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Basic questions about the susceptibility of a first-order phase transition

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I have two basic questions about first-order phase transitions:

1) is the susceptibility divergent at a first-order phase transition?

2) if yes, does it diverge in a universal way as in continuous phase transitions?

My understanding is as follows, and I would like to know if it is correct. The susceptibility diverges at a first phase transition, but in a non-universal way. To be concrete, let us consider a transition signaled by spontaneous symmetry breaking, where the system is symmetric above the transition and the symmetry is broken across the transition. If the transition is first order, right at the transition symmetry-broken states will be degenerate with the symmetric state. Then if an infinitesimal field that couples to the order parameter is exerted, a symmetry-broken state will have the lowest energy and the system will be pinned to that state. Therefore, the susceptibility diverges. However, at a first-order phase transition the correlation length does not diverge, so short distance details will be important and the divergence of the susceptibility will depend on them, thus will be non-universal.

I appreciate if anyone tells me whether my understanding is right.

asked May 8, 2016 in Theoretical Physics by Mr. Gentleman (255 points) [ no revision ]
recategorized May 8, 2016 by Dilaton

At a first order phase transition the susceptibility jumps, rather than diverges.

I think the order parameter jumps at a first-order phase transition, and this is actually why I think the susceptibility should diverge there.

The susceptibilities are one-sided derivatives and therefore jump, too. It makes no sense in practice to treat them as singular distributions (sum of a delta function and a smooth function) since they are response functions, and the response at the jump itself is not measurable.

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