Coherent flux tunneling through NbN nanowires

Abstract

We demonstrate evidence of coherent magnetic flux tunneling through superconducting nanowires patterned in a thin highly disordered NbN film. The phenomenon is revealed as a superposition of flux states in a fully metallic superconducting loop with the nanowire acting as an effective tunnel barrier for the magnetic flux, and reproducibly observed in different wires. The flux superposition achieved in the fully metallic NbN rings proves the universality of the phenomenon previously reported for InO x . We perform microwave spectroscopy and study the tunneling amplitude as a function of the wire width, compare the experimental results with theories, and estimate the parameters for existing theoretical models.

Quantum phase-slip junction under microwave irradiation

“…The results presented in this paper are relevant for recent experiments on Josephson junction chains [43] and nanowires [47,50]. In these works, typical phase-slip energies U 0 are in the range of 1-10 GHz, whereas the environmental inductances L are 50-500 nH.…”

“…The larger L, the smaller E L , and the larger the ratio U 0 /E L , thus favoring a well-defined charge state of the QPSJ. Recent experiments on nanowires [47,50] and chains of Josephson junctions [43] typically achieve U 0 /E L ratios that are of the order of 10 −2 -10 −1 . We will analyze the microwave response of a QPSJ in this regime in detail and study in particular the resolution and accuracy of the dual Shapiro steps.…”

“…The Josephson junction has an exact dual counterpart, the so-called quantum phase-slip junction (QPSJ) [29][30][31][32][33][34][35][36][37][38]. Physical realizations of QPSJ that have been discussed in the literature are a single Josephson junction with a finite capacitance [29][30][31][32][37][38][39] or a linear chain of such Josephson junctions [38,[40][41][42][43], and a narrow superconducting nanowire [44][45][46][47][48][49][50]. With respect to an ordinary JJ, the role of the phase and the charge in a QPSJ is interchanged.…”

Superconductivity in highly disordered NbN nanowires

“…The impact of coherent QPS has been observed unambiguously in qubit-type measurements in NbN rings, containing narrow segments with width 30 nm or lower 42 . Rate of QPS was found to be of order γ QPS =10 GHz for the nanowire with the length of 1 μm made of film with sheet resistance R □ =2 kΩ which corresponds to R ξ(0) =220 Ω if to assume that ξ(0) has the same value as in our samples.…”

“…Taking the directly measured data for γ QPS from Ref. 42 as the reference, it is possible to estimate the expected effective resistance due to QPS process in our samples. One can rewrite Eq.…”