Spin gap, electronic crossover, and charge density waves in Y-Ba-Cu-O superconductors

This paper discusses recent NQR/NMR studies performed on Y-Ba-Cu-O superconductors at the University of Zurich. The work is concerned with normal state properties which are still controversial, for instance the spin-gap effect, i.e. the opening of a pseudo gap in the electron spin excitation spectrum at a temperature T*, which lies above T-c. We will report on the detection of "anomalies" which are displayed in the temperature dependence of several NMR and NQR quantities measured in the normal state of YBa2Cu4O8. These anomalies are interpreted as an electronic crossover which involves a charge redistribution in the CuO2 planes and an enhancement of the charge fluctuations. As a possible mechanism of the crossover, a charge density wave instability is proposed.