Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session C13: DMP Prize SessionInvited
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Sponsoring Units: DMP Chair: Michael E. Flatté, University of Iowa Room: 309 |
Monday, March 14, 2016 2:30PM - 3:06PM |
C13.00001: David Adler Lectureship Award in the Field of Materials Physics: Electrically Tunable Nanoantennas for Control of Absorption, Emission and Scattering Invited Speaker: Harry Atwater Progress in understanding resonant subwavelength structures has fueled an explosion of interest in both fundamental processes and nanophotonic devices for imaging, sensing, solar energy conversion and thermal radiation control.~~Achieving electronic tunability of the optical properties is also an emerging opportunity to bring nanoscale resonators and antennas to life as dynamic objects. While the optical properties of photonic and plasmonic nanostructures are typically fixed at the time of fabrication, gated field effect tuning of the carrier density in conducting oxides and two-dimensional materials enables the optical dispersion of individual structures to be altered from dielectric to plasmonic, yielding active nano-antenna arrays with electrically tunable absorption, radiative emission and scattering properties. [Preview Abstract] |
Monday, March 14, 2016 3:06PM - 3:42PM |
C13.00002: Frank Isakson Prize for Optical Effects in Solids: Optical spectroscopy and mechanisms of superconductivity. Invited Speaker: Dirk van der Marel By its very nature the phenomenon of superconductivity is intimately connected to the electrodynamics properties of a material, both in the normal and in the superconducting state. Optical spectroscopy and electrical transport -corresponding to the zero-frequency limit of the optical response- provide for this reason sensitive tools probing the collective response of a superconducting material. Optical spectroscopy can provide the real and imaginary parts of the optical conductivity of an electron liquid for all frequencies from radiowaves through infrared and visible up to the ultraviolet and even X-ray frequencies. Theory of the optical response is particularly well developed, leading among others to a number of sumrules, providing powerful tools for confronting experiment and theoretical models of superconducting pairing. In this talk examples of sumrules will be discussed relating to the kinetic energy and the Coulomb energy of the paired electrons, and experimental data of addressing these two energies will be presented. The basic understanding of pair formation in the conventional (i.e. BCS) model of superconductivity is, that electrons form pairs as a result of an attractive interaction. On general grounds one than expects the interaction energy to become reduced when the electrons form pairs, while at the same their kinetic energy increases. Superconductivity is a stable state of matter provided that all contributions together result in a lowering of the total (interaction, kinetic plus other terms if relevant) lowering of energy. In this talk I will demonstrate that these two effects can be observed in the cuprate superconductors, that behave according to aforementioned trends for strongly overdoped cuprates, but that the observed effects have the opposite sign for underdoped and optimally doped cuprates. These observations compare favorably with published numerical calculations based on models of strong electron-electron correlation, not involving the vibrations of the lattice, and where the electron-electron interaction is purely repulsive. [Preview Abstract] |
Monday, March 14, 2016 3:42PM - 4:18PM |
C13.00003: Frank Isakson Prize Talk: Superfluid and normal-fluid densities in the cuprate superconductors from infrared spectroscopy Invited Speaker: D.B. Tanner Measurements for a number of cuprate families of optical reflectance over a wide spectral range (far-infrared to ultraviolet) have been analyzed using Kramers-Kronig analysis to obtain the optical conductivity $\sigma(\omega)$ and (by integration of the real part of the conductivity) the spectral weight of low- and mid-energy excitations. For the Kramers-Kronig analysis to give reliable results, accurate high-frequency extrapolations, based on x-ray atomic scattering functions, were used. When the optical conductivities of the normal and superconducting states are compared, a transfer of spectral weight from finite frequencies to the zero-frequency delta-function conductivity of the superconductor is seen. The strength of this delta function gives the superfluid density, $\rho_s$. In a clean metallic superconductor the superfluid density is essentially equal to the conduction electron density. The cuprates in contrast have only about 20\% of the $a$$b$-plane low-energy spectral weight in the superfluid. The rest remains in finite-frequency, midinfrared absorption. In underdoped materials the superfluid fraction is even smaller. There are two ways to measure $\rho_s$, using either the partial sum rule for the conductivity or by examination of $\sigma_2(\omega)$. Comparison of these two estimates of the superfluid density shows that 98\% of the $a$$b$-plane superfluid density comes from energies below 0.15 eV. [Preview Abstract] |
Monday, March 14, 2016 4:18PM - 4:54PM |
C13.00004: Richard L. Greene Dissertation Award in Experimental Condensed Matter or Materials Physics Talk: Ferromagnetic quantum criticality in YbNi4P2 Invited Speaker: Alexander Steppke In a number of strongly correlated electron systems quantum phase transitions can be observed by the suppression of antiferromagnetic order. In contrast the prototypical continuous quantum phase transition of a metallic ferromagnet is often preempted by a first-order transition or a superconducting state. We show that the Kondo lattice system YbNi4P2 exhibits a ferromagnetically ordered phase with a very low Curie temperature of 0.15K. The compound can be tuned to a ferromagnetic quantum critical point by substitution of phosphorus by arsenic. With thermodynamic studies of specific heat, ac susceptibility and thermal expansion we show strong evidence for the ferromagnetic order and the quantum criticality in the YbNi4(P 1-x As x)2 doping series and the existence of a ferromagnetic quantum critical point at zero applied field for small substitutions. [Preview Abstract] |
Monday, March 14, 2016 4:54PM - 5:30PM |
C13.00005: Richard L. Greene Dissertation Award in Experimental Condensed Matter or Materials Physics Talk: Towards single atom magnets Invited Speaker: Susanne Baumann Magnetic anisotropy is a fundamental property of magnetic materials that governs the stability and directionality of their magnetization. At the atomic level, magnetic anisotropy originates from anisotropy in the orbital angular momentum (L) and the spin-orbit coupling that connects the spin moment of a magnetic atom to the spatial symmetry of its ligand field environment. Generally, the ligand field, that is necessary for the anisotropy, also quenches the orbital moment and reduces the total magnetic moment of the atom to its spin component. However, careful design of the coordination geometry of a single atom can restore the orbital moment while inducing uniaxial anisotropy, as we present here for single atoms deposited on top of a thin MgO film. Scanning tunneling spectroscopy and x-ray absorption spectroscopy measurements show a large magnetic anisotropy of 19 meV for iron and 58 meV for cobalt, as well as relaxation times of many milliseconds. These results offer a strategy, based on symmetry arguments and careful tailoring of the interaction with the environment, for the rational design of nanoscopic permanent magnets and single atom magnets. [Preview Abstract] |
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