Cu-spin Correlation in the Electron-overdoped High-Tc Cuprate Thin Films of La_2-x_Ce_x_CuO_4_ Probed by Low-energy Muons

TL;DR

The paper addresses how Cu-spin correlations relate to superconductivity in overdoped electron-doped cuprates. Using depth-resolved low-energy μSR on La$_{2-x}$Ce$_x$CuO$_4$ thin films at $x=0.13$ and $x=0.17$ under varying reduction conditions, the authors show that Cu-spin correlations develop at low $T$ in optimally reduced samples, even in the overdoped regime, with the $x=0.17$ case potentially reflecting antiferromagnetic fluctuations or emergent ferromagnetic order. The results reveal a complex interplay between oxygen reduction, spin dynamics, and transport, including surface vs interior magnetism and depth-dependent oxygen dynamics. These findings challenge simple parallels with hole-doped cuprates and motivate further experiments to disentangle AF versus FM tendencies in heavily overdoped electron-doped cuprates.

Abstract

We investigated the Cu-spin correlation in the overdoped regime of the electron-doped high-Tc cuprate thin films of La_2-x_Ce_x_CuO_4_, changing the reduction condition from muon spin relaxation using low-energy muons. The Cu-spin correlation developed at low temperatures for optimally reduced films with x=0.13 as well as x=0.17 where the superconductivity was almost suppressed. These results are contrary to those observed in the hole-doped high-Tc cuprates where the development of the antiferromagnetic Cu-spin correlation disappears together with the suppression of superconductivity. The Cu-spin correlation developed at low temperatures in x=0.17 may be understood in terms of antiferromagnetism, but it may be related to a ferromagnetic order recently suggested in the nonsuperconducting heavily overdoped La$_2-x_Ce_x_CuO_4_ with x~0.18.

Figures (6)

• Figure 1: (Color online) (a) X-ray diffraction pattern of optimally reduced La$_{2-x}$Ce$_x$CuO$_4$ with $x=0.17$ and (b) Ce concentration dependence of the c-axis lattice constant of La$_{2-x}$Ce$_x$CuO$_4$, together with the previous results. sawasarkar
• Figure 2: (Color online) (a) Temperature dependence of the electrical resistivity of optimally reduced La$_{2-x}$Ce$_x$CuO$_4$ with $x=0.13$ and 0.17. The inset shows the Ce concentration dependence of $T_{\rm c}$, defined as the midpoint temperature of the superconducting transition, together with the previous results. sawa (b) Temperature dependence of the electrical resistivity of La$_{2-x}$Ce$_x$CuO$_4$ with $x=0.17$ under various reduction conditions. The inset shows the change in the onset $T_{\rm c}$, $T_{\rm c}^{\rm onset}$, depending on the reduction condition.
• Figure 3: (Color online) Temperature dependence of the Hall coefficient of under- and optimally reduced La$_{2-x}$Ce$_x$CuO$_4$ with $x=0.13$ and 0.17.
• Figure 4: (Color online) Zero-field $\mu$SR time spectra of (b) under-, (c) optimally, (d) over-reduced La$_{2-x}$Ce$_x$CuO$_4$ with $x=0.17$, changing the implantation energy of muon. The calculated stopping profile of muons for each implantation energy in the film is shown in (a).
• Figure 5: (Color online) (a)-(d) Zero-field $\mu$SR time spectra of under-, optimally, and over-reduced La$_{2-x}$Ce$_x$CuO$_4$ with $x=0.13$ and 0.17 obtained at 23.5 keV. (e) Longitudinal-field $\mu$SR time spectra at 2.6 K for optimally reduced La$_{2-x}$Ce$_x$CuO$_4$ with $x=0.17$ obtained at 23.5 keV.