1,721,493 research outputs found
Current-induced domain wall motion: Domain wall velocity fluctuations
No full textThe spin transfer torque generated by a spin polarized current can generate the domain wall (DW) motion in a magnetic nanowire. We examine fluctuations of a DW velocity induced by uncontrolled edge roughness and defects. In the weak pinning regime (regime I), the fluctuation is small. In the intermediate pinning regime (regime II), the DW velocity fluctuates significantly as the density of pinning centers changes. Moreover the threshold current density for the finite density of pinning centers is found to be noticeably larger than the threshold current density for the escape from a single pinning center. Finally, in the strong pinning regime (regime III), the DW velocity fluctuates as the density of pinning centers changes. But the enhancement of the threshold current density does not occur. Thus the fluctuations of the DW velocity are different in the three pinning regimes. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3125522]open111212sciescopu
Supersymmetry in carbon nanotubes in a transverse magnetic field
No full textElectron properties of carbon nanotubes in a transverse magnetic field are studied using a model of a massless Dirac particle on a cylinder. The problem possesses supersymmetry which protects low-energy states and ensures stability of the metallic behavior in arbitrarily large fields. In metallic tubes we find suppression of the Fermi velocity at half-filling and enhancement of the density of states. In semiconducting tubes the energy gap is suppressed. These features qualitatively persist (although to a smaller degree) in the presence of electron interactions. The possibilities of experimental observation of these effects are discussed.open1142sciescopu
Suppression of Spin Dephasing in a Two-Dimensional Electron Gas with a Quantum Point Contact
No full textSpin-orbit coupling (SOC) is a source of strong spin dephasing in two- and three-dimensional semiconducting systems. We report that spin dephasing in a two-dimensional electron gas can be suppressed by introducing a quantum point contact. Surprisingly, this suppression was not limited to the vicinity of the contact but extended to the entire two-dimensional electron gas. This facilitates the electrical control of the spin degree of freedom in a two-dimensional electron gas through spin-orbit coupling.X11sciescopuskc
A distributed utility max-min flow control algorithm
No full textA fair allocation of utility (application-layer performance) is essential in providing QoS (Quality of Service) guarantee. However, there have been few researches in the literature of utility-fair network resource allocation scheme. In this paper, we propose a distributed utility max-min flow control algorithm which accommodates application diversity in that it does not require the concavity of utility functions, and is scalable in that it does not require any per-flow operation in the network. The algorithm is proved to be convergent under the assumption that there exists a single bottleneck link and the communication delay between any two links in the network is bounded. Although the convergence of the algorithm is analyzed only for the case of a single bottleneck link, we show through simulations that the proposed algorithm works as designed for the case of multiple bottleneck links as well. (c) 2005 Elsevier B.V. All rights reserved.
Interlayer exchange coupling between next nearest neighbor layers
No full textInterlayer exchange coupling (IEC) between next nearest neighbor magnetic layers is investigated. For a multilayer system that contains threemagnetic layers (with magnetization directionsm (m)over-cap(1), (m)(m)over-cap(2), and (m)(m)over-cap(3), respectively) separated by two nonmagnetic layers, the angle dependence of the coupling energy and the thickness dependence of coupling constants were obtained. In addition to the well known nearest neighbor IEC of the form -(J)over-tilde(12)(m)over-cap(1) center dot (m)over-cap(2) and -(J)over-tilde(23)(m)over-cap(2) center dot (m)over-cap(3), we find the next nearest neighbor IEC of the form -(J) over-tilde(123)((m)over-cap(1) center dot (m)over-cap(2))((m)over-cap(2) center dot (m)over-cap(3)), which is different from the Heisenberg type next nearest neighbor coupling -J(13)(m)over-cap(1) center dot (m)over-cap(3). The strength of the next nearest neighbor IEC oscillates with respect to the thickness of both magnetic and nonmagnetic layers. The strength of the next nearest neighbor IEC is generally smaller than the conventional nearest neighbor IEC, but is large enough to allow for experimental detection.open115sciescopu
Transmission zero in a quantum dot with strong electron-electron interaction: Perturbative conductance calculations
No full textA pioneering experiment [E. Schuster, E. Buks, M. Heiblum, D. Mahalu, V. Umansky, and Hadas Shtrikman, Nature 385, 417 (1997)] reported the measurement of the transmission phase of an electron traversing a quantum dot and found the intriguing feature of a sudden phase drop in the conductance valleys. Based on the Friedel sum rule for a spinless effective one-dimensional system, it has been previously argued [H.-W. Lee, Phys. Rev. Lett. 82, 2358 (1999)] that the sudden phase drop should be accompanied by the vanishing of the transmission amplitude, or transmission zero. Here we address roles of strong electron-electron interactions on the electron transport through a two-level quantum dot where one level couples with the leads much more strongly than the other level does [P. G. Silvestrov and Y. Imry, Phys. Rev. Lett. 85, 2565 (2000)]. We perform a perturbative conductance calculation with an explicit account of large charging energy and verify that the resulting conductance exhibits transmission zero, in agreement with the analysis based on the Friedel sum rule.open119sciescopu
EFFECTS OF SYMMETRIES ON SINGLE-CHANNEL SYSTEMS: PERFECT TRANSMISSION AND REFLECTION
No full textWe study the effects of symmetries on resonant transport in single-channel systems with an emphasis on their implications for atomic wires. Employing an expression of the scattering matrix in terms of its poles, it is shown that the mirror reflection and time-reversal symmetries lead generically to transmission and reflection peaks of magnitude I, resulting in perfect transmission and perfect reflection, respectively. In particular, it is demonstrated that even when multiple resonances overlap and resonance shapes are distorted considerably, the perfect transmission (perfect reflection) can still occur provided that a system is free from ''pair annihilation" events of two perfect transmissions (two perfect reflections). It is argued that these results can remain valid even when the employed expression of the scattering matrix becomes inapplicable, because the symmetries create certain topological structures of transmission and reflection amplitudes that stabilize the perfect transmission and reflection. Crossover from resonant transport to ballistic transport is also discussed. Results are discussed in connection with recent works on atomic wires.open1130sciescopu
Correlation-induced resonances and population switching in a quantum-dot coulomb valley
No full textStrong correlation effects on electron transport through a spinless quantum dot are considered. When two single-particle levels in the quantum dot are degenerate, a conserved pseudospin degree of freedom appears for generic tunneling matrix elements between the quantum dot and leads. Local fluctuations of the pseudospin in the quantum dot give rise to a pair of asymmetric conductance peaks near the center of a Coulomb valley. An exact relation to the population switching is provided.open1119sciescopu
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