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Spin Gap, Electronic Crossover, and Charge Density Waves in Y-Ba-Cu-O Superconductors

Abstract This paper discusses recent NQR/NMR studies performed on Y-Ba-Cu-0 superconductors at the University of Zürich. 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 Tc . We will report on the detection of “anomalies” which are dis-played 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 re-distribution 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.