Gateable Suppression of Spin Relaxation in Semiconductors

Abstract

The decay of spin memory in a 2D electron gas is found to be suppressed close to the metal-insulator transition. By dynamically probing the device using ultrafast spectroscopy, relaxation of optically excited electron spin is directly measured as a function of the carrier density. Motional narrowing favors spin preservation in the maximally scattered but nonlocalized electronic states. This implies that the spinrelaxation rate can be both tuned in situ and specifically engineered in appropriate device geometries. DOI: 10.1103/PhysRevLett.86.2150 Emerging technologies for generating and manipulating electronic spins are heralded as the precursor to a new generation of spin-functional devices [1]. One of the key requirements is to find a way to control electron spin in semiconductors which can be independently gated. Much work has been devoted to generating spins in semiconductors either optically [2], by injection through magnetic semiconductors [3][4][5], or within ferromagnetic semiconductors [6], while the transport of spin has also been shown to be feasible [7,8]. However, the relaxation of carrier spin is ascribed to several competing mechanisms and particularly at low temperatures has been found to vary widely between different samples [9][10][11][12][13]. One reason for this is that spin relaxation can be particularly sample dependent since it is strongly influenced by impurities and defects. It is therefore desirable that the same sample be used within a study. However, circumstances may prevent this, as in investigations of the dependence of spin relaxation on background doping level, where different wafers must be used [14]. In addition, such structures have no means for tuning the spin relaxation in a prescribed way.In this Letter, we show an advantageous method which avoids the use of multiple samples to vary the number of carriers in a semiconductor by applying a reverse bias to a semitransparent Schottky contact situated above a charge transport layer. The effect of the gate is to transform the quasi-2D electron gas (2DEG) from a high-density metallic state to a low-density insulating state. At low temperatures it is possible to use this technique to reduce the conductivity by several orders of magnitude. This large change in conductivity is, however, accompanied by a relatively small change in electron density, typically less than an order of magnitude. It has been realized that the remote ionized donors that provide the electrons for the 2DEG play an important role in this drop in conductivity [15][16][17]. Spatial variations in the density of these donors result in random fluctuations in the potential in the plane of the 2DEG. In the conducting regime, the 2DEG acts to screen these fluctuations. However, as the density is reduced the screening becomes less effective and the fluctuations grow. This manifests itself as electron localization and corresponds to a large decrease in conductivity. In this Letter, the spinrelaxation dynamics of photoexcited carriers are investigated as we pass through th...

Spintronics: Fundamentals and applications

“…Similarly to bulk GaAs, spin relaxation in GaAs quantum wells was found to be reduced at carrier concentrations close to the metal-to-insulator transition (nϷ5 ϫ10 10 cm Ϫ2 ; Sandhu et al, 2001).…”

“…Spin relaxation was attributed to Bychkov-Rashba spin splitting in these asymmetric wells, estimating the corresponding ប␣ BR in Eq. (88) to be around 1ϫ10 Ϫ14 eV m (Wilamowski and 84 Here is a list of selected references with useful data on s in GaAs/AlGaAs quantum wells: confinement energy dependence has been studied by Tackeuchi et al (1996); Britton et al (1998) ;Ohno, Terauchi, et al (1999; Endo et al (2000); Malinowski et al (2000); temperature dependence is treated by Wagner et al (1993); Ohno, Terauchi, et al (1999; Malinowski et al (2000); Adachi et al (2001); carrier concentration dependence is studied by Sandhu et al (2001); dependence on mobility is examined by Ohno, Terauchi, et al (1999); and dependence on magnetic field is studied by Zhitomirskii et al (1993). .…”

Dependence of spin dephasing on initial spin polarization in a high-mobility two-dimensional electron system

“…The spin dephasing close to the metalinsulator transition was further studied by Sandhu et al. 9 The dopants act as centers of momentum scattering that enhance spin lifetime due to motional narrowing. On one hand, this is helpful for manipulation of optically-excited spins.…”

Pauli blockade of the electron spin flip in bulk GaAs

“…19 and 6); and (ii) localization centers that are critical in the determination of the spin-relaxation mechanism. 6,11,12 Another fundamental issue that has not been explored until very recently, is the physics of spin-dependent electron many-body processes [20][21][22] and phase-space filling effects. 23,24 Due to the difficulties in the theoretical modeling and in the analysis of the experimental results, the spin relaxation mechanisms in the regime where these effects are important are not well known.…”