Physical Sciences and Mathematics Commons^™

Magnetism Of Fept Nanoclusters In Polyimide, Mircea Chipara, Tom George, Yingfan Xu, Ralph Skmoski, Lanping Yue, Nasar Ali, David J. Sellmyer

David Sellmyer Publications

FePt nanoclusters have been implanted onto polyimide films and subjected to thermal annealing in order to obtain a special magnetic phase (L10) dispersed within the polymer. SQUID measurements quantified the magnetic features of the as-prepared and annealed hybrid films. As-implanted FePt nanoparticles in polyimide films exhibited a blocking temperature of 70 ± 5K. Thermal annealing in zero and 10 kOe applied magnetic field increased the magnetic anisotropy and coercivity of the samples. Wide Angle X-Ray Scattering confirmed the presence of FePt and L10 phase. All samples (as deposited and annealed) exhibited electron spin resonance spectra consisting of two overlapping lines. …

Tomographic Imaging Of Nonsymmetric Multicomponent Tailored Supersonic Flows From Structured Gas Nozzles, Grigory V. Golovin, Sudeep Banerjee, J Zhang, Shouyuan Chen, Cheng Liu, Baozhen Zhao, J Mills, Kevin J. Brown, C Petersen, Donald P. Umstadter

Donald Umstadter Publications

We report experimental results on the production and characterization of asymmetric and composite supersonic gas flows, created by merging independently controllable flows from multiple nozzles. We demonstrate that the spatial profiles are adjustable over a large range of parameters, including gas density, density gradient, and atomic composition. The profiles were precisely characterized using three-dimensional tomography. The creation and measurement of complex gas flows is relevant to numerous applications, ranging from laser-produced plasmas to rocket thrusters.

Compact Source Of Narrowband And Tunable X-Rays For Radiography, Sudeep Banerjee, Shouyuan Chen, Nathan D. Powers, Daniel Haden, Cheng Liu, Grigory V. Golovin, Jun Zhang, Baozhen Zhao, S. Clarke, Sara Pozzi, Jack Silano, H. Karwowski, Donald Umstadter

Donald Umstadter Publications

We discuss the development of a compact X-ray source based on inverse-Compton scattering with a laser-driven electron beam. This source produces a beam of high-energy X-rays in a narrow cone angle (5–10 mrad), at a rate of 108 photons-s_1. Tunable operation of the source over a large energy range, with energy spread of ~50%, has also been demonstrated. Photon energies >10 MeV have been obtained. The narrowband nature of the source is advantageous for radiography with low dose, low noise, and minimal shielding.

All-Laser-Driven Thomson X-Ray Sources, Donald Umstadter

Donald Umstadter Publications

We discuss the development of a new generation of accelerator-based hard X-ray sources driven exclusively by laser light. High-intensity laser pulses serve the dual roles: first, accelerating electrons by laser-driven plasma wakefields, and second, generating X-rays by inverse Compton scattering. Such all-laser-driven X-rays have recently been demonstrated to be energetic, tunable, relatively narrow in bandwidth, short pulsed and well collimated. Such characteristics, especially from a compact source, are highly advantageous for numerous advanced X-ray applications—in metrology, biomedicine, materials, ultrafast phenomena, radiology and fundamental physics.

Electrostatic Charge On Flying Hummingbirds And Its Potential Role In Pollination, Marc Badger, Victor Manuel Ortega-Jimenez, Lisa Von Ribenau, Ashley Smiley, Robert Dudley, Alexei Gruverman , Editor

Alexei Gruverman Publications

Electrostatic phenomena are known to enhance both wind- and insect-mediated pollination, but have not yet been described for nectar-feeding vertebrates. Here we demonstrate that wild Anna's Hummingbirds (Calypte anna) can carry positive charges up to 800 pC while in flight (mean ± s.d.: 66 ± 129 pC). Triboelectric charging obtained by rubbing an isolated hummingbird wing against various plant structures generated charges up to 700 pC. A metal hummingbird model charged to 400 pC induced bending of floral stamens in four plants (Nicotiana, Hemerocallis, Penstemon, and Aloe spp.), and also attracted falling Lycopodium spores at distances of < 2 mm. Electrostatic forces may therefore influence pollen transfer onto nectar-feeding birds.

Spin-Modified Catalysis, R Choudhary, Priyanka Manchanda, Axel Enders, Balamurugan Balamurugan, Arti Kashyap, David J. Sellmyer, Ralph A. Skomski

Ralph Skomski Publications

First-principle calculations are used to explore the use of magnetic degrees of freedom in catalysis. We use the Vienna Ab-Initio Simulation Package to investigate both L10-ordered FePt and CoPt bulk materials and perform supercell calculations for FePt nanoclusters containing 43 atoms. As the catalytic activity of transition-metal elements and alloys involves individual d levels, magnetic alloying strongly affects the catalytic performance, because it leads to shifts in the local densities of states and to additional peaks due to magnetic-moment formation. The peak shift persists in nanoparticles but is surface-site specific and therefore depends on cluster size. Our research indicates that …

Correlations In Rare-Earth Transition-Metal Permanent Magnets, Ralph Skomski, Priyanka Manchanda, Arti Kashyap

Ralph Skomski Publications

It is investigated how electron-electron correlations affect the intrinsic properties of rare-earth transition- metal magnets. Focusing on orbital moment and anisotropy, we perform model calculations for 3d-4f alloys and density-functional theory (DFT) calculations for NdCo5. On an independentelectron level, the use of a single Slater determinant with broken spin symmetry introduces Hund’s rule correlations, which govern the behavior of rare-earth ions and of alloys described by the local spin density approximation (LSDA) and LSDAþU approximations to DFT. By contrast, rareearth ions in intermetallics involve configuration interactions between two or more Slater determinants and lead to phenomena such as spin-charge distribution. …

Magnetic Chiral Spin Textures By Imprinting, Robert Streubel, F. Kronast, U. Rössler, O. G. Schmidt, P. Fischer, D. Makarov

Robert Streubel Papers

The unique properties of non-trivial topological states, e.g. magnetic skyrmions [1] may path the way towards novel spintronic devices [2]. However, these spin textures have only been observed in special classes of materials possessing non-centrosymmetric crystal structure [1,3-6] and at low temperatures, which limits their application potential. We offer an alternate route to design synthetic magnetic heterostructures that resemble swirls, vortices or skyrmions with distinct topological charge densities at room temperature. By vertically stacking two magnetic nanopatterns with in-and out-of-plane magnetization and tailoring the interlayer exchange coupling, non-collinear spin textures with tunable topological charge can be imprinted (Fig. 1a).

Low Trap-State Density And Long Carrier Diffusion In Organolead Trihalide Perovskite Single Crystals, Dong Shi, Valerio Adinolfi, Riccardo Comin, Mingjian Yuan, Erkki Alarousu, Andrei Buin, Yin Chen, Sjoerd Hoogland, Alexander Rothenberger, Khabiboulakh Katsiev, Yaroslav B. Losovyj, Xin Zhang, Peter A. Dowben, Omar F. Mohammed, Edward H. Sargent, Osman M. Bakr

Peter Dowben Publications

The fundamental properties and ultimate performance limits of organolead trihalide MAPbX₃ (MA = CH₃NH₃ +; X = Br^– or I^–) perovskites remain obscured by extensive disorder in polycrystalline MAPbX₃ films. We report an antisolvent vapor-assisted crystallization approach that enables us to create sizable crack-free MAPbX₃ single crystals with volumes exceeding 100 cubic millimeters. These large single crystals enabled a detailed characterization of their optical and charge transport characteristics. We observed exceptionally low trap-state densities on the order of 10⁹ to 10¹⁰ per cubic centimeter in MAPbX₃ single crystals …

Growth Factor And Ultrasound-Assisted Bioreactor Synergism For Human Mesenchymal Stem Cell Chondrogenesis, Sanjukta Guha Thakurta, Gaurav Budhiraja, Anuradha Subramanian

Department of Chemical and Biomolecular Engineering: Faculty Publications

Ultrasound at 5.0 MHz was noted to be chondro-inductive, with improved SOX-9 gene and COL2A1 protein expression in constructs that allowed for cell-to-cell contact. To achieve tissue-engineered cartilage using macroporous scaffolds, it is hypothesized that a combination of ultrasound at 5.0 MHz and transforming growth factor-β3 induces human mesenchymal stem cell differentiation to chondrocytes. Expression of miR-145 was used as a metric to qualitatively assess the efficacy of human mesenchymal stem cell conversion. Our results suggest that in group 1 (no transforming growth factor-β3, no ultrasound), as anticipated, human mesenchymal stem cells were not efficiently differentiated into chondrocytes, judging by …

Effect Of Strain On Ferroelectric Field Effect In Strongly Correlated Oxide Sm0.5nd0.5nio3, L. Zhang, X. G. Chen, H. J. Gardner, M. A. Koten, J. E. Shield, X. Hong

Xia Hong Publications

We report the effect of epitaxial strain on the magnitude and retention of the ferroelectric field effect in high quality PbZr_0.3Ti_0.7O₃ (PZT)/3.8-4.3 nm Sm_0.5Nd_0.5NiO₃ (SNNO) heterostructures grown on (001) LaAlO₃ (LAO) and SrTiO₃ (STO) substrates. For SNNO on LAO, which exhibits a first-order metal-insulator transition (MIT), switching the polarization of PZT induces a 10 K shift in the transition temperature T_MI, with a maximum resistance change between the on and off states of Δ𝑅/𝑅_on ~75%. In sharp contrast, only up to 5% resistance change has …

Multiferroic Tunnel Junctions And Ferroelectric Control Of Magnetic State At Interface, Y. W. Yin, M. Raju, W. J. Hu, John D. Burton, Y.-M. Kim, A. Y. Borisevich, S. J. Pennycook, S. M. Yang, T. W. Noh, Alexei Gruverman, X. G. Li, Z. D. Zhang, Evgeny Y. Tsymbal, Qi Li

Alexei Gruverman Publications

As semiconductor devices reach ever smaller dimensions, the challenge of power dissipation and quantum effect place a serious limit on the future device scaling. Recently, a multiferroic tunnel junction (MFTJ) with a ferroelectric barrier sandwiched between two ferromagnetic electrodes has drawn enormous interest due to its potential applications not only in multi-level data storage but also in electric field controlled spintronics and nanoferronics. Here, we present our investigations on four-level resistance states, giant tunneling electroresistance (TER) due to interfacial magnetoelectric coupling, and ferroelectric control of spin polarized tunneling in MFTJs. Coexistence of large tunneling magnetoresistance and TER has been observed …

Anomalous Nonlinear X-Ray Compton Scattering, Matthias Fuchs, Mariano Trigo, Jian Chen, Shambhu Ghimire, Sharon Shwartz, Michael Kozina, Mason Jiang, Thomas Henighan, Crystal Bray, Georges Ndabashimiye, Philip H. Bucksbaum, Yiping Feng, Sven Herrmann, Gabriella A. Carini, Jack Pines, Philip Hart, Christopher Kenney, Serge Guillet, Sébastien Boutet, Garth J. Williams, Marc Messerschmidt, M. Marvin Seibert, Stefan Moeller, Jerome B. Hastings, David A. Reis

Matthias Fuchs Publications

X-ray scattering is typically used as a weak linear atomic-scale probe of matter. At high intensities, such as produced at free-electron lasers, nonlinearities can become important, and the probe may no longer be considered weak. Here we report the observation of one of the most fundamental nonlinear X-ray–matter interactions: the concerted nonlinear Compton scattering of two identical hard X-ray photons producing a single higher-energy photon. The X-ray intensity reached 4 × 10²⁰ W cm⁻², corresponding to an electric field well above the atomic unit of strength and within almost four orders of magnitude of the quantum-electrodynamic critical …

Unusual Spin Correlations In A Nanomagnet, Balamurugan Balasubramanian, Priyanka Manchanda, Ralph Skomski, Pinaki Mukherjee, Bhaskar Das, T A. George, George C. Hadjipanayis, David J. Sellmyer

David Sellmyer Publications

We show how atomic-scale exchange phenomena can be controlled and exploited in nanoscale itinerant magnets to substantially improve magnetic properties. Cluster-deposition experiments, first-principle simulations, and analytical calculations are used to demonstrate the effect in Co2Si nanoclusters, which have average sizes varying from about 0.6 to 29.5 nm. The cluster-deposited nanoparticles exhibit average magnetic moments of up to 0.70 lB/Co at 10K and 0.49 lB/Co at 300K with appreciable magnetocrystalline anisotropies, in sharp contrast to the nearly vanishing bulk magnetization. The underlying spin correlations and associated cluster-size dependence of the magnetization are explained by a surface induced ferromagnetic spin polarization with …

Chirally-Sensitive Electron-Molecule Interactions, J. M. Dreiling, Timothy J. Gay

Timothy J. Gay Publications

All molecular forms of life have chemically-specific handedness. However, the origin of these asymmetries is not understood. A possible explanation was suggested by Vester and Ulbricht immediately following the discovery of parity violation in 1957: chiral beta radiation in cosmic rays may have preferentially destroyed one enantiomeric form of various biological precursors. In the experiments reported here, we observed chiral specificity in two electronmolecule interactions: quasi-elastic scattering and dissociative electron attachment. Using lowenergy longitudinally spin-polarized (chiral) electrons as substitutes for beta rays, we found that chiral bromocamphor molecules exhibited both a transmission and dissociative electron attachment rate that depended on …

Elasticity Of Differentiated And Undifferentiated Human Neuroblastoma Cells Characterized By Atomic Force Microscopy, Shijia Zhao, Alexander B. Stamm, Jeong Soon Lee, Alexei Gruverman, Jung Yul Lim, Linxia Gu

Department of Mechanical and Materials Engineering: Faculty Publications

Human neuroblastoma (SH-SY5Y) cells, with its ability to differentiate into neurons, have been widely used as the in vitro cell culture model for neuroscience research, especially in studying the pathogenesis of Parkinson’s disease (PD) and developing therapeutic strategies. Cellular elasticity could potentially serve as a biomarker to quantitatively distinguish undifferentiated and differentiated SH-SY5Y cells. The goal of this work is to characterize the retinoic acid (RA) induced alternations of elastic properties of SH-SY5Y cells using atomic force microscopy (AFM). The elasticity was measured at multiple points of a single cell. Results have shown that the differentiation of SH-SY5Y cell led …

Optical Control Of Electron Phase Space In Plasma Accelerators With Incoherently Stacked Laser Pulses, Serguei Y. Kalmuykov, X Davoine, R Lehe, A F. Lifschitz, Bradley Allan Shadwick

Department of Physics and Astronomy: Faculty Publications

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 …

Thresholds For Correcting Errors, Erasures, And Faulty Syndrome Measurements In Degenerate Quantum Codes, Ilya Dumer, Alexey Kovalev, Leonid P. Pryadko

Department of Physics and Astronomy: Faculty Publications

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.

Tunable Monoenergetic Electron Beams From Independently Controllable Laser-Wakefield Acceleration And Injection, Grigory V. Golovin, Shouyuan Chen, Nathan D. Powers, Cheng Liu, Sudeep Banerjee, J Zhang, M Zeng, Z Sheng, Donald P. Umstadter

Donald Umstadter Publications

We report the results of experiments on laser-wakefield acceleration in a novel two-stage gas target with independently adjustable density and atomic-composition profiles.We were able to tailor these profiles in a way that led to the separation of the processes of electron injection and acceleration and permitted independent control of both. This resulted in the generation of stable, quasimonoenergetic electron beams with central energy tunable in 50–300 MeV range. For the first time, we are able to independently control the beam charge and energy spread over the entire tunability range.

Mega-Electron-Volt Ultrafast Electron Diffraction At Slac National Accelerator Laboratory, Stephen Weathersby, G. Brown, Martin Centurion, T. F. Chase, Ryan Coffee, Jeff Corbett, J. P. Eichner, J. C. Frisch, A. R. Fry, M. Gühr, Nick Hartmann, Carsten Hast, R. Hettel, R. K. Jobe, E. N. Jongewaard, J. R. Lewandowski, R. K. Li, A. M. Lindenberg, Igor Makasyuk, J. E. May, D. Mccormick, M. N. Nguyen, Alexander Reid, Xiaozhe Shen, K. Sokolowski-Tinten, Theodore Vecchione, Sharon Vetter, J. Wu, Jie Yang, H. A. Dürr, Xijie Wang

Martin Centurion Publications

Ultrafast electron probes are powerful tools, complementary to x-ray free-electron lasers, used to study structural dynamics in material, chemical, and biological sciences. High brightness, relativistic electron beams with femtosecond pulse duration can resolve details of the dynamic processes on atomic time and length scales. SLAC National Accelerator Laboratory recently launched the Ultrafast Electron Diffraction (UED) and microscopy Initiative aiming at developing the next generation ultrafast electron scattering instruments. As the first stage of the Initiative, a mega-electron-volt (MeV) UED system has been constructed and commissioned to serve ultrafast science experiments and instrumentation development. The system operates at 120-Hz repetition rate …

Self-Assembly Of Strongly Dipolar Molecules On Metal Surfaces, Donna A. Kunkel, James Hooper, Scott Simpson, Daniel P. Miller, Lucie Routaboul, Pierre Braunstein, Bernard Doudin, Sumit Beniwal, Peter A. Dowben, Ralph Skomski, Eva Zurek, Axel Enders

Axel Enders Publications

The role of dipole-dipole interactions in the self-assembly of dipolar organic molecules on surfaces is investigated. As a model system, strongly dipolar model molecules, p-benzoquinonemonoimine zwitterions (ZI) of type C6H2(· · · NHR)2(· · · O)2 on crystalline coinage metal surfaces were investigated with scanning tunneling microscopy and first principles calculations. Depending on the substrate, the molecules assemble into small clusters, nano gratings, and stripes, as well as in two-dimensional islands. The alignment of the molecular dipoles in those assemblies only rarely assumes the lowest electrostatic energy configuration. Based on calculations of the electrostatic energy for various experimentally observed molecular …

Intrinsic Magnetic Properties Of L10 Feni Obtained From Meteorite Nwa 6259, Eric Poirier, Frederick E. Pinkerton, Robert Kubic, Raja K. Mishra, Nina Bordeaux, Arif Mubarok, Laura H. Lewis, Joseph I. Goldstein, Ralph Skomski, Katayun Barmak

Ralph Skomski Publications

FeNi having the tetragonal L10 crystal structure is a promising new rare-earth-free permanent magnet material. Laboratory synthesis is challenging, however, tetragonal L10 FeNi—the mineral “tetrataenite”—has been characterized using specimens found in nickel-iron meteorites. Most notably, the meteorite NWA 6259 recovered from Northwest Africa is 95 vol.% tetrataenite with a composition of 43 at.% Ni. Hysteresis loops were measured as a function of sample orientation on a specimen cut from NWA 6259 in order to rigorously deduce the intrinsic hard magnetic properties of its L10 phase. Electron backscatter diffraction showed that NWA 6259 is strongly textured, containing L10 grains oriented along …

Manipulating Topological States By Imprinting Non-Collinear Spin Textures, Robert Streubel, Luyang Han, Mi Young Im, Florian Kronast, Ulrich K. Rößler, Florin Radu, Radu Abrudan, Gungun Lin, Oliver G. Schmidt, Peter Fischer, Denys Makarov

Robert Streubel Papers

Topological magnetic states, such as chiral skyrmions, are of great scientific interest and show huge potential for novel spintronics applications, provided their topological charges can be fully controlled. So far skyrmionic textures have been observed in noncentrosymmetric crystalline materials with low symmetry and at low temperatures. We propose theoretically and demonstrate experimentally the design of spin textures with topological charge densities that can be tailored at ambient temperatures. Tuning the interlayer coupling in vertically stacked nanopatterned magnetic heterostructures, such as a model system of a Co/Pd multilayer coupled to Permalloy, the in-plane non-collinear spin texture of one layer can be …

Magnetic Soft X-Ray Tomography Of Magnetic Swiss Roll Architectures, Robert Streubel, F. Kronast, P. Fischer, O. G. Schmidt, D. Makarov

Robert Streubel Papers

A further increase in performance of state-of-the-art spin-electronics can be achieved by either miniaturizing its functional components or harnessing the third dimension. The first route, however, faces physical limitations as properties of nanostructures and their response to external stimuli can drastically change in reduced dimensions. An alternative way is to go from planar 2D structures to 3D architectures [1]. Such 3D functional elements can be obtained for example by rolling up initially planar strained thin films into Swiss roll like objects with multiple windings (Fig. 1a). A major advantage of this technology platform is the possibility to fabricate compact multifunctional …

Fast Strain Wave Induced Magnetization Changes In Long Cobalt Bars: Domain Motion Versus Coherent Rotation, S Davis, J A. Borchers, B B. Maranville, Shireen Adenwalla

Department of Physics and Astronomy: Faculty Publications

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 …

Magnetization Pumping And Dynamics In A Dzyaloshinskii-Moriya Magnet, Alexey Kovalev, Utkan Güngördü

Department of Physics and Astronomy: Faculty Publications

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.

Origin Of The Spin Reorientation Transitions In (Fe1–Xcox)2b Alloys, Kirill D. Belashchenko, Liqin Ke, Markus Dane, Lorin X. Benedict, Tej Nath Lamichhane, Valentin Taufour, Anton Jesche, Sergey L. Bud'ko, Paul C. Canfield, Vladimir P. Antropov

Department of Physics and Astronomy: Faculty Publications

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 …

Phys 231: Electric And Electronic Circuits—A Peer Review Of Teaching Project Benchmark Portfolio, Xiaoshan Xu

UNL Faculty Course Portfolios

In this portfolio, I employed a backward design teaching approach on the course PHY231 (Electric and Electronic circuit). I first decide the course objectives and choose the teaching strategies and activities emphasizing student engagement and feedbacks. The effectiveness of the teaching strategies and activities are assessed throughout the course; the adjustments are made accordingly. The results are analyzed at the end of the semester. The assessments indicate that the course objectives are fundamentally achieved and the teaching strategies are effective.

The objectives of the course are:

1. Students will reinforce their understanding in electricity and magnetism.
2. Students will learn theory of …

Continuous Third Harmonic Generation In A Terahertz Driven Modulated Nanowire, Kathleen E. Hamilton, Alexey Kovalev, Amrit De, Leonid P. Pryadko

Department of Physics and Astronomy: Faculty Publications

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 …

Spin Glass Reflection Of The Decoding Transition For Quantum Error Correcting Codes, Alexey Kovalev, Leonid P. Pryadko

Department of Physics and Astronomy: Faculty Publications

We study the decoding transition for quantum error correcting codes with the help of a mapping to random-bond Wegner spin models. Families of quantum low density parity-check (LDPC) codes with a finite decoding threshold lead to both known models (e.g., random bond Ising and random plaquette Z₂ gauge models) as well as unexplored earlier generally non-local disordered spin models with non-trivial phase diagrams. The decoding transition corresponds to a transition from the ordered phase by proliferation of "post-topological" extended defects which generalize the notion of domain walls to non-local spin models. In recently discovered quantum LDPC code families with …