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Transcript
Computational Alchemy @Condensed Matter Theory group: Whisperers at Nano-world Carl Xiangyuan CUI 29/3/2007@tsp class Outline: General Introduction 1. Condensed Matter Physics 2. Nano-sciences 3. Computational Materials Physics 4. Electronic Structure and Density Functional Theory Case studies @cmt: 1. Magnetic semiconductors 2. Native defects in semiconductors 3. Superhard materials J. Slater Condensed Matter Physics Historically, “condensed matter physics” grew out of “solid state physics”, which is now considered one of its main subfields. The term "condensed matter physics" was apparently coined by Philip Anderson when he renamed his research group at Princeton – previously "solid-state theory" to “condensed matter theory” in 1967. In general, condensed phases include solids and liquids, More exotic condensed phases such as superfluid, and the Bose-Einstein Condensate, etc. Philip Anderson Noble Prize Laureate in Physics, 1977 Condensed matter physics is by far the largest field of contemporary physics. Condensed matter physics has a large overlap with chemistry, materials science, nanotechnology and engineering. Nano- “ There’s plenty of room at the bottom ” --- Richard Feynman (1951, Caltech) “… Computers with wires no wider than 100 atoms, a microscope that could view individual atoms, machines that could manipulate atoms 1 by 1, and circuits involving quantized energy levels or the interaction of quantized spins …..” Richard Feynman Nobel Prize Laureate In Physics, 1965 IDEA: To characterize and manipulate individual atoms and molecules WHY: Scaling issues would arise from the changing magnitude of various physical phenomena: gravity would become less important, surface tension and Van der Waals attraction would become more important, etc. Nano-sciences and Nano-technology Nano(meter): 10-9 m. “Quantum Size Effect” where the electronic properties of solids are altered with great reductions in particle size. This effect does not come into play by going from macro to micro dimensions. However, it becomes dominant when the nanometer size range is reached. Small IS Different, and consequently enabling unique applications, for examples: • Opaque substances become transparent (copper); • Inert materials become catalysts (platinum, gold); • Stable materials turn combustible (aluminum); • Solids turn into liquids at room temperature (gold); • Insulators become conductors (silicon). Much of the fascination with nanotechnology stems from these unique quantum and surface phenomena. Computational materials physics (1) Macroscopic properties normally have their microscopic origins; Microscopic physics is generally governed by Quantum Mechanics and Statistical Physics; Despite of their own beauty, unfortunately, both are mathematically very complicated. Quantum mechanics at work Let the computer to solve those equations Computational materials physics (3) Experiment Theory Computational Modelling (perform experiments on computers) Computational materials physics significantly decreases the barrier to apply “theory” to “experiment” Electronic structure and Density functional theory Electronic Structure: Distribution of the electron in spatial space (electron/charge/spin density, etc) energetic space (density of state, bandstructure, etc) Density Functional Theory (DFT): To replace the many-body electronic wavefunction with the electron density as the basic quantity. “Self-consistent Equations including Exchange and Correlation Effects” W. Kohn and L. J. Sham, Phys. Rev. 140, A1133 (1965) Walter Kohn, Nobel Prize in Chemistry, 1998 DFT is among the most popular and versatile methods available in condensed matter physics and electronic materials. Self-Consistent Field Computational Materials Science: Computational Physics Approach to the Electronic Structure of Real Materials Simulation / Modeling Assisted materials design Electronic Structural Superhard materials Mechanical Aerospace Semiconductor electronics Electronic Structure Origin of all properties Optical Information Storage Automotive Permanent Magnets Magnetic Photovoltaics Spintronics Materials from the mind rather than from the mines Some soft-drink beforehand… “The nation that controls magnetism will control the universe!" By the cartoon character, detective Dick Tracy ca. 1940 “One shouldn’t work on semiconductors, that is a filthy mess; who knows whether any semiconductors exist.” by Wolfgang Pauli, ca 1935 Nobel Prize in Physics, 1945 “Semiconductor + impurity” is a “Beauty + the beast” –like combination. What is the Spin? 1. In addition to their mass and electric charge, electrons have an intrinsic quantity of angular momentum called spin, almost as if they were tiny spinning balls. 2. Associated with the spin is a magnetic field like that of a tiny bar magnet lined up with the spin axis. 3. Spin shown as a vector (“up” up or “down”). down 4. In a magnetic field, field electrons with spin “up" and "down" have different energies. energies 5. In an ordinary electric circuit the spins are oriented at random and have no effect on current flow. 6. Spintronic devices create spin-polarized currents and use spin to control current flow. What is Spintronics (or transport spin electronics) • semiconductor for data processing utilizing charge • magnetic material for data storage and recording information utilizing spin. Diluted Magnetic Semiconductor: H. Ohno, Science 281, 951 (1998) One, two, and three Mn atoms in GaN Why a poly-doping investigation? 1 Mn Magnetic, (or assumed to be ferromagnetic) magnetic moment 4μB/Mn 2 Mn Ferromagnetic, (assumed to be isolated pair-Mn) magnetic moment ~ 4μB/Mn 3 Mn Ferrimagnetic magnetic moment ~ 1μB/Mn Spin Density in 96-atom-cell More is Different! “Poly-doping” is necessary! Superhard Materials Applications: Blades; grinding, polishing tools; Impact-resistant coatings; Mining and Petrochemical industry. •Research Goals: •Synthesis superhard materials, although the possibility of producing a bulk materials with hardness exceeding diamond is dubious; •To this end, interfaces between nanocrystalline regions and thin amorphous layers could present a new direction in this field. •To fabricate a superhard materials with other desirable properties (low cost, chemical inertness and thermal stability, etc) Titanium Nitride based coating Interface structures “Model system” – 50-60 GPa for nc-TiN/a-Si3N4 Favoured Oxygen sites DFT calculations reveal that O impurities reduce strength. N vacancies in InN (clustering are favored) structure charge density electron localization function In – In weak bonds form; the metallic character, supports the experimental findings In vacancy in InN (also clustering) N - N bond lengths are 10-23% longer than that of N2 Interstitial Ni _ [1120] (octahedral) O (tetrahedral) T Heroes behind-the-screen: supercomputers Australian Centre for Advanced Computing and Communications (AC3). @Technology Park, Redfern, Sydney. Australian Partnership for Advanced Computing (APAC) National Facility @ANU, Canberra. Carl III-V DMS Aloysius Cu-based Catalysis Xiangmei InN defects Damien Nanowires Mira Lattice Gas Hamiltonian Marco CeO2 Cathy “The Boss” Elvis CeO2 Robyn O/Au(111) Key Collaborators (left to right): Matthias Scheffler (FHI, Berlin) Art Freeman (Northwestern U, Illinois) Bernard Delley (PSI, Switzerland) Chris Van der Walle (UCSB, California) Summary With the advent of supercomputers, computational materials physics, has become an essential tool for electronic materials science. It has revolutionized scientists' approach to the electronic structure of atoms, molecules and solid materials in physics, chemistry and materials science. Where are we now? ---need more re-search-ing.