# SIS

## Seminar

### Collapsing the Bethe ansatz

Seminar Room 1, Newton Institute

#### Abstract

I discuss a system where the Bethe ansatz "collapses", meaning that the nested Bethe equations end up requiring only that the momenta are those of free fermions. The model describes two free-fermion chains coupled via strong attractive nearest-neighbor interactions, akin to a ferromagnet. Despite the collapse of the Bethe ansatz, the system is not free: the spectrum consists of zero-energy fermion pairs (Cooper pairs), and particles obeying mutual exclusion statistics. The extensive degeneracies are described by a (space-time) supersymmetric generalization of the Yangian algebra.

**Related Links**

- http://arxiv.org/abs/cond-mat/0612270 - paper by Fendley and Schoutens

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