The hallmark of the Strange Metal (SM) is a linear-in-temperature resistivity that defies our understanding of electronic scattering in metals, based on the conventional Fermi liquid theory. Remarkably, experimental evidence correlates this T-linear resistivity with the emergence of a universal value of the electronic scattering rate[1], defined as the so-called Planckian limit: 1/τ=αkBT/ℏ, with α∼1. In most systems displaying SM behavior, like heavy fermions or cuprates, antiferromagnetism (AFM) also appears as a key piece of the phase diagram. Therefore, it has been suggested that the presence of strong AFM fluctuations may play a crucial role in the appearance of the Planckian limit and strange metallicity [2].

To understand the connection between AFM fluctuations and the emergence of strange metals, we consider the isostructural delafossites PdCoO₂ and PdCrO₂. While PdCoO₂ is an ultraclean non-magnetic conventional metal, PdCrO₂ is a frustrated antiferromagnet with a Néel temperature of 37,5K, in which AFM fluctuations survive up to high temperatures [3]. In the vicinity of it’s critical point, at temperatures well below its Debye temperature, PdCrO₂ shows a T-linear resistivity consistent with Planckian values of the scattering rate, in total contrast with the conventional Bloch-Grüneisen shape of the resistivity in PdCoO₂[4].

Using Angle-Dependent Magnetoresistance measurements at ultra-high fields, we extract the momentum dependence and temperature dependence of both PdCoO₂ and PdCrO₂. In PdCrO₂, above T_N, the scattering rate is quasi-isotropic at the Planckian limit with α∼1. Only close to T_N we observe some weak anisotropy developing at the folding spots of the Fermi surface caused by the proximity to the antiferromagnetic order. In contrast, we find PdCoO₂ to be completely different, with a purely isotropic and extremely small scattering rate compared to the Planckian limit. The comparison between these almost-identical materials leads us to conclude that antiferromagnetic fluctuations play a crucial role in the emergence of the Planckian limit, but hotspots are not at the origin of the T-linear resistivity.

[1] A. Legros et al, Nature Phys 15, 142–147, 2019

[2] H. Kontani, Rep. Prog. Phys. 71 026501, 2008

[3] H. Takatsu et al, Phys. Rev. B 79 104424, 2009

[4] E. Zhakina et al, Proc. Natl. Acad. Sci. U.S.A. 120 (36) e2307334120, 2023