Charge degree of freedom and single-spin fluid model in YBa_2Cu_4O_8

TL;DR

This study uses 17O NMR to reveal that charge fluctuations, alongside magnetic ones, significantly influence the low-energy excitations in YBa_2Cu_4O_8, especially below 180 K and in the superconducting state.

Contribution

It introduces a novel NMR method to distinguish charge and magnetic fluctuations, demonstrating the presence of charge degrees of freedom in the pseudogap regime of YBa_2Cu_4O_8.

Findings

Charge fluctuations significantly affect relaxation rates below 180 K.

Charge degrees of freedom persist in the pseudogap phase.

Quadrupolar relaxation diminishes faster than magnetic relaxation in the superconducting state.

Abstract

We present a 17O nuclear magnetic resonance study in the stoichiometric superconductor YBa_2Cu_4O_8. A double irradiation method enables us to show that, below around 180 K, the spin-lattice relaxation rate of plane oxygen is not only driven by magnetic, but also significantly by quadrupolar fluctuations, i.e. low-frequency charge fluctuations. In the superconducting state, on lowering the temperature, the quadrupolar relaxation diminishes faster than the magnetic one. These findings show that, with the opening of the pseudo spin gap, a charge degree of freedom of mainly oxygen character is present in the electronic low-energy excitation spectrum.