Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session R36: Focus Session: Optical and X-ray Properties of Nanostructures |
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Sponsoring Units: DMP Chair: John Rehr, University of Washington Room: Baltimore Convention Center 339 |
Wednesday, March 15, 2006 2:30PM - 3:06PM |
R36.00001: Time-Dependent Density Functional Calculations for Optical Excitations in Nanoscale Materials. Invited Speaker: Optical properties play a crucial role in our understanding of the electronic structure of nanoscale materials. Despite significant progress in the development of computational methodologies, first principles calculations for optical excitations in nanomaterials remain extremely difficult. While the ground-state electronic structure formalism is relatively well established, calculations for excited states present major challenges to theoretical methods traditionally employed by condensed matter physics and quantum chemistry. This talk focuses on recent advances in a computational technique based on time-dependent density functional theory (TDDFT). Within the TDDFT method, electronic transition energies and oscillator strengths are obtained in the framework of linear response theory by solving a frequency-dependent equation for the dynamic polarizability. The linear-response TDDFT formalism is applied to compute the excitation energies and absorption spectra for a variety of nanoscale systems, including molecules, atomic clusters, semiconductor quantum dots, functionalized carbon nanotubes, and nanotube-polymer composites. The TDDFT approach is compared to other first principles computational methods for excited states and discussed in terms of accuracy, efficiency, and computational cost. [Preview Abstract] |
Wednesday, March 15, 2006 3:06PM - 3:18PM |
R36.00002: Quantum Monte Carlo Calculations of Excitations in Hydrogenated Germanium Clusters Jordan Vincent, Jeongnim Kim, Richard Martin Quantum Monte Carlo (QMC) calculations are presented for energies of ground and excited states of Ge atom and hydrogen passivated closed-shell molecules and clusters: GeH4, Ge2H6, Ge5H12, Ge10H16 and Ge29H36. We compare the results for excitations with previous QMC and time-dependant Density Functional Theory (TD- DFT) done for the corresponding Silicon clusters [1,2]; in particular; we find that preliminary results for lowest excitation enregy of Ge29H36 5.08[29]eV is lower than the gap ~5.4eV reported for Si[2]. Core-valence partitioning for Ge is implemented by replacing the core-states with a Hartree-Fock pseudopotential plus a Core Polarization Potential (CPP)[3]. Core-valence correlation treated by the CPP is shown to be essential for accurate atomic energies and significant for the molecules, but smaller in the clusters. [1] Porter et. al., PRB 64, 035320 (2001). [2] Williamson et. al., PRL 89, 196803 (2002). [3] Shirley and Martin, PRB 47, 15413 (1993) [Preview Abstract] |
Wednesday, March 15, 2006 3:18PM - 3:30PM |
R36.00003: \textit{Ab initio} X-Ray Absorption Fine Structure Cumulants F. Vila, J.J. Rehr, H.H. Rossner, H.J. Krappe Theoretical calculations of vibrational effects in x-ray absorption spectra typically employ semi-phenomenological models, e.g. empirical force constants or correlated Debye or Einstein models. Instead we introduce an efficient and generally applicable \textit{ab initio} approach based on electronic structure calculations of the dynamical matrix together with the Lanczos recursion algorithm [1] and relations between the cumulants. The approach yields 1) the thermal expansion coefficients (first cumulant of the vibrational distribution function); 2) correlated Debye-Waller factors (second cumulants) and 3) anharmonic contributions (third cumulants). Results are presented for crystalline (Cu, Au, Ge, GaAs) and molecular (GeCl$_4$, C$_6$H$_6$) systems. Our results for the Debye-Waller factors agree well with experiment. \\ \\ {[1]}H.J. Krappe and H.H. Rossner, Phys. Rev. B\textbf{70}, 104102 (2004). [Preview Abstract] |
Wednesday, March 15, 2006 3:30PM - 3:42PM |
R36.00004: Real Space Multiple-Scattering Calculations of Optical Properties M.P. Prange, J.J. Rehr, A.L. Ankudinov, J.A. Soininen We present a method for {\it ab initio} calculations of the dielectric function which is applicable for periodic and non-periodic materials alike. Our approach is a generalization to finite momentum transfer of that implemented in the real space multiple-scattering code FEFF8 and is the real-space analog of the KKR method. The approach includes self-consistent potentials, an energy dependent self-energy and screened core-hole effects. The method yields the dynamic structure factor $S(\vec q,\omega)$ over a broad spectrum from the optical to x-ray energies, and in the long-wavelength limit, yields various optical constants. Results for several materials are presented and compared with experiment. [Preview Abstract] |
Wednesday, March 15, 2006 3:42PM - 3:54PM |
R36.00005: \textit{Ab initio} Quasiparticle Self-Energies and X-ray spectra J. Kas, M. Prange, A. Sorini, J.J. Rehr Present calculations of inelastic losses in x-ray spectra typically employ semi-empirical or highly simplified models, such as the plasmon-pole self-energy, which are only semi-quantitative for near edge spectra. Here we present an efficient {\it ab initio} approach applicable to general materials, starting from a real-space multiple-scattering calculation of the dielectric function,\footnote{M. P. Prange, J. J. Rehr, A. L. Ankudinov and J. A. Soininen, APS March Meeting 2006 (unpublished).} which is fit to a multiple-pole model with of order 10$^2$ poles. This yields multiple-pole GW self-energies, and hence related quantities such as inelastic mean free paths (IMFP). The approach leads to improved amplitudes and phases for core-level x-ray spectra up to photo-electron energies of order 10$^3$ eV. Results for the IMFP are found to be in good agreement with experiment and with other approaches.\footnote{E. L. Shirley, J. A. Soininen and J.J. Rehr, APS March Meeting 2006 (unpublished).}$^{,}$\footnote{C. J. Powell and A. Jablonsky, J. Phys. Chem. Ref. Data {\bf28}, 19 (1999).} [Preview Abstract] |
Wednesday, March 15, 2006 3:54PM - 4:06PM |
R36.00006: Isolating symmetry components of the Mg and Al DOS using momentum-transfer dependent Inelastic X-ray Scattering Tim Fister, Gerald Seidler, John Rehr, Aleksi Soininen, Julie Cross, Albert Macrander We have used the non-resonant inelastic x-ray scattering (NRIXS) from core electrons to measure the dynamic structure factor of the Mg and Al L-edges for momentum transfers ($q$) ranging from 0.8 – 10.1{\AA}$^{-1}$. Changing $q$ alters the measured energy loss spectrum by allowing new, dipole-forbidden transitions to different symmetry final states. Here, we highlight the connection between dynamic structure factor and the projected components of the density of states (ldos) using a modified version of FEFF that incorporates multipole transitions. In addition to \textit{ab initio} theoretical agreement with the observed $q$-dependence, we present preliminary efforts toward obtaining an experimentally-derived ldos. [Preview Abstract] |
Wednesday, March 15, 2006 4:06PM - 4:18PM |
R36.00007: Metal-metal nanolayered structures for generation of hard x-ray radiation Peter Shkolnikov, Alexander Kaplan, Alexander Pokrovsky Our new research effort is aimed at developing a new X-ray source for medical applications, using low-energy electrons. Generation of intense narrow-band X-ray transition radiation (TR) by few-MeV electrons traversing solid multilayer structures, initially proposed for by us $\sim$ 1 keV photons, changes dramatically at energies of interest to medicine, 30-50 keV, because of significant changes in dielectric constants. In particular, the choice of the materials for the multilayer target proposed by us for soft X- ray generation, is no longer applicable. In particular, our original approach was based on choosing a layer of a high-Z material as a ``radiator'' with a chosen $K$-shell transition, and the layer of low-Z material as a neutral ``spacer.'' However, in the 30-50 keV range, TR spectra with such pairs show a spectral $dip$, not a peak,at the chosen $K$-shell. Our new theoretical investigations have shown that the optimal spacer now, as a rule, should have higher Z than the radiator. One example of such combination is Mo/Ag; we predict that its X-ray TR spectrum contains a strong, 1\% wide peak at the K-edge of Mo, $\sim$20 KeV. We present our recent theoretical results of the subject, as well as our experimental results for the Mo/Ag target. [Preview Abstract] |
Wednesday, March 15, 2006 4:18PM - 4:30PM |
R36.00008: Optical conductivity of granular metals Yen Lee Loh, Vikram Tripathi Arrays of metallic grains, which can be fabricated in a number of ways, exhibit interesting behavior in many properties due to the interplay between tunneling and Coulomb blockade effects. One such property is the AC conductivity, which can be measured using optical reflectivity techniques. We present here a calculation of the AC conductivity of a regular granular array. For this purpose we have found it necessary to generalize the Ambegaokar-Eckern-Sch{\"o}n (AES) model to include polarization fluctuations as well as charge fluctuations. In contrast to the DC conductivity, which is determined by inter-grain charge transfer and obeys an Arrhenius law at low temperature, we show that the AC conductivity is dominated by a resonance peak for intra-grain polarization oscillations, which has a power-law tail at low frequencies. Although the resonance frequency agrees with the classical prediction, the resonance width depends on quantum mechanical tunneling and Coulomb blockade parameters, in addition to intra-grain impurity scattering. This additional damping is due to inelastic cotunneling of polarization fluctuations to neighbouring grains. [Preview Abstract] |
Wednesday, March 15, 2006 4:30PM - 4:42PM |
R36.00009: Probing by transport the single-particle energy spectrum up to high energy of one quantum dot with the ground state of an adjacent weakly coupled quantum dot to examine the behavior of magnetic-field induced two- and three-level crossings David Austing, Goulin Yu, James Gupta, Marek Korkusinski, Geoffrey Aers We investigate high bias single electron resonant tunneling through sub-micron gated AlGaAs/InGaAs/AlGaAs/InGaAs/AlGaAs triple barrier structures for which the tunnel coupling energy between the two quantum dots is very weak ($<$0.1meV). The two quantum dot ``disks'' in the vertical diatomic artificial molecule located in the circular device mesa can be almost circular or elliptically deformed. In either case, assuming the lateral confining potential to be strictly parabolic, one would expect the single-particle states of one quantum dot to evolve with magnetic field in a very distinct and recognizable way, and that all energy level crossings are real crossings. We find, however, particularly for two quantum dots with an elliptical deformation ratio of about 4/3, numerous anti-crossings (levels split by up to about 1meV in energy) as well as crossings when two or three levels meet in the spectrum. We show the measured spectrum, attempt to classify the crossing and anti-crossing behavior, and offer possible explanations for this intriguing behavior. [Preview Abstract] |
Wednesday, March 15, 2006 4:42PM - 4:54PM |
R36.00010: Anomalous electronic transport features in a lateral quantum dot array sample Wei Pan, R.G. Dunn, J.L. Reno, J.A. Simmons, D. Li, S.R.J. Brueck We will present in this talk experimental results obtained in a lateral quantum dot array sample, with a pitch size of $\sim$ 350 nm and a designed dot size of $\sim$ 150nm. The starting material is a high quality quantum well with the two-dimensional electron gas buried 200 nm below the surface. The quantum dot array is defined by a Ti/Au metal grid, which was fabricated using the interferometric lithograph and lift-off techniques. Around zero magnetic field, a pronounced positive magnetoresistance is observed, which can be explained by the semi-classical model of magnetic breakdown. The so-called commensurability oscillations together with the usual Shubnikov- de Hass oscillations are also observed. Surprisingly, in a pure DC measurement of longitudinal resistance, an anomalous resistance spike is clearly seen. The magnetic field position of this resistance spike depends on the amplitude of applied DC bias ($V_{ds}$) between source and drain, and shows roughly a $1/V_{ds}$ dependence.\\ Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, March 15, 2006 4:54PM - 5:06PM |
R36.00011: Polarization and orientation effects on energy transfer in semiconductor nanocrystals. Ameenah N. Al-Ahmadi, Sergio E. Ulloa We study the effect of the orientation factor on coherent energy transfer from a donor to an acceptor quantum dot. We calculate the polarization of the acceptor dot as a function of incident light polarization in resonance with different donor levels. We demonstrate that the measurement of the acceptor polarization can be used to obtain information on the relative orientation between the donor-acceptor pair. We use the density matrix to study the dynamics of the luminescence polarization of the QDs in the Lindblad approximation [1]. We use a realistic model of the exciton levels and the oriented dipole transition of each level to predict the importance of the orientation factor on the energy transfer in semiconductor nanocrystals. We consider the band edge fine structure of the exciton in the QDs based on an effective mass description with eight exciton levels [2]. Our results show strong dependence of the polarization of the acceptor dot on the relative orientation between the donor-acceptor pair and the specific donor state under excitation. \\ ~~ \\ $[1]$ A.N. Al-Ahmadi and S.E. Ulloa, Phys.\ Rev.\ B 70, 201302(R) (2004).\\ $[2]$ Al.L. Efros and M. Rosen, Ann.\ Rev.\ Mater.\ Sci.\ 30, 475 (2000). [Preview Abstract] |
Wednesday, March 15, 2006 5:06PM - 5:18PM |
R36.00012: Energy relaxation and phonon bottleneck in semiconductor quantum dots Serguei Goupalov We show explicitly that the so-called phonon bottleneck in energy relaxation in semiconductor quantum dots arises as a limiting case of a more general model originally due to Huang and Rhys [S.V.Goupalov, Phys. Rev. B {\bf 72}, 073301 (2005)]. Therefore, the restrictions on the energy relaxation imposed by the phonon bottleneck are much less fundamental than it is commonly believed. We calculate temperature dependence of the non-radiative multiphonon transition rate within the properly corrected Huang-Rhys model and discuss other models where the phonon bottleneck is circumvented in connection with recent experiments on epitaxial and colloidal quantum dots. [Preview Abstract] |
Wednesday, March 15, 2006 5:18PM - 5:30PM |
R36.00013: Spectroscopic studies of Fe$_{3}$O$_{4}$ nanocrystals Layra Reza, Felicia Manciu, Yudhisthira Sahoo We have used IR spectroscopy to study the temperature dependence of optically active phonon modes of Fe$_{3}$O$_{4}$ nanocrystals, to obtain information about the possible Verwey transition, which is usually manifested in bulk material. The samples were synthesized by colloidal chemistry. The crystallinity and sizes were examined by transmission electron microscopy (TEM) and X-ray diffraction. The TEM pictures show an average size of 6-8 nm for Fe$_{3}$O$_{4}$ nanocrystals. Samples for the IR studies were prepared in the form of pellets, by embedding them in a polycrystalline CsI matrix. The temperature dependence (10K $<$ T $<$ 300K) IR transmission results exhibit a frequency increase upon decreasing the temperature for the infrared-active phonons around 360 cm$^{-1}$ and 574 cm$^{-1}$, but without notable anomalies around the critical temperature (120K) as compared to those previously observed in a single Fe$_{3}$O$_{4 }$crystal. The appearance of a fine structure at low temperatures could account for the lifting of the degeneracy of the phonon modes. This splitting is associated with the degree of the distortion of the symmetry of the system. [Preview Abstract] |
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