Theorists Close In on Improved Atomic Property Predictions.

Source: Physorg.com

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Scientists at the National Institute of Standards and Technology and Indiana University have determined the most accurate values ever for a fundamental property of the element lithium using a novel approach that may permit scientists to do the same for other atoms in the periodic table.

NIST’s James Sims and IU’s Stanley Hagstrom have calculated four excitation energies for the lithium atom approximately 100 times more accurately than any previous calculations or experimental measurements. Precise determination of excitation energy—the amount necessary to raise an atom from a base energy level to the next higher—has intrinsic value for fundamental research into atomic behavior, but the success of the method the team employed has implications that go beyond lithium alone.
The theorists have overcome major computational and conceptual hurdles that for decades have prevented scientists from using quantum mechanics to predict electron excitation energies from first principles. Sims first proposed in the late 1960s that such a quantum approach could be possible, but its application to anything more than two electrons required a fiendishly difficult set of calculations that, until recently, was beyond the capacity of even the world’s fastest computers. In 2006 the team used a novel combination of algorithms, extended precision computing and the increase in power brought about by parallel computing to calculate the most accurate values ever for a simple, two-electron hydrogen molecule.
By making improvements to those algorithms, Sims and Hagstrom now have been able to apply their approach to the significantly more difficult problem of lithium, which has three electrons. Much of the original difficulty with their method stems from the fact that in atoms with more than one electron the mutually repulsive forces among these tiny elementary particles introduces complications that make calculations extremely time-consuming, if not practically impossible.
Sims says that while the lithium calculation is valuable in itself, the deeper import of refining their method is that it should enable the calculation of excitation energies for beryllium, which has four electrons. In turn, this next achievement should enable theorists to predict with greater accuracy values for all of the remaining elements in the second row of the periodic table, from beryllium to neon, and potentially the rest of the periodic table as well. “The mathematical troubles we have with multiple electrons can all be reduced to problems with four electrons,” says Sims, a quantum chemist in the mathematics and computational sciences division. “Once we’ve tackled that, the mathematics for other elements is not any more difficult inherently—there’s just more number-crunching involved.”
To obtain their results, the researchers used 32 parallel processors in a NIST computer cluster, where they are currently working on the calculations for beryllium.
High precision determinations of excitation energies are of interest to scientists and engineers who characterize and model all types of gaseous systems, including plasmas and planetary atmospheres. Other application areas include astrophysics and health physics.
More information: J.S. Sims and S.A. Hagstrom. Hylleraas-configuration-interaction study of the 2 2S ground state of neutral lithium and the first five excited 2S states. Physical Review A, Nov. 19 2009, DOI:10.1103/PhysRevA.80.052507
Provided by National Institute of Standards and Technology

All smoothed out: Hydroxyl radicals remove nanoscopic irregularities on polished gold surfaces.

Source: Physorg.com

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The precious metal gold is the material of choice for many technical applications because it does not corrode - and because it also has interesting electrical, magnetic, and optical properties. Gold is thus one of the most important metals in the electronics industry, miniaturized optical components, and electrochemical processes.

In these applications, it is extremely important that the surface of the gold be completely clean and smooth. However, conventional processes not only “polish” away the undesirable irregularities, but also attack the gold surface. Fritz Scholz and a team from the Universities of Greifswald (Germany) and Warsaw (Poland) have now discovered a technique that can differentiate between the two. As the scientists report in the journal Angewandte Chemie, hydroxyl radicals (OH radicals) rapidly remove all tiny protrusions on mechanically polished gold surfaces, leaving behind an extremely smooth surface.
The researchers treated gold surfaces with Fenton's reagent, which is a mixture of hydrogen peroxide and iron(II) salts that releases OH radicals. It is also used to degrade organic impurities in the purification of waste water. “Actually, it was not expected that the radicals would attack a polished pure gold surface,” says Scholz, “because gold is notoriously difficult to oxidize.” The experiments demonstrated that the hydroxyl radicals oxidize gold very well, though measurable dissolution continues only as long as there are still bumps on the gold surface. Though these results seem contradictory at first glance, the researchers explain that the reaction of the radicals with the highly ordered gold atoms of the completely smooth surface produces a stable layer of gold oxide, which can be reduced back to elemental gold without a significant loss of material. In the protrusions, however, the gold atoms are less ordered and very reactive. During the oxidation, they detach themselves from the atomic structure.
“Because the protrusions are selectively removed, our method is very interesting for polishing gold surfaces for industrial applications,” says Scholz. The process may also find a use in medical technology: gold is used to replace teeth, in tissues for reconstructive surgery, and in electrode implants, such as those used for implanted hearing aids. These release tiny amounts of gold, which enters into the surrounding tissue. This apparently occurs because of an immune reaction that results in the formation of OH radicals or similar species. Pre-treatment of gold implants with Fenton's reagent could inhibit this release of gold into the body.
More information: Fritz Scholz, Hydroxyl Radicals Attack Metallic Gold, Angewandte Chemie International Edition, Permalink: http://dx.doi.org/10.1002/anie.200906358

Provided by Wiley

New quantum cascade lasers emit more light than heat.

Source: Physorg.com

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Northwestern University researchers have developed compact, mid-infrared laser diodes that generate more light than heat - a breakthroughs in quantum cascade laser efficiency.
The results are an important step toward use of quantum cascade lasers in a variety of applications, including remote sensing of hazardous chemicals.
The research, led by Manijeh Razeghi, the Walter P. Murphy Professor of Electrical Engineering and Computer Science at the McCormick School of Engineering and Applied Science, was published online in the journal Nature Photonics on Jan. 10.
After years of research and industrial development, modern laser diodes in the near-infrared (approximately 1 micron) wavelength range are now extremely efficient. However the mid-infrared (greater than 3 microns) is much more difficult to access and has required the development of new device architectures.
The quantum cascade laser (QCL) is a diode laser that is designed on the quantum mechanical level to produce light at the desired wavelength with high efficiency. Unlike traditional diode lasers, the device is unipolar, requiring only electrons to operate. A significant effort has been spent trying to understand and optimize the electron transport, which would allow researchers to improve the laser quality and efficiency.
Despite the special nature of these devices, laser wafer production is done using standard compound semiconductor growth equipment. By optimizing the material quality in these standard tools, researchers at the Center for Quantum Devices (CQD) at Northwestern, led by Razeghi, have made significant breakthroughs in QCL performance.
Previous reports regarding QCLs with high efficiency have been limited to efficiency values of less than 40 percent, even when cooled to cryogenic temperatures.
After removing design elements unnecessary for low-temperature operation, researchers at CQD have now demonstrated individual lasers emitting at wavelengths of 4.85 microns with efficiencies of 53 percent when cooled to 40 Kelvin.
"This breakthrough is significant because, for the very first time, we are able to create diodes that produce more light than heat," says Razeghi. "Passing the 50 percent mark in efficiency is a major milestone, and we continue to work to optimize the efficiency of these unique devices."
Though efficiency is currently the primary goal, the large demonstrated efficiencies also can be exploited to enable power scaling of the QCL emitters. Recent efforts in broad area QCL development have allowed demonstration of individual pulsed lasers with record output powers up to 120 watts, which is up from 34 W only a year ago.
Provided by Northwestern University

Nanoscience Goes 'Big': Discovery Could Lead to Enhanced Electronics.

Source: ScienceDaily
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ScienceDaily (Jan. 11, 2010) — Nanoscience has the potential to play an enormous role in enhancing a range of products, including sensors, photovoltaics and consumer electronics. Scientists in this field have created a multitude of nano scale materials, such as metal nanocrystals, carbon nanotubes and semiconducting nanowires. However, despite their appeal, it has remained an astounding challenge to engineer the orientation and placement of these materials into the desired device architectures that are reproducible in high yields and at low costs -- until now.

Jen Cha, a UC San Diego nanoengineering professor, and her team of researchers, have discovered that one way to bridge this gap is to use biomolecules, such as DNA and proteins. Details of this discovery were recently published in Nature Nanotechology.
"Self-assembled structures are often too small and affordable lithographic patterns are too large," said Albert Hung, lead author of the Nature Nanotechnology paper and a post doc working in Cha's lab. "But rationally designed synthetic DNA nanostructures allow us to access length scales between 5 and 100 nanometers and bridge the two systems.
"People have created a huge variety of unique and functional nanostructures, but for some intended applications they are worthless unless you can place individual structures, billions or trillions of them at the same time, at precise locations," Hung added. "We hope that our research brings us a step closer to solving this very difficult problem."
Hung said the recently discovered method may be useful for fabricating nanoscale electronic or optical circuits and multiplex sensors. "A number of groups have worked on parts of this research problem before, but to our knowledge, we're the first to attempt to address so many parts together as a whole," he said.
One of the main applications of this research that Cha and her group are interested in is for sensing. "There is no foreseeable route to be able to build a complex array of different nanoscale sensing elements currently," said Cha, a former IBM research scientist who joined the UCSD Jacobs School of Engineering faculty in 2008.
"Our work is one of the first clear examples of how you can merge top down lithography with bottom up self assembly to build such an array. That means that you have a substrate that is patterned by conventional lithography, and then you need to take that pattern and merge it with something that can direct the assembly of even smaller objects, such as those having dimensions between 2 and 20 nanometers. You need an intermediate template, which is the DNA origami, which has the ability to bind to something else much smaller and direct their assembly into the desired configuration. This means we can potentially build transistors from carbon nanotubes and also possibly use nanostructures to detect certain proteins in solutions. Scientists have been talking about patterning different sets of proteins on a substrate and now we have the ability to do that."
Cha said the next step would be to actually develop a device based on this research method. "I'm very interested in the applications of this research and we're working our way to get there," she said.
For the last 6years, Cha's research has focused on using biology to engineer the assembly of nanoscale materials for applications in medicine, electronics and energy. One of the limitations of nanoscience is it doesn't allow mass production of products, but Cha's work is focused on trying out how to do that and do it cheaply. Much of her recent work has focused on using DNA to build 2D structures.
"Using DNA to assemble materials is an area that many people are excited about," Cha said. "You can fold DNA into anything you want -- for example, you can build a large scaffold and within that you could assemble very small objects such as nano particles, nano wires or proteins.
"Engineers need to understand the physical forces needed to build functional arrays from functional materials," she added. "My job as a nanoengineer is to out what you need to do to put all the different parts together, whether it's a drug delivery vehicle, photovoltaic applications, sensors or transistors. We need to think about ways to take all the nano materials and engineer them it into something people can use and hold."

Story Source:
Adapted from materials provided by University of California - San Diego.

Journal Reference:
1.Albert M. Hung, Christine M. Micheel, Luisa D. Bozano, Lucas W. Osterbur, Greg M. Wallraff & Jennifer N. Cha. Large-area spatially ordered arrays of gold nanoparticles directed by lithographically confined DNA origami. Nature Nanotechnology, 2009; DOI: 10.1038/nnano.2009.450

Quantum computer calculates exact energy of molecular hydrogen.

Source: Physorg.com

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In an important first for a promising new technology, scientists have used a quantum computer to calculate the precise energy of molecular hydrogen. This groundbreaking approach to molecular simulations could have profound implications not just for quantum chemistry, but also for a range of fields from cryptography to materials science.

"One of the most important problems for many theoretical chemists is how to execute exact simulations of chemical systems," says author Alán Aspuru-Guzik, assistant professor of chemistry and chemical biology at Harvard University. "This is the first time that a quantum computer has been built to provide these precise calculations."
The work, described this week in Nature Chemistry, comes from a partnership between Aspuru-Guzik's team of theoretical chemists at Harvard and a group of experimental physicists led by Andrew White at the University of Queensland in Brisbane, Australia. Aspuru-Guzik's team coordinated experimental design and performed key calculations, while his partners in Australia assembled the physical "computer" and ran the experiments.
"We were the software guys," says Aspuru-Guzik, "and they were the hardware guys."
While modern supercomputers can perform approximate simulations of simple molecular systems, increasing the size of the system results in an exponential increase in computation time. Quantum computing has been heralded for its potential to solve certain types of problems that are impossible for conventional computers to crack.
Rather than using binary bits labeled as "zero" and "one" to encode data, as in a conventional computer, quantum computing stores information in qubits, which can represent both "zero" and "one" simultaneously. When a quantum computer is put to work on a problem, it considers all possible answers by simultaneously arranging its qubits into every combination of "zeroes" and "ones."
Since one sequence of qubits can represent many different numbers, a quantum computer would make far fewer computations than a conventional one in solving some problems. After the computer's work is done, a measurement of its qubits provides the answer.
"Because classical computers don't scale efficiently, if you simulate anything larger than four or five atoms -- for example, a chemical reaction, or even a moderately complex molecule -- it becomes an intractable problem very quickly," says author James Whitfield, research assistant in chemistry and chemical biology at Harvard. "Approximate computations of such systems are usually the best chemists can do."
Aspuru-Guzik and his colleagues confronted this problem with a conceptually elegant idea.
"If it is computationally too complex to simulate a quantum system using a classical computer," he says, "why not simulate quantum systems with another quantum system?"
Such an approach could, in theory, result in highly precise calculations while using a fraction the resources of conventional computing.
While a number of other physical systems could serve as a computer framework, Aspuru-Guzik's colleagues in Australia used the information encoded in two entangled photons to conduct their hydrogen molecule simulations. Each calculated energy level was the result of 20 such quantum measurements, resulting in a highly precise measurement of each geometric state of molecular hydrogen.
"This approach to computation represents an entirely new way of providing exact solutions to a range of problems for which the conventional wisdom is that approximation is the only possibility," says Aspuru-Guzik.
Ultimately, the same quantum computer that could transform Internet cryptography could also calculate the lowest energy conformations of molecules as complex as cholesterol.
More information: Nature Chemistry paper: http://dx.doi.org/10.1038/NCHEM.483
Provided by Harvard University

A solid case of entanglement.

Source: Physorg.com

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This is an SEM image of a typical Cooper pair splitter. The bar is 1 micrometer. A central superconducting electrode (blue) is connected to two quantum dots engineered in the same single wall carbon nanotube (in purple). Entangled electrons inside the superconductor can be coaxed to move in opposite directions in the nanotube, ending up at separate quantum dots, while remaining entangled. Credit: L.G. Herrmann, F. Portier, P. Roche, A. Levy Yeyati, T. Kontos, and C. Strunk

Physicists have finally managed to demonstrate quantum entanglement of spatially separated electrons in solid state circuitry.

For the first time, physicists have convincingly demonstrated that physically separated particles in solid-state devices can be quantum-mechanically entangled. The achievement is analogous to the quantum entanglement of light, except that it involves particles in circuitry instead of photons in optical systems. Both optical and solid-state entanglement offer potential routes to quantum computing and secure communications, but solid-state versions may ultimately be easier to incorporate into electronic devices.
The experiment is reported in an upcoming issue of Physical Review Letters and highlighted with a Viewpoint in the January 11 issue of Physics.
In optical entanglement experiments, a pair of entangled photons may be separated via a beam splitter. Despite their physical separation, the entangled photons continue to act as a single quantum object. A team of physicists from France, Germany and Spain has now performed a solid-state entanglement experiment that uses electrons in a superconductor in place of photons in an optical system.
As conventional superconducting materials are cooled, the electrons they conduct entangle to form what are known as Cooper pairs. In the new experiment, Cooper pairs flow through a superconducting bridge until they reach a carbon nanotube that acts as the electronic equivalent of a beam splitter. Occasionally, the electrons part ways and are directed to separate quantum dots -- but remain entangled. Although the quantum dots are only a micron or so apart, the distance is large enough to demonstrate entanglement comparable to that seen in optical systems.
In addition to the possibility of using entangled electrons in solid-state devices for computing and secure communications, the breakthrough opens a whole new vista on the study of quantum mechanically entangled systems in solid materials.
More information: Carbon Nanotubes as Cooper-Pair Beam Splitters, L. G. Herrmann, F. Portier, P. Roche, A. Levy Yeyati, T. Kontos, and C. Strunk, Phys. Rev. Lett. 104, 026801 (2010) - Published January 11, 2010, Download PDF
Provided by American Physical Society

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