Freezing of charge degrees of freedom across a critical point in CeCoIn5

The presence of a quantum critical point separating two distinct zero-temperature phases is thought to underlie the strange metal state of many high-temperature superconductors. The nature of this quantum critical point, as well as a description of the resulting strange metal, are central open problems in condensed matter physics. In large part, the controversy stems from the lack of a clear broken symmetry to characterize the critical phase transition, and this challenge is no clearer than in the example of the unconventional superconductor CeCoIn5. By direct Hall effect and Fermi surface measurements, in comparison to ab initio calculations, we observe a critical point that connects two Fermi surfaces with different volumes without a finite-temperature symmetry breaking phase transition. Rather, the transition involves an abrupt localization of one sector of the charge degrees of freedom. At low fields and temperatures, this transition causes a divergence in the Hall coefficient that is cut off on diluting either particles or holes. This remarkable behavior is unexpected in the conventional picture of metals but does appear to be a non-trivial prediction of the theory of the fractionalized Fermi liquid. In this framework, the separation of spin and charge leads to a critical point connecting Fermi surfaces with different volumes.