Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session H4: Recent Developments in Semiconductor Heterostructure Spin Physics |
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Sponsoring Units: DCMP Chair: David Awschalom, University of California-Santa Barbara Room: LACC 515A |
Tuesday, March 22, 2005 8:00AM - 8:36AM |
H4.00001: Spin-Orbit Coupling and Spin Polarization in 2D Hole Systems Invited Speaker: The spin degree of freedom of the charge carriers in semiconductors is the subject of great current interest both in fundamental physics and also in applied research due to possible novel applications in the field of spintronics. While electrons in the conduction band have spin $S = 1/2$, the holes in the valence band of semiconductors like GaAs are characterized by an effective spin $S= 3/2$. The dynamics of these hole systems is governed by a strong spin-orbit interaction within the space of $S= 3/2$ states [1] which gives rise to a splitting of heavy and light hole states in 2D systems. A detailed understanding of hole systems is very important in the context of ferromagnetic semiconductors like \mbox{GaMnAs} where the ferromagnetism is mediated by the $S=3/2$ holes in the valence band. We study the spin dynamics of hole systems at magnetic field $B=0$ and $B>0$. It is shown that $S=3/2$ hole systems behave very different from $S=1/2$ electron systems. Due to a competition between spin-orbit coupling and the Zeeman term in an in-plane magnetic field $B$, the spin polarization of 2D hole systems can change its sign at a finite value of $B$ [2]. While $S=1/2$ electron systems are fully spin polarized when the Zeeman energy splitting becomes larger than the Fermi energy of the system, the spin polarization of $S=3/2$ hole systems typically remains much smaller than one in this regime. We discuss possible applications in the field of spintronics. [1] R.~Winkler, \emph{Spin-Orbit Coupling Effects in 2D Electron and Hole Systems} (Springer, Berlin, 2003). [2] R.~Winkler, Phys.~Rev.~B {\bf 70}, 125301 (2004); R.~Winkler, cond-mat/0401067. [Preview Abstract] |
Tuesday, March 22, 2005 8:36AM - 9:12AM |
H4.00002: Spin-polarized reflection of electrons in a two-dimensional electron system Invited Speaker: We present a method to create spin-polarized beams of ballistic electrons by elastic scattering off a lithographic boundary in a mesoscopic geometry, and present experimental data corroborating the method. In the ballistic transport regime, the dominant scattering events involve the device boundaries. Whereas reflection of carriers from device boundaries is specular if spin-orbit interaction can be neglected, in the presence of strong spin-orbit interaction, spin-flip scattering results in different reflection angles for different spin polarizations. Reflection of a spin-unpolarized injected beam from a lithographic barrier gives rise to three reflection angles, hence three beams: two spin-polarized side beams and one unpolarized specular beam. The side beams can be captured through suitably positioned apertures in a mesoscopic geometry. Consequently, if the spin coherence length is longer than the mean free path, spin-orbit interaction together with the device geometry can be exploited for the preparation of spin-polarized carrier states. We present low-temperature experimental results verifying the realization of the method in a high-mobility, MBE-grown InSb/InAlSb heterostructure where spin-orbit interaction is strong. In our geometry, a small perpendicular magnetic field is utilized to sweep the triplet beam structure over an exit aperture, leading to steps in the magnetoresistance data. The data agree with Fermi contours calculated using realistic values of Rashba and Dresselhaus spin-orbit parameters. The multi-beam reflection process can be utilized to create spin-polarized carrier populations, without the use of ferromagnetic contacts (Appl. Phys. Lett., Jan. 05; NSF DMR-0094055, -0080054, -0209371). [Preview Abstract] |
Tuesday, March 22, 2005 9:12AM - 9:48AM |
H4.00003: Spin-Galvanic Effect in Semiconductor Quantum Wells Invited Speaker: The electron spin in a homogeneous spin-polarized two- dimensional electron gas can drive an electric current if some general symmetry requirements are met (for review see [1]). The microscopic origin of the spin- galvanic effect is the inherent asymmetry of spin-flip scattering of electrons in systems with removed $k$-space spin degeneracy of the band structure. The spin-galvanic effect is quite general. It has been observed in various quantum well structures at temperatures varying from 4.2 K to 300 K and at different types of optical excitation in a wide spectral range from visible to the far infrared. Spin-galvanic effect provides new experimental aspect to spin properties of low dimensional semiconductor structures. In particular, the angular dependent measurements of the spin-galvanic current allow the separation of contributions to the band splitting due to Dresselhaus and Rashba terms in the Hamiltonian. Most recently the reversed spin-galvanic effect, i.e. a spin polarization induced by an electric current flow [2], has also been observed [3] demonstrating that a spin polarization can be achieved in non-magnetic semiconductor structures. \begin{enumerate} \item S.D. Ganichev, in series ``Advances in Solid State Physics'', B. Kramer (Ed.) (Springer-Verlag Berlin-Heidelberg), Vol. 43, pp. 427-442 (2003). \item A.G. Aronov and Yu.B. Lyanda-Geller, JETP Lett. 50, 431 (1989). \item S.D. Ganichev et. al, cond-mat/0403641 March 2004. \end{enumerate} [Preview Abstract] |
Tuesday, March 22, 2005 9:48AM - 10:24AM |
H4.00004: Magneto-optical studies of magnetic and non-magnetic narrow-gap semiconductors Invited Speaker: In light of the growing interest in spin-related phenomena and devices, there is now renewed interest in the science and engineering of narrow gap semiconductors. Narrow gap semiconductors (NGS) offer many unique features such as small effective masses, high intrinsic mobilities, large effective g- factors, and large spin-orbit coupling effects. This talk will discuss our recent magneto-optical studies on InSb quantum wells (QWs) and InMnAs ferromagnetic heterostructures. In InSb QWs, we observe spin-resolved cyclotron resonance (CR) caused by the non- parabolicity in conduction band and electron spin resonance in symmetric and asymmetric confinement potentials. The asymmetric wells exhibit a strong deviation in behavior from the symmetric wells at low magnetic fields with far more spin splitting than expected from the bulk g-factor of InSb. In InMnAs/GaSb we observe light and heavy hole CR peaks which demonstrate the existence of delocalized p-like carriers. In addition, In order to increase our understanding of the dynamics of carriers and spins, we performed time resolved measurements such as time- resolved CR spectroscopy on undoped InSb QWs and time-resolved magneto-optical Kerr effect on InMnAs/GaSb. Our results are important for understanding the electronic and magnetic states in NGS. This work was performed in collaboration with M. B. Santos and R. E. Doezema at the Univ. of Oklahoma, J. Wang and J. Kono at Rice Univ., H. Munekata at Tokyo Institute of Technology, C. J. Stanton at the Univ. of Florida, and Y. H. Matsuda and N. Miura at the Univ. of Tokyo. [Preview Abstract] |
Tuesday, March 22, 2005 10:24AM - 11:00AM |
H4.00005: Spin susceptibility of AlAs two-dimensional electron systems Invited Speaker: Bulk AlAs has multiple conduction band minima at the X points of the Brillouin zone, giving rise to ellipsoidal conduction electron Fermi surfaces, similar to those of Si, but with only three full ellipsoids occupied. The AlAs electrons have a larger and anisotropic effective mass ($m_{l}$ = 1.1, $m_{t}$ = 0.21) in comparison to the GaAs electrons ($m$* = 0.067). The effective Lande g-factor of electrons in bulk AlAs ($g$* = 2) is also much larger in magnitude and of a different sign than in GaAs ($g$* = - 0.44). These properties combine to make the AlAs electron system very different from the more commonly studied GaAs system. By confining electrons in modulation-doped AlAs quantum wells we can obtain two-dimensional electron systems (2DESs) with very high low-temperature mobilities. Moreover, by choosing the proper well width, and applying uniaxial stress, we can populate the valleys with their major axis lying either in the 2D plane or out-of-plane. In this talk, we will present results of our measurement of several properties of AlAs 2DESs, with an emphasis on their spin susceptibility. In particular, when the electrons occupy the out-of-plane valley in a very narrow ($<$ 5nm wide) AlAs quantum well, we find that the measured spin susceptibility increases as the density is lowered, quantitatively following the prediction of the quantum Monte Carlo calculations. The measured susceptibility in wider AlAs wells where the electrons occupy the in-plane valleys, however, is puzzling: at a given density, the susceptibility is \textit{larger} when the electrons occupy one valley rather than two valleys. This observation counters the common assumption that a two-valley 2DES is effectively more dilute than a single-valley system because of its smaller Fermi energy. Work performed in collaboration with E.P. DePoortere, O. Gunawan, Y.P. Shkolikov, E. Tutuc, and K. Vakili, and supported by the NSF. [Preview Abstract] |
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