Researchers at Utrecht University have resolved the controversy on ultracold nuclear physics
Two pioneering and yet completely contradictory experiments have recently turned the world of ultracold nuclear physics upside down. Researchers at Utrecht University, FOM Foundation and NWO (Netherlands Organisation for Scientific Research) have thought of an explanation. Both scientific experiments have been performed by using ultracold Fermi gases in two different spin states, at which the number of fermions per spin state can be diversified by experiment. In this way the fermion gas can be polarized. However, it was debated what would have been happening with its superfluidity, as fermions are hardly able to pairing because of the imbalance in the two spin states. The findings of the experiments - performed by renowned research teams at MIT (Massachusetts Institute of Technology) and Rice University - contradicted each other. In November 2006 Utrecht University researchers Gubbels, Romans and Stoof had their theoretical framework to the apparent contradiction published in the scientific journal Physical Review Letters. They also computed a phase diagram that visualizes the differences in the findings of the experiments. This knowledge is also important to other fields in physics, such as superconductivity or neutron galaxies.
Bose-Einstein condensation
Nature knows two kinds of particles, fermions and bosons. Fermions have a half numbered spin and their particles behave 'antisocially'. The fact is that one quantum state may have one fermion at a maximum. However, bosons have a fully numbered spin and they are 'social particles'. They do not mind to be in the same quantum state. Consequently, bosons are able to find themselves in the lowest possible energetic state at low temperatures and they are behaving like one large quantum mechanical object (Bose-Einstein condensation). Bose-Einstein condensation give rise to all kinds of exotic effects, like superfluidity, which means that the condensate is able to flow without friction along an object. Due to their antisocial character fermions are not able to a Bose-Einstein condensation. However, when fermions will have an interactive attraction, they are able to pairing, the so-called Cooper pairs. As a pair of fermions produces a boson, Cooper pairs are indeed able to a Bose-Einstein condensation and by that, give rise to superfluidity. Usually, Cooper pairing occurs between two fermions in various spin states. Besides, in this case the pairing is optimal, as the number of particles in the two spin states is similar. The situation will change, however, when the number of particles becomes dissimilar.
Contradictory observations
The research team led by Wolfgang Ketterle at MIT and the team led by Randy Hulet at Rice University have been succeeded in examining a strongly interagitating Fermi gas of lithium atoms as a function of spin polarization. The findings of these pioneering experiments came as a real bombshell; not in the last place, because of the fact that the findings seem to contradict each other completely. Whereas the MIT-group observes a phase transition between a superfluid and a normal state at a high polarization of seventy percent, the Rice-group just seems to observe a transition between two different superfluid states at a low polarization of nine percent. How is it possible that two excellent studies by experiment at the same systeem produce such different observations, quantitatively as well as qualitatively? This question caused an actual boom of theoretical research, that produced even more various findings, if possible.
Second-order or first-order phase transition?
The researchers at Utrecht University too, have intervened in this discussion and they incorporated their major findings in a phase diagram (figure 2). Following the scientific experiments, the computations were performed through the help of harmonically trapped atoms. As densities are highest in the centre of the trap, the Bose-Einstein condensation will take place there first. Consequently, the Utrecht University researchers distinguish three different phases. The normal phase shows that the gas is not found superfluid anywhere in the trap, not even in the centre.
The so-called Sarma phase and phase-separated phase have a superfluid centre surrounded by gas in the normal phase. The Sarma phase has a specific property: the normal superfluid transition in the trap is of a second-order, i.e. continuous, whereas the transition in the phase-separated phase is of a second-order, i.e. discontinuous. The Sarma phase is called after 'Sarma', because the researchers realised that a polarization, dissimilar to zero in combination with a second-order phase transition, would lead to an exotic form of superfluidity, which had been investigated firstly by Sarma.
The phase diagram in figure 2 visualizes why the observations by the two scientific teams were different: The MIT team have been measuring above the (tri)critical point, on which the three different phases converge. By doing so they measured the transition between the Sarma phase and the normal phase at a high polarization of seventy percent. On the other hand, the Rice team have been working below the (tri)critical point, so that they observed the transition between the Sarma and the phase-separated phase. Meanwhile, this propopsal has encouraged the Rice team in performing new scientific experiments at high temperature, which, actually, have provided for observations analogously to the MIT findings and completely consistent with the phase diagram by the researchers at Utrecht University.
The article is entitled: 'Sarma Phase in Trapped Unbalanced Fermi Gases'. The authors are: Koos Gubbels, Mathijs Romans and Henk Stoof.
For more information, please contact Koos Gubbels, phone: +31 (0)30 253 23 22 or Hernk Stoof, phone: +31 (0)30 253 18 71.