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Stable Atom Clouds Foreshadows New Superconducting Materials

A false-color images showing the Fermi gas size in comparison to the Bose gas at different temperatures: Fermions make up the basic building blocks of matter: electrons, protons and neutrons. Atoms made from an odd number of these basic constituents, like lithium-6 -- composed of three neutrons, three protons and three electrons -- are also Fermions.

Their cousins, the Bosons, are formed from an even number of the building blocks, and include lithium-7, which has an additional neutron. Bosons behave very differently from fermions at ultralow temperatures, where the atoms behave more like waves than as point-like particles. Hulet and others demonstrated in 1995 that when identical Bosons are cooled to near absolute zero (about -460 F), they are happy to coexist in the same location (something Einstein predicted). Their movements fall into step at a single low energy level, and they behave as one unified "super-atom," known as a Bose-Einstein condensate.

In contrast, identical Fermions cannot occupy the same place at the same time, a phenomenon described by the Pauli Exclusion Principle. As Fermions are cooled and their wavelengths become larger, they begin to avoid each other. Because they all can't occupy the lowest energy level, they are forced to "stack up" into higher energy states, like people on a ladder, with at most one to a rung. By keeping their distance from one another, the Fermions create a kind of pressure. In this state, the Fermions have reached a limited size and cannot be compressed any further.

Arlington - March 12, 2001
For the first time, a gas containing both of the fundamental types of atoms (Fermions and Bosons) has been cooled to a temperature just a hair short of absolute zero.

This is the first time in the world of physics, that a state of simultaneous quantum degeneracy -- a very low energy state of matter in which atoms lose their individuality -- has been created, with the potential for highly precise atomic clocks and additional advances in superconducting materials.

ONR funded researchers at Rice University are reporting this work in Science magazine on March 1, 2001. Dr. Randall Hulet, professor of physics at Rice, led the team that created this ultra-cold gas of lithium atoms.

Their research not only illustrates fundamental aspects of quantum mechanics, but also has practical Naval applications. This work could lead to improved gyroscopes, better atomic clocks, higher bandwidth communications networks, and advanced sensors for the Navy and Marine Corps.

Quantum degeneracy isn't new. "What is new is the ability to achieve it simultaneously in these two basically different kinds of matter, and to use one to achieve and act as a diagnostic for the other," says ONR Program Officer, Dr. Peter J. Reynolds.

The facts are familiar to all chemistry students: everything is either a Boson or a Fermion. The difference between the two lies in a quantum property called spin: Bosons have an integral spin, Fermions a half-integral spin.

Bosons and Fermions exhibit degeneracy in dramatically different ways. Fermions obey the Pauli Exclusion Principle -- which states that two Fermions cannot occupy the same state at the same time -- and Bosons do not. This turns out to be very important when matter is cooled to near absolute zero, the theoretical point at which atoms stop moving.

When atoms collide, heat/energy is exchanged. Things cool down when the hottest (fastest) atoms escape. This explains why your cup of coffee gets cold not long after you've poured it from the pot.

The fastest molecules escape while the remainder exchange energy among themselves through collisions, causing some of them to speed up enough to escape, and so on. The collisions are essential to keep the process going.

As the Pauli Exclusion Principle prevents any two Fermions from being in the same quantum state, this makes it tougher for a pair of them to collide, and thus harder to cool them.

Bosons don't resist collision this way, and so researchers have exploited "evaporative cooling" to get them to the point at which they form a Bose-Einstein condensate -- a quantum state in which the atoms collapse into their lowest energy state, losing their individuality in a kind of super-atom.

As Hulet and his team cooled the gas to temperatures as low as 240 nano-Kelvin, less than one-fourth of a millionth of a degree above absolute zero, they observed that the size of the Boson gas shrank, while the Fermion gas stabilized at a particular size.

The resistance to further shrinking is called Fermi pressure. This Fermi pressure is best known for keeping cold, "dead" stars -- known as "white dwarfs" and "neutron stars" -- from collapsing under their own gravitational attraction. But this same phenomenon plays a vital role in the electrical and magnetic properties of ordinary materials.

Drawing on their previous work with Bose-Einstein condensation of lithium-7, Hulet and his team cooled the magnetically trapped Bosons first with lasers and then by removing the hottest atoms through evaporation.

The lithium-6 Fermions, which are difficult to be directly cooled by evaporation because of limitations imposed by the Pauli Exclusion Principle, are instead cooled by contact with the cold Bosons.

Hulet and his team are working to cool the gas further in order to achieve the ultimate goal of this research: to coax the Fermions to form correlated pairs of atoms, called Cooper pairs.

Cooper pairing of electrons is responsible for superconductivity in certain solids, and the same phenomenon has been predicted to occur when lithium-6 atoms are cooled to temperatures near 50 nano-Kelvin. Scientists expect the gas will become a superfluid gas, in which atoms may flow without friction.

Exciting to scientists is the fact that the interactions between atoms in a trapped gas are weak, and that the degree of their strength can be "tuned."

The weakness of the interactions, and the lack of collisions among the very cold Fermions, implies that ultra-precise atomic clocks may one day be developed from such an atomic gas. Such clocks are key elements for navigation, and form the basis of the present-day worldwide GPS system.

Hulet sees no fundamental reason why his team cannot continue to chill the atoms closer to absolute zero. But because of limitations caused by the collapse of the Bose-Einstein condensate in a magnetic trap, further cooling will require the group to switch from a magnetic trap to an optical trap that uses focused laser beams to confine the atoms.

In addition to Hulet, authors on the paper are Rice University post-doctoral scientist Andrew Truscott, graduate students Kevin Strecker and Guthrie Partridge, and William McAlexander who recently received his Ph.D. from Rice and is currently with Agilent Laboratories, Palo Alto, Calif.

Related Links
Randall Hulet's Research
Office of Naval Research
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Magnesium Diboride: A New High-temp Superconducting Metal
Madison - March 7, 2001
The jolt of excitement from the January discovery of a new high-temperature superconducting metal, magnesium diboride, may get another voltage boost this week with evidence that the material can carry electrical currents at high density.

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