Superconducting Diode Effect in Double Quantum Dot Device

TL;DR

This paper theoretically investigates the superconducting diode effect in a double quantum dot system with three superconducting leads, revealing conditions for diode behavior and its enhancement near Dirac points, with potential minimal device applications.

Contribution

It introduces a minimal double quantum dot model exhibiting the superconducting diode effect, highlighting the role of phase differences and electron interactions in enabling diode behavior.

Findings

SDE depends on phase differences between leads and is enhanced near Dirac points.

SDE persists with electron-electron interaction U smaller than the superconducting gap.

Single quantum dot devices do not show the SDE, emphasizing the importance of the double quantum dot setup.

Abstract

Superconducting diode effect (SDE) is theoretically examined in double quantum dot coupled to three superconducting leads, $L$, $R1$ and $R2$. Lead $L$ is commonly connected to two quantum dots (QD1, QD2) while lead $R1$ ($R2$) is connected to QD1 (QD2) only. The phase differences $\varphi_{1}$ between leads $L$ and $R1$ and $\varphi_{2}$ between leads $L$ and $R2$ are tuned independently. The critical current into lead $R1$ depends on its direction unless $\varphi_{2} = 0$, $\pi$, which is ascribable to the formation of Andreev molecule between the QDs. In the absence of electron-electron interaction $U$ in the QDs, the spectrum of the Andreev bound states forms Dirac cones in the $\varphi_{1}-\varphi_{2}$ plane if the energy levels in the QDs are tuned to the Fermi level in the leads. The SDE is enhanced to almost 30\% when $\varphi_{2}$ is set to the value at the Dirac points. In the presence of $U$, the SDE is still observed when $U$ is smaller than the superconducting energy gap in the leads. Our device should be one of the minimal models for the SDE since a similar device with a single QD does not show the SDE.