Thermopower across the stripe critical point of La_(1.6-x)Nd_(0.4)Sr_xCuO_4: Evidence for a quantum critical point in a hole-doped high-T_c superconductor

摘要

The thermopower S of the high-T_c superconductor La_(1.6-x)Nd_(0.4)Sr_xCuO_4 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( I /T) over a decade of temperature. Below p~*, SIT 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.