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ScienceWeek
2004 25 June A3
3. QUANTUM PHYSICS: ON DEGENERATE FERMI GASES
The following points are made by J.E. Thomas and M.E. Gehm (American Scientist 2004 92:238):
1) A degenerate Fermi gas is a remarkable configuration, an assemblage that constitutes a new state of matter and is possibly the closest that researchers will ever come to having on their desktops a neutron star or a piece of the quark matter that made up the early Universe.
2) Although physicists had predicted the existence of degenerate Fermi gases as long ago as the 1930s, nobody had produced a fully independent system in the lab until five years ago. The closest model we had was the cloud of electrons inside an ordinary metal like copper. Even though the metal is solid, the electrons behave very much like a gas: They are free to roam around (which makes the metal conduct electricity), but they have to fit into a strict energy hierarchy -- the same one found in the degenerate Fermi gases we produced.
3) These gases are first cousins to another strange quantum beast that appears at ultracold temperatures, called a Bose-Einstein condensate. The research group of Eric Cornell and Carl Wieman at the University of Colorado at Boulder fashioned the first such condensate in 1995. (In 2001, Cornell and Wieman shared a Nobel prize for their work with Wolfgang Ketterle of the Massachusetts Institute of Technology.)
4) An atomic degenerate Fermi gas is even trickier to create, because it pits two precepts of quantum mechanics against each other. On the one hand is Heisenberg's famous uncertainty principle, which says that the location of any particle becomes more ambiguous as its speed becomes less uncertain. In an ultracold gas, the speed of the atoms is known with unusual precision: It is close to zero. Therefore the atoms get smeared out into blobs that are tens of thousands of times larger than a normal room-temperature atom. This blurring is no problem for a Bose-Einstein condensate, because it is made of "sociable" atoms called bosons, which like to overlap. But degenerate Fermi gases are made from solitary atoms called fermions (like the lithium in our trap), which according to Pauli's exclusion principle cannot share space with their neighbors. As a result, making a degenerate Fermi gas is a lot like trying to pack balloons into a closet.
5) Recently our group was able to probe the quantum behavior of these balloons by using a form of quantum trickery known as "strong interactions" to expand the balloons greatly in size. These interactions make the atoms affect one another at a much greater distance than they ordinarily would. There are exciting indications, not yet confirmed, that the strong interactions cause the atoms to form loose alliances called "Cooper pairs". Superconductivity and some forms of superfluidity are the result of Cooper pairing.(1-5)
References (abridged):
1. Andrews, M. R., C. G. Townsend, H.-J. Miesner, D. S. Durfee, D. M. Kurn and W. Ketterle. 1997. Observation of interference between two Bose condensates. Science 275:637-641
2. Collins, G. P. 2003. The next big chill. Scientific American (October) 289(4):26-28
3. DeMarco, B., and D. S. Jin. 1999. Onset of Fermi degeneracy in a trapped atomic gas. Science 285:1703-1706
4. Granade, S. R., M. E. Gehm, K. M. O'Hara and J. E. Thomas. 2002. All-optical production of a degenerate Fermi gas. Physical Review Letters 88:120405
5. Grimm, R., M. Weidemueller and Y. B. Ovchinnikov. 2000. Optical dipole traps for neutral atoms. Advances in Atomic, Molecular & Optical Physics 42:95-165
American Scientist http://www.americanscientist.org
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Related Material:
ULTRACOLD MATTER: SUPERFLUIDITY IN FERMI GASES
The following points are made by L. Pitaevskii and S. Stringari (Science 2002 298:2144):
1) Over the past decade, studies of ultracold atomic gas clouds have yielded unprecedented insights into the quantum statistical properties of matter, with most studies have focused on boson gases. The elementary constituents of matter can be divided into fermions and bosons. Fermions are particles whose intrinsic angular momentum (or spin) is an odd multiple of h/2(pi), where (h) is the Planck constant. In contrast, the angular momentum of bosons is an even multiple of h/2(pi). The dramatically different thermodynamic properties of fermions and bosons at low temperature are a direct result of quantum statistical effects.
2) The fundamental constituents of atoms (electrons, neutrons, and protons) are fermions. However, pairs of fermions -- and, in general, systems composed of an even number of fermions -- behave like bosons. Because of their bosonic properties, hydrogen and several alkali elements can be used to study the phenomenon of Bose-Einstein condensation (2). But some isotopic species of these alkali atoms, like 6Li and 40K, with an odd number of fermions, instead exhibit fermionic behavior.
3) The first signatures of quantum statistical effects in atomic Fermi gases were reported in 1999 (3). An important motivation for these studies is the search for the transition to the superfluid phase (4), analogous to the transition exhibited by superconductors and liquid 3He. According to the standard theory of fermion superfluidity, this transition should take place at extremely low temperatures, well below the Fermi temperature T(F) (the typical temperature where quantum effects show up). Attempts to reach such temperatures with trapped atomic gases have encountered major difficulties because the cooling mechanisms become less and less efficient with decreasing temperature.
4. In contrast to other systems (such as atomic nuclei, liquid 3He, and superconductors), the trapping and interaction mechanisms in atomic gases can be manipulated in a controlled manner, allowing the interaction between atoms to be tuned (5). By changing the strength of the magnetic field, the value and even the sign of the scattering length can be changed. The scattering length can be extremely large, much larger than the average distance between atoms. As a result, the number of collisions increases dramatically, enhancing the efficiency of the cooling mechanisms, which are based on evaporation.
References (abridged):
1. K. M. O'Hara, S. L. Hemmer, M. E. Gehm, S. R. Granade, J. E. Thomas, Science 298, 2179 (2002)
2. M. H. Anderson et al., Science 269, 198 (1995)
3. B. DeMarco, D. S. Jin, Science 285, 1703 (1999)
4, G. Shlyapnikov, in Proceedings of the 19th International Conference on Atomic Physics, Cambridge, MA (World Scientific Publishing), in press.
5. S. Inouye et al., Nature 392, 151 (1998)
Science http://www.sciencemag.org
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Related Material:
COLLAPSE OF A DEGENERATE FERMI GAS
The following points are made by G. Modugno et al (Science 2002 297:2240):
1) Experimental research on ultracold atoms has highlighted the marked differences in basic properties of bosonic and fermionic dilute quantum gases (1). In the case of a degenerate Fermi gas, confined in a harmonic external potential, the Pauli exclusion principle forbids the multiple occupation of a single quantum state and leads to a strong effective repulsion between the identical atoms. The fermions are arranged in the trap in a cloud with relatively large spatial distribution and large kinetic energy, which can be interpreted as being the result of an outward "Fermi pressure" (2,3). This is a general property of any degenerate Fermi system; for instance, it is the mechanism that stabilizes white dwarfs and neutron stars against gravitational collapse. As a result of this pressure, a dilute atomic Fermi gas is only weakly affected by the actual interactions between particles. Conversely, a Bose-Einstein condensate (BEC) occupies only the ground state of the trap, with a narrow spatial distribution, and the presence of interactions can strongly alter its structure. Indeed, a repulsive interaction broadens the density distribution, whereas an attractive interaction can lead to a collapse for a sufficiently large number of atoms, as observed for lithium (4) and rubidium (5).
2) Another scenario has been opened by the recent production of degenerate boson-fermion mixtures (3). Here also the interspecies interactions can play an important role, and, in particular, the effect of the mutual interaction is predicted to be enhanced for fermions by the higher density of the bosons. Moreover, as shown by the early experiments on mixtures of superfluid 3He and 4He, the presence of an interaction between bosons and fermions can induce an effective attraction between fermions themselves.
3) In summary: The authors report that a degenerate gas of identical fermions is brought to collapse by the interaction with a Bose-Einstein condensate. The authors used an atomic mixture of fermionic potassium-40 and bosonic rubidium-87, in which the strong interspecies attraction leads to an instability above a critical number of particles. The observed phenomenon suggests a direction for manipulating fermion-fermion interactions on the route to superfluidity.
References (abridged):
1. See, for example, J. R. Anglin and W. Ketterle, Nature 416, 212 (2002)
2. B. De Marco and D. S. Jin, Science 285, 1703 (1999)
3. A. G. Truscott, K. E. Strecker, W. I. McAlexander, G. B. Partridge, R. G. Hulet, Science 291, 2570 (2001)
4. C. A. Sackett, J. M. Gerton, M. Welling, R. G. Hulet, Phys. Rev. Lett. 82, 876 (1999)
5. E. A. Donley, et al., Nature 412, 295 (2001)
Science http://www.sciencemag.org
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