Superconducting Nanowires Templated by Single Molecules
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Ultrathin superconducting nanowires (SNWs) can be classified as “weak superconducting links”. They have properties in many ways similar to Josephson junctions. Thus, SNWs can find possible applications in superconducting information processing devices: either classical or quantum. Nanowires can also be used as detectors and mixers of microwave radiation. Fundamentally, the SNW is a model system for understanding coherence and decoherence effects, quantum phase transitions, and macroscopic quantum tunneling phenomena in one-dimensional (1-D) superfluids.
Because of the strong thermal fluctuations in 1-D, the resistance of a nanowire cannot be zero at any finite temperature. The limit of zero temperature is governed by quantum fluctuations, which are not well understood. Therefore, SNWs can fall into one of three different categories: 1) truly superconducting, i.e., with zero resistance in the limit of zero temperature, 2) resistive or normal, with a nonzero but finite resistance (R) at zero temperature (T), and 3) insulating, with R→∞ as T→0. On general grounds, it is expected that extremely thin nanowires should lose their ability to carry a supercurrent. General conditions under which this happens are not known. Many experiments show that nanowires having their normal state resistance, RN, lower than the superconducting quantum resistance, RQ = h/(2e)2 ≈ 6.5 kΩ, obey the predictions of the Langer, Ambegaokar, McCumber, and Halperin (LAMH) theory of thermally activated phase slips (TAPS). Such wires can be considered true superconductors because this theory predicts zero resistance at zero temperature. On the other hand, SNWs with RN > RQ frequently show deviations from LAMH or even an insulating behavior. In some cases, these deviations can be explained by the effect of quantum phase slips (QPS).
Here we focus on superconducting nanowires produced by sputter-coating single linear molecules (carbon nanotubes, DNA) with thin metallic films. Such molecular templating technique results in nanowires that are thinner than 10 nm in diameter. Continuous SNWs were produced with the following two materials: 1) amorphous alloy of MoGe, which is usually used for making extremely thin (down to ∼ 1.5 nm) and homogeneous films, and 2) Nb metal, which finds applications in superconducting electronic circuits. Although Nb wires are polycrystalline, the critical current density in them is ∼ 107 A/cm2, i.e., is similar to bulk practical superconductors (Ref. , p. 372).