Thermopower across the stripe critical point of La1.6-x Nd0.4 Srx CuO4: Evidence for a quantum critical point in a hole-doped high- Tc superconductor

The thermopower S of the high- Tc superconductor La1.6-x Nd0.4 Srx CuO4 was measured as a function of temperature T near its quantum critical point, the critical hole doping p□ where all characteristic temperatures go to zero. Just above p□, S/T varies as ln (1/T) over a decade of temperature. Below p□, S/T undergoes a large increase at low temperature. As with the temperature dependence of the resistivity, which is linear just above p□ and undergoes a large upturn at low temperature, these are typical signatures of a quantum phase transition. This suggests that p□ 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.