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What is the order of the BKT phase transition (first or second), that is the phase transition in 2d in which thermal vortices destroy any order in the system. How is this connected to the "universal jump" of the superfluid stiffness?

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  • $\begingroup$ Minor comment to post (v3): Consider to spell out acronyms. $\endgroup$ – Qmechanic Sep 23 '16 at 2:42
  • $\begingroup$ The BKT phase transition is of infinite order, in that the free energy remains infinitely differentiable, but nonanalytic at the transition. Nevertheless, it has been proved that the spinwave stiffness in the 2d XY is discontinuous (see this paper). This is apparently the analogue of the density jump in superfluids. $\endgroup$ – Yvan Velenik Sep 23 '16 at 11:06
  • $\begingroup$ @YvanVelenik , thank you for your response. I am not understanding of your first sentence. ( It is more math than I can appreciate at the moment) I was under the impression that the spinwave/superfluid/helicity modulus or anything else you want to call it in itself suggested a second order phase transition as it is the second order derivative of the free energy with respect to uniform spin field deformation. It is in Chaikin and Lubensky as one possible reference. But thank you for suggesting this paper $\endgroup$ – Iso43 Sep 23 '16 at 22:28

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