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Search Results: 1 - 10 of 464422 matches for " S. D. Hogan "
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Transmission-line decelerators for atoms in high Rydberg states
P. Lancuba,S. D. Hogan
Physics , 2014, DOI: 10.1103/PhysRevA.90.053420
Abstract: Beams of helium atoms in Rydberg states with principal quantum number $n=52$, and traveling with an initial speed of 1950 m/s, have been accelerated, decelerated and guided while confined in moving electric traps generated above a curved, surface-based electrical transmission line with a segmented center conductor. Experiments have been performed with atoms guided at constant speed, and with accelerations exceeding $10^7$ m/s$^2$. In each case the manipulated atoms were detected by spatially resolved, pulsed electric field ionization. The effects of tangential and centripetal accelerations on the effective trapping potentials experienced by the atoms in the decelerator have been studied, with the resulting observations highlighting contributions from the density of excited Rydberg atoms to the acceleration, deceleration and guiding efficiencies in the experiments.
Rydberg-Stark states in oscillating electric fields
V. Zhelyazkova,S. D. Hogan
Physics , 2015, DOI: 10.1080/00268976.2015.1098741
Abstract: Experimental and theoretical studies of the effects of weak radio-frequency electric fields on Rydberg-Stark states with electric dipole moments as large as 10000 D are reported. High-resolution laser spectroscopic studies of Rydberg states with principal quantum number $n=52$ and $53$ were performed in pulsed supersonic beams of metastable helium with the excited atoms detected by pulsed electric field ionisation. Experiments were carried out in the presence of sinusoidally oscillating electric fields with frequencies of 20~MHz, amplitudes of up to 120~mV/cm, and dc offsets of up to 4.4~V/cm. In weak fields the experimentally recorded spectra are in excellent agreement with the results of calculations carried out using Floquet methods to account for electric dipole couplings in the oscillating fields. This highlights the validity of these techniques for the accurate calculation of the Stark energy level structure in such fields, and the limitations of the calculations in stronger fields where $n-$mixing and higher-order contributions become important.
Probing interactions between Rydberg atoms with large electric dipole moments in amplitude modulated electric fields
V. Zhelyazkova,S. D. Hogan
Physics , 2015, DOI: 10.1103/PhysRevA.92.011402
Abstract: Dipole-dipole interactions between helium atoms in Rydberg-Stark states with principal quantum number $n=53$ and approximately linear Stark energy shifts, resulting from induced electric dipole moments of approximately 7900 D, have been investigated experimentally. The experiments were performed in pulsed supersonic metastable helium beams, with particle number densities of up to $\sim10^9$ cm$^{-3}$. In the presence of amplitude-modulated, radio-frequency electric fields, changes in the spectral intensity distributions associated with the transitions to these states that are attributed to dipole-dipole interactions within the ensembles of excited atoms have been observed. The experimental results are in excellent agreement with calculations of the Rydberg energy level structure carried out using Floquet methods, and excitations shared by up to 4 atoms. The use of these Rydberg-Stark states as sensors for non-resonant broadband radio-frequency electrical noise is also discussed.
Single-color two-photon spectroscopy of Rydberg states in electric fields
T. E. Wall,D. B. Cassidy,S. D. Hogan
Physics , 2014, DOI: 10.1103/PhysRevA.90.053430
Abstract: Rydberg states of atomic helium with principal quantum numbers ranging from n=20 to n=100 have been prepared by non-resonance-enhanced single-color two-photon excitation from the metastable 2 {^3}S{_1} state. Photoexcitation was carried out using linearly and circularly polarized pulsed laser radiation. In the case of excitation with circularly polarized radiation, Rydberg states with azimuthal quantum number |m_{\ell}|=2 were prepared in zero electric field, and in homogeneous electric fields oriented parallel to the propagation axis of the laser radiation. In sufficiently strong electric fields, individual Rydberg-Stark states were resolved spectroscopically, highlighting the suitability of non-resonance-enhanced multiphoton excitation schemes for the preparation of long-lived high-|m_{\ell}| hydrogenic Rydberg states for deceleration and trapping experiments. Applications of similar schemes for Doppler-free excitation of positronium atoms to Rydberg states are also discussed.
Isolated large amplitude periodic motions of towed rigid wheels
D. Takács,G. Stépán. S. J. Hogan
Physics , 2007, DOI: 10.1007/s11071-007-9253-y
Abstract: This study investigates a low degree-of-freedom (DoF) mechanical model of shimmying wheels. The model is studied using bifurcation theory and numerical continuation. Self-excited vibrations, that is, stable and unstable periodic motions of the wheel, are detected with the help of Hopf bifurcation calculations. These oscillations are then followed over a large parameter range for different damping values by means of the software package AUTO97. For certain parameter regions, the branches representing large amplitude stable and unstable periodic motions become isolated following an isola birth. These regions are extremely dangerous from an engineering view-point if they are not identified and avoided at the design stage.
Effects of model chemistry and data biases on stratospheric ozone assimilation
L. Coy, D. R. Allen, S. D. Eckermann, J. P. McCormack, I. Stajner,T. F. Hogan
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2007,
Abstract: The innovations or observation minus forecast (O–F) residuals produced by a data assimilation system provide a convenient metric of evaluating global analyses. In this study, O–F statistics from the Global Ozone Assimilation Testing System (GOATS) are used to examine how ozone assimilation products and their associated O–F statistics depend on input data biases and ozone photochemistry parameterizations (OPP). All the GOATS results shown are based on a 6-h forecast and analysis cycle using observations from SBUV/2 (Solar Backscatter UltraViolet instrument-2) during September–October 2002. Results show that zonal mean ozone analyses are more independent of observation biases and drifts when using an OPP, while the mean ozone O–Fs are more sensitive to observation drifts when using an OPP. In addition, SD O–Fs (standard deviations) are reduced in the upper stratosphere when using an OPP due to a reduction of forecast model noise and to increased covariance between the forecast model and the observations. Experiments that changed the OPP reference state to match the observations by using an "adaptive" OPP scheme reduced the mean ozone O–Fs at the expense of zonal mean ozone analyses being more susceptible to data biases and drifts. Additional experiments showed that the upper boundary of the ozone DAS can affect the quality of the ozone analysis and therefore should be placed well above (at least a scale height) the region of interest.
Broadband Optical Serrodyne Frequency Shifting
D. M. S. Johnson,J. M. Hogan,S. -w. Chiow,M. A. Kasevich
Physics , 2009, DOI: 10.1364/OL.35.000745
Abstract: We demonstrate serrodyne frequency shifting of light from 200 MHz to 1.2 GHz with an efficiency of better than 60 percent. The frequency shift is imparted by an electro-optic phase modulator driven by a high-frequency, high-fidelity sawtooth waveform that is passively generated by a commercially available Non-Linear Transmission Line (NLTL). We also implement a push-pull configuration using two serrodyne-driven phase modulators allowing for continuous tuning between -1.6 GHz and +1.6 GHz. Compared to competing technologies, this technique is simple and robust, and offers the largest available tuning range in this frequency band.
Driving Rydberg-Rydberg transitions from a co-planar microwave waveguide
S. D. Hogan,J. A. Agner,F. Merkt,T. Thiele,S. Filipp,A. Wallraff
Physics , 2011, DOI: 10.1103/PhysRevLett.108.063004
Abstract: The coherent interaction between ensembles of helium Rydberg atoms and microwave fields in the vicinity of a solid-state co-planar waveguide is reported. Rydberg-Rydberg transitions, at frequencies between 25 GHz and 38 GHz, have been studied for states with principal quantum numbers in the range 30 - 35 by selective electric-field ionization. An experimental apparatus cooled to 100 K was used to reduce effects of blackbody radiation. Inhomogeneous, stray electric fields emanating from the surface of the waveguide have been characterized in frequency- and time-resolved measurements and coherence times of the Rydberg atoms on the order of 250 ns have been determined.
Selling Politics? How the Traits of Salespeople Manifest Themselves in Irish Politicians  [PDF]
Dónal ó Mearáin, Roger Sherlock, John Hogan
Open Journal of Political Science (OJPS) , 2013, DOI: 10.4236/ojps.2013.34024
Abstract:

This article seeks to uncover if some of the traits most associated with salespeople manifested themselves in the activities of candidates in the constituency of Dún Laoghaire during the 2007 Irish general election. Such a finding would suggest that just as political parties have looked to the marketing profession for their lead in developing political marketing, politicians are looking to, and adopting the traits of those in the sales profession. This would point to the traits that the modern politician must possess in order to get and remain elected. It would also raise significant questions in terms of how candidates present themselves to the electorate, as well as how they go about campaigning and formulating policy.

Effect of carrier concentration on magnetism and magnetic order in the pyrochlore iridates
M. J. Graf,S. M. Disseler,Chetan Dhital,T. Hogan,M. Bojko,A. Amato,H. Luetkens,C. Baines,D. Margineda,S. R. Giblin,M. Jura,Stephen D. Wilson
Physics , 2014,
Abstract: We present resistivity, magnetization, and zero field muon spin relaxation ($\mu$SR) data for the pyrochlore iridate materials Nd$_{2-x}$Ca$_{x}$Ir$_{2}$O$_{7}$ ($x = 0, 0.06$, and $0.10$) and Sm$_2$Ir$_2$O$_7$. While Nd$_{2}$Ir$_{2}$O$_{7}$ (Nd227) is weakly conducting, Sm$_{2}$Ir$_{2}$O$_{7}$ (Sm227) has slowly diverging resistivity at low temperature. Nd227 and Sm227 exhibit magnetic anomalies at $T_{M} = 105 K$ and $137 K$, respectively. However, zero-field $\mu$SR measurements show that long-range magnetic order of the Ir$^{4+}$ sublattice sets in at much lower temperatures ($T_{LRO} \sim 8 K$ for Nd227 and $70 K$ for Sm227); both materials show heavily damped muon precession with a characteristic frequency near 9 MHz. The magnetic anomaly at $T_{M}$ in Nd227 is not significantly affected by the introduction of hole carriers by Ca-substitution in the conducting Nd$_{2-x}$Ca$_{x}$Ir$_{2}$O$_{7}$ samples, but the muon precession is fully suppressed for both.
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