661 Works

Observation of the local structure of landau bands in a disordered conductor

T. Schmidt, Rolf J. Haug, V.I. Fal'ko, K. V. Klitzing, A. Förster & H. Lüth
The local density of states of heavily doped GaAs is explored at high magnetic fields, where only a single or few Landau bands are occupied. Our experiment is based on resonant tunneling through impurity states and images the local density of states both below and above the Fermi level. Fan-type mesoscopic fluctuations are observed in the energy-magnetic-field plane, which we attribute to the interplay of Landau quantization and quantum interference of scattered electron waves in...

Phonon excitations of composite-fermion Landau levels

F. Schulze-Wischeler, Frank Hohls, U. Zeitler, D. Reuter, A.D. Wieck & Rolf J. Haug
The phonon excitations of fractional quantum Hall (FQH) states at filling factors ν =1÷3,2÷5, 4÷7, 3÷5, 4÷3, and 5÷3 were investigated. The excitations were found to be based on Landau-level transitions of composite fermions (CF). It was observed that at filling factor ν = 2÷3, a linear field dependence of the excitation energy in the high field regime showed a spin transition, which was excited by the phonons. All the transport gaps, except the one...

Impurity effects in quantum dots: Toward quantitative modeling

E. Räsänen, J. Könemann, Rolf J. Haug, M.J. Puska & R.M. Nieminen
We have studied the single-electron transport spectrum of a quantum dot in GaAs/AlGaAs resonant tunneling device. The measured spectrum has irregularities indicating a broken circular symmetry. We model the system with an external potential consisting of a parabolic confinement and a negatively charged Coulombic impurity placed in the vicinity of the quantum dot. The model leads to good agreement between the calculated single-electron eigenenergies and the experimental spectrum. Furthermore, we use the spin-density-functional theory to...

Multiple transitions of the spin configuration in quantum dots

M.C. Rogge, C. Fühner & Rolf J. Haug
Single electron tunneling is studied in a many electron quantum dot in high magnetic fields. For such a system multiple transitions of the spin configuration are theoretically predicted. With a combination of spin blockade and Kondo effect we are able to detect five regions with different spin configurations. Transitions are induced with changing electron numbers. © 2006 The American Physical Society.

Giant anisotropy of zeeman splitting of quantum confined acceptors in Si/Ge

K.-M. Haendel, R. Winkler, U. Denker, O.G. Schmidt & Rolf J. Haug
Shallow acceptor levels in Si/Ge/Si quantum well heterostructures are characterized by resonant-tunneling spectroscopy in the presence of high magnetic fields. In a perpendicular magnetic field we observe a linear Zeeman splitting of the acceptor levels. In an in-plane field, on the other hand, the Zeeman splitting is strongly suppressed. This anisotropic Zeeman splitting is shown to be a consequence of the huge light-hole-heavy-hole splitting caused by a large biaxial strain and a strong quantum confinement...

Observation of an interedge magnetoplasmon mode in a degenerate two-dimensional electron gas

G. Sukhodub, Frank Hohls & Rolf J. Haug
The propagation of edge magnetoplasmons (EMP) in a degenerated two-dimensional electron gas was analyzed. Investigations show that an interedge EMP mode attributed to the excitations of boundary between two regions with constant but different charge densities. The splitting of an incident voltage pulse into several EMP modes propagating with different velocities was also observed. The results show that the internal spatial parameters of the edge structure were derived from the analysis of the propagation velocities...

Phonons in InAs/AlAs single quantum dots observed by optical emission

D. Sarkar, H.P. Van Der Meulen, J.M. Calleja, J.M. Becker, Rolf J. Haug & Klaus Pierz
Photoluminescence emission spectra of self-assembled single InAs/AlAs quantum dots with variable size reveal typical sharp lines below 1.8 eV, corresponding to transitions between fully confined states. Above this energy the sharp peaks disappear due to the Γ-X scattering of conduction electrons. Excitation spectra of the biexciton line in single quantum dots show marked maxima at the optical-phonon frequency of the quantum dot and at its first overtone. The quantum dot phonon energy observed is 31...

Magnetoresistance induced by rare strong scatterers in a high-mobility two-dimensional electron gas

L. Bockhorn, I.V. Gornyi, D. Schuh, C. Reichl, W. Wegscheider & Rolf J. Haug
We observe a strong negative magnetoresistance at nonquantizing magnetic fields in a high-mobility two-dimensional electron gas. This strong negative magnetoresistance consists of a narrow peak around zero magnetic field and a huge magnetoresistance at larger fields. The peak shows parabolic magnetic field dependence and is attributed to the interplay of smooth disorder and rare strong scatterers. We identify the rare strong scatterers as macroscopic defects in the material and determine their density from the peak...

Interaction-induced spin polarization in quantum dots

M.C. Rogge, E. Räsänen & Rolf J. Haug
The electronic states of lateral many-electron quantum dots in high magnetic fields are analyzed in terms of energy and spin. In a regime with two Landau levels in the dot, several Coulomb-blockade peaks are measured. A zigzag pattern is found as it is known from the Fock-Darwin spectrum. However, only data from Landau level 0 show the typical spin-induced bimodality, whereas features from Landau level 1 cannot be explained with the Fock-Darwin picture. Instead, by...

Quantized current source with mesoscopic feedback

Lukas Fricke, Frank Hohls, Niels Ubbelohde, Bernd Kaestner, Vyacheslavs Kashcheyevs, Christoph Leicht, Philipp Mirovsky, Klaus Pierz, Hans W. Schumacher & Rolf J. Haug
We study a mesoscopic circuit of two quantized current sources, realized by nonadiabatic single-electron pumps connected in series with a small micron-sized island in between. We find that quantum transport through the second pump can be locked onto the quantized current of the first one by a feedback due to charging of the mesoscopic island. This is confirmed by a measurement of the charge variation on the island using a nearby charge detector. Finally, the...

Noise at a Fermi-edge singularity in self-assembled InAs quantum dots

Niels Maire, Frank Hohls, T. Lüdtke, Klaus Pierz & Rolf J. Haug
We present noise measurements of self-assembled InAs quantum dots at high magnetic field. In comparison to I-V characteristics at zero magnetic field, we notice a strong current overshoot that is due to a Fermi-edge singularity. We observe an enhanced suppression in the shot noise power simultaneous to the current overshoot that is attributed to the electron-electron interaction at the Fermi-edge singularity. © 2007 The American Physical Society.

Methods for intraoperative, sterile pose-setting of patient-specific microstereotactic frames

Benjamin Vollmann, Samuel Müller, Dennis Kundrat, Tobias Ortmaier & Lüder Alexander Kahrs
This work proposes new methods for a microstereotactic frame based on bone cement fixation. Microstereotactic frames are under investigation for minimal invasive temporal bone surgery, e.g. cochlear implantation, or for deep brain stimulation, where products are already on the market. The correct pose of the microstereotactic frame is either adjusted outside or inside the operating room and the frame is used for e.g. drill or electrode guidance. We present a patientspecific, disposable frame that allows...

Membrane stability during biopreservation of blood cells

Christoph Stoll & Willem F. Wolkers
Storage methods, which can be taken into consideration for red blood cells and platelets, include liquid storage, cryopreservation and freeze-drying. Red blood cells can be hypothermically stored at refrigerated temperatures, whereas platelets are chilling sensitive and therefore cannot be stored at temperatures below 20°C. Here we give an overview of available cryopreservation and freeze-drying procedures for blood cells and discuss the effects of these procedures on cells, particularly on cellular membranes. Cryopreservation and freeze-drying may...

Numerical studies of the melting process in the induction furnace with cold crucible

A. Umbrasko, E. Baake, B. Nacke & A. Jakovics
Purpose - Aims to present recent activities in numerical modeling of cold crucible melting process. Design/methodology/approach - 3D numerical analysis was used for electromagnetic problem and 3D large eddy simulation (LES) method was applied for fluid flow modeling. Findings - The comparative modeling shows, that higher H/D ratio of the melt is more efficient when total power consumption is considered, but this advantage is held back by higher heat losses through the crucible walls. Also,...

Particle transport in recirculated liquid metal flows

M. Kirpo, A. Jakovičs, B. Nacke & E. Baake
Purpose - Aims to present recent activities in numerical modeling of turbulent transport processes in induction crucible furnace. Design/methodology/approach - 3D large eddy simulation (LES) method was applied for fluid flow modeling in a cylindrical container and transport of 30,000 particles was investigated with Lagrangian approach. Findings - Particle accumulation near the side crucible boundary is determined mainly by the ρp/ρ ratio and according to the presented results. Particle settling velocity is of the same...

On primes not dividing binomial coefficients

J.W. Sander
We prove that [Formula Omitted] thus dealing with open problems concerning divisors of binomial coefficients. © 1993, Cambridge Philosophical Society. All rights reserved.

Level anticrossings in quantum dots

J. Könemann, E. Räsänen, M.J. Puska, R.M. Nieminen & Rolf J. Haug
We have studied the single-electron transport spectrum of a localized level embedded into a double-barrier resonant tunneling structure. The experimental spectrum shows clear deviations from the Fock-Darwin model indicating a broken circular symmetry. We interprete our observations in terms of the existence of an additional multiple-charged impurity in the vicinity of the quantum dot. © 2005 American Institute of Physics.

Observation of two modes of edge magnetoplasmons by selective edge channel detection

G. Sukhodub, Frank Hohls & Rolf J. Haug
We study the propagation of edge magnetoplasmons by time-resolved current measurements in the quantum Hall regime. Using selective detection of edge states we distinguish two modes of edge excitations propagating with different velocities along the neighboring compressible strips at filling factors close to v = 3. From the analysis of the propagation velocities of each mode the internal spatial parameters of the edge structure are calculated. © 2005 American Institute of Physics.

Quantum dots formed in a GaAsAlGaAs quantum ring

A. Mühle, W. Wegscheider & Rolf J. Haug
We demonstrate the tunability of a GaAsAlGaAs quantum ring showing coherent Aharonov-Bohm oscillations up to a gate voltage regime where the ring splits into three quantum dots. This is explained based on the structure's special confinement potential. We characterized the dots by analyzing the corresponding sets of Coulomb blockade lines. Additionally, the spatial configuration of the dots was confirmed by charge measurements using an adjacent quantum point contact. © 2008 American Institute of Physics.

High-order cumulants in the counting statistics of asymmetric quantum dots

Christian Fricke, Frank Hohls, Nandhavel Sethubalasubramanian, Lukas Fricke & Rolf J. Haug
Measurements of single electron tunneling through a quantum dot (QD) using a quantum point contact as charge detector have been performed for very long time traces with very large event counts. This large statistical basis is used for a detailed examination of the counting statistics for varying symmetry of the QD system. From the measured statistics we extract high order cumulants describing the distribution. Oscillations of the high order cumulants are observed when varying the...

Phase diagram and critical behavior of the adsorption system O/Ru(001): Comparison with lattice-gas models

P. Piercy, K. De'Bell & Herbert Pfnür
The chemisorption of oxygen on the Ru(001) surface shows ordered p(2×2) and p(1×2) phases at coverages of 1/4 and 1/2 monolayer, respectively, that are observed to undergo apparently continuous transitions to a disordered state upon heating. We present here details of the experimental determination of the O/Ru phase diagram, lattice-gas models that describe it, and some (but not all) of the observed critical behavior. Monte Carlo simulations based on a phenomenological Hamiltonian describing pairwise interactions...

Homocysteine and cobalamin status in German vegans

Annika Waldmann, Jochen W. Koschizke, Claus Leitzmann & Andreas Hahn
Objective: The study aimed to evaluate the homocysteine and cobalamin status of German vegans and determine whether the intake of very small amounts of foods of animal origin can improve this status. Design and setting: Cross-sectional study, Germany. Subjects: The dietary and nutritional intakes of 131 vegans (73 women, 58 men; age range: 20.2-82.1 years) were evaluated using a general questionnaire and two food-frequency questionnaires. Results: The prevalence of inadequate cobalamin status in volunteers of...

Multidimensional transitional dynamics: A simple numerical procedure

Timo Trimborn, Karl-Josef Koch & Thomas M. Steger
We propose the relaxation algorithm as a simple and powerful method for determining the transition process in growth models numerically. This method has a number of important advantages: (1) It can easily deal with a wide range of dynamic systems including stiff differential equations and systems giving rise to a continuum of stationary equilibria. (2) The application of the procedure is fairly user-friendly. The only input required consists of the dynamic system. (3) The variant...

Natural enemies of the maize cob borer, Mussidia nigrivenella (Lepidoptera: Pyralidae) in Benin, West Africa

M. Sétamou, F. Schulthess, G. Goergen, Hans-Michael Poehling & C. Borgemeister
Mussidia nigrivenella Ragonot is a pest of maize cobs in West Africa. It significantly reduces maize yields and grain quality, with quantitative losses of 2-25% at harvest, and up to 10-15% indirect losses due to an increase in storage pest infestation levels. Infestation by M. nigrivenella also significantly increased the susceptibility of maize to Aspergillus flavus infection and subsequent aflatoxin contamination. Surveys conducted in different agro-ecological zones of Benin on cultivated and wild host plants...

Intelligent monitoring and recognition of the short-circuiting gas-metal arc welding process

C.S. Wu, Q.X. Hu, J.S. Sun, T. Polte & D. Rehfeldt
MOE Key Lab of Liquid Structure and Heredity of Materials, Institute of Materials Joining, Shangdong University, 73 Jingshi Road, Jinan 250061, People's Republic of China This paper introduces an intelligent system for monitoring and recognition of process disturbances during short-circuiting gas-metal arc welding. It is based on the measured and statistically processed data of welding electrical parameters. A 12-dimensional array of process features is designed to describe various welding conditions and is employed as input...

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