Physicists find new parallel between cold gases and 'hot' superconductors

Jul 08, 2010
NIST Fellow Deborah Jin in her laboratory at JILA where she studies ultracold atomic gases and their similarities to high-temperature superconductors. The atoms are initially trapped and cooled in a glass cylinder located below the copper coils. Credit: Burrus/NIST

Scientists at JILA, working with Italian theorists, have discovered another notable similarity between ultracold atomic gases and high-temperature superconductors, suggesting there may be a relatively simple shared explanation for equivalent behaviors of the two very different systems.

Described in , the new research lends more support to the idea that JILA studies of superfluidity (flow with zero friction) in atomic gases may help scientists understand far more complicated , solids with zero resistance to electrical current at relatively high temperatures. Known high-temperature superconductors only superconduct well below room temperature, but a detailed understanding of how the materials work may one day lead to practical applications such as more efficient transmission of electricity across power grids.

JILA is operated jointly by the National Institute of Standards and Technology and the University of Colorado at Boulder.

The JILA group studies how atoms in a Fermi gas behave as they "cross over" from acting like a , in which atom pairs form tightly bound molecules, to behaving like pairs of separated electrons in a superconductor. A Fermi gas is a collection of noninteracting particles called fermions, one of two categories of fundamental particles found in nature (bosons are the other). Identical fermions cannot occupy the same place at the same time.

In the new study, JILA scientists applied a technique they developed in 2008 to explore subtle energy properties of . The technique is an adaptation of photoemission spectroscopy, long used to probe the energy of electrons in materials. A superconductor research group recently used electron photoemission spectroscopy to find evidence of electron pairing above the critical temperature where the material switches from a superconductor to a regular conductor. Why this duality occurs is a subject of debate.

The JILA scientists performed comparable measurements for an ultracold gas of potassium atoms at and above temperatures where superfluidity disappears. Like the superconductor group, the JILA team found evidence of atom pairing above the . This demonstrates the existence of a so-called "pseudo-gap region" where the system retains some pairs of correlated but not all characteristics of superfluidity. The findings were made possible in part by significant improvements in the signal strength of the atom photoemission spectroscopy technique since 2008.

"What makes this really interesting is that the two systems are actually very different, with the high-temperature superconductor being much more complicated than atomic gases," says NIST/JILA Fellow Deborah Jin. "The observation of similar behavior with similar measurements suggests that having a pseudogap phase does not require complicated explanations, such as lattice effects, two-dimensionality, or exotic pairing mechanisms."

Explore further: Experiment with speeding ions verifies relativistic time dilation to new level of precision

More information: J.P. Gaebler, J.T. Stewart, T.E. Drake and D.S. Jin, A. Perali, P. Pieri and G.C. Strinati. 2010. Observation of pseudogap behavior in a strongly interacting Fermi gas. Nature Physics. Posted online July 4.

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Jigga
1.5 / 5 (8) Jul 08, 2010
Pseudogap is the state, when material contains islands of superconductive electrons already, but these islands aren't still connected into homogeneous phase, so that no bulk superconductivity can be observed. Electrons are becoming superconductive when they're heavily compressed mutually from all directions, so that their motion becomes driven by quantum noise only in similar way, like at the case of atoms inside of boson condensates. Electrons can be compressed by their attraction to hole stripes, i.e. islands of positivelly charged atoms within superconductor lattice in similar way, like people fighting for money inside of human crowd, when you throw some dollar bank-bills into it.
johanfprins
2 / 5 (4) Jul 09, 2010
How do these atoms pair? By "phonon exchange"? Not on your Ninny! There is NOTHING mysterious in this experiment: What should however be noted is that it does NOT directly relate to the mechanism for superconduction in the "high temperature" ceramics.

In the latter case the gap is NOT a pseudo-gap: It is a gap that exists between donor-sites within the layers, and lower energy "orbitals" which form between the layers when the donor-electrons within the layers de-excite into the spaces between the layers. Although the latter orbitals can be bosons, they are usually NOT. Even so they ALL have the same lowest energy as is required for a superconducting phase. At a low enough temperature their density becomes high enough so that they can transport a current by hopping: The latter is caused by quantum fluctuations.

In fact, I doubt whether ANY superconductor discovered to date has boson-type charge-carriers. Please, BCS, "release us and let us go!! We CANNOT believe you anymore!"
Jigga
1 / 5 (1) Jul 09, 2010
How do these atoms pair? By "phonon exchange"?
It's an electrons, what is paired within superconductors. And they're paired there by their spin in accordance to Pauli's exclusion principle.

BTW Preprint is there: http://arxiv.org/...47v1.pdf
Jigga
2 / 5 (4) Jul 09, 2010
..at a low enough temperature their density becomes high enough so that they can transport a current by hopping..
Such mechanism doesn't explain, why superconductor current doesn't increase gradually with decreasing temperature bellow Tc temperature in accordance to Bose-Einstein distribution law for energy density.
johanfprins
2 / 5 (4) Jul 09, 2010
It's an electrons, what is paired within superconductors. And they're paired there by their spin in accordance to Pauli's exclusion principle.
.
In a very good metal each identical electron-wave accomodates TWO electrons with opposite spins. So they are paired according to Pauli's Exclusion principle. Why do they, according to your argument, not form a codensate that superconducts? In fact the more perfect a metal, to allow such pairing according to Pauli Exclusion Principle, the less it is able to form a superconducting phase. Thus, you are again talking pure unadulterated nonsense!
johanfprins
2.3 / 5 (6) Jul 09, 2010
Such mechanism doesn't explain, why superconductor current doesn't increase gradually with decreasing temperature bellow Tc temperature in accordance to Bose-Einstein distribution law for energy density.

How do you measure this current? When a current flows through a superconductor-element, this current is totally determined by the electronics of the cicuit outside the superconductor: i.e. by the rate at which charge-carriers are injected. If it transfers a current I at a temperature T, it will transfer THE SAME current al lower temperatures when it forms an element within the same circuit.

PLEASE Jigga, how about taking a course in elementary physics?
Jigga
2.3 / 5 (3) Jul 12, 2010
..at a low enough temperature their density becomes high enough so that they can transport a current by hopping...
Like I've said, under such situation the superconductor resistance wouldn't drop gradually - which doesn't occur in nature. It's evident, a more complex mechanism applies here: the superconductor is already in its superconductive state locally, i.e. at pseudogap state above Tc and the superconductive areas are only interconnecting mutually at the Tc, which results in sharp drop of resistance.

Try to read a bit more about pseudogap state to understand the underlying physics here.
johanfprins
1 / 5 (1) Jul 12, 2010
{q]Like I've said, under such situation the superconductor resistance wouldn't drop gradually - which doesn't occur in nature

Who said that the "superconductor resistance would drop gradually?" And under which measuring conditions is this supposed to happen? REALLY Jigga, or whatever you are, pleae define what you are talking about!
Jigga
2.3 / 5 (3) Jul 12, 2010
Who said that the "superconductor resistance would drop gradually?
Me, of course. Can you read?
johanfprins
1 / 5 (1) Jul 13, 2010
Me, of course. Can you read?

But you give as usual no physics-logic to back up your claim! From past exsperience I am not inclined to accept what you say as if you are the oracle.