Zero-field Quantum Critical Point in CeCoIn${}_5$

Quantum criticality in the normal and superconducting state of the heavy-fermion metal CeCoIn${}_5$ is studied by measurements of the magnetic Gr"{u}neisen ratio, ${\mathrm{\Gamma }}_H$, and specific heat in different field orientations and temperatures down to 50 mK. Universal temperature over magnetic field scaling of ${\mathrm{\Gamma }}_H$ in the normal state indicates a hidden quantum critical point at zero field. Within the superconducting state the quasiparticle entropy at constant temperature increases upon reducing the field towards zero, providing additional evidence for zero-field quantum criticality.