A strong anisotropy of electron spin decoherence is observed in GaAs/(AlGa)As quantum wells grown on (110) oriented substrate. The spin lifetime of spins perpendicular to the growth direction is about one order of magnitude shorter compared to spins along (110). The spin lifetimes of both spin orientations decrease monotonically above a temperature of 80 and 120 K, respectively. The decrease is very surprising for spins along (110) direction and cannot be explained by the usual Dyakonov Perel dephasing mechanism. A novel spin dephasing mechanism is put forward that is based on scattering of electrons between different quantum well subbands.The electron spin in semiconductors has recently become a focus of intense research in the context of spinelectronics or spintronics. This new kind of electronics aims to utilize spin for devices with unprecedented properties [1,2,3]. A prime condition for the development of potential applications is the understanding of spin decoherence, i.e. the loss of spin memory, in semiconductor structures [4]. The main reason for spin decoherence at room temperature is the intrinsic spin splitting of the conduction band, which occurs in all binary semiconductors. The spin splitting, which acts as an effective magnetic field, depends on the electron's momentum and is the basis for the Dyakonov-Perel (DP) spin relaxation mechanism [5,6]. Semiconductor heterostructures are in this context of particular interest since spin splitting in conduction and valence band can be controlled via dimensionality and orientation of crystal axes [7]. Ohno et al. observed very long electron spin decoherence times at room temperature in GaAs quantum wells (QWs) grown on (110) oriented substrates that exceeded the coherence times in usual (100) grown QWs by more than one order of magnitude [8,9]. However, slow spin dephasing in (110) QWs had been demonstrated only for electron spins oriented along the crystal growth direction. The dynamics of in-plane spin was left unexplored.Starting point for the theoretical description of the spin dynamics in (110) QWs is the Dresselhaus-Hamilton for binary semiconductorswhere i = x, y, z are the principal crystal axes with i + 3 → i, Γ is the spin-orbit coefficient for the bulk semiconductor, and σ i are the Pauli spin matrices [10]. Comparing eq.(1) with the spin Hamilton for a free electron in a magnetic field (H = 1 2 i µ B σ i B i ) one easily recognizes that random scattering of electrons leads to an effective k dependent random magnetic field with components in x, y, and z direction. This random magnetic field destroys the average spin orientation of an ensemble of electrons by rotating individual spins in different directions. The DP effect increases in bulk semiconductors with temperature due to occupation of higher k-states with larger spin splittings despite a motional narrowing effect at higher temperatures (spin lifetime τ s is inversely proportional to momentum scattering time τ * p ). In (110) QWs, however, the spin splitting (effective magnetic field)

We demonstrate a reduction of the threshold of a semiconductor laser by optically pumping spin-polarized electrons in the gain medium. Polarized electrons couple selectively to one of two possible lasing light modes which effectively reduces the threshold by up to 50% compared to conventional pumping with unpolarized electrons. We theoretically show that our concept can be generalized to an electrically pumped laser.

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Mobile piezoelectric potentials are used to coherently transport electron spins in GaAs (110) quantum wells (QW) over distances exceeding 60 microm. We demonstrate that the dynamics of mobile spins under external magnetic fields depends on the direction of motion in the QW plane. This transport anisotropy is an intrinsic property of moving spins associated with the bulk inversion asymmetry of the underlying GaAs lattice.

We experimentally demonstrate the reduction of the laser threshold of a commercial GaAs∕(AlGa)As vertical-cavity surface-emitting laser (VCSEL) by optical injection of spin-polarized electrons at room temperature. Calculations with a rate-equation model reproduce the measured reduction of 2.5% for injected electrons with 50% spin polarization. The model predicts an improved threshold reduction of 50% in otherwise identical VCSELs grown on a (110) substrate due to the enhanced spin lifetime in such structures.

The electron spin dynamics in n-type wurtzite GaN is studied by time-resolved Kerr rotation for temperatures from 80 to 295 K and magnetic fields up to 1 T. The temperature and magnetic field dependence of the spin-relaxation time are in good agreement with D'yakonov-Perel' theory. We present an analytic expression for the spin-relaxation tensor for semiconductors with wurtzite structure that also includes the interference of Rashba and Dresselhaus contributions.

Electron spin dynamics in n-type c-oriented wurtzite GaN epilayers is studied by time-resolved Kerr-rotation measurements at T=80 K. The electron spin lifetime shows a sudden increase if an external magnetic field is applied in the sample plane. This enhancement is explained by anisotropic Dyakonov–Perel spin relaxation in bulk GaN as a direct consequence of the anisotropy of spin-orbit coupling in semiconductors with wurtzite structure.

We observed a circular photogalvanic effect (CPGE) in GaAs quantum wells at inter-band excitation. The spectral dependence of the CPGE is measured together with that of the polarization degree of the time resolved photoluminescence. A theoretical model takes into account spin splitting of conduction and valence bands. PACS numbers:Spin photocurrents generated by excitation with circularly polarized radiation in quantum wells have attracted considerable attention in the recent decade [1]. Several mechanisms of electric currents driven by optically generated spin polarization are observed in zincblende-structure based bulk semiconductors and quantum wells (QWs). Among these effect are inhomogeneous spin orientation induced currents in bulk GaAs [2, 3], the circular photogalvanic effect (CPGE) and the spingalvanic effect in QWs [4,5], the photovoltaic effect in p − n junctions [6,7] and currents due to quantum interference of one-and two-photon excitations [8,9,10]. Except CPGE in QWs, all other spin photocurrents have been observed at optical excitation across the band gap of the semiconductor. CPGE [11,12,13,14,15] is caused in zinc-blende structure based QWs by homogeneous optical spin orientation of carriers [4]. This effect should also occur at inter-band excitation [4,17], but so far has been detected only at intra-band transitions by excitation with infrared radiation. In the present work we report on the first observation of the CPGE at inter-band excitation in GaAs QWs.The experiments were carried out on (113)Aoriented molecular-beam-epitaxy (MBE) grown p-type GaAs/Al 0.32 Ga 0.68 As structure with 20 QW of 15 nm widths. The free hole density in the sample was 2 · 10 11 cm −2 and the mobility was about 5 · 10 5 cm 2 /Vs at 4.2 K. The sample edges were oriented along the [110]and [332]-directions. Two pairs of ohmic contacts were centered along opposite sample edges pointing in the directions x[110] and y [332] (see Fig. 1). The sample belongs to the symmetry class C s which allows the CPGE at normal incidence of the radiation [1]. For optical inter-band excitation a cw-Ti:sapphire laser and pulsed Ti:sapphire laser were used providing radiation of wavelength in the range between 0.7 µm and 0.85 µm. The power of the cw-laser P was about 80 mW. The pulsed laser provided 1 ps pulses with a repetition rate of 80 MHz and an average power of about 100 mW. One of the main features of the CPGE is that the photocurrent FIG. 1: Photocurrent in QWs normalized by P and a spectrally integrated polarization degree of photoluminescence as a function of the excitation photon energyhω. The inset shows the geometry of the experiment. Normal incidence of radiation on p-type (113)A-grown GaAs/AlGaAs QWs (symmetry class Cs).caused by spin polarization is proportional to the helicity of the incident light P circ = (I σ+ − I σ− )/(I σ+ + I σ− ), where I σ+ and I σ− are intensities of right-(σ + ) and lefthanded (σ − ) polarized radiation. Therefore, the sign of the current changes upon switching from right to left circular polari...