Anomalous Knight shift in the superconducting state of UBe13

We report muon spin rotation measurements of the temperature dependence of the mu(+)-Knight shift in single crystals of UBe13. The dipolar fields prod uced by the uranium Sf-electron moments result in two magnetically inequiva lent mu(+) sites. Below T-c, the magnitude of the Knight shift is observed to decrease with decreasing T at one of the sites, whereas it is found to i ncrease at the other site. The data is explained by an effective exchange i nteraction between the mu(+) and the 5f-electrons which is temperature depe ndent and anisotropic below T approximate to 6 K. (C) 2000 Published by Els evier Science B.V. All rights reserved.