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Non-uniform along thickness spin excitations in magnetic vortex-state nanodots

Fizika Nizkikh Temperatur 2020年第8期46卷 1021-1027页

作者： Kakazei, G.N. Guslienko, K.Y. Verba, R.V. Ding, J. Liu, X.M. Adeyeye, A.O. Departamento de Física e Astronomia Universidade do Porto Porto4169-007 Portugal Departamento de Física de Materiales Universidad del País Vasco San Sebastián20018 Spain IKERBASQUE Basque Foundation for Science Bilbao48011 Spain Institute of Magnetism NAS of Ukraine MES of Ukraine Kyiv03142 Ukraine Information Storage Materials Laboratory Department of Electrical and Computer Engineering National University of Singapore Singapore117576 Singapore Department of Physics Durham University South Rd DurhamDH1 3LE United Kingdom

We summarize our experimental findings in the arrays of Ni80Fe20 circular nanodots with diameter 300 nm and thickness 20 nm ≤ L ≤ 100 nm, probed by broadband ferromagnetic resonance spectroscopy in the absence of external magnetic field. Spin excitation modes related to the vortex core gyrotropic dynamics were observed in the gigahertz frequency range. Micromagnetic simulations revealed that they are flexure oscillations of the vortex core string with n = 0, 1, 2 nodes along the dot thickness. It was found that for L > 70 nm the intensity of more complicated n = 1 vortex gyrotropic mode is unexpectedly higher than the one of the lowest n = 0 gyrotropic mode. This behavior was clarified on the basis of the inhomogeneous vortex mode phase profiles extracted from micromagnetic simulations and calculated analytically. Precise measurements of the dependence of resonance frequency of the vortex n = 0 mode on the dot thickness demonstrated a clear maximum around L = 70 nm, that was theoretically explained by introducing a vortex mass, which is a result of the vortex distortion due to interaction with spin waves having azimuthal indices m = ±1. Finally, several azimuthal spin-wave modes having curled structure at the dot top and bottom faces were found in the spectrum of the dots with thicknesses L ≥ 40 nm. © G.N. Kakazei, K.Y. Guslienko, R.V. Verba, J. Ding, X.M. Liu, and A.O. Adeyeye, 2020

关键词： Ferromagnetic resonance

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Frontispiece: Heat-Free Biomimetic Metal Molding on Soft Substrates

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Angewandte Chemie International Edition 2020年第38期59卷

作者： Julia J. Chang Andrew Martin Chuanshen Du Alana M. Pauls Martin Thuo Iowa State University Department of Materials Science and Engineering Ames IA 50014 USA Micro-Electronics Research Centre Ames IA 50014 USA Iowa State University Department of Electrical and Computer Engineering Ames IA 50014 USA

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Stabilization of Undercooled Metals via Passivating Oxide Layers

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Angewandte Chemie 2020年第11期133卷

作者： Andrew Martin Boyce S. Chang Alana M. Pauls Chuanshen Du Martin Thuo Department of Materials Science and Engineering Iowa State University Ames IA 50010 USA Department of Electrical and Computer Engineering Iowa State University Ames IA 50010 USA Micro-Electronics Research Centre Ames IA 50014 USA

Undercooling metals relies on frustration of liquid–solid transition mainly by an increase in activation energy. Passivating oxide layers are a way to isolate the core from heterogenous nucleants (physical barrier) while also raising the activation energy (thermodynamic/kinetic barrier) needed for solidification. The latter is due to composition gradients (speciation) that establishes a sharp chemical potential gradient across the thin (0.7–5 nm) oxide shell, slowing homogeneous nucleation. When this speciation is properly tuned, the oxide layer presents a previously unaccounted for interfacial tension in the overall energy landscape of the relaxing material. We demonstrate that 1) the integrity of the passivation oxide is critical in stabilizing undercooled particle, a key tenet in developing heat-free solders, 2) inductive effects play a critical role in undercooling, and 3) the magnitude of the influence of the passivating oxide can be larger than size effects in undercooling.

关键词： energy landscape inductive effect metastability phase transformation stable undercooling

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Sensing with Exceptional Surfaces in Order to Combine Sensitivity with Robustness

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Physical Review Letters 2019年第15期122卷 153902-153902页

作者： Q. Zhong J. Ren M. Khajavikhan D. N. Christodoulides Ş. K. Özdemir R. El-Ganainy Department of Physics and Henes Center for Quantum Phenomena Michigan Technological University Houghton Michigan 49931 USA College of Optics & Photonics-CREOL University of Central Florida Orlando Florida 32816 USA Department of Engineering Science and Mechanics The Pennsylvania State University University Park Pennsylvania 16802 USA Materials Research Institute The Pennsylvania State University University Park Pennsylvania 16802-6812 USA Department of Electrical and Computer Engineering Michigan Technological University Houghton Michigan 49931 USA

Exceptional points (EPs) are singularities that arise in non-Hermitian physics. Current research efforts focus only on systems supporting isolated EPs characterized by increased sensitivity to external perturbations, which makes them potential candidates for building next generation optical sensors. On the downside, this feature is also the Achilles heel of these devices: they are very sensitive to fabrication errors and experimental uncertainties. To overcome this problem, we introduce a new design concept for implementing photonic EPs that combine the robustness required for practical use together with their hallmark sensitivity. Particularly, our proposed structure exhibits a hypersurface of Jordan EPs embedded in a larger space, and having the following peculiar features: (1) A large class of undesired perturbations shift the operating point along the exceptional surface (ES), thus, leaving the system at another EP which explains the robustness; (2) Perturbations due to back reflection or backscattering force the operating point out of the ES, leading to enhanced sensitivity. Importantly, our proposed geometry is relatively easy to implement using standard photonics components and the design concept can be extended to other physical platforms such as microwave or acoustics.

关键词： Microcavity & microdisk lasers Nanoparticles Whispering gallery mode resonators

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Frontispiz: Heat‐Free Biomimetic Metal Molding on Soft Substrates

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Angewandte Chemie 2020年第38期132卷

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Angular-dependent spin dynamics of a triad of permalloy macrospins

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Physical Review B 2019年第1期99卷 014415-014415页

作者： Wonbae Bang F. Montoncello M. B. Jungfleisch A. Hoffmann L. Giovannini J. B. Ketterson Department of Physics and Astronomy Northwestern University Evanston Illinois 60208 USA Materials Science Division Argonne National Laboratory Argonne Illinois 60439 USA Dipartimento di Fisica e Scienze della Terra Università di Ferrara Ferrara Italy Department of Physics and Astronomy University of Delaware Newark Delaware 19716 USA Department of Electrical and Computer Engineering Northwestern University Evanston Illinois 60208 USA

We experimentally and theoretically characterize the angular-dependent microwave response of three-macrospin-vertex structures that can serve as a node in various spin ice lattices. The macrospins consist of patterned permalloy thin films with an elliptical cross section together with an in-plane aspect ratio allowing an Ising-like behavior together with bulk modes as low-frequency excitations in the field range of interest. Various branches of the frequency-magnetic field curves display atypical behaviors and discontinuities, together with softening due to macrospin reversals. The overall behavior observed accurately corresponds to a superposition of the spectra of the individual macrospins. The measured ferromagnetic resonance spectra are in good agreement with theoretical modeling. In particular, they reveal a close correlation between the field direction (relative to the individual macrospins), and the corresponding frequency-magnetic field curve, i.e., between the geometry and the magnetic response.

关键词： Magnetization dynamics Spin dynamics Kagome lattice Spin ice Ferromagnetic resonance Micromagnetic modeling Microwave techniques Scanning electron microscopy

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Direct evidence for Cooper pairing without a spectral gap in a disordered superconductor above Tc

arXiv

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arXiv 2021年

作者： Bastiaans, Koen M. Chatzopoulos, Damianos Ge, Jian-Feng Cho, Doohee Tromp, Willem O. van Ruitenbeek, Jan M. Fischer, Mark H. de Visser, Pieter J. Thoen, David J. Driessen, Eduard F.C. Klapwijk, Teunis M. Allan, Milan P. Leiden Institute of Physics Leiden University Niels Bohrweg 2 Leiden2333 CA Netherlands Department of Physics Yonsei University Seoul03722 Korea Republic of Department of Physics University of Zurich Winterthurersstrasse 190 Zurich8057 Switzerland SRON Netherlands Institute for Space Research Sorbonnelaan 2 Utrecht3584 CA Netherlands Kavli Institute of Nanoscience Delft University of Technology Lorentzweg 1 Delft2628 CJ Netherlands Faculty of Electrical Engineering Mathematics and Computer Science Delft University of Technology Mekelweg 4 Delft2628 CD Netherlands Grenoble 300 Rue de la Piscine Saint-Martin-d'Hères38400 France Institute of Topological Materials Julius-Maximilian-Universität Würzburg Sanderring 2 Würzburg97070 Germany Physics Department Moscow State University of Education 1/1 Malaya Pirogovskaya Str. Moscow119991 Russia

The idea that preformed Cooper pairs could exist in a superconductor above its zero-resistance state has been explored for unconventional, interface, and disordered superconductors, yet direct experimental evidence is lacking. Here, we use scanning tunneling noise spectroscopy to unambiguously show that preformed Cooper pairs exist up to temperatures much higher than the zero-resistance critical temperature Tc in the disordered superconductor titanium nitride, by observing a clear enhancement in the shot noise that is equivalent to a change of the effective charge from 1 to 2 electron charges. We further show that spectroscopic gap fills up rather than closes when increasing temperature. Our results thus demonstrate the existence of a novel state above Tc that, much like an ordinary metal, has no (pseudo)gap, but carries charge via paired electrons. Copyright © 2021, The Authors. All rights reserved.

关键词： Superconducting materials

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Environmentally stable room temperature continuous wave lasing in defect-passivated perovskite

arXiv

引用

arXiv 2019年

作者： Moon, Jiyoung Alahbakhshi, Masoud Gharajeh, Abouzar Kwon, Sunah Haroldson, Ross Li, Zhitong Hawkins, Roberta Kim, Moon J. Hu, Walter Zakhidov, Anvar Gu, Qing Department of Materials Science and Engineering University of Texas at Dallas RichardsonTX75080 United States Department of Electrical and Computer Engineering University of Texas at Dallas RichardsonTX75080 United States Department of Physics University of Texas at Dallas RichardsonTX75080 United States Department of Nanophotonics and Metamaterials ITMO University St. Petersburg Moscow Russia

Metal halide perovskites have emerged as promising gain materials for on-chip lasers in photonic integrated circuits (PICs). However, stable continuous wave (CW) lasing behavior under optical pumping at room temperature - a prerequisite for electrically pumped lasing - has not yet been demonstrated. To achieve stable CW operation, we introduce a multiplex of strategies that include morphological, structural and interfacial engineering of CH3NH3PbBr3 (MAPbBr3) thin films to improve perovskite’s intrinsic stability, as well as high quality cavity design to reduce the operational power. We demonstrate for the first time, over 90-minute-long green CW lasing with 9.4W/cm2 threshold from a polycarbonate (PC) -defect-passivated, directly patterned MAPbBr3 two-dimensional photonic crystal (PhC) cavity without any substrate cooling. We also show our approach’s effectiveness on the performance of MAPbBr3 under electrical excitation: we observe a seven-fold current efficiency enhancement by applying our strategies to a MAPbBr3 LED. This work paves the way to the realization of electrically pumped lasing in perovskites. Copyright © 2019, The Authors. All rights reserved.

关键词： Perovskite

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Immunity of nanoscale magnetic tunnel junctions to ionizing radiation

arXiv

引用

arXiv 2019年

作者： Montoya, Eric Arturo Chen, Jen-Ru Ngelale, Randy Lee, Han Kyu Tseng, Hsin-Wei Wan, Lei Yang, En Braganca, Patrick Boyraz, Ozdal Bagherzadeh, Nader Nilsson, Mikael Krivorotov, Ilya N. Department of Physics and Astronomy University of California IrvineCA92697 United States Department of Chemical Engineering and Materials Science University of California IrvineCA92697 United States Department of Chemistry University of California IrvineCA92697 United States Western Digital San JoseCA95135 United States Department of Electrical Engineering and Computer Science University of California IrvineCA92697 United States

Spin transfer torque magnetic random access memory (STT-MRAM) is a promising candidate for next generation memory as it is non-volatile, fast, and has unlimited endurance. Another important aspect of STT-MRAM is that its core component, the nanoscale magnetic tunneling junction (MTJ), is thought to be radiation hard, making it attractive for space and nuclear technology applications. However, studies of the effects of high doses of ionizing radiation on STT-MRAM writing process are lacking. Here we report measurements of the impact of high doses of gamma and neutron radiation on nanoscale MTJs with perpendicular magnetic anistropy used in STT-MRAM. We characterize the tunneling magnetoresistance, the magnetic field switching, and the current-induced switching before and after irradiation. Our results demonstrate that all these key properties of nanoscale MTJs relevant to STT-MRAM applications are robust against ionizing radiation. Additionally, we perform experiments on thermally driven stochastic switching in the gamma ray environment. These results indicate that nanoscale MTJs are promising building blocks for radiation-hard non-von Neumann computing. Copyright © 2019, The Authors. All rights reserved.

关键词： Nanomagnetics

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Artificial synapse with mnemonic functionality using gsst-based photonic integrated memory

arXiv

引用

arXiv 2019年

作者： Miscuglio, Mario Meng, Jiawei Yesiliurt, Omer Zhang, Yifei Prokopeva, Ludmila J. Mehrabian, Armin Hu, Juejun Kildishev, Alexander V. Sorger, Volker J. Department of Electrical and Computer Engineering George Washington University 800 22nd Street NW WashingtonDC20052 United States Birck Nanotechnology Center School of Ece Purdue University West LafayetteIN47907 United States Department of Materials Science and Engineering Massachusetts Institute of Technology CambridgeMA United States

Machine-learning tasks performed by neural networks demonstrated useful capabilities for producing reliable, and repeatable intelligent decisions. Integrated photonics, leveraging both component miniaturization and the wave-nature of the signals, can potentially outperform electronics architectures when performing inference tasks. However, the missing photon-photon force challenges non-volatile photonic device-functionality required for efficient neural networks. Here we present a novel concept and optimization of multi-level discrete-state non-volatile photonic memory based on an ultra-compact ( Copyright © 2019, The Authors. All rights reserved.

关键词： Phase change materials

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