Condensed-Matter Physics & Materials Science Seminar

It is of fundamental importance to establish whether there is a limit to how thin a superconducting wire can be, while retaining its superconducting character ― and if there is a limit, to determine what sets it. This issue may also be of practical importance in defining the limit to miniaturization of superconducting electronic circuits.
In this work, single molecules are used as templates for formation of metallic wires with extremely small diameters, down to ~4 nm in some cases. The process is done be decorating a linear molecule, typically a carbon nanotube or a DNA molecule, with a superconducting alloy of MoGe. A large set of such nanowires exhibit a dichotomy in their transport properties that can be understood as a quantum superconductor-insulator transition. The transition is similar to the Schmid-Bulgadaev transition [1,2,3]. The superconducting phase is well described by the thermal activation of phase slip, at least for short and homogeneous wires. The insulating behavior can be understood in terms of a weak Coulomb blockade [4,5,6]. We also use this technique to fabricate superconducting device, which can be used to measure superconducting phase gradients. Using such nanowire superconducting quantum interference devices (NSQUID) we measured phase gradients generates by external magnetic fields as well as by an external supercurrent. One of the results is the direct confirmation of the Campbell law at the micro-scale [7].

1. A. Schmid, Phys. Rev. Lett. 51, 1506 (1983)
2. S. A. Bulgadaev, JETP Lett. 39, 315 (1984).
3. D. Meidan, Y. Oreg, and G. Refael, Phys Rev. Lett.. 98, 187001 (2007).
4. J. P. Pekola, K. P. Hirvi, J. P. Kauppinen, and M. A. Paalanen, Phys. Rev. Lett. 73, 2903 (1994).
5. Yu. V. Nazarov, Phys. Rev. Lett., 82, 1245 (1999).
6. D. S. Golubev and A. D. Zaikin, Phys. Rev. Lett. 86, 4887 (2001).
7. A. M. Campbell, J. Phys. C 2, 1492 (1969); ibid 4, 3186 (1971); R. Prozorov et al., Phys. Rev. B 67, 184501 (2003).