Bulletin of the American Physical Society
85th Annual Meeting of the APS Southeastern Section
Volume 63, Number 19
Thursday–Saturday, November 8–10, 2018; Holiday Inn at World’s Fair Park, Knoxville, Tennessee
Session H01: Condensed Matter VI |
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Chair: Umesh Kumar, University of Tennessee, Knoxville Room: Holiday Inn Knoxville Downtown Summit |
Friday, November 9, 2018 4:30PM - 4:42PM |
H01.00001: Formation of Metastable Water Chains on Anatase TiO2(101) Arjun Dahal, Zdenek Dohnálek Anatase TiO2 is a pivotal material for energy-harvesting applications and catalysis. In this study, we employ scanning tunneling microscopy (STM) and study water adsorption on most stable TiO2(101) surface of anatase. Well-defined anatase TiO2(101) surface exposes alternating rows of two-fold-coordinated oxygen atoms (O2c) and five-fold-coordinated Ti atoms (Ti5c) along the [010] direction. We show that at 80 K isolated water monomers bind molecularly to the Ti5c sites. The onset of diffusion is found at ~190 K where water monomers diffuse both along and across the Ti5c rows. Surprisingly, we find that at 80 K water molecules start to form linear chains along the Ti5c rows as the coverage is increased. This indicates the presence of transient mobility of water molecules suggesting that the adsorption occurs via a precursor state. When the water chains are annealed at 190 K, they fall apart to monomers that reside on the next-nearest-neighbor Ti5c sites. These results show that the water chains are metastable in nature. This is at odds with many other oxide surfaces where hydrogen-bonded water clusters are energetically preferred over the isolated monomers. |
Friday, November 9, 2018 4:42PM - 4:54PM |
H01.00002: Hydrogen-plasma induced strongly-correlated transparent conducting titanate thin-films John G Connell, Maryam Souri, Namal Wanninayake, Jared Johnson, Justin Thompson, John Gruenewald, Joseph W Brill, Jinwoo Hwang, Doo Young Kim, Ambrose Seo Transparent conducting oxides (TCO’s) are one of the key materials for optoelectronic device applications such as display panels and solar cells. Recently, strongly correlated complex oxide thin-films have been suggested as new TCO’s by utilizing the large effective mass of the conducting carriers. However, conventional chemical doping on complex oxides often results in unwanted impurity bands. Here, we present that epitaxial Ba0.5Sr0.5TiO3 (BST) thin-films exhibit phase transitions from a wide-gap insulator to an Anderson insulator and eventually to a transparent conductor via hydrogen-plasma exposure (HPE). BST thin-films are well-known robust insulators; no chemical doping methods can generate conducting carriers. Our pulsed laser deposited BST thin films on GdScO3 substrates are optically transparent. After about 3 minutes of HPE, the samples exhibit an absorption band at infrared energies. Further HPE up to 5 minutes exhibits a Drude response in the optical conductivity. The transparency of HPE BST thin-films remains the same as unexposed samples below 1.5 eV. We will discuss how HPE can open a new way for generating TCO’s. |
Friday, November 9, 2018 4:54PM - 5:06PM |
H01.00003: Nanosecond pulsed laser deposition of Pb thin film on Si (111) Devon Alexander Loomis, Ilhom Saidjafarzoda, Bektur Abdisatarov, Donald Price, Mikhail Khenner, Ali Oguz Er Pb thin film was deposited onto a Si (111) substrate by pulsed laser deposition (PLD). The Pb target was ablated with a Q-switched 1064 Nd:YAG pulsed laser with 5 nanosecond pulse width, 10 Hz repetition rate, and 1 mm beam diameter. Laser energy density, temperature wavelength and the number of pulses were changed. Different thicknesses of the film ranging from 5 to 70 nm were obtained. Morphological structures of the films were measured using scanning electron microscopy and atomic force microscopy. Our results show that laser energy density, wavelength, and temperature play an important role in morphology. In addition, quantum size effects (QSE) were observed on the ultra-thin films and coarsening effects were observed on the films that underwent high-temperature deposition. Experimental observation is supported by theoretical simulations. Ongoing results of Pb film growth on a copper sample will also be presented. |
Friday, November 9, 2018 5:06PM - 5:18PM |
H01.00004: Neutronic Measurements on Ammonia for Neutron Moderation David Baxter, Dylan R Weaver At many neutron sources today, only one part per billion of the neutrons generated at a source will reach a measurement device within a useful energy band, which leaves plenty of room for improvement in this area of research. The Low Energy Neutron Source (LENS) at Indiana University’s Center for Exploration of Energy and Matter (CEEM) has made multiple contributions to neutron scattering community, with an emphasis on improving neutron instrumentation. One of the efforts that is made at LENS is the analysis of potential neutron moderators for producing larger amounts of cold and thermal neutrons. We have chosen for our experiments to be focused on Ammonia (NH3), which is intriguing to LENS due to its high Hydrogen content, its molecular mass, and its broad, accessible temperature range. For our experiments, we require a cryostat capable of bringing Ammonia to a condensed state (Approx. 220 K) and control the temperature for long periods of time. Here, we present the effectiveness of our cryostat design in terms of accurate temperature control, as well as the results of neutron transmission measurements on Ammonia to determine its total neutron cross section, for neutrons with energies of 0.1 meV – 1 eV. |
Friday, November 9, 2018 5:18PM - 5:30PM |
H01.00005: Magnetic Structure of U2Ni2Sn Chase Evan Cupp, S. Mašková, A. V. Andreev, L. Havela, Farzana Nasreen, Huibo B. Cao, H. Nakotte, Karunakar Kothapalli Uranium intermetallics of the U2T2X (U = Uranium, T = transition metal, X = p-electron ligand) family, in general, contain magnetic moments perpendicular to the shortest U-U distance within the crystal structure. U2Ni2Sn, belonging to this family, crystallizes in the tetragonal Mo2FeB2 structure and has the shortest U-U distance along the basal plane. It orders antiferromagnetically at 26 K with the propagation vector q = (0, 0, 1/2). Previous neutron powder diffraction studies of this compound, albeit somewhat ambiguous, reported U magnetic moments aligned in the basal plane. This result represents an exception to the established trend about orientation of uranium moments in this family of compounds. However, new measurements on high-quality single crystals grown using Czochralski method have raised questions on the direction of the magnetic moment. We will present our single crystal neutron diffraction results on high quality single crystal to resolve the direction of the uranium magnetic moments. Our experiments show that the uranium moments align along the c-axis with a moment ≈ 0.85μB per U-atom at 5 K and help us resolve a long-standing ambiguity and understand better the magnetic anisotropy in these compounds. |
Friday, November 9, 2018 5:30PM - 5:42PM |
H01.00006: Long range antiferromagnetic order and lattice dynamics in a rocksalt high entropy oxide Junjie Zhang, Jiaqiang Yan, Stuart Calder, Qiang Zheng, Douglas Abernathy, Yang Ren, Saul Lapidus, Katharine Page, Hong Zheng, John D Budai, Raphael Pierre Hermann High entropy oxides are of intense interest due to their importance in fundamental research including phase formation and modeling, and potential multifunctional physical properties. Here we report the magnetic, thermal and mechanical properties of the rocksalt high entropy oxide (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O as well as their lattice dynamics for the first time. Long range antiferromagnetism with q=(½, ½, ½) was identified using neutron powder diffraction. The shear modulus obtained from resonant ultrasound spectroscopy is about 1/5 of MgO, 1/3 of CoO and 1/4 of NiO. The thermal conductivity is an order of magnitude smaller than that of MgO. Inelastic neutron scattering results indicate strong magnetic excitations at 100 K that survive up to room temperature. In addition, the phonon density of states (PDOS) is considerably softer (vibrational frequencies) than in MgO. |
Friday, November 9, 2018 5:42PM - 5:54PM |
H01.00007: Role of temperature dependent scattering mechanisms on the electronic transport in freestanding graphene foams Prakash Parajuli, Chuanchang Zeng, Fengjiao Liu, Pooja Puneet, Ramakrishna Podila, Sumanta Tewari, Sriparna Bhattacharya, Apparao Rao We present electronic transport properties of freestanding graphene foam (pristine and nitrogen-doped) using four-probe resistance (R) and Seebeck coefficient (S) measurements in the temperature range ~10-300 K. In pristine graphene, a low value of S ~2 μV/K at room temperature was measured which is attributed to the unique band structure of graphene (symmetric) and low density of states near the Dirac point. With nitrogen doping, the electron-hole symmetry was broken, leading to a six-fold enhancement of S. A theoretical study of temperature dependent scattering mechanisms contributing to the electronic transport characteristics was performed within the framework of Boltzmann transport theory under relaxation time approximation. |
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