Anomalous quantum criticality in the electron-doped cuprates

TL;DR

This study reveals an extended quantum critical regime in electron-doped cuprates, linking thermopower behavior to superconductivity, and identifies a transition to Fermi liquid behavior at higher doping levels.

Contribution

It demonstrates the presence of quantum critical behavior over a wide doping range in electron-doped cuprates and connects it to superconducting properties.

Findings

Quantum critical thermopower behavior observed over doping range x=0.15-0.17.

Slope of S/T versus ln(1/T) scales with Tc.

Fermi liquid behavior found at x=0.19 for T ≤ 40 K.

Abstract

In the physics of condensed matter, quantum critical phenomena and unconventional superconductivity are two major themes. In electron doped cuprates, the low upper critical field allows one to study the putative QCP at low temperature and to understand its connection to the long standing problem of the origin of the high Tc superconductivity. Here we present measurements of the low temperature normal state thermopower (S) of the electron-doped cuprate superconductor La2-xCexCuO4 (LCCO) from x=0.11 to 0.19. We observe quantum critical S divided by 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. 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. Above the superconductivity dome, at x=0.19, a conventional Fermi liquid S proportional to T behavior is found for T less than equal to 40 K.