van der Caspar Wal
University of Groningen
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Publication
Featured researches published by van der Caspar Wal.
Physical Review Letters | 2006
M. V. Costache; M. Sladkov; S. M. Watts; van der Caspar Wal; van Bart Wees
We report direct electrical detection of spin pumping, using a lateral normal-metal/ferromagnet/normal-metal device, where a single ferromagnet in ferromagnetic resonance pumps spin-polarized electrons into the normal metal, resulting in spin accumulation. The resulting backflow of spin current into the ferromagnet generates a dc voltage due to the spin-dependent conductivities of the ferromagnet. By comparing different contact materials (Al and/or Pt), we find, in agreement with theory, that the spin-related properties of the normal metal dictate the magnitude of the dc voltage.
Applied Physics Letters | 2006
M. V. Costache; S. M. Watts; M. Sladkov; van der Caspar Wal; van Bart Wees
The on-chip resonant driving of large cone-angle magnetization precession of an individual nanoscale Permalloy element is demonstrated. Strong driving is realized by locating the element in close proximity to the shorted end of a coplanar strip waveguide, which generates a microwave magnetic field. A frequency modulation method is used to accurately measure resonant changes of the dc anisotropic magnetoresistance. Precession cone angles up to 9° are determined with better than 1° of resolution. The resonance peak shape is well described by the Landau-Lifshitz-Gilbert equation.
Applied Physics Letters | 2006
M. V. Costache; M. Sladkov; van der Caspar Wal; van Bart Wees
The authors measured ferromagnetic resonance of a single submicron ferromagnetic strip, embedded in an on-chip microwave transmission line device. The method used is based on detection of the oscillating magnetic flux due to the magnetization dynamics, with an inductive pickup loop. The dependence of the resonance frequency on applied static magnetic field agrees very well with the Kittel formula, demonstrating that the uniform magnetization precession mode is being driven.
Physical Review B | 2010
S. Z. Denega; Jia Liu; A. Slachter; P. J. Rizo; P.H.M. van Loosdrecht; van Bart Wees; D. Reuter; Andreas D. Wieck; van der Caspar Wal
We report a study of suppressed spin dephasing for quasi-one-dimensional electron ensembles in wires etched into a GaAs/AlGaAs heterojunction system. Time-resolved Kerr-rotation measurements show a suppression that is most pronounced for wires along the [110] crystal direction. This is the fingerprint of a suppression that is enhanced due to a strong anisotropy in spin-orbit fields that can occur when the Rashba and Dresselhaus contributions are engineered to cancel each other. A surprising observation is that this mechanism for suppressing spin dephasing is not only effective for electrons in the heterojunction quantum well but also for electrons in a deeper bulk layer.
Physical Review B | 2008
M. V. Costache; S. M. Watts; van der Caspar Wal; van Bart Wees
We describe electrical detection of spin pumping in metallic nanostructures. In the spin pumping effect, a precessing ferromagnet attached to a normal metal acts as a pump of spin-polarized current, giving rise to a spin accumulation. The resulting spin accumulation induces a backflow of spin current into the ferromagnet and generates a dc voltage due to the spin dependent conductivities of the ferromagnet. The magnitude of such voltage is proportional to the spin-relaxation properties of the normal metal. By using platinum as a contact material we observe, in agreement with theory, that the voltage is significantly reduced as compared to the case when aluminum was used. Furthermore, the effects of rectification between the circulating rf currents and the magnetization precession of the ferromagnet are examined. Most significantly, we show that using an improved layout device geometry, these effects can be minimized.
Journal of Superconductivity and Novel Magnetism | 2007
E. J. Koop; A. I. Lerescu; Jing Liu; van Bart Wees; D. Reuter; Andreas D. Wieck; van der Caspar Wal
Abstract The conductance of a quantum point contact (QPC) shows several features that result from many-body electron interactions. The spin degeneracy in zero magnetic field appears to be spontaneously lifted due to the so-called 0.7 anomaly. Further, the g-factor for electrons in the QPC is enhanced, and a zero-bias peak in the conductance points to similarities with transport through a Kondo impurity. We report here how these many-body effects depend on QPC geometry. We find a clear relation between the enhanced g-factor and the subband spacing in our QPCs, and can relate this to the device geometry with electrostatic modeling of the QPC potential. We also measured the zero-field energy splitting related to the 0.7 anomaly, and studied how it evolves into a splitting that is the sum of the Zeeman effect, and a field-independent exchange contribution when applying a magnetic field. While this exchange contribution shows sample-to-sample fluctuations and no clear dependence on QPC geometry, it is for all QPCs correlated with the zero-field splitting of the 0.7 anomaly. This provides evidence that the splitting of the 0.7 anomaly is dominated by this field-independent exchange splitting. Signatures of the Kondo effect also show no regular dependence on QPC geometry, but are possibly correlated with splitting of the 0.7 anomaly.
Semiconductor Science and Technology | 2013
E. J. Koop; M. J. Iqbal; F. Limbach; M. Boute; van Bart Wees; D. Reuter; Andreas D. Wieck; B.J. Kooi; van der Caspar Wal
Ohmic contacts to a two-dimensional electron gas (2DEG) in GaAs/AlxGa1 ? xAs heterostructures are often realized by annealing of AuGe/Ni/Au that is deposited on its surface. We studied how the quality of this type of ohmic contact depends on the annealing time and temperature, and how optimal parameters depend on the depth of the 2DEG below the surface. Combined with transmission electron microscopy and energy-dispersive x-ray spectrometry studies of the annealed contacts, our results allow for identifying the annealing mechanism. We use this for proposing a model that can predict the optimal annealing time when our commonly applied recipe is used for a certain heterostructure at a certain temperature.
Journal of Applied Physics | 2013
M. J. Iqbal; J. P. de Jong; D. Reuter; Andreas D. Wieck; van der Caspar Wal
We report on developing split-gate quantum point contacts (QPCs) that have a tunable length for the transport channel. The QPCs were realized in a GaAs/AlGaAs heterostructure with a two-dimensional electron gas (2DEG) below its surface. The conventional design uses 2 gate fingers on the wafer surface which deplete the 2DEG underneath when a negative gate voltage is applied, and this allows for tuning the width of the QPC channel. Our design has 6 gate fingers and this provides additional control over the form of the electrostatic potential that defines the channel. Our study is based on electrostatic simulations and experiments and the results show that we developed QPCs where the effective channel length can be tuned from about 200 nm to 600 nm. Length-tunable QPCs are important for studies of electron many-body effects because these phenomena show a nanoscale dependence on the dimensions of the QPC channel.
Physical Review B | 2010
M. Sladkov; A. U. Chaubal; M. P. Bakker; A. R. Onur; D. Reuter; Andreas D. Wieck; van der Caspar Wal
We present measurements of electromagnetically induced transparency with an ensemble of donor-bound electrons in low-doped n-GaAs. We used optical transitions from the Zeeman-split electron-spin states to a bound trion state in samples with optical densities of 0.3 and 1.0. The electron-spin dephasing time T*(2) approximate to 2 ns was limited by hyperfine coupling to fluctuating nuclear spins. We also observe signatures of dynamical nuclear polarization but find these effects to be much weaker than in experiments that use electron-spin resonance and related experiments with quantum dots.
Physical Review Letters | 2008
E. J. Koop; van Bart Wees; D. Reuter; Andreas D. Wieck; van der Caspar Wal
We report electronic control and measurement of an imbalance between spin-up and spin-down electrons in micron-scale open quantum dots. Spin injection and detection were achieved with quantum point contacts tuned to have spin-selective transport, with four contacts per dot for realizing a nonlocal spin-valve circuit. This provides an interesting system for studies of spintronic effects since the contacts to reservoirs can be controlled and characterized with high accuracy. We show how this can be used to extract in a single measurement the relaxation time for electron spins inside a ballistic dot (tau(sf) approximately equal to 300 ps) and the degree of spin polarization of the contacts (P approximately equal to 0.8).