  We consider a finite time quantum heat engine analogous to finite time
classical Carnot heat engine with a working substance of spin half particles.
We study the efficiency at maximum $\dot{\Omega}$ figure of merit of the
quantum heat engine of spin half particles as a working substance in the
presence of external magnetic field. The efficiency of this engine at maximum
$\dot{\Omega}$ figure of merit shows anomalous behavior in certain region of
particles population levels. Further, we find that the efficiency at maximum
$\dot{\Omega}$ figure exceeds all the known bounds and even approaches the
Carnot efficiency at finite time. Our study indicates that the population of
spin half particles plays a crucial role in quantum heat engine whose
collective effect in the quantum regime can provide superior engine performance
with higher efficiency.
