Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session U1: Isakson Prize, Adler Award, Nicholson Medal Session |
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Sponsoring Units: DCMP DMP Chair: David Pine, New York University Room: Morial Convention Center LaLouisiane AB |
Thursday, March 13, 2008 8:00AM - 8:36AM |
U1.00001: TBD Invited Speaker: |
Thursday, March 13, 2008 8:36AM - 9:12AM |
U1.00002: Frank Isakson Prize Talk: Using ultrafast to probe the slow Invited Speaker: The field of ultrafast optics exploded on the scene with the development of mode-locked lasers, and continues to grow as technology evolves. Although often associated with highly-nonequilibrium phenomena, ultrafast experiments can be performed in a low-power regime in which electronic systems are tickled, rather than blasted. The amplitude and phase stability of today's laser oscillators allows detection of very small changes in optical response that result from weak laser excitation. While these changes can be viewed as a form of linear response, they often reveal properties that are not detected by traditional probes such as electrical conductivity or magnetic susceptibility. In this talk I will describe two examples of this approach, in the fields of high-Tc superconductivity and spin propagation in semiconductors. Somewhat paradoxically, the use of ultrafast techniques allows the observation of some rather slow effects. In the high-Tc materials, the lifetime of the photoexcited state diverges as the optical energy per laser pulse is lowered. The slow dynamics in this regime provide a window to the intrinsic inelastic scattering rate of quasiparticles, a new collective mode, and an abrupt transition in dynamics that takes place as a function of doping. In GaAs quantum wells the stability of the laser oscillator enables phase-sensitive detection of a transient spin-polarization wave generated by the interference of two excitation pulses. Measuring dynamics as function of wave vector fully characterizes the spin propagation, revealing effects such as ballistic to diffusive crossover and spin Coulomb drag. In these systems we again have focused on ``slow'' phenomena. I will describe some of our recent attempts to create and detect a long-lived, ``persistent spin-helix'' state, predicted to occur at special points in the spin-orbit coupling parameter space. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:48AM |
U1.00003: Frank Isakson Prize Talk: Optical Probes of $\pi $-Conjugated Polymers Invited Speaker: We review several \textit{optical probes} that have been applied to $\pi $-conjugated polymers over a time period of $\sim $ 30 years. These include linear and nonlinear optical spectroscopies, resonant Raman scattering, transient and steady state photomodulation, photoluminescence and laser action, and optically detected magnetic resonance spectroscopy. The application of these techniques has revealed a myriad of important information on the interaction that govern the optical, electrical and magnetic properties of these materials; including electron-phonon interaction, electron-electron (e-e) and electron-hole (e-h) interactions, interchain coupling, spin-lattice and spin-orbit coupling. These properties are very important for various \textit{optoelectronic applications}, in which the polymers serve as active layers. The following picture of the excited state properties of these polymers has emerged. (i) The e-e and e-h interactions are substantial, and as important as the electron phonon interaction. This leads to relatively large intrachain exciton binding energy of $\sim $0.5 eV, and exchange energy between the singlet and triplet lowest states of $\sim $0.7 eV. (ii) There are few important excited states with odd and even parity symmetry that govern the nonlinear optical spectra of these materials. (iii) The primary photoexcitations are intrachain excitons in isolated chains, and both excitons and polaron pairs in chains coupled by interchain interaction. (iv) The most strongly coupled phonons are amplitude modes of which frequencies and oscillator strengths are very sensitive to the existence of excess charges on the chains. (v) Excess charges are accommodated on the chains in the form of polarons with relatively large relaxation energy ranging from 0.1 to 0.5 eV. (vi) The spin orbit coupling is very weak in these materials, but can be tuned by involving heavy atoms in the polymer building blocks. (vii) The spin relaxation time for spin $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ polarons is relatively long of $\sim $ 1 microsecond. The two latter properties may lead to new applications in the field of Organic Spintronics. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:24AM |
U1.00004: David Adler Lectureship Award Talk: Lattice instabilities and ferroelectricity in complex oxides Invited Speaker: In perovskite oxides, layered perovskites and other complex-structured oxide families, a wide variety of distorted equilibrium structures can be realized, including ferroelectric, antiferroelectric, antiferrodistortive, and mixed-character structures. For an individual material, the equilibrium structure can be understood as being produced by the freezing-in of one or more lattice instabilities of an appropriate high-symmetry reference structure; this concept is central to the long-established soft-mode theory of ferroelectricity. In fact, first-principles phonon-dispersion calculations show that the high-symmetry reference structures of many complex oxides have entire ranges of instabilities that do not contribute to the bulk ground state structure. In this talk, we discuss how the information from first-principles studies of these systems provides guidance for altering the balance of the competition of instabilities of different character through changes in electrical and mechanical boundary conditions characteristic of epitaxial thin films, superlattices, and nanoparticles, leading to the realization of non-bulk phases. For example, it has been shown both theoretically and experimentally that SrTiO$_3$, which has a nonpolar bulk ground state, can be driven ferroelectric by epitaxial strain. To illustrate the further development and application of these ideas, we present results for CaTiO$_3$ and discuss other materials which, while nonpolar in bulk, can be driven in this way through a phase boundary to become ferroelectric. New ferroelectrics thus obtained could have combinations of tunable properties, including switchable polarization, magnetic ordering, and dielectric and piezoelectric response, desirable for current and future technological applications. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 11:00AM |
U1.00005: Nicholson Medal Talk: Simulational Physics in a Shrinking World Invited Speaker: Over the past few decades simulational physics has been elevated to a major new methodology of quantitative science, {\it en par} with theory and experiment. Once, only researchers in the developed world with access to powerful supercomputers were able to carry out state-of-the-art simulational studies; but with the advent of fast, low cost microprocessors, simulational research can now be performed anywhere in the world. In 1986 the Center for Simulational Physics was founded at the University of Georgia, and the subsequent introduction of an annual Workshop has brought together countless individuals from different countries in a highly interactive atmosphere. (2008 marks the 21st such Workshop.) This ongoing Workshop series has brought together countless ``students'' who have gone on to teach other students and also to interact with each other. Similar Workshop series have now been created in Brazil and in China. This talk will review other aspects of the internationalization of simulations research and instruction and will emphasize the importance of human outreach activities at all different levels. [Preview Abstract] |
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