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-usMon, 23 Feb 2015 01:30:30 PST3600Control 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.Observation of the Kapitza-Dirac Effect
http://digitalcommons.unl.edu/physicsfacpub/125
http://digitalcommons.unl.edu/physicsfacpub/125Wed, 16 Jan 2013 13:37:00 PST
In their famous 1927 experiment, Davisson and Germer observed the diffraction of electrons by a periodic material structure, so showing that electrons can behave like waves. Shortly afterwards, Kapitza and Dirac predicted that electrons should also be diffracted by a standing light wave. This Kapitza-Dirac effect is analogous to the diffraction of light by a grating, but with the roles of the wave and matter reversed. The electron and the light grating interact extremely weakly, via the ‘ponderomotive potential,’ so attempts to measure the Kapitza-Dirac effect had to wait for the development of the laser. The idea that the underlying interaction with light is resonantly enhanced for electrons in an atom led to the observation that atoms could be diffracted by a standing wave of light. Deflection of electrons by high-intensity laser light, which is also a consequence of the Kapitza-Dirac effect, has also been demonstrated. But the coherent interference that characterizes wave diffraction has not hitherto been observed. Here we report the diffraction of free electrons from a standing light wave—a realization of the Kapitza-Dirac effect as originally proposed.
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Daniel L. Freimund et al.Feynman’s Relativistic Electrodynamics Paradox and the Aharonov-Bohm Effect
http://digitalcommons.unl.edu/physicsfacpub/124
http://digitalcommons.unl.edu/physicsfacpub/124Wed, 16 Jan 2013 10:47:06 PST
An analysis is done of a relativistic paradox posed in the Feynman Lectures of Physics involving two interacting charges. The physical system presented is compared with similar systems that also lead to relativistic paradoxes. The momentum conservation problem for these systems is presented. The relation between the presented analysis and the ongoing debates on momentum conservation in the Aharonov-Bohm problem is discussed.
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Adam Caprez et al.Simplest Atomic System for Sub-Doppler Laser Cooling
http://digitalcommons.unl.edu/physicsfacpub/123
http://digitalcommons.unl.edu/physicsfacpub/123Mon, 05 Nov 2012 10:16:05 PST
Sub-Doppler laser cooling requires optical pumping among differently light-shifted ground-state sublevels. We describe a study of the simplest possible angular-momentum configuration that permits all sub-Doppler cooling phenomena. The J_{g} = 1 ⟹ J_{e} = 0 angular-momentum configuration shows recoil-limited cooling in the two most well-known types of polarization gradient, magnetically induced laser cooling, velocity-selective resonances, transient cooling, and velocity-selective population trapping.
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R. Gupta et al.Slowing of <sup>85</sup>Rb Atoms with Isotropic Light
http://digitalcommons.unl.edu/physicsfacpub/122
http://digitalcommons.unl.edu/physicsfacpub/122Mon, 05 Nov 2012 10:06:12 PST
We have demonstrated the slowing of a rubidium atomic beam by isotropic monochromatic light. The results agree with a model calculation, thus allowing its use for designing a practical isotropic light slower. The large hyperfine splittings of rubidium lead to natural multifrequency slowing, which is also included in our model.
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Herman Batelaan et al.Electrons, Stern–Gerlach Magnets, and Quantum Mechanical Propagation
http://digitalcommons.unl.edu/physicsfacpub/121
http://digitalcommons.unl.edu/physicsfacpub/121Mon, 05 Nov 2012 10:01:33 PST
Quantum corrections to Newton’s equations are obtained and used to illustrate that classical dynamics is embedded explicitly in quantum dynamics. Originally, the resultant set of dynamical equations has been used to shed light on quantum chaos. We show that the method can provide insight into the dynamics of free particles and the harmonic oscillator. We then use it to determine whether Stern–Gerlach magnets can be constructed for free electrons.
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Herman BatelaanLight Storage with Light of Arbitrary Polarization
http://digitalcommons.unl.edu/physicsfacpub/120
http://digitalcommons.unl.edu/physicsfacpub/120Mon, 05 Nov 2012 09:51:13 PST
We have demonstrated the phase coherence of stored light in Rb vapor with a completely optical technique. Combining this technique with polarization measurements provides strong evidence that arbitrary polarizations can be stored. The fidelity obtained exceeds 95% for all polarizations. We view the capability to store polarizations as a first step towards building a quantum memory in such a system.
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Hong Gao et al.Kapitza-Dirac Diffraction without Standing Waves: Diffraction without a Grating?
http://digitalcommons.unl.edu/physicsfacpub/119
http://digitalcommons.unl.edu/physicsfacpub/119Mon, 05 Nov 2012 09:46:15 PST
We discuss electron diffraction from two counterpropagating light waves with two different frequencies.We show that, even though these waves do not form a standing wave, electron diffraction similar to the conventional Kapitza-Dirac effect, i.e., scattering on a standing wave, is still possible. The nonlinear response of the electron to the laser fields creates a stationary diffraction grating from which the same electron scatters.
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Olga Smirnova et al.Low-Pressure Source of Slow Metastable Rare Gas Atoms
http://digitalcommons.unl.edu/physicsfacpub/118
http://digitalcommons.unl.edu/physicsfacpub/118Mon, 05 Nov 2012 09:41:04 PST
We investigate the properties of a commercial inverted magnetron pressure gauge for use as a source of slow metastable rare gas atoms. We find that the velocity distribution of the atoms as well as the pressure dependence of the output flux agree with a simple model. This shows that the low-velocity output of the source is enhanced over the Maxwell–Boltzmann form due to a velocity-dependent excitation probability. For argon, the center-line intensity per unit area of the source is measured to be greater than 4.2x10^{15} Ar 1s_{5} atoms/(s srm^{2}) at a pressure of 23 mPa. When observing the entire source area, the center-line intensity is at least 2.6x10^{11} Ar 1s_{5} atoms/(s sr).
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M. H.L. van der Velden et al.Electron Diffraction from Free-Standing, Metal-Coated Transmission Gratings
http://digitalcommons.unl.edu/physicsfacpub/117
http://digitalcommons.unl.edu/physicsfacpub/117Mon, 05 Nov 2012 09:31:12 PST
Electron diffraction from a free-standing nanofabricated transmission grating was demonstrated, with energies ranging from 125 eV to 25 keV. Observation of 21 diffraction orders highlights the quality of the gratings. The image charge potential due to one electron was measured by rotating the grating. These gratings may pave the way to low-energy electron interferometry.
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Glen Gronniger et al.Focused-Laser Interferometric Position Sensor
http://digitalcommons.unl.edu/physicsfacpub/116
http://digitalcommons.unl.edu/physicsfacpub/116Mon, 05 Nov 2012 09:16:17 PST
We describe a simple method to measure the position shifts of an object with a range of tens of micrometers using a focused-laser (FL) interferometric position sensor. In this article we examine the effects of mechanical vibration on FL and Michelson interferometers. We tested both interferometers using vibration amplitudes ranging from 0 to 20 μm. Our FL interferometer has a resolution much better than the diffraction grating periodicities of 10 and 14 μm used in our experiments. A FL interferometer provides improved mechanical stability at the expense of spatial resolution. Our experimental results show that Michelson interferometers cannot be used when the vibration amplitude is more than an optical wavelength. The main purpose of this article is to demonstrate that a focused-laser interferometric position sensor can be used to measure the position shifts of an object on a less sensitive, micrometer scale when the vibration amplitude is too large to use a Michelson interferometer.
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Stephen J. Friedman et al.