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All-Electric Quantum Point Contact Spin-Polarizer P ARTICLES PUBLISHED ONLINE: 6 SEPTEMBER 2009 | DOI: 10.1038/NNANO.2009.240 All-electric quantum point contact spin-polarizer P. Debray 1*,S.M.S.Rahman1,J.Wan2,R.S.Newrock1,M.Cahay2,A.T.Ngo3,S.E.Ulloa3, S. T. Herbert4,M.Muhammad1 and M. Johnson5 The controlled creation, manipulation and detection of spin-polarized currents by purely electrical means remains a central challenge of spintronics. Efforts to meet this challenge by exploiting the coupling of the electron orbital motion to its spin, in particular Rashba spin–orbit coupling, have so far been unsuccessful. Recently, it has been shown theoretically that the confining potential of a small current-carrying wire with high intrinsic spin–orbit coupling leads to the accumulation of opposite spins at opposite edges of the wire, though not to a spin-polarized current. Here, we present experimental evidence that a quantum point contact—a short wire—made from a semiconductor with high intrinsic spin–orbit coupling can generate a completely spin-polarized current when its lateral confinement is made highly asymmetric. By avoiding the use of ferromagnetic contacts or external magnetic fields, such quantum point contacts may make feasible the development of a variety of semiconductor spintronic devices. pintronics is one of the most promising paradigms for the current. Here, we present experimental evidence of net spin polariz- development of novel devices for use in the post-complementary ation in side-gated quantum point contacts (QPCs)—short quantum Smetal–oxide–semiconductor (CMOS) era1.Itaimsatdeveloping wires—when the confining potential of the QPC is made sufficiently electronic devices by simultaneous manipulation of electron spin and asymmetric. We suggest that the net spin polarization is triggered by charge and offers the possibility of devices with high speed and very asymmetric lateral spin–orbit coupling and can reach nearly 100% in low power dissipation. However, before this objective can be achieved, the presence of strong electron–electron (e–e) interaction. We ways must be found for the creation, manipulation and detection of present proof-of-concept numerical results obtained from non-equi- spin-polarized currents by purely electrical means. This is a challenge librium Green’s function (NEGF) analysis14 of a model QPC device. facing semiconductor spintronics. The numerical results explain our experimental observations in a Because spin–orbit coupling couples the electron orbital motion coherent manner. Finally, we point out the potential of a side- to its spin, spin–orbit coupling is a possible tool for all-electrical gated quantum point contact as an all-electric generator, detector spin control and generation of spin-polarized current without ferro- and filter of spin-polarized current. magnetic contacts and applied magnetic fields. Considerable research has been directed towards using Rashba spin–orbit coup- Experimental findings ling2,3 to achieve these objectives4,5. Despite this, there has been The QPCs used in this work were made from nominally symmetric no reported success in experimentally achieving spin injection or InAs quantum well structures. Figure 1a shows a scanning electron control by Rashba spin–orbit coupling. micrograph (SEM) of a fabricated QPC, representative of the devices In this work, we present the first experimental study of spin– used in this work. Figure 1b shows a plot of the measured conduc- orbit coupling induced by the in-plane electric field due to the tance at 4.2 K of a QPC device as a function of sweeping voltage VG 6 lateral confining potential of a quantum wire and investigate its applied to both side gates for symmetric (VLG ¼ VRG) and asym- = potential for spin injection and detection. Because this spin–orbit metric (VLG 2 VRG 0) lateral potential confinements of the coupling results from the lateral electric field, we call it lateral device. This plot can be taken as representative of typical device be- spin–orbit coupling. It is distinctly different from Rashba spin– haviour of a number of nominally identical QPCs. Figure 1c illus- orbit coupling, which arises from the electric field due to the asym- trates the case when the confining potential of the QPC is highly metry in the confining potential of a quantum well (QW) structure. asymmetric. Measurements made on several QPC devices gave Recently, it has been theoretically demonstrated that lateral spin– fairly similar and reproducible results, differing only in the pinch- orbit coupling can induce accumulation of opposite spin species off voltages due to differences in the lithographic channel widths. 2 on the opposite edges of a quantum wire when a current flows in In addition to a short plateau at conductance G G0 ¼ 2e /h, 7–9 it . Such spin accumulation also results in the well-known spin another short plateau at conductance G 0.5G0 was observed in Hall effect (SHE)10,11, when a charge current in a material with the absence of any applied magnetic field only when the lateral con- large spin–orbit coupling induces a pure transverse spin current. fining potential of the QPC was made significantly asymmetric by SHE has been extensively studied theoretically11 and observed appropriate voltage biasing of the side gates. We designate the experimentally in two-dimensional systems using optical detection point P (Fig. 1c) of maximum curvature at the onset of the 0.5 12,13 techniques . The spin current predicted by SHE has so far plateau as the G0.5 feature. The 0.5 plateau was also observed eluded electrical detection. The origin of SHE is not related to gra- when the asymmetry of the confining potential was reversed by dients of confining potentials. reversing the constant bias voltages of the gates. None of the theoretical works on lateral spin–orbit coupling The hallmark of ballistic electron transport in a one-dimensional predicts a net spin polarization that can generate a spin-polarized electron system is the quantization of its linear conductance. 1Department of Physics, University of Cincinnati, Cincinnati, Ohio 45221, USA, 2Department of Electrical Engineering, University of Cincinnati, Cincinnati, Ohio 45221, USA, 3 Department of Physics and Astronomy, Ohio University, Athens, Ohio 45701, USA, 4Department of Physics, Xavier University, Cincinnati, Ohio 45207, USA, 5Naval Research Laboratory, Washington DC 20375, USA. *e-mail: [email protected] NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology 1 © 2009 Macmillan Publishers Limited. All rights reserved. ARTICLES NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2009.240 I PS 1.5 QPC2 4.2 K B = 0 V RG VRG LG LG 1.0 ) −1 h AS 2 300 nm e S (2 G 0.5 PD 0.0 VDS −3 −201−1 VG (V) 3.0 QPC1 AlGaAs/GaAs QPC AS = 7.5 V 1.0 plateau T = 70 mK B = 0 2.5 1.0 2.0 ) ) −1 −1 h h B = 0 2 2 e e 1.5 (2 P (2 B = 1.5 T G 0.5 G 1.0 0.5 plateau 0.5 0.5 plateau 0.0 0.0 −3 −20−1 −1.5 −1.2 −0.9 −0.6 −0.3 VG (V) VG (V) Figure 1 | Conductance G of quantum point contact (QPC) devices. a, Scanning electron micrograph (SEM) of the x–y plane of a typical InAs QPC used in this study. LG and RG are, respectively, the left and right side gates. The conductance of the device is determined from measurements of the current I and the voltage difference between the probes PD and PS. The arrow indicates the x-axis direction. b, Conductance of QPC2 measured in zero magnetic field B for symmetric (S) (VLG ¼ VRG) and asymmetric (AS) (VLG 2 VRG ¼ 5 V) transverse confining potential. The 0.5 plateau appears only when the confinement is asymmetric. c, Conductance of QPC1 measured at VLG 2 VRG ¼ AS ¼ 7.5 V, showing the ‘0.5 plateau’ in the absence of applied magnetic field B.Pisthe point of maximum curvature. d, Conductance of a GaAs QPC, showing Zeeman spin-splitting of the first quantized conductance plateau in an external magnetic field B. In all cases, VG is a sweeping voltage applied simultaneously to all gates. The conductance is given by G ¼ N(2e2/h), where N is the number quantized conductance plateaus in our device. Lifetime broadening of occupied one-dimensional subbands15. The factor of two on the and temperature smearing at 4.2 K prevent sharp transitions right-hand side is the spin degeneracy of the electrons. When the between plateaus. Because we made no correction for any series spin degeneracy is removed, for example by a strong external mag- resistance between the voltage probes and the QPC channel, in netic field B, the conductance is quantized at integral values of some devices the plateaus occur at somewhat less than the 0.5G0. The Zeeman spin splitting of the first quantized plateau we quantized values. have observed in the ballistic conductance of a GaAs QPC is We believe the 0.5 plateau observed in the conductance of QPCs shown in Fig. 1d. Although it is not direct evidence, the occurrence results from spontaneous spin polarization induced by lateral spin– of the 0.5 plateau is considered to be a clear signature of complete orbit coupling when the lateral confinement of the QPC is made spin polarization16. The 0.5 plateau observed in Fig. 1b,c results sufficiently asymmetric. from spin splitting of the first quantized conductance plateau. The temperature dependence of the 0.5 plateau was studied to Effects such as fluctuations and resonance due to quantum interfer- obtain an idea of the magnitude of the spin splitting responsible ence caused by impurities, channel wall irregularities and reflections for its occurrence. Lowering the temperature from 4.2 K down to at QPC ends do not result in a plateau in the conductance.
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