322,934 research outputs found
Efficient Linear Scaling Algorithm for Tight-Binding Molecular Dynamics
No full textA novel formulation for tight binding total energy calculations and tight binding molecular dynamics, which scales linearily with the size of the system, is presented. The linear complexity allows us to treat systems of very large size and the algorithm is already faster than the best implementation of classical diagonalization for systems of 64 atoms. In addition, it is naturally parallelizable and it permits us therefore to perform molecular dynamics simulations of systems of unprecedented size. Finite electronic temperatures can also be taken into account. We illustrate this method by investigating structural and dynamical properties of solid and liquid carbon at different densities
Crystal structure prediction based on density functional theory
No full textThe atomic arrangements in solids fundamentally govern the physical properties of a material. In solid state physics, resolving the crystal structure is therefore one of the key approaches when investigating novel materials. However, experimental methods to determine the crystal structure can be very difficult, expensive, or even impossible, depending on the problem and external conditions applied to the material. Examples are high pressure experiments, where accessible pressures are limited to roughly 400 GPa, or investigations of materials with constituents that cannot be detected in X-ray diffraction experiments. Furthermore, investigating crystal structures is not only fundamental in material science, but also in chemistry, biology and pharmacy. Therefore, efficient computational methods for predicting crystal structures based solely on the system's composition would provide a powerful tool with wide scientific applications.
In 1994, Angelo Gavezzotti published an article titled ``Are Crystal Structures Predictable?'', providing simultanously the simple answer: ``no''. Meanwhile, with increasing computational resources, the situation has changed and prediction of crystal structures from first principle calculations has become feasible, while still remaining a demanding task. In 2004, the minima hopping method was developed and has there-since been successfully applied to predict structures in a wide range of non-periodic systems. In this thesis, we present an extended version of the minima hopping method for crystal structure prediction by generalizing the efficient search algorithm for finding the most stable structures within any periodic system. As applications of this approach, we investigated binary Lennard-Jones benchmark mixtures, silicon crystals, high pressure phases of carbon resulting from cold compressed graphite, superconduction phases in disilane and low energy structures in the hydrogen storage material LiAlH4
Atomistic simulations of atomic force microscopy
No full textAn important milestone in exploration of physical phenomena on the nanometer scale
was the invention of scanning tunneling microscopy (STM) in 1982 by G. Binig and H.
Rohrer. Later, using noncontact atomic force microscopy (AFM) atomic resolution has
been achieved so far on a variety of surfaces. A good understanding of the tip-apex
structures is indispensable to the scientists in the field of scanning probe microscopy.
Nowadays, this information is hardly obtained by the experiments, only atomistic
simulations are able to provide detailed insight into the tip-apex structures and also
the atomic relaxations processes induced by the tip-sample interaction. For large scale
simulations such as atomistic simulations of tip-apex structure prediction, one needs
efficient, fast but still accurate tools. We use global optimizations algorithms
together with fast and sufficiently accurate tight binding schemes to investigate
tip-apex structures. In this dissertation, recently developed methods such as P3S,
P3D, and a new Si-H tight-binding scheme are presented. These methods will be of
great help for the atomistic simulations of the atomic force microscopy. The Coulomb
interaction is dominant in ionic systems so that the accurate and efficient evaluation
of Coulomb interactions is crucial for the atomistic simulations of the ionic systems
such as alkali halides, etc
Fast and accurate electronic structure methods : large systems and applications to boron-carbon heterofullerenes
No full textThe interactions among electrons and nuclei, which are the constituents of matter, are governed by the fundamental laws of quantum mechanics. Methods that use these laws in order to determine the properties of matter are called ab-initio approaches. However, due to the enormous complexity of the equations, a straightforward solution is in general not possible, even when evaluated numerically on a computer. Consequently one has to use approximate methods. Kohn-Sham Density Functional Theory (KS-DFT) is one of the most famous approaches due to its good balance between accuracy and speed. Nevertheless the usage of this method is limited to systems containing some hundred atoms due to the cubic scaling with respect to the size of the system. Fortunately this problem can be overcome by the introduction of so-called linear scaling methods, which extend the range for which DFT calculations can be performed. Even though the basic ideas of these methods have been developed already quite a while ago, their implementation is still very challenging.
The first part of this Thesis shows in the beginning the theoretical background of DFT and linear scaling methods and describes then in detail the various steps that had to be taken in order to develop a fully functional code that can perform ab-initio calculations with a time requirement scaling only linearly with respect to the size of the system. The benchmarks done with the code demonstrate its ability to give very accurate results, its appealing speed and its excellent parallelization.
The second part of the Thesis uses an existing DFT code in order to investigate the energy landscape of boron-carbon heterofullerenes. It turned out that there exist many configurations which are much lower in energy than those known so far. Furthermore they exhibit a completely new structural motif. Whereas up to now it has been believed that the boron atoms should be isolated and distributed over the entire surface of the cluster, this new structural motif consists of configurations where the boron atoms are aggregated at one location to form a patch
Optimized Gaussian basis sets for Goedecker-Teter-Hutter pseudopotentials
No full textWe have optimized the exponents of Gaussian s and p basis functions for the elements H, B-F and Al-Cl using the pseudopotentials of (Goedecker, Teter and Hutter 1996 Phys. Rev. B 54 1703) by minimizing the total energy of dimers. We found that this procedure causes the Gaussians to be somewhat more localized than the usual procedure, where the exponents are optimized for atoms. We further found that three exponents, equal for s and p orbitals, are sufficient to reasonably describe the electronic structure of all elements that we have studied. For Li and Be results are presented for pseudopotentials of (Hartwigsen et al 1998 Phys. Rev. B 58 3641). We expect that our exponents will be useful for density functional theory studies where speed is important
Divalent Path to Enhance p-Type Conductivity in a SnO Transparent Semiconductor
No full textThe role of the divalent nature of tin is explored in tin monoxide, revealing a novel path for enhancing p-type conductivity. The consequences of oxygen off-stoichiometry indicate that a defect complex formed by a tin vacancy (V-Sn) and an impurity interstitial (D-i) leads to an increased number of free carriers as well as improved acceptor state stability when compared with the isolated V-Sn. In this study, we identify several elements that are able to stabilize such a defect complex configuration. The enhanced ionization of the resulting complex arises from the divalent nature of Sn, which allows Sn(II) and Sn(IV) oxidation states to form. Such a novel doping mechanism not only offers a path for creating a high-performance p-type transparent SnO, but reveals an as-of-yet unexplored route to improve conductivity in other compounds formed by multivalent elements, for example, Sn(II)-based thermoelectrics
Rare-earth magnetic nitride perovskites
No full textWe propose perovskite nitrides with magnetic rare-earth metals as novel materials with a range of technological applications. These materials appear to be thermodynamically stable and, in spite of possessing different crystal structures and different atomic environments, they retain the magnetic moment of the corresponding elemental rare-earth metal. We find both magnetic metals and semiconductors, with a wide range of magnetic moments and some systems posses record high magnetic anisotropy energies. Further tuning of the electronic and magnetic properties can also be expected by doping with other rare-earths or by creating solid solutions. The synthesis of these exotic materials with unusual compositions would not only extend the accepted stability domain of perovskites, but also open the way for a series of applications enabled by their rich physics
Computational acceleration of prospective dopant discovery in cuprous iodide
No full textThe zinc blende (gamma) phase of copper iodide holds the record hole conductivity for intrinsic transparent p-type semiconductors. In this work, we employ a high-throughput approach to systematically explore strategies for enhancing gamma-CuI further by impurity incorporation. Our objectives are not only to find a practical approach to increase the hole conductivity in CuI thin films, but also to explore the possibility for ambivalent doping. In total 64 chemical elements were investigated as possible substitutionals on either the copper or the iodine site. All chalcogen elements were found to display acceptor character when substituting iodine, with sulfur and selenium significantly enhancing carrier concentrations produced by the native V-Cu defects under conditions most favorable for impurity incorporation. Furthermore, eight impurities suitable for n-type doping were discovered. Unfortunately, our work also reveals that donor doping is hindered by compensating native defects, making ambipolar doping unlikely. Finally, we investigated how the presence of impurities influences the optical properties. In the majority of the interesting cases, we found no deep states in the band-gap, showing that CuI remains transparent upon doping
Diffusive author(s), cohesive author: Analysis of S/N (1994)
No full textThis study indicates the ways in which various aspects of the author(s) are brought forth in Dumb type’s performance art, the S/N production. Previous research has suggested a non-hierarchical organization of Dumb type and the absence of a “privileged author” in Dumb type’s collaborative work, S/N. However, the results that I have investigated from member’s interviews on the creative process of S/N along with my analysis of the recorded images of S/N, indicate a different aspect of the author(s). First, S/N was created through, so to speak, the collective ideas of the members of Dumb type. Further, S/N has at least nine quotations from previous performances, installations, and printed writings, besides the work-in-progress technique. Explicating one of the “author functions” as given by Michel Foucault, each text has plural subjects of the author. However, it has been revealed from members’ interviews that Teiji Furuhashi had a decision-making role in selecting the members’ ideas within the performance. Since then, S/N has had plural subjects of creation; however, Furuhashi is one of the subjects of creation along with the “privileged author.” S/N has plural authors (diffusive authors) yet at the same time, it has a “privileged author,” Teiji Furuhashi (cohesive author)
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