Ultracold experiments heat up quantum research

University of Chicago physicists have experimentally demonstrated for the first time that atoms chilled to temperatures near absolute zero may behave like seemingly unrelated natural systems of vastly different scales, offering potential insights into links between the atomic realm and deep questions of cosmology.

This ultracold state, called "quantum criticality," hints at similarities between such diverse phenomena as the gravitational dynamics of black holes or the exotic conditions that prevailed at the birth of the universe, said Cheng Chin, associate professor in physics at UChicago. The results could even point to ways of simulating cosmological phenomena of the early universe by studying systems of atoms in states of quantum criticality.

"Quantum criticality is the entry point for us to make connections between our observations and other systems in nature," said Chin, whose team is the first to observe quantum criticality in ultracold atoms in optical lattices, a regular array of cells formed by multiple laser beams that capture and localize individual atoms.

UChicago graduate student Xibo Zhang and two co-authors published their observations online Feb. 16 in Science Express and in the March 2 issue of Science.

Quantum criticality emerges only in the vicinity of a quantum phase transition. In the physics of everyday life, rather mundane phase transitions occur when, for example, water freezes into ice in response to a drop in temperature. The far more elusive and exotic quantum phase transitions occur only at ultracold temperatures under the influence of magnetism, pressure or other factors.

"This is a very important step in having a complete test of the theory of quantum criticality in a system that you can characterize and measure extremely well," said Harvard University physics professor Subir Sachdev about the UChicago study.

Physicists have extensively investigated quantum criticality in crystals, superconductors and magnetic materials, especially as it pertains to the motions of electrons. "Those efforts are impeded by the fact that we can't go in and really look at what every electron is doing and all the various properties at will," Sachdev said.

Sachdev's theoretical work has revealed a deep mathematical connection between how subatomic particles behave near a quantum critical point and the gravitational dynamics of black holes. A few years hence, offshoots of the Chicago experiments could provide a testing ground for such ideas, he said.

There are two types of critical points, which separate one phase from another. The Chicago paper deals with the simpler of the two types, an important milestone to tackling the more complex version, Sachdev said. "I imagine that's going to happen in the next year or two and that's what we're all looking forward to now," he said.

Critical Experiments

Other teams at UChicago and elsewhere have observed quantum criticality under completely different experimental conditions. In 2010, for example, a team led by Thomas Rosenbaum, the John T. Wilson Distinguished Service Professor in Physics at UChicago, observed quantum criticality in a sample of pure chromium when it was subjected to ultrahigh pressures.

Zhang, who will receive his doctorate this month, invested nearly two and a half years of work in the latest findings from Chin's laboratory. Co-authoring the study with Zhang and Chin were Chen-Lung Hung, PhD'11, now a postdoctoral scientist at the California Institute of Technology, and UChicago postdoctoral scientist Shih-Kuang Tung.

In their tabletop experiments, the Chicago scientists use sets of crossed laser beams to trap and cool up to 20,000 cesium atoms in a horizontal plane contained within an eight-inch cylindrical vacuum chamber. The process transforms the atoms from a hot gas to a superfluid, an exotic form of matter that exists only at temperatures hundreds of degrees below zero.

"The whole experiment takes six to seven seconds and we can repeat the experiment again and again," Zhang said.

The experimental apparatus includes a CCD camera sensitive enough to image the distribution of atoms in a state of quantum criticality. The CCD camera records the intensity of laser light as it enters that vacuum chamber containing thousands of specially configured ultracold atoms.

"What we record on the camera is essentially a shadow cast by the atoms," Chin explained.

The UChicago scientists first looked for signs of quantum criticality in experiments performed at ultracold temperatures from 30 to 12 nano-Kelvin, but failed to see convincing evidence. Last year they were able to push the temperatures down to 5.8 nano-Kelvin, just billionths of a degree above absolute zero (minus 459 degrees Fahrenehit). "It turns out that you need to go below 10 nano-Kelvin in order to see this phenomenon in our system," Chin said.

Chin's team has been especially interested in the possibility of using ultracold atoms to simulate the evolution of the early universe. This ambition stems from the quantum simulation concept that Nobel laureate Richard Feynman proposed in 1981. Feynman maintained that if scientists understand one quantum system well enough, they might be able to use it to simulate the operations of another quantum system that can be difficult to study directly.

For some, like Harvard's Sachdev, quantum criticality in ultracold atoms is worthy of study as a physical system in its own right. "I want to understand it for its own beautiful quantum properties rather than viewing it as a simulation of something else," he said.

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The above story is reprinted from materials provided by University of Chicago, via Newswise.

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Journal Reference:

X. Zhang, C.-L. Hung, S.-K. Tung, C. Chin. Observation of Quantum Criticality with Ultracold Atoms in Optical Lattices. Science, 2012; 335 (6072): 1070 DOI: 10.1126/science.1217990

SNS, HFIR experiments help refine thin-film solar cells

 Solar cells that convert sunlight into electricity could be a widely used renewable energy source. Getting to that point, though, requires breakthroughs in their cost and their efficiency at turning sunbeams into electric current. Neutron scattering experiments conducted at Oak Ridge National Laboratory are helping solar cell makers obtain the hard data they need to refine their materials and manufacturing processes.

One of the most promising options for lowering costs is to make solar cells from thin films made up of combinations of plastics called polymers. These devices are easy to produce in large numbers because they use conventional industrial processing methods, which are relatively cheap and energy-efficient compared to the processes used to make the silicon solar cells that are most widely used now. Also, panels made from organic solar cells are lighter and less expensive to install than the bulky solar panels made from silicon cells.

The drawback to these easily fabricated thin-film devices is their power conversion efficiency, or how well they convert solar radiation to electricity. They're much less efficient than silicon cells (which are almost 30 percent efficient). To be inexpensive enough to compete with silicon cells, thin-film solar cells must be more than 10 percent efficient, but so far, the best ones are only about 8.3 percent efficient. To make solar cells efficient enough, scientists need to understand the molecular structure of the thin films they're made of, how the structure relates to the efficiency of the solar cell, and how to tailor the structure for the greatest efficiency.

Recent studies of polymer-based solar cells at ORNL's Spallation Neutron Source and High Flux Isotope Reactor revealed important details about their molecular structure and showed that annealing (heat treating) the devices improves their power conversion efficiency. The experiments showed that annealing solar cells appropriately as they are fabricated improves their efficiency by more than 20 percent compared to films that aren't annealed.

"We are trying to use mixtures of photoactive polymers to absorb light over a broad wavelength range to improve efficiency," said principal investigator Thomas Russell of University of Massachusetts-Amherst. Haiyun Lu of U-Mass and Bulent Akgun of the NIST Center for Neutron Research and the University of Maryland are co-investigators. Studies such as this one are key to improving the performance of polymer-based solar cells so that they can compete in the marketplace.

The device studied consisted of two semiconductor materials deposited in a thin film on an underlying plate. The films were examined in their original state after being deposited and then after annealing. The MAGICS magnetism reflectometer at the SNS investigated the vertical arrangement of the layers in the film, and the General Purpose Small-Angle Neutron Scattering instrument at HFIR showed how well the two semiconductors blended.

"Structural characterization of thin films has always represented a challenge for small-angle neutron scattering," said Yuri Melnichenko, lead scientist at GP SANS. A powerful neutron beam is needed to monitor the subtle structural changes that occur during the formation of the film, and HFIR provides one of the strongest neutron beams for SANS in the world. The experiments at HFIR were completed within approximately 24 hours, while similar measurements at less intense neutron sources would require five to seven days, Melnichenko said.

How well the semiconductor materials in the thin film blend is important to their performance. The measurements on MAGICS showed that the blending of the two semiconductors increased steadily as the sample was annealed for up to one minute, said Valeria Lauter, lead scientist for MAGICS. As heating continued beyond one minute, there was little further change in the blending.

The experiments determined that annealing the solar cell at 150 degrees Celsius for one minute at a particular point in the process improved its efficiency by slightly over 20 percent compared to the original film. Annealing for shorter times improved the efficiency by lesser amounts. Annealing for more than a minute caused it to decline, as did annealing it at a different point in the process.

The work is detailed in the paper "Morphological characterization of low-bandgap crystalline polymer: PCBM bulk heterojunction solar cells," in Advanced Energy Materials, available online at http://neutrons.or … _russell.pdf

Provided by Oak Ridge National Laboratory (news : web)

Wave Motion - New Compounds for Molecule Interferometry Experiments

When waves meet, a new single wave is created. This phenomenon is well understood for mechanical waves such as sound, and electro-magnetic waves such as light, and the "interference" of light waves is applied in astronomy, fiber optics, and oceanography. The observation that even individual large organic molecules can delocalize over large distance and interfere — not with each other, but each one with itself — is rather new, and its study requires suitable substances. A team of chemists led by Marcel Mayor at the Universität Basel has recently designed a new series of compounds that were successfully used for interferometry experiments by a group of experimental physicists headed by Markus Arndt at the Universität Wien, as they report in the European Journal of Organic Chemistry.

Chemical functionalization allows the properties of the molecules to be tailored to the needs of the experiments. To be compatible with interferometry, compounds must be highly volatile, stable, and easily ionized. In order to understand the transition between quantum and classical mechanics, it is important to study molecules of increasing mass. The first two criteria can be met by highly fluorinated compounds. To meet the requirements of a high molecular mass and good detectability, the authors judiciously paired the fluorinated moieties to a porphyrin core.

The team presented a modular synthesis of seven fluorinated porphyrins. The aim of the authors was to cover a specific mass range and to optimize the design of the structures towards high volatility; their resulting synthetic strategy is straightforward and easily applied. The fluorine components are coupled to the outer parts of the porphyrins in the last step of the synthesis. They can thus be easily modified to fine-tune the interferometry experiments. Despite the high fluorine content of the porphyrins, these compounds could still be produced by established organic synthesis protocols.

The researchers showed that at least one of their prepared compounds met the criteria for thermal evaporation and stability, and the team plans to adopt the modular synthesis technique reported for the design of more specific, mass-limited, sublimable organic dyes for future molecule interferometry experiments.

Original publication:
Marcel Mayor et al.; "Highly Fluorous Porphyrins as Model Compounds for Molecule Interferometry"; European Journal of Organic Chemistry.

 

New compounds for molecule interferometry experiments

 

When waves meet, a new single wave is created. This phenomenon is well understood for mechanical waves such as sound, and electro-magnetic waves such as light, and the "interference" of light waves is applied in astronomy, fiber optics, and oceanography. The observation that even individual large organic molecules can delocalize over large distance and interfere—not with each other, but each one with itself—is rather new, and its study requires suitable substances.

A team of chemists led by Marcel Mayor at the Universität Basel has recently designed a new series of compounds that were successfully used for interferometry experiments by a group of experimental physicists headed by Markus Arndt at the Universität Wien, as they report in the European Journal of Organic Chemistry.

Chemical functionalization allows the properties of the molecules to be tailored to the needs of the experiments. To be compatible with interferometry, compounds must be highly volatile, stable, and easily ionized. In order to understand the transition between quantum and classical mechanics, it is important to study molecules of increasing mass. The first two criteria can be met by highly fluorinated compounds. To meet the requirements of a high molecular mass and good detectability, the authors judiciously paired the fluorinated moieties to a porphyrin core.

The team presented a modular synthesis of seven fluorinated porphyrins. The aim of the authors was to cover a specific mass range and to optimize the design of the structures towards high volatility; their resulting synthetic strategy is straightforward and easily applied. The fluorine components are coupled to the outer parts of the porphyrins in the last step of the synthesis. They can thus be easily modified to fine-tune the interferometry experiments. Despite the high fluorine content of the porphyrins, these compounds could still be produced by established organic synthesis protocols.

The researchers showed that at least one of their prepared compounds met the criteria for thermal evaporation and stability, and the team plans to adopt the modular synthesis technique reported for the design of more specific, mass-limited, sublimable organic dyes for future molecule interferometry experiments.

More information: Marcel Mayor et al., Highly Fluorous Porphyrins as Model Compounds for Molecule Interferometry, European Journal of Organic Chemistry, http://dx.doi.org/ … oc.201100638

Provided by European Journal of Organic Chemistry

Powerful computers, experiments provide insights into ion's behavior near interfaces

 From renewable energy sources to pharmaceuticals, iodide ions are a common actor, and now, thanks to scientists at Pacific Northwest National Laboratory, the ion's behavior can be better predicted. By considering electrons' subtler choices about where to reside, Dr. Chris Mundy and Dr. Marcel Baer showed the negatively charged iodine ion congregates at the air-water interface. However, the ions gather at a lower concentration than previously predicted.

The team obtained answers about the iodide ion's choices to be at the surface or under bulk solvation, surrounded by liquid, using the laws of quantum mechanics in conjunction with Newton's laws of motion to describe the evolution of aqueous electrolytes, or salty water.  The aforementioned calculations were extensive and required the use of leadership-class computers through the Department of Energy's INCITE award. Previous studies relied on empirical potentials, which are simpler mathematical models of molecular motion that do not explicitly consider quantum mechanics.

Understanding the nature of ions where air and water meet and at similar interfaces could change how we conduct basic energy research, climate studies, and biological investigations. However, the fundamentals of an ion's propensity to be present at an interface and the important interactions that wrap ions in liquid are still not understood. This research sheds new light on the effects of ions in the vicinity of hydrophobic environments.

"Our work shows where some models may fail and where you may have to take into account the more subtle effects of electrons when performing calculations," said Mundy, the physical chemist who co-authored the study.

It begins with large polarizable anions, negatively charged particles where the electrons slosh back and forth around the atom's central core in response to nearby electric fields produced by the motion of surrounding water molecules. The new conventional wisdom since 2002 is that these ions can exist in significant population at the air-water interface.  The now nearly universally accepted results were pioneered by Dr. Liem Dang at PNNL and independently by Profs. Jungwirth and Tobias at the University of California at Irvine. These studies were done using empirical potentials in conjunction with a parameterized model for how electrons respond to different charged environments, namely polarization.

"Simply put, electric fields felt by an ion at the interface are different than those felt in the bulk of the liquid," said Mundy.

The earlier results have influenced a generation of both experimental and theoretical studies dedicated to understanding this phenomenon.  Although there is now a consensus regarding ions at interfaces Mundy and Baer wanted to understand the precise interactions that drive ions to the air-water interface. 

To understand how ions adsorb onto surfaces and provide the more accurate data to scientific models, the researchers integrated experimental research, theory, and leadership-class computing. The researchers performed extensive density functional theory calculations to mathematically represent the electrons and ions and determine their interactions.

To justify the computationally expensive calculations, the team compared the detailed structure of iodide in water to extended x-ray fine structure experiments conducted by John Fulton at PNNL. Results of this joint theoretical and experimental study suggested that quantum mechanical models reproduced the local solvation structure of iodide more accurately than the models based on empirical polarizable interaction potentials, known as multipole expansions.  Here, a multipole expansion breaks down a complicated arrangement of charges into concepts, such as a monopole, dipole, etc., and is a good description when you are looking at charges from far away.

"Multipole expansions are good from far, but far from good," said Mundy. When it comes to the movement of the electrons and where electrons from different atoms overlap, the expansions don't provide the precise answers scientists need.

This study took advantage of the synergy between computational and experimental science. "Our result would not mean anything without the experimental results," said Baer, a Linus Pauling Distinguished Postdoctoral Fellow at PNNL. "It would just be another number with no weight."

The researchers continue to combine electronic structure, statistical mechanics, and leadership-class computing to assist in understanding the effects of iodide and other ions. This research will be continued by Mundy at PNNL and by Baer for the rest of his stay at PNNL and when he returns to Europe.

More information: Baer MD and CJ Mundy. 2011. "Toward an Understanding of the Specific Ion Effect Using Density Functional Theory." Journal of Physical Chemistry Letters 2, 1088-1093. DOI: 10.1021/jz200333b

Provided by Pacific Northwest National Laboratory (news : web)