Atomic, Molecular and Optical Physics Commons^™

Threshold Alignment Reversal And Circularly Polarized Fluorescence In Rotationally Resolved H2, Jack W. Maseberg, Klaus Bartschat, Timothy J. Gay

Timothy J. Gay Publications

Using H₂ and D₂ targets, we have measured the polarization of Fulcher-band fluorescence resulting from spin-polarized electron-impact excitation of vibrationally and rotationally resolved d ³Π_u→ a ³ ∑_g⁺ transitions for incident electron energies from 14.3 to 28.5 eV. Near threshold, the linear polarization P₁ descends from positive values through zero to negative values, indicating a dynamic production of M_N = 0 states. The circular polarization P₃ is measured to be nonzero, indicating the orientation of rotationally resolved molecular states. For Q-branch transitions, P₃ is consistent with theory based …

Optically Pumped Spin-Exchange Polarized-Electron Source, Munir Pirbhai, J. Knepper, E. T. Litaker, D. Tupa, Timothy J. Gay

Timothy J. Gay Publications

We describe the operation of a prototype polarized-electron source. Rubidium vapor, contained in a cell, is optically pumped in the presence of a buffer gas. Unpolarized electrons from a tungsten filament are injected into the cell and extracted after undergoing spin exchange with the Rb atoms. We compare the performance of the source when different buffer gases are used. We measure a decrease in electron polarization as their injection energy increases, but find an unexpected regime at higher injection energies yielding increased electron polarization accompanied by a 40-fold increase in current, suggesting the production of slow secondary electrons in the …

Two Novel Approaches For Electron Beam Polarization From Unstrained Gaas, James L. Mccarter, Nathan B. Clayburn, J. M. Dreiling, Timothy J. Gay, Dominic M. Ryan, John Hansknecht, Matt Poelker, Andrei Afanasev, Ara Kechiantz

Timothy J. Gay Publications

Two novel approaches to producing highly-polarized electron beams from unstrained GaAs were tested using a micro-Mott polarimeter. Based on a suggestion by Nakanishi [1]], twophoton photoemission with 1560 nm light was used with photocathodes of varying thickness: 625m, 0.32m, and 0.18m. For each of these photocathodes, the degree of spin polarization of the photoemitted beam was less than 50%. Polarization via two-photon absorption was highest from the thinnest photocathode sample and close to that obtained from one-photon absorption (using 778 nm light), with values 40.3±1.0% and 42.6±1.0%, respectively. The second attempt to produce highly-polarized electrons used one-photon emission with 778 …

Rhodizonic Acid On Noble Metals: Surface Reactivity And Coordination Chemistry, Donna A. Kunkel, James Hooper, Scott Simpson, Sumit Beniwal, Katie L. Morrow, Douglas C. Smith, Kimberly Cousins, Stephen Ducharme, Eva Zurek, Axel Enders

Stephen Ducharme Publications

A study of the two-dimensional crystallization of rhodizonic acid on the crystalline surfaces of gold and copper is presented. Rhodizonic acid, a cyclic oxocarbon related to the ferroelectric croconic acid and the antiferroelectric squaric acid, has not been synthesized in bulk crystalline form yet. Capitalizing on surface-assisted molecular self-assembly, a two-dimensional analogue to the well-known solution-based coordination chemistry, two-dimensional structures of rhodizonic acid were stabilized under ultrahigh vacuum on Au(111) and Cu(111) surfaces. Scanning tunneling microscopy, coupled with first-principles calculations, reveals that on the less reactive Au surface, extended two-dimensional islands of rhodizonic acid are formed, in which the molecules …

Changing Molecular Band Offsets In Polymer Blends Of (P3ht/P(Vdf–Trfe)) Poly(3-Hexylthiophene) And Poly(Vinylidene Fluoride With Trifluoroethylene) Due To Ferroelectric Poling, Freddy Wong, Godohaldo Perez, Manuel Bonilla, Juan A. Colon-Santana, Xin Zhang, Pankaj Sharma, Alexei Gruverman, Peter A. Dowben, Luis G. Rosa

Peter Dowben Publications

Photoelectron emission and inverse photoemission spectroscopy studies of polymer blends of poly(vinylidene fluoride (70%) – trifluoroethylene (30%)) P(VDF–TrFE 70 : 30) and regio-regular poly(3- hexylthiophene) (P3HT) provide evidence of changes in the molecular band offsets as a result of changes in the ferroelectric polarization in P(VDF–TrFE). Investigation of the blends with higher concentrations of the semiconducting P3HT component revealed that the organic semiconductor component of the blend dominates the electronic structure in the vicinity of the chemical potential. Specifically, the states of P3HT at the conduction band minimum and valence band maximum fall within the HOMO–LUMO gap of the dielectric …

Magnetoelectric Coupling At The Euo/Batio3 Interface, Shi Cao, P Liu, Jinke Tang, Haidong Lu, C W. Bark, Sangjin Ryu, Chang-Beom Eom, Alexei Gruverman, Peter A. Dowben

Peter Dowben Publications

Magnetization modulation by ferroelectric polarization switching is reported for the ferromagnetic-ferroelectric EuO/BaTiO₃heterostructure. The value of the magnetization critical exponent β is consistent with the expected Heisenberg-like ferromagnetism of EuO and reported Curie temperature. The critical exponent is seen to decrease with increased magnetic coupling. The results are discussed in the context of data obtained earlier for epitaxial La_0.67Sr_0.33MnO₃/BaTiO₃ heterostructures, where magnetization increases and critical exponent b also declines with ferroelectric polarization pointing away from ferromagnetic layer. The observed similarity between two systems illustrates an importance of charge doping in magnetoelectric coupling, …

Adsorption Of Tcnqh-Functionalized Quinonoid Zwitterions On Gold And Graphene: Evidence For Dominant Intermolecular Interactions+, Lingmei Kong, Lucie Routaboul, Pierre Braunstein, Hong-Gi Park, Jaewu Choi, John P. Colon Cordova, E Vega, Luis G. Rosa, Bernard Doudin, Peter A. Dowben

Peter Dowben Publications

We experimentally investigate the electronic structure of the strongly dipolar, quinonoid-type molecule obtained by TCNQH-functionalization (TCNQH = (NC)₂CC₆H₄CH(CN)₂) of (6Z)-4-(butylamino)-6-(butyliminio)- 3-oxocyclohexa-1,4-dien-1-olate C₆H₂(...NHR)₂(...O)₂ (where R = n-C₄H₉) to be very similar after deposition from solution on either graphene or gold substrates. These zwitterion adsorbate thin films form structures that are distinct from those formed by related quinonoid molecules previously studied. We argue that adsorbate–adsorbate interactions dominate and lead to a Stranski–Krastanov ‘island growth’ mechanism.

High-Order-Harmonic-Generation Spectroscopy With An Elliptically Polarized Laser Field, M. V. Frolov, N. L. Manakov, T. S. Sarantseva, Anthony F. Starace

Anthony F. Starace Publications

Analytic formulas describing high-order-harmonic generation (HHG) by atoms in an intense laser field with small ellipticity are obtained quantum mechanically in the tunneling limit. The results show that factorization of the HHG yield in terms of an electron wave packet and the photorecombination cross section (PRCS) is valid only for s states of a bound atomic electron, whereas the HHG yield for p states involves two different atomic parameters. For the latter case, elliptic HHG spectroscopy enables one to retrieve both the energy and angular dependence of the PRCS of the target atom, as we illustrate for the case of …

Threshold Effects In Strong-Field Ionization: Energy Shifts And Rydberg Structures, Katarzyna Krajewska, Ilya I. Fabrikant, Anthony F. Starace

Anthony F. Starace Publications

The behavior of strong-field ionization rates of neutral atoms in the vicinity ofmultiphoton ionization thresholds is analyzed using formal collision theory.Our approach,which accounts nonperturbatively for effects of an intense laser field, shows that the ionization rates have a nearly constant behavior below and above each multiphoton threshold and that between such thresholds there are an apparently finite number of rapid oscillations due to resonances with laser-field-modified Rydberg states. This pattern is typical for any atomic target, as we illustrate specifically for hydrogen and neon atoms. The flat behavior of the ionization yield near multiphoton thresholds gives the appearance of an …

Equilibrium Magnetic States In Individual Hemispherical Permalloy Caps, Robert Streubel, Volodymyr P. Kravchuk, Denis D. Sheka, Denys Makarov, Florian Kronast, Oliver G. Schmidt, Yuri Gaididei

Robert Streubel Papers

The magnetization distributions in individual soft magnetic permalloy caps on non-magnetic spherical particles with sizes ranging from 50 to 800nm are investigated. We experimentally visualize the magnetic structures at the resolution limit of the x-ray magnetic circular dichroism photoelectron emission microscopy (XMCD-PEEM). By analyzing the so-called tail contrast in XMCD-PEEM, the spatial resolution is significantly enhanced, which allowed us to explore magnetic vortices and their displacement on curved surfaces. Furthermore, cap nanostructures are modeled as extruded hemispheres to determine theoretically the phase diagram of equilibrium magnetic states. The calculated phase diagram agrees well with the experimental observations. © 2012 American …

Magnetic Vortices On Closely Packed Spherically Curved Surfaces, Robert Streubel, Denys Makarov, Florian Kronast, Volodymyr Kravchuk, Manfred Albrecht, Oliver G. Schmidt

Robert Streubel Papers

We investigate the change of magnetic vortex states driven by curvature. The equilibrium state and magnetization reversal of soft magnetic permalloy (Py, Ni ₈₀Fe ₂₀) caps on self-assembled spherical particles with diameters of 100, 330, and 800 nm are investigated, revealing the vortex ground state for individual caps and closely packed cap arrays. The magnetic coupling between vortices is substantially reduced due to the shape of the cap as apparent in a much weaker dependence of the magnetization reversal process on the separation distance. Interestingly, the remaining coupling is still sufficiently large to introduce chirality frustrated vortex states …

Out-Of-Surface Vortices In Spherical Shells, Volodymyr P. Kravchuk, Denis D. Sheka, Robert Streubel, Denys Makarov, Oliver G. Schmidt, Yuri Gaididei

Robert Streubel Papers

The interplay of topological defects with curvature is studied for out-of-surface magnetic vortices in thin spherical nanoshells. In the case of an easy-surface Heisenberg magnet it is shown that the curvature of the underlying surface leads to a coupling between the localized out-of-surface component of the vortex with its delocalized in-surface structure, i.e., polarity-chirality coupling. © 2012 American Physical Society.

Ultrafast Intense-Field Photoionization And Photofragmentation Of Systematic Series Of Substituted Organic Molecules, Timothy D. Scarborough

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

The abundance and relevance of organic molecules similar to benzene makes their study important. Studying the interactions of such molecules with intense light fields has implications for the generation of short-wavelength radiation, attosecond science, high-harmonic generation, and many other fields. However, the computing power necessary to complete fully ab initio calculations describing molecules of this size does not exist; this leaves theoretical studies to rely on assumptions and approximations just to calculate the energies of the ground state. Including any sort of dynamics in these calculations is prohibitively complicated, and this makes experimental observations important. Since many organic molecules are …

Qpced2.0: A Computer Program For The Processing And Quantification Of Polycrystalline Electron Diffraction Patterns, Xingzhong Li

Nebraska Center for Materials and Nanoscience: Faculty Publications

The processing and quantification of electron diffraction patterns have become vital in advanced electron crystallographic analysis work. A computer program, QPCED2.0, has been developed for the handling of selected-area electron diffraction patterns for polycrystalline materials. QPCED2.0 can be used to enhance the visibility of electron diffraction patterns, to convert electron diffraction patterns into intensity profiles, and to retrieve precisely the lattice d spacings and the integral intensities of the diffraction rings. The design and implementation of QPCED2.0 are elucidated and application examples are given.

Growth Diagram And Magnetic Properties Of Hexagonal Lufe2o4 Thin Films, Wenbin Wang, Zheng Gai, Miaofang Chi, Jason D. Fowlkes, Jieyu Yi, Leyi Zhu, Xuemei Cheng, David J. Keavney, Paul C. Snijders, Thomas Z. Ward, Jian Shen, Xiaoshan Xu

Xiaoshan Xu Papers

Agrowth diagram of Lu-Fe-O compounds on MgO (111) substrates using pulsed laser deposition is constructed based on extensive growth experiments. The LuFe₂O₄ phase can only be grown in a small range of temperature and O₂ pressure conditions. An understanding of the growth mechanism of Lu-Fe-O compound films is offered in terms of the thermochemistry at the surface. Superparamagnetism is observed in the LuFe₂O₄ film and is explained in terms of the effect of the impurity hexagonal LuFeO₃ (h-LuFeO₃) phase and structural defects.

Crystal Field Splitting And Optical Bandgap Of Hexagonal Lufeo3 Films, Wenbin Wang, Hongwei Wang, Xiaoying Xu, Leyi Zhu, Lixin He, Elizabeth Wills, Xuemei Cheng, David J. Keavney, Jian Shen, Xifan Wu, Xiaoshan Xu

Xiaoshan Xu Papers

Hexagonal LuFeO₃ films have been studied using x-ray absorption and optical spectroscopy. The crystal splitting of Fe³⁺ is extracted as E_e' - E_e" = 0.7 eV and E_a'1 - E_e' = 0.9 eV, and a 2.0 eV optical bandgap is determined assuming a direct gap. First-principles calculations confirm the experiments that the relative energies of crystal field splitting states do follow E_a'1 > E_e' > E_e" with slightly underestimated values and a bandgap of 1.35 eV.

Observation And Identification Of Metastable Excited States In Ultrafast Laser-Ionized Pyridine, David B. Foote, Timothy D. Scarborough, Cornelis J. Uiterwaal

C.J.G.J. Uiterwaal Publications

We report on the fragmentation of ionized pyridine (C₅H₅N) molecules by focused 50 fs, 800 nm laser pulses. Such ionization produces several metastable ionic states that fragment within the field-free drift region of a reflectron- type time of flight mass spectrometer, with one particular metastable dissociation being the leading fragmentation process. Because the time of flight is no longer dependent in a simple way on the mass of the ion, the metastable decay is manifested as an unfocused peak on the mass spectrum that appears at a time of flight not corresponding to an integer mass. …

Thermal Quench Effects On Ferroelectric Domain Walls, P. Paruch, A. B. Kolton, X. Hong, C. H. Ahn, T. Giamarchi

Xia Hong Publications

Using piezoresponse force microscopy on epitaxial ferroelectric thin films, we have measured the evolution of domain wall roughening as a result of heat-quench cycles up to 735 ◦C, with the effective roughness exponent ζ changing from 0.25 to 0.5. We discuss two possible mechanisms for the observed ζ increase: a quench from a thermal one-dimensional configuration and from a locally equilibrated pinned configuration with a crossover from a two- to one-dimensional regime. We find that the postquench spatial structure of the metastable states, qualitatively consistent with the existence of a growing dynamical length scale whose ultraslow evolution is primarily controlled …

Transverse Measurements Of Polarization In Optically Pumped Rb Vapor Cells, J. M. Dreiling, E. B. Norrgard, D. Tupa, Timothy J. Gay

Timothy J. Gay Publications

We have developed a simple heuristic method for determining the polarization of an optically pumped alkalimetal vapor. A linearly polarized probe beam traverses a vapor cell perpendicular to the pump-beam propagation direction, and the transmitted beam intensity is monitored for orthogonal linear polarizations. As the probe beam is scanned in frequency across the D₁ transition, its linear-polarization-dependent transmission can be used as a measure of the atomic orientation of the vapor. We analyze these transmission differences and their dependence on the alkali-metal number density in the vapor.

Understanding The Effect Of Ferroelectric Polarization On Power Conversion Efficiency Of Organic Photovoltaic Devices, Yongbo Yuan, Pankaj Sharma, Zhengguo Xiao, Shashi Poddar, Alexei Gruverman, Stephen Ducharme, Jinsong Huang

Stephen Ducharme Publications

It is demonstrated that the power conversion efficiency (PCE) of organic photovoltaic devices can be increased by inserting an ultrathin film of a ferroelectric co-polymer, poly(vinylidenefluoridetrifluoroethylene) (P(VDF-TrFE)), at the metal–organic interface, due to an enhancement of the charge extraction efficiency. Specifically, the effect of P(VDF-TrFE) crystallinity on its function in ferroelectric organic photovoltaic (FE-OPV) devices has been studied by several methods. Highly crystalline and amorphous P(VDF-TrFE) films have been prepared by the Langmuir–Blodgett method and spincoating from acetone solution, respectively. The polymer solar cell devices with a crystalline P(VDFTrFE) interfacial layer at the cathode have larger PCE than the structures …

Laser-Wakefield Accelerators: Glass-Guiding Benefits, Donald P. Umstadter

Donald Umstadter Publications

A main attraction of laser-driven electron accelerators is their absence of cavity walls, which can break down in the presence of intense electric fields. Now it seems that the inclusion of a hollow glass fibre cavity could lead to more efficient acceleration at lower laser intensities. ... Further research will reveal which of the above methods for guiding light will ultimately prove best for future accelerator designs. In any case, although it is perhaps ironic that the relatively old technology of glass waveguides may benefit next-generation accelerators, it is nonetheless satisfying to see such a classic photonic solution come to …

Schottky Barrier Formation At The Au To Rare Earth Doped Gan Thin Film Interface, S. R. Mchale, J. W. Mcclory, J. C. Petrosky, J. Wu, R. Palai, Yaroslav B. Losovyj, Peter A. Dowben

Peter Dowben Publications

The Schottky barriers formed at the interface between gold and various rare earth doped GaN thin films (RE = Yb, Er, Gd) were investigated in situ using synchrotron photoemission spectroscopy. The resultant Schottky barrier heights were measured as 1.68 ± 0.1 eV (Yb:GaN), 1.64 ± 0.1 eV (Er:GaN), and 1.33 ± 0.1 eV (Gd:GaN). We find compelling evidence that thin layers of gold do not wet and uniformly cover the GaN surface, even with rare earth doping of the GaN. Furthermore, the trend of the Schottky barrier heights follows the trend of the rare earth metal work function.

Structure And Magnetism Of Mnau Nanoclusters, X. Wei, Damien Le Roy, Ralph Skomski, Xingzhong Li, Zhiguang Sun, Jeffrey E. Shield, M. J. Kramer, David J. Sellmyer

Nebraska Center for Materials and Nanoscience: Faculty Publications

Equiatomic MnAu clusters with average sizes of 4 and 10 nm are produced by inert-gas condensation. As-produced clusters are used to form both dense cluster films and films with clusters embedded in a W matrix with a cluster volume fraction of 25%. Both structure and magnetism are size-dependent. Structural analysis of the 10 nm clusters indicate a distorted tetragonal body-centered cubic structure with lattice parameters a=0.315 and c=0.329 nm. The 4 nm clusters have a partially ordered tetragonal L10 structure with lattice parameters a=0.410 nm and c=0.395 nm. Magnetic properties of the clusters show evidence at …

Colossal Negative Magnetoresistance In Dilute Fluorinated Graphene, X. Hong, S.-H. Cheng, C. Herding, J. Zhu

Xia Hong Publications

Adatoms offer an effective route to modify and engineer the properties of graphene. In this work, we create dilute fluorinated graphene using a clean, controlled, and reversible approach. At low carrier densities, the system is strongly localized and exhibits an unexpected, colossal negative magnetoresistance. The zero-field resistance is reduced by a factor of 40 at the highest field of 9 T and shows no sign of saturation. Unusual staircaselike field dependence is observed below 5 K. The magnetoresistance is highly anisotropic. These observations cannot be explained by existing theories, but likely require adatom-induced magnetism and/or a metal-insulator transition driven by …

Ultrafast Rempi In Benzene And The Monohalobenzenes Without The Focal Volume Effect, Timothy D. Scarborough, James Strohaber, David B. Foote, Collin J. Mcacy, Cornelis J. Uiterwaal

C.J.G.J. Uiterwaal Publications

We report on the photoionization and photofragmentation of benzene (C₆H₆) and of the monohalobenzenes C₆H₅–X (X = F, Cl, Br, I) under intense-field, single-molecule conditions. We focus 50-fs, 804-nm pulses from a Ti:sapphire laser source, and record ion mass spectra as a function of intensity in the range ~10¹³ W/cm² to ~10¹⁵ W/cm². We count ions that were created in the central, most intense part of the focal area; ions from other regions are rejected. For all targets, stable parent ions (C₆H₅X+) are …

In Memoriam: Robert Katz (1917–2011), M. P. R. Waligorski, Francis A. Cucinotta

Robert Katz Publications

Bob Katz will be well remembered for his enthusiasm and strong personality. In a scientific dispute, few indeed could match his wit or his sense of humor or survive the cutting logic of his arguments. To those who had appreciation for his science and his personality, he was truly a great scientist and a master teacher. There are many people around the world who will remember Bob Katz for what he offered them and for the way he showed them what truth in science is.

The Katz Model, developed at that time for solid-state detectors and for cell cultures, relates …

Magnetism Of Cluster-Deposited Y–Co Nanoparticles, Balamuruga Balamurugan, Ralph Skomski, Xingzhong Li, V. R. Shah, George C. Hadjipanayis, Jeffrey E. Shield, David J. Sellmyer

Nebraska Center for Materials and Nanoscience: Faculty Publications

Nanoparticles of YCo₂, YCo_3, and YCo₅ are produced by cluster-deposition and investigated both structurally and magnetically. The nanoparticles have sizes of less than 10 nm and are superparamagnetic at 300 K, irrespective of stoichiometry. As-produced nanoparticles exhibit disordered structures with magnetic properties differing from those of the bulk particles. The temperature-dependent magnetization curves of the nanoparticles reveal blocking temperatures from 110 to 250 K, depending on stoichiometry. The magnetic anisotropy constant K₁ of disordered YCo₅ nanoparticles of 7.8 nm in size is 3.5×10⁶ergs/cm³, higher than those of the disordered YCo …

Metastability Of Free Cobalt And Iron Clusters: A Possible Precursor To Bulk Ferromagnetism, Xiaoshan Xu, Shuangye Yin, Ramiro Moro, Anthony Liang, John Bowlan, Walt A. De Heer

Xiaoshan Xu Papers

Homonuclear cobalt and iron clusters Co_N and Fe_N measured in a cryogenic molecular beam exist in two states with distinct magnetic moments (μ), polarizabilities, and ionization potentials, indicating distinct valences. The μ is approximately quantized: μ_N ~ 2Nμ_B in the ground states and μ_N* ~ Nμ_B in the excited states for Co; μ_N ~ 3N μ_B and μ_N * ~ Nμ_B for Fe. At a large size, the average μ of the two states converges to the bulk value with diminishing ionization potential differences. …

Mesoscale Flux-Closure Domain Formation In Single-Crystal Batio3, R. G.P. Mcquaid, L. J. Mcgilly, Pankaj Sharma, Alexei Gruverman, J. M. Gregg

Alexei Gruverman Publications

Over 60 years ago, Charles Kittel predicted that quadrant domains should spontaneously form in small ferromagnetic platelets. He expected that the direction of magnetization within each quadrant should lie parallel to the platelet surface, minimizing demagnetizing fields, and that magnetic moments should be configured into an overall closed loop, or flux-closure arrangement. Although now a ubiquitous observation in ferromagnets, obvious flux-closure patterns have been somewhat elusive in ferroelectric materials. This is despite the analogous behaviour between these two ferroic subgroups and the recent prediction of dipole closure states by atomistic simulations research. Here we show Piezoresponse Force Microscopy images of …

Electron Attachment To Dye-Sensitized Solar Cell Components: Cyanoacetic Acid, Alberto Modelli, Paul Burrow

Paul Burrow Publications

The energies of electron attachment associated with temporary occupation of the lower-lying virtual orbitals of cyanoacetic acid (CAA), proposed as a possible component of dye-sensitized solar cells, and its derivative methyl cyanoacetate (MCA) are measured in the gas phase with electron transmission spectroscopy (ETS). The corresponding orbital energies of the neutral molecule, supplied by B3LYP/6-31G(d) calculations and scaled using an empirically calibrated linear equation, are compared with the experimental vertical attachment energies (VAEs). The vertical and adiabatic electron affinities are also evaluated at the B3LYP/6-31+G(d) level as the anion/neutral total energy difference. Dissociative electron attachment spectroscopy …