  The mysterious metallic phase showing perfect $T$-linear resistivity and a
universal scattering rate $1/\tau = \alpha_P k_B T /\hbar$ with a universal
prefactor $\alpha_P \sim 1$ and logarithmic-in-temperature singular specific
heat coefficient, so-called Planckian metal phase was observed in various
overdoped high-$T_c$ cuprate superconductors over a finite range in doping.
Here, we propose a microscopic mechanism for this exotic state based on
quantum-critical bosonic charge Kondo fluctuations coupled to both spinon and a
heavy conduction-electron Fermi surfaces within the heavy-fermion formulation
of the slave-boson $t$-$J$ model. Using a controlled perturbative
renormalization group (RG) analysis, we examine the competition between the
pseudogap phase, characterized by Anderson's Resonating-Valence-Bond
spin-liquid, and the Fermi-liquid state, characterized by the electron hoping
(effective charge Kondo effect). We find a quantum-critical metallic phase with
a universal Planckian $\hbar \omega/k_B T$ scaling in scattering rate near a
localized-delocalized (pseudogap-to-Fermi liquid) charge Kondo breakdown
transition. Our results are in excellent agreement with the recent experimental
observations on optical conductivity (without fine-tuning) in Nat. Commun. 14,
3033 (2023), universal doping-independent field-to-temperature scaling in
magnetoresistance in Nature 595, 661 (2021), and the marginal Fermi-liquid
spectral function observed in ARPES (Science 366, 1099 (2019)) as well as Hall
coefficient in various overdoped cuprates in Nature 595, 661 (2021) and Annu.
Rev. Condens. Matter Phys. 10, 409 (2019). Our mechanism offers a microscopic
understanding of the quantum-critical Planckian metal phase observed in
cuprates d-wave superconducting, and Fermi liquid phases.
