0-π quantum transition in a carbon nanotube Josephson junction: Universal phase dependence and orbital degeneracy
0-π quantum transition in a carbon nanotube Josephson junction: Universal phase dependence and orbital degeneracy
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DOI：
10.1103/physrevb.93.195437
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发表时间：
2016-05-26
影响因子：
3.7
通讯作者：
Deblock, R.
中科院分区：
文献类型：
--
作者：
Delagrange, R.;Weil, R.;Deblock, R.
在π-约瑟夫森结中，超导对在穿过非超导部分时会失相，导致超导电流的符号反转。0-π量子跃迁对电子和磁的相关性非常敏感，提供了强大的探索竞争秩序的工具。在连接到超导储存器的量子点中，跃迁由栅极电压控制。最近的研究表明，在超导邻近效应和近藤关联强竞争的情况下，它也可以被超导相控制。我们在这里研究了一个干净的碳纳米管量子点的电流相位关系，接近轨道简并，在局域电子关联和超导邻近效应之间的强竞争机制。我们表明，过渡的性质取决于至关重要的占领和轨道水平的宽度，这决定了各自的运输的贡献。当库珀对的输运只通过其中一个能级时，我们发现，相位相关的0-π跃迁的相图是零温下不连续能级交叉量子跃迁的普遍特征。在涉及两个能级的情况下，纳米管约瑟夫森电流表现出连续的0-π跃迁，独立于超导相，揭示了不同的物理机制的过渡。
In a pi-Josephson junction, the supercurrent's sign is reversed due to the dephasing of superconducting pairs upon their traversal of the nonsuperconducting part. 0-pi quantum transitions are extremely sensitive to electronic and magnetic correlations, providing powerful exploration tools of competing orders. In a quantum dot connected to superconducting reservoirs, the transition is governed by gate voltage. As shown recently, it can also be controlled by the superconducting phase in the case of strong competition between the superconducting proximity effect and Kondo correlations. We investigated here the current-phase relation in a clean carbon nanotube quantum dot, close to orbital degeneracy, in a regime of strong competition between local electronic correlations and superconducting proximity effect. We show that the nature of the transition depends crucially on the occupation and the width of the orbital levels, which determine their respective contribution to transport. When the transport of Cooper pairs takes place through only one of these levels, we find that the phase diagram of the phase-dependent 0-pi transition is a universal characteristic of a discontinuous level-crossing quantum transition at zero temperature. In the case where the two levels are involved, the nanotube Josephson current exhibits a continuous 0-pi transition, independent of the superconducting phase, revealing a different physical mechanism of the transition.