Doppler velocimetry of spin propagation in a two-dimensional electron gas

Abstract

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

Hanle spin precession in a two-dimensional electron system

“…Thus, the spin-diffusion constant D s in our device is much smaller than the charge diffusion constant D e . This is in line with previous results [9,12,38,40], where this effect was ascribed to the importance of electron-electron interactions in 2D systems [39].…”

“…It is worth noting that τ s can also be estimated from the NLSV signal using λ s = √ D s τ s , but usually one does not know the value of the spindiffusion constant D s , which in general is different from the charge diffusion constant D e , extracted from magnetotransport experiments [38]. This is particularly true for high mobility 2D systems, where electron-electron interaction has to be taken into account [39,40]. Thus Hanle measurements are of utmost importance to get information on spin relaxation.…”

Microscopic theory for Doppler velocimetry of spin propagation in semiconductor quantum wells

“…The cubic Dresselhaus term is speculated to be the main cause for the breaking down. 15 However, this is highly unlikely because q 0 at low temperature only differs by a few percents from the one at high temperature for the narrow QW used in the experiments. It should be pointed out that, since the behaviors of phase in the early stage are the same with or without the CSP, one should be cautious in determining the existence of such CSP from the experimental data in limited time regime, especially when the crossover time t c is large.…”

Doppler velocimetry of spin and charge currents in the 2D Fermi gas

“…Our optical transport measurements show that in a current carrying electron gas, spins and electronhole pairs are not directly affected by an applied electric field, but rather are pushed along by the force exerted on them by the drifting Fermi sea. Spin diffusion is reduced relative to charge diffusion by electron-electron interactions [3], while the spin mobility is precisely equal to the electron mobility, leading to an anomalous rigidity of spin packets [4] (See Fig. 1).…”