Warped Space May Create Escher-like Universe

A new theoretical model posted onto the ArXiv website by Stephen Hawking and colleagues proposes something very curious: that space may actually reflect a curious structure that has more in common with the artistic style of M.C. Escher than the smooth fabric that is often taken as the cornerstone of general relativity. As described in New Scientist (registration required):

The images in question are tessellations, arrangements of repeated shapes, such as the images of interlocking bats and angels seen in?Circle Limit IV. Although these are flat, they serve as "projections" of an alternative geometry called hyperbolic space, rather like a flat map of the world is a projection of a globe. For example, although the bats in the flat projection appear to shrink at an exponential rate at the edges, in hyperbolic space they are all the same size. These distortions in the projection arise because hyperbolic space cannot lie flat. Instead, it resembles a twisting, wiggly landscape of saddle-like hills.

Hawking - together with University of California's James Hartle - have been working on an approach to create a quantum picture of cosmology since the 1980's, and the Escher-like structure fell out of that. The problem with their approach was that the expansion of the universe seems to indicate that the universe has a positive cosmological constant, and that makes their equations unstable and fairly useless.

However, the team (also consisting of Thomas Hertog, from the Institute for Theoretical Physics at Catholic University of Leuven in Belgium) has recently realized that when a negative cosmological constant is put into the wave functions of the universe using their models, it turns out that the model can evolve over time to be very similar to a form of string theory developed by Juan Maldacena in 1997. This famous "Maldacena conjecture" stunningly linked the holographic principle to string theory and is seen by many as having the potential to translate string theory into a format that yields more direct insights, because it links a string theory model together with a quantum physics model and helps reduce the complexities arising from the extra dimensions. (Here is a link to the ArXiv copy of Maldacena's 1997 paper, for those interested in the more technical aspects.)

Hawking, Hertog, and Hartle certainly aren't done with their work ... in many ways, it creates more questions than it solves. That is, if the idea even works at all!

Already, physicist Lubos Motl has indicated on his blog that he doesn't have particularly high regard for the possibilities of this work being decisive and, in fact, it looks like possibly the calculations have been negated in his own comment thread! (Although the Hawking paper on ArXiv has been edited since then, so it's possible that the sign error he's talking about has been fixed and the paper is still there.)

The truth is that these models are sometimes introduced and get a bit of fanfare, but usually they don't really end up going anywhere ... or, if they do go somewhere, it takes years for them to get there. This model is so new that there's very little discussion of it yet, and it's through that discussion that scientists will help to determine whether or not it has any value. At present, I'm suspect, though ... and I'd urge caution from anyone before they begin declaring that our universe has some bizarre geometric structure that isn't experimentally verified!

Still, if the model could be extended and refined to match experimental data, and if it then makes predictions that could be further tested, it might represent some true progress at creating a theory that combines quantum physics with general relativity. And, if that works, then we'll have discovered that, once again, the universe really doesn't look the way that we expect it to ... and that would be a pretty neat discovery.

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Straintronics: Engineers create piezoelectric graphene

Graphene is a wonder material. It is one-hundred-times better at conducting electricity than silicon. It is stronger than diamond. And, at just one atom thick, it is so thin as to be essentially a two-dimensional material. Such promising physics have made graphene the most studied substance of the last decade, particularly in nanotechnology. In 2010, the researchers who first isolated it shared the Nobel Prize.

Yet, while graphene is many things, it is not piezoelectric. Piezoelectricity is the property of some materials to produce electric charge when bent, squeezed or twisted. Perhaps more importantly, piezoelectricity is reversible. When an electric field is applied, piezoelectric materials change shape, yielding a remarkable level of engineering control.

Piezoelectrics have found application in countless devices from watches, radios and ultrasound to the push-button starters on propane grills, but these uses all require relatively large, three-dimensional quantities of piezoelectric materials.

Now, in a paper published in the journal ACS Nano, two materials engineers at Stanford have described how they have engineered piezoelectrics into graphene, extending for the first time such fine physical control to the nanoscale.

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Listen to Stanford engineers Evan Reed and Mitchell Ong discuss their piezoelectric graphene. Credit: ACS Nano

Straintronics

"The physical deformations we can create are directly proportional to the electrical field applied and this represents a fundamentally new way to control electronics at the nanoscale," said Evan Reed, head of the Materials Computation and Theory Group at Stanford and senior author of the study. "This phenomenon brings new dimension to the concept of 'straintronics' for the way the electrical field strains — or deforms — the lattice of carbon, causing it to change shape in predictable ways."

"Piezoelectric graphene could provide an unparalleled degree of electrical, optical or mechanical control for applications ranging from touchscreens to nanoscale transistors," said Mitchell Ong, a post-doctoral scholar in Reed's lab and first author of the paper.

Using a sophisticated modeling application running on high-performance supercomputers, the engineers simulated the deposition of atoms on one side of a graphene lattice — a process known as doping — and measured the piezoelectric effect.

They modeled graphene doped with lithium, hydrogen, potassium and fluorine, as well as combinations of hydrogen and fluorine and lithium and fluorine on either side of the lattice. Doping just one side of the graphene, or doping both sides with different atoms, is key to the process as it breaks graphene's perfect physical symmetry, which otherwise cancels the piezoelectric effect.

The results surprised both engineers.

"We thought the piezoelectric effect would be present, but relatively small. Yet, we were able to achieve piezoelectric levels comparable to traditional three-dimensional materials," said Reed. "It was pretty significant."

Designer piezoelectricity

"We were further able to fine tune the effect by pattern doping the graphene—selectively placing atoms in specific sections and not others," said Ong. "We call it designer piezoelectricity because it allows us to strategically control where, when and how much the graphene is deformed by an applied electrical field with promising implications for engineering."

While the results in creating piezoelectric graphene are encouraging, the researchers believe that their technique might further be used to engineer piezoelectricity in nanotubes and other nanomaterials with applications ranging from electronics, photonics, and energy harvesting to chemical sensing and high-frequency acoustics.

"We're already looking now at new piezoelectric devices based on other 2D and low-dimensional materials hoping they might open new and dramatic possibilities in nanotechnology," said Reed.

Provided by Stanford School of Engineering

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Multiple groups claim to create first atom-thick silicon sheets

Smaller means faster in electronics. Conventional electronic devices based on silicon are being miniaturized, but they start to malfunction as they approach the limits of the single atom scale. Consequently, manufacturers need to find new solutions for faster electronics in the coming years. Because silicene and graphene are essentially two-dimensional they can work at the single atom level.

"This is why silicene is so important. It might take you to that ultimate [size] limit," said Lok C. Lew Yan Voon, a nanomaterial expert from Wright State University in Ohio who coined the term "silicene" in 2007.

According to Lew Yan Voon, electronic devices based on silicene could reliably exhibit the critical on-off function required for transistors, the building blocks of computers. Graphene, however, has struggled to achieve this function, stymieing its practical use as a transistor.

"The electronics industry is fighting against carbon," said Guy Le Lay, a silicene researcher from Aix-Marseille University in France. "To change to graphene, in principle, would be very nice, but it's very complicated."

Because electronics manufacturers have built an infrastructure around silicon, they remain hesitant to fully embrace graphene as the basis for future super-fast processors. Silicene, however, presents an attractive prospect, according to its proponents.

In a forthcoming research paper, Le Lay and his colleagues claim to be the first group to have clear proof of synthesized silicene sheets. The work will appear in Physical Review Letters.

Le Lay hopes that this research will segue into transistors soon.

"The aim is to have a demonstration of a device in two and a half years," said Le Lay.

The advantage silicene transistors would have over their graphene counterparts stems from silicene's unique structure. In silicene, a few silicon atoms exist above and below the main two-dimensional surface and electrons in these two regions have distinct energies. Applying electrical voltages enables electrons to jump across this energy gap and allows silicene to transition between an "on" and "off" state.

Le Lay's team claims to have created the first sheets that pave the way for such applications. But Le Lay and his colleagues have several competitors.

Research groups from China and Japan recently unveiled similar results, and another team announced the synthesis of silicene in an Applied Physics Letters paper published in late 2010. The latter group, however, didn't have enough proof to make that claim, said Le Lay. Le Lay added that the Japanese group does not have as much evidence as his group.

"[Le Lay] wants to own the fatherhood of silicene like it's his thing, but actually it's not," said Abdelkader Kara, a theoretical physicist specializing in silicene at the University of Central Florida in Orlando and coauthor of the 2010 research paper.

Although Kara's group claimed to have synthesized silicene in 2010, they only used one detection method to prove their claim: scanning tunneling microscope images. The device relies on quantum-mechanical effects to produce images on the atomic scale, with greater detail than images from traditional microscopes.

Such images, however, can be deceiving. According to the French team behind the newer research, the 2010 results didn't conclusively indicate that silicene was formed. Instead, something subtle was cropping up in the images, Le Lay's group claims.

Most silicene researchers have been attempting to grow silicene on top of silver because silver atoms and silicon atoms don't interact strongly. While this allows the silicene to form independently from the silver substrate, it can be difficult to differentiate between genuine silicene sheets and silver structures, said Le Lay.

"There's something tricky with the silver surface. The silver surface can mimic the honeycomb surface that looks like silicene," said Patrick Vogt, a silicene researcher at Technical University of Berlin and lead author of the forthcoming research done with Guy Le Lay. "The real silicene structure looks different."

Kara counters that they had sufficient proof for silicene synthesis back in 2010 based on how the silicon atoms aligned themselves at an angle with the silver substrate. The microscope images revealed a honeycomb structure that the silver surface could not have formed alone, said Kara.

"[Scanning tunneling microscope] images of course can be deceiving," said Kara. "We did a very careful job of looking at the silicon, and that's why the paper was accepted [for scientific publication]."

Nonetheless, Le Lay and Vogt weren't satisfied. To ensure that they had truly created silicene, Vogt's research group analyzed their sample in a number of ways. They measured electrical and chemical properties in addition to comparing actual images with simulated theoretical predictions. Proving the synthesis of silicene requires converging evidence from these different sources, said Le Lay.

Also, the team found that the observed distance between the silicon atoms matched theoretical predictions better than the results from 2010.

Kara agrees that Le Lay's team took an important step forward in silicene research, but he believes that they don't deserve too much credit for silicene's discovery.

Kara added that the credit for pioneering experimental silicene research should go to his colleagues, Bernard Aufray and Hamid Oughaddou, who worked on the 2010 paper.

Wright State's Lew Yan Voon, who wasn't affiliated with either Kara's or Le Lay's research, acknowledges that there were some discrepancies between the 2010 paper and theoretical predictions. Consequently, it remains unclear who synthesized silicene first, said Lew Yan Voon.

"The positive note is that more and more groups are reporting [synthesis of silicene]," said Lew Yan Voon. "There was a time when people said you couldn't even create it."

Despite the uncertainty over who created silicene first, researchers agree what needs to be done next. To take full advantage of silicene's properties, physicists need to grow it on an insulating material that won't conduct electricity like silver. Once silicene can be grown on an insulator, it will be much easier to develop silicene transistors and other devices.

Scientists may develop silicene devices that dramatically increase processing speed relatively soon, but large challenges remain, according to Le Lay.

"From this to applications is another big step. It's not trivial," said Le Lay.

Source: Inside Science News Service (news : web)

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