  Phase diagram of an Ising-spin Kondo lattice model on a kagome lattice is
investigated by a Monte Carlo simulation. We find that the system exhibits a
peculiar ferrimagnetic state at a finite temperature, in which each triangle is
in a two-up one-down spin configuration but the spin correlation does not
develop any superstructure. We call this state the loop liquid, as it is
characterized by the emergent degree of freedom, self-avoiding up-spin loops.
We elucidate that the system shows phase transitions from the loop liquid to
ferrimagnetically ordered states and a crossover to a partially ferromagnetic
state by changing the electron density and temperature. These can be viewed as
crystallization and cohesion of the loops, respectively. We demonstrate that
the loop formation is observed in the optical conductivity as a characteristic
resonant peak.
