Thermopower across the stripe critical point of La1.6-xNd0.4SrxCuO4: Evidence for a quantum critical point in a hole-doped high-Tc superconductor

The thermopower S of the high-T-c superconductor La1.6-xNd0.4SrxCuO4 was measured as a function of temperature T near its quantum critical point, the critical hole doping p(star) where all characteristic temperatures go to zero. Just above p(star), S/T varies as ln(1/T) over a decade of temperature. Below p(star), S/T undergoes a large increase at low temperature. As with the temperature dependence of the resistivity, which is linear just above p(star) and undergoes a large upturn at low temperature, these are typical signatures of a quantum phase transition. This suggests that p(star) is a quantum critical point below which some order sets in, causing a reconstruction of the Fermi surface, whose fluctuations are presumably responsible for the linear-T resistivity and logarithmic thermopower. All the evidence points to stripe order, a form of spin/charge modulation known to exist in this material.