms for spin relaxation in semiconductors are reviewed, and the
mechanism prevalent in p-doped semiconductors, namely spin relaxation due to
the electron-hole exchange interaction, is presented in some depth. It is shown
that the solution of Boltzmann-type kinetic equations allows one to obtain
quantitative results for spin relaxation in semiconductors that go beyond the
original Bir-Aronov-Pikus relaxation-rate approximation. Experimental results
using surface sensitive two-photon photoemission techniques show that the spin
relaxation-time of electrons in p-doped GaAs at a semiconductor/metal surface
is several times longer than the corresponding bulk spin relaxation-times. A
theoretical explanation of these results in terms of the reduced density of
holes in the band-bending region at the surface is presented.Comment: 33 pages, 12 figures; earlier submission replaced by corrected and
expanded version; eps figures now included in the tex
Unified Semi-Classical Description of Intrinsic Spin-Hall Effect in Spintronic, Optical, and Graphene Systems
A semi-classical description of the intrinsic spin-Hall effect (SHE) is
presented which is relevant for a wide class of systems. A heuristic model for
the SHE is developed, starting with a fully quantum mechanical treatment, from
which we construct an intuitive expression for the spin-Hall current and
conductivity. Our method makes transparent the physical mechanism which drives
the effect, and unifies the SHE across several spintronic and optical systems.
Finally, we propose an analogous effect in bilayer graphene.Comment: 5 pages, 2 figures, 1 tabl
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