Publications

2020
Zhu Z, Carr S, Massatt D, Luskin M, Kaxiras E. Twisted trilayer graphene: A precisely tunable platform for correlated electrons. Physical review letters. 2020;125 (11) :116404.
Larson DT, Chen W, Torrisi SB, Coulter J, Fang S, Kaxiras E. Effects of structural distortions on the electronic structure of T-type transition metal dichalcogenides. Physical Review B. 2020;102 (4) :045128.
Rhone TD, Chen W, Desai S, Torrisi SB, Larson DT, Yacoby A, Kaxiras E. Data-driven studies of magnetic two-dimensional materials. Scientific reports. 2020;10 (1) :1–11.
Larson DT, Carr S, Tritsaris GA, Kaxiras E. Effects of lithium intercalation in twisted bilayer graphene. Physical Review B. 2020;101 (7) :075407.
Lu Z, Carr S, Larson DT, Kaxiras E. Lithium intercalation in MoS2 bilayers and implications for moiré flat bands. Physical Review B. 2020;102 (12) :125424.
Chaves AS, Antonelli A, Larson DT, Kaxiras E. Boosting the efficiency of ab initio electron-phonon coupling calculations through dual interpolation. Physical Review B. 2020;102 (12) :125116.
2019
Carr S, Fang S, Zhu Z, Kaxiras E. Exact continuum model for low-energy electronic states of twisted bilayer graphene. Physical Review Research. 2019;1 (1) :013001.
Tritsaris GA, Şensoy MG, Shirodkar SN, Kaxiras E. First-principles study of coupled effect of ripplocations and S-vacancies in MoS2. Journal of Applied Physics. 2019;126 (8) :084303.
Defo RK, Kaxiras E, Richardson SL. How carbon vacancies can affect the properties of group IV color centers in diamond: A study of thermodynamics and kinetics. Journal of Applied Physics. 2019;126 (19) :195103.
Klein DR, MacNeill D, Song Q, Larson DT, Fang S, Xu M, Ribeiro RA, Canfield PC, Kaxiras E, Comin R, et al. Enhancement of interlayer exchange in an ultrathin two-dimensional magnet. Nature Physics. 2019;15 (12) :1255–1260.
Neofotistos GN, M.Mattheakis, Barbaris G, Hitzanidi J, Tsironis GP, Kaxiras E. Machine learning with observers predicts complex spatiotemporal behavior. Front. Phys. - Quantum Computing. 2019;7 (24) :1-9. Publisher's VersionAbstract
Chimeras and branching are two archetypical complex phenomena that appear in many physical systems; because of their different intrinsic dynamics, they delineate opposite non-trivial limits in the complexity of wave motion and present severe challenges in predicting chaotic and singular behavior in extended physical systems. We report on the long-term forecasting capability of Long Short-Term Memory (LSTM) and reservoir computing (RC) recurrent neural networks, when they are applied to the spatiotemporal evolution of turbulent chimeras in simulated arrays of coupled superconducting quantum interference devices (SQUIDs) or lasers, and branching in the electronic flow of two-dimensional graphene with random potential. We propose a new method in which we assign one LSTM network to each system node except for {\textquotedblleft}observer{\textquotedblright} nodes which provide continual {\textquotedblleft}ground truth{\textquotedblright} measurements as input; we refer to this method as {\textquotedblleft}Observer LSTM{\textquotedblright} (OLSTM). Wedemonstrate that even a small number of observers greatly improves the data-driven (model-free) long-term forecasting capability of the LSTM networks and provide the framework for a consistent comparison between the RC and LSTM methods. We find that RC requires smaller training datasets than OLSTMs, but the latter require fewer observers. Both methods are benchmarked against Feed-Forward neural networks (FNNs), also trained to make predictions with observers (OFNNs).
Maier M, M.Mattheakis, Kaxiras E, Luskin M, Margetis D. Homogenization of plasmonic crystals: Seeking the epsilon-near-zero behavior. Proceedings of the Royal Society A. 2019;475 (2230). Publisher's VersionAbstract
By using an asymptotic analysis and numerical simulations, we derive and investigate a system of homogenized Maxwell{\textquoteright}s equations for conducting material sheets that are periodically arranged and embedded in a heterogeneous and anisotropic dielectric host.\  This structure is motivated by the need to design plasmonic crystals that enable the propagation of electromagnetic waves with no phase delay (epsilon-near-zero effect). Our microscopic model incorporates the surface conductivity of the two-dimensional (2D) material of each sheet and a corresponding line charge density through a line conductivity along possible edges of the sheets. Our analysis generalizes averaging principles inherent in previous Bloch-wave approaches. We investigate physical implications of our findings. In particular, we emphasize the role of the vector-valued corrector field, which expresses microscopic modes of surface waves on the 2D material. By using a Drude model for the surface conductivity of the sheet, we construct a Lorentzian function that describes the effective dielectric permittivity tensor of the plasmonic crystal as a function of frequency.
2018_homogenization_1809.08276.pdf
Choukroun J, Pala M, Fang S, Kaxiras E, Dollfus P. High performance tunnel field effect transistors based on in-plane transition metal dichalcogenide heterojunctions. NANOTECHNOLOGY. 2019;30 (2).Abstract
In-plane heterojunction tunnel field effect transistors based on monolayer transition metal dichalcogenides are studied by means of self-consistent non-equilibrium Green's functions simulations and an atomistic tight-binding Hamiltonian. We start by comparing several heterojunctions before focusing on the most promising ones, i.e. WTe2-MoS2 and MoTe2-MoS2. The scalability of those devices as a function of channel length is studied, and the influence of backgate voltages on device performance is analyzed. Our results indicate that, by fine-tuning the design parameters, those devices can yield extremely low subthreshold swings (<5 mV/decade) and I-ON/I-OFF ratios higher than 10(8) at a supply voltage of 0.3 V, making them ideal for ultra-low power consumption.
Ma Q, Xu S-Y, Shen H, MacNeill D, Fatemi V, Chang T-rong, Valdivia AMM, Wu S, Du Z, Hsu C-H, et al. Observation of the nonlinear Hall effect under time-reversal-symmetric conditions. NATURE. 2019;565 (7739) :337+.Abstract
The electrical Hall effect is the production, upon the application of an electric field, of a transverse voltage under an out-of-plane magnetic field. Studies of the Hall effect have led to important breakthroughs, including the discoveries of Berry curvature and topological Chern invariants(1,2). The internal magnetization of magnets means that the electrical Hall effect can occur in the absence of an external magnetic field(2); this `anomalous' Hall effect is important for the study of quantum magnets(2-7). The electrical Hall effect has rarely been studied in non-magnetic materials without external magnetic fields, owing to the constraint of timer-eversal symmetry. However, only in the linear response regime-when the Hall voltage is linearly proportional to the external electric field-does the Hall effect identically vanish as a result of time-reversal symmetry; the Hall effect in the nonlinear response regime is not subject to such symmetry constraints(8-10). Here we report observations of the nonlinear Hall effect(10) in electrical transport in bilayers of the non-magnetic quantum material WTe2 under time-reversal-symmetric conditions. We show that an electric current in bilayer WTe2 leads to a nonlinear Hall voltage in the absence of a magnetic field. The properties of this nonlinear Hall effect are distinct from those of the anomalous Hall effect in metals: the nonlinear Hall effect results in a quadratic, rather than linear, current-voltage characteristic and, in contrast to the anomalous Hall effect, the nonlinear Hall effect results in a much larger transverse than longitudinal voltage response, leading to a nonlinear Hall angle (the angle between the total voltage response and the applied electric field) of nearly 90 degrees. We further show that the nonlinear Hall effect provides a direct measure of the dipole moment(10) of the Berry curvature, which arises from layer-polarized Dirac fermions in bilayer WTe2. Our results demonstrate a new type of Hall effect and provide a way of detecting Berry curvature in nonmagnetic quantum materials.
2018
Hoyt RA, Montemore MM, Sykes ECH, Kaxiras E. Anhydrous Methanol and Ethanol Dehydrogenation at Cu(111) Step Edges. JOURNAL OF PHYSICAL CHEMISTRY C. 2018;122 (38) :21952-21962.Abstract
Oxidative methanol dehydrogenation is a major industrial reaction with global formaldehyde production exceeding 30 million tonnes per year. Unfortunately, oxidative dehydrogenation produces water aldehyde mixtures that require subsequent distillation. Anhydrous alcohol dehydrogenation is a promising alternative that produces H-2 instead of water. Pursuant to recent experimental work showing that highly stepped Cu(111) surfaces exhibit anhydrous dehydrogenation activity, we present first-principles density functional theory calculations for methanol and ethanol dehydrogenation at Cu(111) step edges to provide an atomistic understanding of the catalytic mechanism; these sites stabilize all intermediates while reducing activation energies. We find that van der Waals contributions to the energy account for more than 50% of adsorption energies, and essential in achieving good agreement with experimental desorption temperatures. Furthermore, vibrational zero-point energy corrections significantly reduce the activation energy for all reaction steps considered here. Hydrogen bonding among ethanol intermediates at step edges is weakened by geometric frustration. These insights lead us to propose several suggestions for further research on undercoordinated Cu sites as anhydrous alcohol dehydrogenation catalysts.
You J-S, Fang S, Xu S-Y, Kaxiras E, Low T. Berry curvature dipole current in the transition metal dichalcogenides family. PHYSICAL REVIEW B. 2018;98 (12).Abstract
We study the quantum nonlinear Hall effect in two-dimensional (2D) materials with time-reversal symmetry. When only one mirror line exists, a transverse charge current occurs in the second-order response to an external electric field, as a result of the Berry curvature dipole in momentum space. Candidate 2D materials to observe this effect are two-dimensional transition metal dichalcogenides (TMDCs). First, we use an ab initio based tight-binding approach to demonstrate that monolayer T-d-structure TMDCs exhibit a finite Berry curvature dipole. In the 1H and 1T' phase of TMDCs, we show the emergence of a finite Berry curvature dipole with the application of strain and an electrical displacement field, respectively.
Chen W, Cubuk ED, Montemore MM, Reece C, Madix RJ, Friend CM, Kaxiras E. A Comparative Ab Initio Study of Anhydrous Dehydrogenation of Linear-Chain Alcohols on Cu(110). JOURNAL OF PHYSICAL CHEMISTRY C. 2018;122 (14) :7806-7815.Abstract
The catalytic behavior of Cu surfaces in the anhydrous production of aldehydes from alcohols, a process of industrial significance, is puzzling: the two simplest alcohols (methanol and ethanol) show dramatically different decomposition behavior on Cu. Here, we study the thermodynamic and kinetic processes involved in the anhydrous dehydrogenation of linear-chain alcohols including methanol, ethanol, 1-propanol, and 1-butanol on the Cu(110) surface using multiscale approaches. First, we obtain the adsorption structures and energies of the reaction intermediates, in which van der Waals (vdW) interactions play a crucial role. Then, we determine the kinetic barriers for the two dehydrogenation steps, namely, the O-H and the subsequent C-H bond-breaking on Cu. The reaction of methoxy-to-formaldehyde has a rather high-energy transition state, in contrast to that of alkoxide-to-aldehyde in the longer-chain systems. This difference qualitatively explains the lower production efficiency of formaldehyde on Cu. Finally, we simulate the production rates of aldehydes based on which we optimize reaction conditions and propose possible avenues for enhancing the production of anhydrous formaldehyde using Cu-based catalysts.
Bakhta A, Cances E, Cazeaux P, Fang S, Kaxiras E. Compression of Wannier functions into Gaussian-type orbitals. COMPUTER PHYSICS COMMUNICATIONS. 2018;230 :27-37.Abstract
We propose a greedy algorithm for the compression of Wannier functions into Gaussian-polynomials orbitals. The so-obtained compressed Wannier functions can be stored in a very compact form, and can be used to efficiently parameterize effective tight-binding Hamiltonians for multilayer 2D materials for instance. The compression method preserves the symmetries (if any) of the original Wannier function. We provide algorithmic details, and illustrate the performance of our implementation on several examples, including graphene, hexagonal boron-nitride, single-layer FeSe, and bulk silicon in the diamond cubic structure. (C) 2018 Elsevier B.V. All rights reserved.
Cao Y, Fatemi V, Demir A, Fang S, Tomarken SL, Luo JY, Sanchez-Yamagishi JD, Watanabe K, Taniguchi T, Kaxiras E, et al. Correlated insulator behaviour at half-filling in magic-angle graphene superlattices. NATURE. 2018;556 (7699) :80+.Abstract
A van der Waals heterostructure is a type of metamaterial that consists of vertically stacked two-dimensional building blocks held together by the van der Waals forces between the layers. This design means that the properties of van der Waals heterostructures can be engineered precisely, even more so than those of two-dimensional materials(1). One such property is the `twist' angle between different layers in the heterostructure. This angle has a crucial role in the electronic properties of van der Waals heterostructures, but does not have a direct analogue in other types of heterostructure, such as semiconductors grown using molecular beam epitaxy. For small twist angles, the moire pattern that is produced by the lattice misorientation between the two-dimensional layers creates long-range modulation of the stacking order. So far, studies of the effects of the twist angle in van der Waals heterostructures have concentrated mostly on heterostructures consisting of monolayer graphene on top of hexagonal boron nitride, which exhibit relatively weak interlayer interaction owing to the large bandgap in hexagonal boron nitride(2-5). Here we study a heterostructure consisting of bilayer graphene, in which the two graphene layers are twisted relative to each other by a certain angle. We show experimentally that, as predicted theoretically(6), when this angle is close to the `magic' angle the electronic band structure near zero Fermi energy becomes flat, owing to strong interlayer coupling. These flat bands exhibit insulating states at half-filling, which are not expected in the absence of correlations between electrons. We show that these correlated states at half-filling are consistent with Mott-like insulator states, which can arise from electrons being localized in the superlattice that is induced by the moire pattern. These properties of magic-angle-twisted bilayer graphene heterostructures suggest that these materials could be used to study other exotic many-body quantum phases in two dimensions in the absence of a magnetic field. The accessibility of the flat bands through electrical tunability and the bandwidth tunability through the twist angle could pave the way towards more exotic correlated systems, such as unconventional superconductors and quantum spin liquids.
Kolesov G, Kaxiras E, Manousakis E. Density functional theory beyond the Born-Oppenheimer approximation: Accurate treatment of the ionic zero-point motion. PHYSICAL REVIEW B. 2018;98 (19).Abstract
We introduce a method to carry out zero-temperature calculations within density functional theory (DFT) but without relying on the Born-Oppenheimer (BO) approximation for the ionic motion. Our approach is based on the finite-temperature many-body path-integral formulation of quantum mechanics by taking the zero-temperature limit and treating the imaginary-time propagation of the electronic variables in the context of DFT. This goes beyond the familiar BO approximation and is limited from being an exact treatment of both electrons and ions only by the approximations involved in the DFT component. We test our method in two simple molecules, H-2 and benzene. We demonstrate that the method produces a difference from the results of the BO approximation which is significant for many physical systems, especially those containing light atoms such as hydrogen; in these cases, we find that the fluctuations of the distance from its equilibrium position, due to the zero-point motion, is comparable to the interatomic distances. The method is suitable for use with conventional condensed-matter approaches and currently is implemented on top of the periodic pseudopotential code SIESTA.

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