Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session X15: Unconventional Two Dimensional Materials.Focus

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Sponsoring Units: DMP Chair: Luis Jauregui, Harvard University Room: 314 
Friday, March 18, 2016 8:00AM  8:36AM 
X15.00001: Electrons and phonons in layered and monolayer vanadium pentoxide. Invited Speaker: Walter R. L. Lambrecht Vanadium pentoxide (V$_2$O$_5$) is a layered material with the potential for interesting new properties when made in 2D mono or fewlayer form. Its band structure is characterized by a splitoff conduction band. The lowest conduction band is separated from the rest of the conduction bands by about 1 eV and consists of V$d_{xy}$ orbitals, nonbonding to the oxygens by symmetry. This narrow band has dispersion essentially along the direction of chains occurring in the layer. When this band becomes halffilled by doping, spinsplitting occurs accompanied by an antiferromagnetic coupling between nearest neighbors along the chain direction. This situation is well known to occur in the socalled ladder compound NaV$_2$O$_5$ , which was extensively studied in the late 90s as a potential spinPeierls or charge ordering compound. However, the monolayer form of V$_2$O$_5$ may allow for other ways to control the doping by gating, removing vanadyl oxygens, adsorption of alkali metals, nanoribbon formation, etc. Our calculations predict a switch from antiferromagnetic to ferromagnetic coupling for doping slightly less than half filling of the splitoff band. In this talk we will discuss our recent work on the electronic band structure of both bulk and monolayer V$_2$O$_5$ as well as the phonons. We find that the quasiparticle selfconsistent $GW$ method strongly overestimates the band gap. Lattice polarization corrections of the screening are required because of the large LO/TO phonon frequency ratios. Excitonic effects may also be expected to be fairly large. We find that some of the vibrational modes, notably the vanadyloxygen bond stretch perpendicular to the layer, unexpectedly shows a strong blue shift. This is explained in terms of reduced screening affecting the longrange dipole components of the force constants. [Preview Abstract] 
Friday, March 18, 2016 8:36AM  8:48AM 
X15.00002: Doping effects on the electronic and magnetic properties of V$_2$O$_5$ Churna Bhandari, Walter R.L. Lambrecht We study doping of the V$_2$O$_5$ splitoff conduction band using different methods: by adding electrons compensated by an artificial homogeneous background, a virtual crystal approximation(VCA), by changing the atomic number $Z_{v}$ and explicitly by intercalating Na as a dopant. The former two are mathematical models to simulate injected charge by gating, the latter occurs in the vanadium bronze NaV$_2$O$_5$. We also study Na$_{1x}$V$_2$O$_5$ using the VCA by changing $10\leq Z_{\text{Na}}\leq 11$. We discuss the electronic band structure and the optical conductivity using the quasiparticle selfconsistent QS$GW$ method including a lattice polarization effect and the local density functional method with Hubbard$U$ correction (LSDA+$U$) for all these models. We show that the ground state prefers antiferromagnetic order along the chain (crystallographic $b$) direction and extract various near neighbor exchange interactions from total energy differences of different spin configurations. We find that the coupling between the nearest Vneighbors changes from antiferromagnetic to ferromagnetic when the electron concentration is reduced from half filling of the band (1e/V atom) to about 0.88 e/V atom. The magnetic moment gradually decreases with decreasing electron concentration. [Preview Abstract] 

X15.00003: ABSTRACT WITHDRAWN 
Friday, March 18, 2016 9:00AM  9:12AM 
X15.00004: Twodimensional, ordered, double transition metals carbides (MXenes) Paul Kent, Babak Anasori, Yu Xie, Majid Beidaghi, Jun Lu, Brian Hosler, Lars Hultman, Yury Gogotsi, Michel Barsoum We use[1] density functional theory to predict the existence of two new families of 2D ordered carbides (MXenes), M'$_2$M''C$_2$ and M'$_2$M''$_2$C$_3$, where each M is a different early transition metal. Synthesizing Mo$_2$TiC$_2$T$_x$, Mo$_2$Ti$_2$C$_3$T$_x$, and Cr$_2$TiC$_2$T$_x$ (where T is a surface termination), we validated the DFT predictions. Since the Mo and Cr atoms are on the outside, they control the 2D flakes' chemical and electrochemical properties. The latter was proven by showing quite different electrochemical behavior of Mo$_2$TiC$_2$T$_x$ and Ti$_3$C$_2$T$_x$. This work further expands the family of 2D materials, offering additional choices of structures, chemistries, and ultimately useful properties. [1] B. Anasori et al. ACS Nano {\bf 9} 9507 (2015). DOI: 10.1021/acsnano.5b03591 [Preview Abstract] 
Friday, March 18, 2016 9:12AM  9:24AM 
X15.00005: Unusual electronic and magnetic responses from sulfurdecorated graphene. Choongyu Hwang, S. A. Cybart, S. M. Wu, R. C. Dynes, S. J. Shin, E. E. Haller, S. Kim, K. Kim, B. I. Min, T. G. Rappoport, C. Jozwiak, A. V. Fedorov, S. K. Mo, A. H. Castro Neto, D. H. Lee, A. Lanzara Interactions between two different materials can produce strong electronic correlations that do not exist when each material stands alone. We search for such correlations from graphene, a nonmagnetic semimetal, decorated by sulfur, a diamagnetic insulator, using angleresolved photoemission spectroscopy and magnetotransport measurements. Sulfurdecorated graphene exhibits unusual electronic and magnetic responses that are clearly distinguished from clean graphene. Our findings provide intriguing insights on the search for novel quantum phases in graphenebased compounds. [Preview Abstract] 
Friday, March 18, 2016 9:24AM  9:36AM 
X15.00006: Electronic correlations in monolayer VS$_2$ Eric B. Isaacs, Chris A. Marianetti The layered transition metal dichalcogenide vanadium disulfide (VS$_2$), which nominally has 1 electron in the $3d$ shell, is potent for strong correlation physics and is possibly another realization of the oneband Hubbard model beyond the cuprates. Here we investigate the octahedral (OCT) and trigonal prismatic (TP) phases of monolayer VS$_2$ using density functional theory plus Hubbard $U$ calculations. Unlike the OCT phase, the TP phase has an isolated lowenergy band due to the crystal field splitting and the nearestneighbor VV hopping. Within DFT, ferromagnetism spin splits this band leading to a lowbandgap $S=1/2$ ferromagnetic insulating TP phase, which is lower in energy than the OCT phase. The onsite interaction $U$, which we find to be approximately 4 eV via linear response, increases the band gap, leads to Mott insulating behavior, and for sufficiently high values stabilizes the ferromagnetic OCT phase. We explore the impact of charge density waves in monolayer VS$_2$ and discuss the possibility to experimentally realize the TP phase. [Preview Abstract] 
Friday, March 18, 2016 9:36AM  9:48AM 
X15.00007: Competing antiferromagnetism in a quasi2D itinerant ferromagnet: Fe3GeTe2 Zheng Gai, Jieyu Yi, Houlong Zhuang, S.A. Calder, P.R.C. Kent, David Mandrus Fe$_{\mathrm{3}}$GeTe$_{\mathrm{2}}$ is known as an airstable layered metal with itinerant ferromagnetism with a transition temperature of about 220 K. From extensive dc and ac magnetic measurements, we have determined that the ferromagnetic layers of Fe$_{\mathrm{3}}$GeTe$_{\mathrm{2}}$ order antiferromagnetically along the caxis blow 152 K. The antiferromagnetic state was further substantiated by theoretical calculation to be the ground state. A magnetic structure model was proposed to describe the antiferromagnetic ground state as well as competition between antiferromagnetic and ferromagnetic states. Fe$_{\mathrm{3}}$GeTe$_{\mathrm{2}}$ shares many common features with pnictide superconductors and may be a promising system in which to search for unconventional superconductivity. [Preview Abstract] 
Friday, March 18, 2016 9:48AM  10:00AM 
X15.00008: Manybody effects in doped graphene on a piezoelectric substrate. F. Sols, D. G. Gonzalez, I. Zapata, J. Schiefele, F. Guinea We study theoretically the role of piezoelectric acoustic phonons in the context of piezoelectric substrates covered by graphene. They are responsible for effective, substrate dependent electronelectron interactions which can be strong and give rise to novel manybody effects. We present a new derivation of the electronphonon interaction matrix element which generalizes previous calculations made within the simpler and not always justified isotropic approximation. We study several manybody effects, including the temperaturedependent phonon renormalization due to the electron cloud surrounding the lattice vibration, as well as the electron selfenergies arising from the effective electronelectron interactions in the perturbative $G_0W$ approximation. We also perform calculations of the graphene electron mobility on substrates with various levels of piezoelectricity. Finally, we discuss how these piezoelectric phonons can influence the superconducting instability. For completeness, we compare our results with the situation found for the twodimensional electron gas and for conventional threedimensional BCS superconductors. [Preview Abstract] 
Friday, March 18, 2016 10:00AM  10:12AM 
X15.00009: Electrochemical intercalation of lithium ions into NbSe$_{\mathrm{2}}$ nanosheets Emily Hitz, Jiayu Wan, Anand Patel, Yue Xu, Louisa Meshi, Jiaqi Dai, Yanan Chen, Albert Davydov, Liangbing Hu Transition metal dichalcogenides (TMDCs) have been known for decades to have unique properties and recently attracted broad attention for their twodimensional (2D) characteristics. One TMDC that has been studied for its charge density wave transition behavior and superconductivity is metallic NbSe$_{\mathrm{2}}$, yet it is still largely unexplored for device applications in electronics, optics, and batteries. Through this work, we demonstrate successful electrochemical intercalation of lithium ions into layered NbSe$_{\mathrm{2}}$. We present evidence of lithium intercalation as a technique capable of modifying the material properties of hexagonal NbSe$_{\mathrm{2}}$ for further study. We confirm our result through Xray diffraction, showing a unit cell size increase in NbSe$_{\mathrm{2}}$ after intercalation from 12.57 {\AA} to 13.57 {\AA} in the ``c'' lattice dimension. Additionally, planar halfcell microbattery devices are fabricated using ultrathin NbSe$_{\mathrm{2}}$ from platelets to observe Liion intercalation through an increase in the optical transmittance of the material in the visible range. At 550 nm wavelength light, we observed an increase in the optical transmittance of the material by 26{\%} due to electrochemical intercalation. [Preview Abstract] 
Friday, March 18, 2016 10:12AM  10:24AM 
X15.00010: Electronic and magnetic properties of nanoribbons Gayanath Fernando, Zhiwei Zhang, Armen Kocharian We have performed tightbinding calculations with open boundary conditions on a set of twisted nanoribbons (4x100), monitoring the band structure as a function of the twist angle $\theta$. When this angle is zero, the ribbon is rectangular and when it is 60 degrees, the ribbon is cut from a honeycomb lattice. Depending on the parameters of the tightbinding model and the filling factor, semimetallic or insulating behavior is observed. We have also studied the electronic structure of such ribbons due to the adsorption of small atoms such as nitrogen, a magnetic field and the Rashba spinorbit interaction. The role of the adsorbed atoms and the Rashba term with regard to the conducting properties and the symmetry breaking of the ribbons will be discussed in some detail. In addition, the effects of electronic correlations on selected small ribbons will be examined. [Preview Abstract] 
Friday, March 18, 2016 10:24AM  10:36AM 
X15.00011: SU(4) quantum spin liquids in critical Coulomb impurity lattices on MoS2 X. Dou, V. N. Kotov, Bruno Uchoa In the critical regime, massive Dirac fermions are know to form a bound state in the vicinity of a Coulomb impurity. We find that in the presence of electronelectron interactions, the electrons in this bound state will valley and spin polarize. We show that the interaction of the spin and valley polarized electrons bounded to two different Coulomb impurities naturally maps into a Heisenberg model with SU(4) symmetry. We propose that quantum spinorbital liquids with that symmetry can be engineered in artificial Coulomb impurity lattices on the surface of MoS2 monolayer. We discuss possible experiments to detect those states. [Preview Abstract] 
Friday, March 18, 2016 10:36AM  10:48AM 
X15.00012: Crystalline topological insulators in ultrashort optical pulse Seyyedeh Azar Oliaeimotlagh, Vadym Apalkov, Mark Stockman We study theoretically interaction of ultrastrong and ultrashort optical pulse with crystalline topological insulators, which have quadratic band degeneracy at the surface. Coherent electron surface dynamics in such optical pulse is determined by interband dipole coupling, which is highly anisotropic in these materials. Within twoband k.p model of the surface states of topological insulator the electron dynamics is describe in terms of the mixing of two (valence and conduction) bands. Such mixing is characterized by residual, i.e., after the pulse, conduction band population. Residual electron momentum distribution in the conduction band is highly anisotropic and follows the profile of the interband dipole coupling.~ Depending on polarization of the optical pulse, the residual conduction band population has one or two strong peaks in the momentum space, where the shapes of the peaks change with the amplitude of the pulse. The conduction band population is almost one at these peaks. [Preview Abstract] 
Friday, March 18, 2016 10:48AM  11:00AM 
X15.00013: Gate tunable magnetooptical effects in layered antiferromagnets Nikhil Sivadas, Satoshi Okamoto, Di Xiao It has long been believed that the Faraday and the magnetooptic Kerr effects are absent in collinear antiferromagnets due to their vanishing net magnetic moment. To the contrary, using firstprinciples calculations we demonstrate that these effects can be controlled by a perpendicular voltage in bilayer MnPSe$_3$, which has a collinear AFN{\'e}el spin texture. The Kerr rotation can be as high as 10 mrad, and is reversed on the reversal of the polarity of gate voltage. The tunable nature of the magnetooptic effects can result in novel optoelectronic device applications. It also provides a nondestructive way to characterize the magnetic ground state of 2D materials. [Preview Abstract] 
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