arXiv subjects

Sinisa Coh

Publications and source records attributed to Sinisa Coh.

At least 37 records · Page 2Linked to original sources

arXiv2018

What can one learn about material structure given a single first-principles calculation?

We extract a variable $X$ from electron orbitals $Ψ_{n\bf{k}}$ and energies $E_{n\bf{k}}$ in the parent high-symmetry structure of a wide range of complex oxides: perovskites, rutiles, pyrochlores, and cristobalites. Even though calculation was done only in the parent structure, with no distortions, we show that $X$ dictates material's true ground state structure. We propose using Wannier functions to extract concealed variables such as $X$ both for material structure prediction and for high-throughput approaches.

cond-mat.mtrl-sci↗

arXiv2017

Real-space study of the optical absorption in alternative phases of silicon

We introduce a real-space approach to understand the relationship between optical absorption and crystal structure. We apply this approach to alternative phases of silicon, with a focus on the Si$_{20}$ crystal phase as a case study. We find that about 83% of the changes in the calculated low-energy absorption in Si$_{20}$ as compared to Si in the diamond structure can be attributed to reducing the differences between the on-site energies of the bonding and anti-bonding orbitals as well as increasing the hopping integrals for specific Si-Si bonds.

cond-mat.mtrl-sci↗

arXiv2017

Alternative structure of TiO2 with higher energy valence band edge

We propose an alternative structure of TiO2 anatase that has a higher energy oxygen p-like valence band maximum than the pristine TiO2 anatase and thus has a much better alignment with the water splitting levels. This alternative structure is unique when considering a large subspace of possible structural distortions of TiO2 anatase. We propose two routes towards this state and argue that one of them might have been realized in the recently discovered so-called black TiO2.

cond-mat.mtrl-sci↗

arXiv2017

Insulating titanium oxynitride for visible light photocatalysis

We propose a systematic approach to obtain various forms of insulating titanium oxynitrides Ti$_{n}$N$_{2}$O$_{2n-3}$ and we conduct a detailed study on its $n=2$ case, Ti$_{2}$N$_{2}$O. We study the energetics and the electronic structures of Ti$_{2}$N$_{2}$O and compare these results with those of pristine and nitrogen-doped TiO$_{2}$ within the framework of the density-functional theory (DFT) and the GW approximation. We find that Ti$_{2}$N$_{2}$O is semiconducting with the calculated band-gap of 1.81 eV, which is significantly smaller than those of pristine TiO$_{2}$ rutile (3.14 eV) or anatase (3.55 eV). Furthermore, the reduction of the band-gap of Ti$_{2}$N$_{2}$O is realized not by lowering of the conduction-band minimum but by rising the valence-band maximum. Therefore the proposed Ti$_{2}$N$_{2}$O has suitable band-edge alignment for water-splitting photocatalysis. Finally, total energy calculations indicate that Ti$_{2}$N$_{2}$O is potentially easier to synthesize than nitrogen-doped TiO$_{2}$. Based on these results, we propose Ti$_{2}$N$_{2}$O as a promising visible-light photocatalytic material.

cond-mat.mtrl-sci↗

arXiv2017

Design principles for shift current photovoltaics

While the basic principles and limitations of conventional solar cells are well understood, relatively little attention has gone toward maximizing the potential efficiency of photovoltaic devices based on shift currents. In this work, we outline simple design principles for the optimization of shift currents for frequencies near the band gap, derived from the analysis of a general effective model. The use of a novel sum rule allows us to express the band edge shift current in terms of a few model parameters and to show it depends explicitly on wavefunctions via Berry connections in addition to standard band structure. We use our approach to identify two new classes of shift current photovoltaics, ferroelectric polymer films and single-layer orthorhombic monochalcogenides such as GeS. We introduce tight-binding models for these systems, and show that they exhibit the largest shift current responsivities at the band edge reported so far. Moreover, exploring the parameter space of these models we find photoresponsivities that can exceed $100$ mA/W. Our results show how the study of the shift current via effective models allows one to improve the possible efficiency of devices based on this mechanism and better grasp their potential to compete with conventional solar cells.

cond-mat.mes-hall↗

arXiv2016

Antiferromagnetism enables electron-phonon coupling in iron-based superconductors

It is shown that a generic form of an antiferromagnetic wave function opens strong electron-phonon coupling channels in the iron-based superconductors. In the nonmagnetic state these channels exist locally on a single iron atom, but are canceled out between the two iron atoms in the primitive unit cell. Our findings are based on symmetry and the presence of an xz/yz Fermi surface near the M point and thus should be relevant for the known iron-based pnictide or chalcogenide superconductors.

cond-mat.supr-con↗

arXiv2016

Automated construction of maximally localized Wannier functions for bands with nontrivial topology

We show that an optimized projection functions method can automatically construct maximally localized Wannier functions even for bands with nontrivial topology. We demonstrate this method on a tight-binding model of a two-dimensional $\mathbb{Z}_2$ topological insulator, on a three-dimensional strong $\mathbb{Z}_2$ topological insulator, as well as on first-principles density functional theory calculated valence states of Bi$_2$Se$_3$. In all cases, the resulting Wannier functions contain large imaginary components and are more extended than those in the topologically trivial phase.

cond-mat.mtrl-sci↗

arXiv2016

Comparative study of high-Tc superconductivity in H3S and H3P

We report on a comparative study of the electronic structure, phonon spectra, and superconducting properties for recently discovered superconducting hydrides, H3S and H3P. While the electronic structures of these two materials are similar, there are notable changes in the phonon spectra and electron-phonon coupling. The low-frequency bond-bending modes are softened in H3P and their coupling to the electrons at the Fermi surface is enhanced relative to H3S. Nevertheless, coupling to the high-frequency modes is reduced so the resulting calculated superconducting transition temperature is reduced from ~166 K in H3S to ~76 K in H3P.

cond-mat.supr-con↗

arXiv2016

Proposal for a bulk material based on a monolayer FeSe on SrTiO3 high-temperature superconductor

Motivated by the high superconducting transition temperature of monolayer FeSe on SrTiO3, we propose a potential three-dimensional high-temperature superconductor superlattice FeSe-SrTiO3 and study its structural stability and electronic structure using density functional theory. We find that the binding energy between the FeSe and SrTiO3 layers is about 0.7 eV per (Fe2Se2) unit and that it saturates already within a single TiO2 atomic layer of SrTiO3. In addition we analyzed the dynamical stability of the superlattice and compared it to the case of bulk SrTiO3.

cond-mat.supr-con↗

arXiv2016

Inhibiting Klein tunneling in graphene p-n junction without an external magnetic field

We study by first-principles calculations a densely packed island of organic molecules (F4TCNQ) adsorbed on graphene. We find that with electron doping the island naturally forms a p-n junction in the graphene sheet. For example, a doping level of ~ 3 * 10^{13} electrons per cm^2 results in a p-n junction with 800 meV electrostatic potential barrier. Unlike in a conventional p-n junction in graphene, in the case of the junction formed by an adsorbed organic molecular island we expect that the Klein tunneling is inhibited, even without an applied external magnetic field. Here Klein tunneling is inhibited by the ferromagnetic order that spontaneously occurs in the molecular island upon doping. We estimate that the magnetic barrier in the graphene sheet is around 10 mT.

cond-mat.mes-hall↗

arXiv2016

Why do Collapsed Carbon Nanotubes Twist?

We study the collapsing and subsequent spontaneous twisting of a carbon nanotube by in-situ transmission electron microscopy. A custom-sized nanotube is first created in the microscope by selectively extracting shells from a parent multi-wall tube. The few-wall, large-diameter daughter nanotube is driven to collapse via mechanical stimulation, after which the ribbon-like collapsed tube spontaneously twists along its long axis. In-situ diffraction experiments fully characterize the uncollapsed and collapsed tubes. From the experimental observations and associated theoretical analysis, the origin of the twisting is determined to be compressive strain due to charge imbalance.

cond-mat.mes-hall↗

arXiv2015

Automated construction of maximally localized Wannier functions: Optimized projection functions method

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. Here we present an approach based on optimized projection functions that can construct maximally localized Wannier functions without a guess. We describe and demonstrate this approach on several realistic examples.

cond-mat.mtrl-sci↗

arXiv2015

Intrinsic surface dipole in topological insulators

We calculate the local density of states of two prototypical topological insulators (Bi$_2$Se$_3$ and Bi$_2$Te$_2$Se) as a function of distance from the surface within density functional theory. We find that, in the absence of disorder or doping, there is a 2 nm thick surface dipole the origin of which is the occupation of the topological surface states above the Dirac point. As a consequence, the bottom of the conduction band is bent upward by about 75 meV near the surface, and there is a hump-like feature associated with the top of the valence band. We expect that band bending will occur in all pristine topological insulators as long as the Fermi level does not cross the Dirac point. Our results show that topological insulators are intrinsic Schottky barrier solar cells.

cond-mat.mes-hall↗

arXiv2015

Large electron-phonon interactions from FeSe phonons in a monolayer

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 109K). 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. 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 (alpha2F) in our calculations agrees well with inelastic tunneling data.

cond-mat.supr-con↗

arXiv2015

Calculation of the specific heat of optimally K-doped BaFe$_2$As$_2$

The calculated specific heat of optimally K-doped BaFe$_2$As$_2$ in density functional theory is about five times smaller than that found in the experiment. We report that by adjusting the potential on the iron atom to be slightly more repulsive for electrons improves the calculated heat capacity as well as the structural, magnetic, and electronic properties of Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$. Applying the same correction to the antiferromagnetic state, we find that the electron-phonon coupling is strongly enhanced.

cond-mat.supr-con↗

arXiv2013

Atomically perfect torn graphene edges and their reversible reconstruction

The atomic structure of graphene edges is critical in determining the electrical, magnetic, and chemical properties of truncated graphene structures, notably nanoribbons. Unfortunately, graphene edges are typically far from ideal and suffer from atomic-scale defects, structural distortion, and unintended chemical functionalization, leading to unpredictable properties. Here we report that graphene edges fabricated by electron-beam-initiated mechanical rupture or tearing in high vacuum are clean and largely atomically perfect, oriented in either the armchair or zigzag direction. Via aberration-corrected transmission electron microscopy, we demonstrate reversible and extended pentagon-heptagon (5-7) reconstruction at zigzag edges, and explore experimentally and theoretically the dynamics of the transitions between configuration states. Good theoretical-experimental agreement is found for the flipping rates between 5-7 and 6-6 zigzag edge states. Our study demonstrates that simple ripping is remarkably effective in producing atomically clean, ideal terminations thus providing a valuable tool for realizing atomically-tailored graphene and facilitating meaningful experimental study.

cond-mat.mtrl-sci↗

arXiv2013

Theory of the Raman spectrum of rotated double-layer graphene

We study 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 θ. We find a relatively simple dependence of the Raman G peak intensity on the angle θ. On the other hand, the Raman 2D peak position, intensity, and width show a much more complicated dependence on the angle θ. We account for all of these effects, including dependence on the incoming photon energy, in good agreement with the experimental data. We find that it is sufficient to include the interaction between the graphene layers on the electronic degrees of freedom (resulting in the occurrence of van Hove singularities in the density of states). We assume that the phonon degrees of freedom are unaffected by the interaction between the layers. Furthermore, we decompose the Raman 2D peak into two components having very different linewidths; these widths are almost independent of the angle θ. The change in the intensity and the peak position of one of these two components gives insight into the dependence of the overall Raman 2D peak features as a function of the angle θ. Additionally, we show regions in the phonon spectrum giving rise to the Raman 2D peak signal. This work provides an insight into the interplay between the mechanical degree of freedom (angle θ) and the electronic degrees of freedom (singularities in the density of states) in rotated double-layer graphene. Additionally, this work provides a way to establish experimentally the value of the rotation angle θusing Raman spectroscopy measurement. This procedure becomes even more robust if one repeats the Raman spectroscopy measurement with a different incoming photon energy.

cond-mat.mes-hall↗

arXiv2013

Corner states of topological fullerenes

The unusual electronic properties of the quantum spin Hall or Chern insulator become manifest in the form of robust edge states when samples with boundaries are studied. In this work, we ask if and how the topologically non-trivial electronic structure of these two-dimensional systems can be passed on to their zero-dimensional relatives, namely fullerenes or other closed-cage molecules. To address this question, we study Haldane's honeycomb lattice model on polyhedral nano-surfaces. We find that for sufficiently large surfaces characteristic corner states appear for parameters for which the planar model displays a quantized Hall effect. In the electronic structure, these corner states show up as in-gap modes which are well separated from the quasi-continuum of states. We discuss the role of finite size effects and how the coupling between the corner states lifts the degeneracy in a characteristic way determined by the combined Berry phases which leads to an effective magnetic monopole of charge 2 at the center of the nano-surface. Experimental implications for fullerenes in the large spin-orbit regime are also pointed out.

cond-mat.mes-hall↗