We present a generalize Landauer-Büttiker transport theory for multi-terminal spin transport in presence of spin-orbit interaction. It is pointed out that the presence of spin-orbit interaction results in equilibrium spin currents, since in presence of spin-orbit interaction spin is not a conserved quantitative. Further we illustrate the theory by applying it to a three terminal Y-shaped conductor. It is shown that when one of the terminal is a potential probe, there exist nonequilibrium pure spin currents without an accompanying charge current. It is shown that this pure spin currents causes a voltage drop which can be measured if the potential probe is ferromagnetic.PACS numbers: 72. 72.25.Dc, 72.25.Mk Producing and measuring spin currents is a major goal of spintronics. The standard way is to inject spin currents from a Ferromagnet into a semiconductor in a two terminal geometry [1]. However this has a drawback, due to conductivity mismatch, the polarization of injected current is rather small and it always has an accompanying charge current [2]. Also for any spintronics operation spin orbit interaction plays an important role, for e.g., in Datta-Das spin-transistor [3].In light of these development it would be interesting and highly desirable if one can produce spin currents intrinsically. One such possibility is provided by intrinsics spin-orbit interaction. Presence of impurity atom or defects gives rise to spin-orbit interaction of the form [4] [5], [6],where σ is a vector of Pauli matrices ,U(r) is potential due to defects or impurity atoms and k is the momentum wave vector of electrons and λ is spin-orbit interaction strength. For strictly two dimensional case for which the potential U(r) depends on x and y coordinates only the Hamiltonian commutes with σ z , hence z component of the spin is good quantum number. As is well know that this kind of spin-orbit interaction has a polarizing effect on particle scattering [7], i.e, when an unpolarized beam is scattered it gets polarized perpendicular to the plane of scattering. Further scattering of this polarized beam causes asymmetry in scattering processes, i.e. electrons with one particular spin direction, e.g., spin-up electrons have a larger probability to be scattered to the right compared to spin-down electrons [6], [7]. This property of spin-orbit scattering gives rise to novel effects like spin hall effect [4].Here in this article we show that the the above discussed property of spin-orbit scattering can be used to generate and measure spin-currents [5]. Consider a three terminal, two dimensional Y shaped conductor shown in Fig.1. The plane of conductor is xy. Since the conductor is two dimensional which fixes the scattering plane, the scattered electrons will be polarized along z axis (perpendicular to the scattering plane). However the polarization for the two branches of Y junction will be opposite [4]. Hence a three terminal structure would create spin currents from an unpolarized current in presence of spin-orbit interaction [6], [8]. Moreover...

We study spin-relaxation phenomena in a two-dimensional electron gas in the presence of Rashba spin-orbit coupling. A tight-binding model including Rashba spin-orbit coupling is used to study spin relaxation and spin diffusion in a two-dimensional electron gas within Landauer-Büttiker formalism. It is shown that the spindiffusion length is not independent of the mean free path as predicted by the motional narrowing effect. Further it is demonstrated that spin relaxation is anisotropic and can show a nonmonotonic dependence on Fermi energy due to nonparabolicity of the band.

Several physical models have recently been proposed to obtain unidirectional motion of an overdamped Brownian particle in a periodic potential system. The asymmetric ratchetlike form of the periodic potential and the presence of correlated nonequilibrium fluctuating forces are considered essential to obtain such a macroscopic motion in homogeneous systems. In the present work, instead, inhomogeneous systems are considered, wherein the friction coefficient and/or temperature could vary in space. We show that unidirectional motion can be obtained even in a symmetric nonratchetlike periodic potential system in the presence of white noise fluctuations. We consider four different cases of system inhomogeneity We argue that all these different models work under the same basic principle of alteration of relative stability of otherwise locally stable states in the presence of temperature inhomogeneity.

We have considered an open system consisting of a metallic ring coupled to two electron reservoirs. We have recently shown that in the presence of a transport current, circulating currents can flow in such a ring even in the absence of magnetic field. This is related to the current magnification effect in the ring. In our present work we have studied the effect of impurity on the current magnification. We find that the presence of impurity can enhance the current magnification in the loop significantly and thus lead to large circulating currents in certain range of Fermi energies. This is in contrast to the known fact that impurities can only decrease the persistent currents in a closed ring in the presence of magnetic flux.

Two phenomenological models have been proposed in mesoscopic systems to study the role of inelastic scattering on the phase coherent motion of electrons. In the first one, due to Büttiker, one adds a voltage probe into the system ͑or in the scattering matrix͒. The second model invokes the complex ͑or optical͒ potential in the system Hamiltonian. Studying a simple geometry of a metallic loop in the presence of Aharonov-Bohm magnetic flux, we show that the two probe conductance is symmetric in the reversal of the magnetic field in Büttiker's approach. Whereas the two probe conductance within the complex potential model is asymmetric in the magnetic flux reversal contrary to the expected behavior. ͓S0163-1829͑98͒00115-5͔