We have developed a method using a scanning tunneling microscope (STM) probe tip to control the chain polymerization of diacetylene compounds in a self-ordered layer, thereby creating conjugated polydiacetylene nanowires. When a small amount of phthalocyanine was deposited on a molecular layer of diacetylene compound, we found adsorbed and stabilized phthalocyanine single molecules. The initiation of chain polymerization on the diacetylene molecular row to which the single phthalocyanine molecule...
16h
A persistent spin helix (PSH) is a robust helical spin-density pattern arising in disordered 2D electron gases with Rashba α and Dresselhaus β spin-orbit (SO) tuned couplings, i.e., α=±β. Here, we investigate the emergence of a persistent Skyrmion lattice (PSL) resulting from the coherent superposition of PSHs along orthogonal directions-crossed PSHs-in wells with two occupied subbands ν=1, 2. For realistic GaAs wells, we show that the Rashba α_{ν} and Dresselhaus β_{ν} couplings can be simultaneously...
16h
We identify a class of novel low energy phases of the hydrogen storage material LiAlH4 by using the ab initio minima hopping crystal structure prediction method. These phases are, unlike previous predictions and known structures of similar materials, characterized by polymeric networks consisting of Al atoms interlinked with H atoms. The most stable structure is a layered ionic crystal with P21/c symmetry, and it has lower free energy than the previously reported structure over a wide range of temperatures....
16h
A systematic ab initio search for low-enthalpy phases of disilane (Si2H6) at high pressures was performed based on the minima hopping method. We found a novel metallic phase of disilane with Cmcm symmetry, which is enthalpically more favorable than the recently proposed structures of disilane up to 280 GPa, but revealing compositional instability below 190 GPa. The Cmcm phase has a moderate electron-phonon coupling yielding a superconducting transition temperature T-c of around 20 K at 100 GPa, decreasing...
16h
Through a systematic structural search we found an allotrope of carbon with Cmmm symmetry which we predict to be more stable than graphite for pressures above 10 GPa. This material, which we refer to as Z-carbon, is formed by pure sp(3) bonds and it provides an explanation to several features in experimental x-ray diffraction and Raman spectra of graphite under pressure. The transition from graphite to Z-carbon can occur through simple sliding and buckling of graphene sheets. Our calculations predict...
16h
Using the minima hopping global geometry optimization method on the density functional potential energy surface we show that the energy landscape of boron clusters is glasslike. Larger boron clusters have many structures which are lower in energy than the cages. This is in contrast to carbon and boron nitride systems which can be clearly identified as structure seekers. The differences in the potential energy landscape explain why carbon and boron nitride systems are found in nature whereas pure...
16h
Systematic ab initio structure prediction was applied for the first time to predict low energy surface reconstructions by employing the minima hopping method on the alpha-boron (111) surface. Novel reconstruction geometries were identified and carefully characterized in terms of structural and electronic properties. Our calculations predict the formation of a planar, monolayer sheet at the surface, which is responsible for conductive surface states. Furthermore, the isolated boron sheet is shown...
16h
Based on a recently introduced metric for measuring distances between configurations, we introduce distance-energy (DE) plots to characterize the potential energy surface of clusters. Producing such plots is computationally feasible on the density functional level since it requires only a few hundred stable low energy configurations including the global minimum. By using standard criteria based on disconnectivity graphs and the dynamics of Lennard-Jones clusters, we show that the DE plots convey...
16h
Low-energy structures of alanates are currently known to be described by patterns of isolated, nearly ideal tetrahedral [AlH4] anions and metal cations. We discover that the novel polymeric motif recently proposed for LiAlH4 plays a dominant role in a series of alanates, including LiAlH4, NaAlH4, KAlH4, Mg(AlH4)(2), Ca(AlH4)(2), and Sr(AlH4)(2). In particular, most of the low-energy structures discovered for the whole series are characterized by networks of corner-sharing [AlH6] octahedra, forming...
17h
We present state-of-the-art first-principles calculations of the electronic and optical properties of silicon allotropes with interesting characteristics for applications in thin-film solar cells. These new phases consist of distorted sp(3) silicon networks and have a lower formation energy than other experimentally produced silicon phases. Some of these structures turned out to have quasidirect and dipole-allowed band gaps in the range 0.8-1.5 eV, and to display absorption coefficients comparable...
20h
Raman spectroscopy is a powerful tool to study the intrinsic vibrational characteristics of crystals, and, therefore, it is an adequate technique to explore phase transitions of carbon under pressure. However, the diamond-anvil cell, which is used in experiments to apply pressure, appears as a broad intense feature in the spectra. This feature lies, unfortunately, in the same range as the principal modes of recently proposed sp(3) carbon structures. As these modes are hard to distinguish from the...
20h
We show by means of first-principles calculations that in boron nanostructures a large variety of two-dimensional structures can be obtained, all with similar energetic properties. Some of these new structures are more stable than both the B-80 fullerenes initially proposed by Szwacki et al. [Phys. Rev. Lett. 98, 166804 (2007)] and boron nanotubes. At variance from other systems like carbon, disordered configurations are energetically comparable with ordered ones. Cage-like structures that are not...
20h
Density functional and quantum Monte Carlo calculations challenge the existence of a unique ground-state structure for certain Si clusters. For Si clusters with more than a dozen atoms the lowest ten isomers are close in energy and for some clusters entropic effects change the energetic ordering of the configurations. Isotope pure configurations with rotational symmetry and symmetric configurations containing one additional isotope are disfavored by these effects. Comparisons with experiment are...
20h
We study the thermodynamic stability at low temperatures of a series of alkali-metal-zinc double-cation borohydrides, including LiZn(BH4)(3), LiZn2(BH4)(5), NaZn(BH4)(3), NaZn2(BH4)(5), KZn(BH4)(3), and KZn2(BH4)(5). While LiZn2(BH4)(5), NaZn(BH4)(3), NaZn2(BH4)(5), and KZn(BH4)(3) were recently synthesized, LiZn(BH4)(3) and KZn2(BH4)(5) are hypothetical compounds. Using the minima-hopping method, we discover two new lowest-energy structures for NaZn(BH4)(3) and KZn2(BH4)(5) which belong to the C2/c...
20h
We performed a systematic structural search of high-pressure carbon allotropes for unit cells containing from 6 to 24 atoms using the minima hopping method. We discovered a series of new structures that are consistently lower in enthalpy than the ones previously reported. Most of these include (5 + 7)- or (4 + 8)-membered rings and can therefore be placed in the families proposed by H. Niu et al. [Phys. Rev. Lett. 108, 135501 (2012)]. However, we also found three more families with competitive enthalpies...
20h
Electrostatic interactions between the conducting tip of a scanning probe microscope and a flat conductor coated with a thin or thick dielectric layer are treated analytically and numerically. Exact and compact approximate expressions for the capacitance, force, force gradient, electric field profiles, and their effective widths are derived for a spherical model tip by generalizing known solutions for the conducting sphere and sample problem. These expressions allow convenient modeling of various...
20h
We present modifications for the method recently developed by Granot and Baer [J. Chem. Phys. 128, 184111 (2008)]. These modifications significantly enhance the efficiency and reliability of the method. In addition, we discuss some specific features of this method. These features provide important flexibilities which are crucial for a double-ended saddle point search method in order to be applicable to complex reaction mechanisms. Furthermore, it is discussed under what circumstances this methods...
20h
We show that molecular dynamics based moves in the minima hopping method are more efficient than saddle point crossing moves. For binary systems we incorporate identity exchange moves in a way that allows one to avoid the generation of high energy configurations. Using this modified minima hopping method, we re-examine the binary Lennard-Jones benchmark system with up to 100 atoms and we find a large number of new putative global minima.
20h
A structure prediction method is presented based on the minima hopping method. To escape local minima, moves on the configurational enthalpy surface are performed by variable cell shape molecular dynamics. To optimize the escape steps the initial atomic and cell velocities are aligned to low curvature directions of the current local minimum. The method is applied to both silicon crystals and well-studied binary Lennard-Jones mixtures. For the latter new putative ground state structures are presented....
20h
We present a method that gives highly accurate electrostatic potentials for systems where we have periodic boundary conditions in two spatial directions but free boundary conditions in the third direction. These boundary conditions are needed for all kinds of surface problems. Our method has an O(N log N) computational cost, where N is the number of grid points, with a very small prefactor. This Poisson solver is primarily intended for real space methods where the charge density and the potential...
20h
We demonstrate that Daubechies wavelets can be used to construct a minimal set of optimized localized adaptively contracted basis functions in which the Kohn-Sham orbitals can be represented with an arbitrarily high, controllable precision. Ground state energies and the forces acting on the ions can be calculated in this basis with the same accuracy as if they were calculated directly in a Daubechies wavelets basis, provided that the amplitude of these adaptively contracted basis functions is sufficiently...
20h
According to previous interpretations of experimental data, sodium-scandium double-cation borohydride NaSc(BH4)(4) crystallizes in the crystallographic space group Cmcm where each sodium (scandium) atom is surrounded by six scandium (sodium) atoms. A careful investigation of this phase based on ab initio calculations indicates that the structure is dynamically unstable and gives rise to an energetically and dynamically more favorable phase with C222(1) symmetry and nearly identical x-ray diffraction...
20h
In order to characterize molecular structures we introduce configurational fingerprint vectors which are counterparts of quantities used experimentally to identify structures. The Euclidean distance between the configurational fingerprint vectors satisfies the properties of a metric and can therefore safely be used to measure dissimilarities between configurations in the high dimensional configuration space. In particular we show that these metrics are a perfect and computationally cheap replacement...
20h
By adding a nonlinear core correction to the well established dual space Gaussian type pseudopotentials for the chemical elements up to the third period, we construct improved pseudopotentials for the Perdew-Burke-Ernzerhof [J. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 77, 3865 (1996)] functional and demonstrate that they exhibit excellent accuracy. Our benchmarks for the G2-1 test set show average atomization energy errors of only half a kcal/mol. The pseudopotentials also remain highly...
20h
We re-examine the question of whether the geometrical ground state of neutral and ionized clusters are identical. Using a well defined criterion for being "identical" together, the extensive sampling methods on a potential energy surface calculated by density functional theory, we show that the ground states are in general different. This behavior is to be expected whenever there are metastable configurations which are close in energy to the ground state, but it disagrees with previous studies.
20h
We compare evolutionary algorithms with minima hopping for global optimization in the field of cluster structure prediction. We introduce a new average offspring recombination operator and compare it with previously used operators. Minima hopping is improved with a softening method and a stronger feedback mechanism. Test systems are atomic clusters with Lennard-Jones interaction as well as silicon and gold clusters described by force fields. The improved minima hopping is found to be well-suited...
20h
Daubechies wavelets are a powerful systematic basis set for electronic structure calculations because they are orthogonal and localized both in real and Fourier space. We describe in detail how this basis set can be used to obtain a highly efficient and accurate method for density functional electronic structure calculations. An implementation of this method is available in the ABINIT free software package. This code shows high systematic convergence properties, very good performances, and an excellent...
20h
We present a fast and accurate method to calculate the electrostatic energy and forces of interacting particles with the boundary conditions appropriate to surfaces, i.e., periodic in the two directions parallel to the surface and free in the perpendicular direction. In the spirit of the Ewald method, the problem is divided into a short range and a long range part. The charge density responsible for the long range part is represented by plane waves in the periodic directions and by finite elements...
20h
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