Session Y15: Focus Session: Spins in Semiconductors - Spin Currents IV

Electrically-generated electron spin polarization for non-reciprocal integrated photonic devices

Electron spin polarization based photonic devices offer promising advantages over current technologies. Spin orbit coupling allows for an all-electrical means of control over light in semiconductor waveguides. Electrically generated spin polarization results in a circular dichroism near the absorption edge which results in non-reciprocal Faraday rotation. We investigate the requirements for manipulating light in semiconductor waveguides using electrically-generated spin polarization. Ultimately, device performance will be limited by the magnitude of achievable Faraday rotation, birefringence, and absorption. We show that one can limit birefringence by appropriate waveguide design and that substantial Faraday rotation is accessible sufficiently far below the band edge for material absorption to be minimal.   

Spin Control of Drifting Electrons using Local Nuclear Polarization in Ferromagnet-Semiconductor Heterostructures

We demonstrate a spatially-confined magnetic field gate to modulate the Larmor frequency of an optically-injected spin ensemble drifting down a GaAs channel [1]. The gate is activated either optically or electrically and polarizes GaAs nuclear spins at the interface between a lithographically-defined MnAs island and the channel via the ferromagnetic proximity polarization effect. We measure the rotation angle of the spin ensemble as it emerges from the polarized region using time-resolved Kerr rotation. The ensemble's spin rotation angle can be tuned by up to 5$\pi$ radians as the spins travel over 30 $\mu $m by controlling the nuclear field strength and adjusting the drift velocity. \\[4pt] [1] M.E. Nowakowski, et. al., Phys. Rev. Lett. 105, 137206 (2010)   

Large and Small Signal Analyses of Spin Modulation in Lasers

We have developed a set of rate equations for semiconductor spin-lasers that contain spin-polarized carriers (electrons and holes) in the active region due to spin-polarized (electrical or optical) injection. Previous studies consider the steady-state regime [1-5], showing advantages of the spin-lasers over its conventional counterparts such as threshold reduction and enhanced emission intensity [6]. We suggest a further improvement of spin lasers under dynamical operation. We use both large and small signal analyses to show that the spin-polarized injection can lead to an enhanced bandwidth and desirable switching properties of spin-lasers. Supported by ONR, AFOSR, NSF-ECCS CAREER. \\[4pt] [1] J. Rudolph et al., Appl. Phys. Lett. 82, 4516 (2003). \\[0pt] [2] M. Holub et al., Phys. Rev. Lett. 98, 146603 (2007). \\[0pt] [3] S. Hovel et al., Appl. Phys. Lett. 92, 041118 (2008). \\[0pt] [4] C. Gothgen, R. Oszwaldowski, A. Petrou, I. Zutic, Appl. Phys. Lett. 93, 042513 (2008). \\[0pt] [5] I. Vurgaftman et al., Appl. Phys. Lett. 93, 031102 (2008). \\[0pt] [6] J. Lee, W. Falls, R. Oszwaldowski, and I. Zutic, Appl. Phys. Lett. 97, 041116 (2010).   

Mapping between Quantum-Dot and Quantum-Well Spin-Lasers

It has been demonstrated that performance of semiconductor lasers with a quantum-well (QW) active region can be improved by injecting spin-polarized carriers [1-3]. Their rate-equation models have been developed [4-5], however, description of a quantum-dot (QD) spin-laser, demonstrated recently [6], is more complicated [7]. Here, we present a method which allows to employ the simple QW rate equations to study the QD spin-lasers. With this method, one can easily extract QW-like parameters such as differential gain, gain compression factor and time constants. This effort is worthwhile, because the QW spin-laser rate equations have exact analytical solutions, unlike their QD counterparts [7]. Supported by US ONR, AFOSR, DOE-BES, and NSF-ECCS CARRER. [1] J. Rudolph et al., Appl. Phys. Lett. 82, 4516 (2003). [2] M. Holub et al., Phys. Rev. Lett. 98, 146603 (2007). [3] S. Hovel et al., Appl. Phys. Lett. 92, 041118 (2008). [4] C. Gothgen, R. Oszwaldowski, A. Petrou, I. Zutic, Appl. Phys. Lett. 93, 042513 (2008). [5] I. Vurgaftman et al., Appl. Phys. Lett. 93, 031102 (2008). [6] D. Basu et al., Appl. Phys. Lett. 92, 09119 (2008). [7] R. Oszwaldowski, C. Gothgen, and I. Zutic, Phys. Rev. B 82, 085316 (2010).   

High frequency dynamics and output polarization of a spin laser

The dynamic characteristics of a spin laser have been studied theoretically and experimentally. Calculations with the coupled carrier and photon rate equations show that the small signal modulation bandwidth of the preferred polarization mode is enhanced due to spin injection. The large signal modulation characteristics show temporally separated relaxation oscillations corresponding to the two polarization modes. More importantly, it is shown that an output polarization of 100{\%} can be obtained, with appropriate biasing conditions, irrespective of the degree of spin injection. This is experimentally verified in a quantum dot spin-vertical cavity surface emitting laser (spin VCSEL), where an output polarization of $\sim $ 60{\%} is measured with a 5-6{\%} carrier spin polarization in the active region.   

Consequences of spin transport in heterogeneous environments

Understanding behavior of spins in spatially varying environments such as magnetic fields, spin lifetime and gyromagnetic ratio is very important for real spintronic devices [1]. We present here numerical solutions of the spin diffusion equation in such situations. We show that local magnetic fields can be useful as an imaging tool for spin properties such as spin lifetime. It can also complicate the interpretation of experimental results in the case of spin injection from a ferromagnet into a semiconducting channel through a rough interface [1,2]. \\[4pt] [1] S.P.Dash et.al, Electrical creation of spin polarization in silicon at room temperature, Nature 462, 491-494\\[0pt] [2] V.P. Bhallamudi et.al, Spin transport and imaging opportunities in inhomogeneous environments, arXiv:1010.3747v1 [cond-mat.mes-hall]   

Robust Level Coincidences in the Subband Structure of Quasi 2D Systems

Recently, level crossings in the energy bands of crystals have been identified as a key signature for topological phase transitions. In general, three independent parameters must be tuned appropriately to bring two quantum levels into degeneracy. Using realistic models we show that for Bloch electrons in a crystal the parameter space controlling the occurrence of level coincidences has a much richer structure than anticipated previously. In particular, we identify cases where level coincidences depend on only two independent parameters thus making the level coincidences robust, i.e., they cannot be removed by a small perturbation of the Hamiltonian compatible with the crystal symmetry. We consider HgTe/CdTe quantum wells as a specific example. (See arXiv:1011.xxxx)   

Anomalous spin-resolved point-contact transmission of holes due to cubic Rashba spin-orbit coupling

We present experimental and theoretical evidence for the crossing at finite wave vector of the two lowest one-dimensional spin-split subbands in quantum point contacts fabricated from two-dimensional hole gases with strong spin-orbit interaction. We derive the existence of such crossing point from a two-dimensional spin-orbit interaction with a cubic momentum dependence, appropriate for asymmetric quantum wells. This phenomenon provides an explanation for the anomalous sign of the spin polarization filtered by the point contact, as observed in magnetic focusing experiments. Anticrossing in the one-dimensional spin subbands is introduced by a magnetic field parallel to the channel or an asymmetric potential transverse to it. Controlling the magnitude of the spin-splitting affords a novel mechanism for inverting the sign of the spin polarization.   

Energy Spectra and Spin Properties of Electrons in Spin-Orbit Superlattice Quantum Wires

We calculate the energy spectra of electrons in quantum wires with spatially uniform and modulated spin-orbit coupling. The effects of Rashba spin-orbit coupling arising from asymmetric confinement in perpendicular and lateral directions with respect to the plane containing the wire are considered. We investigate the resulting interplay of strong lateral confinement, a periodic one-dimensional superlattice potential, and spin-orbit coupling in two orthogonal directions. The implications for the spin-dependent properties of electrons confined within these quantum wires are discussed. A potential realization of such systems within narrow nanowires at the interface of LaAlO$_{3}$/SrTiO$_{3}$ heterostructures is also described.   

Unexpected Anisotropy of Electron g-factor in GaAs/AlGaAs(110) Quantum Well

Semiconductor spin qubit is a promising candidate for solid state quantum computation. A lot of effort has been devoted to study spin dynamics in semiconductors ever since a revival of research interest in this field in the late 1990s. Spin lifetime longer than 1ns at room temperature has been discovered in GaAs/AlGaAs(110) quantum wells (QW) as a result of the absence of a predominant spin scattering mechansim (DP mechanism), which also leads to a strong anisotropy of electron spin decoherence in such QWs, with the spin lifetime of spins along the growth direction 10 times bigger than that of spins perpendicular to the growth direction. However, not much is known about the (an)isotropy of spin-related processes in the (110) QW plane, despite that it may offer useful information about spin relaxation. Utilizing a time-resolved Kerr rotation (TRKR) system with a rotatable in-plane magnetic field, we studied the spin processes in GaAs/AlGaAs (110) QWs and found an unexpected anisotropy of electron g-factor in such QWs. The g-factor as measured with the magnetic field along the [1-10] axis is some 10\% larger than that along the [001] axis. Such a strong anisotropy is not only unexpected for QWs, but also much bigger than that found in InGaAs/GaAs quantum dots. An explanation for these results is still in demand but it may give some hints to improve our understanding of spin dephasing mechanisms in semiconductors.   

High Resolution Magneto-Optic Measurements in GaAs using a Sagnac Interferometer

The Sagnac Interferometer is a tool which measures the Polar Kerr effect--a direct indicator of magnetism. Using 820 nm light from a superluminescent diode, we probe GaAs structures and measure the Kerr angle with sub-microradian resolution. ~By utilizing diffraction limited optics and a piezoelectric scanner, we also achieve high spatial resolution. Our measurements are performed at cryogenic temperatures and offer a way to measure the Spin Hall Effect in the DC regime along with other forms of magnetic order.   

Temperature-dependent spin- and phase coherence measured via h/e and h/2e quantum oscillations in resistance of mesoscopic ring arrays in an InAs 2DES

We investigate electron spin- and phase coherence in an array of quasi-ballistic InAs quantum well mesoscopic rings through observation of Aharonov-Bohm h/e oscillations (AB) and Altshuler-Aronov-Spivak h/2e oscillations (AAS). The temperature dependence of the AAS oscillations is characterized through a single effective coherence length, $L_{\rm{eff}}$, following the formalism of Dou\c{c}ot and Rammal, from which the phase coherence length, $L_\phi$ and the spin coherence length as limited by spin-orbit interaction, $L_{\rm{SO}}$, are extracted. AB oscillations are also present, and can be separated from AAS by Fourier transformation. We contrast the AAS method of extracting the coherence lengths with analysis of the AB oscillation amplitudes. Previous studies have examined $L_\phi$ from AB signals in single ballistic rings, or by using AAS amplitudes in large networks, or have observed AB and AAS in single rings with spin-orbit interaction. Here the presence of both AB and AAS in an array with spin-orbit interaction allows for study of both $L_\phi$ and $L_{\rm{SO}}$, and enables direct juxtaposition of different quantum coherence phenomena as means for measuring coherence lengths (DOE DE-FG02-08ER46532).   

Weak Antilocalization and Spin-Orbit Coupling in InAlN/AlN/GaN Heterostructures

Spin-orbit coupling is investigated by magnetotransport and weak antilocalization (WAL) measurements in In$_x$Al$_{1-x}$N/AlN/GaN heterostructures in the carrier density ranges extending from 1.22$\times$10$^{13}$ cm$^{-2}$ to 1.41$\times$10$^{13}$ cm$^{-2}$ and from 1.99$\times$10$^{13}$ cm$^{-2}$ to 2.15$\times$10$^{13}$ cm$^{-2}$. By combining the data from AlGaN/AlN/GaN samples, we find that the spin-orbit field is not a constant at high carrier densities and the electron spin-splitting energies show a deviation from linear behavior with Fermi wavefactor. However, the spin-splitting energies extracted from WAL oscillations, even in this high carrier density regime, were found to be much smaller than the previous reports based on Shubnikov-de Haas (SdH) measurements. We will discuss how the nonuniformities in the carrier density can lead to beating features in SdH oscillations, which can then be misinterpreted as large spin-splitting energies. This finding may resolve the long-standing discrepancy between the WAL and SdH results.   

Spin Coulomb Drag in the Hubbard Chain

The spin Coulomb drag is the decay of the spin current in a metal as a consequence of the Coulomb interaction between up- and down-spin carriers and is a distinctive feature of spin- polarized transport. The current of majority spins can induce a current of minority spin carriers via the transresistivity. This friction reduces the current but does not change the spin- polarization.\footnote{I. D'Amico and G. Vignale, Phys. Rev. B {\bf 62}, 4853 (2000).} We calculate the critical exponents of the resistivity for up- and down-spin electrons and the transresistivity for the spin-polarized Hubbard chain with nonmagnetic impurities within the Kubo formalism using (1) bosonization techniques\footnote{P. Schlottmann, Phys. Rev. B {\bf 80}, 205110 (2009).} and (2) the Bethe ansatz solution and conformal invariance.\footnote{P. Schlottmann, Phys. Rev. B {\bf 82}, 075103 (2010).} The charge-spin separation in 1D is strictly valid only in the absence of spin-polarization. Due to the Luttinger liquid properties the temperature dependence of the transport correlation functions follow power laws of $T$ with non-universal exponents. A large spin polarization is more favorable for a sustained spin current than a small magnetization.   

Spin Texture in a Cold Exciton Gas

We report on the observation of a spin texture in a cold exciton gas in a GaAs/AlGaAs coupled quantum well structure. The spin texture is observed around the rings in the exciton emission pattern. The observed phenomena include: a ring of linear polarization, a vortex of linear polarization with polarization perpendicular to the radial direction, an anisotropy in the exciton flux, a skew of the exciton fluxes in orthogonal circular polarizations and a corresponding four-leaf pattern of circular polarization, and a periodic spin texture. These phenomena emerge when the exciton gas is cooled below a few Kelvin.