Theory aims to describe fundamental properties of materials

Gold is shiny, diamonds are transparent, and iron is magnetic. Why is that? The answer lies with a material ’s electronic structure, which determines its electrical, optical, and magnetic properties.

Predicting a material’s properties by first calculating its electronic structure would cut down experimental time and might lead researchers to uncover new materials with unexpected benefits.

But commonly used simulations are inaccurate, especially for materials like silicon, whose strongly correlated electrons influence each other over a distance and make simple calculations difficult.

Now a team of researchers at Sandia National Laboratories may have a solution that offers huge potential. Sergey Faleev and his colleagues applied theoretical innovations and novel algorithms to make a hard-to-use theoretical approach from 1965 amenable to computation. The team ’s approach may open the door to discovering new phases of matter, creating new materials, or optimizing performance of compounds and devices such as alloys and solar cells.

Their paper, “Quasiparticle Self-Consistent GW Theory, ” appeared in the June 9, 2006, issue of Physical Review Letters. GW refers to Lars Hedin ’s 1965 theory that elegantly predicts electronic energy for ground and excited states of materials. “G ” stands for the Greens function — used to derive potential and kinetic energy — and “W ” is the screened Coulomb interaction, which represents electrostatic force acting on the electrons. “Quasiparticles ” are a concept used to describe particle-like behavior in a complex system of interacting particles. Self-consistent means the particle ’s motion and effective field, which determine each other, are iteratively solved, coming closer and closer to a solution until the result stops changing.

“Our code has no approximation except GW itself, ” said Faleev. “It ’s considered to be the most accurate of all GW implementations to date. ”

“It works well for everything in the periodic table, ” adds coauthor Mark van Schilfgaarde, a former Sandian now at Arizona State University. The paper reports results for diverse materials whose properties cannot be consistently predicted by any other theory. The 32 examples include alkali metals, semiconductors, wide band-gap insulators, transition metals, transition metal oxides, magnetic insulators, and rare earth compounds.

Describing force

“Everything in solids is held together by electrostatic forces, ” says van Schilfgaarde. “You can think of this as a huge dance with an astronomically large number of particles, 1023, that is essentially impossible to solve. The raw interactions among the particles are remarkably complex.

“Hedin replaced the raw interactions with ‘dressing ’ the particle with a screened interaction, ” van Schilfgaarde continues, “so the effective charge is much smaller. It becomes much more tractable but the equations become more complicated — you have an infinite number of an infinite number of terms. The hope is that the higher-order terms die out quickly. ”

The researchers ’ use of GW makes the expansion much more rapidly convergent.

“We ’re pretty confident we got the approach right, ” he says. He now would like another group to independently verify this way of framing the task.

Promise and challenges ahead

The researchers use a molecular dynamics code, VASP (Vienna Ab-initio Simulation Package) to model, for example, equations of state in high-energy-density matter. These equations of state depend on quantities like electrical conductivity. Calculating this requires detailed knowledge of the electronic structure — a perfect application for Faleev ’s work. The researchers hope to describe optical spectra, calculate total energy, and account for more than 10 atoms in a unit cell — at 100 times the current speed.

Accelerating the code would facilitate modeling in other research areas at Sandia, such as simulating titanium dioxide used in surface science, or aiding research into carbon nanotubes that might be used in electronic or optical devices.

“To calculate absorption or optical spectra is a huge problem, ” Faleev says with anticipation. “To make it faster is a huge problem. To make it more accurate is a huge problem. To incorporate VASP is a huge problem. ”

Van Schilfgaarde agrees. “It ’s quite an accomplishment to do it at all. It takes someone who is very strong in math, and a clever programmer. We spent easily five to six man-years between us to make it work.

“If we can get the approach right, we can have a theory that ’s universally accurate for anything we want — that ’s really pretty neat, just requiring knowledge of where the atoms are. ”

Van Schilfgaarde believes the theory ’s advantage would be to offer true insight into material behavior. “It ’s kind of like adding night-vision goggles to soldiers working in the dark, ” he says. “Probably in 10 years, ” adds Sergey, “everyone will use this. ”

Source: Sandia National Laboratories

Related stories:

Researchers study best way to solve wicked problems
What’s the best way to solve a wicked problem — by working in a large group sharing ideas via the intranet or as individuals? That’s the question George S. Davidson and his research team at Sandia National Laboratories attempted to resolve this summer.
University helps map the universe
The University of Manchester is developing high-speed data crunching technology that will be crucial to the success of one of the greatest scientific projects of the 21st century.
Electronic displays that fit on clothing could power revolution in lighting
A thin film of plastic which conducts electricity and produces solar power could be the basis for a revolution in the way we light our homes and design clothes.
Researchers predict a new state of matter in semiconductors
Conventional matter exists in three familiar forms-solid, liquid and gas. But under special circumstances, quantum theory predicts exotic states of matter, such as superconductors in which electrons flow with no resistance and Bose-Einstein condensates in which atoms move as a collective whole. Now, in the Dec. 15 issue of the journal Science, three Stanford physicists theorize a new state of matter that may pave the way for electronic devices that dissipate less energy and generate less heat.
It's a wrap, GDC top 12 list
It's been a long week of talking game development tools, dealing with a cold and rainy San Jose, and listening to companies saying "Wait until E3," instead of answering questions. But this correspondent persevered to bring you the top 12 tools and items uncovered at the 2006 Game Developer's Conference.
Computer Simulation, Lab Synthesis Sift Through Universe of Possible Molecules for the Best
Duke University theoretical chemists are investigating a new computer method that could help scientists identify the best molecules for drugs, electronic devices or an array of other uses. Their method would address the "daunting" fact that "that there aren't enough atoms in the universe to make all the reasonable-sized molecules that could be made," said Duke chemistry professor David Beratan.

Britain's top prizes for physics announced
Research that revealed for the first time that the Earth’s core is almost as hot as the Sun, groundbreaking new work on cosmic strings and black holes, and a scientist who believes he could build a viable desktop quantum computer by 2010, have won Britain’s most prestigious prizes for physics, the Institute of Physics Awards 2006.
Breakthrough in high-temperature superconductivity
Scientists at the University of Aberdeen have made a major breakthrough towards the mechanism of high-temperature superconductivity. Results from studies of a crystal structure of a new chemical compound containing copper and ruthenium have provided valuable insight into the mechanism of high-temperature superconductivity. The new results have shown for the first time that the mechanism of high-temperature superconductivity (when there is no resistance to the flow of electrical current) is actually coupled to the crystal lattice.

News discussion:

Physics news