In-plane anisotropy of spin excitations in the normal and superconducting states of underdoped YBa$_2$Cu$_3$O$_{6+x}$

TL;DR

This neutron scattering study reveals that magnetic excitations in underdoped YBa2Cu3O6.6 differ significantly between normal and superconducting states, showing distinct anisotropic dispersion and resonance features.

Contribution

The paper provides detailed measurements of in-plane anisotropy of spin excitations in twin-free YBa2Cu3O6.6, highlighting differences between normal and superconducting states.

Findings

In the superconducting state, spin excitations show upward and downward dispersing branches with modest anisotropy.

In the normal state, the resonance at 37.5 meV disappears, replaced by a steep, strongly anisotropic dispersion.

The results challenge models based on static or dynamic stripe order of spins and charges.

Abstract

A detailed inelastic neutron scattering study of the in-plane anisotropy of magnetic excitations in twin-free YBa$_2$Cu$_3$O$_{6.6}$ ($T_c =$ 61 K) reveals that the spin excitation spectra in the superconducting and normal states are qualitatively different. Below $T_c$, the spectrum consists of upward- and downward-dispersing branches with modest in-plane anisotropy merging at an energy $E_{res} =$ 37.5 meV. In the normal state, the singularity at $E_{res}$ disappears, and the spectrum exhibits a steep dispersion with a strongly anisotropic in-plane geometry. These data have important implications for models based on static or dynamic "stripe" order of spins and charges.