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Full-Text Articles in Physics

Adsorption Of Dysprosium On The Graphite (0001) Surface: Nucleation And Growth At 300 K, Emma J. Kwolek, Huaping Lei, Ann Lii-Rosales, Mark Wallingford, Yinghui Zhou, Cai-Zhuang Wang, Michael C. Tringides, James W. Evans Dec 2016

Adsorption Of Dysprosium On The Graphite (0001) Surface: Nucleation And Growth At 300 K, Emma J. Kwolek, Huaping Lei, Ann Lii-Rosales, Mark Wallingford, Yinghui Zhou, Cai-Zhuang Wang, Michael C. Tringides, James W. Evans

Physics and Astronomy Publications

We have studied nucleation and growth of Dy islands on the basal plane of graphite at 300 K using scanning tunneling microscopy, density functional theory (DFT) in a form that includes van der Waals interactions, and analytic theory. The interaction of atomic Dy with graphite is strong, while the diffusion barrier is small. Experiment shows that at 300 K, the density of nucleated islands is close to the value predicted for homogeneous nucleation, using critical nucleus size of 1 and the DFT-derived diffusion barrier. Homogeneous nucleation is also supported by the monomodal shape of the island size distributions. Comparison with ...


125te Nmr And Seebeck Effect In Bi2te3 Synthesized From Stoichiometric And Te-Rich Melts, E. M. Levin, Trevor M. Riedemann, A. Howard, Na Hyun Jo, Sergey L. Bud’Ko, Paul C. Canfield, Thomas A. Lograsso Nov 2016

125te Nmr And Seebeck Effect In Bi2te3 Synthesized From Stoichiometric And Te-Rich Melts, E. M. Levin, Trevor M. Riedemann, A. Howard, Na Hyun Jo, Sergey L. Bud’Ko, Paul C. Canfield, Thomas A. Lograsso

Materials Science and Engineering Publications

Bi2Te3 is a well-known thermoelectric material and, as a new form of quantum matter, a topological insulator. Variation of local chemical composition in Bi2Te3 results in formation of several types of atomic defects, including Bi and Te vacancies and Bi and Te antisite defects; these defects can strongly affect material functionality via generation of free electrons and/or holes. Nonuniform distribution of atomic defects produces electronic inhomogeneity, which can be detected by 125Te nuclear magnetic resonance (NMR). Here we report on 125Te NMR and Seebeck effect (heat to electrical energy conversion) for two single crystalline samples: (#1) grown from stoichiometric ...


Half-Metallic Co-Based Quaternary Heusler Alloys For Spintronics: Defect- And Pressure-Induced Transitions And Properties, Enamullah, D. D. Johnson, K. G. Suresh, Aftab Alam Nov 2016

Half-Metallic Co-Based Quaternary Heusler Alloys For Spintronics: Defect- And Pressure-Induced Transitions And Properties, Enamullah, D. D. Johnson, K. G. Suresh, Aftab Alam

Ames Laboratory Publications

Heusler compounds offer potential as spintronic devices due to their spin polarization and half-metallicity properties, where electron spin-majority (minority) manifold exhibits states (band gap) at the electronic chemical potential, yielding full spin polarization in a single manifold. Yet, Heuslers often exhibit intrinsic disorder that degrades its half-metallicity and spin polarization. Using density-functional theory, we analyze the electronic and magnetic properties of equiatomic Heusler (L21) CoMnCrAl and CoFeCrGe alloys for effects of hydrostatic pressure and intrinsic disorder (thermal antisites, binary swaps, and vacancies). Under pressure, CoMnCrAl undergoes a metallic transition, while half-metallicity in CoFeCrGe is retained for a limited range. Antisite ...


Big Area Additive Manufacturing Of High Performance Bonded Ndfeb Magnets, Li Ling, Angelica Tirado, I. C. Nlebedim, Orlando Rios, Brian Post, Vlastimil Kunc, R. R. Lowden, Edgar Lara-Curzio, Magnet Applications, Inc., Thomas A. Lograsso, M. Parans Paranthaman Oct 2016

Big Area Additive Manufacturing Of High Performance Bonded Ndfeb Magnets, Li Ling, Angelica Tirado, I. C. Nlebedim, Orlando Rios, Brian Post, Vlastimil Kunc, R. R. Lowden, Edgar Lara-Curzio, Magnet Applications, Inc., Thomas A. Lograsso, M. Parans Paranthaman

Ames Laboratory Publications

Additive manufacturing allows for the production of complex parts with minimum material waste, offering an effective technique for fabricating permanent magnets which frequently involve critical rare earth elements. In this report, we demonstrate a novel method - Big Area Additive Manufacturing (BAAM) - to fabricate isotropic near-net-shape NdFeB bonded magnets with magnetic and mechanical properties comparable or better than those of traditional injection molded magnets. The starting polymer magnet composite pellets consist of 65 vol% isotropic NdFeB powder and 35 vol% polyamide (Nylon-12). The density of the final BAAM magnet product reached 4.8 g/cm3, and the room temperature magnetic properties ...


Laser Angle-Resolved Photoemission As A Probe Of Initial State K(Z) Dispersion, Final-State Band Gaps, And Spin Texture Of Dirac States In The Bi2te3 Topological Insulator, Minna Arrala, Hasnain Hafiz, Daixiang Mou, Yun Wu, Rui Jiang, Trevor Riedemann, Thomas A. Lograsso, Bernardo Barbiellini, Adam Kaminski, Arun Bansil, Matti Lindroos Oct 2016

Laser Angle-Resolved Photoemission As A Probe Of Initial State K(Z) Dispersion, Final-State Band Gaps, And Spin Texture Of Dirac States In The Bi2te3 Topological Insulator, Minna Arrala, Hasnain Hafiz, Daixiang Mou, Yun Wu, Rui Jiang, Trevor Riedemann, Thomas A. Lograsso, Bernardo Barbiellini, Adam Kaminski, Arun Bansil, Matti Lindroos

Ames Laboratory Publications

We have obtained angle-resolved photoemission spectroscopy (ARPES) spectra from single crystals of the topological insulator material Bi2Te3 using a tunable laser spectrometer. The spectra were collected for 11 different photon energies ranging from 5.57 to 6.70 eV for incident light polarized linearly along two different in-plane directions. Parallel first-principles, fully relativistic computations of photointensities were carried out using the experimental geometry within the framework of the one-step model of photoemission. A reasonable overall accord between theory and experiment is used to gain insight into how properties of the initial- and final-state band structures as well as those of ...


Origin Of Magnetic Anisotropy In Doped Ce2co17 Alloys, Liqin Ke, D. A. Kukusta, Duane D. Johnson Oct 2016

Origin Of Magnetic Anisotropy In Doped Ce2co17 Alloys, Liqin Ke, D. A. Kukusta, Duane D. Johnson

Ames Laboratory Publications

Magnetocrystalline anisotropy (MCA) in doped Ce2Co17 and other competing structures was investigated using density functional theory. We confirmed that the MCA contribution from dumbbell Co sites is very negative. Replacing Co dumbbell atoms with a pair of Fe or Mn atoms greatly enhance the uniaxial anisotropy, which agrees quantitatively with experiment, and this enhancement arises from electronic-structure features near the Fermi level, mostly associated with dumbbell sites. With Co dumbbell atoms replaced by other elements, the variation of anisotropy is generally a collective effect and contributions from other sublattices may change significantly. Moreover, we found that Zr doping promotes the ...


Energy Gap Evolution Across The Superconductivity Dome In Single Crystals Of (Ba1−Xkx)Fe2as2, Kyuil Cho, Marcin Kończykowski, Serafim Teknowijoyo, Makariy A. Tanatar, Yong Liu, Thomas A. Lograsso, Warren E. Straszheim, Vivek Mishra, Saurabh Maiti, Peter J. Hirschfeld, Ruslan Prozorov Sep 2016

Energy Gap Evolution Across The Superconductivity Dome In Single Crystals Of (Ba1−Xkx)Fe2as2, Kyuil Cho, Marcin Kończykowski, Serafim Teknowijoyo, Makariy A. Tanatar, Yong Liu, Thomas A. Lograsso, Warren E. Straszheim, Vivek Mishra, Saurabh Maiti, Peter J. Hirschfeld, Ruslan Prozorov

Ames Laboratory Publications

The mechanism of unconventional superconductivity in iron-based superconductors (IBSs) is one of the most intriguing questions in current materials research. Among non-oxide IBSs, (Ba1−xKx)Fe2As2 has been intensively studied because of its high superconducting transition temperature and fascinating evolution of the superconducting gap structure from being fully isotropic at optimal doping (x ≈ 0.4) to becoming nodal at x > 0.8. Although this marked evolution was identified in several independent experiments, there are no details of the gap evolution to date because of the lack of high-quality single crystals covering the entire K-doping range of the superconducting dome. We ...


Asymmetric Mass Acquisition In Labi: Topological Semimetal Candidate, Yun Wu, Tai Kong, Lin-Lin Wang, Duane D. Johnson, Daixiang Mou, Lunan Huang, Benjamin Schrunk, Sergey L. Bud'ko, Paul C. Canfield, Adam Kaminski Aug 2016

Asymmetric Mass Acquisition In Labi: Topological Semimetal Candidate, Yun Wu, Tai Kong, Lin-Lin Wang, Duane D. Johnson, Daixiang Mou, Lunan Huang, Benjamin Schrunk, Sergey L. Bud'ko, Paul C. Canfield, Adam Kaminski

Ames Laboratory Publications

We use our high resolution He-lamp-based, tunable laser-based angle-resolved photoemission spectroscopy measurements and density functional theory calculations to study the electronic properties of LaBi, a binary system that was proposed to be a member of a new family of topological semimetals. Both bulk and surface bands are present in the spectra. The dispersion of the surface state is highly unusual. It resembles a Dirac cone, but upon closer inspection we can clearly detect an energy gap. The bottom band follows roughly a parabolic dispersion. The dispersion of the top band remains very linear, “V” -shape like, with the tip approaching ...


Metallic Layer-By-Layer Photonic Crystals For Linearly-Polarized Thermal Emission And Thermophotovoltaic Device Including Same, Jae-Hwang Lee, Kai-Ming Ho, Kristen P. Constant Jul 2016

Metallic Layer-By-Layer Photonic Crystals For Linearly-Polarized Thermal Emission And Thermophotovoltaic Device Including Same, Jae-Hwang Lee, Kai-Ming Ho, Kristen P. Constant

Iowa State University Patents

Metallic thermal emitters consisting of two layers of differently structured nickel gratings on a homogeneous nickel layer are fabricated by soft lithography and studied for polarized thermal radiation. A thermal emitter in combination with a sub-wavelength grating shows a high extinction ratio, with a maximum value close to 5, in a wide mid-infrared range from 3.2 to 7.8 μm, as well as high emissivity up to 0.65 at a wavelength of 3.7 μm. All measurements show good agreement with theoretical predictions. Numerical simulations reveal that a high electric field exists within the localized air space surrounded ...


Itinerant And Localized Magnetization Dynamics In Antiferromagnetic Ho, L. Rettig, C. Dornes, N. Thielemann-Kühn, Helmholtz-Zentrum Berlin Für Materialien Und Energie Gmbh, H. Zabel, Deborah L. Schlagel, Thomas A. Lograsso, M. Chollet, A. Robert, M. Sikorski, S. Song, J. M. Glownia, S. L. Johnson, U. Staub Jun 2016

Itinerant And Localized Magnetization Dynamics In Antiferromagnetic Ho, L. Rettig, C. Dornes, N. Thielemann-Kühn, Helmholtz-Zentrum Berlin Für Materialien Und Energie Gmbh, H. Zabel, Deborah L. Schlagel, Thomas A. Lograsso, M. Chollet, A. Robert, M. Sikorski, S. Song, J. M. Glownia, S. L. Johnson, U. Staub

Ames Laboratory Publications

Using femtosecond time-resolved resonant magnetic x-ray diffraction at the Ho L 3 absorption edge, we investigate the demagnetization dynamics in antiferromagnetically ordered metallic Ho after femtosecond optical excitation. Tuning the x-ray energy to the electric dipole ( E 1 , 2 p → 5 d ) or quadrupole ( E 2 , 2 p → 4 f ) transition allows us to selectively and independently study the spin dynamics of the itinerant 5 d and localized 4 f electronic subsystems via the suppression of the magnetic (2 1 3 − τ ) satellite peak. We find demagnetization time scales very similar to ferromagnetic 4 f systems, suggesting that the loss ...


Dirac Node Arcs In Ptsn4, Yun Wu, Lin-Lin Wang, Eundeok Mun, Duane D. Johnson, Daixiang Mou, Lunan Huang, Youngbin Lee, Serguei L. Bud’Ko, Paul C. Canfield, Adam Kaminski Apr 2016

Dirac Node Arcs In Ptsn4, Yun Wu, Lin-Lin Wang, Eundeok Mun, Duane D. Johnson, Daixiang Mou, Lunan Huang, Youngbin Lee, Serguei L. Bud’Ko, Paul C. Canfield, Adam Kaminski

Ames Laboratory Publications

In topological quantum materials1–3 the conduction and valence bands are connected at points or along lines in the momentum space. A number of studies have demonstrated that several materials are indeed Dirac/Weyl semimetals4–8 . However, there is still no experimental confirmation of materials with line nodes, in which the Dirac nodes form closed loops in the momentum space2,3 . Here we report the discovery of a novel topological structure—Dirac node arcs—in the ultrahigh magnetoresistive material PtSn4 using laser-based angle-resolved photoemission spectroscopy data and density functional theory calculations. Unlike the closed loops of line nodes, the Dirac ...


Red-Emitting Manganese-Doped Aluminum Nitride Phosphor, Nerine J. Cherepy, Stephen A. Payne, Nicholas M. Harvey, Daniel Åberg, Zachary M. Seeley, Kiel S. Holliday, Ich C. Tran, Fei Zhou, H. Paul Martinez, Jessica M. Demeyer, Alexander D. Drobshoff, Alok M. Srivastava, Samuel J. Camardello, Holly A. Comanzo, Deborah L. Schlagel, Thomas A. Lograsso Apr 2016

Red-Emitting Manganese-Doped Aluminum Nitride Phosphor, Nerine J. Cherepy, Stephen A. Payne, Nicholas M. Harvey, Daniel Åberg, Zachary M. Seeley, Kiel S. Holliday, Ich C. Tran, Fei Zhou, H. Paul Martinez, Jessica M. Demeyer, Alexander D. Drobshoff, Alok M. Srivastava, Samuel J. Camardello, Holly A. Comanzo, Deborah L. Schlagel, Thomas A. Lograsso

Materials Science and Engineering Publications

We report high efficiency luminescence with a manganese-doped aluminum nitride red-emitting phosphor under 254 nm excitation, as well as its excellent lumen maintenance in fluorescent lamp conditions, making it a candidate replacement for the widely deployed europium-doped yttria red phosphor. Solid-state reaction of aluminum nitride powders with manganese metal at 1900 °C, 10 atm N2 in a reducing environment results in nitrogen deficiency, as revealed diffuse reflectance spectra. When these powders are subsequently annealed in flowing nitrogen at 1650 °C, higher nitrogen content is recovered, resulting in white powders. Silicon was added to samples as an oxygen getter to improve ...


Better Band Gaps With Asymptotically Corrected Local Exchange Potentials, Prashant Signh, Manoj K. Harbola, M. Hemanadhan, Abhijit Mookerjee, D. D. Johnson Feb 2016

Better Band Gaps With Asymptotically Corrected Local Exchange Potentials, Prashant Signh, Manoj K. Harbola, M. Hemanadhan, Abhijit Mookerjee, D. D. Johnson

Ames Laboratory Publications

We formulate a spin-polarized van Leeuwen and Baerends (vLB) correction to the local density approximation (LDA) exchange potential [R. van Leeuwen and E. J. Baerends, Phys. Rev. A 49, 2421 (1994)] that enforces the ionization potential (IP) theorem following T. Stein et al. [Phys. Rev. Lett. 105, 266802 (2010)]. For electronic-structure problems, the vLB correction replicates the behavior of exact-exchange potentials, with improved scaling and well-behaved asymptotics, but with the computational cost of semilocal functionals. The vLB + IP correction produces a large improvement in the eigenvalues over those from the LDA due to correct asymptotic behavior and atomic shell structures ...


Formation Mechanism Of Superconducting Phase And Its Three-Dimensional Architecture In Pseudo-Single-Crystal Kxfe2−Yse2, Yong Liu, Qingfeng Xing, Warren E. Straszheim, Jeff Marshman, Pai Pedersen, Richard Mclaughlin, Thomas A. Lograsso Feb 2016

Formation Mechanism Of Superconducting Phase And Its Three-Dimensional Architecture In Pseudo-Single-Crystal Kxfe2−Yse2, Yong Liu, Qingfeng Xing, Warren E. Straszheim, Jeff Marshman, Pai Pedersen, Richard Mclaughlin, Thomas A. Lograsso

Ames Laboratory Publications

We report how the superconducting phase forms in pseudo-single-crystal KxFe2−ySe2. In situ scanning electron microscopy (SEM) observation reveals that, as an order-disorder transition occurs, on cooling, most of the high-temperature iron-vacancy-disordered phase gradually changes into the iron-vacancy-ordered phase, whereas a small quantity of the high-temperature phase retains its structure and aggregates to the stripes with more iron concentration but less potassium concentration compared to the iron-vacancy-ordered phase. The stripes that are generally recognized as the superconducting phase are actually formed as a remnant of the high-temperature phase with a compositional change after an “imperfect” order-disorder transition. It should be ...


Titanium Α−Ω Phase Transformation Pathway And A Predicted Metastable Structure, Nikolai A. Zarkevich, Duane D. Johnson Jan 2016

Titanium Α−Ω Phase Transformation Pathway And A Predicted Metastable Structure, Nikolai A. Zarkevich, Duane D. Johnson

Ames Laboratory Publications

As titanium is a highly utilized metal for structural lightweighting, its phases, transformation pathways (transition states), and structures have scientific and industrial importance. Using a proper solid-state nudged elastic band method employing two climbing images combined with density functional theory DFT + U methods for accurate energetics, we detail the pressure-induced α (ductile) to ω (brittle) transformation at the coexistence pressure. We find two transition states along the minimal-enthalpy path and discover a metastable body-centered orthorhombic structure, with stable phonons, a lower density than the end-point phases, and decreasing stability with increasing pressure.


Physical Properties Of Single Crystalline R Mg 2 Cu 9 ( R = Y , Ce − Nd , Gd − Dy , Yb ) And The Search For In-Plane Magnetic Anisotropy In Hexagonal Systems, Tai Kong, William R. Meier, Qisheng Lin, Scott M. Saunders, S. L. Bud’Ko, Rebecca Flint, Paul C. Canfield Jan 2016

Physical Properties Of Single Crystalline R Mg 2 Cu 9 ( R = Y , Ce − Nd , Gd − Dy , Yb ) And The Search For In-Plane Magnetic Anisotropy In Hexagonal Systems, Tai Kong, William R. Meier, Qisheng Lin, Scott M. Saunders, S. L. Bud’Ko, Rebecca Flint, Paul C. Canfield

Chemistry Publications

Single crystals ofRMg2Cu9 (R = Y, Ce-Nd, Gd-Dy, Yb) were grown using a high-temperature solution growth technique and were characterized by measurements of room-temperature x-ray diffraction, temperature-dependent specific heat, and temperature- and field-dependent resistivity and anisotropic magnetization. YMg2Cu9 is a nonlocal- moment-bearing metal with an electronic specific heat coefficient, γ ∼ 15 mJ/mol K2. Yb is divalent and basically non-moment-bearing in YbMg2Cu9. Ce is trivalent in CeMg2Cu9 with two magnetic transitions being observed at 2.1 K and 1.5 K. PrMg2Cu9 does not exhibit any magnetic phase transition down to 0.5 K. The other members being studied (R = Nd ...


Rapid Acquisition Of Wideline Mas Solid-State Nmr Spectra With Fast Mas, Proton Detection, And Dipolar Hmqc Pulse Sequences, Aaron J. Rossini, Michael P. Hanrahan, Martin M. Thuo Jan 2016

Rapid Acquisition Of Wideline Mas Solid-State Nmr Spectra With Fast Mas, Proton Detection, And Dipolar Hmqc Pulse Sequences, Aaron J. Rossini, Michael P. Hanrahan, Martin M. Thuo

Materials Science and Engineering Publications

The solid-state NMR spectra of many NMR active elements are often extremely broad due to the presence of chemical shift anisotropy (CSA) and/or the quadrupolar interaction (for nuclei with spin I > 1/2). These NMR interactions often give rise to wideline solid-state NMR spectra which can span hundreds of kHz or several MHz. Here we demonstrate that by using fast MAS, proton detection and dipolar hetero-nuclear multiple-quantum (D-HMQC) pulse sequences, it is possible to rapidly acquire 2D spectra which correlate 1H chemical shifts to the indirectly detected wideline MAS powder patterns of dipolar coupled hetero-nuclei. The D-HMQC pulse sequence ...


Polarized Light Microscopy Study On The Reentrant Phase Transition In A (Ba1-Xkx) Fe2as2 Single Crystal With X=0.24, Yong Liu, Makariy A. Tanatar, Erik I. Timmons, Thomas A. Lograsso Jan 2016

Polarized Light Microscopy Study On The Reentrant Phase Transition In A (Ba1-Xkx) Fe2as2 Single Crystal With X=0.24, Yong Liu, Makariy A. Tanatar, Erik I. Timmons, Thomas A. Lograsso

Ames Laboratory Publications

A sequence of structural/magnetic transitions on cooling is reported in the literature for hole-doped iron-based superconductor (Ba1 − xKx)Fe2As2 with x = 0.24. By using polarized light microscopy, we directly observe the formation of orthorhombic domains in (Ba1 − xKx)Fe2As2 (x = 0.24) single crystal below a temperature of simultaneous structural/magnetic transition TN ~ 80 K. The structural domains vanish below ~30 K, but reappear below T = 15 K. Our results are consistent with reentrance transformation sequence from high-temperature tetragonal (HTT) to low temperature orthorhombic (LTO1) structure at TN ~ 80 K, LTO1 to low temperature tetragonal (LTT) structure at ...