  We explore a thermodynamical effect of anharmonicity in quantum mechanical
oscillators. We show that small quartic perturbations to the oscillator
potential lead to an enhancement of performance of quantum refrigerators for
both the Otto and Stirling cycles. A similar nonlinearity driven enhancement of
performance is also observed for an analogous spin-qubit model of quantum
refrigerators. We further demonstrate the robustness of improvement of the
coefficient of performance versus the energy cost for creating anharmonicity.
It is shown that the anharmonicity driven improvement in performance is a
generic effect at the quantum level for the experimentally realizable Otto
refrigerator.
