Evidence for magnetic field-tuned quantum criticality below $H_{c2}$ in CeCoIn$_5$

The existence of a magnetic field-tuned quantum critical point coinciding with the upper critical field for superconductivity in the heavy-fermion superconductor CeCoIn$_5$ has remained a puzzling fact, and has proven difficult to study due to the onset of superconductivity. Here we present low temperature thermal conductivity measurements which probe the approach to the quantum critical point in CeCoIn$_5$ as a function of field from within the superconducting state, revealing new evidence for field- tuned quantum criticality.