We have measured the thermal properties of suspended membranes from 10 to 300 K for two amplitudes of internal stress (about 0.1 and 1 GPa) and for two different thicknesses (50 and 100 nm). The use of the original $3\omega$-Volklein method has allowed the extraction of both the specific heat and the thermal conductivity of each SiN membrane over a wide temperature range. The mechanical properties of the same substrates have been measured at helium temperatures using nanomechanical techniques. Our measurements show that the thermal transport in freestanding SiN membranes is not affected by the presence of internal stress. Consistently, mechanical dissipation is also unaffected even though $Q$'s increase with increasing tensile stress. We thus demonstrate that the theory developed by Wu and Yu [J. Wu and C. C. Yu, Phys. Rev. B 84, 174109 (2011)] does not apply to this amorphous material in this stress range. On the other hand, our results can be viewed as a natural consequence of the “dissipation dilution” argument [Y. L. Huang and P. R. Saulson, Rev. Sci. Instrum. 69, 544 (1998)], which has been introduced in the context of mechanical damping.

Emerging quantum technologies require mastering thermal management, especially at the nanoscale. It is now accepted that thermal metamaterial-based phonon manipulation is possible, especially at sub-kelvin temperatures. In these extreme limits of low temperatures and dimensions, heat conduction enters a quantum regime where phonon exchange obeys the Landauer formalism. Phonon transport is then governed by the transmission coefficients between the ballistic conductor and the thermal reservoirs. Here we report on ultra-sensitive thermal experiments made on ballistic 1D phonon conductors using a micro-platform suspended sensor. Our thermal conductance measurements attain a power sensitivity of 15 attoWatts $$\sqrt {{\mathrm{Hz}}} \,^{ - 1}$$Hz - 1 around 100 mK. Ballistic thermal transport is dominated by non-ideal transmission coefficients and not by the quantized thermal conductance of the nanowire itself. This limitation of heat transport in the quantum regime may have a significant impact on modern thermal management and thermal circuit design.

Sub-nanometer thickness accuracy and excellent conformity make atomic layer deposited films prevalent in modern electronics, continuously shrinking in size. The thermal resistance of these films plays a major role in the overall energy efficiency of miniaturized devices. We report very sensitive thermal conductivity measurements of amorphous Al${}_2$ O${}_3$ thin films grown using atomic layer deposition in the temperature range of 100–300 K. The 3$\omega$ method is used to characterize these films ranging from 17.0 to 119.4 nm in thickness, using a series-resistor model to deconvolve the intrinsic thermal conductivity of the film from thermal boundary resistances inherently present in the multilayer system. The thermal conductivity of amorphous alumina films with a density of $2.77\pm 0.14$ g cm${}^{-3}$ is measured to be $1.73\pm 0.08$ W m${}^{-1}$ K${}^{-1}$ at 300 K. Measurements were carried out on germanium and sapphire substrates, leading to no substrate dependence of the films’ thermal conductivity, within experimental accuracy. On the other hand, thermal boundary resistances of the systems Pt/Al${}_2$ O${}_3$ /substrate are observed to be strongly substrate-dependent, with values ranging from $2.1\times {10}^{-8}$ m${}^2$ K W${}^{-1}$ to $3.7\times {10}^{-8}$ m${}^2$ K W${}^{-1}$ at 300 K for films deposited on sapphire and germanium, respectively. These results provide further insights into the significance of interfaces in thermal transport across layered materials, in particular, for potential germanium-based devices.

Advances in nanomechanics within recent years have demonstrated an always expanding range of devices, from top-down structures to appealing bottom-up MoS₂ and graphene membranes, used for both sensing and component-oriented applications. One of the main concerns in all of these devices is frequency noise, which ultimately limits their applicability. This issue has attracted a lot of attention recently, and the origin of this noise remains elusive to date. In this article we present a very simple technique to measure frequency noise in nonlinear mechanical devices, based on the presence of bistability. It is illustrated on silicon-nitride high-stress doubly clamped beams, in a cryogenic environment. We report on the same T/f dependence of the frequency noise power spectra as reported in the literature. But we also find unexpected damping fluctuations, amplified in the vicinity of the bifurcation points; this effect is clearly distinct from already reported nonlinear dephasing and poses a fundamental limit on the measurement of bifurcation frequencies. The technique is further applied to the measurement of frequency noise as a function of mode number, within the same device. The relative frequency noise for the fundamental flexure δf/f₀ lies in the range 0.5–0.01 ppm (consistent with the literature for cryogenic MHz devices) and decreases with mode number in the range studied. The technique can be applied to any type of nanomechanical structure, enabling progress toward the understanding of intrinsic sources of noise in these devices.