Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session L17: Metal Surfaces and Thin Films |
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Sponsoring Units: DCMP DMP Chair: Theodore Einstein, Univ of Maryland-College Park Room: LACC 306A |
Wednesday, March 7, 2018 11:15AM - 11:27AM |
L17.00001: Metallic Surface Energies Beyond the Random Phase Approximation Jefferson Bates, Niladri Sengupta, Adrienn Ruzsinszky The prediction of surface energies for extended systems is often used as a descriptor for understanding trends in surface properties such as molecular adsorption. Though semilocal functionals can predict either accurate adsorptions or accurate surface energies, non-local functionals, such as the Random Phase Approximation (RPA), are required to deliver accurate predictions simultaneously for both properties. Here we tested the impact of correlation methods beyond RPA to understand the role short-ranged correlation plays in determining the surface energies of several transition metals. We find that RPA overestimates the surface energy due to an overestimation of the bulk correlation energy. Addition of an exchange-correlation kernel corrects this behavior and tends to reduce the predicted surface energies due to an increase in the bulk energy with respect to the surface slab. In comparison to experiment, RPA and beyond-RPA methods both deliver systematically accurate results that are superior to popular semilocal functionals. |
Wednesday, March 7, 2018 11:27AM - 11:39AM |
L17.00002: Morphological transitions in Cu islands grown on strained Cu(100) Ehsan Sabbar, Indiras Khatri, Yunsic Shim, Jacques Amar A variety of experiments on submonolayer heteroepitaxial growth have demonstrated that the combined effects of strain and bonding can lead to shape transitions as well as dislocation formation. In order to separate out the effects of strain from “chemical” effects, we have used temperature-accelerated dynamics simulations to study a simple model system corresponding to the growth of Cu on a strained Cu(100) substrate. In the case of 2% compressive strain we find that - as previously found for Cu/Ni(100) growth [1] - multi-atom “pop-out” events are enhanced, leading to the formation of blobby islands with a mixture of open and closed step-edges. However, for much larger (4%) compressive strain, we find that island formation leads to the formation of dislocations. In contrast, dislocation formation does not occur in the absence of deposition. Finally, we find that - in good agreement with recent theoretical results [2] - a very large (8%) tensile strain leads to the formation of large anisotropic islands. The dependence of the barriers for monomer hopping and exchange on strain will also be discussed. |
Wednesday, March 7, 2018 11:39AM - 11:51AM |
L17.00003: STM study on the copper oxide surfaces on Cu(111) Sanghwa Kim, Trinh Ly, Ganbat Duvjir, Mahmoud Saad, Se Young Jeong, Jungdae Kim It is known that copper oxide has two stable phases which are copper(II) oxide (CuO) and copper(I) oxide (Cu2O). Cu2O has cubic structure with band gap of around 2.1 eV while CuO belongs to monoclinic system with band gap of around 1.2 eV. Due to their semiconducting properties, these copper oxides have been widely studied for potential applications on supercapacitor, solar cell, etc. Copper oxide surfaces formed on Cu(111) are investigated via home-built low temperature scanning tunneling microscope (STM). Epitaxial Cu(111) films are prepared on sapphire substrate. Then, copper oxide is naturally formed on top of Cu(111) under ambient conditions. STM images show well aligned stripe patterns on copper oxide surfaces. It is found that observed stripe structures are due to local strain. Detail analysis on atomic structures and electronic properties of copper oxide surfaces will be discussed. |
Wednesday, March 7, 2018 11:51AM - 12:03PM |
L17.00004: Investigation of Copper’s Unoccupied Bands David Postma, Daniel Finkenstadt, Michael Mehl, Samuel Lambrakos, Andrew Shabaev, Kevin Jensen DFT modeling of optical properties of materials is currently focused on time-dependent and many-body techniques, including Bethe-Salpeter equation (BSE), GW approximation, as well as Hartree-Fock based methods. We explore these methods for Cu, a relevant photocathode material, as well as a seemingly well-understood metal. In the photoexcitation view, a highly excited state may have enhanced lifetime, via quasiparticle modifications of the excited-state band structure, in particular depletion of density-of-states for nearby lower energy levels. This enhances quantum efficiency (QE) for a photoemitter. We will evaluate Cu’s virtual orbitals and the possibilities of engineering a spectral gap. A discussion of Cu as a rugged photocathode and its applications specifically in the areas of the x-ray free electron lasers will also be discussed. |
Wednesday, March 7, 2018 12:03PM - 12:15PM |
L17.00005: A Model of Photoemission Delay with Heterostructure and Coating Barriers Kevin Jensen, John Petillo, Nathan Moody, Daniel Finkenstadt, Andrew Shabaev, Dimitri Panagos, Samuel Lambrakos, Serguei Ovtchinnikov, Aaron Jensen, Hisato Yamaguchi, Fangze Liu High performance Free Electron Lasers (FEL) and x-ray FELs (xFEL) seek high quantum efficiency (QE) photocathodes (typically semiconductors), but simultaneously demand low emittance. Photocathodes incorporating heterostructures and/or coatings such as graphene or boron nitride have been under consideration as a means to improve QE (as well as robustness and lifetime) while allowing for methods beyond simple bulk transport, pulse shaping, and surface modifications to tailor emittance behavior. Modifications induced by coatings and heterostructures have lead to an analysis of the transmission probability of the more complex barrier profiles. The potentials over which emission occurs are provided using a ``macro-averaging'' technique applied to the output of density functional theory simulations. This presentation will analyze the behavior of delayed emission effects for short pulses where field conditions at the surface change over the duration of the pulse. The models are in development for the Particle-in-Cell code MICHELLE used to model beam formation and transport in rf injectors and electron sources. |
Wednesday, March 7, 2018 12:15PM - 12:27PM |
L17.00006: Intermixing in Cu/Ni(110) growth Jacques Amar, Yunsic Shim, Logan Riney Using temperature accelerated dynamics simulations we have examined the intermixing process in the early stages of growth of Cu on Ni(110) and compared with experiments [1]. As in other (110) metal surfaces, the cross-channel hopping barrier is significantly larger than the in-channel barrier, while the barrier for a Cu-Ni exchange process - resulting in the embedding of a Cu atom in the Ni(110) substrate - is somewhat higher. However, this simple diffusion dynamics is significantly modified due to the presence of inherent metastable intermediate states as well as the complex dependence of exchange barriers on the local environment. This leads to relatively small barriers for Ni-Cu exchange as well as a novel exchange mechanism involving nearby islands which promotes intermixing. As a result we find that, while almost 90% of the Cu atoms are embedded in the substrate at coverage θ < 0.1 ML, at higher coverage the fraction of embedded Cu atoms decreases significantly due to reverse Ni-Cu exchange processes. Our results also resolve the apparent discrepancy found in experiment between the fraction of embedded Cu at low coverage and that found at higher coverage. |
Wednesday, March 7, 2018 12:27PM - 12:39PM |
L17.00007: Interplay between Interface Structure and Magnetism in NiFe/Cu/Ni-Based Pseudo-Spin Valves Melissa Loving, Thomas Ambrose, Eric Jones, Henry Ermer, Donald Miller, Ofer Naaman Magnetic pseudo spin valves (PSVs) with superconducting Nb electrodes, have been leading candidates for an energy-efficient memory solution compatible with cryogenic operation of ultra-low power superconducting logic. Integration of these PSV Josephson junctions in a standard multi-layer Nb process requires growing high-quality thin magnetic films on a thick Nb bottom electrode (i.e. ≥1.5kÅ, to achieve bulk superconducting properties). However, as deposited, 1.5kÅ Nb exhibits a rough surface with a characteristic rice grain morphology, which severely degrades the switching properties of subsequently deposited PSVs. Therefore, in order to achieve coherent switching throughout a PSV, the Nb interface must be modified. Here, we demonstrate that the Nb surface morphology and PSV crystallinity can be altered with the incorporation of separate 50Å Cu or 100Å Al/50Å Cu non-magnetic seed layers, and demonstrate their impact on the magnetic switching of a 15Å Ni80Fe20/50Å Cu/20Å Ni PSV, at both room temperature and at 10 K. Most notably, these results show that the incorporation of an Al seed layer leads to an improved face centered cubic templating through the bulk of the PSV, and ultimately to superior magnetic switching. |
Wednesday, March 7, 2018 12:39PM - 12:51PM |
L17.00008: A computational study of the adsorption of Pyridine on Cu, Ag, Au (111) surfaces assessing the role of the van der Waals' interactions Walter Malone, Abdelkader Kara We study the adsorption of pyridine (C5H5N) on Cu, Ag, and Au(111) surfaces using van der Waals (vdW) inclusive density functional theory (vdW-DF). For this study, five difference functionals from the vdW-DF family were used. We explore a variety of adsorption sites with the molecule’s plane both parallel and perpendicular to the surface. We find pyridine prefers to adsorb perpendicular to the surface. Including the vdW interactions tends to enhance the overall adsorption energy of the pyridine molecule. We present along with the adsorption energy, various geometric properties of the substrate/adsorbate system such as adsorption height, tilt angle of the molecule, and buckling of the first layer of the substrate. We then delve into the electronic properties of the substrate/adsorbate system such as change in the surface’s work function, charge transfer, and change in the partial d-band of the atoms that compose the first layer of the substrate upon the adsorption of pyridine. Overall we find pyridine exhibits very similar adsorption characteristics on these three metallic surfaces. We also assess the performance of the five vdW inclusive functionals studied. |
Wednesday, March 7, 2018 12:51PM - 1:03PM |
L17.00009: Probing many-body scattering in Cu(111) via FT-STS:
Understanding local perturbations from the collective signatures of a 2D electron gas Gelareh Farahi, Steven Johnston, Sarah Burke, Doug Bonn Surface states of close-packed noble metals provide one of the simplest scenarios for studying many-body scattering in two dimensions, turning them into a relatively simple platform to test novel spectroscopic techniques. In this talk I will present a study of the Cu(111) surface state with high-resolution Fourier Transform Scanning Tunnelling Spectroscopy (FT-STS), and for the first time demonstrate that the QPI lineshape can be quantitatively fit using many-body effects within the T-matrix formalism. Specifically, the self-energies and the scattering phase-shifts are quantified for both the occupied and unoccupied states, further expanding the capabilities of FT-STS beyond its conventional application as merely a probe of the energy dispersion in scattering space. |
Wednesday, March 7, 2018 1:03PM - 1:15PM |
L17.00010: Distinguishing Attosecond Electron-Electron Scattering and Screening in Transition Metals Yingchao Zhang, Cong Chen, Zhensheng Tao, Adra Carr, Piotr Matybaa, Tibor Szilvásib, Sebastian Emmerichc, Mark Keller, Dmitriy Zusin, Steffen Eich, Markus Rollinger, Wenjing You, Stefan Mathias, Uwe Thumm, Manos Mavrikakis, Martin Aeschlimann, Peter Oppeneer, Henry Kapteyn, Margaret Murnane
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Wednesday, March 7, 2018 1:15PM - 1:27PM |
L17.00011: Exploring semiconductor-to-semimetal transitions in Bi2Se3/TMD heterostructures Ioana Buda, Christopher Lane, Zachariah Hennighausen, Bernardo Barbiellini, Robert Markiewicz, Swastik Kar, Arun Bansil Vertically stacked Van der Waals heterostructures show great promise in the design of novel materials for nanoelectronic, photovoltaic and photonics applications. Recent experiments reveal that growing one quintuple layer of Bi2Se3 on TMD monolayers (2H-MoS2, -WS2, -MoSe2) produces rotationally-aligned heterostructures, despite substantial lattice mismatch, with properties that are distinct from those of the parent films. [1] Intriguingly, our experiments show that the resulting heterostructures can also exhibit finite twisting angles between the stacked layers in a number of cases. In this connection, we have computed electronic structures and their evolution with twist angle of Bi2Se3/TMD and number of Bi2Se3 quintuple layers, within the first-principles density functional theory (DFT) framework. Our analysis reveals rich variations in electronic spectra and a transition from a semiconducting to a semimetallic state as a function of the twist angle, as well as with the number of Bi2Se3 layers. We explore the relationship of this phase transition with effects of interplay between interlayer coupling and strain in the films. |
Wednesday, March 7, 2018 1:27PM - 1:39PM |
L17.00012: Local magnetic anisotropy analysis on manganese boride ultrathin films on transition-metal surfaces studied by first-principles calculations Shogo Nakamura, Yoshihiro Gohda We have developed a method for a local magnetic anisotropy analysis based on second-order perturbation theory. We applied our method to new two-dimensional ferromagnetic nanostructures, MnB/Ag(001) and MnB/Pd(001), using their electronic structures obtained by first-principles calculations. For understanding physical origins of magnetic anisotropy at surfaces, magnetic information considering substrate effects is required. Our method considers hybridization as overlap integrals, and as a result we successfully evaluate the MAE more accurately like in Pd-substrate systems where substrate effects are important. Our method clarifies that perpendicular magnetic anisotropy of MnB/Ag(001) is attributed to virtual transition processes between up-spin occupied states and down-spin unoccupied states, whereas in-plane anisotropy of MnB/Pd(001) comes from transition among down-spin occupied states and up-spin unoccupied states. We also discuss the detailed mechanism of exhibiting the ferromagnetic state taken MnB/Ag(001) as an example. |
Wednesday, March 7, 2018 1:39PM - 1:51PM |
L17.00013: Imaging Magnon Excitation with Spin-Polarized Scanning Tunneling Microscopy Hung-Hsiang Yang, Masayuki Hamada, Yasuo Yoshida, Yukio Hasegawa Inelastic electron scattering plays a crucial role in spintronics devices concerning the spin lifetime of polarized electrons and the amount of spin transfer torque for switching magnetic configurations in magnetic tunnel junctions. One of the fundamental processes is magnon creation, which occurs when injected hot electrons induces spin-flip scattering with an electron in the Fermi sea of the magnetic material. To image and address the magnetic origin of the excitations, we have performed low-temperature spin-polarized inelastic electron tunneling spectroscopy (IETS) on double layer (DL) Mn thin films formed on W(110) substrate. |
Wednesday, March 7, 2018 1:51PM - 2:03PM |
L17.00014: Circuit theory with spin-non-conserving interfaces and first-principles calculations of spin memory loss at magnetic and non-magnetic interfaces Giovanni Baez Flores, Kirill Belashchenko, Alexey Kovalev, Mark Schilfgaarde Spin transport across metallic interfaces is a central process in spintronic device concepts, such as giant magnetoresistance, spin-transfer torque, and spin pumping. Spin-orbit coupling plays an important role in many such devices. In particular, spin current is partially depolarized at an interface due to spin-orbit coupling. This effect has been measured for multiple interfaces. Such systems can be studied using the magnetoelectronic circuit theory equipped with appropriate boundary conditions, and first-principles calculations of the spin-memory loss parameter δ have been reported for the Cu/Pd interface. |
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