Superfluidity in Fermi Gas
Posted Jun23-05 at 07:29 AM by ZapperZ
It appears that there is finally a slam-dunk evidence of superfluidity in a strongly-interacting Fermi gas. The latest results from the Ketterle group at MIT-Harvard have a convincing evidence of superfludity Li6 fermions via the formation of superfluid vortex lattice.[1] While there have been several publications within the past few years that indicated evidence of BE condensate forming in Fermi gasses[2,3,4], this is the most convincing and irrefutable evidence of the formation of such a state.
Why is this a big deal anyway? As Rudolf Grimm stated in his News and Views article in the same issue as Ref. [1], this is a quantum "revolution" of many-body system. There is an important insight into how things behave when their intrinsic spin can no longer be ignored, and when such interactions are quite strong that they form another "phase" of matter. While this is applicable in general to the study of condensed matter physics, what makes the study in THIS particular field is the understanding of how two highly different processes are connected.
At one end of the scale is a boson. The formation of a Bose-Einstein condensate (BEC) is a well-known quantum phenomena. It occurs in He4 and He3, and even occurs in ultracold neutral atomic gas, for which Cornell, Wieman, and Ketterle won the Nobel Prize. The BEC forms a superfluid - a fluid that flows without resistance and viscosity.
At the other end of the scale is the formation of the Bardeen-Cooper-Schriefer (BCS) state. Here, charged fermions, who have no business in being bosons by themselves, pair up. As a pair, they form a "composite" boson under the right condition. When this occurs in electrons (and holes), you get superconductivity. This is where electrical currents flow without impedence.
Now there is a subtle but important difference here between superfludity. Superconductivity is the flow of currents without resistance, while superfludity is the flow of a neutral fluid without resistance. So one is the transport of charge, while the other is the transport of the "particle" (this word is being used very loosely here). One could argue, well what's the difference? Doesn't the particle carry the charge? Well, not necessarily. Keep in mind that one cannot distinguish individual particles in these situations, and so, we cannot assign unique charges to "entities".
In any case, BEC is composed of bosons forming a superfluid, while BCS are composed of paired, charged fermions forming a supercurrent. These two are separate beasts and one cannot go from one to the other - or can we? What if we can take neutral fermions (instead of neutral bosons as in the BEC case) and see if they'll pair up (similar to BCS case) and form a BEC? This would be a "bridge" or the "missing link" between these two extreme state. This is what has been discovered with these recent work. The boundary between BEC and BCS now appears to be a "crossover", rather than an abrupt discontinuity. These experiments show that one can go from supercurrent to superfluid smoothly. This opens up our understanding of a number of phenomena and also opens up a research field far beyond BEC. Its importance here cannot be overstated.
Zz.
[1] M.W. Zwierlein et al, Nature v.435, p.1047 (2005).
[2] C. Regal et al., PRL v.92, p.040403 (2004).
[3] M.W. Zwierlein et al., PRL v.92, p.120403 (2004).
[4] J. Kinast et al., PRL v.92, p.150402 (2004).
Why is this a big deal anyway? As Rudolf Grimm stated in his News and Views article in the same issue as Ref. [1], this is a quantum "revolution" of many-body system. There is an important insight into how things behave when their intrinsic spin can no longer be ignored, and when such interactions are quite strong that they form another "phase" of matter. While this is applicable in general to the study of condensed matter physics, what makes the study in THIS particular field is the understanding of how two highly different processes are connected.
At one end of the scale is a boson. The formation of a Bose-Einstein condensate (BEC) is a well-known quantum phenomena. It occurs in He4 and He3, and even occurs in ultracold neutral atomic gas, for which Cornell, Wieman, and Ketterle won the Nobel Prize. The BEC forms a superfluid - a fluid that flows without resistance and viscosity.
At the other end of the scale is the formation of the Bardeen-Cooper-Schriefer (BCS) state. Here, charged fermions, who have no business in being bosons by themselves, pair up. As a pair, they form a "composite" boson under the right condition. When this occurs in electrons (and holes), you get superconductivity. This is where electrical currents flow without impedence.
Now there is a subtle but important difference here between superfludity. Superconductivity is the flow of currents without resistance, while superfludity is the flow of a neutral fluid without resistance. So one is the transport of charge, while the other is the transport of the "particle" (this word is being used very loosely here). One could argue, well what's the difference? Doesn't the particle carry the charge? Well, not necessarily. Keep in mind that one cannot distinguish individual particles in these situations, and so, we cannot assign unique charges to "entities".
In any case, BEC is composed of bosons forming a superfluid, while BCS are composed of paired, charged fermions forming a supercurrent. These two are separate beasts and one cannot go from one to the other - or can we? What if we can take neutral fermions (instead of neutral bosons as in the BEC case) and see if they'll pair up (similar to BCS case) and form a BEC? This would be a "bridge" or the "missing link" between these two extreme state. This is what has been discovered with these recent work. The boundary between BEC and BCS now appears to be a "crossover", rather than an abrupt discontinuity. These experiments show that one can go from supercurrent to superfluid smoothly. This opens up our understanding of a number of phenomena and also opens up a research field far beyond BEC. Its importance here cannot be overstated.
Zz.
[1] M.W. Zwierlein et al, Nature v.435, p.1047 (2005).
[2] C. Regal et al., PRL v.92, p.040403 (2004).
[3] M.W. Zwierlein et al., PRL v.92, p.120403 (2004).
[4] J. Kinast et al., PRL v.92, p.150402 (2004).
Total Comments 0
