Zooming on the quantum critical point in Nd-LSCO

Recent studies of the high-T_c superconductor La _1.6-xNd_0.4Sr_xCuO₄ (Nd-LSCO) have found a linear-T in-plane resistivity ρab and a logarithmic temperature dependence of the thermopower S/T at a hole doping p=0.24 and a Fermi-surface reconstruction just below p=0.24 [1,2]. These are typical signatures of a quantum critical point (QCP). Here we report data on the c-axis resistivity ρc(T) of Nd-LSCO measured as a function of temperature near this QCP, in a magnetic field large enough to entirely suppress superconductivity. Like ρab,ρc shows an upturn at low temperature, a signature of Fermi surface reconstruction caused by stripe order. Tracking the height of the upturn as it decreases with doping enables us to pin down the precise location of the QCP where stripe order ends, at p=0.235±0.005. We propose that the temperature T_ρ below which the upturn begins marks the onset of the pseudogap phase, found to be roughly twice as high as the stripe-ordering temperature in this material.