Thermal conductivity $\kappa (T)$ of single crystal aluminum nitride grown by physical vapor transport has been measured at temperatures $T$ from 5 to 410 K. The samples exhibit high thermal conductivity with a value of up to 316 W m${}^{-1}$ K${}^{-1}$ at room temperature and about 2800 W m${}^{-1}$ K${}^{-1}$ at a peak of 66 K. At lowest temperatures, $\kappa (T)$ approaches the conductivity limited by the diffuse phonon scattering from sample surfaces. The peculiarities in measured $\kappa (T)$ suggest that the phonon scattering from point defects contributes essentially to the total phonon scattering in samples under investigation at low temperatures. The phonon interaction with electrons and holes bound to neutral donor and acceptor centers is suggested, adding substantially to thermal resistivity near and below the peak in $\kappa (T)$ .

This article describes an improvement of the 3-omega thermal conductivity measurement system facilitating automatic mode measurement. The use of the electrical circuit with negative feedback to compensate the first harmonic in the voltage signal on the heater-thermometer eliminates manual resistance tuning. To test the procedure, measurements of the thermal conductivity of single crystal sapphire Al2O3 were made. The experimental thermal conductivity value is in a good agreement with the reference data.

Nonlinear electrical properties, such as negative differential resistance (NDR), are essential in numerous electrical circuits, including memristors. Several physical origins have been proposed to lead to the NDR phenomena in semiconductor devices over the last more than half a century. Here, we report NDR behavior formation in randomly oriented graphenelike nanostructures up to 37 K and high on-current density up to ${10}^5\mathrm{A}/{\mathrm{cm}}^2$. Our modeling of the current-voltage characteristics, including the self-heating effects, suggests that strong temperature dependence of the low-bias resistance is responsible for the nonlinear electrical behavior. Our findings open opportunities for the practical realization of the on-demand NDR behavior in nanostructures of two- and three-dimensional material-based devices via heat management in the conducting films and the underlying substrates.

Thermal conductivity κ(T) of single-crystal CVD diamond enriched with 13C isotope to 98.16% was measured by the method of steady-state longitudinal heat flow in the temperature range from 6 to 410 K. This crystal with low nitrogen impurity content (<50 ppb) showed thermal conductivity 2010±50 W m−1 K−1 at 300 K (with a maximum of 12 100 W m−1 K−1 at 83 K), which is significantly lower than that of diamond with natural isotopic composition: κ(300K)=2360±50 W m−1 K−1. The measured data were analyzed using first-principles theory and the Callaway model, taking into account phonon scattering in three-phonon processes, scattering at sample boundaries and at isotopes. The first-principles calculations overestimate the thermal conductivity compared to the measured one near and to the right of the κ(T) peak, indicating the presence of significant additional phonon scattering by lattice defects in the studied chemically pure diamond samples. The results of both theoretical approaches for thermal conductivity at 300 K are in good agreement with our measured data and other published experimental data for isotopically modified diamonds. First-principles calculations yield a thermal conductivity ratio of κ12(T)/κ13(T)=1.072 for monoisotopic defect-free 12C and 13C crystals at 300 K. This ratio decreases at high temperatures to a value of 1.041 according to the Leibfried–Schlömann theory and to a value of 0.921 at very low temperatures.