Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session N32: Casimir Forces, Precision Measurements, and Fundamental AMO Interactions |
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Sponsoring Units: DAMOP Chair: Tim Gay, University of Nebraska Room: Colorado Convention Center 402 |
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N32.00001: An Electron EDM Search Using Trapped Molecular Ions Laura Sinclair, John Bohn, Aaron Leanhardt, Edmund Meyer, Russell Stutz, Eric Cornell A sample of trapped molecular ions offers unique possibilities to search for a permanent electron electric dipole moment (EDM). Specifically, we plan to perform this search using the unpaired electron spins in the $^3\Delta_1$ state of trapped HfF$^+$ molecular ions. Ions are easy to trap which will provide the long coherence times necessary to measure the small energy differences associated with an electron EDM. Additionally, the internal electric fields in polarized diatomic molecules can exceed $10^{10}$~V/cm, which will amplify any EDM induced energy splittings. We have created HfF$^+$ ions in a supersonic expansion jet by ablating a Hf target with a pulsed Nd:YAG laser in a He + 1\%SF$_6$ environment. The chemical reaction $\rm{Hf}^+ + \rm{SF}_6 \longrightarrow \rm{HfF}^+ + \rm{SF}_5$ is exothermic and proceeds rapidly. The He buffer gas in the expansion cools the molecular translational, vibrational, and rotational degrees of freedom to $\sim 10$~K. We have measured these temperatures via laser induced fluorescence spectroscopy on known neutral Hf atomic lines and newly identified neutral HfF molecular lines, and are currently searching for the unknown HfF$^+$ electronic transitions. [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N32.00002: Testing local position invariance with four Cesium primary frequency standards and four NIST Hydrogen masers Neil Ashby, Thomas Heavner, Steven Jefferts, Thomas Parker In General Relativity, Local Position Invariance (LPI) implies that if atomic clocks of different structure are placed together and syntonized at a particular location, they will remain syntonized while they move through a variable gravitational potential. In this work we compare four active Hydrogen masers located at the National Institute of Standards and Technology (NIST) with Cesium fountain primary frequency standards at NIST, Physikalische-Technische Bundesanstalt (PTB, Germany), Bureau National de M\'etrologie Syst\`emes de R\'ef\'erence Temps Espace (BNM-SYRTE, France) and Istitute Nazionale di Ricerca Metrologica (INRM, ITALY). For the NIST fountain, comparisons have been conducted for six years, while comparisons with fountains at PTB, BNM-SYRTE, and INRM have been reliably conducted for almost three years. During this time the sun's gravitational potential $\Phi$ changes due to earth's orbital eccentricity $e$, with an amplitude given by $\Delta \Phi/c^2 \approx GM_{\odot}e/(a c^2) \approx 1.66 \times 10^{-10}$, where $a$ is the earth's orbital semimajor axis. The Cs-H maser comparisons show no correlation with variations in the solar potential, within an uncertainty that is about 30 times smaller than the previous most sensitive comparisons. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N32.00003: Absolute optical frequency measurements of Cs two-photon transitions with a femtosecond frequency comb Vela L. Mbele, Jason E. Stalnaker, Vladislav Gerginov, Tara Fortier, Carol E. Tanner, Scott A. Diddams, Leo Hollberg We study by direct excitation with a mode-locked femtosecond optical frequency comb, multiple transitions in Cs atoms in a vapor cell at room temperature. We improve by up to two orders of magnitude the uncertainties in the absolute optical frequency and hyperfine structure of the $6s$ $^2 {\rm S}_{1/2} \rightarrow 8s ^2 {\rm S}_{1/2},9s ^2 {\rm S}_{1/2}$, and $7d ^2 {\rm D}_{3/2,5/2}$ transitions in $^{133}{\rm Cs}$. Cesium is one of the well studied heavy atoms, with atomic structure calculations on the order of 1\%, and has provided a fertile testbed for fundamental tests of atomic theory and QED. This work reports on a simple and novel experimental approach that allows simultaneous recording of multiple transition frequencies. Atoms in a vapor cell at room temperature have a broad Doppler velocity distribution which allow selective excitation by discrete modes of a mode-locked femtosecond comb. This, in turn, results in stepwise multiphoton resonant transitions in the atoms. We model the collected spectra using a standard 2$\gamma$ formula and use least square fitting routines to extract improved values of absolute optical frequencies and coupling constants. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N32.00004: Rb Magnetic Resonance Near Coated Glass Surfaces in an Inhomogeneous Field Kaifeng Zhao, M. Schaden, Z. Wu Evanescent waves are used to measure the rf magnetic resonance signal of Rb spin polarization near Pyrex glass surfaces coated with anti-relaxation coatings in an inhomogeneous magnetic field. The signal shows an asymmetric line shape, with one side having approximately Lorentzian profile and the other side being inhomogeneously broadened. The origin of this asymmetry is due to the diffusion of spins. We studied its dependence on buffer gas pressure, cell thickness, field gradient and rf amplitude modulation rate. A theoretical model is developed to understand this line shape. Interesting characteristics of atom- surface interaction, such as dwell time, collision relaxation rate and de-phasing on the surface, can be estimated by fitting the measured line shape with the calculated one. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N32.00005: Nuclear spin relaxation of $^{129}$Xe due to persistent xenon dimers B. Saam, B.N. Berry-Pusey, B.C. Anger, G. Laicher An understanding of longitudinal relaxation mechanisms (characterized by the time $T_1$) that limit both achievable polarization and sample storage time is critically important to applications of hyperpolarized noble gases. We have measured $T_1$ for $^{129}$Xe in Xe-N$_2$ mixtures at densities $< 0.5 $~amagats in a magnetic field of 8.0 T. The intrinsic relaxation in this regime is due to fluctuations in the intramolecular spin- rotation (SR) and chemical-shift-anisotropy (CSA) interactions, mediated by the formation of $^{129}$Xe-Xe persistent dimers. Our results$^*$ are consistent with previous work done in one case at much lower applied fields where the CSA interaction is negligible and in another case at much higher gas densities where transient Xe dimers mediate the interactions. The 8.0-T field suppresses the persistent-dimer mechanism: we have measured $T_1 > 25$~h at 8.0~T for $^{129}$Xe at room temperature. These data also yield a maximum possible low-field $T_1$ for pure xenon gas at room temperature of $5.45\pm 0.2 $~h.\\ {\rm $^*$B.N.\ Berry-Pusey, {\it et al.}, Phys.\ Rev.\ A {\bf 74}, no.\ 6 (in press).} [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N32.00006: Geometric Weakening of the Casimir Interaction Liviu Mateescu, Martin Schaden We examine the dependence of the Casimir interaction between separate (metallic) bodies on their geometry. From a semi-classical point of view it depends strongly on whether the dominant periodic orbits are stable or unstable and on the number of focal points. We give a very simple semiclassical argument for the theorem [1] that mirror-symmetric periodically corrugated metallic surfaces always attract. Although counter-intuitive because the Van DerWaals interactions between individual pairs of atoms are attractive at long range, we argue that this need not be the case for multi-atom interactions. Semi-classical methods are used to determine the shape of surfaces with minimal Casimir interaction. [1] O.Kenneth, I. Klich, Phys.Rev.Lett. 97, 160401 (2006)~ [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N32.00007: Casimir force measurements between a sphere and a surface with high-aspect ratio, nanoscale channel arrays Yiliang Bao, Ho Bun Chan The Casimir force is a quantum effect that strongly depends on the shape of the boundaries that confines the electromagnetic fields. So far the majority of experiments have concentrated on the simple arrangement of plate-sphere or two parallel plates. Demonstrating the strong shape dependence of the Casimir force would require other geometries with interactions that deviate significantly from the pair-wise summation of two-body potentials. Here we present measurements of the Casimir force between a gold-coated sphere and a silicon plate with an array of nanoscale, high-aspect-ratio rectangular trenches. A micromachined torsional oscillator acts as the force transducer which allows us to measure the interactions between the surfaces at high sensitivity. Channels with widths ranging from 200 nm to 500 nm and depth of 1 um are fabricated on a silicon substrate. We will compare the Casimir interaction between the sphere and these trench arrays with different aspect ratios. Such measurements might open up new possibilities to manipulate the Casimir force by tailoring the shape of the interacting surfaces. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N32.00008: Measurements of the Casimir force in fluids Jeremy Munday, Federico Capasso Confinement of the quantum fluctuations of electromagnetic fields between two grounded, conducting surfaces gives rise to an attractive force first predicted by H. B. G. Casimir. During the past decade, there have been many experimental demonstrations of this force between two metal surfaces in vacuum. While high precision experiments have been performed for this case, few experiments have been done between metallized or dielectric objects in fluids. For this situation, a more general formalism was developed by Lifshitz. If materials are chosen with suitable dielectric response functions, repulsive quantum electrodynamical (QED) forces can also arise. We will discuss experimental results using an atomic force microscope (AFM) to measure the interaction force between a metallized sphere and a plate, made of either metal or dielectric, in fluid. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N32.00009: Casimir force measurements between metal and high-$T_c$ superconductor surfaces Mark B. Romanowsky, Jeremy N. Munday, Richard Schalek, Federico Capasso, Qiang Li, Genda Gu It is well known that the strength of the Casimir force between two objects is controlled by the dielectric properties (or optical conductivity) of the objects. Nearly all precision measurements of Casimir forces to date are between two metals. Here we report measurements of the Casimir force between a metal-coated sphere and a plate made of the high-$T_c$ cuprate superconductor BSCCO-2212, using an atomic force microscope at room temperature. BSCCO has dielectric properties substantially different from metals and indeed most materials, displaying extreme anisotropy in dc and optical conductivity, as well as a ``strange metal'' normal state. The force between metal and BSCCO is compared to the force measured between two metals. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N32.00010: The Casimir force on transparent conductors Limor Spector, Jeremy Munday, Federico Capasso, Nicholas Geisse, Kevin Kit Parker The Casimir force arises from quantum fluctuations of electromagnetic fields in vacuum and is dependent on the dielectric properties of the interacting materials. This force can have a profound impact on the functionality of systems operating on the micro- and nanoscale. As nanotechnology continues to evolve, the ever-present Casimir force will have to be carefully considered during the design stage. Eliminating or greatly reducing this force could be of tremendous importance. To this end, we have performed Casimir force measurements using atomic force microscopy (AFM) between metals (gold and palladium) and transparent conductors (e.g. indium tin oxide). Due to the transparence of these materials, it is expected that the electromagnetic modes will be less well confined, and the Casimir force will be reduced. Experimental results of such studies will be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N32.00011: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N32.00012: Adiabatic change assisted Rabi transitions between Adiabatic change assisted Rabi transitions between decoupled quantum states Xingxiang Zhou, Ari Mizel A periodic perturbation such as a laser field cannot induce transitions between two decoupled states because the transition matrix element vanishes. However, if in addition some system parameters are varied adiabatically, such transitions are possible via the adiabatic change induced excitations to other states. We study such transitions between two decoupled states and show that full amplitude transfer can be achieved. The resulting physics can be understood in terms of the rotation of an effective spin 1/2 in the two-state subspace, but with a rotation angle dependent on the path traversed by the system in the parameter space only. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N32.00013: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N32.00014: Direct Dissociative Recombination of NO$_2$ Daniel J. Haxton, Chris H. Greene We provide estimates for direct dissociative recombination (DR) rates for collisions of NO$_2^+$ + $e^-$ via the 2 $^2\Pi$ and $^2\Phi$ states of the neutral. No calculations or measurements of this rate exist in the literature, despite the fact that NO$_2$ is an important constituent of the atmosphere and DR of the cation may play a role in its atmospheric chemistry. Little is known about the potential energy surfaces of neutral NO$_2$ at energies near the ionization threshold. However, preliminary calculations suggest that the 2 $^2\Pi$ and $^2\Phi$ states may intersect the ground state potential energy curve of the neutral near its Franck-Condon region. R-matrix calculations are employed to obtain the widths of these states, and the direct DR rate is extracted by employing the multidimensional reflection principle along with the formalism of O'Malley. The calculated rates may help to elucidate whether the direct or indirect mechanism plays a larger role in DR of NO$_2^+$. This work was supported in part by the DOE Office of Science. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N32.00015: Neutralization/Ionization of Si Scattered from Adsorbate Sites Xiaojian Chen, Zdenek Sroubek, Jory Yarmoff In low energy ion scattering, ion-surface charge exchange strongly depends on the surface electronic structure and the ionization level of the projectile. Si has an ionization level that overlaps the center of the surface conduction band and is intermediate in energy to that of alkali ions and noble gas ions, which are the projectiles traditionally used. The scattering of Si thus provides new pathways for ion-surface charge exchange. A considerable fraction of the low energy Si$^{+}$ ions backscattered from submonolayers of Cs deposited onto Al(100) are found to be emitted as positive or negative ions. The negative ions result from simple resonant charge transfer (RCT) into the electron affinity level. The formation of Si$^{+}$, however, is in contrast to the expected complete neutralization due to the overlap with the surface bands. It is proposed that valence electron RCT enhanced by the interaction of the Si ionization level with the Cs 5p level is responsible for the ion formation. Positive ions were also produced in Si scattered from I adatoms on Al(100), presumably by a similar mechanism. The ion fractions are smaller than those for scattering from Cs, which suggests that electron tunneling from the occupied I chemisorption states provides an additional neutralization channel. [Preview Abstract] |
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