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THE INTERNATIONAL SERIES OF MONOGRAPHS ON PHYSICS ...

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104 UNIVERSALITY CLASSES <strong>OF</strong> FERMI<strong>ON</strong>IC VACUA<br />

vacuum, but disappear under small deformations violating the symmetry. Zeros<br />

of co-dimension 0 have also been discussed. The homogeneous vacuum state with<br />

such a Fermi band in which all fermions have zero energy (the so-called fermionic<br />

condensate) was suggested by Khodel and Shaginyan (1990). The qauntum phase<br />

transition from the Fermi surface to fermionic condensate can be visualized as<br />

splitting of the vortex in momentum space in Fig. 8.1 representing the Fermi<br />

surface into fractional vortices in Fig. 8.8 connected by the vortex sheet – the<br />

fermionic condensate (Volovik 1991).<br />

The manifold of co-dimension 0 can also be formed by fermion zero modes in<br />

the vortex core (Sec. 23.1.2).

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