Novel spin FETs J. Carlos Egues Departamento de Física e Informática, Instituto de Física de São Carlos, Universidade de São Paulo The spin field-effect transistor (spin FET) first proposed by Datta and Das  more than a decade ago, highlights the possibility of electric control over an intrinsic magnetic degree of freedom: the electron spin. The spin FET is arguably the most popular proposed spintronic device The basic idea of this device (see figure below) is to rotate the spin of the injected carriers as they travel down a low-dimensional channel, connecting two ferromagnetic reservoirs (source and drain). Spin rotation is effected via the so-called Rashba spin-orbit interaction present at the interface where the 2DEG is formed. This particular type of s-o interaction only arises in heterojunctions with structural inversion asymmetry. The Rashba s-o interaction differs fundamentally from its bulk counterpart, the Dresselhaus s-o interaction, in that it can be externally tuned via proper gate electrodes. Roughly speaking, the Rashba s-o coupling α is proportional to “average” electric field normal to the interface. polarizer z analyzer y x (Phys. Today ‘95) Gate control of the s-o coupling constant is a crucial ingredient to the operation of a spin FET. An external gate essentially changes the slope of the confining potential thus altering α . Datta and Das have shown that upon crossing the length L of the channel the electrons undergo a rotation by the Rashba angle θ R = 2mαL / η2 . Ideally, by adjusting the gate voltage the rotation angle can be varied continuously thus modulating the electric current between the source and drain: when the electron spins are rotated so as to be anti-aligned to the ferromagnetic drain lead the current is zero. Other operating modes are possible with source and drain anti-aligned. The original proposal of Datta and Das assumes a ballistic channel connecting the source and drain in a FET geometry. Spin-flip and momentum scattering are detrimental to the operation of a spin transistor since these randomize the spin of the carriers thus destroying the current spin-polarization, vital for the operation of the device. Spin flip due to, say, stray magnetic impurities can in principle be eliminated/minimized. However, the s-o interaction acts as a k-dependent effective magnetic field which the electrons precess about and momentum scattering changes this local direction of precession thus randomizing the electron spin (Dyakonov and Perel mechanism). In my talk I will present two novel spin-FET proposals, namely: (i) a quasi-onedimensional ballistic setup with s-o induced interband coupling which provides enhanced spin control  and (ii) a robust two-dimensional non-ballistic spin transistor  which benefits from a unique interplay of two distinct s-o interactions, Rashba and Dresselhaus. The enhanced capability of our setup (i) relies on the coherent transfer of electrons between two s-o coupled subbands. Spin-polarized electrons can be further spin rotated via additional side gates controlling the width of the channel and hence the s-o interband coupling strength. This extra rotation θ d can, in principle, be tuned independently of that described by θ R . The non-ballistic spin FET proposal (ii) conveniently relaxes the very stringent constraint of ballistic channels in the original proposal of Datta and Das. Due to a ‘partial’ cancellation of the Rashba and Dresselhaus s-o interactions for a particular value of the gatecontrollable α coupling, we can fix the direction of the effective magnetic field the electrons precess around, thus eliminating the Dyakonov-Perel and Eliott-Yafet types of spin dephasing processes. In other words, for tuned Rashba and Dresselhaus couplings the electron spinor is k-independent thus making momentum scattering ineffective in relaxing the electron spin. Finally, I should also briefly discuss transport properties in electron beamsplitters and novel spin-FET geometries with top and side gates . In particular, I will describe the shot noise characteristics of these devices as a means of detecting the electron spin polarization and entanglement of electron pairs.  S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990).  J. Nitta T. Akazaki, H. Takayanagi, and T. Enoki, Phys. Rev. Lett. 78, 1335 (1997); G. Engels, J. Lange, Th. Schäpers, and H. Lüth, Phys. Rev. B 55, R1958 (1997); The highest measured value of α reported to date was by Y. sato, T. Kita, S. Gozu, and S. Yamada, J. Appl. Phys. 89, 8017 (2001).  J. C. Egues, G. Burkard, and D. Loss, Appl. Phys. Lett. 82, 2658 (2003).  J. Schliemann, J. C. Egues, and D. Loss, Phys. Rev. Lett. 90, 146801 (2003).  S. Yamada, private communication.