658,609 research outputs found
Superfrustration of charge degrees of freedom
We review recent results, obtained with P. Fendley, on frustration of quantum
charges in lattice models for itinerant fermions with strong repulsive
interactions. A judicious tuning of kinetic and interaction terms leads to
models possessing supersymmetry. In such models frustration takes the form of
what we call superfrustration: an extensive degeneracy of supersymmetric ground
states. We present a gallery of examples of superfrustration on a variety of 2D
lattices.Comment: 8 pages, 5 figures, contribution to the proceedings of the XXIII
IUPAP International Conference on Statistical Physics (2007) in Genova, Ital
The physical gravitational degrees of freedom
When constructing general relativity (GR), Einstein required 4D general
covariance. In contrast, we derive GR (in the compact, without boundary case)
as a theory of evolving 3-dimensional conformal Riemannian geometries obtained
by imposing two general principles: 1) time is derived from change; 2) motion
and size are relative. We write down an explicit action based on them. We
obtain not only GR in the CMC gauge, in its Hamiltonian 3 + 1 reformulation but
also all the equations used in York's conformal technique for solving the
initial-value problem. This shows that the independent gravitational degrees of
freedom obtained by York do not arise from a gauge fixing but from hitherto
unrecognized fundamental symmetry principles. They can therefore be identified
as the long-sought Hamiltonian physical gravitational degrees of freedom.Comment: Replaced with published version (minor changes and added references
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