Mathias Wagner
Hitachi
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Featured researches published by Mathias Wagner.
Journal of Applied Physics | 1998
M. B. A. Jalil; H. Ahmed; Mathias Wagner
A quantitative analysis is performed of a bidirectional electron pump circuit that incorporates multiple-tunnel junctions (MTJs), with special emphasis on stability diagrams. The number of electrons transferred per cycle is calculated to be in general agreement with experimental data. The frequency and temperature conditions under which the circuit deviates from ideal pumping operation are evaluated, which suggests that the prospect for a metrological application is not improved by the presence of ultrasmall capacitances in the MTJs.
Journal of Applied Physics | 2001
B. W. Alphenaar; K. Tsukagoshi; Mathias Wagner
We study electron spin transport through carbon nanotubes contacted by ferromagnetic electrodes. The resistance of a ferromagnetically contacted multiwalled nanotube switches hysteretically as a function of applied magnetic field, with a maximum resistance change of 9% at 4.2 K. Magnetoresistance measurements of carbon nanotubes having one cobalt contact and one platinum/gold contact, however, show no switching. In addition, we present calculations of the magnetoresistance ratio for the ferromagnetic nanotube device, and predict that a resistance change of 25% is possible.
Physical Review Letters | 2000
Mathias Wagner
A recently demonstrated quantum electron pump is discussed within the framework of photon-assisted tunneling. Because of lack of time-reversal symmetry, different results are obtained for the pump current depending on whether or not final-state Pauli blocking factors are used when describing the tunneling process. While in both cases the current depends quadratically on the driving amplitude for moderate pumping, a marked difference is predicted for the temperature dependence. With blocking factors the pump current decreases roughly linearly with temperature until k(B)T approximately Plancks over 2piomega is reached, whereas without them it is unaffected by temperature, indicating that the entire Fermi sea participates in the transport.
IEEE Transactions on Electron Devices | 2001
Hiroshi Mizuta; Mathias Wagner; Kazuo Nakazato
This paper presents a numerical analysis of the role of tunnel barriers in explaining the experimental I-V characteristics of a new vertical tunnel transistor called phase-state low electron-number drive transistor (PLEDTR), used for constructing a high-speed and high-capacity gain cell. Introducing the characteristic features of tunneling current through ultrathin barriers into a standard two-dimensional (2-D) drift-diffusion (DD) device simulator by way of calibrating it with a self-consistent one-dimensional (1-D) Poisson/Schrodinger equation solver, it is shown that the transistor characteristics at the ON-state are substantially affected by the thickness of the source barrier. Current saturation observed at low source-drain voltages is found to result from tunnelling injection via the source barrier. Asymmetric source and drain barrier (SDBs) structures are found to be responsible for the large asymmetry of the I-V characteristics at large source-drain voltages found experimentally. It is also shown that the central shutter barriers (CSBs) reduce the overall drain current in the subthreshold regime, leading to superior OFF current characteristics.
Journal of Physics: Condensed Matter | 1992
Hiroshi Mizuta; C. J. Goodings; Mathias Wagner; Shirun Ho
Lateral-mode non-conserving resonant tunnelling through a zero-dimensional AlGaAs/InGaAs/AlGaAs resonant tunnelling diode (0D RTD) has been analysed by numerically solving the three-dimensional Schrodinger equation with scattering boundary conditions. The authors report on the effect of lateral-mode mixing, caused by elastic scattering due to an hour-glass-shaped confinement potential, on the multi-mode transmission properties and current-voltage characteristics of the device. The calculated S-matrix clearly shows both new transmission peaks in the off-diagonal components which measure the lateral-mode-non-conserving resonant tunnelling and the related Fano-resonance-type structures in the diagonal components which represent an interference between the second-order resonant tunnelling process and the conventional tunnelling process for the off-resonant condition. The difference between the energy separations of the lateral modes in the cathode region and in the quantum box leads to observable fine current peaks in the current-voltage characteristics due to the lateral-mode-conserving tunnelling.
Physica E-low-dimensional Systems & Nanostructures | 2001
Bruce William Alphenaar; K Tsukagoshi; Mathias Wagner
Abstract Magnetoresistance measurements of multi-walled carbon nanotubes having one or two ferromagnetic contacts are compared. Nanotubes with two cobalt contacts show hysteretic switching in the magnetoresistance, with a maximum change of 9% at 4.2 K , while nanotubes with one cobalt contact and one platinum/gold contact show no magnetoresistance switching. This provides further evidence for coherent transport of electron spin through the multi-walled nanotube. In addition, the magnetoresistance ratio in the ferromagnetic nanotube device is calculated for a two-band ballistic model, and gives a maximum value of 25%.
Applied Physics Letters | 1992
J. Allam; Mathias Wagner
We investigate laterally asymmetric quantum dots (LAQDs) as second‐order nonlinear optical elements in which symmetry‐forbidden intersubband transitions become allowed due to lateral asymmetry. The susceptibility for second‐harmonic generation (χ2ω(2)) of a two‐dimensional LAQD with three equispaced energy levels was studied by calculating the product of the intersubband transition dipole moments. The product was as large as 0.95×10−3 Lx3 for a dot of length Lx confined by an infinite‐potential barrier. The product was increased to 3.0×10−3 Lx3 by varying the aspect ratio (Ly/Lx) of the LAQD and decreasing the barrier height. The lateral asymmetry can be controlled by a gate electrode in semiconductor devices, leading to a device with tunable wavelength and nonlinear coefficients, suitable for quasi‐phase matching in nonlinear optical waveguides.
Journal of Applied Physics | 1999
M. B. A. Jalil; Mathias Wagner; H. Ahmed
A computationally inexpensive approximation is obtained for the retention time of charges stored on a memory node of a multiple-tunnel junction (MTJ) memory device, based on previous simplifying assumptions by Jensen and Martinis. The approximation takes into account both thermally assisted single electron tunneling and higher order processes, or cotunneling and is in good agreement with a full master equation simulation of the device up to a temperature T≈T0/10, where T0=e2/kBC. For the case of a memory device formed within a δ-doped layer in GaAs, it is predicted that leakage due to single tunneling starts to dominate over cotunneling at temperatures above T≈T0/60, and that a sharp reduction in retention time occurs above T≈T0/100. Our analysis also shows that with the typical dimensions of present devices, a memory lifetime of a year requires the stringent condition of an 11-junction MTJ operated at below 1 K.
Journal of Applied Physics | 1996
Philippe Jansen; Hiroshi Mizuta; Ken Yamaguchi; Mathias Wagner
The source resistance of a heterojunction field‐effect transistor (HFET), whose reduction is mandatory for high‐performance devices, consists of an ohmic contact resistance and an access resistance. The access region is located between the geometrical source and the geometrical source side of the gate contact. By means of a quantum‐mechanical modeling program, the effect of changes in layer structure in the access region of a HFET is studied. A new heterojunction structure using a Si planar doped layer is designed to improve the linearity and reduce the access resistance by more than ten times for a specific transistor layout. Thanks to the higher sensitivity of the modeling program to structural information, the contribution of the tunneling current and the change of equilibrium as a function of temperature is investigated.
Journal of Applied Physics | 2000
M. B. A. Jalil; Mathias Wagner
We employ a master equation (ME) approach in the charge transport analysis across a uniform multiple–tunnel junction (MTJ) memory trap, using a much-reduced state list derived from circuit symmetry, and previous assumptions by Jensen and Martinis. This enables all significant single tunneling and higher-order cotunneling sequences to be accounted for, while avoiding the computational cost of the full ME method. The reduced ME method is conceptually simpler and yields greater accuracy, compared with previous approximations based on tunneling probabilities. For an MTJ trap with zero stray capacitance C0, the results obtained are found to agree very closely with the full ME results up to a temperature of T≈3T0/10, where T0=e2/kBC, whereas previous methods break down at T≈T0/10. Furthermore, unlike the earlier methods, the reduced ME approach can be applied to the realistic but less symmetric case of a trap with finite C0, and remains valid up to the trap’s maximum operating temperature of T≈T0/100. Finally, ...