Observation of low energy dispersive modes in un- derdoped (La, Nd) 2--x Sr x CuO 4

TL;DR

This study detects low-energy, non-dispersive and dispersive excitations in underdoped La and Nd-doped cuprates, revealing modes linked to stripe dynamics distinct from phonons, using IXS and INS techniques.

Contribution

It reports the discovery of low-energy excitations in underdoped cuprates that are associated with stripe dynamics, identified through combined IXS and INS measurements.

Findings

Identification of a non-dispersive 9 meV mode in La-based cuprates.

Detection of a lower energy dispersive mode in Nd-free samples.

Modes are distinct from phonons and linked to stripe phenomena.

Abstract

We find excitations lower in energy than known phonon modes in underdoped La$\_{2-x}$Sr$\_x$CuO$\_{4+\delta}$ (x=0.08), with both inelastic X-Ray scattering (IXS) and inelastic neutron scattering (INS). A non dispersive excitation at 9 meV is identified and is also seen by INS in (La,Nd)$\_{2-x}$Sr$\_x$CuO$\_{4+\delta}$, with 40$\%$ Nd substitution. INS also identifies a still lower energy dispersive mode at low q in the Nd free sample. These modes are clearly distinct from the longitudinal acoustic phonon and correspond in energy to the Zone Centre modes measured by optical spectroscopy and associated with stripe dynamics.