Spin excitations coupled with lattice and charge dynamics in La$_{2-x}$Sr$_{x}$CuO$_4$

Abstract

Spin excitations of layered copper oxide show various characteristic features, depending on the carrier concentration. In this study, we conducted inelastic neutron scattering (INS) measurements on La2−xSrxCuO4 (LSCO), with x = 0, 0.075, 0.18, and 0.30 and La2−xSrxNiO4 (LSNO) with 1/3, to clarify the origin of the intensity enhancement in the excitation spectrum of LSCO at the energy (ω) of 16-19 meV [Phys. Rev. B 91, 224404 (2015), ibid. 93, 094416 (2016]. We confirmed the presence of a peak-structure in the ω-dependence of the local spin susceptibility χ (ω) of superconducting (SC) LSCO with a peak energy of 16-19 meV, where the spin excitations intersect optical phonon branches. A comparable peak-structure is not observed in the insulating La2CuO4, LSNO, and heavily overdoped LSCO with x = 0.30. A dome-shaped x-dependence of the integrated intensity around the peak energies is revealed for SC phase by summarizing the present and previously reported results. Furthermore, our phonon calculation on LCO shows the existence of two optical branches at ∼19 meV that could stabilize stripe-alignment of carriers due to out-ofplane vibrations of Cu or O of the CuO2 planes. These results indicate the interplay among spin, charge, and lattice dynamics and suggest that the intensity enhancement is associated with their composite excitations.

Reinvestigation of crystal symmetry and fluctuations in La$_2$CuO$_4$

“…Finally, we have found no evidence for interaction of the antiferromagnetic spin waves with phonons in the 15-20 meV range. Our results are consistent with another recent study that found such interactions in superconducting LSCO but not in LCO [47]. The interaction of phonons with spin stripes in hole-doped CuO 2 planes is an interesting problem that will need further study.…”

“…Our measurements also capture the antiferromagnetic spin waves. We check for a possible interaction between the spin waves and a low-energy optical mode of the type seen in Ba-and Sr-doped LCO [45,46], but find no evidence for such an interaction in the commensurate antiferromagnet, consistent with another recent neutron study [47].…”