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

TL;DR

This study reveals lower-than-expected crystal symmetry and anisotropic bonds in La$_2$CuO$_4$, linking structural features to magnetic and electronic properties, including potential implications for superconductivity and thermal Hall effects.

Contribution

It provides new neutron scattering evidence of structural symmetry lowering and phonon modes in La$_2$CuO$_4$, highlighting their relevance to charge stripe pinning and magnetic phenomena.

Findings

Lower structural symmetry than previously assumed.

Presence of a soft phonon mode related to octahedral tilts.

No interaction between spin waves and optical phonons in the parent compound.

Abstract

New surprises continue to be revealed about La$_2$CuO$_4$, the parent compound of the original cuprate superconductor. Here we present neutron scattering evidence that the structural symmetry is lower than commonly assumed. The static distortion results in anisotropic Cu-O bonds within the CuO$_2$ planes; such anisotropy is relevant to pinning charge stripes in hole-doped samples. Associated with the extra structural modulation is a soft phonon mode. If this phonon were to soften completely, the resulting change in CuO$_6$ octahedral tilts would lead to weak ferromagnetism. Hence, we suggest that this mode may be the "chiral" phonon inferred from recent studies of the thermal Hall effect. We also note the absence of interaction between the antiferromagnetic spin waves and low-energy optical phonons, in contrast to what is observed in hole-doped samples.