  The interplay of spin-orbit interactions and electronic correlations
dominates the physical properties of pyrochlore iridates, R2Ir2O7 (R = Y, rare
earth element), which are typically magnetic insulators. We report an
experimental/theoretical study of single-crystal Bi2Ir2O7 where substitutions
of Bi for R sensitively tips the balance between competing interactions so as
to favor a metallic state with a strongly exchange enhanced paramagnetism. The
ground state is characterized by the following features: (1) A divergent
low-temperature magnetic susceptibility that indicates no long-range order down
to 50 mK; (2) strongly field-dependent coefficients of the low-temperature T-
and T3-terms of the specific heat; (3) a conspicuously large Wilson ratio R_W
\approx 53.5; and (4) unusual temperature and field dependences of the Hall
resistivity that abruptly change below 80 K, without any clear correlation with
the magnetic behavior. All these unconventional properties suggest the
existence of an exotic ground state in Bi2Ir2O7.
