Physics Commons^™

Ferroelectric Instability Under Screened Coulomb Interactions, Yong Wang, Xiaohui Liu, John D. Burton, Sitaram S. Jaswal, Evgeny Y. Tsymbal

Evgeny Tsymbal Publications

We explore the effect of charge carrier doping on ferroelectricity using density functional calculations and phenomenological modeling. By considering a prototypical ferroelectric material, BaTiO₃, we demonstrate that ferroelectric displacements are sustained up to the critical concentration of 0.11 electron per unit cell volume. This result is consistent with experimental observations and reveals that the ferroelectric phase and conductivity can coexist. Our investigations show that the ferroelectric instability requires only a short-range portion of the Coulomb force with an interaction range of the order of the lattice constant. These results provide a new insight into the origin of ferroelectricity …

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 …

Electron-Phonon Coupling And Structural Phase Transitions On Au/Mo(112), Keisuke Fukutani

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

The electronic structures, many-body interactions and Fermi surface topologies of Au/Mo(112) were investigated in detail and were found to play important roles in the newly discovered order-disorder structural phase transition of the system. First, the high-resolution angle-resolved photoemission spectroscopy was utilized to characterize the electronic band structure of Mo(112) in far greater details than before. This elucidated the existence of several surface-derived states and their dispersion relations in high precisions near the Fermi level, as well as the symmetries of the bulk and surface electronic states, which are in good quantitative agreement with the ab-initio calculations. Such thorough understanding of …

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 …

Enhanced Asymmetry In Few-Cycle Attosecond Pulse Ionization Of He In The Vicinity Of Autoionizing Resonances, Jean Marcel Ngoko Djiokap, S. X. Hu, Wei-Chao Jiang, Liang-You Peng, Anthony F. Starace

Anthony F. Starace Publications

By solving the two-active-electron, time-dependent Schrödinger equation in its full dimensionality, we investigate the carrier-envelope phase (CEP) dependence of single ionization of He to the He⁺(1s) state triggered by an intense few-cycle attosecond pulse with carrier frequency ω corresponding to the energy ћω = 36 eV. Effects of electron correlations are probed by comparing projections of the final state of the two-electron wave packet onto field-free highly correlated Jacobi matrix wave functions with projections onto uncorrelated Coulomb wave functions. Significant differences are found in the vicinity of autoionizing resonances. Owing to the broad bandwidths of our 115 and …

Fourier Transform Infrared Spectroscopy And Wide-Angle X-Ray Scattering: Investigations On Polypropylene–Vapor-Grown Carbon Nanofiber Composites, Mircea Chipara, John Hamilton, Alin Cristian Chipara, Tom George, Dorina Magdalena Chipara, Elamin E. Ibrahim, Karen Lozano, David J. Sellmyer

David Sellmyer Publications

Fourier transform infrared (FTIR) spectroscopy and wide-angle X-ray scattering (WAXS) investigations of isotactic polypropylene (iPP)–vapor-grown carbon nanofiber (VGCNF) composites containing various amounts of VGCNFs ranging between 0 and 20 wt %. are reported. The FTIR investigations were focused on the regularity bands of iPP. The FTIR data indicated a drop in the isotac­ticity index as the concentration of nanofibers was increased; this suggested a decrease in the crystallinity. WAXS measurements revealed a dominating α₁ phase, with a small admixture of γ phase or mesophase. The loading of the polymeric matrix with car­bon nanofibers (CNFs) did not induce significant changes …

Propagation Of Ultrasound Through Freshly Excised Human Calvarium, Armando Garcia Noguera

Department of Engineering Mechanics: Dissertations, Theses, and Student Research

The propagation of ultrasound through complex biological media, such as the human calvarium, poses a great challenge for modern medicine. Several ultrasonic techniques commonly used for treatment and diagnosis in most of the human body are still difficult to apply to the human brain, in part, because of the properties of the skull. Moreover, an understanding of the biomechanics of transcranial ultrasound may provide needed insight into the problem of blast wave induced traumatic brain injury (TBI). In the present study, the spatial variability of ultrasonic properties was evaluated for relevant frequencies of 0.5, 1, and 2.25 MHz. A total …

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.

New View Of The Occupied Band Structure Of Mo(112), Keisuke Fukutani, Hirokazu Hayashi, I. N. Yakovkin, Tula R. Paudel, Takafumi Habuchi, Daisuke Hirayama, Jian Jiang, Hideaki Iwasawa, Kenya Shimada, Ning Wu, Evgeny Y. Tsymbal, Yaroslav B. Losovyj, Peter A. Dowben

Evgeny Tsymbal Publications

We present a comprehensive examination of the occupied surface-weighted band structure of Mo(112) along the two high-symmetry directions of the surface Brillouin zone, both from theoretical and experimental perspectives. The band structures are found to be significantly different for the states along the two high-symmetry directions and for the states with even and odd reflection parities with respect to the mirror planes. The present study suggests the existence of a number of surface-weighted bands along both high-symmetry directions. The complexity of the band structure near the Fermi level may impose potential difficulties in experimental determination of the electronphonon coupling parameters …

Mechanical Writing Of Ferroelectric Polarization, Haidong Lu, C.-W. Bark, D. Esque De Los Ojos, J. Alcala, Chang-Beom Eom, G. Catalan, Alexei Gruverman

Alexei Gruverman Publications

Ferroelectric materials are characterized by a permanent electric dipole that can be reversed through the application of an external voltage, but a strong intrinsic coupling between polarization and deformation also causes all ferroelectrics to be piezoelectric, leading to applications in sensors and high-displacement actuators. A less explored property is flexoelectricity, the coupling between polarization and a strain gradient. We demonstrate that the stress gradient generated by the tip of an atomic force microscope can mechanically switch the polarization in the nanoscale volume of a ferroelectric film. Pure mechanical force can therefore be used as a dynamic tool for polarization control …

Robert Fuller Obituary From American Association Of Physics Teachers

Robert G. Fuller Publications

Fuller’s contributions to education will have a continuing impact through the projects he helped lead and the many books and articles he authored or co-authored. AAPT joins the UNL and the physics education community in recognizing the lasting legacy of Dr. Robert G. Fuller.

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 …

An Exploration Of Neutron Detection In Semiconducting Boron Carbide, Nina Hong

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

The ³He supply problem in the U.S. has necessitated the search for alternatives for neutron detection. The neutron detection efficiency is a function of density, atomic composition, neutron absorption cross section, and thickness of the neutron capture material. The isotope ¹⁰B is one of only a handful of isotopes with a high neutron absorption cross section—3840 barns for thermal neutrons. So a boron carbide semiconductor represents a viable alternative to ³He. This dissertation provides an evaluation of the performance of semiconducting boron carbide neutron detectors grown by plasma enhance chemical vapor deposition (PECVD) in order to determine …

Resonant Phenomena In Laser-Assisted Radiative Attachment Or Recombination, A. N. Zheltukhin, A. V. Flegel, M. V. Frolov, N. L. Manakov, Anthony F. Starace

Anthony F. Starace Publications

Resonant enhancements are predicted in cross sections σn for laser-assisted radiative attachment or electron–ion recombination accompanied by absorption of n laser photons. These enhancements occur for incoming electron energies at which the electron can be attached or recombined by emitting μ laser photons followed by emission of a spontaneous photon upon absorbing n + μ laser photons. The close similarity between rescattering plateaus in spectra of resonant attachment/recombination and of high-order harmonic generation is shown based on a general parametrization for σ_n and on numerical results for e−H attachment.

Analytic Theory Of High-Order-Harmonic Generation By An Intense Few-Cycle Laser Pulse, M. V. Frolov, N. L. Manakov, A. M. Popov, O. V. Tikhonova, E. A. Volkova, A. A. Silaev, N. V. Vvedenskii, Anthony F. Starace

Anthony F. Starace Publications

We present a theoretical model for describing the interaction of an electron, weakly bound in a short-range potential, with an intense, few-cycle laser pulse. General definitions for the differential probability of above-threshold ionization and for the yield of high-order-harmonic generation (HHG) are presented. For HHG we then derive detailed analytic expressions for the spectral density of generated radiation in terms of the key laser parameters, including the number N of optical cycles in the pulse and the carrier-envelope phase (CEP). In particular, in the tunneling approximation, we provide detailed derivations of the closed-form formulas presented briefly by M. V. Frolov …

Ferroelectric And Multiferroic Tunnel Junctions, Evgeny Y. Tsymbal, Alexei Gruverman, Vincent Garcia, Manuel Bibes, A. Barthélémy

Evgeny Tsymbal Publications

The phenomenon of electron tunneling has been known since the advent of quantum mechanics, but continues to enrich our understanding of many fi elds of physics, as well as creating sub-fi elds on its own. Spin-dependent tunneling in magnetic tunnel junctions has aroused considerable interest and development. In parallel with this endeavor, recent advances in thin-fi lm ferroelectrics have demonstrated the possibility of achieving stable and switchable ferroelectric polarization in nanometer-thick fi lms. This discovery opened the possibility of using thin-fi lm ferroelectrics as barriers in magnetic tunnel junctions, thus merging the fi elds of magnetism, ferroelectricity, and spin-polarized transport …

Berry Phase From A Randomly Fluctuating Magnetic Field, Ralph Skomski

Ralph Skomski Publications

The quantum-mechanical Berry phase due to a randomly fluctuating magnetic field is calculated by exploiting an analogous random-walk problem in polymer physics. The phase depends on the time correlations of the magnetic field, despite the adiabatic nature of the Berry phase. A probability distribution for the phase is obtained, and how this phase could affect the magnetotransport in granular magnetic nanostructures is briefly discussed.

Magnetoelectric Interactions Between An Organic Ferroelectric And A Transition Metal Ferromagnet, Abhijit Mardana

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

The interaction between ferromagnetic and ferroelectric films, the magnetoelectric effect, is a fascinating fundamental research area as well as having potential applications in magnetic data storage devices. We have investigated magnetoelectric coupling effects in thin film heterostructures, consists of metallic ferromagnet, cobalt, and the polymer ferroelectric [P(VDF-TrFE) 70:30]. The work described here encompasses changes in ferroelectric polarization with magnetic field as well as changes in the magnetic anisotropy with ferroelectric polarization.

In samples of Co overlayers on P(VDF-TrFE), in which the Co is not constrained by the substrate, the polarization shows a large change on application of a perpendicular magnetic …

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.

Measurement Of The Inclusive Jet Cross Section In Pp̅ Collisions At √S= 1.96 Tev, V. M. Abazov, Kenneth A. Bloom, Daniel R. Claes, Kayle Devaughan, Aaron Dominguez, Michael Eads, Ioannis Katsanos, Sudhir Malik, Gregory Snow, P. Svoisky, M. Takahashi, M. Voutilainen

Kenneth Bloom Publications

We present a measurement of the inclusive jet cross section using the Run II cone algorithm and data collected by the D0 experiment in pp̅ collisions at a center-of-mass energy √s= 1.96 TeV, corresponding to an integrated luminosity of 0.70 fb^-1. The jet energy calibration and the method used to extract the inclusive jet cross section are described.We discuss the main uncertainties, which are dominated by the jet energy scale uncertainty. The results cover jet transverse momenta from 50 GeV to 600 GeV with jet rapidities in the range -2.4 to 2.4 and are compared to predictions …

Search For Doubly Charged Higgs Boson Pair Production In Pp̅ Collisions At √S = 1.96 Tev, V. M. Abazov, Kenneth A. Bloom, Daniel R. Claes, Kayle Devaughan, Aaron Dominguez, Michael Eads, D. Johnston, Ioannis Katsanos, Sudhir Malik, Gregory Snow

Kenneth Bloom Publications

We present a search for pair production of doubly charged Higgs bosons in the processes qq̅→H⁺⁺H^-- decaying through H^±±→τ^±τ^±, µ^±τ^±,µ^±µ^±. The search is performed in pp̅ collisions at a center-of-mass energy of √s = 1.96 TeV using an integrated luminosity of up to 7.0 fb-1 collected by the D0 experiment at the Fermilab Tevatron Collider. The results are used to set 95% C.L. limits on the pair production cross section of doubly charged Higgs bosons and on their mass for different H …

Measurement Of The Λb Cross Section And The Λb To Λb Ratio With J/Ψλ Decays In Pp Collisions At √S = 7 Tev, Cms Collaboration, Ekaterina Avdeeva, Kenneth A. Bloom, S. Bose, Jamila Butt, Daniel R. Claes, Aaron Dominguez, Michael Eads, P. Jindal, J. Keller, Ilya Kravchenko, J. Lazo-Flores, Helena Malbouisson, Sudhir Malik, Gregory Snow

Kenneth Bloom Publications

The Λ_b differential production cross section and the cross section ratio σ(Λ̅_b )/σ(Λ_b) are measured as functions of transverse momentum p^Λb_T and rapidity |y^Λb | in pp collisions at √ s = 7 TeV using data collected by the CMS experiment at the LHC. The measurements are based on Λ_b decays reconstructed in the exclusive final state J/ψΛ, with the subsequent decays J/ψ →μ⁺μ⁻ and Λ→pπ, using a data sample corresponding to an integrated luminosity of 1.9 fb⁻¹. The product …

Search For Heavy Long-Lived Charged Particles In Pp Collisions At √S = 7 Tev, Cms Collaboration, Ekaterina Avdeeva, Kenneth A. Bloom, S. Bose, J. Butt, Daniel R. Claes, Aaron Dominguez, Michael Eads, P. Jindal, J. Keller, Ilya Kravchenko, J. Lazo-Flores, H. Malbouisson, Sudhir Malik, Gregory Snow

Kenneth Bloom Publications

The result of a search for heavy long-lived charged particles produced in pp collisions at √s = 7 TeV at the LHC is described. The data sample has been collected using the CMS detector and corresponds to an integrated luminosity of 5.0 fb⁻¹. The inner tracking detectors are used to define a sample of events containing tracks with high momentum and high ionization energy loss. A second sample of events, which have high-momentum tracks satisfying muon identification requirements in addition to meeting high-ionization and long time-of-flight requirements, is analyzed independently. In both cases, the results are consistent …

Spin And Elastic Contributions To Isothermal Entropy Change, Tathagata Mukherjee, Ralph Skomski, Steven A. Michalski, David J. Sellmyer, Christian Binek

David Sellmyer Publications

Statistical considerations of ensembles of localized magnetic moments reveal an upper bound of the isothermal entropy change when only the magnetic degrees of freedom are considered. In this case, the maximum molar isothermal entropy change is determined by the spin multiplicity and is equal to Rln(2J+1), where J is the angular momentum of an individual atom. However, in materials with giant magnetocaloric effect, the isothermal field-induced entropy change goes beyond the spin-multiplicity limit due to field-activated elastic degrees of freedom. Recently, we investigated a model of pairs of exchange-coupled Ising spins with variable real-space positions. We …

Isothermal Entropy Changes In Nanocomposite Co:Ni67Cu33, Steven A. Michalski, Ralph Skomski, Xingzhong Li, Damien Le Roy, Tathagata Mukherjee, Christian Binek, David J. Sellmyer

David Sellmyer Publications

The temperature-dependent magnetic properties of artificial rare-earth, free-magnetic nanostructures are investigated for magnetic cooling. We consider two-phase nanocomposites, where 2 nm nanoclusters of cobalt are embedded in a Ni₆₇Cu₃₃ matrix. Several composite films were produced by cluster deposition. The average Co nanocluster size can be tuned by varying the deposition conditions. Isothermalmagnetization curves weremeasured at various temperatures 150K

Structure And Magnetic Properties Of Co-W Clusters Produced By Inert Gas Condensation, Farhad Reza Golkar-Fard, Matthew J. Kramer, Y. Zhang, R. W. Mccallum, Ralph Skomski, David J. Sellmyer, Jeffrey E. Shield

David Sellmyer Publications

In this article, inert-gas condensation was used to synthesize Co-W clusters. The formation, structure, and magnetic properties of the clusters were investigated. Sub-10-nm clusters were obtained, and the structures and average sizes were strongly dependent on sputtering power. At low sputtering powers, the clusters were predominantly amorphous, while, at high sputtering power, the clusters were crystalline. X ray diffraction and transmission electron microscopy revealed clusters with hcp structure at high sputtering power. The magnetic properties were dependent on the sputtering power and temperature, with the highest coercivity of 810 Oe at 10 K for high sputtering power.

Magnetism Of Directly Ordered Sm-Co Clusters, Balamurugan Balasubramanian, Ralph Skomski, Xingzhong Li, George C. Hadjipanayis, David J. Sellmyer

David Sellmyer Publications

Sm-Co bulk alloys have shown superior permanent-magnet properties, but research on Sm-Co nanoparticles is challenging because of the need to control particle size, size-distribution, crystalline ordering, and phase purity. In the present study, a cluster-deposition method was used to produce Sm-Co nanoparticles having desired crystal structures without the requirement of subsequent high-temperature thermal annealing. Poorly crystallized SmCo₅ nanoparticles exhibit a low room-temperature coercivity of only 100 Oe, whereas crystalline SmCo₅ and Sm₂Co₁₇ nanoparticles show room-temperature coercivities of 2000 and 750 Oe, respectively. The direct synthesis of Sm-Co nanoparticles having sizes of less than 10 nm …

Aligned And Exchange-Coupled L10 (Fe,Co)Pt-Based Magnetic Films, Yi Liu, Thomas A. George, Ralph Skomski, David J. Sellmyer

David Sellmyer Publications

Films of aligned L1₀-structure (Fe,Co)Pt with fcc Fe(Co,Pt) are synthesized by co-sputtering Fe, Co, and Pt on an (001) MgO substrate with in situ heating at 830 degrees C. The nanostructures and magnetic properties of the films are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and superconducting quantum interference device (SQUID). The compositions of the samples (Fe,Co)_xPt_1-x are designed to maintain an atomic Fe: Co ratio of 65: 35 while increasing the Fe,Co content in each successive sample. In samples with low Fe and Co concentration, the XRD patterns exhibit three strong peaks, …