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International audienceWe have measured the thermal properties of suspended membranes from 10 K to 300 K for two amplitudes of internal stress (about 0.1 GPa and 1 GPa) and for two different thicknesses (50 nm and 100 nm). The use of the original 3ω-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 dissip

A suspended system for measuring the thermal properties of membranes is presented. The sensitive thermal measurement is based on the 3ω dynamic method coupled to a Völklein geometry. The device obtained using micro-machining processes allows the measurement of the in-plane thermal conductivity of a membrane with a sensitivity of less than 10nW/K (+/-5×10 −3 Wm −1 K −1 at room temperature) and a very high resolution (∆K/K = 10 −3 ). A transducer (heater/thermometer) centered on the membrane is used to create an oscillation of the heat flux and to measure the temperature oscillation at the third harmonic using a Wheatstone bridge set-up. Power as low as 0.1nanoWatt has been measured at room temperature. The method has been applied to measure thermal properties of low stress silicon nitride and polycrystalline diamond membranes with thickness ranging from 100 nm to 400 nm. The thermal conductivity measured on the polycrystalline diamond membrane support a significant grain size effect on the thermal transport.

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Thermal transport properties of amorphous materials at low temperatures are governed by the interaction between phonons and localized excitations referred to as tunneling two-level systems (TLSs). The temperature variation of the thermal conductivity of these amorphous materials is considered as universal and is characterized by a quadratic power law. This is well described by the phenomenological TLS model even though its microscopic explanation is still elusive. Here, by scaling down to the nanometer-scale amorphous systems much below the bulk phonon-TLS mean free path, we probe the robustness of that model in restricted geometry systems. Using very sensitive thermal conductance measurements, we demonstrate that the temperature dependence of the thermal conductance of silicon nitride nanostructures remains mostly quadratic independently of the nanowire section. It does not follow the cubic power law in temperature as expected in a Casimir-Ziman regime of boundary-limited thermal transport. This shows a thermal transport counterintuitively dominated by phonon-TLS interactions and not by phonon boundary scattering in the nanowires. This could be ascribed to an unexpected high density of TLSs on the surfaces which still dominates the phonon diffusion processes at low temperatures and explains why the universal quadratic temperature dependence of thermal conductance still holds for amorphous nanowires. DOI: 10.1103/PhysRevB.95.165411 Amorphous materials may have significant dispersion in their chemical compositions or their physical structures at the microscopic level. However, at low temperatures, the behavior of the thermal properties of almost all amorphous materials is thought to be universal [1]. These common features include a nearly linear specific heat and a nearly quadratic thermal conductivity in temperature below a few kelvins. As thermal transport is concerned, this universality is not only qualitative but also quantitative; indeed the thermal conductivity of all amorphous materials lies within a factor of twenty in the same order of magnitude called the glassy range [2,3]. Despite much theoretical effort, this universality remains poorly understood and its true microscopic origin is still elusive. Nowadays, the most accepted model is based on the presence of tunneling twolevel systems (TLSs) involving tunneling between different equilibrium positions of an atom or group of atoms [4][5][6]. The scatterings of the phonons on these tunneling sites is assumed to be at the origin of the quadratic variation of thermal conductance in temperature. The phonon heat transport is then characterized by the phonon-TLS mean free path (MFP; the distance between two inelastic collisions), which is on the order of a few hundred micrometers.Phillips suggested that TLSs are likely to form in materials with an open structure and low-coordination regions, and are unlikely in highly dense amorphous systems [4]. Recent experiments give indication of the correlation between the low-density regions, the presence of nanovoid...

We present a specific heat measurement technique adapted to thin or very thin suspended membranes from low temperature (8 K) to 300 K. The presented device allows the measurement of the heat capacity of a 70 ng silicon nitride membrane (50 or 100 nm thick), corresponding to a heat capacity of 1.4x10 −10 J/K at 8 K and 5.1x10 −8 J/K at 300 K. Measurements are performed using the 3ω method coupled to the Völklein geometry. This configuration allows the measurement of both specific heat and thermal conductivity within the same experiment. A transducer (heater/thermometer) is used to create an oscillation of the heat flux on the membrane; the voltage oscillation appearing at the third harmonic which contains the thermal information is measured using a Wheatstone bridge set-up. The heat capacity measurement is performed by measuring the variation of the 3ω voltage over a wide frequency range and by fitting the experimental data using a thermal model adapted to the heat transfer across the membrane. The experimental data are compared to a regular Debye model; the specific heat exhibits features commonly seen for glasses at low temperature. 1 arXiv:1511.07606v1 [cond-mat.mes-hall]

We have characterized the mechanical resonance properties (both linear and nonlinear) of various doubly-clamped silicon nitride nanomechanical resonators, each with a different intrinsic tensile stress. The measurements were carried out at 4 K and the magnetomotive technique was used to drive and detect the motion of the beams. The resonant frequencies of the beams are in the megahertz range, with quality factors of the order of 10 4 . We also measure the dynamic range of the beams and their nonlinear (Duffing) behaviour.

The electrical behavior of diamondlike carbon (DLC) has been measured as a function of depth. The amorphous carbon (a-C) films are deposited by pulsed laser deposition using two complementary setups: a femtosecond (fs) and a nanosecond (ns) pulse lasers. It is demonstrated through four probe resistance measurements and contact resistance mapping that the fs DLC are electrically heterogeneous in thickness. The presence of a thick sp2 rich layer on top is evidenced for fs a-C and is apparently away in the sp3 rich ns a-C. It is attributed to different subplantation processes between ns and fs a-C films.