Download Computational Alchemy @Condensed Matter Theory group

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.