251 research outputs found
Exciton-bound electron-spin relaxation
No full textExciton luminescence polarization studies in semiconductor quantum wells have revealed the coexistence of two main mechanisms of exciton-spin relaxation: a well-known direct relaxation with simultaneous electron and hole spin flip due to the electron-hole exchange interaction and an indirect one with sequential spin flips of the single particles. The rate of exciton spin relaxation in this indirect channel is limited by the slower single-particle spin-flip rate, which is typically the electron one. In this work a theory of exciton-bound electron-spin dynamics driven by the spin-orbit splitting in the conduction band is presented. It is shown that the off-diagonal matrix element between optical active and inactive exciton states that differ only with regard to the electron spin direction represents an effective magnetic field that changes randomly as the exciton is elastically scattered and relaxes its spin. The exchange splitting between the optical active and inactive states acts as a constant external magnetic field, reducing the relaxation. The estimated rate of the bound electron spin flip agrees well with values obtained from previous fittings of the experimental data. Semiconductor heterostructures with real-space indirect excitons, for which the sequential spin-flip relaxation channel becomes the dominant one, are also briefly discussed together with the dependence of-the relaxation time on the well width
Suppressed electron-hole exchange spin flip in cavity polaritons
No full textWe consider theoretically the spin relaxation of exciton-polaritons in semiconductor microcavities in the strong coupling regime. The dominant quantum well exciton polarization relaxation mechanism is typically due to the long-range intra-exciton electron-hole exchange. We estimate perturbatively the corresponding contribution for the lower polaritons as a function of the elastic scattering time in analogy to the Dyakonov-Perel model for electron spin relaxation. We find a strong suppression of the polariton spin flip rate due to the electron-hole exchange with respect to the quantum well exciton case
Rashba spin splitting in semiconductor quantum wires
No full textA general three-dimensional model for asymmetric semiconductor quantum wires is introduced with exact and analytical solutions for the spin-dependent electronic structure. Simple expressions are obtained for the eigenvalues, wave functions, and spin expectation values, valid in both strong and weak Rashba spin-orbit coupling regimes. For III-V quantum wires, the Rashba interaction is shown to be typically in the weak coupling regime and to lead to considerable spin mixing only near the anticrossings, seen only in narrow-gap quantum wires. For realistic wires, the Rashba splitting is shown to decrease with increasing wire confinement
Spin splittings in nanostructures without inversion symmetry
No full textIn a crystal lacking inversion symmetry, in the presence of spin-orbit interaction, the spin degeneracy is in general removed at k not equal 0 even in the absence of an external magnetic field. Such is the case for bulk III-V semiconductors having the zincblende structure. In nanostructures having a reduced symmetry with respect to the bulk, additional zero-field spin-splitting mechanisms are allowed. The interplay between the different spin-orbit interactions is thoroughly investigated and the relevance of the spin-splittings for excitonic properties is addressed. In particular, the competition between intraexciton exchange and single particle spin flips in the relaxation of the exciton luminescence polarization is analysed
Exciton luminescence polarization decay in type II semiconductor heterostructures
No full textIn type II semiconductor heterostructures, one can reduce the electron-hole overlap, without changing much the oscillator strength, so that the space indirect exciton luminescence polarization will decay according to the exciton-bound single-particle spin flip. The exciton spin relaxation rate is then limited by the slower single-particle spin-flip rate which is typically the electron one. We discuss the microscopic theory of the exciton-bound electron spin relaxation driven by the spin-orbit k(e)(3) splitting in the conduction band. The exciton-bound electron spin relaxation rate (Wk) is obtained as a function of the material and structure parameters. The exciton luminescence polarization decay rate in AlSb/GaSb/AlSb/InAs/AlSb heterostructures, where space indirect excitons can be created and controled, is estimated. The InAs electron well width L-c dependence is also discussed. (C) 1998 Elsevier Science B.V. All rights reserved
SPIN-SPLIT SUBBANDS AND MAGNETO-OSCILLATIONS IN III-V ASYMMETRIC HETEROSTRUCTURES
No full textA quantum-mechanical study of the magneto-oscillations in asymmetric heterostructures is presented with the aim of clarifying the origin and the effects of the spin-orbit spin splitting in the conduction subband, The magnetization of the two-dimensional electron gas at the interface of modulation-doped III-V semiconductor heterojunctions is calculated as a function of applied magnetic field and carrier concentration, taking into account both spin-orbit (zero-field) spin-splitting contributions: one due to the k(3) bulk term and one due to the lack of specular symmetry sigma(k) along the growth direction. Regular beating patterns in the amplitude of the oscillations are shown to originate from the latter term. The k(3) term introduces a k-space anisotropy in the zero-held spin splitting. This leads to anomalous beating patterns, related to the occurrence of a magnetic breakdown at special points of the Fermi surface with a small spin splitting. Experimental evidence of regular beating patterns has been found in InAs-based heterostructures. The possibility of observing anomalous beating patterns in GaSb heterojunctions is discussed
Electron-hole symmetry and spin-orbit splitting in IV-VI asymmetric quantum wells
No full textIt is shown that, due to the electron-hole symmetry of the fundamental gap of the lead-salts (PbTe, PbSe and PbS), the Rashba spin splitting in their flat band asymmetric quantum wells is much reduced with the usual equal conduction and valence band-offsets. Different from the III-V case, we find that the important structure inversion asymmetry for the Rashba splitting in IV-VI quantum wells with different left and right barriers is not a material property (i.e., barrier height, effective mass or band gap) but results from the band alignment. This is shown by specific envelope function calculations of the spin-dependent subband structure of Pb1-xEuxTe/PbTe/Pb1-yEuyTe asymmetric quantum wells (x not equal y), based on a simple but accurate four-band kp model for the bulk band structure near the gap, which takes into account band anisotropy, nonparabolicity and multi-valley effects. (C) 2003 Published by Elsevier B.V
Spin-orbit splitting of electronic states in semiconductor asymmetric quantum wells
No full textThe spin-orbit splitting in the dispersion relation for electrons in III-V semiconductor asymmetric quantum wells is studied within the standard envelope-function formalism starting from the eight-band Kane model for the bulk. The Rashba spin-orbit splitting in the different subbands is obtained for both triangular and square asymmetric quantum wells. It is shown, for example, that the Rashba splitting in AlAs/GaAs/Ga1-xAlx square quantum wells is of the order of 1 meV and presents a maximum as a function of the well width. The splitting of the excited subband in square and triangular quantum wells is shown to be bigger and smaller than the splitting in the first subband, respectively. A simple single-band approach, employing spin-dependent boundary conditions and approximate coupling parameters, is also introduced and its range of validity assessed. The discussion presented clarifies the treatment of abrupt interfaces, the Ando argument against the splitting, and the use of common approximations such as neglecting the barrier penetration or the energy-dependent corrections to the parameters. Good agreement is found with available experimental data
Das Große Sterben. 100 Jahre Erster Weltkrieg – Spurensuche an Kriegs- und Gedenkorten in Belgien
No full textIn August 2014 the one hundredth anniversary of the outbreak of the first world war (1914-1918) takes place. In the summer of 1914 Europe unleashed an incredibly atrocious conflict, fought for the first time globally and with consequences which have been shaping our world up to the present day. However, we as Germans are often not very aware of a fact that might cause irritations amongst our neighbours in Europe: In Germany, the millions of dead soldiers and civilians of the war 1914-18 seem to be almost forgotten. Remembrance here is entirely overshadowed by the second world war. By contrast, the Great War, so called by Belgians, the British and the French until today, is deeply rooted in the collective memories of our (west-)European neighbours and other nations such as Australia, New Zealand or Canada. Therefore, a relevant question is how can we raise awareness for the dimensions of this "great seminal catastrophe of the 20th century" (George F. Kennan) and the traces it has left behind to the present? How can we contribute to a (renewed) deeper comprehension of the first world war as part of our own history? How can we get more active in promoting a common - or at least mutually informed European / transnational - way of remembering and commemorating? Faced with this challenge, the authors have developed a concept for a multi-day field visit seminar on World War I, with a Europeanized and transnational perspective, exemplary through the events and experiences of soldiers and civilians at the former western front in Belgium. This article aims to provide an insight into the practice of historical-political educational work, and provides illustrating information about the seminar which has already been conducted on two occasions in Summer 2014. This includes the concept and programme, target group, learning objectives, teaching methods, incl. references, as well as seminar results, impressions and experiences. (DIPF/author
Variational Rashba splitting in two-dimensional electron gases in III-V semiconductor heterojunctions
No full textControl of the Rashba spin-orbit coupling in semiconductor two-dimensional electron gases (2DEGs) is of fundamental interest to the rapidly evolving semiconductor spintronics and depends on the detailed knowledge of the controversial interface and barrier penetration effects. Based on the 8x8 k center dot p Kane model for the bulk, we propose a spin-dependent variational solution for the conduction subbands of III-V heterojuctions, which reveals analytically the different contributions to the Rashba splitting and its dependency on heterostructure and band parameters as the band offset and effective masses. Perturbation expansions are used to derive renormalized parameters for an effective, simple, and yet accurate one band model. Spin-dependent modified Fang-Howard trial functions, which satisfy the spin-dependent boundary conditions, are then introduced. The subband splitting is given as a function of the variational parameter which is obtained minimizing the total energy of the 2DEG. Our calculations applied to InAlAs/InGaAs heterojunctions, where a near 20% increase in the splitting is observed due to the barrier penetration, are in good agreement with both experiment and exact numerical calculations. Well-known expressions in the limit of a perfect insulating barrier are exactly reproduced
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