Jarillo-Herrero Pablo, van Dam Jorden A, Kouwenhoven Leo P
Kavli Institute of Nanoscience, Delft University of Technology, PO Box 5046, 2600 GA, Delft, The Netherlands.
Nature. 2006 Feb 23;439(7079):953-6. doi: 10.1038/nature04550.
Electronic transport through nanostructures is greatly affected by the presence of superconducting leads. If the interface between the nanostructure and the superconductors is sufficiently transparent, a dissipationless current (supercurrent) can flow through the device owing to the Josephson effect. A Josephson coupling, as measured by the zero-resistance supercurrent, has been obtained using tunnel barriers, superconducting constrictions, normal metals and semiconductors. The coupling mechanisms vary from tunnelling to Andreev reflection. The latter process has hitherto been observed only in normal-type systems with a continuous density of electronic states. Here we investigate a supercurrent flowing through a discrete density of states-that is, the quantized single particle energy states of a quantum dot, or 'artificial atom', placed between superconducting electrodes. For this purpose, we exploit the quantum properties of finite-sized carbon nanotubes. By means of a gate electrode, successive discrete energy states are tuned on- and off-resonance with the Fermi energy in the superconducting leads, resulting in a periodic modulation of the critical current and a non-trivial correlation between the conductance in the normal state and the supercurrent. We find, in good agreement with existing theory, that the product of the critical current and the normal state resistance becomes an oscillating function, in contrast to being constant as in previously explored regimes.
通过纳米结构的电子输运受到超导引线存在的极大影响。如果纳米结构与超导体之间的界面足够透明，由于约瑟夫森效应，无耗散电流（超电流）可以流过该器件。已经使用隧道势垒、超导颈缩、普通金属和半导体获得了由零电阻超电流测量的约瑟夫森耦合。耦合机制从隧穿到安德列夫反射各不相同。迄今为止，后一种过程仅在具有连续电子态密度的正常型系统中被观察到。在这里，我们研究流过离散态密度的超电流，即置于超导电极之间的量子点或“人造原子”的量子化单粒子能态。为此，我们利用有限尺寸碳纳米管的量子特性。通过栅电极，连续的离散能态与超导引线中的费米能调谐到共振和非共振状态，导致临界电流的周期性调制以及正常态电导与超电流之间的非平凡相关性。我们发现，与现有理论高度一致的是，临界电流与正常态电阻的乘积成为一个振荡函数，这与之前所研究的情况中该乘积为常数形成对比。
