Carrier Spin Dynamics Research Articles

Excitation of the spin density and current by coherent light pulses in quantum wells

We study the orbital and spin dynamics of charge carriers induced by non-overlapping linearly polarized light pulses in semiconductor quantum wells (QWs). It is shown that such an optical excitation with coherent pulses leads to a spin orientation of photocarriers and an electric current. The effects are caused by the interference of optical transitions driven by individual pulses. The distribution of carriers in the spin and momentum spaces depends on the QW crystallographic orientation and can be efficiently controlled by the pulse polarizations, time delay and phase shift between the pulses, as well as an external magnetic field.

Spin dynamics and spin noise in the presence of randomly varying spin–orbit interaction in a semiconductor quantum wire

Using ensemble Monte Carlo simulation, we have studied hot carrier spin dynamics and spin noise in a multi-subband GaAs quantum wire in the presence of a randomly varying Rashba spin–orbit interaction. The random variation reduces the carrier ensemble’s spin dephasing time due to the D’yakonov–Perel’ mechanism, but otherwise makes no qualitative difference to the temporal spin relaxation characteristics. However, it makes a qualitative difference to the spatial spin relaxation characteristics which change from monotonic and smooth to non-monotonic and chaotic because of a complex interplay between carriers in different subbands. As far as spin fluctuation and spin noise are concerned, the random variation has no major effect except that the low-frequency noise power spectral density increases slightly when the magnitude of the Rashba spin–orbit interaction field is varied randomly while holding the direction constant.

Coherent spin dynamics of electrons and holes in semiconductor quantum wells and quantum dots under periodical optical excitation: Resonant spin amplification versus spin mode locking

The coherent spin dynamics of resident carriers, electrons and holes, in semiconductor quantum structures is studied by periodical optical excitation using short laser pulses and in an external magnetic field. The generation and dephasing of spin polarization in an ensemble of carrier spins, for which the relaxation time of individual spins exceeds the repetition period of the laser pulses, are analyzed theoretically. Spin polarization accumulation is manifested either as resonant spin amplification or as mode-locking of carrier spin coherences. It is shown that both regimes have the same origin, while their appearance is determined by the optical pump power and the spread of spin precession frequencies in the ensemble.

Magneto-optical properties of ZnMnTe/ZnSe quantum dots

The σ+ and σ− circularly polarized time-integrated and time-resolved Photoluminescence (PL) measurements were employed to investigate the carrier spin dynamics of ZnMnTe/ZnSe Quantum Dots (QDs) grown on GaAs substrates by molecular beam epitaxy. The Kohlrausch’s stretching exponential function well correlates both the σ+ and σ− decay profiles. The measured spin relaxation time is about 23ns.

Spin dynamics of charge carriers in the process of their localization in α’-(BEDT-TTF)2IBr2 single crystals

Sharp changes in the integral intensity and linewidth of the ESR spectrum that accompany the localization of the charge carriers have been revealed in α’-(BEDT-TTF)2IBr2 crystals. It has been found that the types of localization in two compounds under investigation are different: charge carriers in β″-(BEDT-TTF)4NH4[Cr(C2O4)3] are localized on irregular defects of the crystal lattice, whereas charge carriers in α’-(BEDT-TTF)2IBr2 are localized at the regular positions of the unit cell. The exchange narrowing of the ESR line and a sharp decrease in the dc and ac magnetic susceptibilities are observed in α’-(BEDT-TTF)2IBr2 at low temperatures T 50 K differ from each other, because the thermally activated hopping frequency of the charge carriers is higher than the frequency of the measuring UHF field of an ESR spectrometer.

Spin dynamics of electrons and holes inInGaAs/GaAsquantum wells at millikelvin temperatures

The carrier spin dynamics in a $n$-doped (In,Ga)As/GaAs quantum well has been studied by time-resolved Faraday rotation and ellipticity techniques in the temperature range down to 430 milliKelvin. These techniques give data with very different spectral dependencies, from which nonetheless consistent information on the spin dynamics can be obtained, in agreement with theoretical predictions. The mechanisms of long-lived spin coherence generation are discussed for the cases of trion and exciton resonant excitation. We demonstrate that carrier localization leads to a saturation of spin relaxation times at 45 ns for electrons below 4.5 K and at 2 ns for holes below 2.3 K. The underlying spin relaxation mechanisms are discussed.

Hole–Nuclear Spin Interaction in Quantum Dots

We have measured the carrier spin dynamics in p-doped InAs/GaAs quantum dots by pump-probe and time-resolved photoluminescence experiments. We obtained experimental evidence of the hyperfine interaction between hole and nuclear spins. In the absence of an external magnetic field, our calculations based on dipole-dipole coupling between the hole and the quantum dot nuclei lead to a hole-spin dephasing time for an ensemble of dots of 14 ns, in close agreement with experiments.

Carrier spin dynamics in GaAs/AlGaAs quantum wells with a laterally localizing electric potential

The spin orientation dynamics in a GaAs quantum well with a laterally nonuniform electric potential generated by a mosaic electrode deposited on the surface of the sample has been investigated using the photoinduced magneto-optical Kerr effect. It has been found that the application of a negative potential higher than 1 V to the electrode leads to more than a hundredfold increase in the spin polarization lifetime in the sample under study. It is concluded that so strong slowing down of the relaxation is caused by a combined action of two effects, namely, the spatial separation of electron and hole, which reduces the radiative recombination rate of electron-hole pair, and the localization of electron, which is accompanied by the suppression of spin relaxation processes caused by electron motion.

Spin Relaxation in InAs Columnar Quantum Dots

We have investigated carrier spin dynamics in InAs columnar quantum dots (CQDs) by time-resolved photoluminescence (PL) measurement. The CQDs were formed by depositing a 1.8 monolayer InAs seed dot layer and a short-period GaAs/InAs superlattice (SL). The spin relaxation time was found to be prolonged from 1.6 to 5.3 ns by increasing the number of SL periods from 3 to 35. The PL decay time also increased from 0.93 to 1.9 ns, indicating a decrease in the spatial overlap of electron and hole wave functions. The changes in both the spin relaxation time and the PL decay time suggest that the Bir–Aronov–Pikus process is the main spin relaxation mechanism.

Exciton and hole spin dynamics in ZnO investigated by time-resolved photoluminescence experiments

The carrier spin dynamics in ZnO is investigated by time-resolved optical orientation experiments. We evidence a clear circular polarization of the donor-bound exciton luminescence in both ZnO epilayer and nonintentionally doped bulk ZnO. This allows us to measure the localized hole spin relaxation time. We find h s 350 ps at T = 1.7 K in the ZnO epilayer. The strong energy and temperature dependences of the photoluminescence polarization dynamics are well explained by the fast free exciton spin relaxation time and the ionization of bound excitons.

Effect of Mn2+ spin frustrations on spin dynamics of charge carriers in (BEDT‐TTF)2Mn[N(CN)2]3 crystals

AbstractThe effect of spin frustrations of Mn2+ planar lattice on the spin relaxation rate in conductive BEDT‐TTF layers was observed. Layer‐to‐layer exchange availability was proved. Critical field of spin flop transition Hc = 32 kOe measured in our experiments makes it possible to estimate the interplanar exchange interaction as ∼10–3 K. Indirect interplanar exchange occurs through BEDT‐TTF layers due to spin polarization of conducting band electrons contributing to the exchange interaction. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Mode-Locking of VECSELs by Crossed-Polarization Gain Modulation

We analyze in detail an approach for inducing passive mode-locking (ML) in vertical-external-cavity surface-emitting lasers (VECSELs) without using any saturable absorber but directly exploiting the lightwave polarization degree of freedom. Part of the VECSEL output, which is forced to be linearly polarized, is reinjected into the optical amplifier after a delay time and rotating its polarization state by 90deg. In the active medium, both polarizations interact via cross-gain modulation. A comprehensive time-domain model including polarization and carrier spin dynamics is derived from fundamental principles. The model is used to evaluate the feasibility and robustness of the ML scheme. We obtain mode-locked pulses of ~ 10-ps duration at repetition rates up to 20 GHz. The reinjection delay time defines resonance conditions where the maximum power and shortest pulsewidth are obtained. Stable fundamental ML is surrounded by regimes of harmonic and incomplete ML. A transition leading from self-pulsation to ML is obtained for sufficiently long reinjection delay times.

Experimental Tests of Possible Mechanisms for the Organic Magnetoresistive Effect

AbstractWe experimentally test three existing models of organic magnetoresistance (OMAR) which are all based on carrier spin dynamics. We first prove that hyperfine field originating from the hydrogen nuclei in organic materials is necessary for observing OMAR by studying C60 sandwich devices using several different electrode materials. We show that C60, unlike many other organic semiconductors, does not exhibit any intrinsic OMAR effect. However, we find that as soon as the carriers in C60 are brought in proximity with hydrogen-containing compounds, either in the form of a polymeric electrode, or side-chain substituents, a weak OMAR effect is observed. Next, we perform charge-induced absorption and electroluminescence spectroscopy in a polyfluorene organic magnetoresistive device. Our experiments allow us to measure the singlet exciton, triplet exciton and polaron densities in a live device under an applied magnetic field, and to distinguish between three models of OMAR. These models are based on different spin-dependent interactions, namely exciton formation, triplet exciton-polaron quenching and bipolaron formation. We show that the singlet exciton, triplet exciton and polaron densities and conductivity all increase with increasing magnetic field. Our data are inconsistent with the exciton formation and triplet-exciton polaron quenching models.

Spin‐dependent type‐II band alignment and carrier spin dynamics in a diluted magnetic double quantum well

We study spin-dependent type-II band alignment and the carrier spin dynamics in a diluted magnetic double quantum well (DQW) composed of a magnetic well (MW) of Zn0.70Cd0.22Mn0.08Se and a Zn0.76Cd0.24Se non-magnetic well (NMW). In this DQW structure, a heavy-hole (hh) level in the MW intersects that in the NMW by applying a magnetic field of 1 T, resulting from the giant Zeeman effect of the hh in the MW. This hh-level crossing induces a type-II band alignment for the spin-polarized hh in the MW and an electron in the NMW. The circularly polarized photoluminescence (PL) energy decreases and the PL lifetime increases simultaneously with an increase in the magnetic field, which is quantitatively explained by the formation of spin-polarized indirect exciton. The magnetic-field dependence of the exciton PL shows the hh-spin dynamics in the transformation from the direct exciton state to indirect one. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic‐field dependence of electron‐spin relaxation in a diluted magnetic semiconductor quantum well studied by pump‐probe absorption spectroscopy

We study the spin dynamics of photo-excited carriers in a diluted magnetic Cd0.95Mn0.05Te quantum well by means of pump-probe absorption spectroscopy. Transient differential absorption intensities, which are discriminated with left/right circular polarizations, indicate the spin-relaxation dynamics after pumping. Time constants of 0.5 and 7 ps are obtained for the heavy-hole (hh)- and electron-spin relaxations at 0 T, where the electron-spin relaxation is mainly caused by the s-d exchange mechanism involving Mn-spins. We observe a systematic increase in the electron-spin relaxation time up to 19 ps with an increase in the magnetic field up to 2.5 T. We attribute this to suppression of the electron-spin relaxation via the s-d exchange mechanism, where the spin transfer from the electron to the Mn-spin requires energy compensation due to the Zeeman splitting of the Mn-spin under magnetic fields. Above 3 T, the electron-spin relaxation time decreases with increasing magnetic field, possibly resulting from LO-phonon scattering involving acoustic phonons and Mn-spins. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin dynamics of carriers and excitons in nanostructure diluted magnetic semiconductors

Spin dynamics of carriers and excitons are studied in nanostructure diluted magnetic semiconductors (DMSs), in which the carrier spins are highly polarized due to the giant Zeeman effects. Spin injection and spatial spin separation are demonstrated in coupled double quantum wells (DQWs) composed of the DMS and non-magnetic wells. Individual spin-polarized tunnelings of electrons and heavy holes (hhs) through a tunneling barrier are directly observed by pump-probe absorption spectroscopy. The spin separation of hhs is realized in a magnetic DQW without a barrier, which is confirmed as a spin-dependent type-II radiative recombination of the excitons. The spin injections are demonstrated in coupled quantum disks fabricated by electron-beam lithography and also in self-assembled quantum dots (SAQDs) coupled with a DMS quantum well. In the coupled SAQD system, efficient spin injection is realized via resonant spin tunneling assisted by LO-phonon scattering.

Carrier spin dynamics in modulation-doped InAs∕GaAs quantum dots

Photoexcited electron and hole spin relaxation was studied in modulation-doped and undoped InAs∕GaAs quantum dots by means of time-resolved photoluminescence. After excitation into the barriers or the wetting layer, the electron spin polarization is preserved during the capture and relaxation in the dots, especially in the p-doped structures, and decays with a characteristic time of about 100ps. Spin state admixture in combination with electron interaction with acoustic phonons is suggested as the spin relaxation mechanism. Rapid spin polarization decay during carrier relaxation in undoped quantum dots is attributed to electron-optical phonon interaction. For carrier excitation directly into the dots, no significant spin polarization was observed, which points to the mixed nature of hole levels in quantum dots. The hole spin polarization randomizes on a much shorter time scale and is not detected in the experiment.

Electron spin resonance in DNA doped polypyrrole

Electron spin resonance (ESR) of electrochemically doped polypyrrole with various concentrations (c) of calf-thymus DNA is reported here. The concentration and thermal dependence of the ESR linewidth (ΔH) clearly depicted a quasi-metallic to metallic transition at a critical doping concentration (c0) of DNA. The thermal and concentration dependences of various ESR parameters are interpreted by considering the possible spin dynamics of carriers and the conformational change in the polypyrrole structure induced by DNA chains.

Spin relaxation and antiferromagnetic coupling in semiconductor quantum dots

We report carrier spin dynamics in highly uniform self-assembled InAs quantum dots and the observation of antiferromagnetic coupling between semiconductor quantum dots. The spin relaxation times in the ground state and the first excited state were measured to be 1.0 and 0.6 ns, respectively, without the disturbance of inhomogeneous broadening. The measured spin relaxation time decreases rapidly from 1.1 ns at 10 K to 200 ps at 130 K. This large change in the spin relaxation time is well-explained in terms of the mechanism of acoustic phonon emission. In coupled quantum dots, the formation of antiferromagnetic coupling is directly observed. Electron spins are found to flip at 80 ps after photoexcitation via the interdot exchange interaction. The antiferromagnetic coupling exists at temperatures lower than 50–80 K. A model calculation based on the Heitler–London approximation supports the finding that the antiferromagnetic coupling is observable at low temperature. These carrier spin features in quantum dots are suitable for the future quantum computation.

Spintronics: Fundamentals and applications

Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.