Superconductor–quantum dot hybrid coolers

We propose a refrigeration scheme in a mesoscopic superconductor--quantum dot hybrid device. The setup can significantly cool down a normal metal coupled to the device by applying a bias voltage across the system. We demonstrate that the cooling power can be as large as $0.05{\mathrm{\ensuremath{\Delta}}}₀²/h$ where ${\mathrm{\ensuremath{\Delta}}}₀$ is the absolute value of the superconducting order parameter. In contrast to previous proposals, our device operates without any magnetic elements, such as ferromagnetic reservoirs or Zeeman splittings. The refrigeration scheme works over a broad parameter range and can be optimized by tuning system parameters, such as level position and bias voltage. Our theory self-consistently determines the temperature drop of the normal reservoir in the nonlinear transport regime including electron-electron interactions at the mean-field level. Finally, we evaluate the refrigeration performance and find efficiencies as large as half of the Carnot bound for realistic values of the coupling strength.