847 research outputs found
Electronic transport in hybrid heterostructures and universal control of spin-orbit interaction in quantum wells
No full textIn recent years, the control of charge and spin in semiconductors has experienced outstanding progress. The extended spin coherence times established in high quality materials together with the advance in elaborate fabrication methods enable coherent spin control, ultimately leading to new spintronic devices. In this thesis, we investigate various aspects of quantum transport with respect to potential spintronic applications in two different kind of 2-dimensional electron gases (2DEGs) in GaAs/AlGaAs crystals.
First, we try to combine the advantages of optically active InAs quantum dots (QDs) and the versatile possibilities of gate tunable 2DEGs in a novel hybrid heterostructure. We have characterized a series of hybrid wafers with different spacings between the inverted 2DEG and Stranski-Krastanov grown self-assembled InAs QDs. Depending on this distance and actual QD nucleation – verified with atomic force microscopy on the wafer surface – the 2DEG mobility is reduced due to scattering induced by the QDs and the InAs wetting layer. For a tunnel barrier of 45 nm, the 2DEG exhibits mobilities exceeding 500′000 cm2/(Vs) despite the presence of InAs QDs, while coherent tunneling between 2DEG and QDs is still permitted. Using a top down approach, lateral quantum point contact gates are aligned precisely to a single, specifically chosen InAs QD. The 1D conductance is not sensitive to QD charging events, but is dominated by a disorder potential, even suppressing quantization.
In the second part of the thesis, we study the control of spin-orbit (SO) interaction. This relativistic coupling of the electron spin to its momentum can be used for coherent spin manipulation, but at the same time also causes spin relaxation. Theory predicts a special symmetry, protecting spin from relaxation in diffusive transport, when the two main contributions of SO interaction in GaAs quantum wells (QW) – the Rashba and the Dresselhaus effect – are of equal strength. We demonstrate broad, independent control of all relevant SO fields, allowing us to tune into this regime. By electrically locking the Rashba and Dresselhaus SO fields via top and back gate, we achieve spin protection for a wide range of voltages on a single QW. We use quantum corrections to 2D conductivity as a sensitive probe of SO coupling. The combination of transport data and numerical calculations allows us to quantify the relevant SO coefficients
Review: Spin photocurrents in quantum wells
No full textSpin photocurrents generated by homogeneous optical excitation with circularly polarized radiation in quantum wells (QWs) are reviewed. The absorption of circularly polarized light results in optical spin orientation due to the transfer of the angular momentum of photons to electrons of a two-dimensional electron gas. It is shown that in QWs belonging to one of the gyrotropic crystal classes a non-equilibrium spin polarization of uniformly distributed electrons causes a directed motion of electrons in the plane of the QW. A characteristic feature of this electric current, which occurs in unbiased samples, is that it reverses its direction upon changing the radiation helicity from left-handed to right-handed and vice versa. Two microscopic mechanisms are responsible for the occurrence of an electric current linked to a uniform spin polarization in a QW: the spin polarization-induced circular photogalvanic effect and the spin-galvanic effect. In both effects the current flow is driven by an asymmetric distribution of spin-polarized carriers in k-space of systems with lifted spin degeneracy due to k-linear terms in the Hamiltonian. Spin photocurrents provide methods to investigate spin relaxation and to reach a conclusion as regards the in-plane symmetry of QWs. The effect can also be utilized to develop fast detectors for determining the degree of circular polarization of a radiation beam. Furthermore, spin photocurrents under infrared excitation were used to demonstrate and investigate monopolar spin orientation of free carriers
Combined atomic force microscopy and photoluminescence imaging to increase the yield of quantum dot photonic devices
No full textMagnetotransport in variable-coupling one-dimensional ballistic constrictions
No full textA scheme to produce closely spaced high-quality one-dimensional electronic channels is presented. It is based on standard nanolithography fabrication applied to a single, wide quantum well AlGaAs/GaAs heterostructure where the spatial separation of the wire modes is given by Coulomb repulsion. The lack of a composition-induced barrier in the double layer leads to electron mobility of 5x10(6) cm(2)/Vs in the starting double two-dimensional system. This in turn yields high-quality one-dimensional channels following nanolithography, as shown by our detailed analysis of the magnetotransport properties of the system. Variable coupling is achieved by split-gate polarization and external magnetic-field intensity and orientation. The relevance for the implementation of coherent nanodevices of this fabrication approach is discussed. (C) 2002 American Institute of Physics
Surface treatments and magnetic properties of Ga1−xMnxAs thin films
No full textAs a diluted magnetic semiconductor (DMS), (Ga,Mn)As is a possible candidate for the realization of spintronics devices, due to its intrinsic compatibility with GaAs based electronics. The low Curie temperature still limits its use for practical devices. Despite the huge knowledge on GaAs surface, the (Ga,Mn)As surface is still not well understood and difficult to handle. Standard surface cleaning techniques have many drawbacks, mainly because thermal treatments changes crystal structure. We will compare the magnetic and spectroscopic properties of differently processed (Ga,Mn)As surfaces with X-ray photoemission (XPS), X-ray absorption (XAS) and X-ray magnetic circular dichroism (XMCD). Samples as-grown, chemically etched, Ar+ sputtered and annealed in oxygen will be compared
Scanning gate imaging of quantum point contacts and the origin of the 0.7 anomaly
No full textThe origin of the anomalous transport feature appearing at a conductance G approximate to 0.7 x (2e(2)/h) in quasi-1D ballistic devices-the so-called 0.7 anomaly-represents a long standing puzzle. Several mechanisms have been proposed to explain it, but a general consensus has not been achieved. Proposed explanations have been based on quantum interference, the Kondo effect, Wigner crystallization, and other phenomena. A key open issue is whether the point defects that can occur in these low-dimensional devices are the physical cause behind this conductance anomaly. Here we adopt a scanning gate microscopy technique to map individual impurity positions in several quasi-1D constrictions and correlate these with conductance characteristics. Our data demonstrate that the 0.7 anomaly can be observed irrespective of the presence of localized defects, and we conclude that the 0.7 anomaly is a fundamental property of low-dimensional systems
Influence of surface treatment on the magnetic properties of GaxMn1-xAs thin films.
No full textWe present a surface science study on carefully characterized GaxMn1-xAs films grown on GaAs(100), where we investigate the influence on magnetic properties of different surface cleaning procedures. X-ray magnetic circular dichroism reveals a net remnant magnetization along the (100) axis, after chemical etching is performed. The analysis of Mn L-2,L-3 XAS line shapes in different environment indicates that Mn in GaxMn1-xAs behaves as a bulk impurity. By exploiting the different depth sensitivity between x-ray photoemission and x-ray absorption we observe the presence of a Mn depleted layer at the surface after etching
Generating tsunami risk knowledge at community level as a base for planning and implementation of risk reduction strategies
No full textMore than 4 million Indonesians live in tsunami-prone areas along the southern and western coasts of Sumatra, Java and Bali. Although there now exists a Tsunami Early Warning Center in Jakarta, installed after the devastating 2004 tsunami, it is essential to develop tsunami risk knowledge within exposed communities as a basis for tsunami disaster management. Communities need to implement risk reduction strategies to mitigate potential consequences.
The major aims of this paper are to present a risk assessment methodology which (1) identifies areas of high tsunami risk in terms of potential loss of life, (2) bridges the gaps between research and practical application, and (3) can be implemented at community level. High risk areas have a high need of action to improve people’s response capabilities towards a disaster and thus to reduce the risk. The methodology developed here is based on a GIS approach and combines hazard probability, hazard intensity, population density and people’s response capability to assess the risk.
Within the framework of the GITEWS (German-Indonesian Tsunami Early Warning System) project, the methodology was applied to three pilot areas; one of which is southern Bali. Bali’s tourism is concentrated for a great part in the communities of Kuta, Legian and Seminyak. Here alone, about 20,000 people live in high and very high tsunami risk areas. The development of risk reduction strategies is therefore of significant interest. A risk map produced for the study area in Bali can be used for local planning activities and the development of risk reduction strategies
Semiclassical transport in semiconductor superlattices with boundaries
No full textIn this thesis the electronic transport properties of semiconductor superlattices with boundaries are studied using the Boltzmann equation in relaxation time approximation.
In the parameter regime that was considered the transport in the electronic minibands in semiconductor superlattices can be described semiclassically with the Boltzmann equation. In the superlattices charged domains which travel through the crystal are caused by the negative differential conductivity. They are created and annihilated at the boundaries. Due to the numerical effort this process can only be described in the relaxation time apporximation of the Boltzmann equation. The results show good agreement with the experiments. The differences between the experiment and the results are attributed to the interaction of the created radiation with the surrounding of the device. In addition to that proposals to avoid the domains were tested. The suppression of the domains would be necessary for the realization of the so-called Blochlaser which would radiate in the THz-regime
Ibuprofen versus fosfomycin for uncomplicated urinary tract infection in women: randomised controlled trial
No full textSTUDY QUESTION: Can treatment of the symptoms of uncomplicated urinary tract infection (UTI) with ibuprofen reduce the rate of antibiotic prescriptions without a significant increase in symptoms, recurrences, or complications? METHODS: Women aged 18-65 with typical symptoms of UTI and without risk factors or complications were recruited in 42 German general practices and randomly assigned to treatment with a single dose of fosfomycin 3 g (n=246; 243 analysed) or ibuprofen 3×400 mg (n=248; 241 analysed) for three days (and the respective placebo dummies in both groups). In both groups additional antibiotic treatment was subsequently prescribed as necessary for persistent, worsening, or recurrent symptoms. The primary endpoints were the number of all courses of antibiotic treatment on days 0-28 (for UTI or other conditions) and burden of symptoms on days 0-7. The symptom score included dysuria, frequency/urgency, and low abdominal pain. STUDY ANSWER AND LIMITATIONS: The 248 women in the ibuprofen group received significantly fewer course of antibiotics, had a significantly higher total burden of symptoms, and more had pyelonephritis. Four serious adverse events occurred that lead to hospital referrals; one of these was potentially related to the trial drug. Results have to be interpreted carefully as they might apply to women with mild to moderate symptoms rather than to all those with an uncomplicated UTI. WHAT THIS PAPER ADDS: Two thirds of women with uncomplicated UTI treated symptomatically with ibuprofen recovered without any antibiotics. Initial symptomatic treatment is a possible approach to be discussed with women willing to avoid immediate antibiotics and to accept a somewhat higher burden of symptoms. FUNDING, COMPETING INTERESTS, DATA SHARING: German Federal Ministry of Education and Research (BMBF) No 01KG1105. Patient level data are available from the corresponding author. Patient consent was not obtained but the data are anonymised and risk of identification is low.Trial registration No ClinicalTrialGov Identifier NCT01488955
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