Doping evolution of the phonon density of states and electron-lattice interaction in Nd$_{2-x}$Ce$_x$CuO$_{4+\delta}$

TL;DR

This study investigates how doping affects phonon density of states and electron-lattice interactions in Nd$_{2-x}$Ce$_x$CuO$_{4+ abla}$, revealing early phonon softening unrelated to superconductivity transition.

Contribution

It provides new insights into the doping-dependent phonon behavior and electron-lattice coupling differences between n-type and p-type high-$T_c$ superconductors.

Findings

Phonon softening occurs within the first few percent of Ce-doping.

Softening is not related to the nonsuperconducting-superconducting transition.

Electron-lattice coupling differs from p-type high-$T_c$ materials.

Abstract

We use inelastic neutron scattering to study the evolution of the generalized phonon density of states (GDOS) of the $n$-type high-$T_c$ superconductor Nd$_{2-x}$Ce$_x$CuO$_{4+\delta}$ (NCCO), from the half-filled Mott-insulator ($x=0$) to the $T_c=24$ K superconductor ($x=0.15$). Upon doping the CuO$_2$ planes in Nd$_2$CuO$_{4+\delta}$ (NCO) with electrons by Ce substitution, the most significant change in the GDOS is the softening of the highest phonon branches associated with the Cu-O bond stretching and out-of-plane oxygen vibration modes. However, the softening occurs within the first few percent of Ce-doping and is not related to the electron doping induced nonsuperconducting-superconducting transition (NST) at $x\approx 0.12$. These results suggest that the electron-lattice coupling in the $n$-type high-$T_c$ superconductors is different from that in the $p$-type materials.