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Beryllium polyhydride Be4H8(H2)2 synthesized at high pressure and temperature

Physical Review materials 2020年第12期4卷 125402-125402页

作者： Takahiro Matsuoka Hiroshi Fujihisa Takahiro Ishikawa Takaya Nakagawa Keiji Kuno Naohisa Hirao Yasuo Ohishi Katsuya Shimizu Shigeo Sasaki Department of Electrical Electronic and Computer Engineering Faculty of Engineering Gifu University Gifu 501-1193 Japan Material Science and Engineering Joint Institute for Advanced Materials (JIAM) The University of Tennessee Knoxville Tennessee 37996 USA National Institute of Advanced Industrial Science and Technology (AIST) Ibaraki 305-8565 Japan The Elements Strategy Initiative Center for Magnetic Materials National Institute for Materials Science Ibaraki 305-0047 Japan Materials Science and Technology Division Graduate School of Engineering Gifu University Gifu 501-1193 Japan Environmental and Renewable Energy Systems Division Gifu University Gifu 501-1193 Japan Japan Synchrotron Radiation Research Institute (JASRI) Hyogo 679-5198 Japan Center for Science and Technology under Extreme Conditions Graduate School of Engineering Science Osaka University Osaka 560-8531 Japan

We report x-ray diffraction and Raman scattering measurements in combination with density functional theory calculations that reveal the formation of beryllium polyhydride Be4H8(H2)2 by the laser heating of a Be-H2 mixture to above 1700 K at pressures between 5 and 8 GPa. Be4H8(H2)2 crystallizes in a P63/mmc structure and consists of the corner-sharing BeH4 tetrahedrons and the H2 molecules that occupy an interstitial site. Be4H8(H2)2 is stable at least to 14 GPa upon compression and stable down to 4 GPa at room temperature. Our ab initio calculations suggest that Be4H8(H2)2 is a metastable phase of the Be-H system.

关键词： Composition Pressure effects Structural properties Hydrides Ab initio calculations Density functional theory Raman spectroscopy X-ray diffraction

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N-Graphene Nanowalls via Plasma Nitrogen Incorporation and Substitution: The Experimental Evidence

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Nano-Micro Letters 2020年第4期12卷 92-108页

作者： Neelakandan MSanthosh Gregor Filipič Eva Kovacevic Andrea Jagodar Johannes Berndt Thomas Strunskus Hiroki Kondo Masaru Hori Elena Tatarova UrošCvelbar Jožef Stefan Institute Jamova cesta 391000 LjubljanaSlovenia Jožef Stefan International Postgraduate School Jamova cesta 391000 LjubljanaSlovenia GREMI CNRS-University of Orleans 14 rue d’Issoudun45067 Orleans Cedex 2France Institute for Materials Science Christian Albrechts University KielKaiserstr224143 KielGermany Department of Electrical Engineering and Computer Science University of NagoyaFuro‑cho Chikusa‑kuNagoyaAichi 464‑8603Japan Instituto de Plasmas e Fusão Nuclear Instituto Superior TécnicoUniversidade de Lisboa1049 LisbonPortugal

Incorporating nitrogen(N)atom in graphene is considered a key technique for tuning its electrical ***,this is still a great challenge,and it is unclear how to build N-graphene with desired nitrogen *** is a lack of experimental evidence to explain the influence and mechanism of structural defects for nitrogen incorporation into graphene compared to the derived DFT ***,this gap is bridged through a systematic study of different nitrogen-containing gaseous plasma post-treatments on graphene nanowalls(CNWs)to produce N-CNWs with incorporated and substituted *** structural and morphological analyses describe a remarkable difference in the plasma–surface interaction,nitrogen concentration and nitrogen incorporation mechanism in CNWs by using different nitrogen-containing *** conductivity measurements revealed that the conductivity of the N-graphene is strongly influenced by the position and concentration of C–N bonding *** findings open up a new pathway for the synthesis of N-graphene using plasma post-treatment to control the concentration and configuration of incorporated nitrogen for application-specific properties.

关键词： Graphene Graphene nanowalls Plasma post-treatment Nitrogen incorporation Raman spectroscopy Vacancy defects

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The debye dielectric behavior of mixed normal and relaxor-ferroelectrics: Monte Carlo investigation

The debye dielectric behavior of mixed normal and relaxor-fe...

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12th International Meeting on Ferroelectricity, IMF-12 and the 18th IEEE International Symposium on Applications of Ferroelectrics, ISAF-18

作者： Laosiritaworn, Yongyut Kanchiang, Kanokwan Ngamjarurojana, Athipong Yimnirun, Rattikorn Guo, Ruyan Bhalla, Amar S. Department of Physics and Materials Science Faculty of Science Chiang Mai University Chiang Mai 50200 Thailand ThEP Center CHE 328 Si Ayutthaya Road Bangkok 10400 Thailand Science and Technology Research Institute Chiang Mai University Chiang Mai 50200 Thailand School of Physics Institute of Science Suranaree University of Technology Nakhon Ratchasima 30000 Thailand Department of Electrical and Computer Engineering University of Texas at San Antonio One UTSA Circle San Antonio TX 78249-1644 United States

In this work, Monte Carlo simulation was used to investigate the Debye dielectric behavior of mixed normal- and relaxor-ferroelectrics. The results show that, with increasing temperature, the dielectric permittivity of the mixed system sharply increases at low temperatures but slowly decays at high temperatures, while still presenting the dielectric peaks close to the relaxor- and normal-ferroelectrics transition temperatures. Further, it is found that the sharpness, the magnitude, and the frequency dispersion of the dielectric peaks can be finely tuned by adjusting relevant normal-ferroelectric parameters in the model. However, the modified Curie-Weiss law shows that its exponent γ is governed by the inverse-temperature dependence as the exponent is very close to 2 regardless of normal-ferroelectric content. The functionalities of the Debye-type dielectric permittivity are then discussed along with the comparison with experiments. © Taylor & Francis Group, LLC.

关键词： Ferroelectric materials

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Room-temperature lasing action in GaN quantum wells in the infrared 1.5 µm region

arXiv

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arXiv 2018年

作者： Ho, V.X. Al tahtamouni, T.M. Jiang, H.X. Lin, J.Y. Zavada, J.M. Vinh, N.Q. Department of Physics Center for Soft Matter and Biological Physics Virginia Tech BlacksburgVA24061 United States Materials Science & Technology Program College of Arts & Sciences Qatar University Doha2713 Qatar Department of Electrical and Computer Engineering Texas Tech University LubbockTX79409 United States Department of Electrical and Computer Engineering New York University BrooklynNY11201 United States

Large-scale optoelectronics integration is strongly limited by the lack of efficient light sources, which could be integrated with the silicon complementary metal-oxide-semiconductor (CMOS) technology. Persistent efforts continue to achieve efficient light emission from silicon in the extending the silicon technology into fully integrated optoelectronic circuits. Here, we report the realization of room-temperature stimulated emission in the technologically crucial 1.5 µm wavelength range from Er-doped GaN multiple-quantum wells on silicon and sapphire. Employing the well-acknowledged variable stripe technique, we have demonstrated an optical gain up to 170 cm-1 in the multiple-quantum well structures. The observation of the stimulated emission is accompanied by the characteristic threshold behavior of emission intensity as a function of pump fluence, spectral linewidth narrowing and excitation length. The demonstration of room-temperature lasing at the minimum loss window of optical fibers and in the eye-safe wavelength region of 1.5 µm are highly sought-after for use in many applications including defense, industrial processing, communication, medicine, spectroscopy and imaging. As the synthesis of Er-doped GaN epitaxial layers on silicon and sapphire has been successfully demonstrated, the results laid the foundation for achieving hybrid GaN-Si lasers providing a new pathway towards full photonic integration for silicon optoelectronics. Driven by the strong need for cheap and integrable Si-based optoelectronic devices for a wide range of applications, continuing endeavors have been made to develop structures for light emission, modulation, and detection in this material system. Recent breakthroughs including the demonstration of a high-speed optical modulator in Si,1-2 photodetectors3, and waveguides4 have brought the concept of transition from electrical to optical interconnects closer to realization. However, the base for silicon photonics, namely, a group IV la

关键词： Semiconductor quantum wells

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Group-equivariant autoencoder for identifying spontaneously broken symmetries

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Physical Review E 2023年第5期107卷 054104-054104页

作者： Devanshu Agrawal Adrian Del Maestro Steven Johnston James Ostrowski Department of Industrial and Systems Engineering University of Tennessee Knoxville Tennessee 37996 USA Department of Physics and Astronomy University of Tennessee Knoxville Tennessee 37996 USA Min H. Kao Department of Electrical Engineering and Computer Science University of Tennessee Knoxville Tennessee 37996 USA Institute for Advanced Materials and Manufacturing University of Tennessee Knoxville Tennessee 37996 USA

We introduce the group-equivariant autoencoder (GE autoencoder), a deep neural network (DNN) method that locates phase boundaries by determining which symmetries of the Hamiltonian have spontaneously broken at each temperature. We use group theory to deduce which symmetries of the system remain intact in all phases, and then use this information to constrain the parameters of the GE autoencoder such that the encoder learns an order parameter invariant to these “never-broken” symmetries. This procedure produces a dramatic reduction in the number of free parameters such that the GE-autoencoder size is independent of the system size. We include symmetry regularization terms in the loss function of the GE autoencoder so that the learned order parameter is also equivariant to the remaining symmetries of the system. By examining the group representation by which the learned order parameter transforms, we are then able to extract information about the associated spontaneous symmetry breaking. We test the GE autoencoder on the 2D classical ferromagnetic and antiferromagnetic Ising models, finding that the GE autoencoder (1) accurately determines which symmetries have spontaneously broken at each temperature; (2) estimates the critical temperature in the thermodynamic limit with greater accuracy, robustness, and time efficiency than a symmetry-agnostic baseline autoencoder; and (3) detects the presence of an external symmetry-breaking magnetic field with greater sensitivity than the baseline method. Finally, we describe various key implementation details, including a quadratic-programming-based method for extracting the critical temperature estimate from trained autoencoders and calculations of the DNN initialization and learning rate settings required for fair model comparisons.

关键词： Phase transitions Structural order parameter Data analysis

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Probing the Low-Temperature Phase Transition in \text{Er}_{x}\mathrm{Y}_{1-x}\text{FeO}_{3} by Terahertz Magnetospectroscopy 45

Probing the Low-Temperature Phase Transition in \text{Er}_{x...

引用

45th International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz 2020

作者： Peraca, N. Marquez Bamba, M. Hayashida, K. Li, X. Ma, X. Makihara, T. Padilla, D. Fallas Huang, C.-L. Morosan, E. Pu, Han Cao, S. Kono, J. Department of Physics and Astronomy Rice University HoustonTX77005 United States Department of Physics Kyoto University Kyoto606-8502 Japan Presto Japan Science and Technology Agency Saitama332-0012 Japan Division of Applied Physics Graduate School and Faculty of Engineering Hokkaido University Sapporo Hokkaido060-8628 Japan Department of Physics International Center of Quantum and Molecular Structures Materials Genome Institute Shanghai University Shanghai200444 China Department of Chemistry Rice University HoustonTX77005 United States Department of Electrical and Computer Engineering Rice University HoustonTX77005 United States Department of Materials Science and NanoEngineering Rice University HoustonTX77005 United States

ISBN: (纸本)9781728166209

We have studied the terahertz response of the rare-earth orthoferrite \text{Er}_{x}\mathrm{Y}_{1-x}\text{FeO}_{3} as a function of magnetic field, temperature, and yttrium composition (x). Through theoretical modelling based on an extended Dicke Hamiltonian, we provide new insight into the role of cooperative, 'superradiant' interactions between Er spins and Fe magnons in the low-temperature phase transition that this magnetic system exhibits. © 2020 IEEE.

关键词： Iron oxides

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Direct measurement of folding angle and strain vector in atomically thin WS2 using second harmonic generation

arXiv

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arXiv 2020年

作者： Khan, Ahmed Raza Liu, Boqing Ma, Wendi Zhang, Linglong Sharma, Ankur Zhu, Yi Lü, Tieyu Lu, Yuerui Research School of Electrical Energy and Materials Engineering College of Engineering and Computer Science Australian National University CanberraACT2601 Australia Department of Physics Institute of Theoretical Physics and Astrophysics Xiamen University Xiamen361005 China

Structural engineering techniques such as local strain engineering and folding provide functional control over critical optoelectronic properties of 2D materials. Accurate monitoring of local strain vector (both strain amplitude and direction) and folding angle in 2D materials is important to optimize the device performance. Conventionally, the accurate measurement of both strain amplitude and direction requires the combined usage of multiple tools, such as atomic force microscopy (AFM), electron microscopy, Raman spectroscopy, etc. Here, we demonstrated the usage of a single tool, polarization-dependent second harmonic generation (SHG) imaging, to determine the folding angle and strain vector accurately in atomically thin tungsten disulfide (WS2). We find that trilayer WS2 folds with folding angle of 600 show 9 times SHG enhancement due to vector superposition of SH wave vectors coming from the individual folding layers. Strain dependent SHG quenching and enhancement is found parallel and perpendicular respectively to the direction of the compressive strain vector. However, despite a variation in strain angle, the total SHG remains constant which allows us to determine the local strain vector accurately using photoelastic approach. We also demonstrate that band-nesting induced transition (C peak) can highly enhance SHG, which can be significantly modulated by strain. Our results would pave the way to enable novel applications of the TMDs in nonlinear optical device. Copyright © 2020, The Authors. All rights reserved.

关键词： Harmonic generation

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Edge-field effects on gate capacitance, threshold voltage and low-field mobility in ALGaN/GaN HEMTs

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Physica Status Solidi (C) Current Topics in Solid State Physics 2011年第7-8期8卷 2448-2450页

作者： Kim, Hyeongnam Schuette, Michael L. Lu, Wu Department of Electrical and Computer Engineering The Ohio State University 2015 Neil Ave. Columbus OH 43210 United States Department of Nanobio Materials and Electronics Gwangju Institute of Science and Technology Gwangju 500-712 Korea Republic of

A methodologybased on output conductance model and pulsed current-voltage characteristics is presented for device parameter extraction in AlGaN/GaN HEMTs. A quiescent bias of (VDSQ, VGS Q) = (0 V, 0 V) and 0.2 μs pule width with 2 ms separation are applied to ensure that all the pulsed measurementsare performed under trap-free operation *** results show that HEMTs with a shorter gate length exhibit a more negative threshold voltage, a lower low-field mobility, and a higher capacitance than the theoretical AlGaN barrier capacitance. This is because edge field effect becomes significant in a shorter gate together with a higher lateral field effect. By comparing the measured and theoretical capacitances of AlGaN barrier, effective gate lengths are also estimated for HEMTs with different gate lengths. It is suggested that our method including the edge-field effect is an effective way for evaluating newly-introduced growth/processing technologies and monitoring device processing. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

关键词： Threshold voltage

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Incoherent magnetization reversal in 30-nm Ni particles

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Physical Review B 2000年第21期62卷 14252-14252页

作者： C. A. Ross R. Chantrell M. Hwang M. Farhoud T. A. Savas Y. Hao Henry I. Smith F. M. Ross M. Redjdal F. B. Humphrey Department of Materials Science and Engineering Massachusetts Institute of Technology Cambridge Massachusetts 02139 Department of Physics University of Durham Durham DH1 3LE United Kingdom Department of Electrical Engineering and Computer Science Massachusetts Institute of Technology Cambridge Massachusetts 02139 Department of Physics Massachusetts Institute of Technology Cambridge Massachusetts 02139 IBM T. J. Watson Research Center Yorktown Heights New York 10598 Department of Electrical and Computer Engineering Boston University Boston Massachusetts 02215

The magnetic properties of a 100-nm-period large-area array of regular, 30-nm polycrystalline nickel particles have been studied. The particles are found to reverse incoherently, and their hysteresis behavior has been compared with a computational model over a range of temperatures. Excellent agreement with the model is obtained, indicating that switching of the particles is dominated by the reversal of approximately 10-nm-diameter volumes within each particle. These switching volumes are identified with the columnar grains in the polycrystalline nickel, showing that the microstructure determines the magnetic behavior of the particles. This explains the anisotropy distribution and the onset of superparamagnetism in the sample. Incoherent reversal occurs even though the particles are only 1.5 times the exchange length in nickel, a size at which nearly uniform rotation is expected to occur if the particles were homogeneous.

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Spectral emission properties of a nitrogen-doped diamond (001) photocathode: Hot electron transport and transverse momentum filtering

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Physical Review B 2023年第16期107卷 165105-165105页

作者： Louis A. Angeloni Sergey V. Baryshev Matthias Mühle W. Andreas Schroeder Department of Physics University of Illinois Chicago Chicago Illinois 60607 USA Department of Electrical and Computer Engineering Michigan State University East Lansing Michigan 48824 USA and Department of Chemical Engineering and Materials Science Michigan State University East Lansing Michigan 48824 USA Fraunhofer USA Inc. Center Midwest East Lansing Michigan 48824 USA

The electron emission properties of a single-crystal nitrogen-doped diamond(001) photocathode inserted in a 10 kV DC photoelectron gun are determined using a tunable (235–410 nm) ultraviolet laser radiation source for photoemission from both the back nitrogen-doped substrate face and the front homoepitaxially grown and undoped diamond crystal face. The measured spectral trends of the mean transverse energy and quantum efficiency of the emitted electrons are both anomalous and nonmonotonic, but are shown to be consistent with (i) the known physics of electron photoexcitation from the nitrogen substitution states into the conduction bands of diamond, (ii) the energy position and dispersion characteristics of the conduction bands of diamond in the (001) emission direction, (iii) the effective electron affinity of the crystal faces, (iv) the strong electron-(optical)phonon coupling in diamond, and (v) the associated hot electron transport dynamics under energy equipartition with the optical phonons. Notably, the observed hot electron emission is shown to be restricted parallel to the photocathode surface by the low transverse effective masses of the emitting band states—a transverse momentum filtering effect.

关键词： Electron emission Electronic structure Jahn-Teller effect Optical phonons Photocurrent Photoinduced effect Doped semiconductors Photoemission

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