Anomalous quantum criticality in the electron-doped cuprates

Abstract

In the physics of condensed matter, quantum critical phenomena and unconventional superconductivity are two major themes. In electron-doped cuprates, the low critical field (H_C2) allows one to study the putative quantum critical point (QCP) at low temperature and to understand its connection to the long-standing problem of the origin of the high-<i>T_C</i> superconductivity. Here we present measurements of the low-temperature normal-state thermopower (<i>S</i>) of the electron-doped cuprate superconductor La_2-<i>x</i> Ce _<i>x</i> CuO₄ (LCCO) from <i>x</i> = 0.11-0.19. We observe quantum critical [Formula: see text] versus [Formula: see text] behavior over an unexpectedly wide doping range <i>x</i> = 0.15-0.17 above the QCP (<i>x</i> = 0.14), with a slope that scales monotonically with the superconducting transition temperature (<i>T_C</i> with H = 0). The presence of quantum criticality over a wide doping range provides a window on the criticality. The thermopower behavior also suggests that the critical fluctuations are linked with <i>T_C</i> Above the superconductivity dome, at <i>x</i> = 0.19, a conventional Fermi-liquid [Formula: see text] behavior is found for [Formula: see text] 40 K.