Low energy charge fluctuations in the presence of the pseudo spin gap in YBa2Cu4O8

We present results from a O-17 nuclear magnetic resonance study in the stoi chiometric superconductor YBa2Cu4O8. A double irradiation method enables us to show that, below around 180 K approximate to T*, the spin-lattice relax ation rate of plane oxygen is not only driven by magnetic, but also signifi cantly by quadrupolar fluctuations, i.e. low-frequency charge fluctuations. From the temperature dependence as well as from the magnitude of the spin- lattice relaxation, we conclude that this low-frequency charge fluctuations are not of simple phononic or sim ple electron-like quasiparticle origin. In the superconducting state, on lowering the temperature, the quadrupolar relaxation diminishes faster than the magnetic one. On the other side, we d o not observe any quadrupolar contributions in the spin-lattice relaxation of plane copper. These findings show that, with the opening of the pseudo s pin gap, a charge degree of freedom of mainly oxygen character is present i n the electronic low-energy excitation spectrum. Different possibilities of charge degrees could explain our findings: (i) Formation of spatially stro ngly correlated holes. (ii) Anomalous strong electron-phonon coupling. Thes e findings suggest that a simple magnetic single spin fluid model is not su fficient to describe the low-energy excitations for temperatures T < T*.