Evidence of good quasiparticles with Plankian scattering*

Strange metals have strange resistivity: linear-in-temperature (T-linear) down to low temperature. Strange metals are found in many families of correlated electron materials, leading to the conjecture that a universal bound - the "Planckian" bound - limits the scattering rate of electrons to a value set by fundamental constants [1]. If the Planckian bound exists, it would provide a natural explanation for why a host of seemingly disparate systems, including high-temperature superconductors and twisted bilayer graphene, all have T-linear resistivity. Perhaps most dramatically, T-linear resistivity suggests that electron-electron interactions are so strong that conventional concepts such as quasiparticles and Boltzmann transport do not apply in strange metals [2]. We have measured the angle-dependent magnetoresistance (ADMR) of Nd-LSCO: a strange metal with perfectly T-linear resistivity down low temperature. We find two remarkable results: 1) we can model our entire data set using Boltzmann transport and a conventional Fermi surface [3]; and 2) we extract a transport scattering rate that saturates the Planckian bound [4]. We show that our extracted Fermi surface and scattering rate are quantitatively consistent with other transport properties, including the Hall effect and the high-field magnetoresistance. These results suggest that quasiparticles are alive and well in this strange metal, despite T-linear resistivity and a scattering rate that saturates the Planckian bound. We present a new idea involving "strange scatterers", originating from two-level systems, that can produce strange-metal behaviour in an otherwise conventional metal, pointing to a resolution of the apparent contraction of Planckian scattering in a metal where Boltzmann transport works.

[1] J.A.N. Bruin et al., Science 339, 804–807 (2013).

[2] S.A. Hartnoll, Nature Physics 11, 54-61 (2015).

[3] Y. Fang et al., Nature Physics 18, 558-564 (2022).

[4] G. Grissonnanche et al., Nature 595, 667-672 (2021).