Faculty Publications, Department of Physics and AstronomyCopyright (c) 2015 University of Nebraska - Lincoln All rights reserved.
http://digitalcommons.unl.edu/physicsfacpub
Recent documents in Faculty Publications, Department of Physics and Astronomyen-usThu, 19 Nov 2015 01:46:09 PST3600Variational formulation of macro-particle plasma simulation algorithms
http://digitalcommons.unl.edu/physicsfacpub/145
http://digitalcommons.unl.edu/physicsfacpub/145Tue, 17 Nov 2015 13:22:23 PST
A variation formulation of macro-particle kinetic plasma models is discussed. In the electrostatic case, the use of symplectic integrators is investigated and found to offer advantages over typical generic methods. For the electromagnetic case, gauge invariance and momentum conservation are considered in detail. It is shown that, while the symmetries responsible for these conservation laws are broken in the presence of a spatial grid, the conservation laws hold in an average sense. The requirements for exact invariance are explored and it is shown that one viable option is to represent the potentials with a truncated Fourier basis
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B. A. Shadwick et al.Magnetization pumping and dynamics in a Dzyaloshinskii-Moriya magnet
http://digitalcommons.unl.edu/physicsfacpub/144
http://digitalcommons.unl.edu/physicsfacpub/144Tue, 17 Nov 2015 13:22:10 PST
We formulate a phenomenological description of thin ferromagnetic layers with inversion asymmetry where the single-domain magnetic dynamics experiences magnon current-induced torques and leads to magnon-motive forces. We first construct a phenomenological theory based on irreversible thermodynamics, taking into account the symmetries of the system. Furthermore, we confirm that these effects originate from Dzyaloshinskii-Moriya interactions from the analysis based on the stochastic Landau-Lifshitz-Gilbert equation. Our phenomenological results generalize to a general form of Dzyaloshinskii-Moriya interactions and to other systems, such as pyrochlore crystals and chiral magnets. Possible applications include spin current generation, magnetization reversal and magnonic cooling.
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Alexey Kovalev et al.Thresholds for Correcting Errors, Erasures, and Faulty Syndrome Measurements in Degenerate Quantum Codes
http://digitalcommons.unl.edu/physicsfacpub/143
http://digitalcommons.unl.edu/physicsfacpub/143Tue, 06 Oct 2015 13:12:05 PDT
We suggest a technique for constructing lower (existence) bounds for the fault-tolerant threshold to scalable quantum computation applicable to degenerate quantum codes with sublinear distance scaling. We give explicit analytic expressions combining probabilities of erasures, depolarizing errors, and phenomenological syndrome measurement errors for quantum low-density parity-check codes with logarithmic or larger distances. These threshold estimates are parametrically better than the existing analytical bound based on percolation.
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Ilya Dumer et al.Spin-dependent two-color Kapitza-Dirac effects
http://digitalcommons.unl.edu/physicsfacpub/142
http://digitalcommons.unl.edu/physicsfacpub/142Tue, 08 Sep 2015 11:29:05 PDT
In this paper we present an analysis of the spin behavior of electrons propagating through a laser field. We present an experimentally realizable scenario in which spin-dependent effects of the interaction between the laser and the electrons are dominant. The laser interaction strength and incident electron velocity are in the nonrelativistic domain. This analysis may thus lead to novel methods of creating and characterizing spin-polarized nonrelativistic femtosecond electron pulses.
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Scot McGregor et al.Discrete Excitation Spectrum of a Classical Harmonic Oscillator in Zero-Point Radiation
http://digitalcommons.unl.edu/physicsfacpub/141
http://digitalcommons.unl.edu/physicsfacpub/141Tue, 08 Sep 2015 11:29:03 PDT
We report that upon excitation by a single pulse, a classical harmonic oscillator immersed in the classical electromagnetic zero-point radiation exhibits a discrete harmonic spectrum in agreement with that of its quantum counterpart. This result is interesting in view of the fact that the vacuum field is needed in the classical calculation to obtain the agreement.
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Wayne Cheng-Wei Huang et al.Quartic force field-derived vibrational frequencies and spectroscopic constants for the isomeric pair SNO and OSN and isotopologues
http://digitalcommons.unl.edu/physicsfacpub/140
http://digitalcommons.unl.edu/physicsfacpub/140Thu, 03 Sep 2015 11:33:26 PDT
The SNO and OSN radical isomers are likely to be of significance in atmospheric and astrochemistry, but very little is known about their gas phase spectroscopic properties. State-of-the-art ab initio composite quartic force fields are employed to analyze the rovibrational features for both systems. Comparison to condensed-phase experimental data for SNO has shown that the 1566.4 cm^{−}^{1} ν_{1} N–O stretch is indeed exceptionally bright and likely located in this vicinity for subsequent gas phase experimental analysis. The OSN ν_{1} at 1209.4 cm^{−}^{1} is better described as the antisymmetric stretch in this molecule and is also quite bright. The full vibrational, rotational, and rovibrational data are provided for SNO and OSN and their single ^{15}N, ^{18}O, and ^{34}S isotopic substitutions in order to give a more complete picture as to the chemical physics of these molecules.
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Ryan C. Fortenberry et al.Continuous third harmonic generation in a terahertz driven modulated nanowire
http://digitalcommons.unl.edu/physicsfacpub/139
http://digitalcommons.unl.edu/physicsfacpub/139Thu, 16 Jul 2015 10:25:59 PDT
We consider the possibility of observing continuous third-harmonic generation using a strongly driven, single-band one-dimensional metal. In the absence of scattering, the quantum efficiency of frequency tripling for such a system can be as high as 93%. Combining the Floquet quasienergy spectrum with the Keldysh Green’s function technique, we derive a semiclassical master equation for a one-dimensional band of strongly and rapidly driven electrons in the presence of weak scattering by phonons. The power absorbed from the driving field is continuously dissipated by phonon modes, leading to a quasi-equilibrium in the electron distribution. We use the Kronig-Penney model with varying effective mass to establish the growth parameters of an InAs/ InP nanowire near optimal for third harmonic generation at terahertz frequency range.
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Kathleen E. Hamilton et al.Optical control of electron phase space in plasma accelerators
with incoherently stacked laser pulses
http://digitalcommons.unl.edu/physicsfacpub/138
http://digitalcommons.unl.edu/physicsfacpub/138Thu, 16 Jul 2015 10:22:22 PDT
It is demonstrated that synthesizing an ultrahigh-bandwidth, negatively chirped laser pulse by incoherently stacking pulses of different wavelengths makes it possible to optimize the process of electron self-injection in a dense, highly dispersive plasma (n0 ~ 1019 c~3). Avoiding transformation of the driving pulse into a relativistic optical shock maintains a quasi-monoenergetic electron spectrum through electron dephasing and boosts electron energy far beyond the limits suggested by existing scaling laws. In addition, evolution of the accelerating bucket in a plasma channel is shown to produce a background-free, tunable train of femtosecond-duration, 35–100 kA, time-synchronized quasi-monoenergetic electron bunches. The combination of the negative chirp and the channel permits acceleration of electrons beyond 1 GeV in a 3mm plasma with 1.4 J of laser pulse energy, thus offering the opportunity of high-repetition-rate operation at manageable average laser power.
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S Y. Kalmuykov et al.Origin of the spin reorientation transitions in (Fe1–xCox)2B alloys
http://digitalcommons.unl.edu/physicsfacpub/137
http://digitalcommons.unl.edu/physicsfacpub/137Thu, 16 Jul 2015 10:17:27 PDT
Low-temperature measurements of the magnetocrystalline anisotropy energy K in (Fe1–xCox)2B alloys are reported, and the origin of this anisotropy is elucidated using a first-principles electronic structure analysis. The calculated concentration dependence K(x) with a maximum near x¼0.3 and a minimum near x¼0.8 is in excellent agreement with experiment. This dependence is traced down to spin-orbital selection rules and the filling of electronic bands with increasing electronic concentration. At the optimal Co concentration, K depends strongly on the tetragonality and doubles under a modest 3% increase of the c/a ratio, suggesting that the magnetocrystalline anisotropy can be further enhanced using epitaxial or chemical strain.
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Kirill D. Belashchenko et al.Fast strain wave induced magnetization changes in long cobalt bars: Domain motion versus coherent rotation
http://digitalcommons.unl.edu/physicsfacpub/136
http://digitalcommons.unl.edu/physicsfacpub/136Thu, 16 Jul 2015 09:41:03 PDT
A high frequency (88 MHz) traveling strain wave on a piezoelectric substrate is shown to change the magnetization direction in 40 lm wide Co bars with an aspect ratio of 103. The rapidly alternating strain wave rotates the magnetization away from the long axis into the short axis direction, via magnetoelastic coupling. Strain-induced magnetization changes have previously been demonstrated in ferroelectric/ferromagnetic heterostructures, with excellent fidelity between the ferromagnet and the ferroelectric domains, but these experiments were limited to essentially dc frequencies. Both magneto-optical Kerr effect and polarized neutron reflectivity confirm that the traveling strain wave does rotate the magnetization away from the long axis direction and both yield quantitatively similar values for the rotated magnetization. An investigation of the behavior of short axis magnetization with increasing strain wave amplitude on a series of samples with variable edge roughness suggests that the magnetization reorientation that is seen proceeds solely via coherent rotation. Polarized neutron reflectivity data provide direct experimental evidence for this model. This is consistent with expectations that domain wall motion cannot track the rapidly varying strain.
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S Davis et al.Control of Majorana edge modes by a <i>g</i>-factor engineered nanowire spin transistor
http://digitalcommons.unl.edu/physicsfacpub/135
http://digitalcommons.unl.edu/physicsfacpub/135Sat, 21 Feb 2015 22:01:59 PST
We propose the manipulation of Majorana edge states via hybridization and spin currents in a nanowire spin transistor. The spin transistor is based on a heterostructure nanowire comprising of semiconductors with large and small g-factors that form the topological and non-topological regions respectively. The hybridization of bound edge states results in spin currents and 4π-periodic torques, as a function of the relative magnetic field angle – an effect which is dual to the fractional Josephson effect. We establish relation between torques and spin-currents in the non-topological region where the magnetic field is almost zero and spin is conserved along the spin–orbit field direction. The angular momentum transfer could be detected by sensitive magnetic resonance force microscopy techniques.
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Amrit De et al.Numerical Techniques for Finding the Distances of Quantum Codes
http://digitalcommons.unl.edu/physicsfacpub/134
http://digitalcommons.unl.edu/physicsfacpub/134Sat, 21 Feb 2015 13:12:12 PST
We survey the existing techniques for calculating code distances of classical codes and apply these techniques to generic quantum codes. For classical and quantum LDPC codes, we also present a new linked-cluster technique. It reduces complexity exponent of all existing deterministic techniques designed for codes with small relative distances (which include all known families of quantum LDPC codes), and also surpasses the probabilistic technique for sufficiently high code rates.
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Ilya Dumer et al.Skyrmionic spin Seebeck effect via dissipative thermomagnonic torques
http://digitalcommons.unl.edu/physicsfacpub/133
http://digitalcommons.unl.edu/physicsfacpub/133Fri, 20 Feb 2015 14:07:04 PST
We derive thermomagnonic torque and its “β-type” dissipative correction from the stochastic Landau-Lifshitz-Gilbert equation. The β-type dissipative correction describes viscous coupling between magnetic dynamics and magnonic current and it stems from spin mistracking of the magnetic order. We show that thermomagnonic torque is important for describing temperature gradient induced motion of skyrmions in helical magnets while dissipative correction plays an essential role in generating transverse Magnus force. We propose to detect such skyrmionic motion by employing the transverse spin Seebeck effect geometry.
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Alexey KovalevDemonstrating entanglement by testing Bell's theorem in Majorana wires
http://digitalcommons.unl.edu/physicsfacpub/132
http://digitalcommons.unl.edu/physicsfacpub/132Fri, 20 Feb 2015 14:01:54 PST
We propose an experiment that would establish the entanglement of Majorana zero modes in semiconductor nanowires by testing the Bell and Clauser-Horne-Shimony-Holt inequalities. Our proposal is viable with realistic system parameters, simple “keyboard” gating, and projective measurement. Theoretical models and simulation results indicate entanglement can be demonstrated with moderately accurate gate operations. In addition to providing further evidence for the existence of the Majorana bound states, our proposal could be used as an experimental stepping stone to more complicated braiding experiments.
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David E. Drummond et al.Parafermion stabilizer codes
http://digitalcommons.unl.edu/physicsfacpub/131
http://digitalcommons.unl.edu/physicsfacpub/131Fri, 20 Feb 2015 13:52:09 PST
We define and study parafermion stabilizer codes, which can be viewed as generalizations of Kitaev’s onedimensional (1D) model of unpaired Majorana fermions. Parafermion stabilizer codes can protect against lowweight errors acting on a small subset of parafermion modes in analogy to qudit stabilizer codes. Examples of several smallest parafermion stabilizer codes are given. A locality-preserving embedding of qudit operators into parafermion operators is established that allows one to map known qudit stabilizer codes to parafermion codes. We also present a local 2D parafermion construction that combines topological protection of Kitaev’s toric code with additional protection relying on parity conservation.
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Utkan Güngördü et al.Spin Transfer of Quantum Information between Majorana Modes and a Resonator
http://digitalcommons.unl.edu/physicsfacpub/130
http://digitalcommons.unl.edu/physicsfacpub/130Fri, 20 Feb 2015 13:02:16 PST
We show that resonant coupling and entanglement between a mechanical resonator and Majorana bound states can be achieved via spin currents in a 1D quantum wire with strong spin-orbit interactions. The bound states induced by vibrating and stationary magnets can hybridize, thus resulting in spin-current induced 4π-periodic torques, as a function of the relative field angle, acting on the resonator. We study the feasibility of detecting and manipulating Majorana bound states with the use of magnetic resonance force microscopy techniques.
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Alexey Kovalev et al.A Low-Power Optical Electron Switch
http://digitalcommons.unl.edu/physicsfacpub/129
http://digitalcommons.unl.edu/physicsfacpub/129Mon, 14 Apr 2014 15:07:36 PDT
An electron beam is deflected when it passes over a silicon-nitride surface, if the surface is illuminated by a low-power continuous-wave diode laser. A deflection angle of up to 1.2 mrad is achieved for an electron beam of 29 μrad divergence. A mechanical beam-stop is used to demonstrate that the effect can act as an optical electron switch with a rise and fall time of 6 μs. Such a switch provides an alternative means to control electron beams, which may be useful in electron lithography and microscopy.
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Wayne Cheng-Wei Huang et al.Dynamics Underlying the Gaussian Distribution of the Classical Harmonic Oscillator in Zero-Point Radiation
http://digitalcommons.unl.edu/physicsfacpub/128
http://digitalcommons.unl.edu/physicsfacpub/128Thu, 03 Apr 2014 15:22:25 PDT
Stochastic electrodynamics (SED) predicts a Gaussian probability distribution for a classical harmonic oscillator in the vacuum field. This probability distribution is identical to that of the ground state quantum harmonic oscillator. Thus, the Heisenberg minimum uncertainty relation is recovered in SED. To understand the dynamics that give rise to the uncertainty relation and the Gaussian probability distribution, we perform a numerical simulation and follow the motion of the oscillator. The dynamical information obtained through the simulation provides insight to the connection between the classic double-peak probability distribution and the Gaussian probability distribution. A main objective for SED research is to establish to what extent the results of quantum mechanics can be obtained. The present simulation method can be applied to other physical systems, and it may assist in evaluating the validity range of SED.
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Wayne Cheng-Wei Huang et al.Magnetic phases of cobalt atomic clusters on tungsten
http://digitalcommons.unl.edu/physicsfacpub/127
http://digitalcommons.unl.edu/physicsfacpub/127Thu, 30 Jan 2014 15:18:49 PST
First-principle calculations are employed to show that the magnetic structure of small atomic clusters of Co, formed on a crystalline W(110) surface and containing 3–12 atoms, strongly deviates from the usual stable ferromagnetism of Co in other systems. The clusters are ferri-, ferro- or non-magnetic, depending on cluster size and geometry. We determine the atomic Co moments and their relative alignment, and show that antiferromagnetic spin alignment in the Co clusters is caused by hybridization with the tungsten substrate and band filling. This is in contrast with the typical strong ferromagnetism of bulk Co alloys, and ferromagnetic coupling in Fe/W(110) clusters.
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Pavel V. Lukashev et al.Phase and Absorption Gratings for Electrons
http://digitalcommons.unl.edu/physicsfacpub/126
http://digitalcommons.unl.edu/physicsfacpub/126Thu, 17 Jan 2013 10:06:39 PST
We report the experimental realization of phase and absorption gratings for electrons. Phase gratings made with standing waves of light with a periodicity of 266 nm are used to diffract 380 eV electrons [1]. Material gratings of 100 and 200 nm periodicity are used to diffract 500 eV electrons. We are exploring the possibility to use these gratings for low energy electron interferometry.
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Hong Gao et al.