Friday, 7 March 2014

Cassini nears 100th Titan flyby with a look back

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Ten years ago, we knew Titan as a fuzzy orange ball about the size of Mercury. We knew it had a nitrogen atmosphere -- the only known world with a thick nitrogen atmosphere besides Earth. But what might lie beneath the hazy air was still just a guess.

via Science Daily

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Mystery of planet-forming disks explained by magnetism

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Astronomers say that magnetic storms in the gas orbiting young stars may explain a mystery that has persisted since before 2006. Researchers using NASA's Spitzer Space Telescope to study developing stars have had a hard time figuring out why the stars give off more infrared light than expected. The planet-forming disks that circle the young stars are heated by starlight and glow with infrared light, but Spitzer detected additional infrared light coming from an unknown source.

via Science Daily

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Public could virtually 'travel' to space for $90 through new project

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Researchers have launched a unique campaign that will enable the public to ‘travel’ to space for the cost of a pair of trainers. Virtual Ride to Space will use cutting-edge virtual technology and a specially designed spacecraft to deliver a three-dimensional, immersive experience, allowing everyone to see what astronauts experience on an ascent to space.

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Faculty Highlight: Raymond C. Ashoori

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The process of scientific discovery often results from hard work and good luck in the lab rather than from theoretical predictions, MIT physics professor Raymond C. Ashoori says.

“A lot of the physics that we do arises from serendipity and luck, and we have interesting tools to [bring to] bear on an interesting system,” Ashoori says. “Let’s see what we see. It’s very rare for me actually to say there is this particular thing I want to test and to study it. Generally, the techniques that we have are pretty different from the commonly used measurements. In a lot of cases, we just don’t know what we’re going to see.”

One technique that took Ashoori more than a decade to develop is time domain capacitance spectroscopy (TDCS), a pulse tunneling method that can send on the order of a million short electrical pulses through a material to study its electronic properties. Until recently, Ashoori's group has focused on using TDCS on semiconductor samples, but they now see an exciting possibility for work in graphene.

Semiconductor systems have been a key focus of Ashoori’s research. Ashoori’s group at MIT recently demonstrated a unique bandgap in graphene coupled to hexagonal boron nitride that could be a precursor to developing the material for functional transistors. The work was a collaboration with Pablo Jarillo-Herrero, the Mitsui Career Development Associate Professor of Physics at MIT, and other researchers in Japan and Arizona.

"In most materials, electrons behave as though they have a mass,” Ashoori says. “In graphene, they look like they don't have a mass. They look like they're behaving like photons. This is good and bad. It means electronic properties of graphene are really different. For instance, it becomes hard to make barriers to impede their flow. You can't make transistors that you can actually turn off completely, because these massless electrons can get through the barriers that you try to put in their way. But if you have an energy gap and massive electrons, then you can actually create transistors that you can turn off."

Recent research in the collaboration between Ashoori and Jarillo-Herrero showed that having a component of the applied magnetic field in the graphene plane forced electrons at the edge of graphene to move in opposite directions based on their spins.

"What we were able to have happen and to see was that at the edges of this piece of graphene, electrons with one spin move in one direction, and electrons with the opposite spin move in the opposite direction,” Ashoori says. “It's the kind of thing that you would naturally want for lots of spintronic devices. You could use them in various kinds of spin filters."

Because of the very high magnetic fields and very low temperatures used, the recent studies aren’t directly applicable to room-temperature semiconductor systems such as computers. Says postdoctoral fellow Benjamin M. Hunt, who was coauthor of papers on both the bandgap and the spin polarized electron work, “The exact system that we’re studying is probably not applicable; however, the physical principles that we are unveiling may be applicable to a system that you can study at much higher temperatures and much lower magnetic fields, something that might be attainable someday at room temperature. It’s really the physical principles that we’re interested in exposing.” Hunt notes that Jarillo-Herrero was an equal contributor to both papers.

Ashoori is also a co-principal investigator for the Center for Integrated Quantum Materials, a five-year project funded by the National Science Foundation and based at Harvard University. “With our group at MIT, we’re all thinking about very similar kinds of problems,” Ashoori says. “It’s a pretty tight fit.” (See related article.)

Technique-driven research

Ashoori’s research is often driven by novel experimental techniques. “What excites me most is seeing a new kind of signal, a new kind of measurement, and that’s where the pulse tunneling for me was really exciting,” Ashoori says. “Nothing like it exists anywhere else. Right now, there is no other place doing it besides our lab, and there is no other measurement that gives you the same information, so it's a pretty unique thing." The technique applies on the order of a million electric pulses, each about 100 nanoseconds in duration, and measures currents during those times.

The research is carried out in dilution refrigerators that operate near absolute zero temperature, which poses its own set of challenges. "Right now we have an amplifier in our dilution fridge that operates with a few microwatts of power, and it has a gain of two-thirds, but that's what was required to make the thing work," Ashoori says.

With postdoc coauthors Andrea Young and Ben Hunt interviewing this year for faculty positions, Ashoori hopes that they’ll take the group’s experimental techniques to other places and create a community of people doing similar research. (See related article.)

“With this tunneling technique, we can look at what is called the single particle spectrum of these materials, which is just asking, ‘If I try to inject an electron, will it go in at this energy, is there room for it, and how much room is there for it?’” Ashoori says. It can also reveal the probability that an electron will move to a sample, and how much energy that will take.

"We've studied the two-dimensional electron system a lot, and people have tried to look at it optically, but it turns out the optical spectrum is often difficult to interpret. What we're doing is much simpler. We're really asking, can this system accept an electron at this energy, period. And that spectrum is a lot simpler to understand than a lot of other ones. It's a very direct way of getting at a lot of basic physics in these systems. That has been a main thrust for us, and we were lucky enough to get funding from the Moore Foundation.”

While the technique has focused largely on semiconductor sample analysis, Ashoori hopes to make it into a much broader technique. "Imagine that you can have any little object, and you don't have to have electrical contact, you just have to be able to place a tunnel barrier on top of it, and then you determine the energies required for adding electrons to the object and also observe the single particle spectrum — the likelihood that an electron can enter the object at a particular energy," he says. “The recent advances in creating and placing exfoliated thin layers of boron nitride and other insulators mean that we can place thin tunnel barriers on top of just about any objects on a surface that we might want to study.”

The Ashoori group's capabilities for making sensitive capacitance measurements were a key to the findings in the recent work on graphene. "In those studies, the capacitance measurements that we made showed the development of energy gaps in those systems,” Ashoori says. “We could sense directly that we were seeing results from having massive fermions, such as electrons, instead of massless ones, which ordinarily is what you have in graphene.”

Serendipitous finding

In a two-dimensional system like graphene — a flat single-atom-thick layer of carbon atoms arranged in a hexagonal lattice pattern — electrostatic potential at any carbon atom is identical to the one next to it. This "sublattice symmetry" results in electrons behaving as if they have no mass. By stacking the graphene on a similarly patterned layer of boron nitride, the researchers found that interactions between carbon and boron atoms on one sublattice and carbon and nitrogen atoms on the other resulted in adequate breaking of the sublattice symmetry to give the electrons an observable mass.

Adjacent identical carbon atoms in the graphene lattice are referred to as A sites and B sites. “If I’m on an A site or a B site and my potential energy is the same, then the electron will have no mass,” Ashoori says. “If I can make those two sites different, the A sites and the B sites, then the electron can have a mass. So that’s what we do. You make them different.”

“We studied many samples and we didn’t see this, we weren’t really looking for it, but what happened was there is this underlying material, boron nitride, which has a very similar structure to graphene, except its A sites and B sites have different energy potential,” Ashoori says.

Still, this only happens in about 1 out of 15 samples when the hexagonal lattices of graphene and boron nitride are in near perfect alignment. Even within those nearly perfectly aligned samples, the alignment will vary across a layer because the lattice spacing in the boron nitride is slightly larger than that of carbon, meaning that as you move farther from a point of alignment, the overlapping lattices will go out of register at other points. "Let's say I make the A sites higher potential than the B sites, and let's say I call that positive mass,” Ashoori says. “Due to the difference in lattice constants between the two materials, this reverses at some other position, so the A sites would be lower potential than the B sites, and then the math ends up calling that negative mass. Between the regions of positive and negative mass, there is a boundary where mass equals zero. The thing that is harder to understand is that, despite this expected variation of mass with position, the mass that we measure is close to the maximum mass, rather than the average of zero."

“A dream in graphene is that you could make transistors that you could turn off,” Ashoori says. “And to make transistors that you can turn off, you need that gap. And in creating the gap, at the same time, we get particles that behave like they have mass, so the two things go hand in hand.”

“The size of the gap that we have is about 30 millivolts,” Ashoori says. “If you translate that to temperature, it’s about 300 degrees Kelvin, which is room temperature. It turns out that to make a room-temperature device that you could turn off, you want that gap to be 10 times bigger. So rather than being 300 Kelvin, it would be nice to have it be 3,000 Kelvin, so you could really shut it off. We’re not there yet. That’s something where if we find a way to enhance the gap just another order of magnitude, then we might be in business.”

Collaborative effort

The graphene-boron nitride work grew out of a collaboration with Jarillo-Herrero's group, which had expertise in building and measuring stacks of graphene and boron nitride and other layered materials. Ashoori's postdoctoral fellow Hunt and Jarillo-Herrero's graduate student Javier Sanchez-Yamagishi made samples with those layering techniques. Young added fabrication techniques he learned as a graduate student at Columbia in the group of Professor Philip Kim, who is moving to Harvard. All three worked together on the measurements. "We ended up having this dream team of really highly qualified people working together," Ashoori says.

“I think things are often still just discovered rather than predicted, and sometimes you can discover things that actually had been predicted and you didn’t realize they had been,” Ashoori says. “I find it really fun to not know what’s going to happen. Our work is really geared up toward exploration.”

via MIT News

A View from the Zone

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Brilliant Venus and the central Milky Way rise in the early morning hours of March 1 in this sea and skyscape. The scene looks out from a beach at Sea Isle City, New Jersey, USA, planet Earth. Of course, Earth orbits well within the solar system's habitable zone, that Goldilocks region not too close and not too far from the Sun where surface temperatures can support liquid water. Similar in size to Earth, Venus lies just beyond the inner boundary of the habitable zone. The watery reflection of light from our inhospitable sister planet is seen along a calm, cold ocean and low cloud bank.

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Kepler marks five years in space

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( —Five years ago today, on March 6, 2009, NASA's Kepler Space Telescope rocketed into the night skies above Cape Canaveral Air Force Station in Florida to find planets around other stars, called exoplanets, in search of potentially habitable worlds.

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Crystals ripple in response to light: First propagating surface phonon polaritons in a van der Waals crystal

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Minuscule waves that propagate across atom -- thin layers of crystal could carry information, light, and heat in nanoscale devices. For the first time, the frequency and amplitude of these waves, called surface phonon polaritons, can be tuned by altering the number of layers of crystals, and they travel far making practical applications for these signals feasible.

via Science Daily

New research could help make 'roll-up' digital screens a reality for all

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New technology could make flexible electronics such as roll-up tablet computers, widely available in the near future. So far, this area of electronic design has been hampered by unreliability and complexity of production.

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Icy wreckage discovered in nearby planetary system

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Astronomers have discovered the splattered remains of comets colliding together around a nearby star. The researchers believe they are witnessing the total destruction of one of these icy bodies once every five minutes.

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