'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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The above story is reprinted from materials provided by University College London - UCL, via AlphaGalileo.

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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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New lightning-fast, efficient nanoscale data transmission

ScienceDaily (Nov. 15, 2011) — A team at Stanford's School of Engineering has demonstrated an ultrafast nanoscale light-emitting diode (LED) that is orders of magnitude lower in power consumption than today's laser-based systems and is able to transmit data at the very rapid rate of 10 billion bits per second. The researchers say it is a major step forward in providing a practical ultrafast, low-power light source for on-chip data transmission.

Stanford's Jelena Vuckovic, an associate professor of electrical engineering, and Gary Shambat, a doctoral candidate in electrical engineering, announced their device in a research paper in the journal Nature Communications.

Vuckovic had earlier this year produced a nanoscale laser that was similarly efficient and fast, but that device operated only at temperatures below 150 degrees Kelvin, about minus-190 degrees Fahrenheit, making it impractical for commercial use. The new device operates at room temperature and could, therefore, represent an important step toward next-generation computer chips.

"Low-power, electrically controlled light sources are vital for next-generation optical systems to meet the growing energy demands of the computer industry," said Vuckovic. "This moves us in that direction significantly."

Single-mode light

The LED in question is a "single-mode LED," a special type of diode that emits light more or less at a single wavelength, similarly to a laser.

"Traditionally, engineers have thought only lasers can communicate at high data rates and ultralow power," said Shambat. "Our nanophotonic, single-mode LED can perform all the same tasks as lasers, but at much lower power."

Nanophotonics is key to the technology. In the heart of their device, the engineers have inserted little islands of the light-emitting material indium arsenide, which, when pulsed with electricity, produce light. These "quantum dots" are surrounded by photonic crystal -- an array of tiny holes etched in a semiconductor. The photonic crystal serves as a mirror that bounces the light toward the center of the device, confining it inside the LED and forcing it to resonate at a single frequency.

"In other words, it becomes single-mode," said Shambat.

"Without these nanophotonic ingredients -- the quantum dots and the photonic crystal -- it is impossible to make an LED efficient, single-mode and fast all at the same time," said Vuckovic.

Engineering ingenuity

The new device includes a bit of engineering ingenuity, too. Existing devices are actually two devices, a laser coupled with an external modulator. Both devices require electricity. Vuckovic's diode combines light transmission and modulation functions into one device, drastically reducing energy consumption.

In tech-speak, the new LED device transmits data, on average, at 0.25 femto-joules per bit of data. By comparison, today's typical "low" power laser device requires about 500 femto-joules to transmit the same bit.

"Our device is some 2,000 times more energy efficient than best devices in use today," said Vuckovic.

Stanford Professor James S. Harris, former PhD student Bryan Ellis and doctoral candidates Arka Majumdar, Jan Petykiewicz and Tomas Sarmiento also contributed to this research.

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The above story is reprinted from materials provided by Stanford School of Engineering. The original article was written by Andrew Myers.

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Nanoscale spin waves can replace microwaves

ScienceDaily (Sep. 10, 2011) — A group of scientists from the University of Gothenburg and the Royal Institute of Technology (KTH), Sweden, have become the first group in the world to demonstrate that theories about nanoscale spin waves agree with observations. This opens the way to replacing microwave technology in many applications, such as mobile phones and wireless networks, by components that are much smaller, cheaper, and that require less resources. The study has been published in the scientific journal Nature Nanotechnology.

"We have been in competition with two other research groups to be the first to confirm experimentally theoretical predictions that were first made nearly 10 years ago. We have been successful due to our method for constructing magnetic nanocontacts and due to the special microscope at our collaborators' laboratory at the University of Perugia in Italy," says Professor Johan Åkerman of the Department of Physics, University of Gothenburg, where he is head of the Applied Spintronics group.

The aim of the research project, which started two years ago, has been to demonstrate the propagation of spin waves from magnetic nanocontacts. Last autumn, the group was able to demonstrate the existence of spin waves with the aid of electrical measurements, and the results were published in the scientific journal Physical Review Letters.

The research group has used one of the three advanced spin wave microscopes in the world, at the university in the Italian town of Perugia, to visualise the motion. The microscope makes it possible to see the dynamic properties of components with a resolution of approximately 250 nanometre.

The results have opened the way for a new field of research known as "magnonics," using nanoscale magnetic waves.

"I believe that our results will signal the start of a rapid development of magnonic components and circuits. What is particularly exciting is that these components are powered by simple direct current, which is then converted into spin waves in the microwave region. The frequency of these waves can be directly controlled by the current. This will make completely new functions possible," says Johan Åkerman, who is looking forward to exciting developments in the next few years.

Its magneto-optical and metallic properties mean that magnonic technology can be integrated with traditional microwave-based electronic circuits, and this will make completely untried combinations of the technologies possible. Magnonic components are much more suitable for miniaturisation than traditional microwave technology.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Gothenburg. The original article was written by Anita Fors.

Journal Reference:

M. Madami, S. Bonetti, G. Consolo, S. Tacchi, G. Carlotti, G. Gubbiotti, F. B. Mancoff, M. A. Yar, J. Åkerman. Direct observation of a propagating spin wave induced by spin-transfer torque. Nature Nanotechnology, 2011; DOI: 10.1038/nnano.2011.140

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