HYPERFINE FIELDS AT THE BA SITE IN YBA2CU4O8 - AN NMR AND NQR STUDY

We report a nuclear quadrupole resonance (NQR) and nuclear magnetic re sonance (NMR) study of the Ba isotopes, (135) Ba and Ba-137, in YBa2Cu 4O8. The Knight shift, at 298 K, is zero indicating that no mobile cha rge carriers are present at the Ba site. The temperature (T) dependenc e of v(Q), which is caused by thermal expansion, can be described by a power law. The electric field gradient tensor at the Ba site is in go od agreement with band structure calculations. The spin-spin relaxatio n rate is nearly constant between 56 and 400 K. The spin-lattice relax ation mechanism is of quadrupolar origin and its T dependence can be d escribed perfectly by a two-phonon Raman process with a Debye temperat ure of 185(20) K, which agrees with the thermal conductivity result. T hus, the Ba site in YBa2Cu4O8 does not sense the spin fluctuations of the CuO2 plane.