Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session U17: Optical Response and Spin-Orbit Coupling in Quantum Dots |
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Sponsoring Units: DCMP Chair: Tigran Shahbazyan, Jackson State University Room: LACC 404B |
Thursday, March 24, 2005 8:00AM - 8:12AM |
U17.00001: Properties of polar optical phonons in wurtzite quantum dots Vladimir A. Fonoberov, Alexander A. Balandin Wurtzite (WZ) ZnO and GaN quantum dots (QDs) have attracted significant attention as promising candidates for optoelectronic applications. To enable the interpretation of the optical response of such QDs, we derive an integral equation that defines both interface and confined polar optical phonon modes in WZ QDs of arbitrary shapes [1]. The analytical solution of the derived equation is found for spheroidal QDs [2]. While the frequency of the confined polar optical phonons in zincblende QDs is equal to that of the bulk crystal, the confined optical phonons in WZ QDs are shown to have a discrete spectrum of frequencies different from those of the bulk crystal. The obtained results have been proven useful for the accurate prediction of interface and confined optical phonon frequencies in the novel class of QDs. This research has been supported in part by the ONR Young Investigator Award to A.A.B. [1] V.A. Fonoberov and A.A. Balandin, Phys. Rev. B 70, in press (Dec. 15, 2004). [2] V.A. Fonoberov and A.A. Balandin, Phys. Stat. Solidi C 1, 2650 (2004). [Preview Abstract] |
Thursday, March 24, 2005 8:12AM - 8:24AM |
U17.00002: Hole-LO phonon interaction in InAs/GaAs quantum dots Vanessa Preisler, Sophie Hameau, Robson Ferreira, Louis-Anne de Vaulchier, Yves Guldner, Aristide Lema\^itre Various experimental and theoretical works demonstrate that electrons confined in quantum dots are strongly coupled to the longitudinal optical(LO) vibrations of the underlying semiconductor lattice \cite{hameau1999}. This leads to the formation of the so-called quantum dot polaron, which are the true excitations of a charged dot. The interaction between holes confined in quantum dots and LO phonons has not, until now, been explored. We present a study of holes confined in InAs/GaAs quantum dots doped with Be. The interaction between the holes in the dots and the LO phonons of the lattice is studied experimentally by spectroscopy in the FIR ($50-700\,\textrm{cm}^{-1}$) energy range and under the influence of a magnetic field (0-15T). We observe several resonances in magneto-transmission around $200\,\textrm{cm}^{-1}$. In order to interpret our experimental results, we calculate the coupling between the hole-phonon states, using the Fr\"ohlich Hamiltonian. The resulting polaron states we find are in good agreement with our experimental results. \begin{thebibliography}{99} \bibitem{hameau1999} S. Hameau {\em et al.}, Phys. Rev. Lett. {\bf 83}, 4152 (1999); S. Hameau {\em et al.}, Phys. Rev. B{\bf 65}, 85316 (2002). \end{thebibliography} [Preview Abstract] |
Thursday, March 24, 2005 8:24AM - 8:36AM |
U17.00003: Aharonov-Bohm Beats in Excitonic Luminescence from Quantum Rings and Type-II Quantum Dots Luis Dias da Silva, Sergio Ulloa, Tigran Shahbazyan We study the absorption spectrum of neutral magnetoexcitons confined in ring-like structures. Despite their neutral character, excitons exhibit strong modulation effects on the energy and oscillator strength in the presence of magnetic fields [1] that have been recently observed [2]. We calculate the absorption coefficient $\alpha$ for neutral excitons confined in circular ring geometries with radii $R_e$ for electrons and $R_h$ for holes. A particularly interesting situation comes about when $R_e \neq R_h$ and a net radial charge polarization arises. In this case, we consider an attractive Coulomb interaction proportional to $(R_e - R_h)^{-1}$ and the excitonic absorption peak shows oscillatory behavior as function of the applied magnetic field both in position and amplitude. Such oscillations strongly depend on the dipole moment $P=e(R_h-R_e)$ of the exciton and on the dielectric constant of the system. Such intensity changes could in principle be experimentally observed with single dot spectroscopy in quantum rings [3]. Supported by the NSF-IMC and NSF-RUI \newline [1] A.O. Govorov et al. Phys. Rev. B 66 081309 (2002); A.O. Govorov et al. Physica E 13, 297 (2002). \newline [2] E. Ribeiro et al. Phys Rev. Lett. 92 126402 (2004). \newline [3] R.J. Warburton et al. Nature 405 (6789) 926 (2000). [Preview Abstract] |
Thursday, March 24, 2005 8:36AM - 8:48AM |
U17.00004: The Aharonov-Bohm effect in self-assembled InGaAs/GaAs quantum rings V.M. Fomin, V.N. Gladilin, J.T. Devreese, P. Offermans, P.M. Koenraad, J.H. Wolter, D. Granados, J.M. Garc\'Ia Based on the structural information from X-STM measurements on buried self-assembled InGaAs/GaAs quantum rings, we calculate the electron energy spectra and the magnetization as a function of the applied magnetic field. Since the lateral size of quantum rings substantially exceeds their height, the lateral electron motion is governed by the adiabatic potential related to the fast electron motion along the growth axis. The electron states are calculated by diagonalizing the adiabatic Hamiltonian. The oscillations of the electron orbital magnetic moment versus magnetic field are analysed as a function of angular modulations of the height, width and slopes of the rim as well as the chemical composition of a quantum-ring structure. Although the realistic quantum-ring shape differs strongly from an idealized circular-symmetric open-ring structure, Aharonov-Bohm oscillations survive. [Preview Abstract] |
Thursday, March 24, 2005 8:48AM - 9:00AM |
U17.00005: AC-Stark effect in a semiconductor self-assembled quantum lens Arezky H. Rodr\'iguez, L. Meza-Montes, C. Trallero-Giner, S.E. Ulloa We present a theoretical study of the effects of an ac electric field applied along the direction of axial symmetry of quantum dots with lens shape. This geometry has been found to realistically describe semiconductor quantum dots grown by self-assembly [1]. Using the Floquet formalism, the time-dependent Hamiltonian in the effective mass approximation is solved. A conformal analytical image is designed to map the quantum dot boundary into a dot with semi- spherical shape, allowing one to obtain a complete set of orthonormal functions to characterize the physical problem, while keeping the full lens symmetry. The Hamiltonian for a carrier confined in the quantum lens is correspondingly mapped into an equivalent operator and the corresponding Dirichlet problem is analyzed. We show that the Hilbert space of solutions is separated into orthogonal subspaces with different z- component of angular momentum. We give an explicit analytical representation for the quasi-energy spectrum and electronic states as function of the lens parameters and electric field intensity. [1] M. Muñoz et al., Appl. Phys. Lett. {\bf 83} 4399 (2003). [Preview Abstract] |
Thursday, March 24, 2005 9:00AM - 9:12AM |
U17.00006: SHG from bulk and surface of nanoparticle composites W. Luis Mochan, Bernardo S. Mendoza Three wave mixing processes such as second harmonic generation (SHG) have proven to be a sensitive probe of buried interfaces. Recently SHG has been employed to monitor the surface of Si nanocrystals within a glass matrix. Due to the macroscopic homogeneity and centrosymmetry of the composite, its bulk SHG signal is produced by the inhomogeneities of the fundamental light beam, and can be enhanced several orders of magnitude by employing two crossed beams [1]. On the other hand, the surface contribution to the SHG of the composite is due to the relatively large inhomogeneities of the surface local field which act on the nanoparticles, and is therefore insensitive to the beam profile and is not enhanced in a two beam geometry. We calculate the SHG from a thin nanocomposite material and compare the relative strength of its surface and bulk contributions when illuminated with one and for two fundamental beams. We employ our results to analyze recent experiments in which the large contrast between the signal produced by the composite and the matrix in the one beam geometry was lost in the two beam geometry [1].\\{} [1]Figliozzi et al., submitted to Phys. Rev. Lett. [Preview Abstract] |
Thursday, March 24, 2005 9:12AM - 9:24AM |
U17.00007: Measurement of the separation dependence of the resonant energy transfer between CdSe nanocrystals Farbod Shafiei, Ricardo S. Decca We have developed an apparatus to study the separation dependence of the interaction between quantum dots (QD). Our measurement scheme is based on depositing isolated QDs on the flat surface of a solid immersion lens (SIL). The photoluminescence (PL) of these dots (around 615 nm) is collected by the SIL and spectroscopically analyzed. The most novel part of our work resides in exciting these QDs by means of resonant energy transfer from smaller ones (emission at 590 nm). The smaller QDs cover the apex of an aperture probe near-field scanning optical microscope, after dipping it on a colloidal suspension. The combination of spectral and positional filtering allows us to measure the interaction between only one of the smaller dots and one of the larger dots at a time. From the analysis of the PL signal as a function of $z $(separation between two QDs), we expect to obtain what part of the energy transfer is dipole induced (F\"{o}rster interaction), and what part is associated with higher order terms (dipole-quadrupole and quadrupole-quadrupole interactions). Results on the progress of resonant energy transfer will be shown. These results improve our knowledge of the QD's wave function and understanding of decoherence phenomena in QDs [Preview Abstract] |
Thursday, March 24, 2005 9:24AM - 9:36AM |
U17.00008: Relaxation dynamics of excitons in a bimodal distribution of CdSe/ZnSSe quantum dots Pradeep Bajracharya, Hans Peter Wagner, Tuan Nguyen, Sebastian Mackowski, Leigh M. Smith, U.W. Pohl , Dieter Bimberg , Martin Strassburg We have studied the dynamical behavior of excitons in a bimodal distribution of CdSe/ZnSSe quantum dots by intensity dependent, temperature dependent and time resolved PL. The effect of exciton localization is investigated and described, both theoretically and experimentally by identifying transfer mechanisms due to thermalization and redistribution of excitons. We observe a dominant exciton emission from smaller dots (QDs1) and weaker emission from wider dots (QDs2) at 10 K and at low optical excitation. At high excitation densities a CdSe precursor state becomes visible at the high energy side of the QDs1 emission. Temperature dependent PL studies reveal a thermally activated exciton transfer between the different types of QDs resulting in a dominant QDs2 emission above 60 K. Time resolved PL measurements allow to estimate the characteristic radiative and non-radiative decay rates as well as the transfer rate of excitons between different QDs. The experimentally observed PL is successfully reproduced by a coupled rate equation model. [Preview Abstract] |
Thursday, March 24, 2005 9:36AM - 9:48AM |
U17.00009: Multi-Configuration Time-Dependent Hartree-Fock theory applied to quantum dots Carlos Destefani, Alexander Pegarkov, Chris McDonald, Thomas Brabec, Pawel Hawrylak The Multi-Configuration Time-Dependent Hartree-Fock method (MCTDHF) is a tool that can be used to study the electron dynamics of several multielectronic systems under strong laser fields. It is based on a variational principle applied to the time-dependent Schrodinger equation, where the many-particle ansatz wavefunction is a sum over configurations, with both coefficients and single-particle functions being time-dependent and optimized at each time step. In this way, electron correlation is taken into account and a faster convergence within a smaller expansion is obtained when compared to other mean-field theories. This method yields two sets of coupled non-linear differential equations to be solved by a certain integration scheme. These so-called working equations are propagated from a given initial state, yielding the wavefunction under the influence of the external field and allowing the study of the multielectronic dynamics under the laser pulse. As a first application of the MCTDHF theory, we analyze the multielectronic dynamics of isolated and coupled harmonic gated quantum dots driven by an intense laser field. [Preview Abstract] |
Thursday, March 24, 2005 9:48AM - 10:00AM |
U17.00010: The universal Hamiltonian of chaotic quantum dots in the presence of spin-orbit interaction Y. Alhassid, Daniel Huertas-Hernando, T. Bebis, T. Rupp In a chaotic dot with a large number of electrons only a few interaction terms survive and constitute the interacting part of the so-called universal Hamiltonian. The universal Hamiltonian was derived in the absence of spin-orbit scattering. The presence of spin-orbit scattering in chaotic (or diffusive) quantum dots was shown to introduce new symmetry limits of the single-particle Hamiltonian [1]. We derive the universal Hamiltonian (i.e., including interaction terms) in the presence of spin-orbit scattering in these new symmetry limits, both in the absence and presence of an orbital magnetic field. We also derive closed expressions for the finite temperature conductance through such dots and use them to study the conductance peak height and peak spacing statistics. We identify interesting statistical signatures of the interplay between spin-orbit and exchange interactions. This work has been supported in part by the Department of Energy grant No. DE-FG-0291-ER-40608. [1] I.L. Aleiner and V.I. Fal'ko, Phys. Rev. Lett. {\bf 87}, 256801 (2001). [Preview Abstract] |
Thursday, March 24, 2005 10:00AM - 10:12AM |
U17.00011: g-tensor evaluation in self-assembled quantum dots F.G.G. Hernandez, T.P. Mayer Alegre, G. Medeiros-Ribeiro In solid state, the first term to be considered in the effective spin Hamiltonian is that representing the electronic Zeeman interaction. In a doublet state with $S=1/2$, the two levels will diverge linearly with the magnetic field ($B$), with slopes $\pm 1/2g\beta B$. In practice, the Zeeman interaction not depends only on the angle between the effective spin vector (\overrightarrow {S}) and $\overrightarrow{B}$ but depends also on the angle that $\overrightarrow{B}$ makes with certain axes defined by the sample symmetry. Taking into account this kind of anisotropy, the effective spin Hamiltonian is $\beta(\overrightarrow{B}\cdot\makebox[0.1cm][l]{\raisebox{1ex} {$\leftrightarrow$}} g\cdot\overrightarrow{S}$), where $\makebox[0.1cm][l]{\raisebox{1ex}{$\leftrightarrow$}}g $ is the g-tensor. Since electrons can be individually trapped into quantum dots (QDs) in a controllable manner, they may represent a good candidate for the successfully implementation of spintronics into semiconductor heterostructures. In this work we realized magneto-capacitance spectroscopy (CV) in order to obtain the localization energies and the evolution of the Zeeman splitting for the s and p electron confined levels in InAs self-assembled quantum dots (SAQDs). The CV experiments were performed at 2K using lock- in amplifiers at a frequency of 7.5KHz. An AC amplitude of 10 mV was superimposed on a varying DC bias ranging from -2 V to 0.5 V with a signal/noise ratio above $10^{4}$. Aligning $\overrightarrow{B}$ with different crystallographic directions, we measured the g-tensor showing the existence of a high anisotropy degree. The g-factor values obtained ranges between 1.9 and 0.7, with $\overrightarrow{B}\parallel001$ and $\overrightarrow{B}\parallel110$ respectively. [Preview Abstract] |
Thursday, March 24, 2005 10:12AM - 10:24AM |
U17.00012: Engineering the g-factor in coupled quantum dots L. Meza-Montes, C. Destefani, Sergio E. Ulloa For applications in spintronics, it is fundamental to control the electron spin within the nanostructures. In this work we propose to use the spin-orbit effect to modulate the g-factor of one electron in coupled quantum dots. Changes in the orbital motion, as obtained by modifying the geometry of the dots, allow to modulate the spin components. The band structure is determined by expanding the wave function in a basis obtained by the Finite Element Method. We will discussed the interplay among the magnetic field and the Rashba and Dresselhaus terms included in the Hamiltonian. We have found that the spin component along the magnetic field can show very strong effects when the coupling between dots is changed or asymmetric dots are used. Results for different semiconductors and the role of Coulomb interaction will also be presented. [Preview Abstract] |
Thursday, March 24, 2005 10:24AM - 10:36AM |
U17.00013: Tuning the g-factor in self assemble quantum dots T. P. Mayer Alegre, F. G. G. Hernandez, G. Medeiros-Ribeiro The knowledge of electron and hole {\it g}-factors, their control and engineering are key for usage of spin degree of freedom for information processing in solid state systems. The electronic {\it g}-factor will be materials dependent, the effect being larger for materials with large spin-orbit coupling. Since electrons can be individually trapped into quantum dots in a controllable manner, they may represent a good platform for the implementation of quantum information processing devices. In this work we explore the effect of a stress on the {\it g}-factor for the electrons trapped in Self- Assembled Quantum Dots (SAQD) in two different samples. The experiments consist on a magneto-capacitance spectroscopy (CV) performed at low temperature (2K) where the direction of magnetic field, as well as the intensity, can be changed. We demonstrated that {\it g}-factor can be increased by as much as fifteen percent. Finally it is also shown that one can achieve the {\it g}-factor assessment, and engineering in SAQDs in a controllable manner. [Preview Abstract] |
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U17.00014: Interband magneto-optical transitions between bound and delocalized states in quantum dots Vanessa Preisler, Francisco Teran, Louis-Anne de Vaulchier, Robson Ferreira, Yves Guldner, Aristide Lemaitre Semiconductor quantum dots are frequently described as artificial atoms. This description implies that the quantum states and exciton transitions are well isolated from their environment. However, since the dots are embedded in a semiconductor material and furthermore sit on top of an underlying wetting layer quantum well, there exists transitions involving the quantum dot states and the delocalized states in the dot's surrounding environment.\footnote{A. Vasanelli {\em et al.}, Phys. Rev. Lett. {\bf 89},216804 (2002).} We present the results of low temperature (4K) interband photoluminescence excitation measurements on an ensemble of InAs/GaAs dots under strong magnetic field (up to 28T) applied along the growth axis. Along with the expected p-p excited state transitions, we observe several other pics in the PLE spectrums. We attribute these pics to crossed transitions between the wetting layer and the discrete states of the quantum dot. A theoretical model is proposed to describe the above transitions. [Preview Abstract] |
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