Magnetic Field Effects on Superconductor/Quantum-Dot/Superconductor Josephson Structures with Intradot Attraction

Chien-Te Wu^1*, Yu-Chun Hsu¹

¹Electrophysics, National Chiao Tung University, Hsinchu, Taiwan

* Presenter:Chien-Te Wu, email:chientewu@nctu.edu.tw

Josephson quantum dot devices have been extensively studied in the literature both theoretically and experimentally [1]. Due to the superconducting proximity effects, the two possible electronic states for the embedded quantum dot are the doublet (linear combinations of up and down states) and the BCS state in the absence of a magnetic field. Depending on the coupling between the quantum dots and the superconducting leads, the ground state is one of the two states and can be determined from the energy competition. In this work, we adopt the self-consistent theoretical method proposed in Ref. [1] to extend our considerations beyond the superconducting atomic limit. Motivated by recent experiments on a quantum dot with intradot attraction, we also investigate the implication of a negative U in electronic structures. With application of an external magnetic field, the degeneracy between up and down states is lifted, and the down state is energetically favorable over the up state. As a result, the BCS state is less likely to be more stable. We find reentrance behavior in the energy phase diagram when U is negative as well as the 0-πphase transitions in the Josephson effects that may offer important switching effects.

Reference:
[1] T. Meng, S.Florens, and P. Simon, Phys. Rev. B 79, 224521 (2009)
[2] G. Cheng, M. Tomczyk, S. Lu, J. P. Veazey, M. Huang, P. Irvin, S. Ryu, H. Lee, C. B. Eom, C. S. Hellberg, and J. Levy, Nature (London) 521, 196 (2015).

Keywords: Josephson Current, Quantum Dot, Kondo Effect