Temperature and electron density dependent measurements of the electron spin relaxation in bulk GaAs are performed using time- and polarization-resolved photoluminescence spectroscopy. The electron spin relaxation time is dominated in the high temperature regime by the D’yakonov-Perel’ [M. I. D’yakonov and V. I. Perel’, Sov. Phys. Solid State 13, 3023 (1972)] spin relaxation mechanism and decreases for negligible electron densities from 42 ps at 300 K to 20 ps at 400 K. The measured spin relaxation times are compared with numerical calculations which include electron-phonon momentum scattering and have no adjustable parameters.

We measure simultaneously the in-plane electron g factor and spin-relaxation rate in a series of undoped inversion-asymmetric (001)-oriented GaAs/AlGaAs quantum wells by spin-quantum beat spectroscopy. In combination the two quantities reveal the absolute values of both the Rashba and the Dresselhaus coefficients and prove that the Rashba coefficient can be negligibly small despite huge conduction-band potential gradients which break the inversion symmetry. The negligible Rashba coefficient is a consequence of the "isomorphism" of conduction-and valence-band potentials in quantum systems where the asymmetry is solely produced by alloy variations. Symmetry is a thread which runs through all of physics, and symmetry reduction discloses basic physical principles. We employ crystallographically engineered symmetry reduction to study the intricate effects of spin-orbit interaction on the electron spin in semiconductor nanostructures. Symmetry reduction is an especially powerful tool in semiconductor physics because the variety of crystallographic directions combined with band-gap engineering allows enormous freedom.The interplay between structure, symmetry, and electron spin in semiconductors directly affects the spin-relaxation rate s and the effective electron Landé factor g. Early studies of s and g focused on bulk zinc-blende material where both entities are isotropic. 1 Subsequently, the reduction in symmetry from T d to D 2d symmetry in symmetrical (001)-oriented quantum wells (QWs) was shown to give rise to anisotropy between the in-plane (x,y) and the out-of-plane (z) directions. 2,3 Further reduction in symmetry to C 2v is achieved in (001) quantum wells by removing the mirror symmetry of the quantum well potential and allows an in-plane, twofold symmetric anisotropy of both s (Ref. 4) and g. 5 Fundamentally, s and g are both determined by spin-orbit interaction but the basic mechanisms for their anisotropies are quite different. Theoretically the in-plane anisotropy of g is proportional to the asymmetry of the electron wave function in the z direction with the proportionality constant given by the Dresselhaus or bulk inversion asymmetry (BIA) spin-splitting coefficient γ . 5,6 In contrast, s is in many cases dominated by the Dyakonov-Perel (DP) spin-relaxation mechanism and the related in-plane anisotropy depends on the ratio (α/β) of the Rashba structural inversion asymmetry (SIA) to the BIA spin splitting. 4 The SIA component is determined in a rather subtle way by the asymmetry of the structure along the z direction. 7,8 In this work, we determine the absolute value of both the Rashba and Dresselhaus coefficients for a series of quantum well structures by simultaneously measuring the in-plane anisotropy of s and g by spin quantum beat spectroscopy. 9 The specially designed undoped (001) quantum well samples, with reduced C 2v symmetry but without external electric fields, illustrate clearly the different origins of the two anisotropies as they possess a strong anisotropy of g and nearly negligible anisot...

We demonstrate by spin quantum beat spectroscopy that in undoped symmetric (110)-oriented GaAs/AlGaAs single quantum wells even a symmetric spatial envelope wavefunction gives rise to an asymmetric in-plane electron Landé-g-factor. The anisotropy is neither a direct consequence of the asymmetric in-plane Dresselhaus splitting nor of the asymmetric Zeeman splitting of the hole bands but is a pure higher order effect that exists as well for diamond type lattices. The measurements for various well widths are very well described within 14 × 14 band k · p theory and illustrate that the electron spin is an excellent meter variable to map out the internal -otherwise hidden-symmetries in two dimensional systems. Fourth order perturbation theory yields an analytical expression for the strength of the g-factor anisotropy, providing a qualitative understanding of the observed effects.PACS numbers: 78.55. Cr,78.47.jd,78.20.Ci,71.18.+y Symmetry is a fundamental principle which runs through all fields of sciences like a common thread. The balance of proportions is attracting great interest ever since reaching from Euclid's geometry theorems and the Archimedes lever principle in ancient times to Mandelbrot sets in present day mathematics and parity violation in modern particle physics. At the beginning of the last century the topic was significantly pushed by Emmy Noether's discovery of the deep connection between symmetry and conservation laws [1] and the classification of nearly all entities in today's physics in terms of its symmetry properties is a very powerful and widely applied method in a vast number of fields. Among the plethora of interesting physical observables the pure quantum mechanical entity spin in connection with the relativistic effect of spin-orbit interaction (SOI) [2] bears an exceeding connection to symmetry. In a free atom, SOI can break the degeneracy of states with the same orbital wave function owing opposite spins. In solids, however, such a splitting interferes with crystal symmetry. The most prominent example is the conduction band Dresselhaus splitting in zinc-blende (ZB) type lattice semiconductors [3], which is not present in their diamond lattice type equivalents [4]. The alteration of the symmetry allows a clear assignment of the investigated spin properties to the symmetry at hand and the change of symmetry properties on micro-and macroscopic scales is easy to produce in solid state physics by the introduction of low dimensional structures, potential gradients, or the choice of peculiar crystallographic quantization axes. This fact has boosted a great interest in recent semiconductor spintronic research [5][6][7] since crystal symmetry yields a control on the spin dynamics [8][9][10][11][12] and contrariwise the entity spin yields jointly with the time-reversal breaking property of a magnetic a unique meter variable to probe internal symmetries which might be inaccessible by other means.In this letter, we exploit the intriguing property that quantum wells (QW) grown with their quantizatio...

We exploit the influence of the Coulomb interaction between electrons and holes on the electron spin relaxation in a (110)-GaAs quantum well to unveil excitonic signatures within the many particle electron-hole system. The temperature dependent time-and polarization-resolved photoluminescence measurements span five decades of carrier density, comprise the transition from localized excitons over quasi free excitons to an electron-hole plasma, and reveal strong excitonic signatures even at relatively high densities and temperatures. 78.55.Cr, 78.67.De Shortly after the big bang, the hot gas of negatively charged electrons and positively charged protons cooled to temperatures below the Rydberg energy and formed a new state of matter, called hydrogen atoms. 13.7 billion years later, researchers shoot short laser pulses on direct semiconductors at liquid helium temperature creating a hot gas of negatively charged electrons in the conduction and positively charged holes in the valence band. The hot carrier gas cools with time by emission of optical and acoustical phonons and forms excitons, i.e., hydrogen like quasi-particles. These quasi-particles have in bulk GaAs a binding energy of 5 meV and strongly influence the photoluminescence (PL) spectrum. Consequently, the PL emission at the exciton transition energy has been used as an indicator for the existence of excitons[1] -as the emission and absorption lines of hydrogen are used in astronomy. However, S. Koch and coworkers demonstrated within the framework of many body semiconductor Bloch equations that distinct excitonic like emission lines from an electron-hole plasma do not proof the existence of excitons but can be explained equally by a Coulomb correlated electron-hole plasma. [2] Manyfold interband pump-probe absorption[3], reflection [4], and four wave mixing experiments[5], high resolution time-resolved PL[6], and experimentally very demanding quasi-particle THz spectroscopy [7] have been carried out to understand the many-body and quantum-optical character and the diligent interaction of excitons in an electron-hole plasma but besides fifty years of intense research the exciton quest remains a fascinating and active field. The challenge in the interpretation of most interband experiments concerning the incoherent exciton population results from the fact that the interaction of classical light and matter is induced by optical polarization and not directly by incoherent population. The challenge in the interpretation of timeresolved PL experiments results generally from the finite excitation density and that thereby the PL does not result from a two but a many particle interaction which strongly depends on the frequency-dependent strength of light-matter interaction and only to some extend on exciton population. Especially THz experiments support that excitonic like PL lines exist without the population of K=0 excitons. [8] In this letter, we present an entirely different experimental approach to study the existence of excitons in a two-dimensional electr...

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