Research

Below are listed some of my research interests. A complete list of my publications is available here.

Molecular self-assembly of like-charged molecules

While F4TCNQ molecules are known to form cohesive 3D solids, the intermolecular interactions that are attractive for F4TCNQ in 3D are repulsive in 2D. Therefore experimental observation of cohesive molecular behavior for F4TCNQ on graphene is unexpected,

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This self-assembly behavior can be explained by a novel solid formation mechanism that occurs when charged molecules are placed in a poorly screened environment. As negatively charged molecules coalesce, the local work function increases, causing electrons to flow into the coalescing molecular island and increase its cohesive binding energy.

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Iron-based superconductors

We show that electron-phonon coupling can induce strong electron pairing in an FeSe monolayer on a SrTiO3 substrate (experimental indications for superconducting Tc are between 65 and 109 K). The role of the SrTiO3 substrate in increasing the coupling is two-fold. First, the interaction of the FeSe and TiO2 terminated face of SrTiO3 prevents the FeSe monolayer from undergoing a shear-type (orthorhombic, nematic) structural phase transition.

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Second, the substrate allows an anti-ferromagnetic ground state of FeSe which opens electron-phonon coupling channels within the monolayer that are prevented by symmetry in the non-magnetic phase. The spectral function for the electron-phonon coupling (\alpha^2 F) in our calculations agrees well with inelastic tunneling data.

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Automated construction of Wannier functions

Maximally localized Wannier functions are widely used in electronic structure theory for analyses of bonding, electric polarization, orbital magnetization, and for interpolation. The state of the art method for their construction is based on the method of Marzari and Vanderbilt. One of the practical difficulties of this method is guessing functions (initial projections) that approximate the final Wannier functions. We found an approach based on optimized projection functions (OPF) that can construct maximally localized Wannier functions without a guess.

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Solid nanocrystal moving through a constriction in the carbon nanotube

Under the application of electrical currents, metal nanocrystals inside carbon nanotubes can be bodily transported. We examine how an iron nanocrystal can pass through a constriction in the carbon nanotube with a smaller cross-sectional area than the nanocrystal itself. Remarkably we find that, while passing through a constriction, the nanocrystal remains largely solid and crystalline and the carbon nanotube is unaffected. We account for this behavior by a pattern of iron atom motion and rearrangement on the surface of the nanocrystal.

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Animations:

Tuning electronic structure of graphene

We studied theoretically the Raman spectrum of the rotated double-layer graphene, consisting of two graphene layers rotated with respect to each other by an arbitrary angle \theta. The Raman 2D peak position, intensity, and width show a complicated dependence on the angle \theta. We account for all of these effects, including dependence on the incoming photon energy, in good agreement with the experimental data.

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In addition, we investigated the influence of SiO2 , Au, Ag, Cu, and Pt substrates on the Raman spectrum of graphene. Experiments reveal particularly strong modifications to the Raman signal of graphene on platinum, compared to that of suspended graphene. The modifications strongly depend on the relative orientation of the graphene and platinum lattices. These observations are theoretically investigated and shown to originate from hybridization of electronic states in graphene and d orbitals in platinum. It is expected that, quite generally, hybridization between graphene and any material with d orbitals near the Fermi level will result in an imprint on the graphene Dirac cone, which depends sensitively on the relative orientation of the respective lattices.

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Geometric quantities in the electronic structure context

Electron wavefunctions in a periodic solid are labeled by a wavevector k which lives on a torus (Brillouin zone). Only recently it has been realized that there are some specific properties of periodic solids that can be calculated from the knowledge of relative phases (A) of electron wavefunctions between neighboring points on a torus. These are the: electronic polarization (1993), the anomalous Hall conductivity (2002), and the Chern-Simons component of orbital magnetoelectric coupling (2008).

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Figure above shows how one can calculate these quantities by integrating relative phase A in various ways.

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Recently we performed a systematic representation-theory-based search for the simplest structures allowing isotropic Chern-Simons magnetoelectric coupling. We find 44 such structures, all sharing a common pattern of atomic displacements in the direction of atomic magnetic moments. We focus on one of these canonical structures and find that it is generically realized in a class of fractionally substituted pyrochlore compounds with an all-in-all-out magnetic order.

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Isosymmetric transition in epitaxially strained perovskites

Using first-principles density functional theory calculations, we discover anomalously large bi-axial strain-induced reorientation of the axis about which the oxygen octahedral framework rotates in orthorhombic perovskites with tendency towards rhombohedral symmetry. The transition between crystallographically equivalent (isosymmetric) structures with different octahedral rotation magnitudes originates from both the strong strain-octahedral rotation coupling available to perovskites and the energetic hierarchy among competing octahedral tilt patterns. By elucidating these criteria, we suggest many functional perovskites would exhibit the transition in thin film form, thus offering a new landscape in which to tailor highly anisotropic electronic responses.

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Figure above shows how the energy landscape topology changes with epitaxial strain for a system we analyzed.

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Si-compatible high-K dielectrics among Pbnm perovskites

In a collaboration with experimental groups we analyze both experimentally (where possible) and theoretically from first-principles the dielectric tensor components and crystal structure of five classes of Pbnm perovskites. All of these materials are believed to be stable on silicon and are therefore promising candidates for high-K dielectrics.

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Figure above shows photographs of single crystals various Si-compatible Pbnm perovskites while the graphs show calculated (red) and measured (blue, where available) structural and dielectric properties of systems we analyzed.

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Polarization of a Chern insulator

We have extended the Berry-phase concept of polarization to insulators having a non-zero value of the Chern invariant. The generalization to such Chern (anomalous Hall) insulators requires special care because of the partial occupation of chiral edge states. We show how the integrated bulk current arising from an adiabatic evolution can be related to a difference of bulk polarizations. We also show how the surface charge can be related to the bulk polarization, but only with a knowledge of the wavevector at which the occupancy of the edge state is discontinuous.

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Figure above on the left shows how for two different choices of Brillouin zone one would naively expect to get a different value of polarization in a Chern insulator. Figure on the right shows population of edge states in a Chern insulator after an adiabatic evolution.

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Structure of SiO2 cristobalite

Among the phases of SiO2 are \alpha- and \beta-cristobalites, which have a long and somewhat controversial history of proposed structural assignments and phase-transition mechanisms. Motivated by experimental result by Zhang and Scott (2007) we use first-principles calculations to investigate the energy, structure, and local stability of \alpha and \beta structures.

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We argue that the various \alpha and \beta enantiomorphs can be grouped into three clusters, each of which is identified with a three-dimensional manifold of structures of P2_12_12_1 symmetry in which the \alpha and \beta appear as higher-symmetry special cases. We find that there are relatively high energy barriers between manifolds, but low barriers within a manifold. Exploring the energy landscape within one of these manifolds, we find a minimal-energy path connecting \alpha and \beta structures with a surprisingly low barrier of 5 meV per formula unit. This landscape is indicated in the figure above and the minimal energy path within that landscape is shown in the following video.

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Animation:

Video of the minimal energy path in \alpha- and \beta-cristobalites