49 research outputs found
Adiabatische und kohärente Elektronendynamik in lichtgetriebenen, resonanten Festkörpersystemen
No full textDiese Arbeit untersucht die Elektronendynamik in resonanten Festkörpersystemen mittels Detektion der dritten Harmonischen. Im ersten Teil wird die kohärente Kontrolle in CdTe-Quantenpunkten analysiert. Es zeigt sich, dass Quanteninterferenzeffekte und die nichtlineare Suszeptibilität dritter Ordnung stark von der Partikelgröße abhängen und ein direkter Zusammenhang zwischen der Elektronendynamik im Leitungsband und temporären Brechungsindexänderungen hergeleitet werden kann. Der zweite Teil erforscht die Elektronendynamik an lokalisierten atomaren Fehlstellen in Quarzkristallen. Durch lokalisierte Synchronisation der Elektronengeschwindigkeit mit der Magnetfeldkomponente des treibenden Feldes werden ausgeprägte spektrale Modulationen beobachtet, die auf topologische Prozesse wie die Berry-Phase zurückgeführt werden können. Erstmals konnte die Verschränkung von Valenz- und Leitungsband in Propagationsrichtung über den Landau-Zener-Tunneleffekt bestimmt werden, was einen neuen Zugang zu korrelierten Elektronen eröffnet.This work investigates electron dynamics in resonant solid-state systems via third-harmonic generation detection. The first part explores coherent control in CdTe quantum dots, revealing that quantum interference effects and the third-order nonlinear susceptibility are highly size-dependent. A direct link between conduction band electron dynamics and transient refractive index changes is established. The second part studies electron dynamics at localized atomic defects in quartz crystals. Pronounced spectral modulations are observed, attributed to the localized synchronization of electron velocity with the magnetic field component of the driving field. This allows access to topological processes like the Berry phase. For the first time, entanglement between valence and conduction bands in the propagation direction was determined via the Landau-Zener tunneling effect, providing a novel pathway to correlated electrons
Evidence for a Magnetic Proximity Effect up to Room Temperature at Fe/(Ga,Mn)As Interfaces
No full textWe report x-ray magnetic circular dichroism and superconducting quantum interference device magnetometry experiments to study magnetic order and coupling in thin Fe/(Ga,Mn)As(100) films. We observe induced magnetic order in the (Ga,Mn)As layer that extends over more than 2 nm, even at room temperature. We find spectroscopic evidences of a hybridized d configuration of Mn atoms in Fe/(Ga,Mn)As, with negligible Mn diffusion and/or MnFe intermixing. We show by experiment as well as by theory that the magnetic moment of the Mn ions couples antiparallel to the moment of the Fe overlayer
Morphology and flexibility of graphene and few-layer graphene on various substrates
No full textWe report on detailed microscopy studies of graphene and few-layer graphene produced by mechanical exfoliation on various semiconducting substrates. We demonstrate the possibility to prepare and analyze graphene on (001)-GaAs, manganese p-doped (001)-GaAs, and InGaAs substrates. The morphology of graphene on these substrates was investigated by scanning electron and atomic force microscopies and compared to layers on SiO2. It was found that graphene sheets strongly follow the texture of the sustaining substrates independent on doping, polarity, or roughness. Furthermore resist residues exist on top of graphene after a lithographic step. The obtained results provide the opportunity to research the graphene-substrate interactions
Investigations on a dilute magnetic semicondutor (Ga1−xMnxAs) by conventional TEM and EELS
No full textOverflow of a dipolar exciton trap at high magnetic fields
No full textWe study laterally trapped dipolar exciton ensembles in coupled GaAs quantum wells at high magnetic fields in the Faraday configuration. In photoluminescence experiments, we identify three magnetic field regimes. At low fields, the exciton density is increased by a reduced charge carrier escape from the trap, and additionally, the excitons' emission energy is corrected by a positive diamagnetic shift. At intermediate fields, magnetic field dependent correction terms apply which follow the characteristics of a neutral magnetoexciton. Due to a combined effect of an increasing binding energy and lifetime, the exciton density is roughly doubled from zero to about 7 T. At the latter high field value, the charge carriers occupy only the lowest Landau level. In this situation, the exciton trap can overflow independently from the electrostatic depth of the trapping potential, and the energy shift of the excitons caused by the so-called quantum confined Stark effect is effectively compensated. Instead, the exciton energetics seem to be driven by the magnetic field dependent renormalization of the many-body interaction terms. In this regime, the impact of parasitic in-plane fields at the edge of trapping potential is eliminated. (C) 2017 Elsevier Ltd. All rights reserved
Tunneling anisotropic spin polarization in lateral (Ga,Mn)As/GaAs spin Esaki diode devices
No full textWe report here on anisotropy of spin polarization obtained in lateral all-semiconductor, all-electrical spin injection devices employing p+-(Ga,Mn)As/n+-GaAs Esaki diode structures as spin aligning contacts, resulting from the dependence of the efficiency of spin tunneling on the orientation of spins with respect to different crystallographic directions. We observed an in-plane anisotropy of 8% in the case of spins oriented either along [1[overline 1]0] or [110] direction and 25% anisotropy between in-plane and perpendicular-to-plane orientations of spins
Strong localization effect in magnetic two-dimensional hole systems
No full textWe report an extensive study of the magnetotransport properties of magnetically doped two-dimensional hole systems. Inverted manganese modulation doped InAs quantum wells with localized manganese ions providing a magnetic moment of S=5/2 were grown by molecular beam epitaxy. Strong localization effect found in low-field magnetotransport measurements on these structures can either be modified by the manganese doping density or by tuning the two-dimensional hole density p via field effect. The data reveal that the ratio between p and manganese ions inside or in close vicinity to the channel enlarges the strong localization effect. Moreover, asymmetric broadening of the doping layer due to manganese segregation is significantly influenced by strain in the heterostructure
Coexistence of ferromagnetism and quantum Hall effect in Mn modulation-doped two-dimensional hole systems
No full textModulation doping using Mn as an acceptor has been applied to the molecular beam epitaxial grown compressively strained InAs channels. Strain engineering has been accomplished by the growth of a graded, fully relaxed InxAl1-xAs buffer layer on GaAs(0 0 1) substrates in which the In content x was increased from 0% to 75%. Transmission electron microscopical investigation of the heterostructures reveals the high quality of the heterostructures grown on top of the metamorphic buffer layer. Significant segregation and diffusion of the Mn doping profile is confirmed by secondary ion mass spectroscopical analysis. Both normal as well as inverted doped quantum well structures show pronounced Shubnikov-de Haas oscillations and quantum Hall effect plateaus. However, for the inverted heterostructures where a significant Mn concentration within the InAs channel can be detected, a strong resistance maximum at B=0 T accompanied by a hysteretic behaviour and reproducible resistance jumps indicative of ferromagnetic ordering is observed
Annealing-induced transition from a (311)A-oriented Ga0.98Mn0.02As alloy to a GaMnAs/MnAs hybrid structure studied by angle-dependent magnetotransport
No full textThe angle-dependent behavior of the magneto-resistance of a series of low-temperature molecular beam epitaxy grown Ga(0.98)Mn(0.02)As samples on (311)A-GaAs-substrates is studied. The samples are annealed at different temperatures in a range from 300 degrees C to 500 degrees C, which leads to a redistribution of the manganese in the sample and finally to the formation of MnAs clusters. As a consequence, the angle-dependence of the magneto-resistance changes with increasing annealing temperature and vanishes finally at an annealing temperature of 400 degrees C. The observed anisotropy of the magneto-resistance can be correlated with the magnetic anisotropy of the magnetization. The parameters describing this magnetic anisotropy and their changes due to annealing are extracted from the experimental magneto-resistance data by a fitting procedure. The magnetocrystalline anisotropy of the samples can be described by the sum of cubic and uniaxial contributions. The former are not affected by the annealing whereas the latter change considerably. (c) 2008 American Institute of Physics
