Strange metals – which exhibit unusual properties such as a resistivity that scales linearly with temperature – challenge our understanding of charge transport in metals. A general and puzzling feature of the strange metal is a linear-in-T resistivity existing over a wide region of the phase diagram in the limit of low temperature. In contrast, linear resistivity down to the lowest T is observed in quantum critical metals, but only at a singular parameter in the phase diagram. A common ingredient for theories of strange metals is often the existence of a low energy degree of freedom that can effectively couple to charge carriers down to the lowest T. In high-Tc cuprate superconductors, the nature and origin of these low-lying excitations remains elusive.
We use fields as high as 86 T to explore the physics of strange metals in the cuprate superconductor La2-xSrxCuO4 (LSCO). Close to the critical doping of the pseudogap p* = 0.19, we discover that T-linear resistivity exists down to the lowest T over an extended range of magnetic field between 60 T and 70 T or so, and disappears above (see Fig. 1a). Indeed, above 70 T, a spin glass phase gradually appears and causes the end of strange metallicity, as shown schematically in the phase diagram of Fig. 1b. In the region where low T magnetic fluctuations exist, as demonstrated by previous NMR and ultrasound measurements, linear-in-T resistivity appears over an extended range of magnetic field (green area in Fig. 1b). Our results show that the strange metal can be controlled with a field, via spin dynamics, and that the strange metal phase is closely linked to low energy magnetic fluctuations that persist at the lowest temperatures.
We show that the field-dependent magnetism drives the magnetoresistance. Resistivity upturns, a signature of the metal-insulator crossover of LSCO, appear at low temperatures in the spin glass phase. This shows that the metal-insulator crossover of LSCO is linked to the freezing of spins.
Figure 1: a) Resistivity of LSCO p= 0.188, near the pseudogap critical doping, as a function of temperature at various magnetic fields. Dashed lines are fit to the data. b) False color plot of the exponent n of the temperature dependent in-plane resistivity ρ(T) = ρ0 + aTn for LSCO p = 0.188. It is obtained from interpolation of (dln(ρ(T)-ρ0)/dlnT) calculated for different magnetic fields. The white area corresponds to the superconducting phase and the resistive transition. ρ0 is the residual resistivity extrapolated to T=0 from linear fits as shown in panel a.
Reference : Impact of low-energy spin fluctuations on the strange metal in a cuprate superconductor
D. J. Campbell, M. Frachet, V. Oliviero, T. Kurosawa, N. Momono, M. Oda, J. Chang, D. Vignolles, C. Proust, D. LeBoeuf, Nature Physics (2025). https://doi.org/10.1038/s41567-025-03034-0