Friday, August 17, 2012

1208.3266 (Ralph M. Kaufmann et al.)

Re-gauging groupoid, symmetries and degeneracies for Graph Hamiltonians
and applications to the Gyroid wire network

Ralph M. Kaufmann, Sergei Khlebnikov, Birgit Wehefritz-Kaufmann
Motivated by Harper Hamiltonians on skeletal graphs and their $C^*$--geometry, we study a certain class of graph Hamiltonians. These Hamiltonians can be thought of as a finite groupoid representation in separable Hilbert spaces. Here the groupoid is the path groupoid of a finite graph. Given such a setup, we consider the possible matrix versions of the Hamiltonian, which are indexed by the choice of a rooted spanning tree and an order of the vertices. The first result is that all the matrix representations are linked to each other via the conjugation action of a re--gauging groupoid. We furthermore show that the symmetries of the underlying graph give rise to an action on the Hamiltonians of a group of extended symmetries. The new concept for the extension is to allow phase transformations on the vertices. In the commutative case, we prove that the extended symmetries act via a projective representation giving rise to isotypical decompositions and super--selection rules. We then apply these results to the PDG and honeycomb graphs using representation theory for projective groups and show that all the degeneracies in the spectra are consequences of these enhanced symmetries. This includes the Dirac points of the Gyroid and the honeycomb.
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