  The heavy-fermion compound URu2Si2 exhibits a hidden-order phase below the
temperature, ~ 17.5 K. In spite of intense research for past three decades, no
consensus on the order parameter exists and the nature has posed a
long-standing mystery. Here we report the discovery of a Griffiths phase within
the hidden-order phase, characterized by residual short-range correlations on
the collapse of long-range orders due to the dilution effects. In the Griffiths
phase scenario, strong evidence are provided for those cluster-like spins, such
as the unique power-law behavior of magnetic susceptibility and specific heat
as well as the frequency dispersion of AC susceptibility. In this way, the
existence of an order parameter is excluded, and the hidden order has a
significant kinship with the long-range large-moment antiferromagnetism which
is accessible by tuning the hydrostatic pressure or the chemical pressure
(i.e., isoelectronic Fe doping). Moreover, an unidirectional anisotropy of
resistivity measurements in rotating magnetic fields is observed in the
hidden-order phase. The anisotropic magnetoresistance and the associated broken
symmetries directly reflect the freezing behavior of at least part of magnetic
clusters. Thus, the demonstrations of the Griffiths phase as an alternative
proposal for the hidden-order phase of URu2Si2 are very promising, challenging
the understanding of exotic electronic states in correlated matter and quantum
materials.
