Faculty Publications, Department of Physics and AstronomyCopyright (c) 2016 University of Nebraska - Lincoln All rights reserved.
http://digitalcommons.unl.edu/physicsfacpub
Recent documents in Faculty Publications, Department of Physics and Astronomyen-usWed, 10 Aug 2016 19:27:48 PDT3600Magnetoelectric domain wall dynamics and its implications for magnetoelectric memory
http://digitalcommons.unl.edu/physicsfacpub/169
http://digitalcommons.unl.edu/physicsfacpub/169Thu, 14 Jul 2016 14:07:05 PDT
Domain wall dynamics in a magnetoelectric antiferromagnet is analyzed, and its implications for magnetoelectric memory applications are discussed. Cr_{2}O_{3} is used in the estimates of the materials parameters. It is found that the domain wall mobility has a maximum as a function of the electric field due to the gyrotropic coupling induced by it. In Cr_{2}O_{3}, the maximal mobility of 0.1 m/(s Oe) is reached at E = 0.06 V/nm. Fields of this order may be too weak to overcome the intrinsic depinning field, which is estimated for B-doped Cr_{2}O_{3}. These major drawbacks for device implementation can be overcome by applying a small in-plane shear strain, which blocks the domain wall precession. Domain wall mobility of about 0.7 m/(s Oe) can then be achieved at E = 0.2 V/nm. A split-gate scheme is proposed for the domain-wall controlled bit element; its extension to multiple-gate linear arrays can offer advantages in memory density, programmability, and logic functionality.
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Kirill Belashchenko et al.Spin torque and Nernst effects in Dzyaloshinskii-Moriya ferromagnets
http://digitalcommons.unl.edu/physicsfacpub/168
http://digitalcommons.unl.edu/physicsfacpub/168Thu, 14 Jul 2016 13:55:32 PDT
We predict that a temperature gradient can induce a magnon-mediated intrinsic torque in systems with a nontrivial magnon Berry curvature. With the help of a microscopic linear response theory of nonequilibrium magnon-mediated torques and spin currents we identify the interband and intraband components that manifest in ferromagnets with Dzyaloshinskii-Moriya interactions and magnetic textures. To illustrate and assess the importance of such effects, we apply the linear response theory to the magnon-mediated spin Nernst and torque responses in a kagome lattice ferromagnet.
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Alexey Kovalev et al.Stability of skyrmion lattices and symmetries of quasi-two-dimensional chiral magnets
http://digitalcommons.unl.edu/physicsfacpub/167
http://digitalcommons.unl.edu/physicsfacpub/167Thu, 14 Jul 2016 13:42:51 PDT
Recently there has been substantial interest in realizations of skyrmions, in particular in quasi-two-dimensional (2D) systems due to increased stability resulting from reduced dimensionality. A stable skyrmion, representing the smallest realizable magnetic texture, could be an ideal element for ultradense magnetic memories. Here we use the most general form of the quasi-2D free energy with Dzyaloshinskii-Moriya interactions constructed from general symmetry considerations reflecting the underlying system. We predict that the skyrmion phase is robust and it is present even when the system lacks the in-plane rotational symmetry. In fact, the lowered symmetry leads to increased stability of vortex-antivortex lattices with fourfold symmetry and in-plane spirals, in some instances even in the absence of an external magnetic field. Our results relate different hexagonal and square cell phases to the symmetries of materials used for realizations of skyrmions. This will give clear directions for experimental realizations of hexagonal and square cell phases, and will allow engineering of skyrmions with unusual properties. We also predict striking differences in gyrodynamics induced by spin currents for isolated skyrmions and for crystals where spin currents can be induced by charge carriers or by thermal magnons. We find that under certain conditions, isolated skyrmions can move along the current without a side motion which can have implications for realizations of magnetic memories.
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Utkan Güngördü et al.Theory of magnon motive force in chiral ferromagnets
http://digitalcommons.unl.edu/physicsfacpub/166
http://digitalcommons.unl.edu/physicsfacpub/166Thu, 14 Jul 2016 13:42:47 PDT
We predict that magnon motive force can lead to temperature dependent, nonlinear chiral damping in both conducting and insulating ferromagnets. We estimate that this damping can significantly influence the motion of skyrmions and domain walls at finite temperatures. We also find that in systems with low Gilbert damping moving chiral magnetic textures and resulting magnon motive forces can induce large spin and energy currents in the transverse direction.
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Utkan Güngördü et al.Diffractive imaging of a rotational wavepacket
in nitrogen molecules with femtosecond
megaelectronvolt electron pulses
http://digitalcommons.unl.edu/physicsfacpub/165
http://digitalcommons.unl.edu/physicsfacpub/165Fri, 17 Jun 2016 12:54:36 PDT
Imaging changes in molecular geometries on their natural femtosecond timescale with sub-Angstro¨m spatial precision is one of the critical challenges in the chemical sciences, as the nuclear geometry changes determine the molecular reactivity. For photoexcited molecules, the nuclear dynamics determine the photoenergy conversion path and efficiency. Here we report a gas-phase electron diffraction experiment using megaelectronvolt (MeV) electrons, where we captured the rotational wavepacket dynamics of nonadiabatically laser-aligned nitrogen molecules. We achieved a combination of 100 fs root-mean-squared temporal resolution and sub-Angstrom (0.76Å) spatial resolution that makes it possible to resolve the position of the nuclei within the molecule. In addition, the diffraction patterns reveal the angular distribution of the molecules, which changes from prolate (aligned) to oblate (anti-aligned) in 300 fs. Our results demonstrate a significant and promising step towards making atomically resolved movies of molecular reactions.
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Jie Yang et al.Chemical ordering suppresses large-scale
electronic phase separation in doped manganites
http://digitalcommons.unl.edu/physicsfacpub/164
http://digitalcommons.unl.edu/physicsfacpub/164Fri, 17 Jun 2016 12:32:55 PDT
For strongly correlated oxides, it has been a long-standing issue regarding the role of the chemical ordering of the dopants on the physical properties. Here, using unit cell by unit cell superlattice growth technique, we determine the role of chemical ordering of the Pr dopant in a colossal magnetoresistant (La_{1-y}Pry)_{1-x}Ca_{x}MnO_{3} (LPCMO) system, which has been well known for its large length-scale electronic phase separation phenomena. Our experimental results show that the chemical ordering of Pr leads to marked reduction of the length scale of electronic phase separations. Moreover, compared with the conventional Pr-disordered LPCMO system, the Pr-ordered LPCMO system has a metal–insulator transition that is ~100 K higher because the ferromagnetic metallic phase is more dominant at all temperatures below the Curie temperature.
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Yinyan Zhu et al.Coherence in laser-induced Compton scattering
http://digitalcommons.unl.edu/physicsfacpub/163
http://digitalcommons.unl.edu/physicsfacpub/163Fri, 17 Jun 2016 12:08:44 PDT
The concept of the electron mass dressing by a powerful laser pulse is discussed. It is shown, by considering the coherent frequency combs generated out of the Compton radiation, how the electron dressed mass can be determined experimentally. This also opens a possibility to measure properties of extremely intense pulses for which the previously developed methods, working at moderate intensities, are not applicable. Namely, one can determine these properties from the properties of coherent Compton radiation.
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F. Cajiao V´elez et al.Magnetic and magneto-transport studies of substrate
effect on the martensitic transformation in a NiMnIn
shape memory alloy
http://digitalcommons.unl.edu/physicsfacpub/162
http://digitalcommons.unl.edu/physicsfacpub/162Fri, 17 Jun 2016 11:56:45 PDT
The effect of substrates on the magnetic and transport properties of Ni_{2}Mn_{1.5}In_{0.5} ultra-thin films were studied theoretically and experimentally. High quality 8-nm films were grown by laser-assisted molecular beam epitaxy deposition. Magnetotransport measurements revealed that the films undergo electronic structure transformation similar to those of bulk materials at the martensitic transformation. The temperature of the transformation depends strongly on lattice parameters of the substrate. To explain this behavior, we performed DFT calculations on the system and found that different substrates change the relative stability of the ferromagnetic (FM) austenite and ferrimagnetic (FiM) martensite states. We conclude that the energy difference between the FM austenite and FiM martensite states in Ni_{2}Mn_{1.5}In_{0.5} films grown on MgO (001) substrates is ΔE = 0.20 eV per NiMnIn f.u, somewhat lower compared to ΔE = 0.24 eV in the bulk material with the same lattice parameters. When the lattice parameters of Ni_{2}Mn_{1.5}In_{0.5} film have values close to those of the MgO substrate, the energy difference becomes ΔE = 0.08 eV per NiMnIn f.u. These results suggest the possibility to control the martensitic transition in thin films through substrate engineering.
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Andrei Sokolov et al.Eikonal perturbation theory in photoionization
http://digitalcommons.unl.edu/physicsfacpub/161
http://digitalcommons.unl.edu/physicsfacpub/161Fri, 17 Jun 2016 11:29:26 PDT
The eikonal perturbation theory is formulated and applied to photoionization by strong laser pulses. A special emphasis is put on the first order approximation with respect to the binding potential, which is known as the generalized eikonal approximation [2015Phys. Rev. A 91 053417]. The ordinary eikonal approximation and its domain of applicability is derived from the generalized eikonal approximation. While the former approach is singular for the electron trajectories which return to the potential center, the generalized eikonal avoids this problem. This property makes it a promising tool for further investigations of rescattering and high-order harmonic generation processes.
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F. Cajiao Vélez et al.A time-implicit numerical method and benchmarks for the relativistic
Vlasov–Ampere equations
http://digitalcommons.unl.edu/physicsfacpub/160
http://digitalcommons.unl.edu/physicsfacpub/160Fri, 17 Jun 2016 11:14:54 PDT
We present a time-implicit numerical method to solve the relativistic Vlasov–Ampere system of equations on a two dimensional phase space grid. The time-splitting algorithm we use allows the generalization of the work presented here to higher dimensions keeping the linear aspect of the resulting discrete set of equations. The implicit method is benchmarked against linear theory results for the relativistic Landau damping for which analytical expressions using the Maxwell-Juttner distribution function are derived. We note that, independently from the shape of the distribution function, the relativistic treatment features collective behaviours that do not exist in the nonrelativistic case. The numerical study of the relativistic two-stream instability completes the set of benchmarking tests.
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Michael Carrié et al.Unphysical kinetic effects in particle-in-cell modeling of laser wakefield accelerators
http://digitalcommons.unl.edu/physicsfacpub/159
http://digitalcommons.unl.edu/physicsfacpub/159Mon, 11 Jan 2016 11:22:17 PST
Unphysical heating and macroparticle trapping that arise in the numerical modeling of laser wakefield accelerators using particle-in-cell codes are investigated. A dark current free laser wakefield accelerator stage, in which no trapping of background plasma electrons into the plasma wave should occur, and a highly nonlinear cavitated wake with self-trapping, are modeled. Numerical errors can lead to errors in the macroparticle orbits in both phase and momentum. These errors grow as a function of distance behind the drive laser and can be large enough to result in unphysical trapping in the plasma wake. The resulting numerical heating in intense short-pulse laser-plasma interactions grows much faster and to a higher level than the known numerical grid heating of an initially warm plasma in an undriven system. The amount of heating, at least in the region immediately behind the laser pulse, can, in general, be decreased by decreasing the grid size, increasing the number of particles per cell, or using smoother interpolation methods. The effect of numerical heating on macroparticle trapping is less severe in a highly nonlinear cavitated wake, since trapping occurs in the first plasma wave period immediately behind the laser pulse.
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Estelle Cormier-Michel et al.HfCo<sub>7</sub>-Based Rare-Earth-Free Permanent-Magnet Alloys
http://digitalcommons.unl.edu/physicsfacpub/158
http://digitalcommons.unl.edu/physicsfacpub/158Mon, 11 Jan 2016 11:18:42 PST
This study presents the structural and magnetic properties of melt-spun HfCo_{7}, HfCo_{7-x}Fe_{x }(0.25 < x < 1), and HfCo_{7}Si_{x} (0.2 < x < 1.2) alloys. Appreciable permanent-magnet properties with a magnetocrystalline anisotropy of about 9.6–16.5 Mergs/cm^{3}, a magnetic polarization J_{s} = 7.2 –10.6 kG, and coercivities H_{c} = 0.5–3.0 kOe were obtained by varying the composition of these alloys. Structural analysis reveals that the positions of x-ray diffraction peaks of HfCo_{7 }show good agreement with those corresponding to an orthorhombic structure having lattice parameters of about a = 4.719 A, b = 4.278 A, and c = 8.070 A. Based on these results, a model crystal structure for HfCo_{7} is developed and used to estimate the magnetic properties of HfCo_{7 }using density-functional calculations, which agree with the experimental results.
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B. Das et al.Quasi Monoenergetic and Tunable X-rays by Laser
Compton Scattering from Laser Wakefield e-beam
http://digitalcommons.unl.edu/physicsfacpub/157
http://digitalcommons.unl.edu/physicsfacpub/157Mon, 11 Jan 2016 11:07:22 PST
Quasi monoenergetic and tunable x-ray beams are reported by inverse-Compton scattering from laser wakefield accelerated electrons. The high peak brightness, ultrashort duration, and small size of the source make it uniquely suitable for many applications.
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Nathan D. Powers et al.Variational Formulation of Macroparticle Models
for Electromagnetic Plasma Simulations
http://digitalcommons.unl.edu/physicsfacpub/156
http://digitalcommons.unl.edu/physicsfacpub/156Mon, 11 Jan 2016 10:59:43 PST
A variational method is used to derive a selfconsistent macroparticle model for relativistic electromagnetic kinetic plasma simulations. Extending earlier work, discretization of the electromagnetic Low Lagrangian is performed via a reduction of the phase-space distribution function onto a collection of finite-sized macroparticles of arbitrary shape and discretization of field quantities onto a spatial grid. This approach may be used with lab frame coordinates or moving window coordinates; the latter can greatly improve computational efficiency for studying some types of laser-plasma interactions. The primary advantage of the variational approach is the preservation of Lagrangian symmetries, which in our case leads to energy conservation and thus avoids difficulties with grid heating. In addition, this approach decouples particle size from grid spacing and relaxes restrictions on particle shape, leading to low numerical noise. The variational approach also guarantees consistent approximations in the equations of motion and is amenable to higher order methods in both space and time. We restrict our attention to the 1.5-D case (one coordinate and two momenta). Simulations are performed with the new models and demonstrate energy conservation and low noise.
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Alexander B. Stamm et al.A Time-Implicit Algorithm for Solving the
Vlasov–Poisson Equation
http://digitalcommons.unl.edu/physicsfacpub/155
http://digitalcommons.unl.edu/physicsfacpub/155Mon, 11 Jan 2016 10:55:40 PSTB. A. Shadwick et al.Bayesian Spectrum Analysis for
Laser Vibrometry Processing
http://digitalcommons.unl.edu/physicsfacpub/154
http://digitalcommons.unl.edu/physicsfacpub/154Mon, 11 Jan 2016 10:53:24 PST
Laser vibration sensing provides a sensitive non-contact means of measuring vibrations of objects. These measurements are used in industrial quality control and wear monitoring as well as the analysis of the vibrational characteristics of objects. In laser vibrometry, the surface motion is monitored by heterodyne laser Doppler velocimetry, and the received heterodyne signal is sampled to produce a time-series which is processed to obtain a vibrational spectrum of the object under test. Laser vibrometry data has been processed with a traditional FM discriminator approach and by spectrogram and time-frequency distribution processing techniques. The latter techniques have demonstrated improved performance over the FM discriminator method, but do not take full advantage of the prior knowledge one has about the signal of interest. We consider here a statistical signal processing approach to laser vibrometry data. In this approach the quantities of interest are the frequencies of vibration, while the phase and quadrature amplitudes are considered nuisance parameters. Because of the optimal use of prior knowledge about the laser vibrometry signal, the frequencies can be determined with much greater precision and greater noise immunity than using Fourier- or time-frequency-based approaches. Furthermore, the statistical approach is known to have superior performance when the data extends over a small number of vibrational periods. We illustrate the method with data from a beroptic laser Doppler velocimeter. Our results show that while the choice of processing method for determining the instantaneous velocity is relatively unimportant, the Bayesian method exhibits superior performance in determining the vibrational frequency.
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Walter F. Buell et al.Higher-Order Explicit Methods for Laser-Plasma
Interactions
http://digitalcommons.unl.edu/physicsfacpub/153
http://digitalcommons.unl.edu/physicsfacpub/153Mon, 11 Jan 2016 10:46:04 PST
The evolution of a short, intense laser pulse propagating in an underdense plasma is of particular interest for laser-plasma accelerator physics and, in some circumstances, is well-modeled by the cold Maxwell-fluid equations. Solving this system using conventional second-order explicit methods in a three-dimensional simulation over experimentally-relevant configurations is prohibitively expensive. This motivated a search for more efficient numerical methods to solve the fluid equations. Explicit methods tend to suffer from stability constraints which couple the maximum allowable time step to the spatial grid size. If the dynamics of the system evolves on a time scale much larger than the constrained time step, an explicit method would require many more update cycles than is physically necessary. In these circumstances implicit methods, which tend to be unconditionally stable, may be attractive. But when physical situations (e.g., Raman processes) need to resolve the fast dynamics, implicit methods are unlikely to exhibit much improvement over explicit methods. Thus, we look for higher-order explicit methods in space that would allow coarser spatial grids and larger time steps. We restrict our discussion to the one-dimensional case and present a comprehensive survey of a wide range of numerical methods to solve the fluid equations, including methods of order two through six in space and two through eight in time. A systematic approach to determine the stability condition is presented using linear stability analysis of numerical dispersion relations. Three higher-order methods are implemented to show their behavior, in terms of numerical stability and energy conservation.
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J. Paxon Reyes et al.Higher-Order Explicit Numerical Methods for Laser-
Plasma Interactions
http://digitalcommons.unl.edu/physicsfacpub/152
http://digitalcommons.unl.edu/physicsfacpub/152Mon, 11 Jan 2016 10:44:05 PST
The evolution of a short, intense laser pulse propagating in an underdense plasma is of particular interest for laser-plasma accelerator physics. This case is well-modeled by the cold, Maxwell–fluid equations but, using conventional second-order explicit methods, a three-dimensional simulation for experimentally relevant configurations is prohibitively expensive. This motivated a search for numerical methods that might be used to solve the fluid equations more efficiently. Explicit methods tend to suffer from stability constraints which couple the maximum allowable time step to the spatial grid size. If the dynamics of the system evolves on a time scale much larger than the constrained time step, an explicit method may require many more update cycles than is physically necessary. In these circumstances implicit methods, which tend to be unconditionally stable, may be attractive. However, in many physical situations (e.g., Raman processes) it is necessary to fully-resolve the fast dynamics. In this case, implicit methods are unlikely to exhibit much improvement over explicit methods. Thus, we look for methods of higherorder in space that would allow the use of coarser spatial grids and thus larger time steps.
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J. Paxon Reyes et al.OPTICAL CONTROL OF ELECTRON TRAPPING
AND ACCELERATION IN PLASMA CHANNELS:
APPLICATION TO TUNABLE, PULSED SOURCES OF
MULTI-COLOR THOMSON GAMMA-RAYS
http://digitalcommons.unl.edu/physicsfacpub/151
http://digitalcommons.unl.edu/physicsfacpub/151Mon, 11 Jan 2016 10:41:01 PSTS. Y. Kalmykov et al.Accordion Effect in Plasma Channels:
Generation of Tunable Comb-Like Electron Beams
http://digitalcommons.unl.edu/physicsfacpub/150
http://digitalcommons.unl.edu/physicsfacpub/150Mon, 11 Jan 2016 10:37:53 PST
Propagating a short, relativistically intense laser pulse in a plasma channel makes it possible to generate comb-like electron beams for advanced radiation sources. The ponderomotive force of the leading edge of the pulse expels all electrons facing the pulse. The bare ions attract the ambient plasma electrons, forming a closed bubble of electron density confining the pulse tail. The cavity of electron density evolves slowly, in lock-step with the optical driver, and readily traps background electrons. The combination of a bubble (a self-consistently maintained, “soft” hollow channel) and a preformed channel forces transverse flapping of the laser pulse tail, causing oscillations in the bubble size. The resulting periodic injection produces a sequence of background-free, quasi-monoenergetic bunches of femtosecond duration. The number of these spectral components, their charge, energy, and energy separation is sensitive to the channel radius and pulse length. Accumulation of noise (continuously injected charge) can be prevented using a negatively chirped drive pulse with a bandwidth close to a one-half of the carrier wavelength. As a result of dispersion compensation, self-steepening of the pulse is reduced, and continuous injection almost completely suppressed. This level of control on a femtosecond time scale is hard to achieve with conventional accelerator techniques. These comb-like beams can drive high-brightness, tunable, multi-color -ray sources.
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Serge Y. Kalmykov et al.