Electronic crossover in the normal state of YBa2Cu4O8

By performing nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR), respectively, with Cu, O, and Y isotopes in the normal state of the high-temperature superconductor ${\mathrm{YBa}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$, we have found the following major results: (i) The Y, Cu, and O shift data support, for planes and chains, the validity of the “single-spin fluid model.” (ii) Around $T^{†}=180\mathrm{}$ K, “anomalies” in the temperature behavior of several NMR/NQR quantities have been detected. (iii) The anomalies do not arise from a structural phase transition; instead they reflect an electronic crossover which involves enhanced charge fluctuations in planes and chains accompanied by a charge (hole) transfer from chain to plane. (iv) As a possible mechanism of the crossover, a charge-density-wave instability is proposed.