Articles you may be interested inTwo-dimensional electron gases in strained quantum wells for AlN/GaN/AlN double heterostructure field-effect transistors on AlN High mobility two-dimensional hole system in Ga As ∕ Al Ga As quantum wells grown on (100) GaAs substrates Appl.Extremely high electron mobility of pseudomorphic In 0.74 Ga 0.26 As ∕ In 0.46 Al 0.54 As modulation-doped quantum wells grown on ( 411 ) A InP substrates by molecular-beam epitaxy Appl. Phys. Lett. 85, 4043 (2004); 10.1063/1.1807023Transport and quantum electron mobility in the modulation Si δ-doped pseudomorphic GaAs/In 0.2 Ga 0.8 As/Al 0.2 Ga 0.8 As quantum well grown by metalorganic vapor phase epitaxyThe authors present the fabrication details of completely undoped electron-hole bilayer devices in a GaAs/ AlGaAs double quantum well heterostructure with a 30 nm barrier. These devices have independently tunable densities of the two-dimensional electron gas and two-dimensional hole gas. The authors report four-terminal transport measurements of the independently contacted electron and hole layers with balanced densities from 1.2ϫ 10 11 cm −2 down to 4 ϫ 10 10 cm −2 at T = 300 mK. The mobilities can exceed 1 ϫ 10 6 cm 2 V −1 s −1 for electrons and 4 ϫ 10 5 cm 2 V −1 s −1 for holes.

Controlling the flow of electrons by manipulation of their spin is a key to the development of spin-based electronics. While recent demonstrations of electrical-gate control in spin-transistor configurations show great promise, operation at room temperature remains elusive. Further progress requires a deeper understanding of the propagation of spin polarization, particularly in the high mobility semiconductors used for devices. Here we report the application of Doppler velocimetry to resolve the motion of spin-polarized electrons in GaAs quantum wells driven by a drifting Fermi sea. We find that the spin mobility tracks the high electron mobility precisely as a function of T.However, we also observe that the coherent precession of spins driven by spin-orbit interaction, which is essential for the operation of a broad class of spin logic devices, breaks down at temperatures above 150 K for reasons that are not understood theoretically.The transistor, the iconic invention of 20 th century science, is a semiconductor device in which the flow of electrons is modulated by voltages applied via electrodes known appropriately as gates. In a conventional transistor the gate electrode controls the number of mobile electrons in the current carrying pathway, or "channel." In pursuit of transistors with faster response and lower rates of energy dissipation, there has been intense investigation aimed at modulating current through manipulation of spin by applied electric fields [1,2], a coupling that occurs because of the spin-orbit (SO) interaction. Recently, gate-controlled modulation of current via SO coupling has been demonstrated in prototype device structures that operate below room temperature [3,4].Further progress towards spintronic logic requires a deeper understanding of the basic physical principles upon which such devices are based. Essentially the question is this: how far, and how fast, can spin polarization propagate in a current-carrying electron gas? This question was first addressed in pioneering work that used magneto-optic imaging to follow the drift of spin polarization packets in real space [5]. These experiments were enabled by the enhanced spin lifetimes (in excess of 10 ns) that arise near the metal-insulator transition of a doped semiconductor at the expense of electron mobility, . However, the high electron gas needed for fast devices is in a very different dynamical regime, where spin lifetimes are ~ 10-100 ps, during which time spin may propagate only 10-100 nm (depending on the temperature, T, and

We use phase-resolved transient grating spectroscopy to measure the drift and diffusion of electronhole density waves in a semiconductor quantum well. The unique aspects of this optical probe allow us to determine the frictional force between a two-dimensional Fermi liquid of electrons and a dilute gas of holes. Knowledge of electron-hole friction enables prediction of ambipolar dynamics in highmobility electron systems. The motion of electrons and holes is crucial to the operation of virtually all semiconductor devices and is a central topic of the classic semiconductor texts [1,2]. In particular the coupled motion of electronhole (e-h) packets in applied electric fields, known as ambipolar transport, is discussed in depth. However, it has been known for some time, although not perhaps widely appreciated, that the motion of e-h packets in the high-mobility electron gases found in semiconductor quantum wells and heterojunctions violates the predictions of the standard theory. Insufficient understanding of ambipolar dynamics poses a problem for the development of a spin-based electronics, as many prospective devices are based on spin currents carried by spin polarized e-h packets subjected to electric fields [3][4][5].In the standard textbook description of ambipolar transport in a doped semiconductor, electrons and holes interact only through the long-range Coulomb interaction. Momentum relaxation occurs by scattering on impurities and phonons and there is no exchange of momentum between electrons and holes. On the basis of these assumptions it is predicted that in an n-type semiconductor, for example, an eh packet drifts in direction of the force on the holes, opposite to the motion of the Fermi sea of electrons. However, by photoluminescence imaging, Höpfel et al. discovered that in GaAs quantum wells a drifting e-h packet moves in the direction of the majority, rather than minority carrier, an effect they termed "negative ambipolar mobility" [6]. They recognized that this effect originates from the scattering between electrons and holes, neglected in the standard versions of ambipolar transport. * To whom all correspondence should be addressed. Email: jworenstein@lbl.gov The scattering that dominates ambipolar transport in a single quantum well is precisely analogous to the Coulomb drag effect that has been studied intensively in systems in which layers of electron gases are in close proximity [7][8][9]. In such systems, the strength of the Coulomb interaction between layers can be determined with precision via the transresistance, which is the ratio of the voltage induced in one layer to a current in the other . The transresistance is a direct measure of the rate of momentum exchange (or frictional force) between the two coupled electronic systems. Unfortunately, this technique cannot be used to probe the much stronger frictional force between electrons and holes in the same layer, which plays a crucial role in ambipolar dynamics.In the experiments reported here we perform the first complete characteriza...

We use phase-resolved transient grating spectroscopy to measure the propagation of spin helices in a high mobility n-GaAs/AlGaAs quantum well with an applied in-plane electric field. At relatively low fields helical modes crossover from overdamped excitations where the spin-precession period exceeds the spin lifetime, to a regime of coherent propagation where several spin-precession periods can be observed. We demonstrate that the envelope of a spin polarization packet reaches a current-driven velocity of 10(7) cm s(-1) in an applied field of 70 V cm(-1).

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