Novel Phenomena With Berezinskii-Kosterlitz-Thouless Transition, Topological Fluctuations, and Phase Slips in Superconducting NbN Nanoflms

This thesis presents a systematic investigation of phase fluctuation effects in supercon- ducting niobium nitride (NbN) nanofilms, exploring the rich physics that emerges dur- ing dimensional crossover from three-dimensional to two-dimensional and quasi-one- dimensional behavior. The experimental foundation of this study is based on the careful optimization of DC magnetron sputtering parameters, enabling the fabrication of high- quality NbN films with thicknesses ranging from 5 nm to 100 nm on various substrates including MgO, Al2O3 (r-cut and c-cut), and SiO2. To ensure precise electrical measure- ments, the fabrication process was refined through the development of Hall bar geometries using optical lithography and reactive ion etching techniques, with particular attention to preserving film quality during processing. Our systematic electrical transport measurements reveal two distinct manifestations of phase fluctuations in these films. In the two-dimensional regime, we observe the emer- gence of the Berezinskii-Kosterlitz-Thouless (BKT) transition in ultra-thin NbN films (d < 15 nm), characterized by a universal jump in the superfluid phase stiffness at TBKT . The application of perpendicular magnetic fields (3-150 Gauss) leads to an interesting behav- ior: a field-induced crossover from Halperin-Nelson fluctuations to a BCS-like state with Aslamazov-Larkin fluctuations. This transition exhibits distinct regimes, with the BKT signature preserved up to 5 Gauss, gradually smearing between 5-50 Gauss, and com- pletely suppressed by 100 Gauss, where conventional current-voltage behavior emerges. The crossover mechanism is attributed to field-induced free vortices screening the logarithmic vortex-antivortex interactions. As the dimensionality is further reduced, we observe the emergence of phase-slip phe- nomena in the quasi-1D regime. The intrinsic disorder within the granular NbN matrix leads to the formation of channels, whose dimensions critically influence the nature of phase fluctuations. Through careful analysis of temperature-dependent resistivity and current-voltage characteristics, we establish a quantitative correlation between the type of phase slips (quantum vs. thermal) and the size of these pathways relative to the supercon- ducting coherence length. Most notably, we discover the unprecedented coexistence of BKT transition and thermally activated phase slips within the same system, highlighting the subtle interplay between nano-conducting path dimensions and coherence length. These investigations employ comprehensive characterization techniques, including resis- tivity measurements, current-voltage characteristics, and magnetic field-dependent stud- ies, all conducted using a specially designed pulsed measurement technique to minimize heating effects. Observing these coexisting phenomena, together with their tunability through temperature, thickness, and magnetic field, not only advances our understand- ing of phase fluctuation mechanisms in low-dimensional superconductors but also opens new possibilities for controlling quantum phase transitions in advanced superconducting devices.