Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session F45: Surface Physics |
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Sponsoring Units: DCMP Chair: Steven Erwin, United States Naval Research Laboratory Room: BCEC 211 |
Tuesday, March 5, 2019 11:15AM - 11:27AM |
F45.00001: Atomic and Electronic Structures in Intermetallic CaBi2 Thin Films Yanfeng Lyu, Hanming Yuan, Samira Daneshmandi, Shuyuan Huyan, Paul C. W. Chu Bismuth-based alloys and compounds are promising candidates for novel topological nontrivial materials due to the strong spin-orbit coupling effect in the heavy bismuth element. The search for topological superconductivity and Majorana fermions in these materials have attracted significant interest. Recently, intermetallic CaBi2 single crystal was reported to be superconducting, while the atomic, electronic, and topological properties have never been investigated. Using molecular beam epitaxy, we have successfully grown CaBi2 thin film on TiO2-terminated SrTiO3(100) substrate. The lattice and electronic structures were probed by using in-situ reflection high-energy electron diffraction, scanning tunneling microscopy and ex-situ X-ray diffraction. We found the epitaxial CaBi2 thin film has a Volmer-Weber growth mode and the surface has four totally different terminations: two are non-reconstructed, and the others are reconstructed to 1×2 and c(5×5), respectively. The different electronic density of states and physical properties relevant to these different surface terminations were also investigated. |
Tuesday, March 5, 2019 11:27AM - 11:39AM |
F45.00002: Surface phases on Te (0001) single crystal by low temperature STM Michael Dreyer, Pengke Li, Ian Appelbaum, Robert E Butera Tellurium has a curious crystal structure where one dimensional covalently bound chains are held together by van der Waals interactions in a hexagonal pattern. Additionally, theory predicts that the (0001) surface (perpendicular to the chains) of tellurium is a candidate to host a zero-energy bound state. Since evaporated thin films tend to grow in the (1000) direction, we use a Te single crystal to examine the (0001) surface by low temperature scanning tunneling microscopy/spectroscopy. The samples were cut from a larger single crystal and chemo-mechanically polished. After introduction into the ultra-high vacuum system the samples were carefully cleaned by Ar ion sputtering and annealing cycles. We found two surface structures as candidates for the native (0001) surface. The first is a row structure with a period of twice the lattice constant hinting at dimerization of the dangling bonds. The second is more complicated with rows separated by terraces of variable width. This might be a higher-index surface or induced by residual contamination . Initial spectroscopy measurements will also be discussed. |
Tuesday, March 5, 2019 11:39AM - 11:51AM |
F45.00003: Quantum saturation of capacitance in metals Carlos Untiedt, Bernat Olivera, Joaquin Fernandez-Rossier There are two contributions to the electronic capacitance in between two electrodes. First, there is the classical capacitive effect as an electric field is being developed in between these. Another contribution, which has often been neglected for metals, is related to the Pauli exclusion principle for the difficulty to accumulate charges on the electrode surface, the so-called quantum capacitance [T. Christen and M. Büttiker. Phys. Rev. Letters 77 (1996), 1]. Here we report the use of a Scanning Tunneling Microscope at low temperatures to study the variations of the capacitance as we approach two metallic electrodes made of Pt or Au. Three regimes are clearly visible in this process: A classical increase of capacitance which at short distances turn to saturation to the quantum capacitance limit, and finally, a leak of capacitance due to quantum tunneling. |
Tuesday, March 5, 2019 11:51AM - 12:03PM |
F45.00004: Comparison of Features for Au and Ir Adsorbed on the Ge (110) Surface Shirley Chiang, Ruikuan Xie, Huai Zhong Xing, Talat Rahman, Ching Yao Fong Single atoms of Au and Ir adsorbed, respectively, on the Ge(110) surface are studied by a first-principles algorithm based on density functional theory. The surface is modeled by a slab consisting of 108 Ge atoms with a 10 Å vacuum region. Hydrogen atoms are used to saturate the dangling orbitals at the other side of the vacuum region. The relaxed surface with the Au or Ir adatom shows the following distinct features: (1) Au is located at the top of the surface while Ir is located below the top layer of the Ge atoms, as a results of the smaller 1.423Å atomic radius of Ir vs. 1.503Å for Au. (2) No bond formation occurs between the metallic adatoms and the Ge atoms because the more tightly bound nature of the d-states in the metal elements do not allow hybridization to form sp3-type directional orbitals, as seen in charge density plots. (3) The Ir charge density is more deformed than Au, because the partially filled d-shell in Ir can be more easily polarized than the completely filled d- states of Au. Comparisons will be made to Ir/Ge(111) and Au/Ge(110) data measured by STM and LEEM. |
Tuesday, March 5, 2019 12:03PM - 12:15PM |
F45.00005: Step-Spacing Distributions Revisited: Curved Crystals Bring Many Opportunities and Challenges to Analysis Theodore L. Einstein, J. Enrique Ortega, Frederik Schiller, Martina Corso, Ignacio Piquero-Zulaica, Jorge Lobo-Checa, Aitor Mugarza While the properties of vicinal surfaces with close-packed steps are long well understood, recent experiments using curved crystals invite examination of many orientations under the same temperature and other conditions [1]. In addition to having simultaneously a range of geometries, each of which may be preferred for specific epitaxial growth and chemical reactions, one can now test scaling theories of terrace-width distributions (TWDs) based on fundamental theories and the existence of a single characteristic length, the mean terrace width. For close-packed steps, TWDs are well described by a single-parameter Wigner distribution. For fully-kinked steps, the stiffness tends to vanish, and some of the underlying assumptions of that analysis fail [2]. Hence, TWDs typically do not scale. Furthermore, surface states introduce a new length, λF, which can confound the scaling analysis. For large terrace widths, a description in terms of quantum well states offers a novel accounting of the TWD. Other subtleties and open questions are discussed. |
Tuesday, March 5, 2019 12:15PM - 12:27PM |
F45.00006: Tunneling Spectroscopy of the excitonic insulator phase in Ta2NiSe5 Jinwon Lee, Chang-Jong Kang, ManJin Eom, Jun Sung Kim, Byung Il Min, Han Woong Yeom The excitonic insulator phase is one of the novel electronic ground states driven by many-body interactions. Despite its theoretical proposal about 60 years ago, its material realization is still unclear. Ta2NiSe5 is a strong candidate, exhibiting a temperature-driven phase transition without any other competing phase such as the charge-density-wave. Experimental evidence so far includes an anomalous increase of the electric resistivity, the flat valence band maximum, and optical gap measurements. We provide further evidence of the excitonic insulator phase in Ta2NiSe5 using scanning tunneling microscopy/spectroscopy. At 78 K, the single-particle gap is measured as 300 meV and the orbital characters at the valence band maximum (-175 meV) are inverted from the high-temperature phase. These results indicate the strong interband (or electron-hole band) interaction in the gap opening, which is in a good agreement with the excitonic scenario. Moreover, our model calculation reveals that the non-interacting phase in Ta2NiSe5 has a semimetallic band structure, implying the excitonic condensation in Ta2NiSe5 is close to the Bardeen-Cooper-Schrieffer (BCS) regime. |
Tuesday, March 5, 2019 12:27PM - 12:39PM |
F45.00007: STM investigation of oxygen dissociation on Ag/Cu(111) near surface alloy Laura Cramer, Charlie Emile Sykes Both experiment and theory have demonstrated the ability of near surface alloys (NSAs) to desirably tune catalyst properties. These alloys consist of a solute metal confined to the first few atomic layers and present in lower concentrations than the host metal. This alters the electronic and thus catalytic properties of the alloy. Deposition of Ag on Cu(111) leads to a well-characterized NSA with Ag confined to the uppermost layer. The presence of Ag in the topmost layer forms a dislocation loop in the underlying Cu(111) due to strain from lattice mismatch. Ag-based catalysts are the current industrial standard for many partial oxidation reactions, guiding our interest in oxidation dissociation on Ag/Cu(111). Herein, we report on the observation of oxygen dissociation on extended Ag ensembles on Cu(111) via room-temperature scanning tunneling microscopy (STM). Atomic oxygen coverage was observed to scale with increasing oxygen exposure. Through time-lapse STM images, we observed localized diffusion of oxygen on Ag ensembles. These results hint at the potential for this surface to perform partial oxidation reactions with enhanced selectivity and activity. |
Tuesday, March 5, 2019 12:39PM - 12:51PM |
F45.00008: Phase Transitions of Condensates Investigated in an Atom by Atom Way Aisha Ahsan, S. Fatemeh Mousavi, Thomas Nijs, Sylwia Nowakowska, Olha Popova, Aneliia Wäckerlin, Jonas Björk, Lutz H Gade, Thomas Jung Condensation processes and phase transitions are investigated in an atom-by-atom way in on-surface nanosized confinements. Structural transformations are induced thermally and by local probe excitation. The pores of a metal-organic network occupied with 1 up to 9 Xe atoms have been investigated in their propensity to undergo 'condensed solid' to 'confined fluid' transitions. Different transition temperatures are identified, which depend on the number of Xe atoms in the condensate and relate to the stability of the Xe clustering in the condensed 'phase'. This work is of fundamental interest and reveals the feature-rich behaviour of transitions of confined planar condensates which provide a showcase towards future 'phase-transition' storage media patterned by self-assembly. |
Tuesday, March 5, 2019 12:51PM - 1:03PM |
F45.00009: Macroscopic Transport Signatures of Alkali Metal Surface Doping in Quantum Materials Christopher Parzyck, Brendan Faeth, Kyle M Shen Carrier injection by surface deposition of alkali metals has become a staple technique in the investigation of doping effects in many materials. Owing to their extremely low ionization potentials, alkali metals act readily as electron donors and since the resulting free carriers are localized to the material surface this technique has traditionally been used in conjunction with surface sensitive probes of the electronic structure, namely ARPES and STM. Here we present the use of macroscopic electrical transport measurements, in conjunction with potassium surface dosing, to examine doping effects in bulk single crystals of Ba(Fe1-xCox)2As2 and MoS2 as well as thin film FeSe. This technique allows measurement of the effects of doping on macroscopic electrical properties while maintaining a high level of crystalline order in the doped structure. Additionally, it provides a promising route to studying the evolution of electronic properties with carrier concentration in materials less amenable to electrostatic gating techniques, namely superconductors with a metallic parent state. |
Tuesday, March 5, 2019 1:03PM - 1:15PM |
F45.00010: A Comprehensive Computational Study of Adatom Diffusion on the Aluminum (1 0 0) Surface James Chapman, Rohit Batra, Blas Pedro Uberuaga, Ghanshyam Pilania, Ramamurthy Ramprasad The complexity of adatom diffusion on the Al (100) surface is reflected by the existence of several low-energy non-trivial atomic exchange or vacancy formation mechanisms. Interestingly, these mechanisms have energy barriers lower than or comparable to that of the simple (and intuitive) hopping mechanism. While prior studies mainly used classical potentials to understand diffusion processes active on Al (100) surface, here we use accurate (and expensive) density functional theory (DFT) computations to estimate barriers associated with nine low-energy adatom diffusion mechanisms. We find that there exist several exchange mechanisms with energy barriers less than or equal to that of the trivial hop mechanism, thereby highlighting mechanisms that can be relevant during surface/crystal growth. Our results paint a highly complex picture of the diffusion landscape on Al (100) and provide insights into how such mechanisms may contribute toward large length- and time-scale surface phenomena. Further, we show that some of the commonly used interatomic potentials fail to accurately capture the details of adatom diffusion on Al (100). |
Tuesday, March 5, 2019 1:15PM - 1:27PM |
F45.00011: Novel electronic junctions in an atomic wire array: interfaces between metallic and charge density wave ordered electronic phases Samad Razzaq, Sun Kyu Song, Han Woong Yeom, Stefan Wippermann The Si(111)-(4x1)In atomic wire array is an extremely popular model for one-dimensional electronic systems. It features a reversible, temperature-induced metal insulator transition into a charge density wave (CDW) ordered ground state with (8x2) translational symmetry. Close to the phase transition temperature, both phases can coexist and form novel types of electronic junctions between the metallic (4x1) phase and the insulating CDW-ordered (8x2) phase. Combining scanning tunneling microscopy/spectroscopy (STM/STS) and ab initio molecular dynamics calculations, we explore the microscopic structure of interfaces between distinct electronic phases at the atomic scale. Specific defects allow to modify and control the structure of these electronic interfaces. We explain the atomistic mechanism behind the junction formation and its tunability from first principles. |
Tuesday, March 5, 2019 1:27PM - 1:39PM |
F45.00012: Ab initio investigation of ultrathin films of phase-change materials Riccardo Mazzarello, Ider Ronneberger, Zeila Zanolli, Matthias Wuttig Chalcogenide phase change materials such as GeTe and GeSbTe are capable of switching between the crystalline and the amorphous state on a nanosecond time scale at elevated temperatures. Furthermore, the two states exhibit a large electrical contrast, which is exploited in novel storage-class memory devices. The continuous miniaturization of such devices requires an understanding of the structural and electronic properties of these compounds in the quasi two-dimensional limit. Besides, few-layer crystalline films of GeTe and similar monochalcogenides have recently drawn attention because they display a peculiar in-plane ferroelectricity. In this work, we investigate thin layers of selected chalcogenides with varying thickness by first principles calculations based on density functional theory. We study the changes in said properties as a function of film thickness. We rationalize these changes in terms of bonding mechanisms and effects of depolarizing fields. |
Tuesday, March 5, 2019 1:39PM - 1:51PM |
F45.00013: Electronic structure of the LaB6 (001)-surface Florian Sohn, Philipp Buchsteiner, Jan Voigt, Martin Wenderoth, Peter E Bloechl Lanthanum hexaboride (LaB6) is the first compound among the rare earth hexaborides. Due to its low work function and high melting point, LaB6 is widely used for thermionic electron emission. Recently, the LaB6 (001)-cleavage plane has been investigated with scanning tunneling microscopy (STM) and spectroscopy (STS). Our data shows a mainly 2×1 reconstructed surface. We rationalize the constant current topographies and differential conductance spectra by simulations based on density functional theory (DFT). Our simulations of a LaB6 (001)-surface, terminated by chains of lanthanum ions, show, that in STM measurements with positive bias voltage the d-orbitals of the topmost lanthanum ions are addressed. In contrast, at small negative bias voltages, a broad feature in the local density of states below the Fermi energy could be successfully resolved by both STS and DFT and is traced back to surface boron orbitals. Our study shows the necessity of surface calculations in order to understand the experimental data in detail and can be applied to various model systems. |
Tuesday, March 5, 2019 1:51PM - 2:03PM |
F45.00014: Quantum nutcracker for near-room-temperature H2 dissociation Lei Tao, Wei Guo, Yuyang Zhang, Shixuan Du, Sokrates T Pantelides Here we provide the theoretical foundations for an alternative paradigm of molecular dissociation -- a “quantum nutcracker”. The two nutcracker jaws are transition-metal phthalocyanine and a metal substrate such as Cu(111) or Au(111), all of which are relatively inert on their own. Density-functional-theory calculations demonstrate that, when a H2 molecule enters the channel between the jaws, it splits into two H atoms by quantum interactions and a gentle mechanical squeeze. Au-based nutcrackers are predicted to operate at room-temperature, while less-expensive Cu-based ones are predicted to be active at a slightly elevated temperature. Indirect experimental evidence is consistent with the present predictions. Such in silico design holds promise for inexpensive, high-performance heterogeneous catalysts for H2 dissociation and may inspire new approaches to other complex reactions. |
Tuesday, March 5, 2019 2:03PM - 2:15PM |
F45.00015: Structural stability and electronic properties of the chiral topological superconductor Pb3Bi/Ge(111): A first-principles study Leiqiang Li, Wei Qin, Shang Ren, Ping Cui, Zhenyu Zhang In a separate study, we have shown that a hole-doped Pb3Bi monolayer can serve as a highly appealing new platform for realizing two-dimensional (2D) intrinsic chiral topological superconductivity. Using first-principles calculations, here we systematically investigate the structural stability of Pb3Bi grown on Ge(111) substrate, and find two nearly degenerate lattice configurations labeled as T1 and H3. For the T1 structure, the appearance of type-II van Hove singularity in the density of states arises from the Bi doping, where the p orbit of Bi hybridizes with that of Pb, leading to the emergence of saddle-like band structures. Moreover, for both the T1 and H3 structures, Bi doping gives rise to larger Rashba-type splittings of the energy bands. We also find a sizable energy barrier of ~0.30 eV per formula unit from the T1 to H3 structure. These findings are expected to stimulate new research activities in searching for 2D intrinsic topological superconductors. |
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