Nano News: 06/10/15

Near absolute zero, molecules may start to exhibit exotic states of matter

The air around us is a chaotic superhighway of molecules whizzing through space and constantly colliding with each other at speeds of hundreds of miles per hour. Such erratic molecular behavior is normal at ambient temperatures. But scientists have long suspected that if temperatures were to plunge to near absolute zero, molecules would come to a screeching halt, ceasing their individual chaotic motion and behaving as one collective body. This more orderly molecular behavior would begin to form very strange, exotic states of matter — states that have never been observed in the physical world.

MIT researchers have successfully cooled a gas of sodium potassium (NaK) molecules to a temperature of 500 nanokelvin. In this artist's illustration, the NaK molecule is represented with frozen spheres of ice merged together: the smaller sphere on the left represents a sodium atom, and the larger sphere on the right is a potassium atom. (Illustration: Jose-Luis Olivares/MIT) Now experimental physicists at MIT have successfully cooled molecules in a gas of sodium potassium (NaK) to a temperature of 500 nanokelvins — just a hair above absolute zero, and over a million times colder than interstellar space. The researchers found that the ultracold molecules were relatively long-lived and stable, resisting reactive collisions with other molecules. The molecules also exhibited very strong dipole moments — strong imbalances in electric charge within molecules that mediate magnet-like forces between molecules over large distances. Martin Zwierlein, professor of physics at MIT and a principal investigator in MIT's Research Laboratory of Electronics, says that while molecules are normally full of energy, vibrating and rotating and moving through space at a frenetic pace, the group’s ultracold molecules have been effectively stilled — cooled to average speeds of centimeters per second and prepared in their absolute lowest vibrational and rotational states. “We are very close to the temperature at which quantum mechanics plays a big role in the motion of molecules,” Zwierlein says. “So these molecules would no longer run around like billiard balls, but move as quantum mechanical matter waves. And with ultracold molecules, you can get a huge variety of different states of matter, like superfluid crystals, which are crystalline, yet feel no friction, which is totally bizarre. This has not been observed so far, but predicted. We might not be far from seeing these effects, so we’re all excited.” Zwierlein, along with graduate student Jee Woo Park and postdoc Sebastian Will — all of whom are members of the MIT-Harvard Center of Ultracold Atoms — have published their results in the journal ("Ultracold Dipolar Gas of Fermionic Na23K40 Molecules in Their Absolute Ground State"). Sucking away 7,500 kelvins Every molecule is composed of individual atoms that are bonded together to form a molecular structure. The simplest molecule, resembling a dumbbell, is made up of two atoms connected by electromagnetic forces. Zwierlein’s group sought to create ultracold molecules of sodium potassium, each consisting of a single sodium and potassium atom. However, due to their many degrees of freedom — translation, vibration, and rotation — cooling molecules directly is very difficult. Atoms, with their much simpler structure, are much easier to chill. As a first step, the MIT team used lasers and evaporative cooling to cool clouds of individual sodium and potassium atoms to near absolute zero. They then essentially glued the atoms together to form ultracold molecules, applying a magnetic field to prompt the atoms to bond — a mechanism known as a “Feshbach resonance,” named after the late MIT physicist Herman Feshbach. “It’s like tuning your radio to be in resonance with some station,” Zwierlein says. “These atoms start to vibrate happily together, and form a bound molecule.”

ultracold molecules of sodium potassium have been cooled to their lowest vibrational and rotational energies

Researchers have created ultracold molecules of sodium potassium that have been cooled to their lowest vibrational and rotational energies. (Image: Jee Woo Park and Sebastian Will) The resulting bond is relatively weak, creating what Zwierlein calls a “fluffy” molecule that still vibrates quite a bit, as each atom is bonded over a long, tenuous connection. To bring the atoms closer together to create a stronger, more stable molecule, the team employed a technique first reported in 2008 by groups from the University of Colorado, for potassium rubidium (KRb) molecules, and the University of Innsbruck, for non-polar cesium (Ce2) molecules. For this technique, the newly created NaK molecules were exposed to a pair of lasers, the large frequency difference of which exactly matched the energy difference between the molecule’s initial, highly vibrating state, and its lowest possible vibrational state. Through absorption of the low-energy laser, and emission into the high-energy laser beam, the molecules lost all their available vibrational energy. With this method, the MIT group was able to bring the molecules down to their lowest vibrational and rotational states — a huge drop in energy. “In terms of temperature, we sucked away 7,500 kelvins, just like that,” Zwierlein says. Chemically stable In their earlier work, the Colorado group observed a significant drawback of their ultracold potassium rubidium molecules: They were chemically reactive, and essentially came apart when they collided with other molecules. That group subsequently confined the molecules in crystals of light to inhibit such chemical reactions. Zwierlein’s group chose to create ultracold molecules of sodium potassium, as this molecule is chemically stable and naturally resilient against reactive molecular collisions. “When two potassium rubidium molecules collide, it is more energetically favorable for the two potassium atoms and the two rubidium atoms to pair up,” Zwierlein says. “It turns out with our molecule, sodium potassium, this reaction is not favored energetically. It just doesn’t happen.” In their experiments, Park, Will, and Zwierlein observed that their molecular gas was indeed stable, with a relatively long lifetime, lasting about 2.5 seconds. “In the case where molecules are chemically reactive, one simply doesn’t have time to study them in bulk samples: They decay away before they can be cooled further to observe interesting states,” Zwierlein says. “In our case, we hope our lifetime is long enough to see these novel states of matter.” By first cooling atoms to ultralow temperatures and only then forming molecules, the group succeeded in creating an ultracold gas of molecules, measuring one thousand times colder than what can be achieved by direct cooling techniques. To begin to see exotic states of matter, Zwierlein says molecules will have to be cooled still a bit further, to all but freeze them in place. “Now we’re at 500 nanokelvins, which is already fantastic, we love it. A factor of 10 colder or so, and the music starts playing.”

Stretchy sensors can detect deadly gases and UV radiation

RMIT University researchers have created wearable sensor patches that detect harmful UV radiation and dangerous, toxic gases such as hydrogen and nitrogen dioxide (, "Stretchable and Tunable Microtectonic ZnO-Based Sensors and Photonics"). These transparent, flexible electronics – which can be worn as skin patches or incorporated into clothing - are bringing science fiction gadgets closer to real life. Dr Madhu Bhaskaran, project leader and co-leader of the RMIT Functional Materials and Microsystems Research Group, said the sensors can be placed on work and safety gear to detect dangerous gases. “Hydrogen leaks can lead to explosions as happened with the Hindenburg disaster and nitrogen dioxide is a major contributor to smog,” she said. “The ability to monitor such gases in production facilities and coal-fired power stations gives vital early warning of explosions, while the ability to sense nitrogen dioxide allows for a constant monitoring of pollution levels in crowded cities.” The latest development follows RMIT’s MicroNano Research Facility breakthrough in bendable electronics which has paved the way for flexible mobile phones. Lead author, PhD researcher Philipp Gutruf, says the unbreakable, stretchy electronic sensors are also capable of detecting harmful levels of UV radiation known to trigger melanoma. Much like a nicotine patch, they can be worn on the skin. In future, they will be able to link to electronic devices to continuously monitor UV-levels and alert the user when radiation hits harmful levels. Gutruf said the research used zinc oxide - present in most sunscreens as a fine powder mixed into a lotion - as the UV sensing material. Zinc oxide was used in the form of very thin coatings over a hundred times thinner than a sheet of paper. “This thin zinc oxide layer is engineered with a plate-like structure that we call micro-tectonics, these plates can slide across each other bit like geological plates that form the earth’s crust allowing for high sensitivity and the ability to bend and flex the devices,” he said. Dr Bhaskaran said the sensors are cheap and durable – attributes which will see flexible electronics and sensors become an integral part of everyday life.

How structural flaws dictate failure strength and deformation in nanosized alloys

A study from A*STAR reveals that designers of metallic-glass-based nanodevices must account for tiny flaws in alloy frameworks to avoid unpredictable catastrophic failure ("Mechanisms of Failure in Nanoscale Metallic Glass"). Understanding how nanoscale metallic glass fractures and fails when subjected to external stress is critical to improving its reliability in devices and composites.

Experimental measurements (left and right) and molecular dynamics simulations (middle) of metallic glass nanopillars reveal that structural flaws play important roles in determining material strength. (© American Chemical Society) Recently, researchers have found evidence that artificial flaws — miniscule notches carved into the alloy — do not affect the material’s overall tensile strength. But other work has shown that such notches may actually induce the formation of localized cracks. Mehdi Jafary-Zadeh and co-workers from the A*STAR Institute of High Performance Computing, in collaboration with researchers in the United States, used a combination of physical experiments and computational simulations to study nanoscale flaw tolerance with in-depth precision. First, the researchers fabricated nickel–phosphorous metallic glass into narrow ‘nanopillars’ bearing tiny notches and mushroom-shaped endcaps that served as tension grips (see image). Guided by high-resolution scanning electron microscopy, they systematically pulled the structures apart until they cracked — an action that consistently occurred at the notched zone, and at failure strengths 40 per cent lower than those for unflawed nanopillars. The team then turned to massive molecular dynamics simulations to explain these physical results. “Simulating failure modes in the nanopillar metallic glasses required large-scale, three-dimensional models containing millions of atoms,” says Jafary-Zadeh. “Performing simulations at these scales is pretty daunting, but we overcame this challenge with the help of the A*STAR Computational Resource Centre.” When the researchers modeled atomic strain during nanopillar elongation, they found that the un-notched structures failed via a plastic type of deformation known as shear banding. However, the notched structures were brittle and failed through crack propagation from the flaw point at tensile strengths significantly smaller than the un-notched samples (see video). These observations suggest that ‘flaw insensitivity’ may not be a general feature of nanoscale mechanical systems.

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“The theory of flaw insensitivity postulates that the strength of materials that are intrinsically brittle or have limited plastic deformation modes approaches a theoretical limit at the nanoscale, and does not diminish due to structural flaws,” explains Jafary-Zadeh. “However, our results show that failure strength and deformation in amorphous nanosolids depend critically on the presence of flaws.” Jafary-Zadeh notes that the excellent agreement between experimental results and the simulations is exciting and demonstrates how such computations can bridge the knowledge gap between macroscopic mechanical fracturing and the hidden corresponding mechanisms taking place at atomistic time and length scales.