Thermoelectric evidence of anomalous quantum criticality in the electron-doped cuprates*

In electron-doped cuprates, the low upper critical field allows one to study the putative quantum critical phenomena at low temperature and to understand its connection to the long standing problem of the origin of high-Tc superconductivity. We have measured the low temperature normal state thermopower (S) of the electron-doped cuprate superconductor La_2-xCe_xCuO₄ (LCCO) from x=0.11 to 0.19. We observe quantum critical S/T versus ln(1/T) behavior over an unexpectedly wide doping range x = 0.15 - 0.17 above the putative QCP (x=0.14) with a slope that scales monotonically with the superconducting transition temperature. We find similar behaviour in another n-type cuprate, PCCO, strongly indicating that this is a universal behaviour in electron-doped cuprates. The presence of quantum criticality over a wide doping range provides a new window on the criticality. The thermopower behavior also suggests that the critical fluctuations are linked with Tc. For x = 0.11 and 0.13, S/T has a completely different behavior, an indication of a Fermi surface reconstruction. Above the superconductivity dome, at x=0.19, a conventional Fermi-liquid S∝T behavior is found for T≤40 K.