Physics Commons^™

Nonlinear Dichroism In Back-To-Back Double Ionization Of He By An Intense Elliptically Polarized Few-Cycle Extreme Ultraviolet Pulse, Jean Marcel Ngoko Djiokap, N. L. Manakov, A. V. Meremianin, S. X. Hu, L. B. Madsen, Anthony F. Starace

Anthony F. Starace Publications

Control of double ionization of He by means of the polarization and carrier-envelope phase (CEP) of an intense, few-cycle extreme ultraviolet (XUV) pulse is demonstrated numerically by solving the six-dimensional two-electron, time-dependent Schrödinger equation for He interacting with an elliptically polarized XUV pulse. Guided by perturbation theory (PT), we predict the existence of a nonlinear dichroic effect (∝ I^3/2) that is sensitive to the CEP, ellipticity, peak intensity I, and temporal duration of the pulse. This dichroic effect (i.e., the difference of the two-electron angular distributions for opposite helicities of the ionizing XUV pulse) originates from interference of …

Electron Matter Interferometry And The Electron Double-Slit Experiment, Roger Bach

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

Quantum mechanics has fundamentally changed the way scientists think about the world. Quantum mechanical theory has found it's way into our everyday lives through advances in technology. In this dissertation a fundamental quantum mechanical demonstration and the technological development of a new quantum mechanical device are presented.

Double-slit diffraction is a corner stone of quantum mechanics. It illustrates key features of quantum mechanics: interference and the particle-wave duality of matter. Here we demonstrate the full realization of Richard Feynman's famous thought experiment. By placing a movable mask in front of a double-slit to control the transmission through the individuals slits. …

X-Ray Second Harmonic Generation, S. Shwartz, Matthias Fuchs, J. B. Hastings, Y. Inubushi, T. Ishikawa, T. Katayama, D. A. Reis, T. Sato, K. Tono, M. Yabashi, S. Yudovich, S. E. Harris

Matthias Fuchs Publications

We report clear experimental evidence for second harmonic generation at hard x-ray wavelengths. Using a 1.7 Å pumping beam generated by a free electron laser, we observe second harmonic generation in diamond. The generated second harmonic is of order 10 times the background radiation, scales quadratically with pump pulse energy, and is generated over a narrow phase-matching condition. Of importance for future experiments, our results indicate that it is possible to observe nonlinear x-ray processes in crystals at pump intensities exceeding 10¹⁶ W/cm².

A Low-Power Optical Electron Switch, Wayne Cheng-Wei Huang, Roger Bach, Peter Beierle, Herman Batelaan

Department of Physics and Astronomy: Faculty Publications

An electron beam is deflected when it passes over a silicon-nitride surface, if the surface is illuminated by a low-power continuous-wave diode laser. A deflection angle of up to 1.2 mrad is achieved for an electron beam of 29 μrad divergence. A mechanical beam-stop is used to demonstrate that the effect can act as an optical electron switch with a rise and fall time of 6 μs. Such a switch provides an alternative means to control electron beams, which may be useful in electron lithography and microscopy.

Resonant Electron-Atom Bremsstrahlung In An Intense Laser Field, A. N. Zheltukhin, A. V. Flegel, M. V. Frolov, N. L. Manakov, Anthony F. Starace

Anthony F. Starace Publications

We analyze a resonant mechanism for spontaneous laser-assisted electron bremsstrahlung (BrS) involving the resonant transition (via either laser-assisted electron-ion recombination or electron-atom attachment) into a laser-dressed intermediate quasibound state (corresponding, respectively, to either a field-free neutral atom or a negative-ion bound state) accompanied by ionization or detachment of this state by the laser field. This mechanism leads to resonant enhancement (by orders of magnitude) of the BrS spectral density for emitted photon energies corresponding to those for laser-assisted recombination or attachment. We present an accurate parametrization of the resonant BrS amplitude in terms of the amplitudes for nonresonant BrS, for …

Nonlinear Transport In Nanoscale Phase Separated Colossal Magnetoresistive Oxide Thin Films, V. R. Singh, L. Zhang, A. K. Rajapitamahuni, N. Devires, X. Hong

Xia Hong Publications

We report a study of the I-V characteristics of 2.5–5.4 nm epitaxial La_1-xSr_xMnO₃ (x=0.33 and 0.5) and La_0.7Ca_0.3MnO₃ thin films. While La_0.67Sr_0.33MnO₃ films exhibit linear conduction over the entire temperature and magnetic field ranges investigated, we observe a strong correlation between the linearity of the I-V relation and the metal-insulator transition in highly phase separated La_0.5Sr_0.5MnO₃ and La_0.7Ca_0.3MnO₃ films. Linear I-V behavior has been observed in the high temperature …

Spin Density Waves In Periodically Strained Graphene Nanoribbons, Nabil M. Al-Aqtash, Renat F. Sabirianov

Nebraska Center for Materials and Nanoscience: Faculty Publications

Zigzag graphene nanoribbons (ZGNRs) are antiferromagnetic in the ground state with zero net magnetization due to the compensation of contributions from opposite edges. Uniform deformations (both shear and axial) do not produce magnetization due to symmetry restrictions. However, we report the results of first-principles calculations that predict the induction of spin density waves (SDWs) in ZGNRs under non-uniform periodic strain. Using the density functional theory (DFT) method, we show that a sinusoidal magnetization variation along the axis of the ribbon occurs under a sinusoidal transversal shear strain. SDWs appear due to the presence of a strain gradient that induced asymmetry …

Cms Use Of A Data Federation, Kenneth A. Bloom, C.

Kenneth Bloom Publications

CMS is in the process of deploying an Xrootd based infrastructure to facilitate a global data federation. The services of the federation are available to export data from half the physical capacity and the majority of sites are configured to read data over the federation as a back-up. CMS began with a relatively modest set of use-cases for recovery of failed local file opens, debugging and visualization. CMS is finding that the data federation can be used to support small scale analysis and load balancing. Looking forward we see potential in using the federation to provide more flexibility in the …

Temporary Anion States Of Three Herbicide Families, Adam M. Scheer, Kayvan Aflatooni, Gordon A. Gallup, Paul Burrow

Paul Burrow Publications

Electron scattering studies are used to locate the energies of temporary negative ion states of three chloro-substituted molecular families of herbicidal importance: salicylic and phenoxyacetic acids and acetamides. The correlation between these energies and the computed virtual orbital energies of the compounds is examined and used to put the latter on an absolute energy scale. Such scaling of orbital energies permits the anion states of other members of these families, for which experimental data may not be available, to be estimated from the calculated orbital energies. Studies of electron reduction rates often rely on calculated LUMO energies as molecular descriptors. …

Chirally Sensitive Electron-Induced Molecular Breakup And The Vester-Ulbricht Hypothesis, J. M. Dreiling, Timothy J. Gay

Timothy J. Gay Publications

We have studied dissociative electron attachment in sub-eV collisions between longitudinally polarized electrons and chiral bromocamphor molecules. For a given target enantiomer, the dissociative Br anion production depends on the helicity of the incident electrons, with an asymmetry that depends on the electron energy and is of order 3 × 10⁻⁴. The existence of chiral sensitivity in a well-defined molecular breakup reaction demonstrates the viability of the Vester-Ulbrict hypothesis, namely, that the longitudinal polarization of cosmic beta radiation was responsible for the origins of biological homochirality.

The Spin State Of A Molecular Adsorbate Driven By The Ferroelectric Substrate Polarization†, Xin Zhang, Tatiana Palamarciuc, Jean-François Létard, Patrick Rosa, Eduardo Vega Lozada, Fernand Torres, Luis G. Rosa, Bernard Doudin, Peter A. Dowben

Peter Dowben Publications

The spin state of [Fe(H₂B(pz)₂)₂(bipy)] thin films is mediated by changes in the electric field at the interface of organic ferroelectric polyvinylidene fluoride with trifluoroethylene (PVDF–TrFE). Signatures of the molecular crossover transition are evident in changes in the unoccupied states and the related shift from diamagnetic to paramagnetic characteristics. This may point the way to the molecular magneto-electric effect on devices.

Laboratory Manual For Physics Of Lasers And Modern Optics, 13th Ed, Stephen Ducharme

Stephen Ducharme Publications

You will encounter challenging puzzles and explore new and exciting physical phenomena. You will be provided with sufficient tools, guidance and other resources, but what you learn depends on your inquisitiveness and creativity. This laboratory course is designed to serve two purposes: 1) To explore a variety of physical principles using the fascinating and diverse behavior of light. 2) To learn some of the principles behind the pervasive and expanding area of optical and laser technology. This course is suitable for students of science, life sciences, and engineering, or any student who is curious about light. The prerequisites are the …

Comment On ‘Dynamics Of Formation Of Anthracene Anions In Molecular Clouds And Protoplanetary Atmospheres’, Paul Burrow, Gordon A. Gallup

Paul Burrow Publications

The shape resonances computed in the title paper are shown to be approximately 1.8 eV too high.

Garcia-Sanz et al [1] describe electron scattering calculations on anthracene in the gas phase and report the energies of seven temporary negative ion states, i.e. shape resonances, associated with electron attachment into the normally unoccupied π* molecular orbitals. The calculations were carried out at the ground state geometry of the neutral molecule, and thus the resonance energies represent the vertical attachment energies (VAEs) of the neutral. The purpose of this comment is to point out that their energies do not agree well with …

Reconstruction Of Three-Dimensional Molecular Structure From Diffraction Of Laseraligned Molecules, Jie Yang, Varun Makhija, Vinod Kumarappan, Martin Centurion

Martin Centurion Publications

Diffraction from laser-aligned molecules has been proposed as a method for determining 3-D molecular structures in the gas phase. However, existing structural retrieval algorithms are limited by the imperfect alignment in experiments and the rotational averaging in 1-D alignment. Here, we demonstrate a two-step reconstruction comprising a genetic algorithm that corrects for the imperfect alignment followed by an iterative phase retrieval method in cylindrical coordinates. The algorithm was tested with simulated diffraction patterns. We show that the full 3-D structure of trifluorotoluene, an asymmetric-top molecule, can be reconstructed with atomic resolution.

Tilted Femtosecond Pulses For Velocity Matching In Gas-Phase Ultrafast Electron Diffraction, Ping Zhang, Jie Yang, Martin Centurion

Martin Centurion Publications

Recent advances in pulsed electron gun technology have resulted in femtosecond electron pulses becoming available for ultrafast electron diffraction experiments. For experiments investigating chemical dynamics in the gas phase, the resolution is still limited to picosecond time scales due to the velocity mismatch between laser and electron pulses. Tilted laser pulses can be used for velocity matching, but thus far this has not been demonstrated over an extended target in a diffraction setting. We demonstrate an optical configuration to deliver high-intensity laser pulses with a tilted pulse front for velocity matching over the typical length of a gas jet. A …

Reconstruction Of Three-Dimensional Molecular Structure From Diffraction Of Laser-Aligned Molecules, Jie Yang, Varun Makhija, Vinod Kumarappan, Martin Centurion

Martin Centurion Publications

Diffraction from laser-aligned molecules has been proposed as a method for determining 3-D molecular structures in the gas phase. However, existing structural retrieval algorithms are limited by the imperfect alignment in experiments and the rotational averaging in 1-D alignment. Here, we demonstrate a two-step reconstruction comprising a genetic algorithm that corrects for the imperfect alignment followed by an iterative phase retrieval method in cylindrical coordinates. The algorithm was tested with simulated diffraction patterns. We show that the full 3-D structure of trifluorotoluene, an asymmetric-top molecule, can be reconstructed with atomic resolution.

Tilted Femtosecond Pulses For Velocity Matching In Gas-Phase Ultrafast Electron Diffraction, Ping Zhang, Jie Yang, Martin Centurion

Martin Centurion Publications

Recent advances in pulsed electron gun technology have resulted in femtosecond electron pulses becoming available for ultrafast electron diffraction experiments. For experiments investigating chemical dynamics in the gas phase, the resolution is still limited to picosecond time scales due to the velocity mismatch between laser and electron pulses. Tilted laser pulses can be used for velocity matching, but thus far this has not been demonstrated over an extended target in a diffraction setting. We demonstrate an optical configuration to deliver high-intensity laser pulses with a tilted pulse front for velocity matching over the typical length of a gas jet. A …

Coplanar Switching Of Polarization In Thin Films Of Vinylidene Fluoride Oligomers, Pankaj Sharma, Alexandra Fursina, Shashi Poddar, Stephen Ducharme, Alexei Gruverman

Stephen Ducharme Publications

Switching characteristics of vinylidene fluoride oligomer thin films with molecular chains aligned normal to the substrate and exhibiting a preferential in-plane polarization have been investigated using coplanar geometry of inter-digital electrodes via high-resolution piezoresponse force microscopy. It has been shown that in-plane switching proceeds via non-180 rotation of dipoles mediated by non-stochastic nucleation, expansion, and coalescence of domains. Asgrown multidomain configuration is found to be strongly pinned aided by charged domain walls, and the electrically induced (in-plane) mono-domain states relax to the as-grown state. The observed coercive field (approximately 0.6 MV/m) is two to three orders of magnitude smaller than …

Multiferroic Hexagonal Ferrites (H-Rfeo3, R=Y, Dy-Lu): An Experimental Review, Xiaoshan Xu, Wenbin Wang

Xiaoshan Xu Papers

Hexagonal ferrites (h-RFeO₃, R=Y, Dy-Lu) have recently been identified as a new family of multiferroic complex oxides. The coexisting spontaneous electric and magnetic polarizations make h-RFeO₃ rare-case ferroelectric ferromagnets at low temperature. Plus the room-temperature mul- tiferroicity and predicted magnetoelectric effect, h-RFeO₃ are promising materials for multiferroic applications. Here we review the structural, ferroelectric, magnetic, and magnetoelectric properties of h-RFeO₃. The thin film growth is also discussed because it is critical in making high quality single crystalline materials for studying intrinsic properties.

Structural And Electronic Origin Of The Magnetic Structures In Hexagonal Lufeo3, Hongwei Wang, Igor V. Solovyev, Wenbin Wang, Xiao Wang, Philip J. Ryan, David J. Keavney, Jong-Woo Kim, Thomas Z. Ward, Leyi Zhu, Jian Shen, X. M. Cheng, Lixin He, Xiaoshan Xu, Xifan Wu

Xiaoshan Xu Papers

Using combined theoretical and experimental approaches, we studied the structural and electronic origin of the magnetic structure in hexagonal LuFeO₃. Besides showing the strong exchange coupling that is consistent with the high magnetic ordering temperature, the previously observed spin reorientation transition is explained by the theoretically calculated magnetic phase diagram. The structural origin of this spin reorientation that is responsible for the appearance of spontaneous magnetization, is identified by theory and verified by x-ray diffraction and absorption experiments.

Active Control Of Magnetoresistance Of Organic Spin Valves Using Ferroelectricity, Dali Sun, Mei Fang, Xiaoshan Xu, Lu Jiang, Hangwen Guo, Yanmei Wang, Wenting Yang, Lifeng Yin, Paul C. Snijders, T. Z. Ward, Zheng Gai, X. -G. Zhang, Ho Nyung Lee, Jian Shen

Xiaoshan Xu Papers

Organic spintronic devices have been appealing because of the long spin lifetime of the charge carriers in the organic materials and their low cost, flexibility and chemical diversity. In previous studies, the control of resistance of organic spin valves is generally achieved by the alignment of the magnetization directions of the two ferromagnetic electrodes, generating magnetoresistance. Here we employ a new knob to tune the resistance of organic spin valves by adding a thin ferroelectric interfacial layer between the ferromagnetic electrode and the organic spacer: the magnetoresistance of the spin valve depends strongly on the history of the bias voltage, …

Positive Exchange Bias In Epitaxial Permalloy/Mgo Integrated With Si (100), S. S. Rao, J. T. Prater, Fan Wu, S. Nori, D. Kumar, Lanping Yue, Sy_Hwang Liou, Jagdish Narayan

Sy-Hwang Liou Publications

In magnetic random access memory (MRAM) devices, soft magnetic thin film elements such as permalloy (Py) are used as unit cells of information. The epitaxial integration of these elements with the technologically important substrate Si (100) and a thorough understanding of their magnetic properties are critical for CMOS-based magnetic devices. We report on the epitaxial growth of Ni_82.5Fe_17.5 (permalloy, Py) on Si (100) using a TiN/MgO buffer layer. Initial stages of growth are characterized by the formation of discrete islands that gradually merge into a continuous film as deposition times are extended. Interestingly, we find that the …