Physics Commons^™

Magnetocrystalline Anisotropy Of "-Fe2o3, Imran Ahamed, Rohilt Pathak, Arti Kashyap

Nebraska Center for Materials and Nanoscience: Faculty Publications

The epsilon Fe₂O₃ phase of iron oxide has been studied to understand the spin structure and the magnetocrystalline anisotropy in the bulk and in thin films of "-Fe₂O₃ and Co-doped "-Fe₂O₃. The preferential magnetization direction in the nanoparticles of "-Fe₂O₃ is along the a-axis [M. Gich et al., Chem. Mater. 18, 3889 (2006)]. Compared to the bulk band gap of 1.9 eV, the thin-film band gap is reduced to 1.3 eV in the Co-free films and to 0.7 eV in the film with partial …

Texture Development And Coercivity Enhancement In Cast Alnico 9 Magnets, Wenyong Zhang, Shah Valloppilly, Xingzhong Li, Lanping Yue, Ralph Skomski, Iver Anderson, Matthew Kramer, Wei Tang, Jeff Shield, David J. Sellmyer

Nebraska Center for Materials and Nanoscience: Faculty Publications

The effect of Y addition and magnetic field on texture and magnetic properties of arc-melted alnico 9 magnets has been investigated. Small additions of Y (1.5 wt.%) develop a (200) texture for the arc-melted alnico 9 magnet. Such a texture is hard to form in cast samples. To achieve this goal, we set up a high-field annealing system with a maximum operation temperature of 1250⁰ C. This system enabled annealing in a field of 45 kOe with subsequent draw annealing for the solutionized buttons; we have been able to substantially increase remanence ratio and coercivity, from 0.70 and 1200 …

Angle-Resolved Observation Of X-Ray Second Harmonic Generation In Diamond, Björn Senfftleben

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

This thesis reports angularly-resolved observation of X-ray second harmonic generation (XSHG) in diamond at several phase-matching geometries. The XSHG signal was produced by ultra-short, highly intense X-ray pulses with a photon energy of 9.831 keV generated by a free-electron laser. In some geometries for high pulse energies more than 10 second harmonic photons per pulse were generated.

Different phase-matched geometries were used for XSHG to investigate the angular dependence of the efficiency of the process. Furthermore, for each phase-matching condition, the quadratic dependence for second harmonic generation at each geometry was verified and the crystal rocking curves were measured. The …

A Measurement Of The Wγ Cross Section At √S = 8 Tev In Pp Collisions With The Cms Detector, Ekaterina Avdeeva

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

A measurement of cross section of the Wγ → lνγ production in proton-proton collisions using 19.6 fb − 1 of LHC data collected by CMS detector at the center- √ of-mass collision energy of s = 8 TeV is reported. The W bosons are identified in their electron and muon decay modes. The process of Wγ production in the Standard Model (SM) involves a pure gauge boson coupling, a WWγ vertex, which allows one to test the electroweak sector of the SM in a unique way not achievable by studies of other processes. In addition to the total cross section, …

Design And Construction Of A High-Current Femtosecond Gas-Phase Electron Diffraction Setup, Omid Zandi

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

We designed and constructed a state-of-the-art high current ultrafast gas electron diffraction experimental setup, which resolved two main challenges that constraint temporal resolution in previous setups. These aforementioned bottlenecks were: the space charge effect due to the Coulomb expansion, and the velocity mismatch between the sub-relativistic electrons (probe) and the exciting laser pulse (pump). In our setup, the problem of space charge effect was ameliorated by compressing 90 keV photo-emitted electron pulses using a radio-frequency electric field. The compression allowed us to increase the beam current by almost two orders of magnitude higher than previously reported. We developed a laser-activated …

From Quantum To Classical Interactions Between A Free Electron And A Surface, Peter Beierle

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

Quantum theory is often cited as being one of the most empirically validated theories in terms of its predictive power and precision. These attributes have led to numerous scientific discoveries and technological advancements. However, the precise relationship between quantum and classical physics remains obscure. The prevailing description is known as decoherence theory, where classical physics emerges from a more general quantum theory through environmental interaction. Sometimes referred to as the decoherence program, it does not solve the quantum measurement problem. We believe experiments performed between the microscopic and macroscopic world may help finish the program. The following considers a free …

Doubly-Excited State Effects On Two-Photon Double Ionization Of Helium By Time-Delayed, Oppositely Circularly-Polarized Attosecond Pulses, Jean Marcel Ngoko Djiokap, Anthony F. Starace

Anthony F. Starace Publications

We study two-photon double ionization (TPDI) of helium by a pair of time-delayed (non-overlapping), oppositely circularly-polarized attosecond pulses whose carrier frequencies are resonant with ¹P^o doubly-excited states. All of our TPDI results are obtained by numerical solution of the two-electron time-dependent Schrödinger equation for the six-dimensional case of circularly-polarized attosecond pulses, and they are analyzed using perturbation theory (PT). As compared with the corresponding nonresonant TPDI process, we find that the doubly-excited states change the character of vortex patterns in the two-electron momentum distributions for the case of back-to-back detection of the two ionized electrons in the polarization …

Imaging Electronic Motions By Ultrafast Electron Diffraction, Hua-Chieh Shao, Anthony F. Starace

Anthony F. Starace Publications

Recently ultrafast electron diffraction and microscopy have reached unprecedented temporal resolution, and transient structures with atomic precision have been observed in various reactions. It is anticipated that these extraordinary advances will soon allow direct observation of electronic motions during chemical reactions. We therefore performed a series of theoretical investigations and simulations to investigate the imaging of electronic motions in atoms and molecules by ultrafast electron diffraction. Three prototypical electronic motions were considered for hydrogen atoms. For the case of a breathing mode, the electron density expands and contracts periodically, and we show that the time-resolved scattering intensities reflect such changes …

Energy-Resolved Coherent Diffraction From Laser-Driven Electronic Motion In Atoms, Hua-Chieh Shao, Anthony F. Starace

Anthony F. Starace Publications

We investigate theoretically the use of energy-resolved ultrafast electron diffraction to image laser-driven electronic motion in atoms. A chirped laser pulse is used to transfer the valence electron of the lithium atom from the ground state to the first excited state. During this process, the electronic motion is imaged by 100-fs and 1-fs electron pulses in energy-resolved diffraction measurements. Simulations show that the angle-resolved spectra reveal the time evolution of the energy content and symmetry of the electronic state. The time-dependent diffraction patterns are further interpreted in terms of the momentum transfer. For the case of incident 1-fs electron pulses, …

Alignment Of The (3D104S5S)3S1 State Of Zn Excited By Polarized Electron Impact, Nathan B. Clayburn, Timothy J. Gay

Timothy J. Gay Publications

We measure the integrated Stokes parameters of light from Zn (4s4p)4³P_0,1-(4s5s)5³S₁ transitions excited by a transversely polarized electron impact at energies between 7.0 and 8.5 eV. Our results for the electron-polarization-normalized linear polarization Stokes parameter P₂, between incident electron energies 7.0 and 7.4 eV, are consistent with zero, as required by basic angular-momentum coupling considerations and by recent theoretical calculations. They are in qualitative disagreement with previous experimental results for the P₂ parameter.

Three-Dimensional Nanomagnetism, Amalio Fernández-Pacheco, Robert Streubel, Olivier Fruchart, Riccardo Hertel, Peter Fischer, Russell P. Cowburn

Robert Streubel Papers

Magnetic nanostructures are being developed for use in many aspects of our daily life, spanning areas such as data storage, sensing and biomedicine. Whereas patterned nanomagnets are traditionally two-dimensional planar structures, recent work is expanding nanomagnetism into three dimensions; a move triggered by the advance of unconventional synthesis methods and the discovery of new magnetic effects. In three-dimensional nanomagnets more complex magnetic configurations become possible, many with unprecedented properties. Here we review the creation of these structures and their implications for the emergence of new physics, the development of instrumentation and computational methods, and exploitation in numerous applications.

Ionization Enhancement And Suppression By Phase-Locked Ultrafast Pulse Pairs, David B. Foote, Y. Lin, Liang-Wen Pi, Jean Marcel Ngoko Djiokap, Anthony F. Starace, W. T. Hill

Anthony F. Starace Publications

We present the results of a study of ionization of Xe atoms by a pair of phase-locked pulses, which is characterized by interference produced by the twin peaks. Two types of interference are considered: ordinary optical interference, which changes the intensity of the composite pulse and thus the ion yield, and a quantum interference, in which the excited electron wave packets interfere. We use the measured Xe+ yield as a function of the temporal delay and/or relative phase between the peaks to monitor the interferences and compare their relative strengths. We model the interference with a pulse intensity function and …

Nanosecond X-Ray Photon Correlation Spectroscopy On Magnetic Skyrmions, M. H. Seaberg, B. Holladay, J. C.T. Lee, M. Sikorski, A. H. Reid, S. A. Montoya, G. L. Dakovski, J. D. Koralek, G. Coslovich, S. Moeller, W. F. Schlotter, R. Streubel, S. D. Kevan, P. Fischer, E. E. Fullerton, J. L. Turner, F. J. Decker, S. K. Sinha, S. Roy, J. J. Turner

Robert Streubel Papers

We report an x-ray photon correlation spectroscopy method that exploits the recent development of the two-pulse mode at the Linac Coherent Light Source. By using coherent resonant x-ray magnetic scattering, we studied spontaneous fluctuations on nanosecond time scales in thin films of multilayered Fe/Gd that exhibit ordered stripe and Skyrmion lattice phases. The correlation time of the fluctuations was found to differ between the Skyrmion phase and near the stripe-Skyrmion boundary. This technique will enable a significant new area of research on the study of equilibrium fluctuations in condensed matter.

Adiabatic-Limit Coulomb Factors For Photoelectron And High-Order-Harmonic Spectra, M. V. Frolov, N. L. Manakov, A. A. Minina, S. V. Popruzhenko, Anthony F. Starace

Anthony F. Starace Publications

A momentum-dependent Coulomb factor in the probability for nonlinear ionization of atoms by a strong low-frequency laser field is calculated analytically in the adiabatic approximation. Expressions for this Coulomb factor, valid for an arbitrary laser pulse waveform, are obtained and analyzed in detail for the cases of linear and circular polarizations. The dependence of the Coulomb factor on the photoelectron momentum is shown to be significant in both cases. Using a similar technique, the Coulomb factor for emission of high-order harmonics by an atom in a bichromatic laser field is also calculated. In contrast to the case of a single-frequency …

Kinematical Vortices In Double Photoionization Of Helium By Attosecond Pulses, Jean Marcel Ngoko Djiokap, A. V. Meremianin, N. L. Manakov, S. X. Hu, L. B. Madsen, Anthony F. Starace

Anthony F. Starace Publications

Two-armed helical vortex structures are predicted in the two-electron momentum distributions produced in double photoionization (DPI) of the He atom by a pair of time-delayed elliptically polarized attosecond pulses with opposite helicities. These predictions are based upon both a first-order perturbation theory analysis and numerical solutions of the two-electron, time-dependent Schrödinger equation in six spatial dimensions. The helical vortex structures originate from Ramsey interference of a pair of ionized two-electron wave packets, each having a total angular momentum of unity, and appear in the sixfold differential DPI probability distribution for any energy partitioning between the two electrons. The vortex structures …

Simulation Of Alnico Coercivity, Liqin Ke, Ralph Skomski, Todd D. Hoffman, Lin Zhoue, Wei Tang, Duane D. Johnson, Matthew J. Kramer, Iver E. Anderson, C.Z. Wang

Nebraska Center for Materials and Nanoscience: Faculty Publications

Micromagnetic simulations of alnico show substantial deviations from Stoner-Wohlfarth behavior due to the unique size and spatial distribution of the rod-like Fe-Co phase formed during spinodal decomposition in an external magnetic field. The maximum coercivity is limited by single-rod effects, especially deviations from ellipsoidal shape, and by interactions between the rods. Both the exchange interaction between connected rods and magnetostatic interaction between rods are considered, and the results of our calculations show good agreement with recent experiments. Unlike systems dominated by magnetocrystalline anisotropy, coercivity in alnico is highly dependent on size, shape, and geometric distribution of the Fe-Co phase, all …

Elastic Properties Of Superconductors And Materials With Weakly Correlated Spins, Christian Binek

Christian Binek Publications

It is shown that in the ergodic regime, the temperature dependence of Young’s modulus is solely determined by the magnetic properties of a material. For the large class of materials with paramagnetic or diamagnetic response, simple functional forms of the temperature derivative of Young’s modulus are derived and compared with experimental data and empirical results. Superconducting materials in the Meissner phase are ideal diamagnets. As such, they display remarkable elastic properties. Constant diamagnetic susceptibility gives rise to a temperature independent elastic modulus for ceramic and single crystalline superconductors alike. The thermodynamic approach established in this report, paves the way to …

Vortex Circulation Patterns In Planar Microdisk Arrays, Sven Velten, Robert Streubel, Alan Farhan, Noah Kent, Mi Young Im, Andreas Scholl, Scott Dhuey, Carolin Behncke, Guido Meier, Peter Fischer

Robert Streubel Papers

We report a magnetic X-ray microscopy study of the pattern formation of circulation in arrays of magnetic vortices ordered in a hexagonal and a honeycomb lattice. In the honeycomb lattice, we observe at remanence an ordered phase of alternating circulations, whereas in the hexagonal lattice, small regions of alternating lines form. A variation in the edge-to-edge distance shows that the size of those regions scales with the magnetostatic interaction. Micromagnetic simulations reveal that the patterns result from the formation of flux closure states during the nucleation process.

Ferroelectric-Domain-Patterning-Controlled Schottky Junction State In Monolayer Mos2, Zhiyong Xiao, Jingfeng Song, David K. Ferry, Stephen Ducharme, Xia Hong

Stephen Ducharme Publications

We exploit scanning-probe-controlled domain patterning in a ferroelectric top layer to induce nonvolatile modulation of the conduction characteristic of monolayer MoS₂ between a transistor and a junction state. In the presence of a domain wall, MoS₂ exhibits rectified I-V characteristics that are well described by the thermionic emission model. The induced Schottky barrier height Φ^eff_B varies from 0.38 to 0.57 eV and is tunable by a SiO₂ global back gate, while the tuning range of Φ^eff_B depends sensitively on the conduction-band-tail trapping states. Our work points to a new route to achieving programmable …

Implementation And Modeling Of A Femtosecond Laser-Activated Streak Camera, Omid Zandi, Kyle J. Wilkin, Martin Centurion

Martin Centurion Publications

8 June 2017) A laser-activated streak camera was built to measure the duration of femtosecond electron pulses. The streak velocity of the device is 1.89 mrad/ps, which corresponds to a sensitivity of 34.9 fs/pixels. The streak camera also measures changes in the relative time of arrival between the laser and electron pulses with a resolution of 70 fs RMS. A full circuit analysis of the structure is presented to describe the streaking field and the general behavior of the device. We have developed a general mathematical model to analyze the streaked images. The model provides an accurate method to extract …

Discontinuities In The Electromagnetic Fields Of Vortex Beams In The Complex Source-Sink Model, Andrew Vikartofsky, Liang-Wen Pi, Anthony F. Starace

Anthony F. Starace Publications

An analytical discontinuity is reported in what was thought to be the discontinuity-free exact nonparaxial vortex beam phasor obtained within the complex source-sink model. This discontinuity appears for all odd values of the orbital angular momentum mode. Such discontinuities in the phasor lead to nonphysical discontinuities in the real electromagnetic field components. We identify the source of the discontinuities, and provide graphical evidence of the discontinuous real electric fields for the first and third orbital angular momentum modes. A simple means of avoiding these discontinuities is presented.

High Current Table-Top Setup For Femtosecond Gas Electron Diffraction, Omid Zandi, Kyle J. Wilkin, Martin Centurion

Martin Centurion Publications

We have constructed an experimental setup for gas phase electron diffraction with femtosecond resolution and a high average beam current. While gas electron diffraction has been successful at determining molecular structures, it has been a challenge to reach femtosecond resolution while maintaining sufficient beam current to retrieve structures with high spatial resolution. The main challenges are the Coulomb force that leads to broadening of the electron pulses and the temporal blurring that results from the velocity mismatch between the laser and electron pulses as they traverse the sample. We present here a device that uses pulse compression to overcome the …

Enhancing High-Order-Harmonic Generation By Time Delays Between Two-Color, Few-Cycle Pulses, Dian Peng, Liang-Wen Pi, M. V. Frolov, Anthony F. Starace

Anthony F. Starace Publications

Use of time delays in high-order-harmonic generation (HHG) driven by intense two-color, few-cycle pulses is investigated in order to determine means of optimizing HHG intensities and plateau cutoff energies. Based upon numerical solutions of the time-dependent Schrõdinger equation for the H atom as well as analytical analyses, we show that introducing a time delay between the two-color, few-cycle pulses can result in an enhancement of the intensity of the HHG spectrum by an order of magnitude (or more) at the cost of a reduction in the HHG plateau cutoff energy. Results for both positive and negative time delays as well …

Effect Of Disorder On The Resistivity Of Cofecral Films, Y. Jin, R. Skomski, P. Kharel, S.R. Valloppilly, D. J. Sellmyer

Nebraska Center for Materials and Nanoscience: Faculty Publications

Structural and electron-transport properties of thin films of the ferrimagnetic Heusler compound CoFeCrAl have been investigated to elucidate structure-property relationships. The alloy is, ideally, a spin-gapless semiconductor, but structural disorder destroys the spin-gapless character and drastically alters the transport behavior. Two types of CoFeCrAl films were grown by magnetron sputtering deposition at 973 K, namely polycrystalline films on Si substrates and epitaxial films on MgO (001) substrates. The resistivity decreases with increasing temperature, with relatively small temperature coefficients of –0.19 cm=K for the polycrystalline films and –0.12 cm=K for the epitaxial films. The residual resistivity of the polycrystalline films deposited …

Half-Metallic Magnetism In Ti3co5-Xfexb2, Rohit Pathak, Imran Ahamed, W. Y. Zhang, Shah Vallopilly, D. J. Sellmyer, Ralph Skomski, Arti Kashyap

Nebraska Center for Materials and Nanoscience: Faculty Publications

Bulk alloys and thin films of Fe-substituted Ti₃Co₅B₂ have been investigated by first-principle density-functional calculations. The series, which is of interest in the context of alnico magnetism and spin electronics, has been experimentally realized in nanostructures but not in the bulk. Our bulk calculations predict paramagnetism for Ti₃Co₅B₂, Ti₃Co₄FeB₂ and Ti₃CoFe₄B₂, whereas Ti₃Fe₅B₂ is predicted to be ferromagnetic. The thin films are all ferromagnetic, indicating that moment formation may be facilitated at nanostructural …

Cooperative And Noncooperative Magnetization Reversal In Alnicos, Raplh Skomski, Liqin Ke, Matthew J. Kramer, Iver E. Anderson, C.Z. Wang, W.Y. Zhang, Jeff E. Shield, D. J. Sellmyer

Nebraska Center for Materials and Nanoscience: Faculty Publications

It is investigated how magnetostatic interactions affect the coercivity of alnicotype magnets. Starting from exact micromagnetic relations, we analyze two limits, namely cooperative reversal processes operative on short lengths scales and noncooperative reversal processes on long length scales. In alnicos, intrawire interactions are predominantly cooperative, whereas interwire effects are typically noncooperative. However, the transition between the regimes depends on feature size and hysteresis-loop shape, and interwire cooperative effects are largest for nearly rectangular loops. Our analysis revises the common shape-anisotropy interpretation of alnicos.

Two-Color Multiphoton Emission From Nanotips, Wayne Cheng-Wei Huang, Maria Becker, Joshua Beck, Herman Batelaan

Department of Physics and Astronomy: Faculty Publications

Two-color multiphoton emission from polycrystalline tungsten nanotips has been demonstrated using two-color laser fields. The two-color photoemission is assisted by a three-photon multicolor quantum channel, which leads to a twofold increase in quantum efficiency. Weak-field control of two- color multiphoton emission was achieved by changing the efficiency of the quantum channel with pulse delay. The result of this study complements two-color tunneling photoemission in strong fields, and has potential applications for nanowire-based photonic devices. Moreover, the demonstrated two-color multiphoton emission may be important for realizing ultrafast spin-polarized electron sources via optically injected spin current.

Epitaxial Strain Controlled Magnetocrystalline Anisotropy In Ultrathin Ferh/Mgo Bilayers, Guohul Zheng, San-Huang Ke, Maosheng Miao, Jinwoong Kim, R, Ramesh, Nicholas Kioussis

Nebraska Center for Materials and Nanoscience: Faculty Publications

Using ab initio electronic structure calculations we have investigated the effect of epitazial strain on the magnetocrystalline anisotropy (MCA) of ultrathin FeRh/MgO heterostructures. Analysis of the energy- and k-resolved distribution of the orbital character of the band structure reveals that MCA largely arises from the spin-orbit coupling (SOC) between d_x2-y2 and d_xz/d_yz orbitlas of Fe atoms at the FeRh/MhO interface. We demonstrate that the strain has significant effects on the MCA: It not only affects the value of the MCA but also induces a switching of the magnetic easy axis from perpendicular on in-plane direction. The …

First-Principle Investigation Of Structural, Electronic And Magnetic Properties Of Co2vin And Covin Heusler Compounds, Zipporah Muthui, Rohit Pathak, Robinson Musembi, Julius Mwabora, Ralph Skomski, Arti Kashyap

Nebraska Center for Materials and Nanoscience: Faculty Publications

Investigation of the structural, electronic and magnetic properties of full-Heusler Co₂VIn as well as half-Heusler CoVIn Cobalt based Heusler compounds using density functional theory (DFT) leads to the general conclusion that Co₂VIn and CoVIn are half-metallic materials with a gap at the Fermi level in the minority states and majority states respectively. A Hubbard-like Coulomb correlation term U has been included in the DFT (DFT+U) for the computation of the electronic and magnetic properties of the compounds. The structural properties have been calculated for the paramagnetic and ferromagnetic phases, and both Co₂VIn …

Effect Of Disorder On The Magnetic And Electronic Structure Of A Prospective Spin-Gapless Semiconductor Mncrval, P. Kharel, J. Herran, P. Lukashev, Y. Jin, J. Waybright, S. Gilbert, B. Staten, P. Gray, S. Valloppilly, Y. Huh, D. J. Sellmyer

Nebraska Center for Materials and Nanoscience: Faculty Publications

Recent discovery of a new class of materials, spin-gapless semiconductors (SGS), has attracted considerable attention in the last few years, primarily due to potential applications in the emerging field of spin-based electronics (spintronics). Here, we investigate structural, electronic, and magnetic properties of one potential SGS compound, MnCr-VAl, using various experimental and theoretical techniques. Our calculations show that this material exhibits ≈ 0.5 eV band gap for the majority-spin states, while for the minority-spin it is nearly gapless. The calculated magnetic moment for the com- pletely ordered structure is 2.9 μ_B/f.u., which is different from our experimentally measured value …