Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session J23: Spectroscopic Studies of 2D Electron Gas and Other Low Dimensional Semiconductor Structures |
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Sponsoring Units: DCMP Chair: Aron Pinczuk, Columbia University Room: 325 |
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J23.00001: Coexistence of quantum phases in the quantum Hall regime of the 2$^{nd}$ Landau Level Trevor Rhone, Jun Yan, Yann Gallais, Aron Pinczuk, Loren Pfeiffer, Ken West We report the experimental study of spin excitation modes in the regime of quantum hall phases of the 2$^{nd}$ Landau Level. In the ferromagnetic state at $\nu $=3 the long wavelength spin wave mode is seen at the bare Zeeman energy. At low temperatures and at filling factors slightly lower ($\nu \sim $2.97), the spin wave attenuates and a broad `overdamped' continuum of low-lying excitations emerges. Under these conditions, sharp and broad modes coexist, suggesting the presence of mixed quantum phases. At slightly elevated temperatures the continuum disappears. Further away from filling factor three, near the odd-denominator state at $\nu \cong $8/3, the continuum dominates at low temperature. However, the sharp spin wave is recovered at T$>$1K. For even lower filling factors, such as $\nu \cong $5/2, low temperature spectra display only the broad continuum of low-lying excitations. At high temperatures (T$\sim $2K) a sharp spin wave is recovered while the broad continuum persists, indicating the emergence of phase coexistence. The interplay between sharp and `overdamped' modes may manifest tendencies towards loss of full spin polarization in the N=1 Landau level and may indicate that spin degrees of freedom have significant impact on the physics of quantum Hall states with 3$\ge \nu \ge $ 2. [Preview Abstract] |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J23.00002: The Virtual Scanning Tunneling Microscope: Induced Tunneling in Bilayer Two-Dimensional Electron Systems Adam Sciambi, Matthew Pelliccione, David Goldhaber-Gordon, Seth Bank, Arthur Gossard, Michael Lilly, John Reno We propose a novel probe technique, the virtual scanning tunneling microscope (VSTM), which will spatially and spectroscopically map two-dimensional electron systems (2DESs) in semiconductor heterostructures. The probe overcomes the typical inaccessibility of a buried 2DES by having a second parallel ``probe'' 2DES grown nearby. A biased tip overhead can then induce local tunneling from the probe 2DES into the original by adjusting the interlayer potential barrier. Prior bilayer studies have shown that a tunneling signal is dominated by the overlap of the layers' Fermi surfaces, hindering VSTM-induced tunneling and obscuring any spectroscopy. We show, however, in widely-space bilayers systems where interlayer inelastic scattering is more prominent that the previous energy-momentum constraints are relaxed. In GaAs/AlGaAs samples grown by two different sources, we show we can increase interlayer tunneling by an order of magnitude with gating, setting the stage for spectroscopy. [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 11:51AM |
J23.00003: Tunneling spectroscopy of a 2D-2D tunnel junction: Towards a local spectroscopic probe of 2D electron systems Matthew Pelliccione, Adam Sciambi, David Goldhaber-Gordon, Seth Bank, Arthur Gossard, John Reno, Michael Lilly We present measurements on GaAs/AlGaAs bilayer two-dimensional electron systems (2DES) that exhibit inelastic tunneling between the 2D electron layers. Due to a relatively large interlayer separation, scattering allows for tunneling events between states of different energy and momentum, which are not observed in similar systems with a small interlayer separation. This behavior can be used to measure spectroscopic information about the 2DES that is obscured when tunneling events conserve energy and momentum exclusively. We study the bulk behavior of this system in the integer quantum Hall regime on samples from different sources, and provide a model to explain the observed tunneling dynamics. We also discuss the prospect of using this system as a virtual scanning tunneling microscope (VSTM), where a scanning probe is used to locally induce tunneling between the 2D electron layers. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J23.00004: Unusual Cyclotron Resonance Line Broadening in Ultra-High Mobility Two-Dimensional Electron Gas System Li-Chun Tung, Changli Yang, L.N. Pfeiffer, K.W. West, R.R. Du, Yong-Jie Wang Microwave induced resistance oscillation in the ultra high mobility two-dimensional electron gas system has attracted an intense interest in recent years. Under the illumination of an intense microwave radiation in the millimeter regime, the system exhibits an unique resistance oscillation with a microwave-frequency-dependent period. We have carried out a FIR magneto-optical study up to 33T on a set of GaAs/AlAs ultra-high mobility heterostructure samples at 4K. At low magnetic field, ultra-sharp electron CR has been observed as expected. At high magnetic field, the halfwidth of CR grows rapidly with increasing magnetic field and this phenomenon is uniquely found in the 2DEG in the heterojunction. At the same time, there is no indication of a reduced mobility at high field in the transport measurement. The sudden broadening of the CR line shape cannot be simply interpreted by either the short- or long-range random potential. The halfwidth changes by at least an order, thus it can hardly be explained by either magnetophonon effect, inhomogeniety nor magnetic oscillation. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J23.00005: Evidence of Auger Satellite Lines in Landau-Level Photoluminescence Spectroscopy in a Two Dimensional Electron Gas S.K. Lyo, W. Pan, J.L. Reno Landau-level spectroscopy has provided a powerful tool for investigating the electronic structure and the scattering dynamics in a two-dimensional electron gas (2DEG) in the past. In this paper, we present theoretical and experimental evidence for Auger satellite lines in the magnetoluminescence of the Landau-level spectroscopy from a 2DEG under a perpendicular magnetic field $B$ at low temperatures. These new anomalous lines with a weak intensity appear below the energy gap in the form of radial ``spokes'' with negative slopes in the so-called fan (energy vs. $B$) diagram, in contrast to the well-known standard spokes of the fan diagrams of the spectral lines which appear above the band gap energy with positive slopes. [S. K. Lyo, E. D. Jones, and J. F. Klem, Phys. Rev. Lett. 61, 2265 (1988)]. Our theoretical predictions yield reasonable agreement with observed low-temperature data from GaAs quantum wells. The satellite lines can be used to determine the conduction (valence) band mass in n-doped (p-doped) systems. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J23.00006: Localization - Delocalization Transition of Indirect Excitons in Lateral Electrostatic Lattices M. Remeika, J. Graves, A.T. Hammack, L.V. Butov, M. Hanson, A.C. Gossard We report on a study of transport of indirect excitons in GaAs/AlGaAs coupled quantum wells in linear lateral lattices created by laterally modulated gate voltage. The localization-delocalization transition for the excitons was observed with increasing density in the directions along and across the lattice. At high lattice amplitudes, the density corresponding to the transition across the lattice was found to linearly depend on the lattice amplitude. Screening of the potential by repulsive exciton-exciton interaction was demonstrated. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J23.00007: Giant upconversion of photoluminescence in semiconductor heterostructures Angelo Mascarenhas, Brian Fluegel, David Snoke, Gamani Karunasiri, Loren Pfeiffer Photoluminescence (PL) upconversion is observed in GaAs/AlGaAs multiple quantum wells at photon energies up to 90 meV above the exciting laser and with an efficiency up to 27\% relative to normal Stokes PL excited from the two-dimensional continuum. The effect is due to electric field-induced bipolar tunneling. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J23.00008: Phase Separation of Bright and Dark Excitons in Coupled Quantum Wells Nicholas Sinclair, Zoltan Voros, Jeff Wuenschell, David Snoke, Kenneth West, Loren Pfeiffer The diversity and complexity of solid-state environments suggests that Bose-Einstein Condensation (BEC) of excitations in a solid might manifest in a variety of interesting ways, in correspondence with the diversity of features among ground states of these systems. The pursuit of excitonic BEC is both enriched and obfuscated by this flexibility of condensate character. Previous research pursuing BEC of interwell excitons in GaAs coupled quantum wells (CQWs) has focused attention on observing unusual luminescence from `bright', dipole-coupled (J=1) excitons to detect a BEC. However, theorists have recently predicted a `dark' (J=2) ground state for interwell excitons in GaAs. Our recent work with interwell excitons confined in stress-induced, in-plane traps shows the critical onset of a dark spot in the exciton-recombination luminescence at trap center, suggestive of a dense population of dark excitons and a phase separation between the dark/bright species. The critical temperature vs. density in the low temperature regime matches well with ideal 2D harmonic trap BEC criteria, and preliminary theoretical work suggests that this degree of species separation cannot be explained by a model based on classical statistical level occupation using the bright/dark state energy separation. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J23.00009: Ultrafast Relaxation Dynamics of a High Density Electron-Hole Plasma in High Magnetic Fields Jinho Lee, Dave H. Reitze, Junichiro Kono, Alexey Belyanin, Glenn Solomon, Steve McGill We study the inter-Landau level relaxation dynamics of a dense electron-hole plasma in high magnetic fields (up to 31 T). Intense 150 fs pump pulses create carrier densities approaching 10$^{13}$/cm$^{2}$ in In$_{0.2}$Ga$_{0.8}$As/GaAs multiple quantum wells. Relaxation dynamics are probed as a function of Landau level (LL) and magnetic field using time-resolved transient absorption (TRTA) and time-resolved photoluminescence (TRPL), which provide complementary information about the relaxation processes. Manifestly non-exponential decays of the TRTA signals are observed at high fields (above 15 T). TRPL emissions measured in the plane of the wells reveal the presence of multiple emission bursts from the LLs at high magnetic fields, suggesting a complicated relaxation process mediated by the field whereby carriers get trapped in a specific LL, emit PL though recombination, and then `reload' as the carriers relax down to the previously occupied LLs. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J23.00010: Spin splitting of the valence band Landau levels in GaAs quantum wells I. Khan, T. Ali, M. Yasar, A. Petrou, A. Hanbicki, G. Kioseoglou, C. Li, B. Jonker We have studied as function of magnetic field the electroluminescence spectra from an n-i-p LED that incorporates three GaAs quantum wells in the intrinsic region. This device has excess n-type doping and as a result, the quantum wells are populated by a two-dimensional electron gas. The broad zero field emission band evolves into a series of discrete features in the presence of a magnetic field. These are identified as interband transitions between the $\ell $ = 0, 1, and 2 Landau levels associated with the e$_{1}$ and h$_{1}$ subbands, with the selection rule $\Delta \ell $ = 0. The EL spectra were analyzed in their $\sigma $+(LCP) and $\sigma $- (RCP) components. An energy splitting between the two polarized components is observed for each Landau level transition, which is equal to the sum of the conduction and valence band spin splittings. Since the electron g-factor value is known ($g$ = - 0.44) we were able to determine the valence band spin splittings. Our experimental values are compared to calculated values (1) and were found to be in reasonable agreement. Work at SUNY was supported by ONR and NSF 1. H.A. Nickel et al., Phys. Rev. B, \underline {62}, 2773, (2000) [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J23.00011: Bose Einstein Condensation in Trapped Polaritons Versus Lasing Effects Ryan Balili, Bryan Nelsen, David Snoke, Loren Pfieffer, Kenneth West Evidence for Bose Einstein condensation (BEC) of exciton-polaritons have been presented recently by several groups in a variety semiconductor microcavity geometries (eg. [1). Objections nevertheless remain as experimental evidence for polariton BEC bear striking similarity to observed behavior in a regular photon laser or microcavity in weak coupling regime [2]. Latest results however show that a both BEC and lasing transitions can occur and are distinguishable in the stress trapped case [3]. \\[3pt] [1] Balili, R., Hartwell, V., Snoke, D., Pfeiffer L., and West, K. Science 316, 1007 (2007). \\[0pt] [2] Bajoni, D., Senellart, P, Lemaitre, A., and Bloch, J. Phys. Rev. B. 76, 201305(R) (2007). \\[0pt] [3] Balili, R., Nelsen, B., Snoke, D., Pfeiffer L., and West, K. arXiv:0808.1861v2 [cond-mat.other](2008). [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J23.00012: Study of Optical Bistability in Coupled Microdisks S.N. Ghosh, Y.K. Verma, B.B. Buckley, X. Li, N. Samarth, D.D. Awschalom, S. Ghosh Semiconductor microcavities offer unique means of controlling light-matter interactions in confined geometries, resulting in a wide range of applications in optical communications. We report bi-stable lasing in coupled GaAs microdisks with quantum wells and interface-fluctuation quantum dots in the active region. The inter-disk coupling results in mode-splitting, with the higher energy resonance persistently achieving higher mode Q ($\sim $ 4000). The bi-stability manifests in the form of hysteresis in the intensity of the coupled modes on non-uniform excitation and can be attributed to saturable absorption. This property in the lasing characteristics of coupled cavities gives us a control on the gain modulation and mode-switching and would be useful for applications in optical memories and computing, and in next generation of low-threshold optoelectronic devices. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J23.00013: Stress Induced Splitting of Polariton States in Semiconductor Microcavities Bryan Nelsen, Ryan Balili, David Snoke, Lauren Pfeiffer, Kenneth West Photoluminescence and reflectance measurements of microcavity polaritons indicate that there is an energy splitting of the lower polariton branch on the order of 10 to 100 $\mu $eV. When stress is applied with a pin to the microcavity samples, this energy splitting is found to increase. The states have varying degrees of polarization which depend on the applied stress. Experimental results will be presented, along with theoretical mechanisms that may cause such a splitting. These results may have implications for the spontaneous emission of polarized light associated with the Bose-Einstein condensation transition observed in microcavity polaritons.[1] \newline \newline 1. R. Balili, V. Hartwell, D.W. Snoke,~ L. Pfeiffer and K. West, Science 316, 1007 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J23.00014: Polarization dependence of coherent LO phonon excitation in Si Anca-Monia Constantinescu, Hrvoje Petek The coherent LO phonon mode excitation dependence on the angle $\theta$ between pump and probe polarizations is systematically investigated by transient electro-optic sampling measurement for both $\Gamma_{25}$ and $\Gamma_{12}$ symmetries\footnote{M. Hase, M. Kitajima, A. M. Constantinescu, and H. Petek, \textit{Nature} \textbf{426}, 51 (2003).} We find that for $\Gamma_{25}$ symmetry the phonon exhibits a sin(2$\theta)$ dependence in the amplitude, while for $\Gamma_{12}$ symmetry the signal is considerably weaker and has a cos(2$\theta)$ dependence. The LO phonon maximum amplitude for $\Gamma_{25}$ is larger than the maximum amplitude for $\Gamma_{12}$ by about a factor of 6. We will discuss the results in the context of coherent phonon excitation mechanisms. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J23.00015: ABSTRACT WITHDRAWN |
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