Current perpendicular to plane Giant Magnetoresistance (GMR) in laminated nanostructures

We theoretically studied spin dependent electron transport perpendicular-to-plain (CPP) in magnetic laminated multilayered structures by using Kubo formalism. We took into account not only bulk scattering, but the interface resistance due to both specular and diffuse reflection and also spin conserving and spin-flip processes. It was shown that spin-flip scattering at interfaces substantially reduces the value of GMR. This can explain the experimental observations that the CPP GMR ratio for laminated structures only slightly increases as compared to non-laminated ones despite lamination induces a significant increase in CPP resistance.Comment: 4 pages, 2 figure

Hydrogenic Spin Quantum Computing in Silicon: A Digital Approach

We suggest an architecture for quantum computing with spin-pair encoded qubits in silicon. Electron-nuclear spin-pairs are controlled by a dc magnetic field and electrode-switched on and off hyperfine interaction. This digital processing is insensitive to tuning errors and easy to model. Electron shuttling between donors enables multi-qubit logic. These hydrogenic spin qubits are transferable to nuclear spin-pairs, which have long coherence times, and electron spin-pairs, which are ideally suited for measurement and initialization. The architecture is scalable to highly parallel operation.Comment: 4 pages, 5 figures; refereed and published version with improved introductio

Zero-bias Anomaly of Tunneling into the Edge of a 2D Electron System

We investigate the electron tunneling into the edge of a clean weakly interacting two-dimensional electron gas. It is shown that the corresponding differential conductance $G(V)$ has a cusp at zero bias, and is characterized by a universal slope $|dG/dV|$ at $V=0$. This singularity originates from the electron scattering on the Friedel oscillation caused by the boundary of the system.Comment: 10 pages, uuencoded compressed Postscript file, to appear in Phys. Rev. B (Rapid Communications

Quantum Hall line junction with impurities as a multi-slit Luttinger liquid interferometer

We report on quantum interference between a pair of counterpropagating quantum Hall edge states that are separated by a high quality tunnel barrier. Observed Aharonov-Bohm oscillations are analyzed in terms of resonant tunneling between coupled Luttinger liquids that creates bound electronic states between pairs of tunnel centers that act like interference slits. We place a lower bound in the range of 20-40 $\mu$m for the phase coherence length and directly confirm the extended phase coherence of quantum Hall edge states.Comment: 4 pages, 3 figures, 1 tabl

Spin Relaxation in a Quantum Dot due to Nyquist Noise

We calculate electron and nuclear spin relaxation rates in a quantum dot due to the combined action of Nyquist noise and electron-nuclei hyperfine or spin-orbit interactions. The relaxation rate is linear in the resistance of the gate circuit and, in the case of spin-orbit interaction, it depends essentially on the orientations of both the static magnetic field and the fluctuating electric field, as well as on the ratio between Rashba and Dresselhaus interaction constants. We provide numerical estimates of the relaxation rate for typical system parameters, compare our results with other, previously discussed mechanisms, and show that the Nyquist mechanism can have an appreciable effect for experimentally relevant systems.Comment: v2: New discussion of arbitrary gate setups (1 new figure), more Comments on experiments; 6 pages, 4 figure

Reply to the Comment on "Enhancement of the Tunneling Density of States in Tomonaga-Luttinger Liquids"

In their comment Fabrizio and Gogolin dispute our result of the enhancement of the tunneling density of states in a Tomonaga-Luttinger liquid at the location of a backward scattering defect [Phys. Rev. Lett. 76, 4230(1996); cond-mat/9601020]. They state that the anticommutativity of the fermion operators of the left and right moving electrons was not considered properly in the Letter. We show in the Reply that the result of the Letter can be reproduced following the Comment when its calculations are performed correctly. This clearly indicates that the question about the anticommutation relations was raised by Fabrizio and Gogolin without serious grounds.Comment: Published in PRL as a Reply to the Comment by Fabrizio and Gogolin (cond-mat/9702080

Transport through quasi-ballistic quantum wires: the role of contacts

We model one-dimensional transport through each open channel of a quantum wire by a Luttinger liquid with three different interaction parameters for the leads, the contact regions and the wire, and with two barriers at the contacts. We show that this model explains several features of recent experiments, such as the flat conductance plateaux observed even at finite temperatures and for different lengths, and universal conductance corrections in different channels. We discuss the possibility of seeing resonance-like structures of a fully open channel at very low temperatures.Comment: revtex, 5 pages, 1 eps figure; clarifications added in light of new experiment

Topological Insulators with Inversion Symmetry

Topological insulators are materials with a bulk excitation gap generated by the spin orbit interaction, and which are different from conventional insulators. This distinction is characterized by Z_2 topological invariants, which characterize the groundstate. In two dimensions there is a single Z_2 invariant which distinguishes the ordinary insulator from the quantum spin Hall phase. In three dimensions there are four Z_2 invariants, which distinguish the ordinary insulator from "weak" and "strong" topological insulators. These phases are characterized by the presence of gapless surface (or edge) states. In the 2D quantum spin Hall phase and the 3D strong topological insulator these states are robust and are insensitive to weak disorder and interactions. In this paper we show that the presence of inversion symmetry greatly simplifies the problem of evaluating the Z_2 invariants. We show that the invariants can be determined from the knowledge of the parity of the occupied Bloch wavefunctions at the time reversal invariant points in the Brillouin zone. Using this approach, we predict a number of specific materials are strong topological insulators, including the semiconducting alloy Bi_{1-x} Sb_x as well as \alpha-Sn and HgTe under uniaxial strain. This paper also includes an expanded discussion of our formulation of the topological insulators in both two and three dimensions, as well as implications for experiments.Comment: 16 pages, 7 figures; published versio

Variations micrométriques des tiges de jeunes plants de Acacia senegal (L.) Wildd dans différents régimes hydriques

L’influence des régimes d’alimentation hydrique sur les variations du diamètre des tiges et la réponse de jeunes plants de Acacia senegal (L.) Wildd après une réhydratation a été appréciée par la méthode des capteurs. 1-En condition d’alimentation hydrique optimale, Acacia senegal (L.) Wildd subit des variations nycthémérales: déshydratation au cours de la journée et double récupération hydrique pendant la nuit et en début de matinée. 2-L’état hydrique du sol détermine les variations d’amplitude des diamètres des jeunes plants qui sont d’autant plus importantes que la contrainte est sévère. 3-Les micro-variations des diamètres déterminent la croissance des jeunes plants qui est d’autant plus importante que le traitement est humide. 4-La réhydratation dans les traitements hydriques modéré et sévère se traduit par une reprise de la croissance avec des récupérations nocturnes en condition modérée.Mots clés: Acacia senegal, capteurs, traitements hydriques, variations de diamètre des tige