Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session U52: Surfaces and Thin Films of Novel Electronic Materials |
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Sponsoring Units: DCMP Chair: Daniel Dougherty, North Carolina State University Room: Mile High Ballroom 1E |
Thursday, March 5, 2020 2:30PM - 2:42PM |
U52.00001: Exploring the origin of the thickness-dependent metal-insulator transition in thin-film systems with band structures Sungsoo Hahn, Byungmin Sohn, Changyoung Kim Metal-insulator transition (MIT) has been studied in decades and is well known to be induced by temperature, pressure, and dopants. However, the origin of MIT in thin films which occur as metallic thin films become thinner is not explained. In order to investigate the thickness-dependent MIT, transport measurements and photoemission spectroscopy have been performed, however, the conclusion is not to be determined yet. Here, we observed band structures of SrRuO3 ultrathin films near the MIT-critical thickness by using in-situ angle-resolved photoemission spectroscopy (ARPES). We expect our experimental observation sheds light on the thickness-dependent MIT in thin-film systems. |
Thursday, March 5, 2020 2:42PM - 2:54PM |
U52.00002: Scanning tunneling spectroscopy of rare earth hexaborides Philipp Buchsteiner, Florian Sohn, Lisa Harmsen, Peter E Bloechl, Martin Wenderoth The surface physics of rare earth hexaborides is of great interest for both technological applications and fundamental research. For example, LaB6 is a widely used electron emitter due to its low work function and the 4f-electron system SmB6 is a Kondo insulator with proposed topologically protected surface states. In order to systematically investigate the influence of 4f-electrons on the hexaboride surface physics, we performed scanning tunneling microscopy (STM) on the (001)-cleavage planes of LaB6 [1] and PrB6. We found that on both crystal surfaces a chain-like (2x1) reconstruction is present. Furthermore, we found uniform terminations on PrB6. By performing slab calculations within the framework of density functional theory (DFT) we rationalize this reconstruction as parallel chains of rare earth ions on top of a B6 framework. Using scanning tunneling spectroscopy (STS) we find a spectral feature below EF, which can be described as boron dangling-bond state. For PrB6, an additional contribution is visible in the spectra, which we tentatively attribute to the 4f-electrons. |
Thursday, March 5, 2020 2:54PM - 3:06PM |
U52.00003: Synchrotron-Based Ultrafast Optical Pump/X-Ray Probe Measurements of Lattice Dynamics in Photo-Excited Pt Thin Films Matthew Forbes DeCamp, Anthony DiChiara, Karl Unruh A synchrotron-based optical pump/x-ray probe measurement technique has been used to study the structural evolution of elemental Pt thin films following photo-excitation by ps duration optical laser pulses. High quality x-ray diffraction (XRD) patterns were obtained with a time-resolution of less than 100 ps (corresponding to the duration of a single x-ray pulse) and with time delays relative to the pump pulse ranging from 10’s of ps to ms. These measurements revealed a very rapid lattice expansion normal to the sample for several 100 ps following photo-excitation followed by a much less rapid decrease. The film response to excitation energies less than about 50 mJ/cm2 was nearly reversible while greater excitation energies produced irreversible stress relaxation, and eventually film oblation. Assuming that the lattice expansion can be modelled in terms of a bulk thermal expansion coefficient, the maximum observed lattice temperature was about 1500 K. |
Thursday, March 5, 2020 3:06PM - 3:18PM |
U52.00004: Ex-Situ Ozone Cleaning of SrTiO3 Substrates for Molecular Beam Epitaxy Camille Bean, Tatiana A Webb, Ruizhe Kang, Jennifer E. Hoffman
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Thursday, March 5, 2020 3:18PM - 3:30PM |
U52.00005: Benchmarking various levels of theory for first principles modeling of iron oxide surfaces and interfaces Suvadip Das, Efstratios Manousakis, Andrew J Medford Surfaces of iron and its oxides are scientifically relevant due to their performance as photo-anodes for water oxidation as well as their formation during corrosion processes. Finding an accurate ab-initio approach to appropriately describe surface phenomenon of iron and its oxides is critical to enable modeling of the charge accumulation, energetics and bond formation mechanisms governing the reactivity of iron oxide surfaces. Prior work has established that incorporation of the random-phase approximation (RPA) improves the accuracy of adsorption and surface energy calculations but these techniques have not been systematically applied to iron oxides. Here, we present a benchmarking study for various levels of theory such as onsite Coulomb interaction, hybrid functionals and quasiparticle GW methodology to appropriately modulate surface phenomenon in oxides. A detailed study of GW convergence from various starting points is presented for iron monoxide establishing hybrid-level orbitals as the optimum starting point for self-consistent GW calculations. The study also establishes the hybrid functional approach as an ideal trade-off between accuracy and computational cost towards efficient convergence of electronic wavefunction and formation energy for electrochemical applications. |
Thursday, March 5, 2020 3:30PM - 3:42PM |
U52.00006: Spin Polarized Electron Emission from Magnetite Half-metals Determined by First-principles Calculations Liangliang Xu, Nan Zhao, Ming-Chieh Lin, Tsan-Chuen Leung, Hua-Yi Hsu Magnetite is a mineral and one of the main iron ores. With the chemical formula Fe3O4, it is one of the oxides of iron. Magnetite is the earliest discovered magnet, around 1500 B.C. It crystallizes in the inverse cubic spinel structure (Fd3m) above the so-called Verwey transition temperature, which is about 120 K. In this work, we study the electronic properties of magnetite (100), (110), and (111) surfaces using first principles or ab initio calculations based on density functional theory. With a +U calculation, most of the surfaces with different terminations show half-metallic, different from those predicted by a without +U calculation. It is proposed that the magnetite as a half-metal can possibly be used as a spin-polarized electron source. The spin polarized electrons can be obtained via the field emission process as described by the Fowler–Nordheim equation. From the first principles calculations, the surfaces suitable for spin polarized emission are found. |
Thursday, March 5, 2020 3:42PM - 3:54PM |
U52.00007: Domain Walls in the Electronic Polarization of Silver Bismuthate Fred Florio, Ravishankar Sundararaman, Jian Shi, Yuwei Guo Ternary bismuth oxides are a new class of atypical ferroelectric materials whose polar states are primarily induced by electronic charge disproportionation rather than atomic displacement. Silver bismuthates are particularly exciting as ferroelectric semiconductors with low band gaps. Previous research has focused primarily on bulk geometry and electronic structure of thee materials, identifying three structural phases of Ag2BiO3 as a function of temperature. Understanding domain wall structures and energetics in this material is now critical to facilitate prediction of polarization and domain switching dynamics for applications in memory and opto-electronics. |
Thursday, March 5, 2020 3:54PM - 4:06PM |
U52.00008: Understanding Electrical and Thermal Transport in Single Strand Cu-Coated Carbon Fiber Hojoon Yi, Ji Yeon Kim, Hamza Zad Gul, Seungsu Kang, Giheon Kim, Eunji Sim, Hyunjin Ji, Jungwon Kim, Young Chul Choi, Won Seok Kim, Seongchu Lim Carbon fiber is used in many parts of high-performance cars to improve its performance. Furthermore, there are many researches to improve the properties of cables that have reduced weight by coating metal on carbon fiber cores. But there is a lack of research on thermal properties, one of the important properties. |
Thursday, March 5, 2020 4:06PM - 4:18PM |
U52.00009: Structure and electronic states of Pt/SrTiO3 (001) interface using X-ray standing wave excited photoelectron emission Yanna Chen, Anusheela Das, Tien-Lin Lee, Michael J Bedzyk In this work, we focus on Pt/SrTiO3 heterostructure to discover the interfacial interaction between metal catalysts and metal oxide supports. Pt nano single crystals were grown on a SrTiO3 (001) substrate using pulsed laser deposition. The Pt/SrTiO3 heterostructure was characterized using X-ray standing wave (XSW) excited photoelectron emission collected in the Bragg back-reflection geometry at beamline I09 of the Diamond Light Source. Different electronic core levels of Pt 4f, 4d, 3d were collected, which showed differences in their responses to the XSW excitation. O 1s, Ti 2p, and Sr 3d core levels were also collected to understand the interfacial interaction between Pt and SrTiO3. Focusing on Pt 4f, O 1s, Ti 2p, and Sr 3d, three symmetry inequivalent hkl Bragg peaks were measured to produce 3D interfacial atomic maps for each species via Fourier inversion. Among other things, these results should be beneficial for the design of oxide supported catalysts. |
Thursday, March 5, 2020 4:18PM - 4:30PM |
U52.00010: Evolution of picosecond surface electric fields generated by photon-induced charge emission from La0.67Sr0.33MnO3 thin films at 300 K and 77 K Runze Li, Peter M. Rentzepis The spatial-temporal distribution of picosecond surface electric fields, associated with the femtosecond-laser-induced photoemission from La0.67Sr0.33MnO3 (LSMO) single crystal thin films, at 300 K and 77 K, were interrogated by sub-picosecond, 30 keV, electron pulses and reconstructed by a “three-layer” theoretical model. Around a few tens of picoseconds after femtosecond laser illumination, a surprisingly strong surface electric field on the order of hundreds kV/m was observed on the LSMO film at room temperature. This electric field strength is on the same order as the field(s) used for altering the orbital domains in lanthanum strontium manganite films. The experimental data of this study imply that the self-induced transient electric fields, generated during photoemission, may be a promising means for the control of ultrafast processes in materials that are mainly achievable through strong THz excitations. |
Thursday, March 5, 2020 4:30PM - 4:42PM |
U52.00011: Role of substrate vicinity on structural evolution and functional properties of lightly doped La1-xSrxMnO3 thin films Binod Paudel, Aiping Chen, Heinrich Nakotte Epitaxial engineering has been considered an important approach to tune functional properties of oxide thin films. However, there is always a quest of other approaches that can provide more knobs to tailor and explore wide range of novel functional properties. Film substrate symmetry mismatch, thermal coefficient mismatch, substrate termination, and miscut angle and directions are also important factors to modulate growth, structure and functional properties. Here, we present the study on structural and functional properties of lightly doped La0.9Sr0.1MnO3 (LSMO) films on SrTiO3 substrates with different miscut angles (0.5°, 2°, 4° and 8°). Coherent growth was observed for films on 0.5° miscut substrate and in-plane periodic twining was observed for films grown on higher miscut angles. The out-of-plane periodic twinning was seen for films on for 0.5° miscut substrate while higher miscut substrates promote tilted domains formation. These structural distortions are attributed to monoclinic or triclinic distortion of orthorhombic LSMO films on cubic STO substrates. Curie temperature systematically reduces while magnetoresistance and temperature coefficient increase due to the reduction of domain variants and increase of out of plane tilt angles with the increase of miscut angles. |
Thursday, March 5, 2020 4:42PM - 4:54PM |
U52.00012: Surface structure determination of a two dimensional Au-Si-alloy formed on Au(110) by x-ray photoelectron spectroscopy (XPS) and x-ray photoelectron diffraction (XPD) Marie Schmitz, Peter Roese, Malte Schulte, Carsten Westphal The metal-semiconductor interface has caused multiple interest in solid state physics especially in methods of metal silicides for various applications, for instance photonic and electronic devices.[1] |
Thursday, March 5, 2020 4:54PM - 5:06PM |
U52.00013: Geometric models and electronic structure of Pb overlayers on Ge(111): a first-principles study Andre Childs, Shree Ram Acharya, Duy Le, Ching Yao Fong, Shirley Chiang, Talat S. Rahman Lead deposition on Ge(111) shows several low dimensional phases [1]: the (√3×√3)R30o Pb structure forms at coverages of 1/3 monolayer (ML) (α phase) and 4/3 ML (β phase), while at 1 ML the overlayer is (1 × 1). In all cases the Ge(111) surface is unreconstructed. The geometric and electronic structure of the phases are not yet fully understood. We present results from our density functional theory based calculations with and without inclusion of spin-orbit coupling (SOC) to shed light onto atomic and electronic structures of these phases and the nature of their binding to the substrate. We find that Pb atoms adsorb at the T4 sites at a height of 2.94 Å, at the T4 and H3 sites at height of 2.84 Å, and the off-centered T1 sites at height of 2.84, from the underlying Ge surface, on the α, β and 1 ML phases of the structure, respectively, and at the T1 site at a height of 2.76 Å directly above the nearest Ge on the Pb(1 × 1) phase. We find that these phase are metallic. Our results reinforce the importance of SOC in reproducing geometric structures of the Pb over-layers, in good agreement with experimental observations. |
Thursday, March 5, 2020 5:06PM - 5:18PM |
U52.00014: Giant Nonlinear Tunneling Current in HfO2-based Anti-Ferroelectric Tunnel Junction Jinho Byun, Taewon Min, Jaekwang Lee Owing to the recent advances in the oxide growth technology, ferroelectricity has been stabilized even in a few nm-thick films, which makes it possible to realize the ultrathin oxides-based ferroelectric tunneling junctions (FTJs) useful for the next generation switchable diode. Among various ferroelectric oxides, HfO2 is the most promising material for FTJ devices since it has the great advantage of complementary metal-oxide-semiconductor (CMOS) process compatibility. Despite their numerous advantages, low non-linearity and tunneling current have hindered their applications to electronic devices. Here, combining density functional theory (DFT) calculations and numerical tunneling current simulations of the new type of tunneling potential, we find that the antiferroelectric-like head-to-head and tail-to-tail polarizations significantly enhance the non-linearity (>103) and tunneling current at the same time, which will be an essential guideline to design high density and low power consumption electronics applications. |
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