Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session N40: Semiconductors: Magneto-optics and Dynamics |
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Sponsoring Units: FIAP Chair: David Reitze, University of Florida Room: Colorado Convention Center 503 |
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N40.00001: Cyclotron Dephasing Times in the Quantum Hall Regime X. Wang, D. J. Hilton, L. Ren, D. M. Mittleman, J. Kono, J. L. Reno, M. M. Fogler We have used time-domain THz spectroscopy to study a high-mobility GaAs two-dimensional electron gas in quantizing magnetic fields. We observe very long-lived (up to $\sim $30 ps) coherent cyclotron oscillations, which can be viewed as the free induction decay of a coherent superposition between the lowest-unfilled Landau level and the highest-filled Landau level induced by the incident coherent THz pulse. From the frequency and decay time of these oscillations, we can directly determine the cyclotron mass and dephasing time. The magnetic field dependence of the dephasing time shows an oscillatory behavior, which can be due to the filling factor dependent dielectric screening of long-range potentials. The temperature dependence of the dephasing time at a fixed magnetic field shows three pronounced regions where different scattering potentials dominate. The two high temperature regions have standard interpretations while the lowest temperature range has not been observed before, to our knowledge. A sharp increase in dephasing time is observed below $\sim $ 5 K, which could be due to strong screening enhanced by the spin splitting of Landau levels. We provide detailed theoretical calculations to explain these observations. [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N40.00002: Theory of Cyclotron Resonance in Si,Ge,Zn and Cd Shigeji Fujita, Robert Simion, Rohit Singh, Rohit Singh, Seichi Watanabe, Salvador Godoy A quantum theory is developed for the cyclotron resonance (CR) in Si and Ge by introducing the concept of the Cyclotronic Planes (CP) in which the conduction electrons (``electrons'',``holes'') complete circulations. The angular dependent CR peaks for heavy ``holes'' are analyzed, using the Dresselhaus-Kip-Kittel (DKK) formula: $\omega=(\omega_{t}^{2}cos^2\theta + \omega_{t}\omega_{l}sin^2 \theta)$, $\omega_{t}\equiv eB/m_{t}$, $\omega_{l}\equiv eB/m_ {l}$.The Fermi surfaces for Si(Ge) are spheroids oriented along $\left\langle 100\right\rangle$ axes with the transverse mass $m_{t}=0.46(0.29)m$ and the longitudinal mass $m_{l}=1.03(0.78) m$.The fluted energy surfaces used by DKK were avoided.The angular-independent CR peaks for light ``holes'' in Ge(Si) arise from the spherical Fermi surface with the effective mass $m_{l} =1.03(0.78)m$ with the CP $\left\{100\right\}$. The reason why there are light and heavy ``holes'' with the same CP in $\left\{100\right\}$ is explained by decomposing the fcc lattice in two sets of sublattices. The theory is extended to treat the CR peaks in Zn and Cd, both hexagonal-close-packed metals. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N40.00003: Infrared longitudinal and Hall conductivity of SrRuO$_3$ and Ga$_{1-x}$Mn$_x$As films obtained by magneto-polarization measurements M.-H. Kim, G. Acbas, M.-H. Yang, J. Cerne, I. Ohkubo, H. Christen, D. Mandrus, M.A. Scarpulla, O.D. Dubon, Z. Schlesinger By measuring the changes in the polarization of transmitted (Faraday effect) and reflected (Kerr effect) mid-infrared (MIR: 115-366 meV) light induced by an external magnetic field, we determine the complete complex magneto-conductivity tensor in SrRuO$_3$ and Ga$_{1-x}$Mn$_x$As films. Thick film transmission and reflection equations are used to convert the measured complex Faraday and Kerr angles into the MIR complex longitudinal conductivity $\sigma_{xx}$ and the complex transverse (Hall) conductivity $\sigma_{xy}$. The resulting $\sigma_{xx}$ is consistent with the values obtained from conventional transmittance and reflectance measurements on these films, as well as the results from Kramers-Kronig analysis of reflectance measurements on similar films. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N40.00004: Time-resolved Faraday rotation measurements of spin relaxation in InGaAs/GaAs quantum dots Jennifer Robb, Ye Chen, Adam Timmons, Oleg Shchekin, Dennis Deppe, Kimberley Hall We report measurements of room temperature spin dynamics in InGaAs quantum dots using time-resolved differential transmission and Faraday rotation techniques. We observe an enhancement of the electron spin lifetime by an order of magnitude for direct optical pumping of the quantum dot ground state compared to optical pumping of the GaAs barriers. These findings indicate that the optical excitation conditions can have a critical influence on the spin kinetics, a result which may account for the wide variation of spin lifetimes reported to date. The observed enhancement in spin lifetime is attributed to the reduction of phonon-mediated spin-flip scattering. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N40.00005: Measurement of the GaSb surface band bending potential from the magnetotransport characteristics of GaSb-InAs-AlSb quantum wells Patrick Folkes, Godfrey Gumbs, Wen Xu, M. Taysing-Lara Low-temperature magnetotransport measurements on GaSb/InAs/AlSb coupled quantum well structures with a GaSb cap layer and self-consistent calculations of their electronic structure have led to the determination of the Fermi level at the surface, E$_{FS}$, of undoped molecular-beam-epitaxy-grown GaSb. E$_{FS}$ is pinned around 0.2 eV above the top of the GaSb valence band when the GaSb cap layer is width is greater than 900 {\AA}. For smaller GaSb cap widths, E$_{FS}$ decreases with the GaSb width. The heterostructures' Fermi level is determined by bulk donor defects in the AlSb layer adjacent to the InAs quantum well. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N40.00006: Magnetoexcitons in InSb Quantum Wells William Gempel, Taroshani Kasturiarachchi, Ryan Doezema, Madhavie Edirisooriya, Tetsuya Mishima, Michael Santos, Gary Sanders, Christopher Stanton We report on an experimental and theoretical study of excitons in strained InSb quantum wells with Al$_{x}$In$_{1-x}$Sb barriers. Perpendicular magnetic fields of 0$<$B$<$8T are applied in far infrared transmission measurements at a temperature of 4.2K. We observe rich spectra of excitonic transitions between hole and electron subbands in square and parabolic wells. Strain and confinement lift the degeneracy between the light and heavy holes. These data are in the large B regime since the electron effective mass is small (0.014m$_{o})$ and the dielectric constant is large (18) in InSb. Spin splitting is well resolved due to the large g-factor (-51) for electrons in InSb. We use a modified Bauer-Ando theory to identify the magnetoexciton transitions and to explain observed anti-crossing behavior. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N40.00007: Time-resolved infrared spectroscopy of a DMS InGa(Mn)As film H. Zhang, D.H. Reitze, C.J. Stanton, D.B. Tanner, G. Khodaparast, J. Kono, H. Munekata, G.L. Carr We report a two-color, time-resolved, differential absorption study of the dilute magnetic semiconductor InGa(Mn)As. We varied the temperatures above and below the nominal Curie point of 50 K. The sample is an (In0.53Ga0.47)0.87Mn0.13As layer grown on InGaAs buffer layers on InP. The study was performed using near-IR pulses from a Ti:sapphire laser to photoexcite the sample and pulsed far-infrared synchrotron radiation (at beamline U12IR of the NSLS) to probe the relaxation process. Our time resolution is $\sim $200 ps. A two-step decay process was observed, with the initial decay in the few ns range followed by a much slower decay lasting $\sim $100 ns or longer. The relaxation time for the faster decay was found to increase as the temperature decreased below 60 K. The slower decay may be due to thermal relaxation of the sample into its surroundings. Details of the photo-induced far-infrared spectra, as well as the dependence on photoexcitation fluence and wavelength, will also be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N40.00008: Effects of a parallel magnetic field on the Fermi-energy in two-dimensional electron systems Y.D. Jho, X. Wang, D.H. Reitze, J. Kono, S.A. Crooker, C. Kadow, A.C. Gossard, M. Semtsiv, W.T. Masselink The transport properties in 2DEG systems are determined by the electrons near the Fermi energy, and whether those electrons are localized or delocalized determines the character of the material, be it metallic or insulating. Here, we investigate whether there is any optical process that is sensitive to the conductivity change of the electrons near the Fermi energy, and report results of correlated experimental studies of interband optical spectra and transport properties in a two-dimensional electron gas placed in an in-plane magnetic field up to 45 T. Our magneto-absorption experiments show an initial red shift of the Fermi energy with increasing magnetic field, while photoluminescence strength at the Fermi-energy edge singularity reveals a quenching behavior only at low temperature along with resistivity increment as the multiple electron-electron scattering is being suppressed. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N40.00009: Magnetic field induced Raman excitations in Zn$_{1-x}$Cr$_{x}$Te, Cd$_{1-x}$Cr$_{x}$Te and Cd$_{1-x}$Cr$_{x}$Se X. Lu, S. Tsoi, I. Miotkowski, S. Rodriguez, A.K. Ramdas, H. Alawadhi, T.M. Pekarek Raman electron paramagnetic resonance(Raman-EPR) of the transitions due to the $\Delta $m$_{s}=\pm $1 spin-flip of the 3d electrons of Cr$^{+}$ in Zn$_{1-x}$Cr$_{x}$Te and Cd$_{1-x}$Cr$_{x}$Te are observed at $\omega _{PM}$=g(Cr$^{+})\mu _{B}$B, g(Cr$^{+})$=2.0041$\pm $ 0.0095 and 2.0039 $\pm $ 0.0093, respectively. Raman lines appear at $\omega _{LO}\pm $ n$\omega _{PM}$, n=1,2 and 3, resulting from the strong Fr\"{O}hlich interaction with LO phonon. The intensity of $\omega _{PM}$ can be enhanced through the photo-generation process Cr$^{2+}\to $ Cr$^{+}$; photoluminescence spectra reveal signatures of excitons bound to Cr$^{+}$ acceptors in Zn$_{1-x}$Cr$_{x}$Te. The resonance profile of $\omega _{PM }$shows that the strong resonant enhancement is mediated via an exciton bound to a neutral acceptor. Spin flip Raman scattering (SFRS) at $\omega _{SFR}$ from donor-bound electrons in Cd$_{1-x}$Cr$_{x}$Se, as well as in pure CdSe, are observed, in turn yielding the s-d exchange energy. The magnetization of Cd$_{1-x}$Cr$_{x}$Se is intermediate between van Vleck and a B$_{2}$ Brillouin paramagnetism. The linear dependence of the s-d exchange energy as a function of magnetization yields the s-d exchange constant in Cd$_{1-x}$Cr$_{x}$Se, $\alpha $N$_{0}$=(213.7 $\pm $13) meV. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N40.00010: Aharonov-Bohm oscillations in Type-II ZnSe$_{1-x}$Te$_{x}$ excitons. I.R. Sellers, V.R. Whiteside, B.D. McCombe, I.L. Kuskovsky, W. MacDonald, A.O. Govorov The luminescence of ZnSe$_{1-x}$Te$_{x }$has been studied extensively and it is generally accepted that the emission of this system is dominated by excitons bound to isoelectronic centers [1]. We will show that when these isoelectronic centers group into large clusters they appear to form type-II quantum dot-like structures (QDs). In such structures the hole is strongly localised within the dot while the electron is localised outside through coulombic attraction. These localised excitons are analogous to the hydrogen atom with the electron orbiting the confined hole in a `ring-like' geometry. We will show that as a result of this behaviour we observe Aharonov-Bohm (AB) oscillations in the magneto-photoluminescence of excitons bound to QDs formed with various configurations of isoelectronic centers (atoms, clusters) [2]. The behaviour of QD-like structures of different energy will be presented and it will be shown that in the case of QDs formed by larger Te-clusters the AB oscillations are preserved up to 180K. Finally, we will also present the optically detected resonance for these structures, in which a FIR laser is used to probe resonances related to the atomic-like higher order shells of the bound excitons. [1] I. L. Kuskovsky et al. Phys. Rev. B. (2001) [2] I. L. Kuskovsky et al. Cond-mat/0606752 [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N40.00011: NMR Probing Spin Excitations in the Ring-Like Structure of a Two-Subband System Xinchang Zhang, Gavin Scott, Hongwen Jiang Resistively detected nuclear magnetic resonance (NMR) is observed inside the ring-like structure, with a quantized Hall conductance of 6$e^2/h$, in the phase diagram of a two subband electron system. The NMR signal persists up to 400 mK and is absent in other states with the same quantized Hall conductance. The nuclear spin-lattice relaxation time, $T_1$, is found to decrease rapidly towards the ring center. These observations are consistent with the assertion of the ring-like region being a ferromagnetic state that is accompanied by collective spin excitations. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N40.00012: The dynamics of Bloch electrons in 2D hexagonal systems M. J. Rave, W. C. Kerr We investigate the semiclassical equations of motion (EOM) for Bloch electrons in a 2D hexagonal lattice. When a system has a non-zero Berry curvature, there are additional terms in these EOM beyond the familiar group velocity and classical electric and magnetic force terms. We investigate the existence and consequences of these terms for Bloch electrons in a 2D hexagonal tight-binding model, parameterized so that it can represent either graphene or a single layer of boron nitride (h-BN). We calculate these terms in the vicinity of a K point in the first Brillouin Zone of h-BN using perturbation theory carried to sufficiently high order to give a non-constant Berry curvature. These terms can be written in terms of momentum operator matrix elements, which we estimate using the tight-binding wave functions. We show the effects of these terms on electron trajectories in the presence of external fields. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N40.00013: Optical phonon spectra of CdS crosslinked sulfonate polystyrene nanocomposites Jayesh Govani, Felicia Manciu, S. Ortiz-Col\'on, Matthew Espe, Ronald Ziolo We have used IR transmission and FT-Raman spectroscopy to study optically active phonon modes of CdS nanoparticles synthesized in sulfonated polystyrene resin and obtained information about the morphology, crystallinity, and surface interactions. The dominant feature in the far-infrared region of CdS/polystyrene nanocomposites spectra is a sharp peak centered at 255 cm$^{-1}$, which could be assigned to the transversal optical mode at the L edge of the Brillouin zone of CdS nanoparticles. Also, this vibrational line, based on theoretical core-shell model calculation, could be attributed to the presence of a very thin CdS shell layer. HRTEM images of the CdS nanocomposites show CdS nanoparticles of about 2.5 nm aligned in rows or strings on the polymer surface. Amorphous CdS is also present and may be seen surrounding the nanocrystalline regions. Complementary solid state $^{113}$Cd NMR analysis will be presented as well. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N40.00014: Phonon-assisted coherent control of injected carriers in indirect bandgap semiconductors Julien Rioux, Fred Nastos, John E. Sipe Charge and spin currents can be generated in direct semiconductors by quantum interference between one- and two-photon absorption\footnote{M.J. Stevens, R.D.R. Bhat, A. Najmaie, H.M. van Driel, J.E. Sipe and A.L. Smirl, in \emph{Optics of Semiconductors and Their Nanostructures}, edited by H. Kalt and M. Hetterich (Springer, Berlin, 2004), vol. 146 of Springer Series in Solid-State Sciences, p. 209.}. For semiconductors such as Si and Ge, optical injection of carriers over the indirect bandgap must be assisted by momentum transfer from phonon scattering. We consider the optical properties for such 1+2 photon processes in the presence of the electron-phonon interaction. The latter is modelled by acoustic deformation potential. Indirect transitions involve double Brillouin zone integrations, which are computed by a linearized tetrahedron method. We compare our results to those for bulk GaAs. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N40.00015: \textit{Ab Initio} XAS Debye-Waller Factors Beyond the Harmonic Approximation Fernando Vila, H. H. Rossner, H. J. Krappe, J. J. Rehr We introduce an \textit{ab initio} approach to calculate the temperature dependent vibrational effects in x-ray absorption spectra beyond the harmonic approximation. Instead of relying on empirical models, we apply electronic structure theory to determine the dynamical matrix of the system, from which the appropriate vibrational densities of state can be obtained using a Lanczos recursion algorithm [2]. By combining thermodynamic perturbation theory and the quasi-harmonic approximation we obtain x-ray absorption fine structure (XAFS) cumulants such as the mean square relative displacement (2nd cumulant), the thermal expansion (first cumulant), the asymmetry of the distribution (third cumulant) and the perpendicular motion contribution to the DW factor. Other quantities of interest such as mean square atomic displacements are also discussed. {[2]}H.J. Krappe and H.H. Rossner, Phys. Rev. B\textbf{70}, 104102 (2004). [Preview Abstract] |
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