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Japanese Journal of Applied Physics

Current Reversal of Photon-assisted Tunneling in a Quantum Dot Connected to Superconducting Leads

Sam Young Cho, Kicheon Kang, Chang‐Mo Ryu, Chul Koo Kim

Published Jul 1, 2001• 0 citations
Publisher

Abstract

We study the current reversal of the photon-assisted electron transport in a single level quantum dot connected to two superconductors. It is found that the direction of photon-assisted tunneling (PAT) current sensitively changes depending on the external frequency and on the electron energy level of the quantum dot. The frequency region for the reversal of the PAT current is determined by a small applied dc-bias or the energy gap difference between the two connecting leads.

DOI
10.1143/jjap.40.4559

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The combined action of a dc bias and a microwave drive on the transport characteristic of a superconductor-quantum dot-superconductor junction is investigated. To cope with time dependent nonequilibrium effects and interactions in the quantum dot, we develop a general formalism for the dynamics of the density operator based on a particle conserving approach to superconductivity. Without invoking a broken U(1) symmetry, we identify a dynamical phase connected to the coherent transfer of Cooper pairs across the junction. In the weak-coupling limit, we show that besides quasiparticle transport, proximity induced superconducting correlations manifest in anomalous pair tunneling involving the transfer of a Cooper pair. The resulting generalized master equation in presence of the microwave drive showcases the characteristic bichromatic response due to the combination of the ac Josephson effect and an ac voltage. Analytical expressions for all harmonics in the driving frequency of both the current and the reduced dot operator are given for arbitrary driving strength. For the net dc current, the resulting photon assisted processes give rise to rich current-voltage characteristics. In addition to photon assisted subgap transport, we find regions of total current inversion in the stability diagram. There, the junction acts as a pump with the net dc current flowing against the applied dc bias. The first harmonic of the current, being closely related to the nonlinear dynamic susceptibility of the junction, is discussed at finite applied dc bias.
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We theoretically study the photon-assisted transport (PAT) in a double quantum dot (QD) which is conditionally controlled by the optical excitation in an adjacent QD. Based on a real three-QD model, a double-electron level configuration needed for the conditional PAT is designed. Interesting conditional dynamics are demonstrated for the double QD and its adjacent QD. Depending on various conditional frequencies, the conditional behaviors can be manifested as the current peaks or collapses of the PAT. They correspond to coherent excitations or trapping of double-electron states. Our results present a possible way to optically manipulate electron states and monitor them via the signals of current spectrum.
In this study, a model of a double-quantum-dot system driven by two-mode microwave photons is presented. The quantum master equation is derived from the system’s Hamiltonians, and the expression for the steady-state current is obtained. Electronic tunneling properties are then analyzed. The results revealed that different two-mode quantum microwave photons have varying effects on the tunneling current within the double-quantum-dot system, with a steplike current trend emerging. The tunneling current showed pronounced negative differential conductance for both coherent and squeezed microwave photons. Furthermore, the tunneling current was significantly influenced by changing the squeezing coefficient and phase. The asymmetric evolution of the tunneling current under varying bias voltages also depends on the asymmetry in system parameters. These findings are crucial for manipulating the transport properties of double-quantum-dot systems in nanostructured devices.