Spiraling laser pulses could change graphene

Thu, 05/28/2015 - 11:46am

SLAC National Accelerator Laboratory

This illustration depicts the structure of graphene, which consists of a single layer of carbon atoms arranged in a honeycomb pattern. A new simulation suggests that spiraling pulses of polarized laser light could change graphene's nature, turning it from a metal to an insulator. Led by researchers at SLAC and Stanford, the study paves the way for experiments that create and control new states of matter with this specialized form of light. Image: AlexanderAlUS via Wikimedia Commons

A new study predicts that researchers could use spiraling pulses of laser light to change the nature of graphene, turning it from a metal into an insulator and giving it other peculiar properties that might be used to encode information.

The results, published in Nature Communications, pave the way for experiments that create and control new states of matter with this specialized form of light, with potential applications in computing and other areas.

“It’s as if we’re taking a piece of clay and turning it into gold, and when the laser pulse goes away the gold goes back to clay,” said Thomas Devereaux, a professor at the U.S. Dept. of Energy (DOE)’s SLAC National Accelerator Laboratory and director of the Stanford Institute for Materials and Energy Sciences (SIMES), a joint SLAC/Stanford institute.

“But in this case,“ he said, “our simulations show that we could theoretically change the electronic properties of the graphene, flipping it back and forth from a metallic state, where electrons flow freely, to an insulating state. In digital terms this is like flipping between zero and one, on and off, yes and no; it can be used to encode information in a computer memory, for instance. What makes this cool and interesting is that you could make electronic switches with light instead of electrons.”

Devereaux led the study with Michael Sentef, who began the work as a postdoctoral researcher at SLAC and is now at the Max Planck Institute for the Structure and Dynamics of Matter in Germany.

Tweaking a wonder material

Graphene is a pure form of carbon just one atom thick, with its atoms arranged in a honeycomb pattern. Celebrated as a wonder material since its discovery 12 years ago, it’s flexible, nearly transparent, a superb conductor of heat and electricity and one of the strongest materials known. But despite many attempts, scientists have not found a way to turn it into a semiconductor—the material at the heart of microelectronics.

An earlier study demonstrated that it might be possible to take a step in that direction by hitting a material with circularly polarized light—light that spirals either clockwise or counterclockwise as it travels, a quality that can also be described as right- or left-handedness. This would create a “band gap,” a range of energies that electrons cannot occupy, which is one of the hallmarks of a semiconductor.

In the SIMES study, theorists used the DOE’s National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory to perform large-scale simulations of an experiment in which graphene is hit with circularly polarized pulses a few millionths of a billionth of a second long.

Getting as close to real as possible

“Previous studies were based on analytical calculations and on idealized situations,” said Martin Claassen, a Stanford graduate student in Devereaux’s group who made key contributions to the study. “This one tried to simulate what happens in as close to real experimental conditions as you can get, right down to the shape of the laser pulses. Doing such a simulation can tell you what types of experiments are feasible and identify regions where you might find the most interesting changes in those experiments.”

The simulations show that the handedness of the laser light would interact with a slight handedness in the graphene, which is not entirely uniform. This interaction leads to interesting and unexpected properties, said SLAC staff scientist and study co-author Brian Moritz. Not only does it produce a band gap, but it also induces a quantum state in which the graphene has a so-called “Chern number” of either one or zero, which results from a phenomenon known as Berry curvature and offers another on/off state that scientists might be able to exploit.

Insights go beyond graphene

While this study does not immediately open ways to make electronic devices, it does give researchers fundamental insights that advance the science in that direction. The results are also relevant to materials called dichalcogenides, which are also two-dimensional sheets of atoms arranged in a honeycomb structure.

Dichalcogenides are the focus of intense research at SIMES and around the world because of their potential for creating “valleytronic” devices. In valleytronics, electrons move through a two-dimensional semiconductor as a wave with two energy valleys whose characteristics can be used to encode information. Possible applications include light detectors, low-energy computer logic and data storage chips and quantum computing. In addition to the work on graphene, members of the research team have also been simulating experiments involving the interaction of light with dichalcogenides.

“Ultimately,” Moritz said, “we’re trying to understand how interaction with light can alter a material’s character and properties to create something that’s both new and interesting from a technological point of view.”

Source: SLAC National Accelerator Laboratory

Recycling nuclear waste

Fri, 05/29/2015 - 11:12am

Kate McAlpine, Univ. of Michigan

The simulation of the reactor core confirms that the dead zones allow the reactor to operate safely. This image shows where atoms split, or fission. The fuel rods run vertically, with the red, high-fission fuel regions and blue, low-fission dead zones. Image: Seker et al, Univ. of Michigan

The simulation of the reactor core confirms that the dead zones allow the reactor to operate safely. This image shows where atoms split, or fission. The fuel rods run vertically, with the red, high-fission fuel regions and blue, low-fission dead zones. Image: Seker et al, Univ. of Michigan

An advanced nuclear reactor under development by Hitachi could help solve the nuclear waste problem, and Univ. of Michigan researchers were involved in verifying its safe performance through computer simulations.

The U-M team worked with colleagues at the Massachusetts Institute of Technology and the Univ. of California, Berkeley. After more safety analysis, Hitachi plans to move forward with a prototype of the "resource-renewable boiling water reactor" in the next few years.

One of the major technological hurdles for nuclear energy is developing systems to dispose of the waste produced by typical reactors. It must be sealed away for hundreds of millennia while the radioactivity naturally decreases.

Hitachi's new design would burn off the longest-lived radioactive materials, called transuranics, shortening that isolation period to a few centuries. This would recycle the nuclear waste to produce yet more energy and reduce the amount that must be stowed away.

"Because of transuranics, we're talking about lifetimes for storing fuel that we can't even fathom," said Thomas Downar, U-M professor of nuclear engineering and radiological sciences. "You get this down to a hundred years, then you're talking about the ability to engineer a container that you have confidence will last that long."

In the conventional boiling water reactors that currently produce about 30 percent of all the nuclear-generated electricity in the U.S., the neutrons that split uranium atoms have been slowed by the boiling water. In contrast, the Hitachi design uses fast neutrons since they are more likely to split, or fission, transuranic atoms.

Prototype fast reactors have been running since the 1970s, but they use a sodium coolant. Sodium burns when it comes into contact with air and reacts violently with water. This is one of the reasons why U.S. utilities that operate reactors have been hesitant to consider sodium-cooled designs.

A water-cooled fast reactor, though, could offer safer and more familiar operation. The challenge was designing a water-cooled core that would stop itself if it started overheating and the water turned to steam. In conventional reactors, the water's slowing action acts as a failsafe because steam is less effective at decelerating neutrons. Since fewer neutrons are at the right speed to cause fissions, the reaction rate slows down too.

For a boiling water reactor that's burning transuranics, this scenario is trickier. The faster neutrons could mean a faster fission rate, creating more heat, steam and fast neutrons.

"If something goes wrong and the power increases, you want to have the fission rate decrease," Downar said.

To create this safety feature in their reactor, Hitachi engineers plan large dead zones in the fuel rods, made of materials with a much lower probability of fissioning with fast neutrons.

Hitachi calculated that as the presence of steam reduced the density of the water, fast neutrons were likely to travel further. By keeping the active regions of the fuel assembly small, more neutrons would be lost to these "blanket" regions in an overheating scenario, slowing the fission rate.

Before beginning the expensive process of prototyping, Hitachi wanted to confirm with outside experts that the design would perform as expected.

With funding from the Department of Energy, members of Downar's group spent the last three years developing codes that could simulate the more complex layout and physics of Hitachi's reactor core design. For example, uranium fission reactions are reasonably steady and easy to predict, but transuranic reactions are irregular and difficult to calculate accurately.

The U-M team developed a method to generate data that simulates the way transuranics burn. They then applied this data to established codes currently used for boiling water reactor analysis. By looking at what happened when the steam bubbles appeared, the team found that the fast neutrons tended to leave the reactive part of the fuel assembly, slowing the reaction rate as planned.

Now, the university teams are about to begin a careful comparison of their methods with the predictions from the Hitachi computer codes to discover any differences in the simulation of the advanced reactor's performance. Hitachi will fund the teams at U-M, MIT and Berkeley for the next phases of the project.

Source: Universty of Michigan