37 research outputs found
Electrical control of ferromagnetism in Mn-doped semiconductor heterostructures
The interplay of tunneling transport and carrier-mediated ferromagnetism in
narrow semiconductor multi-quantum well structures containing layers of GaMnAs
is investigated within a self-consistent Green's function approach, accounting
for disorder in the Mn--doped regions and unwanted spin-flips at
heterointerfaces on phenomenological ground. We find that the magnetization in
GaMnAs layers can be controlled by an external electric bias. The underlying
mechanism is identified as spin-selective hole tunneling in and out of the
Mn-doped quantum wells, whereby the applied bias determines both hole
population and spin polarization in these layers. In particular we predict
that, near resonance, ferromagnetic order in the Mn doped quantum wells is
destroyed. The interplay of both magnetic and transport properties combined
with structural design potentially leads to several interrelated physical
phenomena, such as dynamic spin filtering, electrical control of magnetization
in individual magnetic layers, and, under specific bias conditions, to
self-sustained current and magnetization oscillations
(magneticmulti-stability). Relevance to recent experimental results is
discussed.Comment: 10 pages, 8 figure
Spin entanglement using coherent light and cavity-QED
A scheme for probabilistic entanglement generation between two distant single
electron doped quantum dots, each placed in a high-Q microcavity, by detecting
strong coherent light which has interacted dispersively with both subsystems
and experienced Faraday rotation due to the spin selective trion transitions is
discussed. In order to assess the applicability of the scheme for distant
entanglement generation between atomic qubits proposed by T.D. Ladd et al. [New
J. Phys. 8, 184 (2006)] to two distant quantum dots, one needs to understand
the limitations imposed by hyperfine interactions of the quantum dot spin with
the nuclear spins of the material and by non-identical quantum dots.
Feasibility is displayed by calculating the fidelity for Bell state generation
analytically within an approximate framework. The fidelity is evaluated for a
wide range of parameters and different pulse lengths, yielding a trade-off
between signal and decoherence, as well as a set of optimal parameters.
Strategies to overcome the effect of non-identical quantum dots on the fidelity
are examined and the timescales imposed by the nuclear spins are discussed,
showing that efficient entanglement generation is possible with distant quantum
dots. In this context, effects due to light hole transitions become important
and have to be included. The scheme is discussed for one- as well as for
two-sided cavities, where one must be careful with reflected light which
carries spin information. The validity of the approximate method is checked by
a more elaborate semiclassical simulation which includes trion formation.Comment: 17 pages, 13 figures, typos corrected, reference update
Linear optical absorption spectra of mesoscopic structures in intense THz fields: free particle properties
We theoretically study the effect of THz radiation on the linear optical
absorption spectra of semiconductor structures. A general theoretical
framework, based on non-equilibrium Green functions, is formulated, and applied
to the calculation of linear optical absorption spectrum for several
non-equilibrium mesoscopic structures. We show that a blue-shift occurs and
sidebands appear in bulk-like structures, i.e., the dynamical Franz-Keldysh
effect [A.-P. Jauho and K. Johnsen, Phys. Rev. Lett. 76, 4576 (1996)]. An
analytic calculation leads to the prediction that in the case of superlattices
distinct stable steps appear in the absorption spectrum when conditions for
dynamical localization are met.Comment: 13 Pages, RevTex using epsf to include 8 ps figures. Submitted to
Phys. Rev. B (3 April 97
Monte Carlo simulation of ultrafast processes in photoexcited semiconductors: Coherent and incoherent dynamics
The ultrafast dynamics of photoexcited carriers in a semiconductor is investigated by using a Monte Carlo simulation. In addition to a ‘‘conventional’’ Monte Carlo simulation, the coherence of the external light field and the resulting coherence in the carrier system are fully taken into account. This allows us to treat the correct time dependence of the generation process showing a time-dependent linewidth associated with a recombination from states off resonance due to stimulated emission. The subsequent dephasing of the carriers due to scattering processes is analyzed. In addition, the simulation contains the carrier-carrier interaction in Hartree-Fock approximation giving rise to a band-gap renormalization and excitonic effects which cannot be treated in a conventional Monte Carlo simulation where polarization effects are neglected. Thus the approach presents a unified numerical method for the investigation of phenomena occurring close to the band gap and those typical for the energy relaxation of hot carriers
Coupled free-carrier and exciton relaxation in optically excited semiconductors
The energy relaxation of coupled free-carrier and exciton populations in semiconductors after low-density ultrafast optical excitation is studied through a kinetic approach. The set of semiclassical Boltzmann equations, usually written for electron and hole populations only, is complemented by an additional equation for the exciton distribution. The equations are coupled by reaction terms describing phonon-mediated exciton binding and dissociation. All the other relevant scattering mechanisms, such as carrier-carrier, carrier-phonon, and exciton-phonon interactions, are also included. The resulting system of rate equations in reciprocal space is solved by an extended ensemble Monte Carlo method. As a first application, we show results for the dynamics of bulk GaAs in the range from 1 to ∼200 ps after photoexcitation. The build-up of an exciton population and its sensitivity to the excitation conditions are discussed in detail. As a consequence of the pronounced energy dependence of the LO-phonon-assisted transition probabilities between free-pair states and excitons, it is found that the efficiency of the exciton-formation process and the temporal evolution of the resulting population are sensitive to the excitation energy. We discuss the effects on luminescence experiments
Semiconductor Spintronics
Spintronics refers commonly to phenomena in which the spin of electrons in a
solid state environment plays the determining role. In a more narrow sense
spintronics is an emerging research field of electronics: spintronics devices
are based on a spin control of electronics, or on an electrical and optical
control of spin or magnetism. This review presents selected themes of
semiconductor spintronics, introducing important concepts in spin transport,
spin injection, Silsbee-Johnson spin-charge coupling, and spindependent
tunneling, as well as spin relaxation and spin dynamics. The most fundamental
spin-dependent nteraction in nonmagnetic semiconductors is spin-orbit coupling.
Depending on the crystal symmetries of the material, as well as on the
structural properties of semiconductor based heterostructures, the spin-orbit
coupling takes on different functional forms, giving a nice playground of
effective spin-orbit Hamiltonians. The effective Hamiltonians for the most
relevant classes of materials and heterostructures are derived here from
realistic electronic band structure descriptions. Most semiconductor device
systems are still theoretical concepts, waiting for experimental
demonstrations. A review of selected proposed, and a few demonstrated devices
is presented, with detailed description of two important classes: magnetic
resonant tunnel structures and bipolar magnetic diodes and transistors. In most
cases the presentation is of tutorial style, introducing the essential
theoretical formalism at an accessible level, with case-study-like
illustrations of actual experimental results, as well as with brief reviews of
relevant recent achievements in the field.Comment: tutorial review; 342 pages, 132 figure
Theoretical investigation of spin-filtering in CrAs/GaAs heterostructures
The electronic structure of bulk fcc GaAs, fcc and tetragonal CrAs, and
CrAs/GaAs supercells, computed within LMTO local spin-density functional
theory, is used to extract the band alignment (band offset) for the [1,0,0]
GaAs/CrAs interface in dependence of the spin orientation. With the lateral
lattice constant fixed to the experimental bulk GaAs value, a local energy
minimum is found for a tetragonal CrAs unit cell with a slightly ( 2%)
reduced longitudinal ([1,0,0]) lattice constant. Due to the identified
spin-dependent band alignment, half-metallicity of CrAs no longer is a key
requirement for spin-filtering. Encouraged by these findings, we study the
spin-dependent tunneling current in [1,0,0] GaAs/CrAs/GaAs heterostructures
within the non-equilibrium Green's function approach for an effective
tight-binding Hamiltonian derived from the LMTO electronic structure. Results
indicate that these heterostructures are probable candidates for efficient
room-temperature all-semiconductor spin-filtering devices.Comment: published in JAP, Vol.114, Issue 2