Subtle electronic effect in magnetite discovered: Long-standing puzzle in study of magnetism finally solved

ScienceDaily (Dec. 21, 2011) — A fundamental problem that has long puzzled scientists has been solved after more than 70 years. An international team of researchers has discovered a subtle electronic effect in magnetite, the most magnetic of all naturally occurring minerals. The effect causes a dramatic change to how this material conducts electricity at very low temperatures.

The discovery gives new insight into the mineral in which magnetism was discovered, and it may enable magnetite and similar materials to be exploited in new ways.

Ancient knowledge

Magnetite's properties have been known for more than 2000 years and gave rise to the original concepts of magnets and magnetism. The mineral has formed the basis for decades of research into magnetic recording and information storage materials.

The research was led by the University in collaboration with the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, where the experiments were conducted. Their results were published in Nature.

Unexplained behaviour

In 1939, Dutch scientist Evert Verwey discovered that the electrical conductivity of magnetite decreases abruptly and dramatically at low temperatures. At about 125 Kelvin, or minus 150 degrees Celsius, the metallic mineral turns into an insulator.

Despite many efforts, until now the reason for this transition has been debated and remained controversial.

X-ray experiment

The team of scientists fired an intense X-ray beam at a tiny crystal of magnetite at very low temperatures. Their results enabled them to understand a subtle rearrangement of the mineral's chemical structure. Electrons are trapped within groups of three iron atoms, where they can no longer transport an electrical current.

"We have solved a fundamental problem in understanding the original magnetic material, upon which everything we know about magnetism is built," said Professor Paul Attfield, Centre for Science at Extreme Conditions. "This vital insight into how magnetite is constructed and how it behaves will help in the development of future electronic and magnetic technologies."

The research was funded by the Science and Technology Facilities Council, the Engineering and Physical Sciences Research Council, and the Leverhulme Trust.

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Journal References:

Mark S. Senn, Jon P. Wright, J. Paul Attfield. Charge order and three-site distortions in the Verwey structure of magnetite. Nature, 2011; DOI: 10.1038/nature10704J. Paul Attfield. Condensed-matter physics: A fresh twist on shrinking materials. Nature, 2011; 480 (7378): 465 DOI: 10.1038/480465a

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'Graphene earns its stripes': New nanoscale electronic state discovered on graphene sheets

Researchers from the London Centre for Nanotechnology (LCN) have discovered electronic stripes, called 'charge density waves', on the surface of the graphene sheets that make up a graphitic superconductor. This is the first time these stripes have been seen on graphene, and the finding is likely to have profound implications for the exploitation of this recently discovered material, which scientists believe will play a key role in the future of nanotechnology. The discovery is reported in Nature Communications, 29th November.

Graphene is a material made up of a single sheet of carbon atoms just one atom thick, and is found in the marks made by a graphite pencil. Graphene has remarkable physical properties and therefore has great technological potential, for example, in transparent electrodes for flat screen TVs, in fast energy-efficient transistors, and in ultra-strong composite materials. Scientists are now devoting huge efforts to understand and control the properties of this material.

The LCN team donated extra electrons to a graphene surface by sliding calcium metal atoms underneath it. One would normally expect these additional electrons to spread out evenly on the graphene surface, just as oil spreads out on water. But by using an instrument known as a scanning tunneling microscope, which can image individual atoms, the researchers have found that the extra electrons arrange themselves spontaneously into nanometer-scale stripes. This unexpected behavior demonstrates that the electrons can have a life of their own which is not connected directly to the underlying atoms. The results inspire many new directions for both science and technology. For example, they suggest a new method for manipulating and encoding information, where binary zeros and ones correspond to stripes running from north to south and running from east to west respectively.

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Journal Reference:

K.C. Rahnejat, C.A. Howard, N.E. Shuttleworth, S.R. Schofield, K. Iwaya, C.F. Hirjibehedin, Ch. Renner, G. Aeppli, M. Ellerby. Charge density waves in the graphene sheets of the superconductor CaC6. Nature Communications, 2011; 2: 558 DOI: 10.1038/ncomms1574

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Planet with 2 suns discovered

The planet is similar to Saturn, but denser, and orbits the two small stars every 229 days. The smaller of the two stars is just a fifth the mass of the sun. (JPL-Caltech/NASA)

A planet that orbits two suns at the same time — just like Tatooine in Star Wars — has been found for the first time.

"The discovery is stunning," said Alan Boss, co-author of the paper describing the findings, in a statement. "Once again, what used to be science fiction has turned into reality."

Boss is a researcher at the Carnegie Institution for Science in Washington, D.C. The study was published online Thursday in Science, and NASA was to hold a news conference at 2 p.m. ET.

The research was led by Laurance Doyle, a researcher at the SETI Institute, a non-profit research group in Mountain View, Calif., that describes itself as dedicated to exploring the origin, nature and prevalence of life in the universe. His group specializes in looking for eclipsing binaries — systems of two stars that orbit each other and eclipse each other every time one passes in front of the other.

The researchers combed through data collected by the Kepler telescope, which is focused on the part of the sky containing the constellations Cygnus and Lyra and is designed to look for planets outside our solar system. It does so by precisely measuring the brightness of stars and any dimming caused by an orbiting planet passing in front of the star.

Doyle's team identified 2,000 eclipsing binaries, then carefully looked at them one at a time.

"My eye was drawn to some extra eclipses that occurred out of sequence," Doyle said in a podcast interview with Science. Some were caused by a third star — that is, the two-star system turned out to be a three star system.

But in one case, the two stars were very small — just 20 per cent and 69 per cent as massive as the sun respectively — and the extra "eclipses" dimmed the stars only very slightly, indicating that the object that caused the dips was planet-sized.

Based on careful measurements, the team figured out that the two stars orbit each other every 41 days, and the Saturn-sized planet orbits the two stars every 229 days. The planet is denser than Saturn, suggesting that it contains rock as well as gas, and its distance from the centre of its orbit is about 70 per cent of the distance between the Earth and the sun.

The planet is thought to be cold and not habitable.

Kepler was launched into space in March 2009. As of February, the telescope had found more than 1,200 "planet candidates" or data suggesting possible planets.

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