WHY ARE CUPRATE SUPERCONDUCTORS SO ATTRACTIVE FOR NMR/NQR RELAXATION STUDIES?

Detlef Brinkmann

Physik-Institut, University of Zürich, CH-8057 Zürich, Switzerland

Nuclear Magnetic Resonance (NMR) and Nuclear Quadrupole Resonance (NQR) are two spectroscopic techniques which employ magnetic nuclei to study, among others, the dynamics of a condensed matter sample at the atomic level. Thus, NMR and NQR have successfully been applied in the research field of high-temperature superconductors.

Among the most thoroughly studied structures are the cuprate compounds YBa2Cu3O7 and YBa2Cu4O8. They are so attractive for NMR/NQR for two reasons. First, each atom of these compounds has one or even two magnetic nuclei suitable for NMR investigations: 89Y, 135Ba, 137Ba, 63Cu, 65Cu, and 17O. The Ba and Cu isotopes posses, in addition to their magnetic moment, a sufficiently large nuclear quadrupole moment that allows NQR experiments. Secondly, the different atoms occupy structurally quite different positions in the unit cell: both Cu and O are located in chains and planes, Y separates the two CuO2 planes, and Ba occupies voids between chains and planes. Thus, the magnetic nuclei can monitor different dynamic aspects at the atomic level.

In order to demonstrate this, we will review some NMR/NQR relaxation experiments performed in YBa2Cu3O7 and YBa2Cu4O8. We will start with a discussion of a phenomenological and of microscopic models that explain the temperature dependence of the spin-lattice relaxation of Cu, O (and this for plane and chain sites), and Y. We then show how relaxation studies can detect peculiar processes at the atomic level: the spin gap and the isotope effect of the spin gap opening temperature T*, an electronic crossover (in the neighborhood of T*) and a possible relation to a charge density wave. We also briefly deal with some relaxation studies in the superconducting state (e.g. concerning the symmetry of the pair wave function). Finally, the Ba relaxation is an example which demonstrates the importance of phonons in these compounds.