Spin dynamics in the pseudogap state of a high-temperature superconductor

TL;DR

This study reveals that spin excitation dispersions in the pseudogap state of a high-temperature superconductor differ significantly from those in the superconducting state, indicating a complex interplay and competition between these phases.

Contribution

It provides the first detailed comparison of spin dynamics in the pseudogap and superconducting states using inelastic neutron scattering, highlighting qualitative differences in magnetic dispersion relations.

Findings

The 'hour glass' magnetic dispersion is absent in the pseudogap state.

The pseudogap state exhibits an unusual 'vertical' dispersion with in-plane anisotropy.

High-energy excitations are similar in both states, obeying lattice symmetry.

Abstract

The pseudogap is one of the most pervasive phenomena of high temperature superconductors. It is attributed either to incoherent Cooper pairing setting in above the superconducting transition temperature Tc, or to a hidden order parameter competing with superconductivity. Here we use inelastic neutron scattering from underdoped YBa(2)Cu(3)O(6.6) to show that the dispersion relations of spin excitations in the superconducting and pseudogap states are qualitatively different. Specifically, the extensively studied "hour glass" shape of the magnetic dispersions in the superconducting state is no longer discernible in the pseudogap state and we observe an unusual "vertical" dispersion with pronounced in-plane anisotropy. The differences between superconducting and pseudogap states are thus more profound than generally believed, suggesting a competition between these two states. Whereas the high-energy excitations are common to both states and obey the symmetry of the copper oxide square lattice, the low-energy excitations in the pseudogap state may be indicative of collective fluctuations towards a state with broken orientational symmetry predicted in theoretical work.