Phonon Localization in Boron Nitride Nanotubes: Mixing Effect of 10B Isotopes and Vacancies

We explored the mixing effect of 10B isotopes and boron (B) or nitrogen (N) vacancies on the atomic vibrational properties of (10, 0) single-wall boron nitride nanotubes (BNNT). The forced oscillation technique was employed to evaluate the phonon modes for the entire range (0-100%) of 10B isotopes and atomic vacancy densities ranging from 0 to 30%. With increasing isotope densities, we noticed a blue-shift of the Raman active A1 phonon peak, whereas an increased density of mixed or independent B and N vacancies resulted in the emergence of a new low-frequency peak and the annihilation of the A1 peak in the phonon density-of-states. High-energy optical phonons were localized as a result of both 10B isotopes and the presence of mixing defects. We generated typical mode patterns for different defects to show the phonon localization processes due to the defects. We found an asymmetrical nature of the localization length with increasing 10B isotope content, which corresponds well with the isotope inherited localization length of carbon nanotubes and mono-layer graphene. The localization length falls abruptly with the increase in concentration of both atomic vacancies (B or N) and mixing defects (10B isotope and vacancies). These findings are critical for understanding heat conduction and nanoscopic vibrational investigations like tip-enhanced Raman spectra in BNNT, which can map local phonon energies.

Temperature oscillation in one-dimensional superlattice induced by phonon localization

Thermal transport properties and thermodynamic quantities often present anomalous behaviors in low-dimensional systems. In this paper, we find that temperature oscillates spatially in one dimensional harmonic and weakly anharmonic superlattice. With the increase of anharmonicity, the temperature oscillation gradually disappears and a normal temperature gradient forms. Further analysis reveals that the formation of temperature oscillation is due to the localization of high frequency phonons which cannot be thermalized. Moreover, the localized modes interact weakly with heat reservoirs, thus, their contributions to local temperature remain negligible while varying the temperatures of heat reservoirs. The oscillated temperature profile is in a good agreement with Visscher's formula. These discoveries of temperature oscillation phenomenon have great potential in applications of phononic devices for heat manipulation.