Featured Research
Quantum Signatures of Chaos
A team led by Physics and Optical Science professor Poul Jessen has seen directly how the presence of classical chaos affects the behavior of a quantum system. Chaos is common in our everyday world where it affects wide range of phenomena, including electron transport, chemical reactions, neural networks, population dynamics, weather systems, and the motion of planetary bodies. Remarkably, it is still debated how chaos can emerge from quantum mechanics, which is believed to be our most complete theory of nature. This fundamental disconnect has motivated decades of theoretical study, but there remains a near-total lack of experiments that probe quantum-classical correspondence in chaotic systems. Jessen and his team have now realized a popular model system for chaos, the kicked top, using the spin of a single Cs atom. By measuring the entire spin quantum state they have made stop motion movies of the evolving quantum kicked top, and observed directly how it respects the very same boundaries between stable and chaotic motion that characterize a classical kicked top. It has recently been proposed that classical chaos will reflect itself in the degree to which the constituent parts of a quantum system become entangled. Jessen and his team has obtained the first experimental evidence for this hypothesis, by measuring the amount of entanglement between the nuclear and electron spins that make up the total spin of their Cs atom. Their data show how the spins remain largely unentangled if the classical motion is stable, and rapidly become entangled if it is chaotic.
The team published their results in the Oct. 8 issue of the journal Nature.
"Quantum Signatures of Chaos", S. Chaudhury, A. Smith, B. E. Anderson, S. Ghose, and P. S. Jessen, Nature 461, 768 (2009).
http://www.nature.com/nature/journal/v461/n7265/edsumm/e091008-04.html
The Physics of Solar Energy
Alexander Cronin, a UA associate professor of physics and optical sciences, is working with a team of students working with panels at the Tucson Electric Power solar test yard. The UA team has been studying various types of solar panels as part of an effort to ultimately determine how long they will last and which is most cost efficient. For an online video interview go to http://uanews.org/node/24896/
Prof.Cronin is convinced that careful measurements of solar panel performance will lead to solving the grand challenges of solar energy generation. He sums these up with the mnemonic CREST — cost, reliability, efficiency, storage and transmission. One could argue that currently, solar energy costs too much to compete with energy developed from fossil fuels, and the reliability of recently developed systems is untested. The efficiency of conversion of solar to electric power of solar panels, working in a system, is today still below 12 percent. Electricity storage is needed because the resource is only available when the sun is shining. The problem of how to send power efficiently from where the sun is shining to where it is not also needs solving. That will all take years and many minds, says Cronin, but careful physical measurement of panel performance in industrial environment is an important step towards achieving some of these goals.
For more related information see the article in Arizona Daily Star of Sunday, April 5, 2009 page B8, the web edition at http://www.azstarnet.com/allheadlines/287467.php
Closing in on the Higgs boson
Researchers at Fermilab are narrowing the search for the Higgs boson, the missing piece of the standard model of particle physics. For the first time, we have enough evidence to show that the mass of the Higgs is not between 160 and 170 GeV. Among the researchers is Erich Varnes, Associate Professor of Physics. For additional information on the hunt for the Higgs boson see these articles:
Arizona Daily Star: http://www.azstarnet.com/metro/284343
UANews.org: http://uanews.org/node/24568
Fermilab:
http://www.fnal.gov/pub/presspass/press_releases/Higgs-mass-constraints-20090313.html
Researchers explain how patches of photosynthetic plankton form at sea
From UAnews.org
Tiny photosynthetic plankton sometimes swim into the watery equivalent of Rod Serling's Twilight Zone: a sharp variation in marine currents that traps billions of the organisms until a shift in wind or tide sets them free by altering the currents.
A research team that includes John O. Kessler of The University of Arizona now explains how some spontaneously developing ocean currents set the stage for swimming single-celled organisms known as phytoplankton to accumulate.
Adjacent layers of water moving at different speeds produce a "shear" flow that traps the tiny swimmers between the layers, according to the new research.
Concentrating the organisms requires currents that move horizontally in opposite directions, such as would happen if the wind blows the upper layer one way while the tide moves another.
The aggregations of the tiny organisms form in the ocean's top 50 yards. The thickness of the patches can range from less than an inch to several yards. The patches can span as much as several miles horizontally and last hours, days or weeks.
The new finding from Kessler and his colleagues at MIT may help predict the occurrence of algal accumulations, including harmful ones such as red tides.
"These algal blooms come and go and up to now, no one realized that physical, for example, hydrodynamical, mechanisms can cause these organisms to concentrate themselves," said Kessler, a UA professor emeritus of physics. "This is a piece of the puzzle."
He emphasized that these processes only work on phytoplankton that are swimming.
First author William M. Durham said, "Many species can swim, but this fact is often neglected by researchers because phytoplankton are slow compared to ocean currents."
Phytoplankton are the base of marine food chains. Such concentrations are analogous to watering holes in a savanna, said co-author Roman Stocker. The layers of phytoplankton "draw a wide range of organisms and thus play a disproportionate role in the ecological landscape."
Durham, a doctoral student at MIT in Cambridge, Mass., Kessler, and Stocker, the Doherty Associate Professor of Ocean Utilization at MIT, published their paper, "Disruption of Vertical Motility by Shear Triggers Formation of Thin Phytoplankton Layers," in the Feb. 20 issue of the journal Science.
The National Science Foundation, the U.S. Department of Energy and the MIT Earth Systems Initiative funded the research.
Kessler has been working on the physics of how one-celled organisms move in aquatic environments for more than 20 years.
"From the beautiful swimming patterns made by the algae, I figured out how you could hydrodynamically choreograph the swimming directions of the algae," he said.
About two years ago, he was visiting MIT to give a seminar and talked to Stocker about the biological physics of currents and algae.
As a result, Stocker and Durham began laboratory experiments based on mathematical models extending Kessler's earlier results. Using video-microscopy, Durham and Stocker tracked the movements of individual cells as they become trapped in the layers of shear.
The team's research showed that the swimming cells cannot escape the layers on their own. Once trapped, phytoplankton are at the mercy of the flow and must wait for the shear to decrease before they can swim out of the Twilight Zone.
"Phytoplankton are incredibly small. You would have to stack about 10 back to back to equal the width of a single human hair," Durham said. "Despite their small size, they play an outsized role in the environment. They form the base of the marine food web and cumulatively produce half the world's oxygen."
Because phytoplankton have different shapes and swimming abilities, one species may be able to swim through a layer of shear that will capture another. Therefore, each species could be trapped in a different level of shear, creating an oceanic layer-cake effect - a boon for other organisms that feed on specific species.
When a toxic species of phytoplankton becomes trapped, that can spawn a harmful algal bloom - an explosion in the population of toxic phytoplankton that sickens or kills larger animals that ingest the cells.
Harmful algal blooms occur near coastal areas and are a major source of social and economic concern, because they cause billions of dollars in annual losses to fishing and recreational industries worldwide.
This story is modified from one written by Denise Brehm of the MIT department of civil & environmental engineering.
Extreme Measures - Atom Interferometry's precision could make it the Swiss Army Knife of Physics
Ewen Callaway
Science News
(Reprinted from the February 16, 2008 issue)
In spring 2010, the military plans to embark on a road trip across the country to test a new way of navigating. Instead of taking a path marked by a dog-eared road atlas, a compass, or even global positioning satellites, the vehicle will follow one mapped by supercold cesium atoms.
This cross-country trek will be a field test for the Defense Department's Precision Inertial Navigation Systems program to navigate by measuring the Earth's rotation using atoms that behave like waves. The vehicle won't drive blind, but the machine guiding it could make such a feat possible. And someday the new system could also improve the accuracy of gyroscope navigation in airplanes 200-fold, says Air Force Lt. Col. Jay Lowell, who is leading the project.
The atoms' direction-finding powers come from a technique called atom interferometry. Once a lab curiosity, atom interferometry is now becoming the Swiss Army knife of physics. It has the potential to steer airplanes and submarines, uncover buried caches of oil and diamonds, and perhaps hunt down cave-dwelling terrorists. The technology is also helping scientists probe the very nature of the universe, from detecting theoretical waves of gravity sent out by exploding stars to measuring deviations in the strength of gravity at superclose distances. Physicists are even rallying to put an atom interferometer in orbit to test theories like Einstein's general relativity with unparalleled exactness.
"In the last 10 years, atom interferometry has gone from inventions and demonstrations into precision measurement tools," says physicist Alex Cronin, an expert on the technique at the University of Arizona in Tucson.
The path less traveled
At its heart, atom interferometry is similar to light interferometry, a 200-year-old technique that itself improved the accuracy of many measurements.
Shine a light through a half-silvered glass plate and half the waves pass through, while half bounce off at an angle. A couple of regular mirrors can reflect the two beams back together. If one wave travels a little bit further than the other, the recombined waves will be slightly out of sync. With visible light, this effect produces a pattern of white-and-black stripes: white stripes correspond to areas where the waves line up and black stripes to where they cancel each other out. Physicists use this effect—called an interference pattern—to calculate differences in the distance each beam travels.
During the late 1800s, two American physicists, Albert Michelson and Edward Morley, used a light interferometer to try to detect the "luminiferous ether," then thought to occupy all space. Just as sound consists of vibrations in air, light was supposedly a vibration of the ether, scientists thought. If so, the apparent direction of Earth's motion through the ether would alter the velocities of light beams taking different paths. However, Michelson and Morley's experiment found no such difference, casting doubt on the ether's existence.
Classical physics explains the interference of light waves just fine. Atom interferometry, however, hinges on the bizarre behavior of atoms predicted by quantum mechanics, the math that describes how matter works at sub-microscopic scales. Just as waves of light can sometimes act like particles called photons, atoms can be coaxed into showing off their inner waves. In this condition, an atom can exist in two or more places at once, called a superposition. "It's just weird and you finally end up saying this is a very weird theory," says physicist David Pritchard, a pioneer in the field who works at the Massachusetts Institute of Technology.
One advantage of using atoms for interferometry is their tiny wavelengths. In the 1920s, French physicist Louis-Victor de Broglie proposed that a particle could behave as a wave, and the wavelength would be determined by the particle's speed and mass; the heavier and faster the particle, the shorter its wavelength. Shortly thereafter, experiments proved de Broglie right. The atoms used in interferometry have wavelengths around a hundredth of a nanometer, while the wavelength of visible light measures from 400 to 700 nanometers. Atom waves split into two paths can be used to detect much smaller differences than light. If the path an atom takes varies by even a thousandth of a nanometer (a picometer) an atom interferometer can spot the difference, Cronin says.
Another benefit of atoms is the breadth of their physical characteristics, which include mass, magnetic sensitivity, and ability to hold an electric charge. And atoms feel the pull of gravity. If light interferometry is an old wooden meter stick, then atom interferometry is a modern tape measure, scale, and voltmeter rolled into one.
Yet these beneficial traits also make atoms tough to observe as waves, Cronin says. Atoms flitting about at room temperature tend to bump into one another. To better detect the waves, physicists cool the atoms to a few millionths of a degree above absolute zero—the temperature at which all atomic motion virtually stops—and manipulate them to fly in the same direction.
Hitting pay dirt
The first atom interferometers were developed in the early 1990s by four independent teams working in Germany and the United States. Initial applications, such as exacting measurements of the Earth's gravity and rotation, were obvious, says Pritchard, whose MIT lab developed one of the first atom interferometers. "When we made it, we certainly had a pretty good list of things we wanted to do," he says. "What I think was a little surprising to me was the rapidity with which the precision developed."
Many researchers, including Pritchard, credit Mark Kasevich, a Stanford University physicist, with pushing the frontiers of atom interferometry, especially its more practical uses. "Mark's a very good scientist, but he's an off-the-scale good engineer," Pritchard says.
Kasevich's lab is using an atom interferometer to sense tiny fluctuations in the Earth's gravity. High school physics students learn that every second an object is in free fall, it speeds up another 9.8 meters per second. This isn't exactly true, says Kasevich. The acceleration from gravity gets smaller the further you are from the Earth's center. The difference in gravity—the gravity gradient—between sea level and the top of Mount Everest is about 0.3 percent.
On the Earth's surface, gravity's acceleration changes depending on the composition of the rock (or other substance) beneath it. "Anytime you have missing dirt, you can ask if you can find the gravity gradient," Kasevich says. It's a task perfect for an atom interferometer. "One of the killer applications is using it to find oil and minerals," he says.
One hundred thousand metric tonnes of oil buried a kilometer underground will decrease the effect of gravity by a few hundred thousandths of a percent compared with the surrounding rock—small enough for an atom interferometer to detect. The international mining company BHP Billiton has used conventional gravity gradiometry to hunt for diamonds in Australia and Canada's Northwest Territories. Atom interferometry could make these searches even more accurate, Kasevich says.
Oil reserves and diamond caches aren't the only buried structures that atom interferometers could find, says Lowell, program manager for atom interferometry applications at the Defense Advanced Research Projects Agency (DARPA), an arm of the military that supports speculative research. DARPA will fund about $5 million in atom interferometry work this year, Lowell says.
Underground bunkers or even caves might someday be detected with an atom interferometer.
Better navigation may be the most immediate commercial application for atom interferometry. GPS can't reach every nook and cranny of the Earth—deep underwater, for instance—and planes need to have a way of navigating that doesn't count on spotty satellite signals. Today, most jumbo jets have navigation systems that measure the change in the Earth's rotation relative to a fixed point. But these gyroscopes lose about 1 kilometer of accuracy every hour. The military's atom interferometry system aims to cut the loss to 5 m every hour, Lowell says. The cross-country road trip planned for 2010 will test this accuracy by comparing the measurements of an onboard atom interferometer to known landmarks.
Such precision has become routine in lab experiments, but building an interferometer that can take the bumps of an airplane or helicopter ride demands changes to lab models, Lowell says. "We have systems that are now smaller and more compact and work with the flip of a switch, instead of a system that takes up massive chunks of tabletops and a rack of equipment, and takes armies of grad students to keep operating," he says.
A constant struggle
On a sprawling 20-by-20-foot table at the National Institute of Standards and Technology (NIST) in Gaithersburg, Md., physicist Kris Helmerson shows off an instrument that does in fact demand an army of graduate students.
On one end of the gigantic table, the eerie glow of an orange laser dances around a dozen little lenses and mirrors set at different angles and orientations. The laser will chill sodium atoms to a hair above absolute zero. In this state, hundreds of thousands of atoms together act like one big wave, called a Bose-Einstein condensate.
Just creating the condensate earned Nobel prizes in 2001 for the three physicists who accomplished the feat in 1995. Helmerson, who works in NIST's Advanced Measurement Laboratory, uses atom interferometry to measure more esoteric properties of Bose-Einstein condensates. In 2000, for instance, he and colleague Bill Phillips put a kind of sound wave, called a soliton, into their condensate. And in 2006, the team created a whirlpool-like vortex out of a condensate.
Less exotic pursuits keep other atom interferometers busy. The most common experiments are measurements of physical constants, the numbers describing gravity, electricity, and other forces that make the universe tick. Over the past two centuries, light interferometry refined these figures, and atom measurements will do the same, says physicist Steven Chu, director of the Lawrence Berkeley National Laboratory in California.
Chu, whose lab is based at Stanford, has shown that atom interferometry can achieve the same precision as conventional methods in measuring the acceleration due to gravity on Earth. With technological improvements, atom interferometry could do even better.
Atom interferometry could also refine another measure of gravity—the constant that describes the gravitational tug between any two objects, which physicists call "big G." Currently, G's value is known only to one part in 10,000. That's equivalent to knowing the length of 100 meters (roughly a football field) down to a centimeter—not very precise by the usual standards for fundamental constants of nature. "It's kind of embarrassing," Cronin says.
In a paper in the Feb. 8 Physical Review Letters, a team of Italian physicists suggests that atom interferometry could improve the precision of G by at least a factor of 10. In another paper, the same team used atom interferometry to measure gravity between objects separated by a few millionths of a meter. Such measurements could reveal a breakdown in Newton's laws of gravity, which could have implications for theories suggesting the existence of extra dimensions of space.
Adding an extra digit or two to constants like G may sound like the scientific equivalent of dotting i's and crossing t's, but physicists like Chu think otherwise. "It's not chasing after the last digits," he says. "It's chasing after the first digit of something radically new." Better telescope measurements of celestial orbits buttressed Newton's theory of gravity, and Einstein's theory of special relativity explained the results of Michelson and Morley's light experiments. Likewise, atom interferometry could augur the next scientific revolution, Chu says. "Whenever you make something orders of magnitude better and extend your ability to see, you're going to stumble onto something new."
Colliding frontiers
A basement physics laboratory at Stanford might produce such a discovery. Kasevich's team is nearly finished building the world's biggest atom interferometer, a 10 m deep well. Several factors determine an interferometer's precision, but the distance the atoms can travel is a big part of it. The first experiments planned for the device include testing whether objects of different mass fall at the same rate, echoing Galileo's legendary experiment at the leaning tower of Pisa. Kasevich plans to toss two different kinds of rubidium atoms down his interferometer. The atoms differ slightly in mass because one contains more neutrons than the other.
Even with his giant interferometer set to begin experiments in 6 months, Kasevich is thinking bigger. A planned underground laboratory in the abandoned Homestake gold mine in Lead, S.D., has the physicist licking his lips. "We've been brainstorming about what we can do if we have a kilometer rather than 10 meters," he says.
Yet all interferometers on Earth, however deep, suffer from environmental noise, such as ocean tides and earthquakes. Physicists go to great extremes to shield their instruments from these effects, but as precision increases, environmental noise eventually drowns out results. "For the ultimate in precision we would like to consider space," says Savas Dimopoulos, a theoretical physicist at Stanford who hopes to use atom interferometry to detect gravitational waves radiating from pairs of black holes and other binary systems. He's working with Kasevich to perform the experiments on Earth, but a space experiment offers an even better chance to see the gravitational waves, which could provide a glimpse of the early universe. Any launch is at least a decade away, Dimopoulos says.
While atom interferometry probes the universe with growing meticulousness, another frontier of physics is taking a different approach to unlocking its secrets. Deep below ground crossing Switzerland's border with France, an army of physicists and engineers is readying the world's largest particle accelerator, the Large Hadron Collider (LHC), for a planned summer startup. With the LHC, physicists will smash protons together at speeds within a sliver of the speed of light, hoping to find new forms of matter.
These two approaches—precision and high energy—overlap in the questions they can answer, Dimopoulos says. For instance, the LHC could detect what are called supersymmetric particles—sparticles—that he and other physicists have proposed lurk beside the quarks, neutrinos, and other elementary particles that fill the universe. Atom interferometry, by measuring the charge on an atom's electrons with great sensitivity, could support the existence of those supersymmetric particles. "In the end we are looking for a new theory," he says. "This one theory has the same consequences at high energy and high precision."
UA Physicists Discover 'Super Crystals' in a Semiconductor
University of Arizona physicists have discovered that "super crystals" -- crystals which are hundreds to thousands times larger than conventional crystals -- exist in certain organic semiconducting solids.
Pure super-crystalline organic semiconductors will conduct electricity much differently than conventional solids. Super-crystalline semiconductors, for example, could create splashes of current on electrical contacts, even in a uniform electric field, say UA physicist Andrei Lebed and graduate student Si Wu.
Most people understand how liquids freeze as solid crystals when temperatures become cold enough, like water droplets crystallizing into snowflakes or molten glass hardening into solid glass. Snowflakes are homogenous solids formed by a repeating, three-dimensional pattern of molecules that have fixed distances between the repeating molecular units. Solid glass approaches a perfect crystalline pattern, too, after a few hundred years, Lebed said.
Latter 20th-century physicists realized that at low enough temperatures, most liquids that exist in nature become energetically unstable as they solidify. Scientists discovered solids that don't have the commonly known, regular crystalline and glass phases - things like liquid crystals, quasi-crystals and charge-density waves. Charge-density waves are systems that display interesting physics, such as metals becoming insulators.
Understanding the physical nature of a solid phase is one of the most important problems in condensed matter physics, both from a fundamental point of view and from an applications point of view, Lebed and Wu said.
Leading American and Soviet physicists first predicted more than 25 years ago that some organic metals should be made up of "super crystals," Lebed said. Nobel laureate Robert Schrieffer, physicist Lev Gor'kov, who is a pioneer in superconductivity, and other members of the National Academy of Sciences were among the first to predict super crystals.
In super crystals, not only do the patterns of atoms or molecules repeat, there is also a periodically repeating super-structure of plane traps for electrons, Lebed said. "The distance between these plane traps, which are called soliton walls, are typically hundreds to thousands times greater than the distances between the organic molecules." (See the accompanying graphic, above, that illustrates this concept.)
U.S., Soviet and Japanese scientists, including Lebed, collaborated in research to discover the soliton wall superlattices, or super crystals, in organic metals. "Unfortunately, so far no one has discovered super crystals in organic metals," Lebed said.
Lebed and Wu are among the solid state theorists who collaborate with experimentalists in studying other materials that might possibly be super-crystalline.
"Our hopes for a discovery of a long awaited super-crystalline phase were raised after we started to analyze experimental data of James Brooks' group," Lebed said.
Brooks directs condensed matter experiments at the National High Magnetic Field Laboratory in Tallahassee, Fla. Three years ago, physicists there discovered a mysterious solid-state phase in a semiconductor made up of very complicated organic molecules, molecules of perylene (Per) and maleonitriledithiolate (mnt), in high magnetic fields.
"When Wu and I, who are theorists, analyzed the experimental data, what we found was a complete surprise to us," Lebed said. "Our theoretical calculations showed that the only way to explain the appearance of a mysterious high magnetic field state was to suggest that it appears inside a super-crystalline phase."
Lebed and Wu published their study in the July 13 issue of Physical Review Letters.
Future experiments are needed to confirm the theoretical discovery, Lebed added.
If experiments do confirm Lebed's and Wu's results, the novel, exotic solid phase in organic semiconductors promises important technological applications. Such semiconductors will conduct electricity in novels ways. Another striking feature of the super-crystalline semiconductor is that its period and electronic properties might be tuned by changing the strength of the external magnetic field, Lebed said.
Emergent Behavior and Hydrodynamics of Active Bioparticles
Researchers at Argonne National Laboratory and University of Arizona, Tucson, have developed a new approach to control the concentration and separation of swimming bacteria in confined geometries, such as thin fluid films and channels.
The method relies on the collective response of actively swimming bacteria, such as Bacillus Subtilis, E. Coli among others, and on the local change of pH levels induced by the transmission of a small electric current through the fluid. As a result of the pH variation, the living cells swim away from the electrodes and concentrate at the middle of the cell, whereas the dead bacteria remain immobile. The concentrated bacteria excite large-scale hydrodynamic flows in the fluid film, and after some time form a dense biofilm. This emergent behavior is captured in the framework of a mathematical model formulated in terms of a two-dimensional equation for local bacteria orientation coupled to the low Reynolds number Navier-Stokes equation for the fluid flow velocity.
The collective motion of the bacteria is represented by an additional source term in the Navier-Stokes equation. It is demonstrated that this system exhibits spontaneous formation of large-scale patterns with the characteristic scale determined by the density of the bacteria. The primary mechanism of instability is associated with the shear flow induced deflection of the orientation of the bacteria.
Significance
Our interdisciplinary studies, performed at the intersection of biology and condensed matter physics, have wide-ranging fundamental significance and will give insights into the origin of emergent behavior in systems characterized by competing long-range and short range interactions that are far from equilibrium. Our work may result in entirely new approaches to separate, control, and manipulate bacterial populations and other biological particles. Apart from the fundamental scientific issues, there are several areas of practical application upon which the proposed research would impact directly. Selective self-assembly and manipulation of microorganisms by electric and/or magnetic fields can be achieved in the near future, opening a wide variety of important applications in biomaterials and nanotechnology. The results were selected for an invited talk at the 2006 APS March Meeting. A joint ANL-University of Arizona seed proposal to BES DOE, utilizing these ideas of collective biohydrodynamics, was funded in 2006.
Performers
Andrey Sokolov and Igor Aronson (ANL-MSD)
Raymond Goldstein and John Kessler (Dept. of Physics, University of Arizona)
Arizona Radiocarbon Dating Lab Turns 25
By Lori Stiles
April 23, 2007
Lots of evidence, some priceless, has gone up in smoke at the University of Arizona since April 26, 1982.
That's when the National Science Foundation - Arizona Accelerator Mass Spectrometry (AMS) Lab officially dated its first sample by the radiocarbon dating technique.
On Thursday, April 26 -- exactly a quarter-century and roughly 75,000 radiocarbon measurements later -- more than 100 scientists will celebrate the pioneering facility's 25th anniversary with a mini workshop and reception at the Westward Look Resort, 245 E. Ina Road. The workshop and reception are open by invitation only.
In the radiocarbon dating technique, researchers burn the sample and convert the carbon dioxide given off by combustion to graphite. They use a huge machine, the accelerator mass spectrometer, to measure how much radioactive carbon, or carbon 14, is present in the graphite sample. Radioactive carbon decays at a known rate, giving scientists the object's radiocarbon age. They convert radiocarbon years to known calendar years by using the calibrated tree-ring record.
Among the most publicized individual objects dated at the UA lab are the Shroud of Turin, the Vinland Map, the Dead Sea Scrolls, the Gospel of Judas.
But the bulk of the lab's work is in geosciences, said Professor and AMS Lab Director A. J. Tim Jull. "Sediments and oceanographic samples, followed by anthropological and archaeological samples, are most of what we date," Jull said. He was one of three who initially staffed the lab in 1982.
The facility now has the capability to study a wide range of climatic, geologic and archaeological records using three other isotopes, beryllium 10, aluminum 26 and iodine 129, as well as carbon 14. The laboratory is jointly run by the UA geosciences and physics departments. In 2005, the Arizona AMS Lab dated objects for 265 scientists from more than 100 universities, 27 government laboratories and dozens of museums in the United States and abroad.
UA scientists on the lab's staff of 20 rely on the facility for their research on ocean corals, cave deposits, lake sediments and, increasingly, in tracing groundwater supplies. Some of the earliest dating was done for air pollution studies, Jull said. Future projects might include tracing nuclear materials for homeland security reasons, he added.
Not the least of its mission, the Arizona AMS Lab is an educational training ground for students and scientists. The Arizona lab's student education and public outreach mission sets it apart from the commercial laboratories that aim to provide analyses for profit, Jull said. For many years the Arizona AMS Facility has run a student intern program which provides free, hands-on training for graduate students doing research. The lab has also trained scientists from all over the world. As a result, places like China and Eastern Europe have recently established their own new isotope dating facilities, Jull added.
The Point of Icicles
By Ryan Krug
Contemplating some of nature's cool creations is always fun. Now a team of scientists from The University of Arizona in Tucson has figured out the physics of how drips of icy water can swell into the skinny spikes known as icicles.
Deciphering patterns in nature is a specialty of UA researchers Martin B. Short, James C. Baygents and Raymond E. Goldstein. In 2005, the team figured out that stalactites, the formations that hang from the ceilings of caves, have a unique underlying shape described by a strikingly simple mathematical equation.
However, stalactites aren't the only natural formations that look like elongated carrots. Once the researchers had found a mathematical representation of the stalactite's shape, they began to wonder if the solution applied to other similarly shaped natural formations caused by dripping water.
So the team decided to investigate icicles. Although other scientists have studied how icicles grow, they had not found a formula to describe their shape.
Surprisingly, the team found that the same mathematical formula that describes the shape of stalactites also describes the shape of icicles.
"Everyone knows what an icicle is and what it looks like, so this research is very accessible. I think it is amazing that science and math can explain something like this so well. It really highlights the beauty of nature," Short said.
The finding is surprising because the physical processes that form icicles are very different from those that form stalactites. Whereas heat diffusion and a rising air column are keys to an icicle's growth, the diffusion of carbon dioxide gas fuels a stalactite's growth.
Short, a doctoral candidate in UA's physics department, Baygents, a UA associate professor of chemical and environmental engineering, and Goldstein, a UA professor of physics and the Schlumberger Professor of Complex Physical Systems at the University of Cambridge in England, published their article, "A Free-Boundary Theory for the Shape of the Ideal Dripping Icicle," in the August 2006 issue of Physics of Fluids. The National Science Foundation funded the research.
As residents of cold climates know, icicles form when melting snow begins dripping down from a surface such as the edge of a roof. For an icicle to grow, there must be a constant layer of water flowing over it.
The growth of an icicle is caused by the diffusion of heat away from the icicle by a thin fluid layer of water and the resulting updraft of air traveling over the surface. The updraft of air occurs because the icicle is generally warmer than its surrounding environment, and thus convective heating causes the air surrounding the icicle to rise. As the rising air removes heat from the liquid layer, some of the water freezes, and the icicle grows thicker and elongates.
"At first, we focused only on the thin water layer covering the icicle, just like we did with stalactites," said Short. "It was only later that we examined the layer of rising air, which is technically more correct. Strangely though, both methods lead to the same mathematical shape for icicles."
The resulting shape turns out to be described by the same mathematical equation that describes stalactites. One could call it the Platonic form.
The team wanted to compare the predicted shape to real icicles. Because icicles are scarce in Tucson, the scientists naturally turned to the Internet. They were able to compare pictures of actual icicles with their predicted shape.
The team found that it doesn't matter how big or small the actual icicles were, they could all fit to the shape generated by the mathematical equation.
"Fundamentally, just like in the early stalactite work, it's a result that implies that the shape of an icicle, at least in its ideal, pristine form, ought to be described by this mathematical equation. And we found, examining images of icicles, that it is a very good fit," senior author Goldstein said.
The team's next step will be to solve the problem of how ripples are formed on the surfaces of both stalactites and icicles.
UA Physicists Invent 'QuIET'- Single Molecule Transistors
By Lori Stiles
University of Arizona physicists have discovered how to turn single molecules into working transistors. It's a breakthrough needed to make the next-generation of remarkably tiny, powerful computers that nanotechnologists dream of.
They have applied for a patent on their device, called Quantum Interference Effect Transistor, nicknamed "QuIET." The American Chemical Society publication, "Nano Letters," has published the researchers' article about it online at Nano Letters. The research is planned as the cover feature in the print edition in November.
A transistor is a device that switches electrical current on and off, just like a valve turns water on and off in a garden hose. Industry now uses transistors as small as 65 nanometers. The UA physicists propose making transistors as small as a single nanometer, or one billionth of a meter.
"All transistors in current technology, and almost all proposed transistors, regulate current flow by raising and lowering an energy barrier," University of Arizona physicist Charles A. Stafford said. "Using electricity to raise and lower energy barriers has worked for a century of switches, but that approach is about to hit the wall."
Transistors can't shrink much smaller than 25 nanometers, or 1/40,000 the width of a pinhead, because scaling down further creates intractable energy problems, Stafford said. Even if it were possible to build an ultra-advanced laptop computer with molecule-sized transistors using current transistor technology, it would take a city's worth of electricity to run the laptop, and the thing would get so hot it would probably vaporize.
Stafford, UA physicist Sumit Mazumdar and David Cardamone, who received his doctorate from UA in 2005, began thinking about the problem of next-generation transistor technology three years ago. They realized that quantum mechanics can solve the problem of how to regulate current flow in a single-molecule transistor that would work at room temperature.
"Our approach is a little more finesse than brute force," Cardamone said. "We don't put up a wall to stop current. It's just that we can regulate how electron waves combine to turn the transistor on or off."
The simplest molecule they propose for a transistor is benzene, a ring-like molecule. They propose attaching two electrical leads to the ring to create two alternate paths through which current can flow.
They also propose attaching a third lead opposite one of the electrical leads. Other researchers have succeeded in attaching two contacts to a molecule this small, but attaching the third is the trick -- and the point. The third lead is what turns the device on and off, the "valve."
"In classical physics, the two currents through each arm of the ring would just add," Stafford said. "But quantum mechanically, the two electron waves interfere with each other destructively, so no current gets through. That's the 'off' state of the transistor."
The transistor is turned on by changing the phase of the waves so they don't destructively interfere with each other, opening up addiitonal paths through the third lead.
"It took a while to go from the idea of how this could work to developing realistic calculations of this kind of system," Stafford said. "We were able to do the simplest kind of quantum chemical calculations which neglect interactions between different electrons within a few weeks. But it took some time to put in all the electron interactions that demonstrate this really is a very robust device."
According to the Semiconductor Research Corp. it typically takes a dozen years for a new idea to go from initial scientific publication to commercial technological application, Stafford noted.
"That means if the computer industry is to continue its recent pace in making smaller-scale computers, we should have had this idea yesterday, " Cardamone said.
Why all this effort to make incomprehensibly small computers? Why expend so much brainpower on nanocomputing?
More computing power will result in more realistic simulations, whether you're a scientist modeling global warming or supernovae explosions, or an entertainment industry animator creating believable emotion in a simulated human face, Stafford said. 
Nanocomputers could have a major impact in medicine, Cardamone said. "These machines could operate in solution, in vivo. There already are clinical trials of nanoparticles to deliver medicinal drugs. Imagine how much more powerful those little nanoparticles or nanorobots would be if they could count, or do simple computation. With our transistors packed at maximum density, you could put a microprocessor as powerful as the top-of-the-line workstation on the back of an E. coli."
"Have you seen the movie, Fantastic Voyage?" Stafford asked. A nano-sized surgical team journeyed through a human body in the 1966 sci-fi flick. That's a different story, but with a similar theme.
"We're not futurists at all and can't predict it, but imagine that you could make an artificial intelligence, that you could have this little submarine that goes inside somebody's arteries and capillaries to repair them," Stafford said.
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