Carlos Egues

In recent years, spin-related effects have emerged as the key ingredient underlying many fundamental spin-dependent phenomena in nanoscale condensed-matter systems. In this context, the 5th International Conference on Physics and Applications of Spin-related Phenomena in Semiconductors (PASPS V) took place in the city of Foz do Iguaçu, Brazil, on August 3-6, 2008. PASPS V followed the successful series of conferences held in Japan (Sendai, 2000, 2006), Germany (Wurzburg, 2002), and The United States (Santa Barbara, 2004).

In the quantum Hall regime, the longitudinal resistivity \rho_xx plotted as a density-magnetic-field (n_2D-B) diagram displays ringlike structures due to the crossings of two sets of spin split Landau levels from different subbands [see, e.g., Zhang et al., in Phys. Rev. Lett. 95:216801, 2005. For tilted magnetic fields, some of these ringlike structures 'shrink' as the tilt angle is increased and fully collapse at \theta_c ~ 6 degrees. Here we theoretically investigate the topology of these structures via a non-interacting model for the 2DEG. We account for the inter Landau-level coupling induced by the tilted magnetic field via perturbation theory. This coupling results in anticrossings of Landau levels with parallel spins. With the new energy spectrum, we calculate the corresponding n_2D - B diagram of the density of states (DOS) near the Fermi level. We argue that the DOS displays the same topology as \rho_xx in the n_2D - B diagram. For the ring with filling factor \nu = 4, we find that the anticrossings make it shrink for increasing tilt angles and collapse at a large enough angle. Using effective parameters to fit the theta=0 data, we find a collapsing angle \theta_c ~ 3.6 degrees. Despite this factor-of-two discrepancy with the experimental data, our model captures the essential mechanism underlying the ring collapse.

We investigate the spin Hall conductivity \sigma_xy^z of a clean 2D electron gas formed in a two-subband well. We determine \sigma_xy^z as arising from the inter-subband induced spin-orbit (SO) coupling \eta (Calsaverini et al., Phys. Rev. B 78:155313, 2008) via a linear-response approach due to Rashba. By self-consistently calculating η for realistic wells, we find that \sigma_xy^z presents a non-monotonic (and non-universal) behavior and a sign change as the Fermi energy varies between the subband edges. Although our σ xy z is very small (i.e., < < e/4Pi), it is non-zero as opposed to linear-in-k SO models.

We apply the master equation technique to calculate shot noise in a system composed of single level quantum dot attached to a normal metal lead and to a ferromagnetic lead (NM-QD-FM). It is known that this system operates as a spin-diode, giving unpolarized currents for forward bias and polarized current for reverse bias. This effect is observed when only one electron can tunnel at a time through the dot, due to the strong intradot Coulomb interaction. We find that the shot noise also presents a signature of this spin-diode effect, with a super-Poissonian shot noise for forward and a sub-Poissonian shot noise for reverse bias voltages. The shot noise thus can provide further experimental evidence of the spin-rectification in the NM-QD-FM geometry.

The longitudinal resistivity ρ_xx of two-dimensional electron gases formed in wells with two subbands displays ringlike structures when plotted in a density--magnetic-field diagram, due to the crossings of spin-split Landau levels (LLs) from distinct subbands. Using spin density functional theory, we investigate the shape and spin polarization of these structures as a function of the temperature and the magnetic-field tilt angle. We find that (i) some of the rings "break" at sufficiently low temperatures due to a quantum Hall ferromagnetic phase transition, thus exhibiting a high degree of spin polarization (∼ 50%) within, consistent with the NMR data of Zhang et al. [Phys. Rev. Lett. 98, 246802 (2007)], and (ii) for increasing tilting angles the interplay between the anticrossings due to inter-LL couplings and the exchange-correlation (XC) effects leads to a collapse of the rings at some critical angle θ_c, in agreement with the data of Guo et al. [Phys. Rev. B 98, 246802 (2008)].

We investigate the intrinsic spin Hall effect in two-dimensional electron gases in quantum wells with two subbands, where a new intersubband-induced spin-orbit coupling is operative. The bulk spin Hall conductivity $\sigma_xy^z$ is calculated in the ballistic limit within the standard Kubo formalism in the presence of a magnetic field $B$ and is found to remain finite in the B=0 limit, as long as only the lowest subband is occupied. Our calculated $\sigma_xy^z$ exhibits a non-monotonic behavior and can change its sign as the Fermi energy (the carrier areal density $n_2D$) is varied between the subband edges. We determine the magnitude of $\sigma_xy^z$ for realistic InSb quantum wells by performing a self-consistent calculation of the intersubband-induced spin-orbit coupling.

Recently, we have found an additional spin-orbit (SO) interaction in quantum wells with two subbands [Phys. Rev. Lett. 99, 076603 (2007)]. This new SO term is non-zero even in symmetric geometries, as it arises from the intersubband coupling between confined states of distinct parities, and its strength is comparable to that of the ordinary Rashba. Starting from the $8 \times 8$ Kane model, here we present a detailed derivation of this new SO Hamiltonian and the corresponding SO coupling. In addition, within the self-consistent Hartree approximation, we calculate the strength of this new SO coupling for realistic symmetric modulation-doped wells with two subbands. We consider gated structures with either a constant areal electron density or a constant chemical potential. In the parameter range studied, both models give similar results. By considering the effects of an external applied bias, which breaks the structural inversion symmetry of the wells, we also calculate the strength of the resulting induced Rashba couplings within each subband. Interestingly, we find that for double wells the Rashba couplings for the first and second subbands interchange signs abruptly across the zero bias, while the intersubband SO coupling exhibits a resonant behavior near this symmetric configuration. For completeness we also determine the strength of the Dresselhaus couplings and find them essentially constant as function of the applied bias.

We study spin polarization induced by an applied bias in a bilayer quantum well system with interlayer spin-orbit coupling. The bias is incorporated via the non-equilibrium Green's function formalism, which allows us to handle a variety of system configurations. We shall focus on the component of the spin density perpendicular to the bilayer and compare our results to those obtained for a single layer system.

Using non-equilibrium Green functions we calculate the spin-polarized current and shot noise in a ferromagnet--quantum-dot--ferromagnet (FM-QD-FM) system. Both parallel (P) and antiparallel (AP) magnetic configurations are considered. Coulomb interaction and spin-flip are taken into account within the dot. We find that the interplay between Coulomb interaction and spin accumulation in the dot can result in a bias-dependent current polarization $\wp$. In particular, $\wp$ can be suppressed in the P alignment and enhanced in the AP case depending on the bias voltage. The spin-flip can also result in a switch of the current polarization from the emitter to the collector lead. Interestingly, for a particular set of parameters it is possible to have a polarized current in the collector and an unpolarized current in the emitter lead. We also found a suppression of the Fano factor to values well below 0.5 due to spin-flip.

A general approach to the measurement of an observable with pre- and post-selection is presented. The limit of weak measurement is studied in detail, and it is shown that the phase of the probe, including a Hamiltonian contribution to it, gives rise to observable effects, since the coherence of the probe is essential for the concept of complex weak value to be meaningful. As a particular example, the measurement of a spin component is considered. We find that the contribution of the imaginary part of the weak value is sizeable in this case.

Recently, we have introduced a novel inter-subband-induced spin-orbit (s-o) coupling (Phys. Rev. Lett. 99, 076603 (2007); cond-mat/0607218) arising in symmetric wells with at least two subbands. This new s-o coupling gives rise to an usual zitterbewegung -- i.e. the semiconductor analog to the relativistic trembling motion of electrons -- with cycloidal motion without magnetic fields. Here we complement these findings by explicitly deriving expressions for the corresponding zitterbewegung in spin space.

Using canonical transformations we diagonalize approximately the Hamiltonian of a gaussian wire with Rashba spin-orbit interaction. This proceedure allows us to obtain the energy dispersion relations and the wavefunctions with good accuracy, even in systems with relatively strong Rashba coupling. With these eigenstates one can calculate the non-equilibrium spin densities induced by applying bias voltages across the sample. We focus on the $z$-component of the spin density, which is related to the spin Hall effect.

We report a study of spin dependent transport in a system composed of a quantum dot coupled to a normal metal lead and a ferromagnetic lead (NM-QD-FM). We use the master equation approach to calculate the spin-resolved currents in the presence of an external bias and an intra-dot Coulomb interaction. We find that for a range of positive external biases (current flow from the normal metal to the ferromagnet) the current polarization $\wp=(I_\uparrow-I_\downarrow)/(I_\uparrow+I_\downarrow)$ is suppressed to zero, while for the corresponding negative biases (current flow from the ferromagnet to the normal metal) $\wp$ attains a relative maximum value. The system thus operates as a rectifier for spin--current polarization. This effect follows from an interplay between Coulomb blockade and nonequilibrium spin accumulation in the dot. In the parameter range considered, we also show that the above results can be obtained via nonequilibrium Green functions within a Hartree-Fock type approximation.

Motivated by recent experiments [Zhang \textit{et al.}, Phys. Rev. Lett. \textbf{95}, 216801 (2005) and Ellenberger \textit{et al.}, cond-mat/0602271] reporting novel ringlike structures in the density--magnetic-field ($n_2D$\emph{--B}) diagrams of the longitudinal resistivity $\rho_xx$ of quantum wells with two subbands, we investigate theoretically here the magneto-transport properties of these quantum-Hall systems. We determine $\rho_xx$ via both the Hartree and the Kohn-Sham self-consistent schemes plus the Kubo formula. While the Hartree calculation yields diamond-shaped structures in the $n_2D$\emph{--B} diagram, the calculation including exchange and correlation effects (Kohn-Sham) more closely reproduces the ringlike structures in the experiments.

We investigate the spin-orbit (s-o) interaction in two-dimensional electron gases (2DEGs) in quantum wells with two subbands. From the $8\times 8$ Kane model, we derive a new inter-subband-induced s-o term which resembles the functional form of the Rashba s-o -- but is non-zero even in \emph{symmetric} structures. This follows from the distinct parity of the confined states (even/odd) which obliterates the need for asymmetric potentials. We self-consistently calculate the new s-o coupling strength for realistic wells and find it comparable to the usual Rashba constant. Our new s-o term gives rise to a non-zero ballistic spin-Hall conductivity, which changes sign as a function of the Fermi energy ($\varepsilon_F$), and can induce an unusual \emph{zitterbewegung} with cycloidal trajectories \textit{without} magnetic fields.

In this review we discuss a recent proposal to perform partial Bell-state (parity) measurements on two-electron spin states for electrons confined to quantum dots. The realization of this proposal would allow for a physical implementation of measurement-based quantum computing. In addition, we consider the primary sources of energy relaxation and decoherence which provide the ultimate limit to all proposals for quantum information processing using electron spins in quantum dots. We give an account of the Hamiltonians used for the most important interactions (spin-orbit and hyperfine) and survey some of the recent work done to understand dynamics, control, and decoherence under the action of these Hamiltonians. We conclude the review with a table of important decay times found in experiment, and relate these time scales to the potential viability of measurement-based quantum computing.

We extend our previous work on shot noise for entangled and spin polarized electrons in a beam-splitter geometry with spin-orbit (\textit{s-o}) interaction in one of the incoming leads (lead 1). Besides accounting for both the Dresselhaus and the Rashba spin-orbit terms, we present general formulas for the shot noise of singlet and triplets states derived within the scattering approach. We determine the full scattering matrix of the system for the case of leads with \textit{two} orbital channels coupled via weak \textit{s-o} interactions inducing channel anticrossings. We show that this interband coupling coherently transfers electrons between the channels and gives rise to an additional modulation angle -- dependent on both the Rashba and Dresselhaus interaction strengths -- which allows for further independent coherent control of the electrons traversing the incoming leads. We derive explicit shot noise formulas for a variety of correlated pairs (e.g., Bell states) and lead spin polarizations. Interestingly, the singlet and \textit{each} of the triplets defined along the quantization axis perpendicular to lead 1 (with the local \textit{s-o} interaction) and in the plane of the beam splitter display distinctive shot noise for injection energies near the channel anticrossings; hence, one can tell apart all the triplets, in addition to the singlet, through noise measurements. We also find that spin-orbit induced backscattering within lead 1 reduces the visibility of the noise oscillations, due to the additional partition noise in this lead. Finally, we consider injection of two-particle wavepackets into leads with multiple discrete states and find that two-particle entanglement can still be observed via noise bunching and antibunching.

Editorial summary. Electrons not only have mass and charge, they also have magnetic properties directly related to their intrinsic spin. These spins can combine into quantum states of different spin parity, and such states may be useful as qubits in future quantum computers. As Egues discusses in his Perspective, Engel and Loss [ Science 309, 586 (2005)] report in this issue a method for "fingerprinting" the spin states of electrons contained in quantum dot structures. By allowing the electrons to leak from one dot to another, and then using a nanowire to sense the presence of the spin states, the authors were able to perform a nondestructive spin-parity measurement. Such a spin-parity detector should permit the manipulation of quantum dot qubits for quantum computation.

We have reconsidered the relevant problem of spin injection across ferromagnet/non-magnetic-semiconductor (FM/NMS) and dilute-magnetic-semiconductor/non-magnetic-semiconductor interfaces, for structures with \textit{finite} magnetic layers (FM or DMS). By using appropriate physical boundary conditions, we find new expressions for the resistances of these structures which are in general different from previous results in the literature. The results obtained can be important for the interpretation of the experimental data in the case of DMS/NMS structures.

We use spin-density-functional theory to study recently reported hysteretic magnetoresistance $\rho_xx$ spikes in Mn-based 2D electron gases [Jaroszy\'{n}ski \textit{et al.} Phys. Rev. Lett. \textbf{89}, 266802 (2002)]. We find hysteresis loops in our calculated Landau fan diagrams and total energies signaling quantum-Hall-ferromagnet phase transitions. Spin-dependent exchange-correlation effects are crucial to stabilize the relevant magnetic phases arising from \emph{distinct}symmetry-broken excited- and ground-state solutions of the Kohn-Sham equations. Besides hysteretic spikes in $\rho _xx$, we predict \textit{hysteretic dips} in the Hall resistance $ \rho _xy$. Our theory, \textit{without} domain walls, satisfactorily explains the recent data.

In this paper we theoretically investigate the magnetic-field and temperature dependences of the Shubnikov-de Haas oscillations in group II-VI modulation-doped Digital Magnetic Heterostructures. We self-consistently solve the effective-mass Schroedinger equation within the Hartree approximation and calculate the electronic structure and the magneto-transport properties. Our results show i) a shift of the Shubnikov-de Haas minima to lower magnetic fields with increasing temperature, and ii) an anomalous oscillation which develops when two opposite Landau levels cross near the Fermi energy. Both of these are consistent with recent magneto-transport measurements in such heterostructures [R. Knobel et al., Phys. Rev. B 65, 235327 (2002)].

Using the Keldysh nonequilibrium technique, we study current and the tunnelling magnetoresistance (TMR) in a quantum dot coupled to two ferromagnetic leads (FM-dot-FM). The current is calculated for both parallel and antiparallel lead alignments. Coulomb interaction and spin-flip scattering are taken into account within the quantum dot. Interestingly, we find that these interactions play a contrasting role in the TMR: there is a parameter range where spin flip suppresses the TMR, while Coulomb correlations enhance it, due to Coulomb blockade.

We review our recent work towards quantum communication in a solid-state environment with qubits carried by electron spins. We propose three schemes to produce spin-entangled electrons, where the required separation of the partner electrons is achieved via Coulomb interaction. The non-product spin-states originate either from the Cooper pairs found in a superconductor, or in the ground state of a quantum dot with an even number of electrons. In a second stage, we show how spin-entanglement carried by a singlet can be detected in a beam-splitter geometry by an increased (bunching) or decreased (antibunching) noise signal. We also discuss how a local spin-orbit interaction can be used to provide a continuous modulation of the noise as a signature of entanglement. Finally, we review how one can use a quantum dot as a spin- lter, a spin-memory read-out, a probe for single-spin decoherence and ultimately, a single-spin measurement apparatus.

Digital magnetic heterostructures (DMH) are semiconductor structures with magnetic monolayers. Here we study electronic and magnetotransport properties of shallow modulation-doped (ZnSe/ZnCdSe) DMHs with spin-5/2 Mn impurities. We compare the 'reservoir' model, possibly relevant to shallow geometries, to the usual 'constant-density' model. Our results are obtained by solving the Kohn-Sham equations within the local spin density approximation (LSDA). In the presence of a magnetic field, we show that both models exhibit characteristic behaviors for the electronic structure, two-dimensional carrier density, Fermi level and transport properties. Our results illustrate the relevance of exchange and correlation effects in the study of shallow heterostructures of the group II-VI.

An electronic beam splitter with a local Rashba spin-orbit coupling can serve as a detector for spin-polarized currents. The spin-orbit coupling plays the role of a tunable spin rotator and can be controlled via a gate electrode on top of the conductor. We use spin-resolved scattering theory to calculate the zero-temperature current fluctuations (shot noise) for such a four-terminal device and show that the shot noise is proportional to the spin polarization of the source. Moreover, we analyze the effect of spin-orbit-induced intersubband coupling, leading to an additional spin rotation.

We propose a spin field-effect transistor based on spin-orbit (s-o) coupling of both the Rashba and the Dresselhaus types. Differently from earlier proposals, spin transport through our device is tolerant against spin-independent scattering processes. Hence the requirement of strictly ballistic transport can be relaxed. This follows from a unique interplay between the Dresselhaus and the (gate-controlled) Rashba interactions; these can be tuned to have equal strengths thus yielding k-independent eigenspinors even in two dimensions. We discuss implementations with two-dimensional devices and quantum wires. In the latter, our setup presents strictly parabolic dispersions which avoids complications arising from anticrossings of different bands.

We review our recent contributions on shot noise for entangled electrons and spin-polarized currents in novel mesoscopic geometries. We first discuss some of our recent proposals for electron entanglers involving a superconductor coupled to a double dot in the Coulomb blockade regime, a superconductor tunnel-coupled to Luttinger-liquid leads, and a triple-dot setup coupled to Fermi leads. We calculate current and shot noise for spin-polarized currents and entangled/unentangled electron pairs in a beam-splitter geometry with a \textit{local} Rashba spin-orbit (s-o) interaction in the incoming leads. We find \textit{continuous} bunching and antibunching behaviors for the \textit{entangled} pairs -- triplet and singlet -- as a function of the Rashba rotation angle. In addition, we find that unentangled triplets and the entangled one exhibit distinct shot noise. Shot noise for spin-polarized currents shows sizable oscillations as a function of the Rashba phase. This happens only for electrons injected perpendicular to the Rashba rotation axis; spin-polarized carriers along the Rashba axis are noiseless. We find an additional spin rotation for electrons with energies near the crossing of the bands where s-o induced interband coupling is relevant. This gives rise to an additional modulation of the noise for both electron pairs and spin-polarized currents. Finally, we briefly discuss shot noise for a double dot near the Kondo regime.

We consider a two-channel spin transistor with weak spin-orbit induced interband coupling. We show that the coherent transfer of carriers between the coupled channels gives rise to an \textit{additional} spin rotation. We calculate the corresponding spin-resolved current in a Datta-Das geometry and show that a weak interband mixing leads to enhanced spin control.

Using the Keldysh nonequilibrium technique we calculate current, noise and Fano factor in a ferromagnetic(FM)-quantum dot-ferromagnetic(FM) system with Coulomb interaction and spin-flip scattering in the dot. The lead polarizations are considered in both parallel P and antiparallel AP alignments. We show that spin-flip can increase both AP-current and AP-noise, while the P-current and P-noise are almost insensitive to it. This fact leads to a suppression of the tunnelling magnetoresistance with increasing spin-flip rate.

We have recently calculated shot noise for entangled and spin-polarized electrons in novel beam-splitter geometries with a local Rashba s-o interaction in the incoming leads. This interaction allows for a gate-controlled rotation of the incoming electron spins. Here we present an alternate simpler route to the shot noise calculation in the above work and focus on only electron pairs. Shot noise for these shows continuous bunching and antibunching behaviors. In addition, entangled and unentangled triplets yield distinctive shot noise oscillations. Besides allowing for a direct way to identify triplet and singlet states, these oscillations can be used to extract s-o coupling constants through noise measurements. Incoming leads with spin-orbit interband mixing give rise an additional modulation of the current noise. This extra rotation allows the design of a spin transistor with enhanced spin control.

We investigate the nu=1 quantum Hall ferromagnet in the presence of spin-orbit coupling of the Rashba or Dresselhaus type by means of Hartree-Fock-typed variational states. In the presence of Rashba (Dresselhaus) spin-orbit coupling the fully spin-polarized quantum Hall state is always unstable resulting in a reduction of the spin polarization if the product of the particle charge $q$ and the effective $g$-factor is positive (negative). In all other cases an alternative variational state with O(2) symmetry and finite in-plane spin components is lower in energy than the fully spin-polarized state for large enough spin-orbit interaction. The phase diagram resulting from these considerations differs qualitatively from earlier studies.

We study shot noise for spin-polarized currents and entangled electron pairs in a four-probe (beam splitter) geometry with a local Rashba spin-orbit (s-o) interaction in the incoming leads. Within the scattering formalism we find that shot noise exhibits Rashba-induced oscillations with continuous bunching and antibunching. We show that entangled states as well as triplet states can be identified via their Rashba phase in noise measurements. For two-channel leads we find an additional spin rotation due to s-o induced interband coupling which provides additional spin control. We show that the s-o interaction determines the Fano factor which provides a direct way to measure the Rashba coupling constant via noise.

We study conductance and spin-polarization fluctuations in one-dimensional wires with spin-5/2 magnetic impurities. Our tight-binding Green function approach goes beyond mean field thus including s-d exchange-induced spin-flip scattering exactly. In a certain parameter range, we find that spin flip suppresses conductance fluctuations while enhancing spin-polarization fluctuations. More importantly, spin-polarization fluctuations attain a universal value 1/3 for large enough spin flip strengths.

Within a hydrodynamic approach we investigate the dynamics of an inhomogeneous electron-hole gas coupled to phonons in Te and the corresponding THz emission. We find that the {\em longitudinal} {\em % inhomogeneity} of the photogenerated electron-hole gas -- due to short absorption lengths in Te -- gives rise to {\em screening ineffectiveness} for non-zero wave-vector modes. This allows for THz dynamics and emission at the bare LO phonon frequency $\nu _LO$ even at high carrier densities. Screening ineffectiveness thus provides an appealingly simple explanation for the existence of bare modes at $\nu _LO$ in longitudinally inhomogeneous systems such as Te; no lateral inhomogeneity of the excitation spot is needed here.

We present a normal-mode analysis of coupled photocarrier-phonon dynamics in Te. We consider a dielectric function which accounts for LO phonons and the electron-hole gas within the Debye-Huckel model and RPA. Our main finding is the existence of a bare LO phonon mode in the system even at high carrier density. This oscillation is an unscreened L- mode arising from ineffective screening at large wave vectors. This mode is consistent with the bare LO-phonon peak in recent THz-emission spectra of Te.

Here we investigate the spin-dependent subband structure of newly-developed Mn-based modulation-doped quantum wells. In the presence of an external magnetic field, the s-d exchange coupling between carriers and localized d electrons of the Mn impurities gives rise to large spin splittings resulting in a magnetic-field dependent subband structure. Within the framework of the effective-mass approximation, we self-consistently calculate the subband structure at zero temperature using Density Functional Theory (DFT) with a Local Spin Density Approximation (LSDA). We present results for the magnetic-field dependence of the subband structure of shallow ZnSe/ZnCdMnSe modulation doped quantum wells. Our results show a significant contribution to the self-consistent potential due to the exchange-correlation term. These calculations are the first step in the study of a variety of interesting spin-dependent phenomena, e.g., spin-resolved transport and many-body effects in polarized two-dimensional electron gases.

Shot noise is a time-dependent current fluctuation due to the discrete character of the electron charge. Here we investigate shot noise in a spin-resolved tunneling system under the influence of spin-flip scattering. We find that the average current $\langle I\rangle $ and Fano factor $\gamma $ (``normalized noise'') present contrasting behavior for differing spin-flip time ratios: $\langle I\rangle $\ decreases while $\gamma $\increases for $\tau_{{\uparrow }{\downarrow }}>\tau_{{\downarrow }{\uparrow }}$\ as compared to the $\tau _{{\uparrow }{\downarrow }}=\tau_{{\downarrow }{\uparrow }}$ case and vice versa for $\tau_{{\uparrow }{\downarrow }}<\tau_{{\downarrow }{\uparrow}}$.

We theoretically investigate magnetoresistance (MR) effects in connection with spin filtering in quantum-coherent transport through tunnel junctions based on non-magnetic/semimagnetic heterostructures. We find that spin filtering in conjunction with the suppression/enhancement of the spin-dependent Fermi seas in semimagnetic contacts gives rise to (i) spin-split kinks in the MR of single barriers and (ii) a robust beating pattern in the MR of double barriers with a semimagnetic well. We believe these are unique signatures for quantum filtering.

We investigate spin-dependent fluctuations in spin-polarized electronic currents in a semimagnetic double-barrier tunneling structure. We use both quantum-coherent and semiclassical approaches to study the effects of spin-flip scattering on shot noise. In a limited parameter range, we find that for a fully spin-polarized incoming beam both descriptions yield shot-noise suppression.

We investigate the ultrafast dynamics of an electron-hole plasma coupled to phonons in bulk GaAs excited with femtosecond laser pulses. Our approach is based on balance equations directly derived from the Boltzmann equation within the relaxation-time approximation. Poisson's equation together with a phenomenological driven-harmonic-oscillator equation supplements our description by accounting for time-dependent electric and vibrational fields. Our calculated internal fields show oscillations at frequencies characteristic of those of coupled plasmon-phonon modes. Our results are consistent with recent experimental data.

This work addresses spin-dependent magnetotransport in a band-gap-matched ZnSe/Zn1-xMnxSe heterostructure. In an external magnetic field the paramagnetic layer behaves as a potential well for spin-down electrons and a potential barrier for spin-up ones. My current-density calculation shows a strong suppression of the spin-up component of the current density for increasing magnetic fields; the total current density is dominated by the spin-down component for B > 2 T. This result gives rise to the possibility of devising spin filters relevant for spin-dependent optoelectronics.

Digital-magnetic heterostructures (DMH's), II-VI quantum wells with "planes" of Mn, exhibit strongly spin-dependent physics. We investigate the magnetic-field (B) dependence of exchange-induced energy splittings and spin-flip scattering in DMH's. We find that Mn clustering is relevant to explain the magnitude and the concentration dependence of recently observed splittings. Our calculated electron spin-Rip times show "branching," i.e., tau(sf)(up-->down) decreases and tau(sf)(down-->up) increases with increasing B. This feature, consistent with recent data, is due to the B-field dependence of the available phase space.