Vanishing Hall number at a quantum critical point*

Certain types of quantum critical points, phase transitions at zero-temperature, have long been thought to underlie unconventional superconductivity in a variety of systems. We study the effect of particle-hole dilution in the unconventional superconductor CeCoIn₅, and discover an unusual critical point that connects two distinct Fermi liquids with different Fermi surface volumes without any apparent symmetry breaking. The signature of this transition is pronounced in the Hall number, which nearly vanishes at the phase boundary between the two Fermi liquids and exhibits a strong dependence on the applied magnetic field. The experiment provides evidence for a type of quantum critical point that fractionalizes conventional metallic quasiparticles into gapless spin excitations and gapped charge carrying excitations. Calculations are presented which suggest that the experimentally measured Hall resistivity in this material reflects the motion of charged particles in the fractionalized Fermi fluid.