Saturday, December 17, 2011

S-t-r-e-t-c-h-i-n-g electrical conductance to the limit

 Individual molecules have been used to create electrical components like resistors, transistors and diodes that mimic the properties of familiar semiconductors. But according to Nongjian (NJ) Tao, a researcher at the Biodesign Institute at ASU, unique properties inherent in single molecules also may allow clever designers to produce novel devices whose behavior falls outside the performance observed in conventional electronics.

In research appearing in a recent issue of Nature Nanotechnology, Tao describes a method for mechanically controlling the geometry of a single molecule, situated in a junction between a pair of gold electrodes that form a simple circuit. The manipulations produced over tenfold increase in conductivity.

The unusual, often non-intuitive characteristics of single molecules may eventually be introduced into a broad range of microelectronics, suitable for applications including biological and chemical sensing electronic and mechanical devices.

Delicate molecular manipulations requiring patience and finesse are routine for Tao, whose research at Biodesign's Center for Bioelectronics and Biosensors has included work on molecular diodes, graphene behavior and molecular imaging techniques. Nevertheless, he was surprised at the outcome described in the current paper: "If you have a molecule attached to electrodes, it can stretch like a rubber band," he says. "If it gets longer, most people tend to think that the conductivity will decrease. A longer wire is less conductive than a shorter wire."

Indeed, diminishing conductivity through a molecule is commonly observed when the distance between the electrodes attached to its surface is increased and the molecule becomes elongated. But according to Tao, if you stretch the molecule enough, something unexpected happens: the conductance goes up -- by a huge amount. "We see at least 10 times greater conductivity, simply by pulling the molecule."

As Tao explains, the intriguing result is a byproduct of the laws of quantum mechanics, which dictate the behavior of matter at the tiniest scales: "The conductivity of a single molecule is not simply inversely proportional to length. It depends on the energy level alignment."

In the metal leads of the electrodes, electrons can move about freely but when they come to an interface -- in this case, a molecule that sits in the junction between electrodes -- they have to overcome an energy barrier. The height of this energy barrier is critical to how readily electrons can pass through the molecule. By applying a mechanical force to the molecule, the barrier is lowered, improving conductance.

"Theoretically, people have thought of this as a possibility, but this is a demonstration that it really happens," Tao says. "If you stretch the molecule and geometrically increase the length, it energetically lowers the barrier so electrons can easily go through. If you think in optical terms, it becomes more transparent to electrons."

The reason for this has to do with a property known as force-induced resonant tunneling. This occurs when the molecular energy moves closer to the Fermi level of the electrodes -- that is, toward the region of optimal conductance. Thus, as the molecule is stretched, it causes a decrease in the tunneling energy barrier.

For the experiments, Tao's group used 1,4'-Benzenedithiol, the most widely studied entity for molecular electronics. Further experiments demonstrated that the transport of electrons through the molecule underwent a corresponding decrease as the distance between the electrodes was reduced, causing the molecule's geometry to shift from a stretched condition to a relaxed or squeezed state. "We have to do this thousands of times to be sure the effect is robust and reproducible."

In addition to the discovery's practical importance, the new data show close agreement with theoretical models of molecular conductance, which had often been at variance with experimental values, by orders of magnitude.

Tao stresses that single molecules are compelling candidates for a new types of electronic devices, precisely because they can exhibit very different properties from those observed in conventional semiconductors.

Microelectromechanical systems or MEMS are just one domain where the versatile properties of single molecules are likely to make their mark. These diminutive creations represent a $40 billion a year industry and include such innovations as optical switches, gyroscopes for cars, lab-on-chip biomedical applications and microelectronics for mobile devices.

"In the future, when people design devices using molecules, they will have a new toolbox they can use."

The above story is reprinted from materials provided by Arizona State University.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Christopher Bruot, Joshua Hihath, Nongjian Tao. Mechanically controlled molecular orbital alignment in single molecule junctions. Nature Nanotechnology, 2011; DOI: 10.1038/nnano.2011.212

Slow road to stability for emulsions

 By studying the behavior of tiny particles at an interface between oil and water, researchers at Harvard have discovered that stabilized emulsions may take longer to reach equilibrium than previously thought.

Much longer, in fact.

"We were looking at what we thought would be a very simple phenomenon, and we found something very strange," says principal investigator Vinothan Manoharan, Associate Professor of Chemical Engineering and Physics at the Harvard School of Engineering and Applied Sciences (SEAS).

"We knew that the particle would stick to the interface, and other researchers had assumed this event happened instantaneously," he says. "We actually found that the timescale for this process was months to years."

The findings, published in Nature Materials (online) on December 4, have important implications for the manufacturing processes used in pharmaceuticals, cosmetics, and foods, among other chemical industries.

An emulsion is a mixture of two or more insoluble liquids -- usually oil and water. A simple emulsion like vinaigrette takes energy to create (for example, by shaking it), and over time it will separate out, as the oil or water molecules cluster together again.

To give products like mayonnaise and sunscreen a reasonable shelf life, manufacturers typically add stabilizing particles to create Pickering emulsions. Ice cream, for example, is stabilized by tiny ice crystals that cling to the interfaces between the fat and water droplets, creating a rigid physical barrier between the two. In traditional mayonnaise, proteins from the egg yolk perform the same role.

When the oil and water in these types of emulsions are completely mixed and stable, the particles are said to be at equilibrium.

"There are certain rules for making different types of emulsions," explains Manoharan. "For example, do you get oil droplets in water, or water droplets in oil? The conventional rules are based on the properties of the materials, but our results suggest that it also has to do with time and the energy you put into the system."

To study Pickering emulsions, Manoharan and his colleagues used holography to gain a three-dimensional view of microscopic polystyrene balls while they approached an interface between oil and water. The researchers used light from a focused laser (optical tweezers) to gently push a particle toward the interface, hoping to watch it settle into its predicted equilibrium point, straddling the oil-water boundary.

To their surprise, none of the particles reached equilibrium during the experimental timeframe. Instead, they breached the interface quickly, but then slowed down more and more as they crossed into the oil. Mathematically extrapolating the logarithmic behavior they did observe, Manoharan's team discovered that the particles would stabilize on a time frame much longer than anyone had predicted.

"Our experiments only went on for a few minutes, but for the system to reach equilibrium would take at least weeks to months, and possibly years," explains lead author David Kaz, Ph.D. '11, who earned his degree in physics at Harvard's Graduate School of Arts and Sciences.

The finding is unlikely to affect any time-tested culinary recipes, but many other applications rely on very precise predictions of the particles' behavior.

In biomedical engineering, for example, Pickering emulsions are used to create colloidosomes -- microscale capsules that could deliver precise concentrations of drugs to specific targets in the human body. Understanding the behavior of particles at liquid interfaces is also relevant to many aspects of chemical engineering, water purification, mineral recovery techniques, and the manufacture of nanostructured materials.

The new research suggests that the models currently used to predict and optimize these systems may be too simplistic.

"It has always been assumed that the particles moved almost instantly to their equilibrium contact angle or height, and then Young's law would apply," says co-author Michael Brenner, Glover Professor of Applied Mathematics and Applied Physics at SEAS. "What we found, though, is that equilibrium might take much, much longer to achieve than the time scale at which you're using your product."

"If you're really stirring hard, maybe you can get the particles to reach equilibrium faster," Brenner adds, "But what we're saying is that the process matters."

Co-authors Ryan McGorty (Ph.D. '11, physics) and Madhav Mani (Ph.D. '10, applied mathematics), also contributed to the research, which was funded by the National Science Foundation (NSF) and the NSF-supported Materials Research Science and Engineering Center at Harvard.

Story Source:

The above story is reprinted from materials provided by Harvard School of Engineering and Applied Sciences.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

David M. Kaz, Ryan McGorty, Madhav Mani, Michael P. Brenner, Vinothan N. Manoharan. Physical ageing of the contact line on colloidal particles at liquid interfaces. Nature Materials, 2011; DOI: 10.1038/nmat3190

Atoms dressed with light show new interactions, could reveal way to observe enigmatic particle

Physicists at the National Institute of Standards and Technology (NIST) have found a way to manipulate atoms' internal states with lasers that dramatically influences their interactions in specific ways. Such light-tweaked atoms can be used as proxies to study important phenomena that would be difficult or impossible to study in other contexts.

Their most recent work, appearing in Science, demonstrates a new class of interactions thought to be important to the physics of superconductors that could be used for quantum computation.

Particle interactions are fundamental to physics, determining, for example, how magnetic materials and high temperature superconductors work. Learning more about these interactions or creating new "effective" interactions will help scientists design materials with specific magnetic or superconducting properties.

Because most materials are complicated systems, it is difficult to study or engineer the interactions between the constituent electrons. Researchers at NIST build physically analogous systems using supercooled atoms to learn more about how materials with these properties work.

"Basically, we're able to simulate these complicated systems and observe how they work in slow motion," says Ian Spielman, a physicist at NIST and fellow of the Joint Quantum Institute (JQI), a collaborative enterprise of NIST and the University of Maryland.

According to Ross Williams, a postdoctoral researcher at NIST, cold atom experiments are good for studying many body systems because they offer a high degree of control over position and behavior of the atoms.

"First, we trap rubidium-87 atoms using magnetic fields and cool them down to 100 nanokelvins," says Williams. "At these temperatures, they become what's known as a Bose-Einstein condensate. Cooling the atoms this much makes them really sluggish, and once we see that they are moving slowly enough, we use lasers to 'dress' the atoms, or mix together different energy states within them. Once we have dressed the atoms, we split the condensate, collide the two parts, and then see how they interact."

According to Williams, without being laser-dressed, simple, low-energy interactions dominate how the atoms scatter as they come together. While in this state, the atoms bang into each other and scatter to form a uniform sphere that looks the same from every direction, which doesn't reveal much about how the atoms interacted.

When dressed, however, the atoms tended to scatter in certain directions and form interesting shapes indicative of the influence of new, more complicated interactions, which aren't normally seen in ultracold atom systems. The ability to induce them allows researchers to explore a whole new range of exciting quantum phenomena in these systems.

While the researchers used rubidium atoms, which are bosons, for this experiment, they are modifying the scheme to study ultracold fermions, a different species of particle. The group hopes to find evidence of the Majorana fermion, an enigmatic, still theoretical kind of particle that is involved in superconducting systems important to quantum computation.

"A lot of people are looking for the Majorana fermion," says Williams. "It would be great if our approach helped us to be the first."

Story Source:

The above story is reprinted from materials provided by National Institute of Standards and Technology (NIST).

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

R. A. Williams, L. J. Leblanc, K. Jiménez-García, M. C. Beeler, A. R. Perry, W. D. Phillips, I. B. Spielman. Synthetic Partial Waves in Ultracold Atomic Collisions. Science, 2011; DOI: 10.1126/science.1212652

The impact of quantum matter

"Dressing" atoms with laser light allows high angular momentum scattering to be seen for the first time in long-lived atomic Bose-Einstein condensates at ultracold temperatures.

Scientists at the Joint Quantum Institute (JQI) have for the first time engineered and detected the presence of high angular momentum collisions between atoms at temperatures close to absolute zero. Previous experiments with ultracold atoms featured essentially head-on collisions. The JQI experiment, by contrast, is able to create more complicated collisions between atoms using only lasers. This innovation may facilitate the creation of exotic quantum states that can be exploited for practical applications like quantum computing. The key to the JQI approach is to alter the atoms' environment with laser light. They "dress" rubidium atoms by bathing them in a pair of laser beams, which force the atoms to have one of three discrete values of momentum. In the JQI experiment, rubidium atoms comprise a Bose-Einstein condensate (BEC). BECs have been collided before. But the observation of high-angular-momentum scattering at such low energies is new.

The new JQI results are being reported in the journal Science.


One of the cardinal principles of quantum science is that matter must be simultaneously thought of as both particles and waves. When the temperature of a gas of atoms is lowered, the wavelike nature of the atom emerges, and the idea of position becomes fuzzier. While an atom at room temperature might spread over a hundredth of a nm, atoms at nano-kelvin temperatures have a typical wavelength of about 100 nm. This is much larger than the range of the force between atoms, only a few nm. Atoms generally collide only when they meet face to face.

However, to study certain interesting quantum phenomena, such as searching for Majorana particles -- hypothetical particles that might provide a robust means of encoding quantum information -- it is desirable to engineer inter-atomic collisions beyond these low-energy, head-on type. That's what the new JQI experiment does.

Partial Waves

Scattering experiments date back to the discovery of the atomic nucleus 100 years ago, when Ernest Rutherford shot alpha particles into a foil of gold. Since then other scattering experiments have revealed a wealth of detail about atoms and sub-atomic matter such as the quark substructure of protons.

A convenient way of picturing an interaction between two particles is to view their relative approach in terms of angular momentum. Quantized angular momentum usually refers to the motion of an electron inside an atom, but it necessarily pertains also to the scattering of the two particles, which can be thought of as parts of a single quantum object.

If the value of the relative angular momentum is zero, then the scattering is designated as "s-wave" scattering. If the pair of colliding particles has one unit of angular momentum, the scattering is called p-wave scattering. Still more higher-order scattering scenarios are referred to by more letters: d-wave, f-wave, g-wave, and so on. This model is referred to as the partial waves view.

In high energy scattering, the kind at accelerators, these higher angular-momentum scattering scenarios are important and help to reveal important structure information about the particles. In atomic scattering at low temperatures, the s-wave interactions completely swamp the higher-order scattering modes. For ultralow-temperature s-wave scattering, when two atoms collide, they glance off each other (back to back) at any and all angles equally. This isotropic scattering doesn't reveal much about the nature of the matter undergoing collision; it's as if the colliding particles were hard spheres.

This has changed now. The JQI experiment is the first to create conditions in which d-wave and g-wave scattering modes in an ultracold experiment could be seen in otherwise long-lived systems.

Quantum Collider

Ian Spielman and his colleagues at the National Institute for Standards and Technology (NIST) chill Rb atoms to nano-kelvin temperatures. The atoms, around half a million of them, have a density about a millionth that of air at room temperature. Radiofrequency radiation places each atom into a superposition of quantum spin states. Then two (optical light) lasers impart momentum (forward-going and backward-going motion) to the atoms.

If this were a particle physics experiment, we would say that these BECs-in-motion were quantum beams, beams with energies that came in multiples of the energy kick delivered by the lasers. The NIST "collider" in Gaithersburg, Maryland is very different for the CERN collider in Geneva, Switzerland. In the NIST atom trap the particles have kinetic energies of a hundred pico-electron-volts rather than the trillion-electron-volt energies used at the Large Hadron Collider.

At JQI, atoms are installed in their special momentum states, and the collisions begin. Outward scattered atoms are detected after the BEC clouds are released by the trap. If the atoms hadn't been dressed, the collisions would have been s-wave in nature and the observed scattered atoms would have been seen uniformly around the scattering zone.

The effect of the dressing is to screen the atoms from s-wave scattering in the way analogous to that in some solid materials, where the interaction between two electrons is modified by the presence of trillions of other electrons nearby. In other words, the laser dressing effectively increased the range of the inter-atom force such that higher partial wave scattering was possible, even at the lowest energies.

In the JQI experiment, the observed scattering patterns for atoms emerging from the collisions was proof that d-wave and g-wave scattering had taken place. "The way in which the density of scattered atoms is distributed on the shell reflects the partial waves," said Ian Spielman. "A plot of scattered-density vs. spherical polar angles would give the sort of patterns you are used to seeing for atomic orbitals. In our case, this is a sum of s-, p-, and d- waves."

Simulating Solids Using Gases

Ultracold atomic physics experiments performed with vapors of atoms are excellent for investigating some of the strongly-interacting quantum phenomena usually considered in the context of condensed matter physics. These subjects include superconductivity, superfluids, the quantum Hall effect, and topological insulators, and some things that haven't yet been observed, such as the "Majorana" fermions.

Several advantages come with studying these phenomena in the controlled environment of ultracold atoms. Scientists can easily manipulate the landscape in which the atoms reside using knobs that adjust laser power and frequency. For example, impurities that can plague real solids can be controlled and even removed, and because (as in this new JQI experiment) the scattering of atoms can now (with the proper "dressing") reveal higher-partial-wave effects. This is important because the exotic quantum effects mentioned above often manifest themselves under exactly these higher angular-momentum conditions.

"Our technique is a fundamentally new method for engineering interactions, and we expect this work will stimulate new directions of research and be of broad interest within the physics community, experimental and theoretical," said Spielman. "We are modifying the very character of the interactions, and not just the strength, by light alone."

Story Source:

The above story is reprinted from materials provided by Joint Quantum Institute.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

R. A. Williams, L. J. Leblanc, K. Jiménez-García, M. C. Beeler, A. R. Perry, W. D. Phillips, I. B. Spielman. Synthetic Partial Waves in Ultracold Atomic Collisions. Science, December 8 2011 DOI: 10.1126/science.1212652