Quantum dots are nanoscale systems where electrons are confined in the three spatial dimensions. This unique feature allows us to control the current that passes through a quantum dot, which thus shows properties similar to a transistor. A remarkable transport phenomenon that can be studied using dots is the Coulomb drag effect. This effect is produced when two dots are coupled together but only one of them is connected to a current source. A drag current then arises through the other dot as a result of the Coulomb repulsion between electrons. This mechanism might have practical applications in quantum computers or for ultrasmall charge sensors.
A team of scientists, including an IFISC (CSIC, UIB) researcher, has published a paper in Physical Review Letters in which they analyze the effect of replacing one of the normal electrodes with a superconducting one. To obtain the electron transport properties under these conditions, they carry out numerical simulations using two different methods and determine which physical processes were relevant when producing the drag current.
In the paper, entitled Andreev-Coulomb Drag in Coupled Quantum Dots, they demonstrate that for low temperatures and sufficiently strong couplingx, the Coulomb drag effect is mediated by Andreev processes, in which an electron is converted into a hole while at the same time a pair of electrons forms inside the superconductor. This novel Andreev-Coulomb mechanism can be distinguished from the conventional drag mechanism by observing the sign changes in the generated supercurrent. The authors find that the Andreev-Coulomb effect can be observed for the typical values of the systems currently employed in experiments, which implies that the effect will be likely detected in the near future.
Andreev-Coulomb Drag in Coupled Quantum Dots. S. Mojtaba Tabatabaei, David Sánchez, Alfredo Levy Yeyati, and Rafael Sánchez. Phys. Rev. Lett. 125, 247701 (2020)