Universal Planckian relaxation in the strange metal state of the cuprates

Immediately following the discovery of the high-transition-temperature (Tc) superconducting cuprates, Anderson proposed that Mott physics plays a crucial role in understanding their phase diagrams. Specifically, he suggested that, much like the ‘almost-localized’ Fermi liquid in 3He, the effective mass renormalization in the cuprates is driven by the physics of a doped Mott insulator, scaling inversely with doping as the system moves away from half-filling. Here, we report a compre- hensive survey of calorimetry and resistivity data in the cuprates across a broad range of dopings and temperatures, extending into the strange metal state at high temperatures. We find that the experimentally determined mass renormalization scales inversely with doping, indicating a strong reduction in the quasiparticle spectral weight approaching the Mott insulating state. This estab- lishes Mott physics as the primary driver of mass renormalization across the entire doping range of the strange metal state. In fact, it is only through such Mott scaling that the T-linear slope of the relaxation rate ħ/τ is independent of doping in the strange metal state. The experimental evidence for true universality of the Planckian relaxation rate across the entire doping range of the cuprates is not merely qualitative but also quantitative: i.e. ħ/τ = αkBT with α ≈ 1.