  The pyrochlore iridates have become ideal platforms to unravel fascinating
correlated and topolog?ical phenomena that stem from the intricate interplay
among strong spin-orbit coupling, electronic correlations, lattice with
geometric frustration, and itinerancy of the 5d electrons. The all-in-all?out
antiferromagnetic state, commonly considered as the magnetic ground state, can
be dramatically altered in reduced dimensionality, leading to exotic or hidden
quantum states inaccessible in bulk. Here, by means of magnetotransport,
resonant elastic and inelastic x-ray scattering experiments, we discover an
emergent quantum disordered state in (111) Y2Ir2O7 thin films (thickness less
than 30 nm) per?sisting down to 5 K, characterized by dispersionless magnetic
excitations. The anomalous Hall effect observed below an onset temperature near
135 K corroborates the presence of chiral short-range spin configurations
expressed in non-zero scalar spin chirality, breaking the macroscopic
time-reversal symmetry. The origin of this chiral state is ascribed to the
restoration of magnetic frustration on the pyrochlore lattice in lower
dimensionality, where the competing exchange interactions together with
enhanced quantum fluctuations suppress any long-range order and trigger
spin-liquid-like behavior with degenerate ground-state manifold.
