Person: Kaxiras, Efthimios
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Kaxiras
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Efthimios
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Kaxiras, Efthimios
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Publication Dirac Fermions and Flat Bands in the Ideal Kagome Metal FeSn(Springer Science and Business Media LLC, 2019-12-09) Fang, Shiang; Han, Minyong; Graff, David; Kaxiras, Efthimios; Kang, Mingu; Ye, Linda; You, Jhih-Shih; Levitan, Abe; Facio, Jorge; Jozwiak, Chris; Bostwick, Aaron; Rotenberg, Eli; Chan, Mun; McDonald, Ross; Kaznatcheev, Konstantine; Vescovo, Elio; Bell, David; van den Brink, Jeroen; Richter, Manuel; Prasad Ghimire, Madhav; Checkelsky, Joseph; Comin, RiccardoA kagome lattice of 3d transition metal ions is a versatile platform for correlated topological phases hosting symmetry-protected electronic excitations and magnetic ground states. However, the paradigmatic states of the idealized two-dimensional kagome lattice – Dirac fermions and flat bands – have not been simultaneously observed. Here, we utilize angle-resolved photoemission spectroscopy and de Haas-van Alphen quantum oscillations to reveal coexisting surface and bulk Dirac fermions as well as flat bands in the antiferromagnetic kagome metal FeSn, that has spatially-decoupled kagome planes. Our band structure calculations and matrix element simulations demonstrate that the bulk Dirac bands arise from in-plane localized Fe-3d orbitals, and evidence that coexisting Dirac surface state realizes a rare example of fully spin-polarized two-dimensional Dirac fermions due to spin-layer locking in FeSn. The prospect to harness these prototypical excitations in kagome lattice is a frontier of great promise at the confluence of topology, magnetism, and strongly-correlated physics.Publication Quantum nanomagnets in on-surface metal-free porphyrin chains(Springer Science and Business Media LLC, 2022-10-24) Zhao, Yan; Jiang, Kaiyue; Li, Can; Liu, Yufeng; Zhu, Gucheng; Pizzochero, Michele; Kaxiras, Efthimios; Guan, Dandan; Li, Yaoyi; Zheng, Hao; Liu, Canhua; Jia, Jinfeng; Qin, Mingpu; Zhuang, Xiaodong; Wang, ShiyongAbstract Quantum nanomagnets exhibit collective quantum behaviors beyond the usual long range ordered states due to the interplay of low dimension, competing interactions and strong quantum fluctuations. Despite numerous theoretical works treating quantum magnetism, the experimental study of individual quantum nanomagnets remains very challenge, greatly hindering the development of this cutting-edge field. Here, we demonstrate an effective strategy to realize individual quantum nanomagnets in metal-free porphyrins by using combined on-surface synthesis and atom manipulation approaches, with the ultimate ability to arrange coupled spins one by one as envisioned by Richard Feynman 60 years ago. A series of metal-free porphyrin nanomagnets have been constructed on Au(111) and their collective magnetic properties have been thoroughly characterized on the atomic scale by scanning probe microscopy together with theoretical calculations. Our results reveal that the constructed S=1/2 antiferromagnets host a gapped excitation in consistent with isotropic Heisenberg antiferromagnets S=1/2 model, while the S=1 antiferromagnets with odd-number units exhibit two zero-mode end states due to quantum fluctuations. Our achieved strategy not only provides a unique testing bed to study the strongly correlated effects of quantum magnetism in purely organic materials, but expands the functionalities of porphyrins with implications for quantum technological applications.Publication Heterointerface Effects in the Electro-Intercalation of Van Der Waals Heterostructures(Springer Nature, 2018-06-21) Kim, Philip; Rezaee, Mehdi; Yoo, Hyobin; Larson, Daniel; Zhao, Frank; Taniguchi, Takashi; Watanabe, Kenji; Brower-Thomas, Tina; Kaxiras, Efthimios; Bediako, KwabenaMolecular-scale manipulation of electronic/ionic charge accumulation in materials is a preeminent challenge, particularly in electrochemical energy storage. Layered van der Waals (vdW) crystals exemplify a diverse family of materials that permit ions to reversibly associate with a host atomic lattice by intercalation into interlamellar gaps. Motivated principally by the search for high-capacity battery anodes, ion intercalation in composite materials is a subject of intense study. Yet the precise role and ability of heterolayers to modify intercalation reactions remains elusive. Previous studies of vdW hybrids represented ensemble measurements at macroscopic films/powders, which do not permit the isolation and investigation of the chemistry at individual 2-dimensional (2D) interfaces. Here, we demonstrate the intercalation of lithium at the level of individual atomic interfaces of dissimilar vdW layers. Electrochemical devices based on vdW heterostructures comprised of deterministically stacked hexagonal boron nitride, graphene (G) and molybdenum dichalcogenide (MoX2; X = S, Se) layers are fabricated, enabling the direct resolution of intermediate stages in the intercalation of discrete heterointerfaces and the extent of charge transfer to individual layers. Operando magnetoresistance and optical spectroscopy coupled with low-temperature quantum magneto-oscillation measurements show that the creation of intimate vdW heterointerfaces between G and MoX2 engenders over 10-fold accumulation of charge in MoX2 compared to MoX2/MoX2 homointerfaces, while enforcing a more negative intercalation potential than that of bulk MoX2 by at least 0.5 V. Beyond energy storage, our new combined experimental and computational methodology to manipulate and characterize the electrochemical behavior of layered systems opens up new pathways to control the charge density in 2D (opto)electronic devices.Publication Theory of Graphene Raman Scattering(American Chemical Society (ACS), 2016) Heller, Eric; Yang, Yuan; Kocia, Lucas; Chen, Wei; Fang, Shiang; Borunda, Mario; Kaxiras, EfthimiosRaman scattering plays a key role in unraveling the quantum dynamics of graphene, perhaps the most promising material of recent times. It is crucial to correctly interpret the meaning of the spectra. It is therefore very surprising that the widely accepted understanding of Raman scattering, i.e., Kramers–Heisenberg–Dirac theory, has never been applied to graphene. Doing so here, a remarkable mechanism we term“transition sliding” is uncovered, explaining the uncommon brightness of overtones in graphene. Graphene’s dispersive and fixed Raman bands, missing bands, defect density and laser frequency dependence of band intensities, widths of overtone bands, Stokes, anti-Stokes anomalies, and other known properties emerge simply and directly.Publication Computational Caches(ACM Press, 2013) Waterland, Amos; Angelino, Elaine Lee; Cubuk, Ekin; Kaxiras, Efthimios; Adams, Ryan Prescott; Appavoo, Jonathan; Seltzer, MargoCaching is a well-known technique for speeding up computation. We cache data from file systems and databases; we cache dynamically generated code blocks; we cache page translations in TLBs. We propose to cache the act of computation, so that we can apply it later and in different contexts. We use a state-space model of computation to support such caching, involving two interrelated parts: speculatively memoized predicted/resultant state pairs that we use to accelerate sequential computation, and trained probabilistic models that we use to generate predicted states from which to speculatively execute. The key techniques that make this approach feasible are designing probabilistic models that automatically focus on regions of program execution state space in which prediction is tractable and identifying state space equivalence classes so that predictions need not be exact.Publication Hydrokinetic Approach to Large-Scale Cardiovascular Blood Flow(Elsevier, 2010) Melchionna, Simone; Bernaschi, Massimo; Succi, Sauro; Kaxiras, Efthimios; Rybicki, Frank John; Mitsouras, Dimitris; Coskun, Ahmet U.; Feldman, Charles LawrenceWe present a computational method for commodity hardware-based clinical cardiovascular diagnosis based on accurate simulation of cardiovascular blood flow. Our approach leverages the flexibility of the Lattice Boltzmann method to implementation on high-performance, commodity hardware, such as Graphical Processing Units. We developed the procedure for the analysis of real-life cardiovascular blood flow case studies, namely, anatomic data acquisition, geometry and mesh generation, flow simulation and data analysis and visualization. We demonstrate the usefulness of our computational tool through a set of large-scale simulations of the flow patterns associated with the arterial tree of a patient which involves two hundred million computational cells. The simulations show evidence of a very rich and heterogeneous endothelial shear stress pattern (ESS), a quantity of recognized key relevance to the localization and progression of major cardiovascular diseases, such as atherosclerosis, and set the stage for future studies involving pulsatile flows.Publication Graphene Hydrate: Theoretical Prediction of a New Insulating Form of Graphene(IOP Publishing, 2010) Wang, Wei; Kaxiras, EfthimiosUsing first-principles calculations, we show that the formation of carbohydrates directly from carbon and water is energetically favored when graphene is subjected to an unequal chemical environment across the two sides, with a difference in the chemical potential of protons and hydroxyl groups. The resultant carbohydrate structure is two-dimensional (2D), with the hydrogen atoms exclusively attached on one side of the graphene and the hydroxyl groups on the other side, the latter forming a herringbone reconstruction that optimizes hydrogen bonding. We show that graphene undergoes a metal–insulator transition upon hydration that is readily detectable from the significant shift in the vibration spectrum. The hydrate form of graphene offers new applications for graphene in electronics, either deposited on a substrate or in solution.Publication A Flexible High-Performance Lattice Boltzmann GPU Code for the Simulations of Fluid Flows in Complex Geometries(Wiley-Blackwell, 2010) Bernaschi, Massimo; Fatica, Massimiliano; Melchionna, Simone; Succi, Sauro; Kaxiras, EfthimiosWe describe the porting of the Lattice Boltzmann component of MUPHY, a multi-physics/scale simulation software, to multiple graphics processing units using the Compute Unified Device Architecture. The novelty of this work is the development of ad hoc techniques for optimizing the indirect addressing that MUPHY uses for efficient simulations of irregular domains.Publication Embrittlement of Metal by Solute Segregation-Induced Amorphization(American Physical Society, 2010) Chen, Hsiu-Pin; Kalia, Rajiv K.; Kaxiras, Efthimios; Lu, Gang; Nakano, Aiichiro; Nomura, Ken-ichi; van Duin, Adri C. T.; Vashishta, Priya; Yuan, ZaoshiImpurities segregated to grain boundaries of a material essentially alter its fracture behavior. A prime example is sulfur segregation-induced embrittlement of nickel, where an observed relation between sulfur-induced amorphization of grain boundaries and embrittlement remains unexplained. Here, \(48×10^6\)-atom reactive-force-field molecular dynamics simulations provide the missing link. Namely, an order-of-magnitude reduction of grain-boundary shear strength due to amorphization, combined with tensile-strength reduction, allows the crack tip to always find an easy propagation path.Publication Is the Nature of Magnetic Order in Copper-Oxides and in Iron-Pnictides Different?(Elsevier, 2010) Manousakis, Efstratios; Ren, Jun; Meng, Sheng; Kaxiras, EfthimiosWe use the results of first-principles electronic structure calculations and a strong coupling perturbation approach, together with general theoretical arguments, to illustrate the differences in super-exchange interactions between the copper-oxides and iron-pnictides. We provide a possible explanation for the two magnetic ground states within the same theoretical foundation. Contrary to the emerging view that magnetic order in the iron-pnictides is of itinerant nature, we argue that the observed magnetic moment is small because of frustration introduced by the electrons of the Fe orbitals as they compete to impose their preferred magnetic ordering.