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We study the thermodynamics of a lattice of half vortices and strings. The response of a superfluid to mechanical rotation demonstrates one of the most remarkable features of these systems. Rather than rotate like a classical fluid, a superfluid will instead nucleate quantised vortices which carry angular momentum. In the limit of many vortices, a vortex lattice will form. Depending on the type of superfluid and the experimental parameters, different kinds of vortices can be found as the equilibrium configuration of the rotating superfluid: integer vortices and half vortices. In the half vortex lattice with the presence of an easy-axis anisotropy, a state which is phenomenologically equivalent to having strings with finite tension pairing up half vortices arises. Hence, we develop a description of the system based on interacting point vortices and strings.
In Plain English
If you take a glass of water and stir it with a spoon, you find that the water rotates creating a swirl in the middle. A superfluid is a fluid with some special properties. Among others, when you set it in rotation, it creates many swirls instead of just one. Furthermore, under certain experimental conditions that can be tuned in the lab, these vortices appear to be pulled together in pairs as if they were linked by strings. We study the behaviour of such a superfluid at different temperatures and different string tensions.
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