HOUSTON — For the first time, a mixed gas of the two fundamental types of atoms has been cooled to a state of simultaneous quantum degeneracy--a state of matter in which atoms behave like waves--dramatically illustrating the difference between the two types at ultralow temperatures, Rice University physicists report in research published by Science on March 2.
In addition to illustrating a fundamental aspect of quantum mechanics, study of this system could lead to a better understanding of phenomena such as superconductivity and superfluity.
Randall Hulet, the Fayez Sarofim Professor of Physics at Rice University, led a team that created an ultracold gas of lithium atoms, consisting of bosons and fermions, the two types of quantum particles found in nature.
As the gas was cooled to temperatures as low as 240 nano-Kelvin, less than one-fourth of a millionth of a degree above absolute zero, the Rice team observed that the size of the boson gas shrank, while the fermion gas stabilized at a particular size. The researchers independently imaged the two gases, and at the lowest temperatures, the size difference is clearly distinguishable.
This quantum effect, known as Fermi degeneracy, plays a vital role in the electrical properties of metals. The corresponding Fermi pressure is best known for stabilizing cold, "dead" stars, known as white dwarf and neutron stars, against collapse under their own gravitational attraction.
Drawing on their previous work with Bose-Einstein condensation of lithium-7, Hulet and his team cooled the magnetically trapped bosons by removing the hottest atoms through a process of evaporation. The lithium-6 fermions, which cannot be directly cooled by evaporation because of limitations imposed by the Pauli exclusion principle, are 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."
"This will enable exploration of the underlying theory of superconductivity," Hulet said, "and by adjusting the strength of the interactions between atoms we hope to understand its implications in a way not previously possible."
Hulet sees no fundamental reason that they can’t continue to chill the atoms closer to absolute zero. Because of limitations caused by the collapse of the Bose-Einstein condensate in the magnetic trap, however, further progress 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.
This research was supported by the Office of Naval Research, NASA, the National Science Foundation and the R.A. Welch Foundation.
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. 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 known as 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.
Editors: For false-color images showing the Fermi gas size in comparison to the Bose gas at different temperatures, see http://atomcool.rice.edu/atomstar.jpg.
For more information about Randall Hulet’s research, see: http://atomcool.rice.edu.
Rice University is consistently ranked one of America’s best teaching and research universities. It is distinguished by its: size--2,700 undergraduates and 1,500 graduate students; selectivity--10 applicants for each place in the freshman class; resources—an undergraduate student-to-faculty ratio of 5-to-1, and the fourth largest endowment per student among American universities; residential college system, which builds communities that are both close-knit and diverse; and collaborative culture, which crosses disciplines, integrates teaching and research, and intermingles undergraduate and graduate work. Rice’s wooded campus is located in the nation’s fourth largest city and on America’s South Coast.
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