Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session P19: Spectroscopy of Semiconductor Nanostructures II |
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Sponsoring Units: DCMP Chair: Giti Khodaparast, Virginia Tech Room: LACC 406B |
Wednesday, March 23, 2005 11:15AM - 11:27AM |
P19.00001: Dynamics of the Inter-Landau-Level Magnetoplasmon Coherence in a Quantum Hall system Keshav Dani, Jerome Tignon, Michael Breit, Daniel Chemla, Eleftheria Kavousanaki, Ilias Perakis Collective many-electron behavior of the cold 2DEG in a magnetic field leads to novel Quantum Hall (QH) effects and electronic excitations like the inter-Landau-level magnetoplasmon (MP). Using 3-pulse four-wave mixing (FWM) spectroscopy, we study coherent MP dynamics in a QH system. By delaying the arrival of one pulse relative to the others, the FWM signal shows striking beats for short time delays ($\sim $300fs), followed by a rise ($\sim $10ps) and then a decay ($\sim $200ps). We analyze the experiment in the coherent regime (short time delays) by extending the Dynamics Controlled Truncation Scheme to the case of a strongly correlated ground state. We infer that the beats are due to quantum interference of the MP and magnetoexciton coherences. The decay of the beats gives the decay of the MP coherence. We perform a comprehensive study of these effects as a function of excitation frequency, magnetic field, excitation intensity and temperature. [Preview Abstract] |
Wednesday, March 23, 2005 11:27AM - 11:39AM |
P19.00002: Infrared spectroscopy of 2D electron gas in high magnetic field: a case study of graphite Zhiqiang Li, Willie Padilla, Sasa Dordevic, Pablo Esquinazi, C.C. Homes, Dimitri Basov We present the first systematic investigation of the optical constants of HPOG graphite in magnetic fields up to 17T. The ab plane magneto-reflectance in the frequency range 15-3000 cm$^{-1}$ was measured with the field in c-axis. The optical conductivity was obtained from Kramers-Kronig analysis augmented with ellipsometry data. These experiments have allowed us to monitor the field-induced transfer of the electronic spectral weight from the Drude mode to cyclotron resonance (CR) modes. In applied fields, the conductivity in the limit of $\omega \to $0 is depleted by several orders of magnitude in accord with notoriously large positive magneto-resistance of graphite. A close examination of the lineshape of CR modes is indicative of the coexistence of carriers with 3D and 2D character. The latter mode reveals a $\sqrt H $ dependence of the cyclotron frequency long anticipated for Dirac quasiparticles with linear dispersion. In this fashion, our magneto-optics experiments have allowed us to explore novel aspects of charge dynamics in this prototypal quasi-2D material. [Preview Abstract] |
Wednesday, March 23, 2005 11:39AM - 11:51AM |
P19.00003: Observation of Optical Gain in InAs Nanocrystals Gang Chen, Ronen Rapaport, Dan Fuchs, Sahar Vilan, Assaf Aharoni, Uri Banin We developed processes that enable the inclusion of InAs nanocrystals, emitting at 1.55 microns, into a transparent polymer matrix while preserving their optical properties. This provides a flexible platform for integrating the functionality of the nanocrystals into the current photonic circuit technologies. Using three-beam, time-resolved pump-probe measurements, we observed strong evidence of optical gain in a polymer film containing InAs nanocrystals for the first time. We measure the gain dynamics in that system, extracting the gain lifetime and the gain recovery time. These processes and measurements are the first step towards incorporating semiconductor nanocrystals into active devices, such as lasers and amplifiers, on an integrated photonic circuit. [Preview Abstract] |
Wednesday, March 23, 2005 11:51AM - 12:03PM |
P19.00004: Cathodoluminescence study of thermal activation of carriers in InAs/GaAs(001) self-assembled quantum dots D. H. Rich, S. Khatsevich, O. Moshe, E. Kim, A. Madhukar We have examined state-filling and thermal activation of carriers in buried InAs/GaAs(001) self-assembled quantum dots (SAQDs) with excitation-dependent cathodoluminescence (CL) imaging and spectroscopy. The dependence of the CL intensity of the ground and various excited state transitions on excitation density was studied. The measurements reveal that carriers escape and are recaptured as excitons or correlated electron-hole pairs. At sufficiently high excitations, state filling and spatial smearing effects are observed together with a sublinear dependence of the CL intensity on excitation. Thermal quenching of the CL intensity of the QD ground and first excited state transitions at low excitations in $\sim $230 to 300 K temperature range is attributed to dissociation of excitons from the QD states into the InAs wetting layer. At high excitations, significantly lower activation energies of the ground and excited states are obtained, suggesting thermal reemission of single holes from QD states into the GaAs matrix is responsible for the observed temperature dependence of the QD luminescence in $\sim $230 to 300 K temperature range. [Preview Abstract] |
Wednesday, March 23, 2005 12:03PM - 12:15PM |
P19.00005: Exciton Determination of Strain Parameters in InSb/AlInSb Quantum Wells T. Kasturiarachchi, X.H. Zhang, F. Brown, N. Dai, R.E. Doezema, N. Goel, S.J. Chung, M.B. Santos Excitons in semiconductors can be used as a tool to probe various material and structural properties. We studied strain-related material parameters in the strained and nonparabolic InSb /AlInSb quantum well system. The strain present in the InSb wells alters the spectrum from that for unstrained systems by introducing a shift in both the heavy and light hole band gaps. With the change of Al concentration in the barrier layers, the strain in the quantum well system can be tuned continuously. Using FTIR spectroscopy, we were able to trace the strain-induced shifts of the exciton energies. The different strain dependence of the light-and heavy-hole band edges allows us to determine deformation potentials a and b simultaneously. Our samples were nominally undoped InSb/AlInSb quantum wells with Al concentration ranging from $5$ to$15\%$ grown by molecular beam epitaxy on GaAs substrates. This work is supported by the National Science Foundation under Grants No. DMR-0080054 and DMR-0209371 [Preview Abstract] |
Wednesday, March 23, 2005 12:15PM - 12:27PM |
P19.00006: Carrier Relaxation in Self-Assembled ZnTe/ZnSe Quantum Dots Der-Jun Jang, Y. C. Yeh, S.K. Lu, C.L. Wu, C.-S. Yang, W.C. Chou, M.E Lee Carrier relaxation in the type II self-assembled ZnTe/ZnSe quantum dots have been studied using ultrafast photoluminescence upconversion technique. We found that PL exhibits fast decay for ZnTe/ZnSe QDs grown in Volmer-Weber mode than that in Stranski-Krastanow mode. The dependence of PL decay time on energy was found only for QDs grown in Stranski-Krastanow mode. We attribute the different behaviors of PL decay for these two modes to the wetting layers. [Preview Abstract] |
Wednesday, March 23, 2005 12:27PM - 12:39PM |
P19.00007: Carrier Relaxation in Multi-Stacked InAs/GaAs Quantum Dots C.L. Wu, D.-J. Jang, S.-K. Lu, C.M. Lai, J.S. Wang, K.Y. Hsei, M.-E. Lee We report the ultrafast time-resolved photoluminescence study of multi-stacked InAs/GaAs quantum dots (MSQD) using the photoluminescence upconversion technique. MSQD with thicknesses of GaAs spacer of 30, 15, and 10 nm were studied to elucidate the dynamics of carrier coupling in both growth and lateral directions. The PL decay time decreases with the thickness of the GaAs spacer. The PL exhibits fast decay as the energy increases. We attribute the energy dependence of PL decay time to carrier tunneling in growth direction. We also found that the carrier tunneling is less effective for GaAs spacer of thickness 30 nm. [Preview Abstract] |
Wednesday, March 23, 2005 12:39PM - 12:51PM |
P19.00008: Green's Function Monte Carlo Study of Two-Dimensional Quantum Dots Kenneth Graham, James Anderson, Jainendra Jain Two-dimensional quantum dots of interacting electrons in zero magnetic field have been studied with various Monte Carlo methods for some time. To our knowledge, however, Green's function Monte Carlo (GFMC) has not been used. We show that GFMC is a useful method of calculating the ground state of these quantum dot systems. We calculate ground state energies for two and four electrons using a weighted average of a trial wave function's local energy, $(\mbox{\^{H}}\Psi_{T})/\Psi_T$. Our trial wave function $\Psi_T$ uses a two-body (electron-electron) Jastrow function to incorporate correlations between electrons and is variance-minimized. The GFMC method used is an exact-cancellation method which locates the nodes of the true ground state wave function. This method is in principle ``exact,'' outside of a statistical error common to all Monte Carlo methods. The energies obtained are slightly lower than those of earlier diffusion and variational Monte Carlo work. [Preview Abstract] |
Wednesday, March 23, 2005 12:51PM - 1:03PM |
P19.00009: Study of the Trion Correlations in Quantum Dots with the Tight Binding Method Gian Franco Sacco, Paul von Allmen The existence of a negatively or positively charged exciton bound state, also known as trion, has been proposed by Lampert in 1958. The binding energy of this complex is only of the order of a meV or less, making the task of observing such state and studying its properties particularly challenging. The importance of studying the properties of trions resides in the fact that this quasi-particle can be manipulated both electrically and optically. Many electro-optical devices utilizing the properties of trions have been proposed, such as a mobile light emitter, a memory device, and even a quantum computer. We study the properties of the trion system by means of the empirical tight-binding method, where the trion Hamiltonian is explicitly described at an atomistic level and the correlation among the constituents is included in the configuration interaction. This approach has been successfully applied to study the binding energy of excitons in quantum dots (QDs). We will extend this previous approach to describe the binding energy of trions in QDs. [Preview Abstract] |
Wednesday, March 23, 2005 1:03PM - 1:15PM |
P19.00010: Exciton Confinement in Traps Formed by a Laterally Modulated Gate Voltage A.T. Hammack, G.O. Andreev, Sen Yang, N.A. Gippius, L.V. Butov, M. Hanson, A.C. Gossard In semiconductor materials, cold gases of bosons can be realized in the system of indirect excitons in coupled quantum well structures. Boson confinement in potential traps improves the critical temperature for Bose-Einstein condensation and allows manipulation of the bosons by varying the trap potential [1,2]. Here, we present in-plane potential traps for indirect excitons, where the traps are formed by a laterally modulated gate voltage. The calculated trap design allows effective exciton confinement as well as in situ manipulation of excitons by the gate electrodes that control the confining potential. The design also ensures that the electric fields caused by the confining potential are well below the threshold for the exciton dissociation. Experiments with indirect excitons in the traps are presented as well. 1. E.A. Cornell, C.E. Wieman, Rev. Mod. Phys. 74, 875 (2002). 2. W. Ketterle, Rev. Mod. Phys. 74, 1131 (2002). [Preview Abstract] |
Wednesday, March 23, 2005 1:15PM - 1:27PM |
P19.00011: Condensation of microcavity polaritons with a disorder induced distribution of oscillator strengths Francesca Maria Marchetti, Jonathan Keeling, Marzena H. Szymanska, Peter B. Littlewood Partly motivated by recent experiments $[1,2]$, a model for condensation of disordered excitons coupled to cavity photons is investigated. The inhomogeneous broadening of the excitonic energies is combined with a distribution of oscillator strengths, allowing for correlations between the exciton energies and their coupling to light. Results are discussed in terms of the mean-field phase diagram, the spectrum of excitations and the polariton photoluminescence. \\[1em] \noindent $[1]$ P.~G.~Lagoudakis {\it et al.}, J.Appl.Phys. {\bf 95}, 2487 (2004)\\ \noindent $[2]$ M. Richard {\it et al.}, J. Phys. C {\bf 16}, S3683 (2004) [Preview Abstract] |
Wednesday, March 23, 2005 1:27PM - 1:39PM |
P19.00012: Polariton Condensation with Localised Excitons and Propagating Photons Jonathan Keeling, Paul Eastham, Marzena Szymanska, Peter Littlewood We calculate the condensation temperature for a model of microcavity polaritons, constructed from localised excitons and propagating photons. This condensation may be described in two different ways. At low densities, it may be considered as B.E.C. of weakly interacting bosons; at high densities a mean-field theory of self-consistent polarisation and optical fields is more appropriate. Considering fluctuations on top of the mean-field theory causes a crossover of the phase boundary from a B.E.C-like $T_c \propto \rho$ at low densities to the mean field theory when $T_c$ reaches the Rabi splitting. Due to the photon component of polaritons this regime occurs at densities much lower than those at which excitons overlap and becomes relevant for current experiments aimed at polariton B.E.C. From the excitation spectrum of fluctuations, one can also predict a number of experimentally accessible signatures which could indicate the presence of a condensate. In particular, we discuss the excitation spectrum, and momentum distribution of emitted photons. [Preview Abstract] |
Wednesday, March 23, 2005 1:39PM - 1:51PM |
P19.00013: Condensation of interacting excitons in a microcavity Anson Cheung, Ben Simons, Peter Littlewood We consider the ground state of excitons interacting both via their Coulomb forces and via photons in a microcavity. We propose a mean field ansatz for the wavefunction, generalised from [NOZ1], that encapsulates the physics of both high and low densities. We discuss the phase diagram in regimes where the photon- or Coulomb-mediated coupling is dominant, and the excitations have a character that is either excitonic, or polaritonic, or an electron-hole plasma. \newline \newline [NOZ1] Comte, C. and Nozi\`{e}res, P., J. Physique \textbf{43}, 1069--1081 (1982) [Preview Abstract] |
Wednesday, March 23, 2005 1:51PM - 2:03PM |
P19.00014: Characterization of Macroscopic Ordering in Exciton Rings Sen Yang, A.T. Hammack, L.V. Butov, L.S. Levitov, B.D. Simons, A.C. Gossard Recently observed complex PL patterns in 2D QW structures exhibit the inner [1,3] and the outer [1-4] exciton rings, localized bright spots [1,3], and the macroscopically ordered exciton state (MOES) [1,3]. The latter appears at the outer ring via its fragmentation into a periodic array of aggregates. While the gross features have been explained within classical framework, attributing the inner rings to nonradiative exciton transport and cooling [1], and the outermost rings and the bright spots to macroscopic charge separation [3,4], the origin of the MOES remains unidentified [5]. Here, for the first time, we report experiments demonstrating the exciton energy modulation over the MOES as well as the phase diagram of MOES in exciton density and temperature coordinates. The experiments shed new light on the dynamical origin of MOES. Besides, we present the studies of dynamical processes within MOES including the observation of aggregate instabilities and bifurcations that point to the spontaneous character of the instability.[1] L.V. Butov, A.C. Gossard, D.S. Chemla, Nature 418, 751 (2002). [2] D. Snoke, S. Denev, Y. Liu, L. Pfeiffer, K. West, Nature 418, 754 (2002). [3] L.V. Butov, L.S. Levitov, A.V. Mintsev, B.D. Simons, A.C. Gossard, D.S. Chemla PRL 92, 117404 (2004). [4] R. Rapaport, G. Chen, D. Snoke, S.H. Simon, L. Pfeiffer, K. West, Y. Liu, S. Denev PRL 92, 117405 (2004). [5] L.S. Levitov, B.D. Simons, L.V. Butov, cond-mat/0403377. [Preview Abstract] |
Wednesday, March 23, 2005 2:03PM - 2:15PM |
P19.00015: Spin relaxation of stress-split orthoexcitons in cuprous oxide* Joon Jang, Jim Wolfe By applying Hertzian strain fields of $\sigma $ = 0 -- 1.1 kbar along (100) direction, we break the cubic crystal symmetry of the cuprite, causing the triply-degenerate orthoexciton to split into a singlet and doublet. The interconversion rate between the orthoexciton singlet and lower-lying doublet is measured at temperatures from 2 to 15 K by using sub-nanosecond time-resolved luminescence. Based on the experimentally observed stress and temperature dependence, we propose that this transition process occurs via TA-phonon scattering associated with a shear tensor field, in contrast to the vector field scattering mechanism for the ortho-para conversion. At high excitation levels, both singlet and doublet transients are well explained by a density-dependent Auger recombination process. *Supported by DOE Grant DEFG02-96ER45439 [Preview Abstract] |
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P19.00016: Theoretical study of electronic properties of Cu2O and Cu1.5Mn0.5O Hao Zhu, John Q. Xiao, Liqing Pan, Chongfei Fan Cu$_{2}$O has excellent optical properties and is believed to be a good host to achieve diluted magnetic semiconductors because doped Cu$_{2}$O is a p-type and direct wide bandgap semiconductor. We present electronic structure calculations of Cu$_{2}$O and Cu$_{1.5}$Mn$_{0.5}$O using density functional theory (DFT) within the generalized gradient approximation (GGA). In ground state of Cu$_{2}$O, the calculated lattice constant $a$ = 4.295A and bulk modulus $B_{o}$ = 109GPa are in excellent agreement with experimental values of $a$ = 4.267A and $B_{o}$ = 102GPa. The density of states and band structure show that Cu$_{2}$O has \textit{0.5eV} direct band gap at \textit{$\Gamma $} point. Very interestingly, Cu$_{1.5}$Mn$_{0.5}$O shows half-metal behaviors with \textit{0.6eV} band gap for spin up band at \textit{$\Gamma $} point. The magnetic moment for each manganese atom is about \textit{4$\mu $}$_{B}$, obtained from spin polarized calculation. [Preview Abstract] |
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