Strange metal transport, ferromagnetism and the Planckian scattering rate limit in electron-doped cuprates

I will discuss recent transport studies on thin films of the electron-doped cuprate system La2-xCexCuO4 that exhibit strange metal transport in the normal state down to 35 mK. These results strongly suggest that the high-Tc superconductivity emerges from this strange metal normal state. The strange metal behavior is manifested as a linear-in-T resistivity from 35 mK to 20K and a ~T^2 resistivity from 50K to 400K over a range of doping above and below a Fermi surface reconstruction at x = 0.14 [1-2]. Other indications of strange metal behavior include a low temperature linear-in-H magnetoresistance and a low temperature lnT thermopower [3-5] over the same range of doping. At the present time these results have not been explained and they represent a challenge to the theory of the cuprates.

In addition, I will discuss a study of these films via a combination of dc conductivity and optical conductivity [6], which shows that the electron scattering rate far exceeds the conjectured Planckian bound on inelastic scattering at temperatures above 20K. This suggests that recent highly publicized claims of a Planckian bound on transport in solids are not universal and that the temperature dependence of the normal state resistivity of the cuprates remains an unexplained mystery. If time permits I will discuss experiments that suggest ferromagnetism exists below 4K for electron-doping beyond the superconducting dome [7].

References [1]. T. Sarkar et al., Phys. Rev. B 103, 224501 (2021) [2]. T. Sarkar et al., Phys. Rev. B 98, 224503 (2018) [3]. R. L. Greene et al., Ann Rev Cond. Matt. Phys. 11, 213 (2020) [4]. T. Sarkar et al., Sci. Adv. 5, eeav6753 (2019) [5]. P. R. Mandal et. al., PNAS 116, 5991 (2019) [6]. N. R. Poniatowski et al., Phys. Rev. B 104, 235138 (2021) [7]. T. Sarkar et al., Science 368, 532 (2020).