| Models for metallic anti-ferromagnets map to the dissipative XY model as do
XY ferromagnets, the superconductor-insulator transition, and the model for
loop-current order in Cuprates. The spectral function of the quantum-critical
fluctuations for this model in 2D for a range of parameters is determined by
topological defects - instantons and 2D vortices; it is a separable function of
space and time, with a 1/τ dependence at criticality. The marginal fermi-liquid
properties for the fermions follow from coupling to such fluctuations.
These fluctuations are directly measured by inelastic neutron scattering in the
quantum-critical region of the new Fe- based compounds, in a 2D ferromagnet
and in heavy-Fermions, and deduced in cuprates through analysis of ARPES.
ARPES also reveals the coupling function of the fermions to the fluctuations
so as to give nearly angle-independent normal self-energy but d-wave pairing
energy. |