ΔK=0 M1 Excitation Strength of the Well-Deformed Nucleus ^{164}Dy from K Mixing.

The size of a Delta K = 0 M1 excitation strength has been determined for the first time in a predominantly axially deformed even-even nucleus. It has been obtained from the observation of a rare K-mixing situation between two close-lying J(pi) = 1(+) states of the nucleus Dy-164 with components characterized by intrinsic projection quantum numbers K = 0 and K = 1. Nuclear resonance fluorescence induced by quasimonochromatic linearly polarized gamma-ray beams provided evidence for K mixing of the 1(+) states at 3159.1(3) and 3173.6(3) keV in excitation energy from their gamma-decay branching ratios into the ground-state band. The Delta K = 0 transition strength of B(M1;0(1)(+) -> 1(K=0)(+) = 0.008(1)mu(2)(N) was inferred from a mixing analysis of their M1 transition rates into the ground-state band. It is in agreement with predictions from the quasiparticle phonon nuclear model. This determination represents first experimental information on the M1 excitation strength of a nuclear quantum state with a negative R-symmetry quantum number.