Theoretical study of pressure and temperature variations of the electric-field gradients in YBa2Cu4O8

C.O. RODRIGUEZ; G. FABRICIUS; M.G. STACHIOTTI; N.E. CHRISTENSEN

PHYSICAL REVIEW B

AMER PHYSICAL SOC

Lugar: New York; Año: 1997 vol. 56 p. 14833 - 14837

1098-0121

The behavior of the electric-field gradients (EFG?s) at all nuclear sites for YBa2Cu4O8 (Y124) under pressure has been obtained within the local-density approximation (LDA) to density-functional theory, using the full-potential linear-augmented plane-wave method. We have also calculated their changes with temperature by including solely effects due to the crystallographic temperature modifications. The need for using an extended basis to deal with errors due to extended core states or linearization of narrow states is demonstrated by comparing our results to those obtained using the more standard two-panel scheme. The calculated EFG?s at ambient pressure for plane and chain Cu and O sites agree with already published theoretical values, but those of Y and Ba are quite different. The EFG?s for which there exist experimental determinations are reproduced except for planar Cu, where a factor of three discrepancy is found. This is similar to the case of YBa2Cu3O6.9, where the EFG?s of the planar Cu were off by a factor of two. The smaller predicted anisotropy has been attributed to a real failure of the LDA. As pressure is increased the calculated EFG?s at planar Cu sites remain practically constant. Since many properties of Y124, including Tc, are very sensitive to pressure it has been speculated that there should be a corresponding charge redistribution in the Cu-O planes and this, in principle, should be reflected in changes in the components of the EFG?s. This does not happen. Experimentally pressure changes in EFG?s at Cu site are small. From our analysis of the p and d contributions to the EFG at planar Cu sites it is found that charge redistribution does indeed occur but in such a manner that the contribution to changes to the EFG cancels.