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Reversing the direction of the supercurrent in a controllable Josephson junction

机译：在可控的约瑟夫森结中反转超电流的方向

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When two superconductors are connected by a weak link, a supercurrent flows, the magnitude of which is determined by the difference in the macroscopic quantum phases of the superconductors. This phenomenon was discovered by Josephson for the case of a weak link formed by a thin tunnel barrier: the supercurrent, I, is related to the phase difference, φ, through the Josephson current-phase relation, I= I_c sinφ, with I_c being the critical current which depends on the properties of the weak link. A similar relation holds for weak links consisting of a normal metal, a semiconductor or a constriction. In all cases, the phase difference is zero when no supercurrent flows through the junction, and increases monotonically with increasing supercurrent until the critical current is reached. Here we use nanolithography techniques to fabricate a Josephson junction with a normal-metal weak link in which we have direct access to the microscopic current-carrying electronic states inside the link. We find that the fundamental Josephson relation can be changed from I = I_c sinφ to I = I_c sin(φ +π)—that is, a π-junction—by controlling the energy distribution of the current-carrying states in the normal metal. This fundamental change in the way these Josephson junctions behave has potential implications for their use in superconducting electronics as well as in (quantum) logic circuits based on superconductors.
机译：当两个超导体通过薄弱环节连接时，会流过一个超电流，其大小取决于超导体宏观量子相中的差异。约瑟夫森发现这种现象是由于薄隧道势垒形成的薄弱环节：超电流I通过约瑟夫森电流-相位关系I = I_csinφ与相位差φ有关，其中I_c为取决于弱连接特性的临界电流。类似的关系适用于由普通金属，半导体或颈缩构成的薄弱环节。在所有情况下，当没有超电流流过结时，相位差为零，并且随着超电流的增加而单调增加，直到达到临界电流为止。在这里，我们使用纳米光刻技术来制造具有正常金属弱连接的约瑟夫逊结，在该连接中，我们可以直接访问该连接内部的微观载流电子态。我们发现，通过控制正常金属中载流态的能量分布，基本的约瑟夫森关系可以从I = I_csinφ变为I = I_c sin（φ+π）-即π结。这些约瑟夫森结的行为方式的根本变化对其在超导电子器件以及基于超导体的（量子）逻辑电路中的使用具有潜在的影响。

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