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Post Info TOPIC: SUPERFLUIDITY


L

Posts: 131433
Date:
Doubly Charged Particles
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Are Doubly Charged Particles Lurking in High-T Superconductors?
One of the greatest unsolved problems in condensed matter physics is explaining how electrons pair up in the copper-oxide materials that superconduct at temperatures above 100 K. Some theorists believe that the place to start in straightening out this mystery is to understand better how the cuprates behave at normal temperatures, long before they become superconducting.

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Posts: 131433
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Superconductor breakthrough
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Boffins from Canada have had a breakthrough in the field of superconductors and are now predicting a technological revolution.
The boffins - from the University of British Columbia and the University of Sherbrooke - detected an elusive signature of electrons within a high-temperature superconductor.
According to the popular science mag Nature, this solves a decades-old mystery surrounding metals that carry electricity without resistance.
It will open the door for everyday trains that levitate on magnetic fields, ultrapowerful quantum computers and big savings for utilities.
Nature thinks that it will lead to the introduction of room temperature superconductors with 10 years and could be a revolution similar to the invention of the transistor.

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Posts: 131433
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Quantum mechanics
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If you want to really see quantum mechanics in action, you've got to turn the temperature down so low that even atoms stop moving. Physicists have come close to achieving this "absolute zero" state by using precision-tuned lasers, but the technique has only allowed researchers to freeze small groups of atoms at a time. Now members of an international team say they have managed to cool a dime-sized mirror to within one degree of absolute zero, the lowest laser-induced freeze yet achieved with a visible object.
One of the greatest enigmas in physics is how matter can be governed by the four basic forces of nature--electromagnetism, which governs light, heat and electricity; the strong and weak nuclear forces, which bind atoms together; and gravity--and still follow the rules of quantum mechanics, which operate only at the subatomic level. In other words, scientists want to know how solid objects keep from flying apart when their atoms are also influenced by the chaotic nature of quantum physics. The major research obstacle has been that natural forces overwhelm quantum effects. The only way to cancel those forces entirely is to cool an atom down to absolute zero (-237 degrees Celsius), where quantum forces apply exclusively.

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L

Posts: 131433
Date:
Fermions
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Fermions tend to avoid each other and cannot "travel" in close proximity. Demonstrated by a team at the Institut d'optique (CNRS/Université Paris 11, Orsay-Palaiseau), this result is described in detail in the January 25, 2007 issue of Nature. It marks a major advance in our understanding of phenomena at a quantum scale.
For many years, the theory of quantum mechanics stipulated that certain particles, the fermions , were incapable of "travelling" in close proximity. For example, in a jet of identical particles, the theory supposed that the distance between them was always greater than a given value, called the "correlation length".
Scientists in the Charles Fabry Laboratory at the Institut d'optique, working with a team from the Free University in Amsterdam, have recently shown that this "anti-bunching" property, which it had never been possible to demonstrate hitherto, does indeed exist. It is as if the particles repel each other, even though interactions between them are negligible. In fact, this "anti-bunching" is due to quantum interferences which forbid the probability of finding two very close particles.
To arrive at this conclusion, the scientists compared the behaviour of fermions with that of bosons , under identical conditions. Amongst the latter, the same interferences led on the contrary to a "bunching" effect, and thus an increased probability of finding two particles together. The experiments at the Institut d'optique were performed using the same system (which ensured identical conditions) on two helium isotopes. In this situation, the scientists demonstrated the correlation length of fermions, which was close to a millimetre. This effect was anticipated, but its demonstration constitutes an advance in our ability to detect correlations between atoms, and thus a further step towards understanding the behaviour of matter at the quantum scale.

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L

Posts: 131433
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New phase transition at absolute zero
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The atomic constituents of matter are never still, even at absolute zero (-273.15 degrees Celsius). This consequence of quantum mechanics can result in continuous transition between different material states. Physicists at the Max Planck Institute for Chemical Physics of Solids have studied this phenomenon using ytterbium, rhodium and silicon at very low temperatures under the varying influence of a magnetic field. Until now, it has been assumed that the properties of a transition of this nature can be described completely with the fluctuations of one parameter, in this case, magnetic order. However, the experiments that have now been published reveal, completely unexpectedly, an additional change to the electronic properties of the transition. It confirms again that quantum effects can result in phenomena that are inconceivable in classical physics. On the one hand, the results extend the general understanding of phase transitions and, on the other, are also relevant to complex systems, such as high-temperature superconductors (Science, February 2007

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-- Edited by Blobrana at 07:08, 2007-03-10

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L

Posts: 131433
Date:
Bose–Einstein condensation
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BoseEinstein condensation is one of the most fascinating phenomena predicted by quantum mechanics. It involves the formation of a collective quantum state composed of identical particles with integer angular momentum (bosons), if the particle density exceeds a critical value. To achieve BoseEinstein condensation, one can either decrease the temperature or increase the density of bosons. It has been predicted that a quasi-equilibrium system of bosons could undergo BoseEinstein condensation even at relatively high temperatures, if the flow rate of energy pumped into the system exceeds a critical value. Here we report the observation of BoseEinstein condensation in a gas of magnons at room temperature. Magnons are the quanta of magnetic excitations in a magnetically ordered ensemble of magnetic moments. In thermal equilibrium, they can be described by BoseEinstein statistics with zero chemical potential and a temperature-dependent density. In the experiments presented here, we show that by using a technique of microwave pumping it is possible to excite additional magnons and to create a gas of quasi-equilibrium magnons with a non-zero chemical potential. With increasing pumping intensity, the chemical potential reaches the energy of the lowest magnon state, and a Bose condensate of magnons is formed.

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L

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RE: SUPERFLUIDITY
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For the first time, MIT scientists have directly observed the transition of a gas to a superfluid, a form of matter closely related to the superconductors that allow electrical currents to travel without resistance.

Observations of superfluids may help solve lingering questions about high-temperature superconductivity, which has widespread applications for magnets, sensors and energy-efficient transport of electricity.
The superfluid gas created at MIT can also serve as an easily controlled model system to study properties of neutron stars or the quark-gluon plasma that existed in the early universe.
The work, reported in the July 6 issue of Nature and in the July 18 issue of Physical Review Letters, was led by Nobel laureate Wolfgang Ketterle, the John D. MacArthur Professor of Physics and a principal investigator in MIT's Research Laboratory of Electronics.
The team observed the transition to superfluidity of a gas of so-called fermionic atoms. Fermionic atoms are atoms with an odd number of neutrons, protons and electrons. They can become superfluid only if they form pairs. These pairs then have an even number of basic constituents and can form a kind of Bose-Einstein condensate, a type of matter where all pairs act as a giant matter wave, "march in lockstep" and flow without friction.
For several years, research groups around the world have seen the transition of a Fermi gas to superfluidity only indirectly because this transition was not accompanied by any change in the appearance of the gas cloud.
The new trick used by the MIT group was to have an unequal number of two kinds of fermions, sometimes labelled as spin up and spin down. In this situation, not all the atoms can find a partner to form a pair, and the difference between the paired superfluid and the gas of unpaired atoms is clearly visible.
One may regard the two kinds of fermions as women and men on the dance floor who have to pair up to perform a superfluid dance. At first, it was not clear what would happen if the men outnumbered the women. Would the single men take part in the dance, would they stay at the side of the dance floor or would their presence cause everyone to stop dancing?
The dance ends when the ratio of men to women exceeds six to one -- this breakdown of superfluidity was observed by the MIT researchers in 2005.

For a smaller ratio, the superfluid dance continues.
In the current work, the MIT team has found that for the superfluid dance to go on, the single men (excess atoms) must be expelled from the dance floor. This expulsion is directly observed as the shell of excess atoms surrounding the superfluid core. When the atoms are cooled down, the appearance of superfluidity is accompanied by a sudden change in the shape of the cloud.

"To see directly how the superfluid core forms in the center is quite amazing. Our results challenge state-of-the-art theory" - Martin Zwierlein, physics graduate student.

"The features we have observed are very difficult to reproduce in calculations"Yong-Il Shin, Postdoctoral associate.

Ketterle's team members, in addition to Zwierlein and Shin, were MIT physics graduate students André Schirotzek and Christian Schunck. All are members of MIT's Center for Ultracold Atoms.
The team observed fermionic superfluidity by cooling the gas close to about 50 billionths of 1 Kelvin, very close to absolute zero (-273 degrees C or -459 degrees F). By using phase-contrast imaging -- a standard microscopy technique -- in a novel way, they could directly observe the superfluid core and the shell of excess atoms around it.
Properties of superfluid ultracold fermions are also being studied by teams at the University of Colorado at Boulder, the University of Innsbruck in Austria, the École Normale Supérieure in Paris, Duke University and Rice University. The Rice group has also studied imbalanced Fermi mixtures.

The MIT research was supported by the National Science Foundation, the Office of Naval Research and NASA.

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L

Posts: 131433
Date:
Superconductivity
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After an exhaustive data search for new compounds, researchers at Duke University's Pratt School of Engineering have discovered a theoretical "metal sandwich" that is expected to be a good superconductor. Superconductive materials have no resistance to the flow of electric current.

The new lithium monoboride (LiB) compound is a "binary alloy" consisting of two layers of boron -- the "bread" of the atomic sandwich -- with lithium metal "filling" in between, the researchers said. Once the material is synthesised, it should be superconductive at a higher temperature than other superconductors in its class, according to their results.

The researchers reported their findings in the May 5 online edition of the journal Physical Review B, Rapid Communications.

"To the best of our knowledge, this alloy structure had not been considered before. We have been able to identify synthesis conditions under which the LiB compound should form. And we believe that if the material can be synthesised, it should superconduct at a higher temperature, perhaps more than 10 percent greater, than any other binary alloy superconductor." - Stefano Curtarolo, professor of mechanical engineering and materials sciences at Duke's Pratt School.

"The significance of the work is not only the discovery of lithium monoboride itself, but also that this opens the door to finding derivatives that could aid in the search for additional novel superconductors" - Aleksey Kolmogorov, lead author of the study and a postdoctoral fellow at the Pratt School.

He said that once a new superconductive material is identified, scientists typically can manipulate the substance -- twisting it or doping it with other elements – to create related structures that might have even more appealing properties.
Superconductors have the potential to produce more efficient electronics and electric generators, according to the researchers. The materials also have unique magnetic capabilities that may enable their use in transportation applications, such as "levitated" trains that glide over their tracks with virtually no friction.
However, today's superconductors perform only when cooled to extremely low temperatures near absolute zero, which is -459.67 degrees Fahrenheit, or 0 degrees Kelvin. This requirement makes their use prohibitively expensive, the researchers said.
The first superconductive material was identified in 1911 when a Dutch scientist cooled mercury to 4 degrees Kelvin, the temperature of liquid helium. Since then, scientists have discovered superconductivity in various materials, including other pure elements, complex ceramics, and binary alloys.
Since 1986, ceramics have held the overall record for highest superconducting temperature -- currently 138 degrees Kelvin. Among pure elements, lithium, when contained under pressure, holds the record at 20 degrees Kelvin.

Recently, scientists scored an unexpected breakthrough with the discovery of superconductivity in the simple binary alloy magnesium diboride (MgB2), Curtarolo said. This compound holds the current temperature record for its class at 39 degrees Kelvin, and it has attracted much attention because it can be produced relatively easily from two abundant elements.


In new binary alloy, two layers of boron 'bread' surround a 'filling' of lithium metal.
Credit Duke University



"The physics of the superconductivity in MgB2 is now well understood. However, MgB2 has been shown to be such a unique superconductor -- finely tuned by nature -- that attempts to improve it or use it as a model for finding even better superconducting materials have so far been fruitless"- Aleksey Kolmogorov.

Curtarolo and Kolmogorov decided it was time to try something else. Using a theoretical data-mining method developed by Curtarolo, the pair scoured a database of experimental and hypothetical compounds, looking for other possible configurations of binary alloys and tweaking their compositions.

In the process, the team stumbled onto "a path to a new metal sandwich structure consisting of stacks of metal and boron layers," - Stefano Curtarolo.

Additional calculations identified the binary alloy lithium monoboride as a promising candidate that might be both structurally stable and superconductive at temperatures that exceed those of the current binary alloy record-holder.

"It's a very thin line, because as you try to increase the temperature at which a material becomes superconducting, the material tends to lose its stability. But we think lithium monoboride should be stable and superconduct at temperatures greater than 39 degrees Kelvin" - Aleksey Kolmogorov

"It was like spotting a $100 bill on the street. It seemed impossible that this could be real and that no one had seen it before" - Stefano Curtarolo

The researchers are now conducting more precise theoretical calculations of LiB's "critical temperature" -- that is, the temperature at which it becomes superconductive -- with computational support from the San Diego Supercomputer Centre at the University of California, San Diego.
The material will have to be synthesised before experimental tests can confirm any of the theoretical results, the researchers said. They added that this won't be an easy process, as manufacturing lithium monoboride will require extremely high temperatures and pressures.

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L

Posts: 131433
Date:
Fermion Superfluid
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In quantum physics, every tiny spec of matter has something called "spin" -- an intrinsic trait that cannot be changed and which dictates, very specifically, what other bits of matter the spec can share quantum space with. When fermions, the most antisocial type of quantum particle, do get together, they pair up in a wondrous dance that enables such things as superconductivity.

For the first time, researchers at Rice University have succeeded in creating and observing an elusive and long-sought quantum state - a superfluid of fermions with mismatched numbers of dance partners.
Despite more than 40 years of theoretical musings about what would occur in such a case, the result - a cluster of matched pairs surrounded by a cloud of would-be dance partners - was largely unexpected.
The research, which appears online this week, is slated to appear in an upcoming issue of the journal Science, together with a paper from MIT reporting related results. The experiments offer physicists a new window into two of the least understood and most intriguing phenomena in physics - superconductivity and superfluidity.
Both phenomena result from a change in the phase of matter. Anyone who has seen ice melt has seen matter change phases, and when electrons, atoms and other specs of matter change quantum phases, they behave just as differently as do ice and water in a glass.
Superconducting and superfluid phases of matter occur in fermions - the antisocial particles that can't share quantum space - only when quantum forces become dominant. Because thermodynamic forces are typically so powerful that they overwhelm quantum interactions - like loud music overwhelms the whisper of someone nearby - superconductivity and superfluidity only occur in extreme cold.


This is a 3-dimensional projection of an image of a phase separated atomic cloud. The tall central (semi-transparent) region consists of paired fermionic 6 Li atoms, and is believed to be a superfluid. The shorter (opaque) peaks on either side, as well as the faint ring around the bottom, are unpaired atoms which have been expelled from the paired central region. The light in the background is a representation of the probe laser beam used to image this cloud.
Image credit: Rice University.


In the Rice experiment, when temperatures drop to within a few billionths of a degree of absolute zero, fermions with equal but opposite spin become attracted to one another and behave, in some respects, like one particle. Like a couple on the dance floor, they don't technically share space, but they move in unison. In superconductors, these dancing pairs allow electrical current to flow through the material without any resistance at all, a property that engineers have long dreamed of harnessing to eliminate "leakage" in power cables, something that costs billions of dollars per year in the U.S. alone.

The superconducting and superfluid phases are analogous except that superconductivity happens with particles carrying an electrical charge and superfluidity occurs in electrically neutral particles. In superfluids, fermionic pairing leads to a complete absence of viscosity - like a wave rippling back and forth in a swimming pool without ever diminishing.

"Conventional theory tells us superconductivity or superfluidity occurs only in the presence of an equal number of spin-up and spin-down particles. Physicists have speculated for almost 50 years about what would happen if this condition were not met. Because of the pristine and controlled nature of our ultracold atoms, we're able to offer definitive evidence of what happens with mismatched numbers of spin-up and spin-down particles" - lead researcher Randy Hulet, Fayez Sarofim Professor of Physics and Astronomy.

Ultracold experiments at temperatures just a few billionths of a degree above absolute zero are Hulet's specialty. It's only been technically possible to chill atoms to these temperatures for the past 10 years, but in that time, this ability has proved remarkably useful for testing the predictions of quantum mechanics and for exploring the properties of what physicists call "many-body phenomena," including superconductivity and superfluidity.
In the latest experiments, Hulet's team - which includes graduate students Guthrie Partridge, Ramsey Kamar and Yean-an Liao and postdoctoral researcher Wenhui Li - cooled a mixture of fermionic lithium-6 atoms to about 30-billionths of a degree above absolute zero. That's far colder than any temperature in nature - even in deepest interstellar space - and it's sufficient to quell virtually all thermodynamic interaction in the atoms, leaving them subject to superfluid quantum pairing.
Using radio waves, Hulet's team altered the ratio of spin-up and spin-down atoms in the cooled atoms with great precision. They found that the superfluid was able to tolerate an excess of up to 10 percent unpaired fermions with no detrimental effects.
Hulet's team found that increasing the ratio of spin-up to spin-down atoms eventually caused a phase change. When unpaired spin-up atoms rose above 10 percent of the total sample, the unpaired loners were suddenly expelled, leaving a core of superfluid pairs surrounded by a shell of excess spin-up atoms.
It is the unbalanced yet seemingly unaffected superfluid, however, that is capturing most of the scientific attention at the moment.

"The gas behaves as if it is still perfectly paired, which is quite remarkable given the excess of spin-up atoms. This was unexpected, and it could signal a new, exotic form of superfluidity that may be akin to the electron pairings in unconventional superconductors or to the quark soup that's predicted to exist at the heart of the densest neutron stars" - Randy Hulet.

In the largest neutron stars - known as "quark stars" - a mass about five times greater than the sun is pressed into a space smaller than the island of Manhattan. Some physics theorists believe gravity is so strong at the heart of these stars that it creates something called a "strange matter," a dense superfluid of up quarks, down quarks and strange quarks.

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L

Posts: 131433
Date:
RE: SUPERFLUIDITY
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Whirling Atoms Dance into Physics Textbooks

NASA-funded researchers at the Massachusetts Institute of Technology, Cambridge, Mass., have created a new form of superfluid matter. This research may lead to improved superconducting materials, useful for energy-efficient electricity transport and better medical diagnostic tools.

The research marks the first time scientists have positively created a friction-free superfluid using a gas of fermionic atoms, atoms with an odd number of electrons, protons and neutrons. The breakthrough happened on the night of April 13.

"It's a night I won't forget. It was overwhelming to watch on our computers as the lithium atoms behaved in a way that no one had ever seen before" - Dr. Wolfgang Ketterle, a Nobel prize-winning physics professor at MIT who led the team of researchers.

To accomplish this experiment, Ketterle's team cooled a gas cloud of lithium atoms to nearly absolute zero (about minus 459 degrees Fahrenheit). They used an infrared laser beam to trap the gas, then a green laser to spin it.

A normal gas simply spins, but a superfluid can rotate only by forming quantum whirlpools. A rotating superfluid looks like Swiss cheese; the holes are the cores of the whirlpools. This is exactly what the MIT physicists observed that night.

In 1995, Ketterle and his team were among the first to create a Bose-Einstein condensate, composed of bosonic atoms that have an even number of electrons, neutrons and protons. In Bose-Einstein condensates, particles act as one big wave, a phenomenon predicted by Albert Einstein in 1925. That discovery earned Ketterle a shared Nobel Prize in Physics in 2001. Bose-Einstein condensates were later shown to be superfluids.

The new frontier became fermions. Fermions must pair up to have an even number of electrons, neutrons and protons, which allows them to form a Bose-Einstein condensate. Breakthroughs at MIT and several other institutions, including Duke University, Durham, North Carolina, produced Bose-Einstein condensation of fermion pairs loosely bound as molecules, but found no concrete evidence of superfluidity.

Over the past two years researchers have been looking for the "smoking gun" for fermionic superfluidity. Despite some hints and indirect evidence, it was not found until this research team's discovery.

Superconductivity is superfluidity for charged particles instead of atoms. High-temperature superconductivity is not fully understood, but the MIT observations open up opportunities to study the microscopic mechanisms behind this phenomenon.

"Pairing electrons in the same way as our fermionic atoms would result in room-temperature superconductors. It is a long way to go, but room-temperature superconductors would find many real-world applications, from medical diagnostics to energy transport" - Dr. Wolfgang Ketterle.
Superfluid Fermi gas might also help scientists test ideas about other Fermi systems, like spinning neutron stars and the primordial soup of the early universe.

The MIT research was supported by the National Science Foundation, the Office of Naval Research, the Army Research Office, and NASA's Fundamental Physics in Exploration Systems Mission Directorate, in support of the Vision for Space Exploration. NASA's Jet Propulsion Laboratory, Pasadena, Calif., Pasadena, manages the Fundamental Physics program.

The research was published in the June 23 issue of Nature. Ketterle's co-authors include grad students Schirotzek, Schunck and Zwierlein, and former grad student Abo-Shaeer. They are all members of the NSF-funded MIT-Harvard Centre for Ultracold Atoms.

nasa press release

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