Charge Freezing in Zig-zag Chain Cuprates PrBa2Cu4O8 Observed by Cu Nuclear Quadrupole Resonance

We report nuclear quadrupole resonance (NQR) studies on the chain Cu sites of PrBa${}_2$Cu${}_4$O${}_8$, a quasi-one-dimensional conductor with a nearly quarter-filled band. The nuclear spin-lattice relaxation rate $1\mathrm{/}{T}_1$ shows a pronounced peak near 100 K caused by fluctuations of electric field gradient (EFG). Similar peak was observed for the spin-echo decay rate $1\mathrm{/}{T}_2$, however, at a different temperature near 50 K. These results and broadening of the NQR spectrum at low temperatures indicate that slow charge fluctuations of either electronic or ionic origin freeze gradually at low temperatures.