Superconducting Resonators from Ultrathin NbN Films

Author: Borah, Hrishikesh

Affiliation: Max Planck Institute for Chemical Physics of Solids

Type: Poster

Display Dates: 20.07.2026 - 21.07.2026

Board: MT-057

We investigate the transport and microwave response of ultrathin niobium nitride (NbN) films with thicknesses ranging from 25 nm down to 2.8 nm. Transport measurements on the thinnest film confirm two-dimensional superconductivity through the observation of a Berezinskii–Kosterlitz–Thouless transition. In this ultrathin limit, the inertia of Cooper pairs, manifested as kinetic inductance, becomes dominant; notably, the 2.8 nm film exhibits a large sheet kinetic inductance of 300 pH/sq. This strongly influences the behavior of superconducting microwave resonators patterned from these films. Temperature-dependent microwave measurements show that the superfluid density follows a power-law dependence at low temperatures, evolving toward a more BCS-like behavior as the film thickness increases. In the thinnest films, energy losses at low temperatures are dominated by two-level system (TLS) defects, while at higher temperatures they are governed by Cooper-pair breaking. These results highlight ultrathin NbN as a promising platform for compact, high-impedance devices for quantum circuits and sensitive detectors.