Faculty Publications, Department of Physics and AstronomyCopyright (c) 2018 University of Nebraska - Lincoln All rights reserved.
https://digitalcommons.unl.edu/physicsfacpub
Recent documents in Faculty Publications, Department of Physics and Astronomyen-usSun, 18 Mar 2018 01:42:30 PDT3600Stabilization and control of Majorana bound states with elongated skyrmions
https://digitalcommons.unl.edu/physicsfacpub/186
https://digitalcommons.unl.edu/physicsfacpub/186Sat, 17 Mar 2018 09:43:00 PDT
We show that elongated magnetic skyrmions can host Majorana bound states in a proximity-coupled two-dimensional electron gas sandwiched between a chiral magnet and an s-wave superconductor. Our proposal requires stable skyrmions with unit topological charge, which can be realized in a wide range of multilayer magnets, and it allows quantum information transfer by using standard methods in spintronics via skyrmion motion. We also show how braiding operations can be realized in our proposal.
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Utkan Güngördü et al.Magnetic skyrmion bubble motion driven by surface acoustic waves
https://digitalcommons.unl.edu/physicsfacpub/185
https://digitalcommons.unl.edu/physicsfacpub/185Thu, 15 Mar 2018 11:24:51 PDT
We study the dynamical control of a magnetic skyrmion bubble by using counter-propagating surface acoustic waves (SAWs) in a ferromagnet. First, we determine the bubble mass and derive the force due to SAWs acting on a magnetic bubble using Thiele's method. The force that pushes the bubble is proportional to the strain gradient for the major strain component. We then study the dynamical pinning and motion of magnetic bubbles by SAWs in a nanowire. In a disk geometry, we propose a SAWs-driven skyrmion bubble oscillator with two resonant frequencies.
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Rabindra Nepal et al.Reversible spin texture in ferroelectric HfO2
https://digitalcommons.unl.edu/physicsfacpub/184
https://digitalcommons.unl.edu/physicsfacpub/184Thu, 07 Dec 2017 16:06:34 PST
Spin-orbit coupling effects occurring in noncentrosymmetric materials are known to be responsible for nontrivial spin configurations and a number of emergent physical phenomena. Ferroelectric materials may be especially interesting in this regard due to reversible spontaneous polarization making possible a nonvolatile electrical control of the spin degrees of freedom. Here, we explore a technologically relevant oxide material, HfO2, which has been shown to exhibit robust ferroelectricity in a noncentrosymmetric orthorhombic phase. Using theoretical modelling based on density-functional theory, we investigate the spin-dependent electronic structure of the ferroelectric HfO2 and demonstrate the appearance of chiral spin textures driven by spin-orbit coupling. We analyze these spin configurations in terms of the Rashba and Dresselhaus effects within the k · p Hamiltonian model and find that the Rashba-type spin texture dominates around the valence-band maximum, while the Dresselhaus-type spin texture prevails around the conduction band minimum. The latter is characterized by a very large Dresselhaus constant λD = 0.578 eV A° , which allows using this material as a tunnel barrier to produce tunneling anomalous and spin Hall effects that are reversible by ferroelectric polarization.
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L. L. Tao et al.Distance Verification for Classical and Quantum LDPC Codes
https://digitalcommons.unl.edu/physicsfacpub/183
https://digitalcommons.unl.edu/physicsfacpub/183Wed, 06 Dec 2017 13:29:13 PST
The techniques of distance verification known for general linear codes are first applied to the quantum stabilizer codes. Then, these techniques are considered for classical and quantum (stabilizer) low-density-parity-check (LDPC) codes. New complexity bounds for distance verification with provable performance are derived using the average weight spectra of the ensembles of LDPC codes. These bounds are expressed in terms of the erasure-correcting capacity of the corresponding ensemble. We also present a new irreducible-cluster technique that can be applied to any LDPC code and takes advantage of parity-checks’ sparsity for both the classical and quantum LDPC codes. This technique reduces complexity exponents of all existing deterministic techniques designed for generic stabilizer codes with small relative distances, which also include all known families of the quantum stabilizer LDPC codes.
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Ilya Dumer et al.Pumping of magnons in a Dzyaloshinskii-Moriya ferromagnet
https://digitalcommons.unl.edu/physicsfacpub/182
https://digitalcommons.unl.edu/physicsfacpub/182Thu, 25 May 2017 12:57:54 PDT
We formulate a microscopic linear response theory of magnon pumping applicable to multiple-magnonic-band uniform ferromagnets with Dzyaloshinskii-Moriya interactions. From the linear response theory, we identify the extrinsic and intrinsic contributions where the latter is expressed via the Berry curvature of magnonic bands. We observe that in the presence of a time-dependent magnetization Dzyaloshinskii-Moriya interactions can act as fictitious electric fields acting on magnons. We study various current responses to this fictitious field and analyze the role of Berry curvature. In particular, we obtain an analog of the Hall-like response in systems with nontrivial Berry curvature of magnon bands. After identifying the magnon-mediated contribution to the equilibrium Dzyaloshinskii-Moriya interaction, we also establish the Onsager reciprocity between the magnon mediated thermal torques and heat pumping. We apply our theory to the magnonic heat pumping and torque responses in honeycomb and kagome lattice ferromagnets.
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Alexey Kovalev et al.Performance of two Askaryan Radio Array stations and first results in the search for ultrahigh energy neutrinos
https://digitalcommons.unl.edu/physicsfacpub/181
https://digitalcommons.unl.edu/physicsfacpub/181Thu, 06 Apr 2017 12:20:09 PDT
Ultrahigh energy neutrinos are interesting messenger particles since, if detected, they can transmit exclusive information about ultrahigh energy processes in the Universe. These particles, with energies above 10^{16} eV, interact very rarely. Therefore, detectors that instrument several gigatons of matter are needed to discover them. The ARA detector is currently being constructed at the South Pole. It is designed to use the Askaryan effect, the emission of radio waves from neutrino-induced cascades in the South Pole ice, to detect neutrino interactions at very high energies. With antennas distributed among 37 widely separated stations in the ice, such interactions can be observed in a volume of several hundred cubic kilometers. Currently three deep ARA stations are deployed in the ice, of which two have been taking data since the beginning of 2013. In this article, the ARA detector “as built” and calibrations are described. Data reduction methods used to distinguish the rare radio signals from overwhelming backgrounds of thermal and anthropogenic origin are presented. Using data from only two stations over a short exposure time of 10 months, a neutrino flux limit of 1.5 × 10^{−}^{6} GeV/cm^{2}/s/sr is calculated for a particle energy of 10^{18} eV, which offers promise for the full ARA detector.
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P. Allison et al.Constraints on the ultra-high-energy neutrino flux from Gamma-Ray bursts from a prototype station of the Askaryan radio array
https://digitalcommons.unl.edu/physicsfacpub/180
https://digitalcommons.unl.edu/physicsfacpub/180Thu, 06 Apr 2017 11:54:03 PDT
We report on a search for ultra-high-energy (UHE) neutrinos from gamma-ray bursts (GRBs) in the data set collected by the Testbed station of the Askaryan Radio Array (ARA) in 2011 and 2012. From 57 selected GRBs, we observed no events that survive our cuts, which is consistent with 0.12 expected background events. Using NeuCosmA as a numerical GRB reference emission model, we estimate upper limits on the prompt UHE GRB neutrino fluence and quasi-diffuse flux from 10^{7} to 10^{10} GeV. This is the first limit on the prompt UHE GRB neutrino quasi-diffuse flux above 10^{7} GeV.
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P. Allison et al.High-Flux Femtosecond X-Ray Emission from Controlled Generation of Annular
Electron Beams in a Laser Wakefield Accelerator
https://digitalcommons.unl.edu/physicsfacpub/179
https://digitalcommons.unl.edu/physicsfacpub/179Thu, 06 Apr 2017 11:31:28 PDT
Annular quasimonoenergetic electron beams with a mean energy in the range 200–400 MeV and charge on the order of several picocoulombs were generated in a laser wakefield accelerator and subsequently accelerated using a plasma afterburner in a two-stage gas cell. Generation of these beams is associated with injection occurring on the density down ramp between the stages. This well-localized injection produces a bunch of electrons performing coherent betatron oscillations in the wakefield, resulting in a significant increase in the x-ray yield. Annular electron distributions are detected in 40% of shots under optimal conditions. Simultaneous control of the pulse duration and frequency chirp enables optimization of both the energy and the energy spread of the annular beam and boosts the radiant energy per unit charge by almost an order of magnitude. These well-defined annular distributions of electrons are a promising source of high-brightness laser plasma-based x rays.
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T. Z. Zhao et al.Supercontinuum in ionization by relativistically intense and short laser pulses: Ionization without interference and its time analysis
https://digitalcommons.unl.edu/physicsfacpub/178
https://digitalcommons.unl.edu/physicsfacpub/178Thu, 06 Apr 2017 11:25:42 PDT
Ionization by relativistically intense laser pulses of finite duration is considered in the framework of strongfield quantum electrodynamics. We show that the resulting ionization spectra change their behavior from the interference-dominated oscillatory pattern to the interference-free smooth supercontinuum, the latter being the main focus of this paper. More specifically, when studying the energy distributions of photoelectrons ionized by circularly polarized and short pulses, we observe the appearance of broad structures lacking the interference patterns. These supercontinua extend over hundreds of driving photon energies, thus corresponding to high-order nonlinear processes. Their positions on the electron energy scale can be controlled by changing the pulse duration. The corresponding polar-angle distributions show asymmetries which are attributed to the radiation pressure experienced by photoelectrons. Moreover, our time analysis shows that the electrons comprising the supercontinuum can form pulses of short duration. While we present the fully numerical results, their interpretation is based on the saddle-point approximation for the ionization probability amplitude.
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Katarzyna Krajewska et al.Effects of biaxial strain on the improper multiferroicity in <i>h</i>-LuFeO<sub>3</sub> films studied using
the restrained thermal expansion method
https://digitalcommons.unl.edu/physicsfacpub/177
https://digitalcommons.unl.edu/physicsfacpub/177Thu, 06 Apr 2017 11:21:37 PDT
Elastic strain is potentially an important approach in tuning the properties of the improperly multiferroic hexagonal ferrites, the details of which, however, have been elusive due to experimental difficulties. Employing the method of restrained thermal expansion, we have studied the effect of isothermal biaxial strain in the basal plane of h-LuFeO_{3} (001) films. The results indicate that a compressive biaxial strain significantly enhances the K_{3} structural distortion (the order parameter of the improper ferroelectricity), and the effect is larger at higher temperatures. The compressive biaxial strain and the enhanced K_{3} structural distortion together cause an increase in the electric polarization and a reduction in the canting of the weak ferromagnetic moments in h-LuFeO3, according to our first principles calculations. These findings are important for understanding the strain effect as well as the coupling between the lattice and the improper multiferroicity in h-LuFeO_{3}. The experimental elucidation of the strain effect in h-LuFeO_{3} films also suggests that the restrained thermal expansion can be a viable method to unravel the strain effect in many other thin film materials.
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Kishan Sinha et al.Structure Evolution and Multiferroic Properties in Cobalt
Doped Bi<sub>4</sub>NdTi<sub>3</sub>Fe<sub>1-x</sub>Co<sub>x</sub>O<sub>15</sub>-
Bi<sub>3</sub>NdTi<sub>2</sub>Fe<sub>1-x</sub>CoxO<sub>12-δ</sub> Intergrowth Aurivillius Compounds
https://digitalcommons.unl.edu/physicsfacpub/176
https://digitalcommons.unl.edu/physicsfacpub/176Thu, 06 Apr 2017 11:09:21 PDT
Here, we report the structure evolution, magnetic and ferroelectric properties in Co-doped 4- and 3-layered intergrowth Aurivillius compounds Bi_{4}NdTi_{3}Fe_{1-x} Co_{x}O_{15}-Bi_{3}NdTi_{2}Fe_{1-x}Co_{x}O_{12-δ}. The compounds suffer a structure evolution from the parent 4-layered phase (Bi_{4}NdTi_{3}FeO_{15}) to 3-layered phase (Bi_{3}NdTi_{2}CoO_{12-δ}) with increasing cobalt doping level from 0 to 1. Meanwhile the remanent magnetization and polarization show opposite variation tendencies against the doping level, and the sample with x = 0.3 has the largest remanent magnetization and the smallest polarization. It is believed that the Co concentration dependent magnetic properties are related to the population of the Fe^{3+} -O-Co^{3+} bonds, while the suppressed ferroelectric polarization is due to the enhanced leakage current caused by the increasing Co concentration. Furthermore, the samples (x = 0.1–0.7) with ferromagnetism show magnetoelectric coupling effects at room temperature. The results indicate that it is an effective method to create new multiferroic materials through modifying natural superlattices.
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D. L. Zhang et al.Molecular dynamics simulation of superionicity in neighborite, NaMgF<sub>3</sub>
https://digitalcommons.unl.edu/physicsfacpub/175
https://digitalcommons.unl.edu/physicsfacpub/175Mon, 03 Apr 2017 13:23:17 PDT
Superionicity in neighborite, NaMgF_{3}, has been studied by molecular dynamics using parameter free Gordon-Kim potentials. These simulations show that this halide-based perovskite has a superionic phase. The onset temperature for superionicity is ~50-100 K below the melting point. This is a novel type of superionicity in a halide, occurring as it does in a perovskite rather than a fluorite. Superionicity is demonstrated to occur whether one uses the full ionicity for Mg ^{2+} and F^{-} or employs reduced charges derived from a Quantum Chemistry calculation. The relevance of these findings to predictions of superionicity in the oxide perovskite MgSiO_{3} [Matsui and Price, 1991] is discussed
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L. X. Zhou et al.Hydrogen-induced ferromagnetism in two-dimensional Pt dichalcogenides
https://digitalcommons.unl.edu/physicsfacpub/174
https://digitalcommons.unl.edu/physicsfacpub/174Mon, 03 Apr 2017 13:17:19 PDT
Electronic, structural, and magnetic properties of Pt dichalcogenide monolayers are investigated using firstprinciple calculations.We find that hydrogenation lifts the spin degeneracy in narrow antibonding Pt 5d subband electrons and transforms the nonmagnetic semiconductors PtX_{2} (X = S,Se,Te) into ferromagnetic metals, PtX_{2}-1H; neither strain nor thin-film edges are necessary to support the transition. The trend towards ferromagnetism is most pronounced for X = S, decreasing with increasing atomic weight of the chalcogens.
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P. Manchanda et al.Electron acceleration and generation of high-brilliance x-ray radiation in kilojoule, subpicosecond laser-plasma interactions
https://digitalcommons.unl.edu/physicsfacpub/173
https://digitalcommons.unl.edu/physicsfacpub/173Mon, 03 Apr 2017 13:13:30 PDT
Petawatt, picosecond laser pulses offer rich opportunities in generating synchrotron x-rays. This paper concentrates on the regimes accessible with the PETAL laser, which is a part of the Laser Megajoule (LMJ) facility. We explore two physically distinct scenarios through Particle-in-Cell simulations. The first one realizes in a dense plasma, such that the period of electron Langmuir oscillations is much shorter than the pulse duration. Hallmarks of this regime are longitudinal breakup (“self-modulation”) of the picosecond-scale laser pulse and excitation of a rapidly evolving broken plasma wake. It is found that electron beams with a charge of several tens of nCcan be obtained, with a quasi-Maxwellian energy distribution extending to a few-GeVlevel. In the second scenario, at lower plasma densities, the pulse is shorter than the electron plasmaperiod. The pulse blows out plasma electrons, creating a single accelerating cavity, while injection on the density downramp creates a nC quasi-monoenergetic electron bunch within the cavity. This bunch accelerates without degradation beyond 1 GeV. The x-ray sources in the self-modulated regime offer a high number of photons (∼10^{12}) with the slowly decaying energy spectra extending beyond 60 keV. In turn, quasimonoenergetic character of the electron beam in the blowout regime results in the synchrotron-like spectra with the critical energy around 10MeVand a number of photons> 10^{9}.Yet, much smaller source duration and transverse size increase the x-ray brilliance by more than an order of magnitude against the self-modulated case, also favoring high spatial and temporal resolution in x-ray imaging. In all explored cases, accelerated electrons emit synchrotron x-rays of high brilliance, B > 10^{20} photons/s/mm^{2}/mrad^{2}/0.1%BW. Synchrotron sources driven by picosecond kilojoule lasers may thus find an application in x-ray diagnostics on such facilities such as the LMJ or National Ignition Facility (NIF).
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J. Ferri et al.Magnon Spin Nernst Effect in Antiferromagnets
https://digitalcommons.unl.edu/physicsfacpub/172
https://digitalcommons.unl.edu/physicsfacpub/172Fri, 18 Nov 2016 13:15:44 PST
We predict that a temperature gradient can induce a magnon-mediated spin Hall response in an antiferromagnet with nontrivial magnon Berry curvature. We develop a linear response theory which gives a general condition for a Hall current to be well defined, even when the thermal Hall response is forbidden by symmetry. We apply our theory to a honeycomb lattice antiferromagnet and discuss a role of magnon edge states in a finite geometry.
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Vladimir Zyuzin et al.Theory of Spin Loss at Metallic Interfaces
https://digitalcommons.unl.edu/physicsfacpub/171
https://digitalcommons.unl.edu/physicsfacpub/171Fri, 18 Nov 2016 13:11:42 PST
Interfacial spin-flip scattering plays an important role in magnetoelectronic devices. Spin loss at metallic interfaces is usually quantified by matching the magnetoresistance data for multilayers to the Valet-Fert model, while treating each interface as a fictitious bulk layer whose thickness is δ times the spin-diffusion length. By employing the properly generalized circuit theory and the scattering matrix approaches, we derive the relation of the parameter δ to the spin-flip transmission and reflection probabilities at an individual interface. It is found that δ is proportional to the square root of the probability of spin-flip scattering. We calculate the spin-flip scattering probabilities for flat and rough Cu/Pd interfaces using the Landauer-Büttiker method based on the first-principles electronic structure and find δ to be in reasonable agreement with experiment.
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Kirill D. Belashchenko et al.Measurement of the differential cross section and charge asymmetry for inclusive pp → W<sup>±</sup> + <i>X</i> production at √<i>s</i> = 8 TeV
https://digitalcommons.unl.edu/physicsfacpub/170
https://digitalcommons.unl.edu/physicsfacpub/170Thu, 29 Sep 2016 07:17:50 PDT
The differential cross section and charge asymmetry for inclusive pp → W^{±} + X → μ^{±}ν + X production at √s = 8 TeV are measured as a function of muon pseudorapidity. The data sample corresponds to an integrated luminosity of 18.8 fb^{−1} recorded with the CMS detector at the LHC. These results provide important constraints on the parton distribution functions of the proton in the range of the Bjorken scaling variable x from 10^{−3} to 10^{−1}.
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Ekaterina Avdeeva et al.Magnetoelectric domain wall dynamics and its implications for magnetoelectric memory
https://digitalcommons.unl.edu/physicsfacpub/169
https://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 D. Belashchenko et al.Spin torque and Nernst effects in Dzyaloshinskii-Moriya ferromagnets
https://digitalcommons.unl.edu/physicsfacpub/168
https://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
https://digitalcommons.unl.edu/physicsfacpub/167
https://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.