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Conventionally the Cherenkov sound is governed by {\it orbital} degrees of freedom and is excited by {\it supersonic} particles. Additionally, it usually has a {\it forward} nature with a conic geometry known as the Cherenkov cone whose axis is oriented {\it along} the {\it supersonic} particle motion. Here we predict Cherenkov sound of a unique nature entirely resulting from the electronic {\it spin} degree of freedom and demonstrate a fundamentally distinct Cherenkov effect originating from essentially {\it subsonic} electrons in two-dimensional gases with both Bychkov-Rashba and Dresselhaus spin-orbit interactions. Specifically, we show that the axis of the conventional {\it forward} Cherenkov cone gets a nontrivial {\it quarter-turn} and at the same time the sound distribution strongly localizes around this rotated axis being now {\it orthogonal} to the {\it subsonic} particle motion. Apart from its fundamentally appealing nature, the orthogonal Cherenkov sound could have applications in planar semiconductor technology combining spin and acoustic phenomena to develop, {\it e.g.}, acoustic amplifiers or sound sources with a flexible spin dependent orientation of the sound propagation., 7 pages, 4 figures

We analyze universal transport properties of a strongly interacting quantum dot in the Kondo regime when the quantum dot is placed in an external magnetic field. The quantum dot is described by the asymmetric Anderson model with the spin degeneracy removed by the magnetic field resulting in the Zeeman splitting. Using an analytical expression for the tunneling density of states found from a Keldysh effective field theory, we obtain in the whole energy range the universal differential conductance and analytically demonstrate its Fermi-liquid and logarithmic behavior at low- and high-energies, respectively, as a function of the magnetic field. We also show results on the zero temperature differential conductance as a function of the bias voltage at different magnetic fields as well as results on finite temperature effects out of equilibrium and at a finite magnetic field. The modern nonequilibrium experimental issues of the critical magnetic field, at which the zero bias maximum of the differential conductance starts to split into two maxima, as well as the distance between these maxima as a function of the magnetic field are also addressed., Comment: Published version; First submission to NJP: 27.03.2013; Second submission to NJP: 06.05.2013

We calculate spin transport in two-dimensional waveguides in the presence of spatially modulated Zeeman-split energy bands. We show that in a regime where the spin evolution is predominantly adiabatic the spin backscattering rate can be tuned via diabatic Landau-Zener transitions between the spin-split bands [C. Betthausen et. al., Science 337, 324 (2012)]. This mechanism is tolerant against spin-independent scattering processes. Completely spin-polarized systems show full spin backscattering, and thus current switching. In partially spin-polarized systems a spatial sequence of Landau-Zener transition points enhances the resistance modulation via reoccupation of backscattered spin-polarized transport modes. We discuss a possible application as a spin transistor., Theory related to the adiabatic spin transistor concept (C. Betthausen et al., Science 337, 324 (2012), DOI: 10.1126/science.1221350)

High-mobility two-dimensional electron systems may be one of the bases of future spintronic devices, where the spin states of ballistic electrons could be manipulated via external gate voltages and the resulting spin-orbit fields. Therefore, the knowledge of spin dynamics in such systems is of technical interest and offers on the other side an exciting view on the underlying physics. Here, we present time-resolved Faraday rotation measurements in a high-mobility two-dimensional electron system in a GaAs/AlGaAs quantum well structure grown along the [001] direction. Even without applied external magnetic fields the optically generated spin ensemble shows a coherent precession about the effective spin-orbit field. If a nonquantizing magnetic field is applied normal to the sample plane, the effective spin-orbit fields are rotated by the cyclotron motion of the electrons. This rotation leads to fast oscillations in the spin polarization about a nonzero value and an strong increase of the spin dephasing times. The measurement data is in excellent agreement with a model based on a kinetic equation approach., AIP Conf. Proc. 1399

The existence of a spin-orbit coupling (SOC) induced by the gradient of the effective mass in low-dimensional heterostructures is revealed. In structurally asymmetric quasi-two-dimensional semiconductor heterostructures the presence of a mass gradient across the interfaces results in a SOC which competes with the SOC created by the electric field in the valence band. However, in graded quantum wells subjected to an external electric field, the mass-gradient induced SOC can be finite even when the electric field in the valence band vanishes.

We propose a method to determine the relative strength of Rashba and Dresselhaus spin-orbit interaction from transport measurements without the need of fitting parameters. To this end, we make use of the conductance anisotropy in narrow quantum wires with respect to the directions of an in-plane magnetic field, the quantum wire and the crystal orientation. We support our proposal by numerical calculations of the conductance of quantum wires based on the Landauer formalism which show the applicability of the method to a wide range of parameters., Comment: 4 pages, 4 figures

Electron spin relaxation in paramagnetic Ga(Mn)As quantum wells is studied via the fully microscopic kinetic spin Bloch equation approach where all the scatterings, such as the electron-impurity, electron-phonon, electron-electron Coulomb, electron-hole Coulomb, electron-hole exchange (the Bir-Aronov-Pikus mechanism) and the $s$-$d$ exchange scatterings, are explicitly included. The Elliot-Yafet mechanism is also incorporated. From this approach, we study the spin relaxation in both $n$-type and $p$-type Ga(Mn)As quantum wells. For $n$-type Ga(Mn)As quantum wells where most Mn ions take the interstitial positions, we find that the spin relaxation is always dominated by the DP mechanism in metallic region. Interestingly, the Mn concentration dependence of the spin relaxation time is nonmonotonic and exhibits a peak. This behavior is because that the momentum scattering and the inhomogeneous broadening have different density dependences in the non-degenerate and degenerate regimes. For $p$-type Ga(Mn)As quantum wells, we find that Mn concentration dependence of the spin relaxation time is also nonmonotonic and shows a peak. Differently, this behavior is because that the $s$-$d$ exchange scattering (or the Bir-Aronov-Pikus) mechanism dominates the spin relaxation in the high Mn concentration regime at low (or high) temperature, whereas the DP mechanism determines the spin relaxation in the low Mn concentration regime. The Elliot-Yafet mechanism also contributes the spin relaxation at intermediate temperature. The spin relaxation time due to the DP mechanism increases with Mn concentration due to motional narrowing, whereas those due to the spin-flip mechanisms decrease with Mn concentration, which thus leads to the formation of the peak.... (The remaining is omitted due to the space limit), 12 pages, 8 figures, Phys. Rev. B 79, 2009, in press

Der bei tiefen Temperaturen ferromagnetische Halbleiter (Ga,Mn)As ist in den letzten Jahren verstärkt in den Mittelpunkt des wissenschaftlichen Interesses gerückt. In der vorliegenden Arbeit standen vor allem die magnetischen Anisotropien im Vordergrund. Die ersten Versuche epitaktischen Wachstums dünner (Ga,Mn)As-Schichten (typisch 20 .. 200 nm) erfolgten auf einem (001)-GaAs-Substrat. Magnetfeldabhängige in plane Widerstandsmessungen (Magnetfeld liegt in der Ebene) in Hallgeometrie (Querwiderstand) führten im Jahre 2003 zur Entdeckung des Giant-Planar-Hall-Effektes (kurz GPHE). Sowohl der Halleffekt als auch der bereits seit langem bekannte anisotrope Magnetowiderstand (kurz AMR) haben ihre Ursache in der Spin-Bahn-Kopplung. Entscheidend für eine Beschreibung des Ummagnetisierungsvorganges ist der Verlauf der Hallspannung auf Grund der Abhängigkeit des Widerstandes vom Winkel zwischen der Magnetisierung und dem Strompfad. Die Arbeit stellt neben Anisotropieuntersuchungen an Proben, die Abmessungen im Mikrometerbereich besitzen, auch Ergebnisse von Proben mit Strukturgrößen im Nanometerbereich vor. Die mit Hilfe der Elektronenstrahllithographie hergestellten und nur wenige hundert Nanometer breiten (Ga,Mn)As-Streifen zeigten eine starke zusätzliche uniaxiale magnetische Anisotropie, die in Abhängigkeit vom Aspektverhältnis der Struktur Einfluss auf den Ummagnetisierungsvorgang nahm. Durch gezieltes Zusammensetzen einzelner (Ga,Mn)As-Streifen zu sog. Zick-Zack-Strukturen konnte der Nachweis eines positiven Domänenwandwiderstandes in diesem Materialsystem erbracht werden. Im zweiten Teil der Arbeit wurden (Ga,Mn)As-Schichten untersucht, die durch Wachstum auf (311)A-GaAs-Substrat entstanden. Bei B-feldabhängigen Messungen zeigten die Hallwiderstandsverläufe gegenüber (001)-(Ga,Mn)As abweichende magnetische Anisotropieeigenschaften. Die Lage der magnetisch leichten Achsen liegen nicht wie beim (001)-(Ga,Mn)As in der Probenebene, sondern abhängig von der Schichtdicke leicht geneigt außerhalb der (311)A-Fläche. Dies führte auch bei einem in plane angelegten Magnetfeld auf Grund einer nicht verschwindenden z-Komponente (senkrecht zur Probenoberfläche) der Magnetisierung während des Ummagnetisierungsprozesses zu einem ausgeprägten AHE. Durch Auswertung der magnetfeldabhängigen Widerstandsänderungen mittels Ableitung konnte eine sehr genaue Bestimmung der Schaltfelder erfolgen. Mit Hilfe von sog. Minor-Loops wurde die Richtigkeit des Verfahrens bestätigt. Sowohl der GPHE als auch der AHE wurden aus den Messkurven extrahiert. Dieser Schritt ließ eine sehr genaue Interpretation des Ummagnetisierungsprozesses zu. Gestützt wurden die Transportergebnisse mit Hilfe von SQUID und FMR-Messungen. Neben der Temperaturabhängigkeit wurde auch der Einfluss der Schichtdicke auf die magnetischen Anisotropien von (311)A-(Ga,Mn)As untersucht. Die Erkenntnisse aus den Anisotropieuntersuchungen zum (311)A-(Ga,Mn)As halfen in ersten Untersuchungen auch bei der Interpretation von Hallmessungen an (110)-(Ga,Mn)As. Mit Hilfe von Transport- und FMR-Daten konnte der B-feldabhängige Ummagnetisierungsvorgang erklärt werden. (110)-(Ga,Mn)As zeichnet sich bei T = 4,2 K durch eine rein kubische Anisotropie mit Lage der magnetisch leichten Achsen entlang der -Richtungen aus., In recent years the in low temperatures ferromagnetic semiconductor (Ga,Mn)As has gotten more and more into the focus of scientific interests. In the present work especially the magnetic anisotropies are concentrated upon. The first trials of epitactical growth of thin (Ga,Mn)As layers were successful on (001) GaAs substrate (typical thickness 20 .. 200 nm). In 2003 the Giant Planar Hall effect (GPHE) was discovered by resistance measurements in Hall geometry (transverse resistance) with an applied in plane magnetic field. The spin orbit coupling is accountable for both GPHE and the long known anisotropic magneto resistance (AMR). The gradient of the Hall voltage is important for the description of the resetting of magnetization because the resistance depends on the angle between the magnetization and the current path. This work presents results of anisotropy study on samples with a structure size in micro- and nanometer dimensions. The only a few hundred nanometer wide (Ga,Mn)As stripes were made by means of electron beam lithography and show strong additional uniaxiale magnetic anisotropy. The aspect ratio of the structure also influenced the resetting of magnetization. With the aid of compounding individual (Ga,Mn)As stripes (in a so-called zigzag structure) the evidence of a positive domain wall resistance in this material system could be shown. The second part of this work presents studies of (Ga,Mn)As layers grown on (311)A GaAs substrate. Here the B-field dependent Hall resistance measurements showed anomalous magnetic anisotropy in variance to (001)-(Ga,Mn)As. The orientation of the magnetic easy axes of (311)A (Ga,Mn)As depends � in contrast to (001)-(Ga,Mn)As � on the thickness of the magnetic layer and is tilted outside the (311)A surface. By an in plane applied magnetic field the AHE was also observable by the resetting of magnetization based on a non evanescent z-component perpendicular to the surface of this. An exact determination of the switching fields could take place by derivation of the B-field dependent resistance changing. This method was confirmed with minor loops. The GPHE as well as the AHE was extracted from the measurements curves. This step allowed an excellent interpretation of the resetting of magnetization. More measurements with SQUID and FMR supported these transport experiments. In addition to temperature dependence the influence of the layer thickness to the magnetic anisotropy of (311)A-(Ga,Mn)As was examined. The findings of these anisotropy studies of (311)A-(Ga,Mn)As were also very helpful in interpreting the Hall measurement on (110)-(Ga,Mn)As. With the help of transport- and FMR data the illustration of the B-field dependent resetting of the magnetization could take place. (110)-(Ga,Mn)As has an absolute magnetic cubic anisotropy at T = 4,2 K. The orientation of the magnetic easy axes are along .

Single electron confined in a quantum dot is studied. A special emphasis is laid on the spin properties and the influence of spin-orbit interactions on the system. The study is motivated by a perspective exploitation of the spin of the confined electron as a qubit, a basic building block of in a foreseen quantum computer. The electron is described using the single band effective mass approximation, with parameters typical for a lateral electrostatically defined quantum dot in a GaAs/AlGaAs heterostructure. The stemming data for the analysis are obtained by numerical methods of exact diagonalization, however, all important conclusions are explained analytically. The work focuses on three main areas -- electron spectrum, phonon induced relaxation and electrically and magnetically induced Rabi oscillations. It is shown, how spin-orbit interactions influence the energy spectrum, cause finite spin relaxation and allow for all-electrical manipulation of the spin qubit. Among the main results is the discovery of easy passages, where the spin relaxation is unusually slow and the qubit is protected against parasitic electrical fields connected with manipulation by resonant electromagnetic fields. The results provide direct guide for manufacturing quantum dots with much improved properties, suitable for realizing single electron spin qubits., In dieser Arbeit werden die Spineigenschaften eines einzelnen Elektrons, dass in einem Quantenpunkt eingesperrt ist, diskutiert. Insbesondere wird die Bedeutung der Spin-Bahn-Kopplung eingehend untersucht. Der Spinzustand eines Elektrons stellt ein denkbar einfaches quantenmechanisches Zweizustandssystem dar, dass man als mögliche physikalische Realisierung eines Quantenbits (Qubits) nutzen möchte. Solch ein Qubit bildet den grundlegenden Baustein für die Informationsverarbeitung in zukünftigen Quantencomputern. Die theoretische Beschreibung des Elektrons basiert auf der effektiven Masse-Näherung für ein einzelnes Leitungsband mit Parametern die typisch für elektrostatisch definierte, laterale Quantenpunkte in GaAs/AlGaAs Heterostrukturen sind. Die numerischen Ergebnisse werden mit Hilfe der exakten Diagonalisierung gewonnen und alle wichtigen Schlussfolgerungen werden mit analytischen Modellen unterlegt und physikalisch erklärt. Die Arbeit konzentriert sich auf den folgenden physikalischen Schwerpunkten: das Energiespektrum des Elektrons im Quantenpunkt, die phononeninduzierte Spinrelaxation und die elektrisch und magnetisch induzierten Rabioszillationen. Im Detail wird gezeigt, wie die Spin-Orbit Kopplung das Energiespektrum beeinflusst, eine endliche Spinrelaxation bewirkt und eine rein elektronische Manipulation des Spin-Qubits erlaubt. Das Hauptergebnis der Arbeit ist die Entdeckung von sogenannten easy passages, bei denen die Spinrelaxation ungewöhnlich langsam erfolgt und der Spin-Qubit von parasitären elektrischen Feldern geschützt ist. Die Ergebnisse der Arbeit geben Auskunft darüber in welcher Art und Weise Quantenpunkte hergestellt werden sollten, um verbesserte physikalische Eigenschaften zu erreichen, die notwendig sind um nützliche Spin-Qubits zu erzeugen.

We develop a semiclassical theory for spin-dependent quantum transport to describe weak (anti)localization in quantum dots with spin-orbit coupling. This allows us to distinguish different types of spin relaxation in systems with chaotic, regular, and diffusive orbital classical dynamics. We find, in particular, that for typical Rashba spin-orbit coupling strengths, integrable ballistic systems can exhibit weak localization, while corresponding chaotic systems show weak antilocalization. We further calculate the magnetoconductance and analyze how the weak antilocalization is suppressed with decreasing quantum dot size and increasing additional in-plane magnetic field., 5 pages

We develop a semiclassical theory for spin-dependent quantum transport in ballistic quantum dots. The theory is based on the semiclassical Landauer formula, that we generalize to include spin-orbit and Zeeman interaction. Within this approach, the orbital degrees of freedom are treated semiclassically, while the spin dynamics is computed quantum mechanically. Employing this method, we calculate the quantum correction to the conductance in quantum dots with Rashba and Dresselhaus spin-orbit interaction. We find a strong sensitivity of the quantum correction to the underlying classical dynamics of the system. In particular, a suppression of weak antilocalization in integrable systems is observed. These results are attributed to the qualitatively different types of spin relaxation in integrable and chaotic quantum cavities., Comment: 20 pages

We review our recent optical experiments on two-dimensional electron systems at temperatures below 1 K and under high magnetic fields. The two-dimensional electron systems are realized in modulation-doped GaAs-AlGaAs single quantum wells. Via gate electrodes the carrier density of the two-dimensional electron systems can be tuned in a quite broad range between about 1×10^{10} cm^{-2} and 2×10^{11} cm^{-2}. In dilute two-dimensional electron systems, at very low electron densities, we observe the formation of negatively charged excitons in photoluminescence experiments. In this contribution we report about the observation of a dark triplet exciton, which is observable at temperatures below 1 K and for electron filling factors

Laterale Halbleiter-Übergitter eignen sich hervorragend für das Studium der Physik von Elektronen im periodischen Potential. Diese mesoskopischen Systeme werden in Halbleiter-Heterostrukturen realisiert, indem einem zweidimensionalen Elektronensystem in der Ebene der freien Bewegung ein periodisches Potential aufgeprägt wird. Für das Elektronensystem in der Halbleiter-Heterostruktur existiert eine Spin-Bahn-Wechselwirkung (Rashba-Term), die neben den Materialparametern des Halbleiters von der Asymmetrie des Confinementpotentials bestimmt wird. Mit dieser Wechselwirkung kann der Spin-Freiheitsgrad der Elektronen durch ein externes elektrisches Feld manipuliert werden, wodurch sie für den Einsatz in der Spintronik von großem Interesse ist. Im Mittelpunkt der vorliegenden Arbeit steht die quantenmechanische Rechnung zum Magnetotransport in lateralen Halbleiter-Übergittern und der Einfluß der Spin-Bahn-Wechselwirkung in zweidimensionalen Elektronensystemen ohne und mit Modulation. Die quantenmechanische Berechnung des Magnetotransports, welche sowohl die periodische Modulation als auch das externe Magnetfeld im Systemhamiltonian berücksichtigt, liefert wesentliche Beiträge zum Verständnis für die der Leitfähigkeit zugrundeliegenden physikalischen Mechanismen., Lateral semiconductor superlattices are an excellent tool to study the physic of electrons in a periodic potential. These mesoscopic systems are realized in semiconductor heterostructures, when a two-dimensional electron system is subjected to a periodic potential in the plain of the free motion. For the electron system in the semiconductor heterostructure a spin-orbit interaction exist which depends on material parameters and the asymmetry of the confinement potential. This spin-orbit interaction plays a key role in spintronics as it allows to manipulate the spin by an external electric field. The presented work focus on the quantum mechanical calculation of magneto transport properties in lateral superlattices and the influence of the spin-orbit interaction in two-dimensional electron systems witout and with modulation. The quantum mechanical calculation of the magneto transport, which takes into account the lateral modulation and the external electric field in the hamiltonian of the system, contributes to the understanding of the basic transport mechanisms.

Generalization of the Gutzwiller trace formula for the oscillating part of the density of states for systems with spin-orbit interaction is derived using the spin-coherent states and path-integral representation for the quantum propagator. The limit of large spin as well as the limits of weak and strong coupling are considered. Applications to systems with Rashba-type spin-orbit interaction are presented., Eine Verallgemeinerung der Gutzwillerschen Spurformel für den oszillierenden Anteil der Zustandsdichte von Systemen mit Spin-Bahn-Wechselwirkung wird hergeleitet basierend auf spin-kohärenten Zuständen und der Pfadintegral- Darstellung des quantemechanischen Propagators. Der Grenzwert grossen Spins sowie der von schwacher und starker Kopplung werden untersucht. Anwendungen auf Systeme mit Spin-Bahn-Wechselwirkung vom Rashba-Typ werden vorgestellt.

Energy transfer in RbMnF3 was studied by time-resolved spectroscopy of 4T1g exciton and trap luminescence. Uniaxial stress was used to lift the orientational degeneracy of the Jahn-Teller distorted MnF6 octahedra. Stress also reduces the excitonic transfer rates considerably. The most drastic reduction of transfer and a nonexponential decay was observed with (110) stress. Evidence for two-dimensional transfer under (110) stress is given by computer simulation of random-walk processes and by a calculation of nearest-neighbour transfer integrals.