Signature of the Reduced Sound Velocity in Thin NbN Films

• Published in: IEEE transactions on applied superconductivity Vol. 34; no. 3; pp. 1 - 5
• Main Authors: Baeva, Elmira, Soldatenkova, Maria, Titova, Nadezhda, Kolbatova, Anna Igorevna, Gol'tsman, Gregory
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.05.2024; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Acoustic velocity; Critical temperature; Electrical resistance measurement; Kapitza resistance; Low temperature; low-temperature detectors; Measuring instruments; Niobium compounds; niobium nitride; phonon heat capacity; phonon mean free path; Phonons; Relaxation time; sound velocity; Substrates; superconducting thin films; Temperature measurement; thermal boundary resistance; Thermal resistance; Thermodynamic properties; Thin films
• ISSN: 1051-8223; 1558-2515
• DOI: 10.1109/TASC.2024.3350586

Here, we investigate thermal transport and phonon dynamics in the 30-nm thick NbN device. By measuring the thermal resistance <inline-formula><tex-math notation="LaTeX">Z_{\text{2}D}</tex-math></inline-formula> at temperatures beyond the critical temperature <inline-formula><tex-math notation="LaTeX">T_{c}</tex-math></inline-formula> and the resistance relaxation time <inline-formula><tex-math notation="LaTeX">\tau _{R}</tex-math></inline-formula> in the vicinity of <inline-formula><tex-math notation="LaTeX">T_{c}</tex-math></inline-formula>, we obtain thermal parameters that are consistent with the acoustic mismatch model. Using the acoustic mismatch model and the experimental values of <inline-formula><tex-math notation="LaTeX">Z_{\text{2}D}</tex-math></inline-formula> and <inline-formula><tex-math notation="LaTeX">\tau _{R}</tex-math></inline-formula>, we determine the phonon heat capacity, which also gives access to the sound velocity <inline-formula><tex-math notation="LaTeX">v_{s}</tex-math></inline-formula> in NbN. The obtained value of <inline-formula><tex-math notation="LaTeX">v_{s}</tex-math></inline-formula> is two times lower than the value in bulk NbN, and it corresponds to the velocity estimates obtained by other experimental methods. Our findings can be useful for modeling and improving the performance of low-temperature devices based on NbN films.