  Mott insulators have surprised us many times by hosting new and diverse
quantum phenomena when the frozen electrons are perturbed by various stimuli.
Superconductivity, metal-insulator transition, and colossal magnetoresistance
induced by element substitution, pressure, and magnetic field are prominent
examples. Here we report a novel phenomenon, namely giant diamagnetism, in the
Mott insulator Ca2RuO4 induced by electric current. With application of 1 A/cm2
current, the strongest diamagnetism among all nonsuperconducting materials is
induced as the system is tuned to a semimetallic state. The origin lies in the
emergence of indirect Dirac cones in the many-body spectrum and associated
monopole-like anomaly in the momentum dependent susceptibility. This
record-breaking and switchable diamagnetism is a new class of non-equilibrium
quantum phenomena on the verge of Mott insulating states.
