Optimal Single Quantum Dot Heat-to-pure-spin-current Converters

We delve into the conditions under which a quantum dot thermoelectric setup may be tuned to realize an optimal heat-to-pure-spin-current converter. It is well known that a heat-to-pure-spin-current converter may be realized using a non-interacting quantum dot with a spin-split energy spectrum under particle hole symmetry conditions. However, with the inclusion of Coulomb interaction $U$, ubiquitous in typical quantum dot systems, the relevant transport physics is expected to be altered. In this work, we provide a detailed picture of thermoelectric pure spin currents at various Coulomb interaction parameters $U$ and describe the conditions necessary for an exact cancellation of charge transport between energy levels $\epsilon$ and their Coulomb-charged partner levels $\epsilon+U$, so as to yield the largest terminal pure spin currents. A non-trivial aspect pointed out here is that at sufficiently large values of $U$ ($\ge U_0$), pure spin currents tend to optimize at points other than where the particle-hole symmetry occurs. It is also ascertained that a global maximum of pure spin current is generated at a typical value of the interaction parameter $U$. These optimum conditions may be easily realized using a typical gated quantum dot thermoelectric transport setup