Abstract
The mysterious Planckian metal state, showing perfect T-linear resistivity associated with universal scattering rate, 1/τ = αk(B)T/ℏ with α ~ 1, has been observed in the normal state of various strongly correlated superconductors close to a quantum critical point. However, its microscopic origin and link to quantum criticality remains an outstanding open problem. Here, we observe quantum-critical T/B-scaling of the Planckian metal state in resistivity and heat capacity of heavy-electron superconductor Ce(1-x)Nd(x)CoIn(5) in magnetic fields near the edge of antiferromagnetism at the critical doping x(c) ~ 0.03. We present clear experimental evidences of Kondo hybridization being quantum critical at x(c). We provide a generic microscopic mechanism to qualitatively account for this quantum critical Planckian state within the quasi-two dimensional Kondo-Heisenberg lattice model near Kondo breakdown transition. We find α is a non-universal constant and depends inversely on the square of Kondo hybridization strength.