  Muon spin rotation and relaxation experiments in the pyrochlore iridate
Eu2Ir2O7 yield a well-defined muon spin precession frequency below the
metal-insulator/antiferromagnetic transition temperature TM = 120 K, indicative
of long-range commensurate magnetic order and thus ruling out quantum spin
liquid and spin-glass-like ground states. The dynamic muon spin relaxation rate
is temperature-independent between 2 K and ~TM and yields an anomalously long
Ir^{4+} spin correlation time, suggesting a singular density of low-lying spin
excitations. Similar behavior is found in other pyrochlores and geometrically
frustrated systems, but also in the unfrustrated iridate BaIrO3. Eu2Ir2O7 may
be only weakly frustrated; if so, the singularity might be associated with the
small-gap insulating state rather than frustration.
