Previous experiments have shown that in high-temperature superconductors known as cuprates, electrons bind together in pairs. The energy required to pull a pair apart -- called the energy gap -- is different in different directions; a plot of energy vs. direction forms a cloverleaf pattern. The explanation for this so-called "pseudogap" has so far eluded physicists.
The new work finds that in a cuprate that is not a superconductor at any temperature the same cloverleaf-shaped energy gap appears. The surprise for physicists is that the same materials in two very different states apparently have identical energy-gap structures.
"This may provide a key to understanding the superconducting phenomenon," said J.C. Séamus Davis, Cornell professor of physics, who collaborated in the work with Brookhaven physicist Tonica Valla. "This is the first time that it has been possible to measure the electronic structure of this very important material. The big surprise is that we go to this state where it's not superconducting, and we measure the electronic structure, and lo and behold, it's the same [as the superconductor]."
Their experiments were described Nov. 16 in the online journal Science Express and will appear in a future print edition of Science.
Superconductors conduct electricity with virtually no resistance. The phenomenon was first discovered in materials cooled to near absolute zero by immersion in liquid helium. Certain oxides of copper called cuprates that have been "doped" with small amounts of other elements become superconducting at temperatures up to 134 kelvins (degrees above absolute zero) or more, depending on pressure. These materials can be cooled with much less expensive liquid nitrogen and are in wide use in industry.
"Doping" disrupts the crystal structure of the copper oxide, creating "holes" where electrons ought to be, and this somehow facilitates superconductivity. Physicists have been puzzled by the fact that at a certain low level of doping, many cuprates cease to superconduct, yet at levels above and below this, superconductivity returns.
Valla, Davis and co-workers studied a version of a cuprate known as LBCO that ceases to superconduct when just one-eighth of its electrons have been removed. Previous measurements have shown that in this material the electrons arrange themselves in alternating "stripes" about four atoms wide, and this somehow seems to inhibit superconductivity.
The researchers studied samples cooled to near absolute zero -- where the material is still not a superconductor -- to observe the simplest or "ground" state. This was, the researchers said, the first measurement of the electronic structure of a cuprate in which the material's superconductivity did not interfere.
Valla's group measured the energy and momentum of the electrons in the non-superconducting LBCO by photoemission spectroscopy, in which X-rays are used to knock electrons off the surface for measurement. Davis and colleagues at Cornell studied a piece of the same crystal with a specially built scanning tunneling microscope so sensitive that it can detect the arrangement of electrons in the material. They were amazed to find that in both kinds of measurements, all low-energy electronic signatures were the same in the "striped" material as in superconducting cuprates.
Valla speculated that the difference lies in the way electrons form pairs, that they might sometimes pair too strongly for superconductivity to work. Davis declined to speculate, simply saying, "The electronic structure we observe ... appears to indicate that the 'striped' state is intimately related to the superconducting state -- perhaps they are two sides of the same coin."
It will be up to theorists, he said, to revise their theories to account for these results.
The research was supported by the U.S. Department of Energy, the Office of Naval Research and Cornell.
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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Cornell University.
Monday, May 31, 2010
'Stripes' And Superconductivity: Two Faces Of The Same Coin?
World's Smallest Superconductor Developed: Sheet of Four Pairs of Molecules Less Than One Nanometer Wide
"Researchers have said that it's almost impossible to make nanoscale interconnects using metallic conductors because the resistance increases as the size of wire becomes smaller. The nanowires become so hot that they can melt and destruct. That issue, Joule heating, has been a major barrier for making nanoscale devices a reality," said lead author Saw-Wai Hla, an associate professor of physics and astronomy with Ohio University's Nanoscale and Quantum Phenomena Institute.
Superconducting materials have an electrical resistance of zero, and so can carry large electrical currents without power dissipation or heat generation. Superconductivity was first discovered in 1911, and until recently, was considered a macroscopic phenomenon. The current finding suggests, however, that it exists at the molecular scale, which opens up a novel route for studying this phenomenon, Hla said. Superconductors currently are used in applications ranging from supercomputers to brain imaging devices.
In the new study, which was funded by the U.S. Department of Energy, Hla's team examined synthesized molecules of a type of organic salt, (BETS)2-GaCl4, placed on a surface of silver. Using scanning tunneling spectroscopy, the scientists observed superconductivity in molecular chains of various lengths. For chains below 50 nanometers in length, superconductivity decreased as the chains became shorter. However, the researchers were still able to observe the phenomenon in chains as small as four pairs of molecules, or 3.5 nanometers in length.
To observe superconductivity at this scale, the scientists needed to cool the molecules to a temperature of 10 Kelvin. Warmer temperatures reduced the activity. In future studies, scientists can test different types of materials that might be able to form nanoscale superconducting wires at higher temperatures, Hla said.
"But we've opened up a new way to understand this phenomenon, which could lead to new materials that could be engineered to work at higher temperatures," he said.
The study also is noteworthy for providing evidence that superconducting organic salts can grow on a substrate material.
"This is also vital if one wants to fabricate nanoscale electronic circuits using organic molecules," Hla added.
Collaborators on the paper include Kandal Clark, a doctoral student in the Russ College of Engineering and Technology at Ohio University; Sajida Khan, a graduate student in the Department of Physics and Astronomy at Ohio University; Abdou Hassanien, a researcher with the Nanotechnology Research Institute, Advanced Industrial Science and Technology (AIST) and the Japan Science and Technology Agency's Core Research of Evolutional Science & Technology (JST-CREST) in Japan who conducted the work as a visiting scientist at Ohio University; Hisashi Tanaka, a scientist at AIST and JST-CREST who synthesized the molecules; and Kai-Felix Braun, a scientist with the Physikalisch Technische Bundesanstalt in Braunschweig, Germany, who conducted the calculations at the Ohio Supercomputing Center.
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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Ohio University. The original article was written by Andrea Gibson.
Flight Of The Bumble Bee Is Based More On Brute Force Than Aerodynamic Efficiency
In recent years scientists have modelled how insect wings interact with the air around them to generate lift by using computational models that are relatively simple, often simplifying the motion or shape of the wings.
"We decided to go back to the insect itself and use smoke, a wind tunnel and high-speed cameras to observe in detail how real bumblebee wings work in free flight," said Dr Richard Bomphrey of the Department of Zoology, co-author of a report of the research published this month in Experiments in Fluids. ‘We found that bumblebee flight is surprisingly inefficient – aerodynamically-speaking it’s as if the insect is ‘split in half’ as not only do its left and right wings flap independently but the airflow around them never joins up to help it slip through the air more easily.’
Such an extreme aerodynamic separation between left and right sets the bumblebee [Bombus terrestris] apart from most other flying animals.
"Our observations show that, instead of the aerodynamic finesse found in most other insects, bumblebees have a adopted a brute force approach powered by a huge thorax and fuelled by energy-rich nectar," said Dr Bomphrey. "This approach may be due to its particularly wide body shape, or it could have evolved to make bumblebees more manoeuvrable in the air at the cost of a less efficient flying style."
Professor Adrian Thomas of Oxford’s Department of Zoology, co-author of the report, said: "a bumblebee is a tanker-truck, its job is to transport nectar and pollen back to the hive. Efficiency is unlikely to be important for that way of life."
Observing insects in free – as opposed to tethered – flight is a considerable challenge. The Oxford team trained bumblebees to commute from their hive to harvest pollen from cut flowers at one end of a wind tunnel. They then used the wind tunnel to blow streams of smoke passed the flying bees, to reveal vortices in the air, and recorded the results with high-speed cameras taking up to 2000 images per second. From these images the team were able to visualise the airflow over flapping bumblebee wings.
The old myth that "bumblebees shouldn’t be able to fly" was based on calculations using the aerodynamic theory of 1918-19, just 15 years after the Wright brothers made the first powered flight. These early theories suggested that bumblebee wings were too small to create sufficient lift but since then scientists have made huge advances in understanding aerodynamics and how different kinds of airflow can generate lift.
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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Oxford.
Journal Reference:
- Richard James Bomphrey, Graham K. Taylor and Adrian L. R. Thomas. Smoke visualization of free-flying bumblebees indicates independent leading-edge vortices on each wing pair. Experiments in Fluids, 2009; 46 (5): 811 DOI: 10.1007/s00348-009-0631-8
Monday, May 3, 2010
Theorists and experimentlists
A theorist should pay close attention to experimental facts. He should lay greater weight on facts than on the thoughts from his peers. It is the facts that say the last word, and that constitute the objects which he has to be directly faced with. He should not do simply cobbler and tailor type jobs. He must try to figure out what is hidden behind phenomenology. In a sense, the thoughts from his contemporaries are just secondary. So, theorists, please attend meticulously to facts.
Similarly, an experimentalist should listen carefully to speculative views. Because, ignoring these views make him blind to the meanings of his experiments. It is a theory that confers essence upon observations. Without theory, it is futile and pointless to make observations.
Henceforth, a real physicist should intend to be complete.