We propose a spin field-effect transistor based on spin-orbit (s-o) coupling of both the Rashba and the Dresselhaus types. Differently from earlier proposals, spin transport through our device is tolerant against spin-independent scattering processes. Hence the requirement of strictly ballistic transport can be relaxed. This follows from a unique interplay between the Dresselhaus and the (gate-controlled) Rashba interactions; these can be tuned to have equal strengths thus yielding kindependent eigenspinors even in two dimensions. We discuss implementations with two-dimensional devices and quantum wires. In the latter, our setup presents strictly parabolic dispersions which avoids complications arising from anticrossings of different bands.In the recent years research in semiconductor physics has been focused on the emerging field of spintronics. This key word refers to the variety of efforts to use the electron spin rather than, or in combination with, its charge for information processing; or, even more ambitiously, quantum information processing [2]. Among the most prominent device proposals is the spin field-effect transistor (FET) due to Datta and Das [3]. This proposal uses the Rashba spin-orbit coupling to perform controlled rotations of spins of electrons passing through an FETtyped device. This particular spin-orbit interaction is due to the inversion-asymmetry of the confining potential and is of the form [4]where p is the momentum of the electron confined in a two-dimensional geometry, and σ the vector of Pauli matrices. The coefficient α is tunable in strength by the external gate of the FET. Due to the dependence on the momentum, the Rashba spin-orbit coupling can be viewed as a wave vector-dependent Zeeman field which can change drastically if the electron is scattered into a different momentum state. Therefore, such scattering events readily randomize the electron spin thus limiting the range of operation of the Datta-Das spin-FET to the regime of ballistic transport where such processes do not occur.In the present work we propose a modified version of the spin-FET in which the electrons are not only subject to spin-orbit interaction of the Rashba but also of the Dresselhaus type [5]. The latter is present in semiconductors lacking bulk inversion symmetry. When restricted to a two-dimensional semiconductor nanostruture with appropriate growth geometry this coupling is of the form [6,7] where the coefficient β is determined by the semiconductor material and the geometry of the sample. Below we show that our proposed device is robust against spin-independent scattering and hence can also operate in a non-ballistic (or diffusive) regime. This unique feature follows from the possibility of tuning the Rashba (via proper gating) and the Dresselhaus terms so that they have equal strengths α = β. In this case, we show quite generally below that the electron spinor is k-independent in two dimensions -even in the presence of (spin-independent) scatterers. Tuned Rashba and Dresselhaus terms. Consider the Ham...

We investigate quantum transport through a quantum dot connected to source and drain leads and side coupled to a topological superconducting nanowire (Kitaev chain) sustaining Majorana end modes. Using a recursive Green's-function approach, we determine the local density of states of the system and find that the end Majorana mode of the wire leaks into the dot, thus, emerging as a unique dot level pinned to the Fermi energy ε F of the leads. Surprisingly, this resonance pinning, resembling, in this sense, a "Kondo resonance," occurs even when the gate-controlled dot level ε dot (V g ) is far above or far below ε F . The calculated conductance G of the dot exhibits an unambiguous signature for the Majorana end mode of the wire: In essence, an off-resonance dot [ε dot (V g ) = ε F ], which should have G = 0, shows, instead, a conductance e 2 /2h over a wide range of V g due to this pinned dot mode. Interestingly, this pinning effect only occurs when the dot level is coupled to a Majorana mode; ordinary fermionic modes (e.g., disorder) in the wire simply split and broaden (if a continuum) the dot level. We discuss experimental scenarios to probe Majorana modes in wires via these leaked/pinned dot modes.

We investigate the spin-orbit (SO) interaction in two-dimensional electron gases in quantum wells with two subbands. From the 8 8 Kane model, we derive a new intersubband-induced SO term which resembles the functional form of the Rashba SO but is nonzero even in symmetric structures. This follows from the distinct parity of the confined states (even or odd) which obliterates the need for asymmetric potentials. We self-consistently calculate the new SO coupling strength for realistic wells and find it comparable to the usual Rashba constant. Our new SO term gives rise to a nonzero ballistic spin-Hall conductivity, which changes sign as a function of the Fermi energy (" F ) and can induce an unusual Zitterbewegung with cycloidal trajectories without magnetic fields. The rapidly developing field of spintronics has generated a great deal of interest in spin-orbit (SO) coupling in semiconductor nanostructures [1]. For an n-doped zincblende semiconductor quantum well with only the lowest subband occupied, i.e., in a strictly 2D situation, there are two main contributions to the interaction of the spin and orbital degrees of freedom of electrons. One contribution is the Dresselhaus term, which results from the lack of inversion symmetry of the underlying zinc-blende lattice [2] and is to lowest order linear in the crystal momentum [3]. This linearity is shared by the other contribution known as the Rashba term [4], which is due to structural inversion asymmetry and can be tuned by an electric gate across the well [5]. These two contributions can lead to an interesting interplay in spintronic systems [6].In this Letter we consider yet another type of electronic SO coupling which, as we show, occurs in III-V (or II-VI) zinc-blende semiconductor quantum wells with more than one subband. We derive a new intersubband-induced SO interaction which resembles that of the ordinary Rashba model; however, in contrast to the latter, ours is nonzero even in symmetric structures (Fig. 1). We self-consistently determine the strength of this new SO coupling for realistic single and double wells and find it comparable to the Rashba constant [Figs. 2(a) and 2(b)]. We have investigated the spin-Hall effect and the dynamics of spinpolarized electrons due to this new SO term. We find (i) a nonzero ballistic spin-Hall conductivity which changes sign as a function of " F and (ii) an unusual Zitterbewegung [7] with cycloidal trajectories without magnetic fields (Fig. 3). As derived below, for a symmetric well with two subbands our 4 4 electron Hamiltonian is