  While much is known about repulsive quantum impurity models, significantly
less attention has been devoted to their attractive counterparts. This
motivated us to study the attractive SU(N) Anderson impurity model. While for
the repulsive case, the phase diagram features mild N dependence and the ground
state is always a Fermi liquid, in the attractive case a Kosterlitz-Thouless
charge localization phase transition is revealed for N > 2. Beyond a critical
value of attractive interaction an abrupt jump appears in the number of
particles at the impurity site, and a singular Fermi liquid state emerges,
where the scattering of quasiparticles is found to exhibit power law behavior
with fractional power. The capacity diverges exponentially at the quantum
critical point, signaling the Kosterlitz-Thouless transition.
