On Thu, Apr 11, 2013 at 1:45 AM, Kevin O'Malley <[email protected]> wrote:
> > > > Superheated Bose-Einstein condensate exists above critical temperature > > April 10, 2013 by Lisa Zyga > > Physicists created a BEC that can persist at up to 1.5 times hotter than > the critical temperature at which it normally decays. The BEC can survive > in the superheated regime for more than a minute when different components > of the boson gas are not in equilibrium. > > Credit: Alexander L. Gaunt, et al. ©2013 Macmillan Publishers Limited > (Phys.org) — > > At very low temperatures, near absolute zero, multiple particles called > bosons can form an unusual state of matter in which a large fraction of the > bosons in a gas occupy the same quantum state—the lowest one—to form a > Bose-Einstein condensate (BEC). In a sense, the bosons lose their > individual identities and behave like a single, very large atom. But while > previously BECs have only existed below a critical temperature, scientists > in a new study have shown that BECs can exist above this critical > temperature for more than a minute when different components of the gas > evolve at different rates. > > The physicists, Alexander L. Gaunt, Richard J. Fletcher, Robert P. Smith, > and Zoran Hadzibabic at the University of Cambridge in the UK, have > published their study on the superheated BEC in a recent issue of Nature > Physics. As the physicists explain, a superheated BEC is reminiscent of > superheated distilled water (water that has had many of its impurities > removed), which remains liquid above 100 °C, the temperature at which it > would normally boil into a gas. In both cases, the temperature—as defined > by the average energy per particle (boson or water molecule)—rises above a > critical temperature at which the phase transition should occur, and yet it > doesn't. > > In BECs and distilled water, the inhibition of a phase transition at the > critical temperature occurs for different reasons. In general, there are > two types of phase transitions. The boiling of water is a first-order phase > transition, and it can be inhibited in clean water because, in the absence > of impurities, there is in an energy barrier that "protects" the liquid > from boiling away. On the other hand, boiling a BEC is a second-order phase > transition. > > In this case, superheating occurs because the BEC component and the > remaining thermal (non-condensed) component decouple and evolve as two > separate equilibrium systems. The physicists explain how this mechanism > works in more detail. > > In equilibrium, a BEC can only exist below a critical transition > temperature. If the temperature is increased towards the critical value, > the BEC should gradually decay into the thermal component. The particles > flow between the two components until they have the same chemical potential > (a measure of how much energy it takes to add a particle to either > component), or in other words, until they are in equilibrium with each > other. However, maintaining this equilibrium relies on the interactions > between the particles. > > Here, the researchers demonstrated that in an optically trapped > potassium-39 gas the strength of interactions can be reduced just enough so > that the two components remain at the same temperature, but the particle > flow between them is slowed down and their chemical potentials decouple. > This condition makes it possible for the BEC to maintain a higher chemical > potential than the surrounding thermal component, and thus survive far > above its equilibrium transition temperature. > > > > "The thing that prompted this work was a previous paper of ours on > measuring the equilibrium BEC transition temperature as a function of the > interparticle interaction strength," Smith told Phys.org. "At the time we > noticed that something funny was happening at very low interaction > strengths: the transition temperature seemed higher than it should be by up > to 5%. We realized that this was probably due to non-equilibrium effects, > but could not explain it fully. Also, the effect was much smaller than we > demonstrated now. Only after fully understanding the equilibrium properties > of a BEC in an interacting gas we could come back to this problem, > demonstrate a much clearer effect, and explain it quantitatively." > > In the new study, the physicists experimentally demonstrated that a BEC > could persist in the superheated regime (at temperatures above the critical > temperature) for more than a minute. They also showed that that they could > cause the BEC to rapidly boil away by strengthening the interatomic > interactions to their normal levels, confirming the presence of the > superheated state. > > The scientists predict that extending a BEC's lifetime by tuning the > interactions could have several applications. "Generally, atomic BECs are > increasingly used for applications such as atom interferometry and > precision measurements, and might also find applications in quantum > information processing and computing," Smith said. "For all those > applications one wishes to preserve the coherent BEC for as long as > possible, e.g., to perform a longer (hence more precise) measurement or > more quantum-information type operations. Our work shows that it is > possible to significantly extend the lifetime of a coherent BEC exposed to > the experimentally unavoidable decohering thermal environment." > > In the future, the researchers plan to further investigate the physical > mechanism behind superheating. "We are primarily interested in further > fundamental understanding of the superheating phenomenon," Smith said. "The > funny thing is that the system is simultaneously in equilibrium in some > respects (e.g., the BEC and the thermal component have the same > temperature, the BEC has an equilibrium shape for the given number of > condensed atoms, etc.) and out of equilibrium in other ways (primarily the > fact that the number of condensed atoms is much higher than expected in > equilibrium). This poses new question about how we define equilibrium in a > quantum system, which we would like to understand better. Practical > applications might come later, fully exploiting their potential being > reliant on more complete fundamental understanding. " > > > > “Also, it turns out that condensation in 2D systems is even more > interesting than in 3D, and we plan to study superheating and other > non-equilibrium phenomena for an ultracold 2D Bose gas." > > > > More information: Alexander L. Gaunt, et al. "A superheated Bose-condensed > gas." > Nature Physics. DOI: 10.1038/NPHYS2587 Related: R. P. Smith, et al. > "Effects of interactions on the critical temperature of a trapped Bose > gas." > Phys. Rev. Lett. 106, 250403 (2011) > DOI: 10.1103/PhysRevLett.106.250403Journal reference: Nature Physics > Physical Review Letters Copyright 2013 Phys.org All rights reserved. This > material may not be published, broadcast, rewritten or redistributed in > whole or part without the express written permission of Phys.org. > > Read more at: > http://phys.org/news/2013-04-superheated-bose-einstein-condensate-critical-temperature.html#jCp > >
