Physical Sciences and Mathematics Commons^™

Magnetism Of Cr-Doped Diamond-Like Carbon, J. A. Colón Santana, Ralph Skomski, V. R. Singh, V. Palshin, A. Petukhov, Yaroslav B. Losovyj, Andrei Sokolov, Peter A. Dowben, Ihor Ketsman

Peter Dowben Publications

Chromium-doped hydrogenated diamond-like carbon (Cr-DLC) and chromium carbide hydrogenated DLC alloys were synthesized by plasma-assisted vapor deposition and investigated by x-ray absorption near edge structure spectroscopy, extended x-ray absorption fine structure, and superconducting quantum interference device (SQUID) magnetometry. Structural and magnetic properties of the doped and alloy materials were investigated as a function of the Cr concentration (0.1–20 at. %). Toward the upper end of the concentration range, Cr precipitates in the form of chromium carbide (Cr₃C₂) nanoclusters. For low Cr concentrations, the systems are ferromagnetic at very low temperatures, whereas the chromium carbide clusters appear …

Comparison Of N-Type Gd2O3 And Gd-Doped Hfo2, Yaroslav B. Losovyj, David Wooten, Juan Colon Santana, Joonhee Michael An, Kirill D. Belashchenko, N. Lozova, J. C. Petrosky, Andrei Sokolov, Jinke Tang, Wendong Wang, Navamoney Arulsamy, Peter A. Dowben

Peter Dowben Publications

Gd₂O₃ and Gd-doped HfO₂ films were deposited on p-type silicon substrates in a reducing atmosphere. Gd 4f photoexcitation peaks at roughly 7 and 5 eV below the valence band maximum have been identified using the resonant photoemission of Gd₂O₂ and Gd-doped HfO₂ films, respectively. In the case of Gd₂O₃, strong hybridization with the O 2p band is demonstrated, and there is evidence that the Gd 4f weighted band exhibits dispersion in the bulk band structure. The rectifying (diode-like) properties of Gd-doped HfO₂–silicon and Gd₂O …

Radiation-Induced Decomposition Of The Metal-Organic Molecule Bis(4-Cyano-2,2,6,6-Tetramethyl-3,5-Heptanedionato)Copper(Ii), David Wisbey, Ning Wu, Yaroslav B. Losovyj, Ihor Ketsman, A. N. Caruso, Danqin Feng, John Belot, Elio Vescovo, Peter A. Dowben

Peter Dowben Publications

The effects of vacuum ultraviolet radiation on the adsorbed copper center molecule bis(4-cyano-2,2,6,6- tetramethyl-3,5-heptanedionato)copper(II) (or Cu(CNdpm)₂), (C₂₄H₃₆N₂O₄Cu, Cu(II)) was studied by photoemission spectroscopy. Changes in the ultraviolet photoemission spectra (UPS) of Cu(CNdpm)₂, adsorbed on Co(1 1 1), indicate that the ultraviolet radiation leads to decomposition of Cu(CNdpm)₂ and this decomposition is initially dominated by loss of peripheral hydrogen.

Induced Spin Polarization Of Copper Spin 1/2 Molecular Layers, David Wisbey, Ning Wu, Danqin Feng, A. N. Caruso, J. Belot, Yaroslav B. Losovyj, Elio Vescovo, Peter A. Dowben

Peter Dowben Publications

Thin films of the metal organic molecule bis(4-cyano-2,2,6,6-tetramethyl-3,5-heptanedionato)copper(II) (or Cu(CNdpm)₂), (C₂₄H₃₆N₂O₄Cu, Cu(II)), deposited on ferromagnetic Co(111) at 40 K, exhibit a finite electron spin polarization. The spin polarization magnitude and sign for Cu(CNdpm)₂ deposited on Co(111) is coverage dependent, but deviates from the mean field expectations for a simple paramagnet on a ferromagnetic substrate. The spin asymmetry is seen to favor select molecular orbitals, consistent with the predicted single molecule density of states. The overlayer polarization observed indicates a strong influence of the ferromagnetic Co(111) substrate and some extra-molecular …

4f Hybridization And Band Dispersion In Gadolinium Thin Films And Compounds, Takashi Komesu, H.-K. Jeong, David Wooton, Yaroslav B. Losovyj, J. N. Crain, M. Bissen, F. J. Himpsel, J. C. Petrosky, Jinke Tang, Wendong Wang, I. N. Yakovkin, Peter A. Dowben

Peter Dowben Publications

There is interplay between intra-atomic orbital hybridization and extra-atomic hybridization in various gadolinium systems, which affects magnetic coupling and electron itinerancy (localization). The results do not always follow expectation. The experimental band structure of thin Gd(0001) films, grown on the Mo(112) surface, along the Γ − M does not agree with expectations even qualitatively. In particular, the dispersion of the gadolinium band, with strong 5d weight near 2 eV binding energy provides considerable evidence to support the case for 4f- 5d hybridization, with increasing 5d localization. On the other hand, there is also evidence of extra-atomic Gd 4f hybridization leading …

The Effect Of Defects On The Electronic Structure Of Long Chain Ferroelectric Polymers, Jie Xiao, Xin Zhou, Q. M. Zhang, Peter A. Dowben

Peter Dowben Publications

The apparent molecular density of states from combined photoemission and inverse photoemission spectroscopies is compared to the bulk conductivity and frequency dependent dielectric measurements for a variety of ferroelectric polymer poly(vinylidene fluoride) (PVDF) copolymers and terpolymers. While some extra chemical groups introduced into the PVDF long chains increase the measured conductivities at room temperature and can cause significant changes in the dielectric properties, few changes are observed in the overall experimental molecular density of states. The exception appears to be the very short polymer chains. The significance of this comparison is that profound changes can occur in the overall dielectric …

The Minority Spin Surface Bands Of Cos2(001), Ning Wu, Renat F. Sabirianov, Wai-Ning Mei, Yaroslav B. Losovyj, N. Lozova, M. Manno, C. Leighton, Peter A. Dowben

Peter Dowben Publications

Angle-resolved photoemission was used to study the surface electronic band structure of high quality single crystals of ferromagnetic CoS₂ (below 120 K). Strongly dispersing Co t_2g bands are identified along the (100) k _ǁ direction, the Γ-X line of the surface Brillouin zone, in agreement with model calculations. The calculated surface band structure includes corrections for the previously determined surface structure of CoS₂(001) and is in general agreement with the experimental photoemission spectra in the region of the Fermi level. There is evidence of the existence of several minority spin surface states, falling into a gap …

The Surface Relaxation And Band Structure Of Mo(112), Ning Wu, Yaroslav B. Losovyj, Zhaoxian Yu, Renat F. Sabirianov, Wai-Ning Mei, N. Lozova, J. A. Colón Santana, Peter A. Dowben

Peter Dowben Publications

The experimental and theoretical surface band structures of Mo(112) are compared. This surface band structure mapping is presented with corrections included for the lattice relaxation of the Mo(112) surface. Quantitative low energy electron diffraction (LEED) has been used to determine the details of the Mo(112) surface structure. The first layer contraction is 14.9% by LEED intensity versus voltage analysis and is in general agreement with the 17.6% contraction found from total surface energy optimization. The electronic band structure is mapped out along Γ-X and Γ–Y of the surface Brillouin zone (SBZ). There is strong evidence of electron–phonon coupling particularly in …

Microstructures And Magnetic Properties Of Rapidly Solidified Ni54Fe27−2xGa19+2x Ferromagnetic Heusler Alloys, S. Aich, S. Das, I. A. Al-Omari, P. Alagarsamy, S. G. Chowdhury, M. Chakraborty, Jeffrey E. Shield, David J. Sellmyer

David Sellmyer Publications

Rapidly solidified Ni₅₄Fe_27−2xGa_19+2x (x=0, 1, 2, 3, and 4) ferromagnetic shape memory alloys were made by melt-spinning with variation of Fe and Ga contents to report on the martensitic phase transformation, microstructures, and magnetic properties. Rapid solidification produced pure L2₁ phase by preventing the formation of γ-phase. To study the effect of heat treatment on the phase transitions, microstructures, and the magnetic properties, the melt-spun ribbons were partly heat treated at different temperatures of 800, 900, 1000, 1100, and 1200 K with holding times of 5, 10, and 15 min followed by either water …

Use Of Partial-Wave Decomposition To Identify Resonant Interference Effects In The Photoionization–Excitation Of Argon, Timothy J. Gay, Chris H. Greene, J. R. Machacek, K. W. Mclaughlin, H. W. Van Der Hart, Orhan Yenen, Duane H. Jaecks

Timothy J. Gay Publications

We have studied simultaneous photoionization and excitation of Ar in the range of incident photon energies between 36.00 and 36.36 eV, where the resonant production of doubly excited neutral Ar states imbedded in the ionization continuum is dominant. By measuring the relative Stokes parameters of the fluorescence from residual Ar^+∗ (3p⁴ [³P] 4p) ions (²P_1/2, 465.8 nm transition; ²P_3/2, 476.5 nm; ²D_3/2, 472.7 nm; ²D_5/2, 488.0 nm; ⁴P_5/2, 480.6 nm; ⁴D_5/2, 514.5 nm) we demonstrate a technique …

Self-Assembly Of Magnetic Ni Nanoparticles Into 1d Arrays With Antiferromagnetic Order, Valery Bliznyuk, Srikanth Singamaneni, Sarbeswar Sahoo, Srinivas Polisetty, Xi He, Christian Binek

Christian Binek Publications

In this paper, we report on the magnetic properties of isolated nanoparticles and interacting nanochains formed by the self-assembly of Ni nanoparticles. The magnetic properties were studied using superconducting quantum interference device (SQUID) magnetometry and magnetic force microscopy (MFM). We demonstrate that single-domain Ni nanoparticles spontaneously form one-dimensional (1D) chains under the influence of an external magnetic field. Furthermore, such magnetic field-driven self-assembly in conjunction with surface templating produces regular arrays of 1D nanochains with antiferromagnetic intra-chain order. The antiferromagnetic order, which is in striking contrast to what is found for non-interacting nanoparticle assemblies within the chains, can be evidenced …

Electron Attachment To Trans-Azobenzene, Alberto Modelli, Paul Burrow

Paul Burrow Publications

The temporary anion states of gas-phase trans-azobenzene are characterized by means of electron transmission spectroscopy (ETS) in the 0–6 eV range. The measured energies of vertical electron attachment are compared with the energies of the π* virtual orbitals of the neutral molecule supplied by HF (at MP2 optimized geometries) and B3LYP calculations. The calculated energies, scaled with empirical equations, reproduce quantitatively the energies of the corresponding spectral features and predict a positive vertical electron affinity of 0.83 eV.

The total anion current at the walls of the collision chamber and the mass-selected molecular anion current are also reported as …

Development Of A Source Of Quasi-Monochromatic Mev Energy Photons, Donald Umstadter, Sudeep Banerjee, Viswanathan Ramanathan, Nathan D. Powers, Nathaniel Cunningham, Nathan A. Chandler-Smith

Donald Umstadter Publications

We report current progress on a project to develop an all-optically-driven x-ray photon source. A laser pulse with 40-50 TW of peak power is focused on a supersonic helium nozzle to drive a relativistic plasma wave. Electron beams with energies of 320 MeV (+/- 28 MeV) are accelerated by means of laser wakefield acceleration. Remarkably, the acceleration region is only 3 mm in length. This accelerator is currently being employed to demonstrate the generation of MeV- energy x-ray by means of all-optical Thomson scattering. By this mechanism, a lower power, laser pulse (from the same laser system) is focused onto …

Prediction Of A Spin-Polarized Two-Dimensional Electron Gas At The Laalo3/Euo(001) Interface, Yong Wang, Manish K. Niranjan, John D. Burton, Joonhee M. An, Kirill D. Belashchenko, Evgeny Y. Tsymbal

Evgeny Tsymbal Publications

First-principles calculations predict the existence of a spin-polarized two-dimensional electron gas (2DEG) at the LaO/EuO interface in a LaAlO₃/EuO(001) heterostructure. This polar interface favors electron doping into the Eu-5d conduction bands resulting in a 2DEG formed at the interface.

Prediction Of Electrically Induced Magnetic Reconstruction At The Manganite/Ferroelectric Interface, John D. Burton, Evgeny Y. Tsymbal

Evgeny Tsymbal Publications

The control of magnetization via the application of an electric field, known as magnetoelectric coupling, is among the most fascinating and active research areas today. In addition to fundamental scientific interest, magnetoelectric effects may lead to new device concepts for data storage and processing. There are several known mechanisms for magnetoelectric coupling that include intrinsic effects in single-phase materials, strain-induced coupling in two-phase composites, and electronically driven effects at interfaces. Here we explore a different type of magnetoelectric effect at a ferromagnetic-ferroelectric interface: magnetic reconstruction induced by switching of electric polarization.

Prediction Of A Switchable Two-Dimensional Electron Gas At Ferroelectric Oxide Interfaces, Manish K. Niranjan, Yong Wang, Sitaram S. Jaswal, Evgeny Y. Tsymbal

Evgeny Tsymbal Publications

The demonstration of a quasi-two-dimensional electron gas (2DEG) in LaAlO₃=SrTiO₃ heterostructures has stimulated intense research activity in recent years. The 2DEG has unique properties that are promising for applications in all-oxide electronic devices. For such applications it is desirable to have the ability to control 2DEG properties by external stimulus. Here, based on first-principles calculations we predict that all-oxide heterostructures incorporating ferroelectric constituents, such as KNbO₃=ATiO₃ (A = Sr, Ba, Pb), allow creating a 2DEG switchable between two conduction states by ferroelectric polarization reversal. The effect occurs due to the screening charge at …

Ultrashort Pulse Electron Gun With A Mhz Repetition Rate, D. Wytrykus, Martin Centurion, P. Reckenthaeler, F. Krausz, A. Apolonski, E. Fill

Martin Centurion Publications

We report the construction of an electron gun emitting ultrashort pulses with a repetition rate of 2.7 MHz. The gun works at an acceleration voltage of 20 kV and is operated with a laser oscillator having an ultralong cavity. A low number of electrons per pulse eliminates space charge broadening. Electron yield and beam profiles are measured for operation with laser wavelengths of 800, 400, and 266 nm. The initial energy spread of the electrons is determined for these three wavelengths, and pulse durations of 600, 390, and 270 fs are inferred from the data.

Complex Magnetic Phase In Submonolayer Fe Stripes On Pt(997), Jan Honolka, T. Y. Lee, K. Kuhnke, D. Repetto, V. Sessi, Peter Wahl, A. Buchsbaum, P. Varga, S. Gardonio, C. Carbone, S. R. Krishnakumar, P. Gambardella, M. Komelj, R. Singer, M. Fähnle, K. Fauth, G. Schütz, Axel Enders, K. Kern

Axel Enders Publications

Correlations between magnetism and morphology of iron nanostructures of monatomic height on Pt(997) substrates are studied using x-ray magnetic circular dichroism as well as scanning tunneling microscopy and helium scattering. A drastic collapse of the average magnetization by more than a factor of 4 is observed when increasing the iron coverage from 0.1 to 0.2 ML. This effect goes along with a softening of the magnetic anisotropy energy and a gradual reorientation of the magnetic easy axis from in plane to out of plane. The experimental findings together with electronic density-functional calculations suggest the formation of a complex magnetic phase …

Electron Attachment To Cl2 From 300 To 1100 K: Experiment And Theory, Jeffrey Friedman, Thomas Miller, Linda Schaffer, A. A. Viggiano, Ilya I. Fabrikant

Ilya Fabrikant Publications

Rate constants for dissociative electron attachment to Cl₂ have been measured from 300 to 1100 K in a high-temperature flowing-afterglow Langmuir-probe apparatus. R-matrix calculations have been carried out which compare well with the present measurements with possible deviation at the highest temperatures. The attachment rate constants do not show Arrhenius behavior. The temperature dependence of the calculated rate constants for successive vibrational levels provides insight as to this behavior. While the lowest vibrational level of Cl₂ dominates attachment at low temperatures, the rate constant is not flat with temperature because of the p-wave character of the …

Magnetism Of Tio And Tio2 Nanoclusters, Xiaohui Wei, Ralph Skomski, Balamurugan Balasubramanian, Z. G. Sun, Stephen Ducharme, David J. Sellmyer

Ralph Skomski Publications

Nanoclusters of rocksalt TiO, anatase TiO₂, and rutile TiO₂ were produced by cluster deposition and examined with transmission-electron microscopy, x-ray diffraction, and magnetization measurements. The clusters are all magnetic at room temperature, but the magnetization is structure-dependent. The hysteresis loops show coercivities that are of the order of 100 Oe and all films show a preferential in-plane magnetization direction. The size dependence of the magnetization was investigated for rutile clusters with average sizes from about 15 to 40 nm. The analysis of the measurements indicates that the magnetism is predominantly located near the surface of the clusters …

Graded Permanent Magnets, Ralph Skomski, J. Hadjipanayis, David J. Sellmyer

Ralph Skomski Publications

The effect of semihard magnetic phases and interfaces on the performance of nanostructured two-phase permanent magnets is investigated by model calculations. In addition to the trivial coercivity increase due to the replacement of soft regions by semihard regions, there is a coercivity enhancement even if the volume-averaged anisotropy is kept constant during the introduction of the semihard phase. A variational approach is used to derive analytical results for representative anisotropy profiles. The improvement is operative on length scales slightly larger than that of the soft phase in hard-soft composites, but the main challenge is to find semihard light or heavy …

Temporal Lenses For Attosecond And Femtosecond Electron Pulses, Shawn A. Hilbert, Cornelis J. Uiterwaal, Brett E. Barwick, Herman Batelaan, Ahmed H. Zewail

C.J.G.J. Uiterwaal Publications

Here, we describe the ‘‘temporal lens’’ concept that can be used for the focus and magnification of ultrashort electron packets in the time domain. The temporal lenses are created by appropriately synthesizing optical pulses that interact with electrons through the ponderomotive force. With such an arrangement, a temporal lens equation with a form identical to that of conventional light optics is derived. The analog of ray diagrams, but for electrons, are constructed to help the visualization of the process of compressing electron packets. It is shown that such temporal lenses not only compensate for electron pulse broadening due to velocity …

Mining Sky Surveys For Astrophysically Interesting Variable Stars: The Cepheid Period Range, Edward G. Schmidt

Edward Schmidt Publications

We have undertaken a project to discover new type II Cepheids in two large-area sky surveys. However, most of the candidates from the surveys differ from type II Cepheids in their amplitude and period distributions and they form a separate, cooler sequence relative to the Cepheids in a temperature-gravity plot. We refer to these objects as "Cepheid-lke" stars. Many exhibit unstable pulsation which we illustrate with representative light curves.

Preface To The Proceedings Of The Xxvi International Conference On Photonic, Electronic And Atomic Collisions, Ann Orel, Anthony F. Starace, Dragan Nikolić, Nora Berrah, Thomas Gorczyca, Emanuel Kamber, John A. Tanis

Anthony F. Starace Publications

The XXVIth International Conference on Photonic, Electronic and Atomic Collisions was held on the campus of Western Michigan University (WMU) in Kalamazoo during 22-28 July 2009. Kalamazoo, the home of a major state university amid pleasant surroundings, was a delightful place for the conference. The 473 scientific participants, 111 of whom were students, had many fruitful discussions and exchanges that contributed to the success of the conference. Participants from 43 countries made the conference truly international in scope.

The plenary lectures and the progress and special reports bridged the gap between the different fields of collision physics, making it possible …

First-Principles Analysis Of Spin-Disorder Resistivity Of Fe And Ni, Aleksander L. Wysocki, Renat F. Sabirianov, Mark Van Schilfgaarde, Kirill D. Belashchenko

Department of Physics and Astronomy: Faculty Publications

Spin-disorder resistivity of Fe and Ni and its temperature dependence are analyzed using noncollinear density functional calculations within the supercell method. Different models of thermal spin disorder are considered, including the mean-field approximation and the nearest-neighbor Heisenberg model. If the local moments are kept frozen at their zero-temperature values, very good agreement with experiment is obtained for Fe but for Ni the resistivity at elevated temperatures is significantly overestimated. Agreement with experiment for Fe is improved if the local moments are made self-consistent. The effect of short-range order on spindisorder resistivity is more pronounced in Ni compared to Fe but …

Magnetism Of Chromia From First-Principles Calculations, Siqi Shi, Aleksander L. Wysocki, Kirill D. Belashchenko

Department of Physics and Astronomy: Faculty Publications

The electronic structure and magnetism of chromia (corundum-type Cr₂O_{3) are studied using full-potential first-principles calculations. The electronic correlations are included within the LSDA+U method. The energies of different magnetic configurations are very well fitted by the Heisenberg Hamiltonian with strong exchange interaction with two nearest neighbors and additional weak interaction up to the fifth neighbor shell. These energies are insensitive to the position of the oxygen states, indicating that magnetism in Cr2O3 is dominated by direct exchange. The Néel temperature is calculated using the pair-cluster approximation for localized quantum spins of magnitude 3/2. …}

Voltage-Induced Switching With Magnetoresistance Signature In Magnetic Nano-Filaments, Andrei Sokolov, Renat F. Sabirianov, Ildar F. Sabiryanov, Bernard Doudin

Department of Physics and Astronomy: Faculty Publications

Large hysteretic resistance changes are reported on sub-100 nm diameter metallic nanowires including thin dielectric junctions. Bi-stable 50% switching in a double junction geometry is modeled in terms of an occupation-driven metal–insulator transition in one of the two junctions, using the generalized Poisson expressions of Oka and Nagaosa (2005 Phys. Rev. Lett. 95 266403). It illustrates how a band bending scheme can be generalized for strongly correlated electron systems. The magnetic constituents of the nanowires provide a magnetoresistive signature of the two resistance states, confirming our model and enabling a four states device application.

Feynman’S Relativistic Electrodynamics Paradox And The Aharonov-Bohm Effect, Adam Caprez, Herman Batelaan

Department of Physics and Astronomy: Faculty Publications

An analysis is done of a relativistic paradox posed in the Feynman Lectures of Physics involving two interacting charges. The physical system presented is compared with similar systems that also lead to relativistic paradoxes. The momentum conservation problem for these systems is presented. The relation between the presented analysis and the ongoing debates on momentum conservation in the Aharonov-Bohm problem is discussed.

Nonlinear Laser Energy Depletion In Laser-Plasma Accelerators, Bradley Allan Shadwick, Carl B. Schroeder, Eric Esarey

Department of Physics and Astronomy: Faculty Publications

Energy depletion of intense, short-pulse lasers via excitation of plasma waves is investigated numerically and analytically. The evolution of a resonant laser pulse proceeds in two phases. In the first phase, the pulse steepens, compresses, and frequency redshifts as energy is deposited in the plasma. The second phase of evolution occurs after the pulse reaches a minimum length at which point the pulse rapidly lengthens, losing resonance with the plasma. Expressions for the rate of laser energy loss and rate of laser redshifting are derived and are found to be in excellent agreement with the direct numerical solution of the …

A Wide-Angle Electron Grating Bi-Prism Beam Splitter, Adam Caprez, Roger Bach, Scot Mcgregor, Herman Batelaan

Department of Physics and Astronomy: Faculty Publications

We demonstrate a wide-angle electron beam splitter capable of producing 1 cm beam separation at the detection plane. The beam splitter utilizes a nanofabricated periodic grating in combination with a bi-prism element. In contrast to devices utilizing only bi-prism elements, the use of the periodic grating causes amplitude, and not wavefront, splitting. Even at maximum separation, beam profiles remain undistorted, providing evidence that coherence is intact. This is a step towards the realization of a large area electron interferometer using such a grating bi-prism combination.