J. Rodríguez-Viejo

Suppression of tunneling two-level systems in ultrastable glasses of indomethacin

Significance Glasses are disordered solids usually obtained by supercooling a liquid bypassing crystallization. A remarkable feature of glasses is that, independently of their nature and composition, they exhibit universal properties in the low-temperature range. Of interest here, the specific heat is characterized by a linear term below 1 K, ascribed to quantum tunneling between two states of similar energy. We have investigated if this ubiquitous behavior also applies to so-called “ultrastable glasses,” directly synthesized from the vapor phase into low-energy positions of the potential-energy landscape. Interestingly, we find a full suppression of the linear term in the specific heat, which questions the current view of the popular tunneling model and sheds light on the microscopic origin of two-level systems in glasses. Glasses and other noncrystalline solids exhibit thermal and acoustic properties at low temperatures anomalously different from those found in crystalline solids, and with a remarkable degree of universality. Below a few kelvin, these universal properties have been successfully interpreted using the tunneling model, which has enjoyed (almost) unanimous recognition for decades. Here we present low-temperature specific-heat measurements of ultrastable glasses of indomethacin that clearly show the disappearance of the ubiquitous linear contribution traditionally ascribed to the existence of tunneling two-level systems (TLS). When the ultrastable thin-film sample is thermally converted into a conventional glass, the material recovers a typical amount of TLS. This remarkable suppression of the TLS found in ultrastable glasses of indomethacin is argued to be due to their particular anisotropic and layered character, which strongly influences the dynamical network and may hinder isotropic interactions among low-energy defects, rather than to the thermodynamic stabilization itself. This explanation may lend support to the criticisms by Leggett and others [Yu CC, Leggett AJ (1988) Comments Condens Matter Phys 14(4):231–251; Leggett AJ, Vural DC (2013) J Phys Chem B 117(42):12966–12971] to the standard tunneling model, although more experiments in different kinds of ultrastable glasses are needed to ascertain this hypothesis.

In situ nanocalorimetry of thin glassy organic films

In this work, we describe the design and first experimental results of a new setup that combines evaporation of liquids in ultrahigh vacuum conditions with in situ high sensitivity thermal characterization of thin films. Organic compounds are deposited from the vapor directly onto a liquid nitrogen cooled substrate, permitting the preparation and characterization of glassy films. The substrate consists of a microfabricated, membrane-based nanocalorimeter that permits in situ measurements of heat capacity under ultrafast heating rates (up to 10(5) K/s) in the temperature range of 100-300 K. Three glass forming liquids-toluene, methanol, and acetic acid-are characterized. The spikes in heat capacity related to the glass-transition temperature, the fictive temperature and, in some cases, the onset temperature of crystallization are determined for several heating rates.

Review on measurement techniques of transport properties of nanowires.

Physical properties at the nanoscale are novel and different from those in bulk materials.

Heat transfer in symmetric U-shaped microreactors for thin film calorimetry

We describe the results of two-dimensional finite difference analysis of the thermal profile, in both transient and steady state, of a symmetric U-shape designed high-sensitive nanocalorimeter. The thin film calorimeter, with a heat capacity of 100 nJ K−1 at room temperature, consists of a 180 nm thick freestanding silicon-rich nitride membrane on which thin film heaters and sensors are deposited. Simulated temperature profiles are in good agreement with in situ experimental data obtained at the heater and sensor locations. The first-order solid-to-liquid transition of indium films, from a few A to hundreds of nm thick, was used as an experimental reference of the thermal profiles obtained from the 2D modeling. Temperature differences inside the sample region induced by the symmetric U-shape design of the Pt heaters limit the use of the nanocalorimeter to two different heating rate regimes. At low heating rates, β < 10 K s−1, especially with a thermal layer, the temperature profile is reasonably flat so that small samples can be characterized in power compensation mode. At heating rates faster than 4 × 104 K s−1 the nanocalorimeter works in adiabatic mode and measures transitions occurring in the sample directly loaded underneath the heater.

Glass transition in ultrathin films of amorphous solid water

Nanocalorimetry at ultrafast heating rates is used to investigate the glass transition of nanometer thick films of metastable amorphous solid water grown by vapor deposition in an ultrahigh vacuum environment. Apparent heat capacity curves exhibit characteristic features depending on the deposition temperature. While films grown at T ≥ 155 K are completely crystallized, those deposited at 90 K show a relaxation exotherm prior to crystallization. Films grown between 135 and 140 K and subsequently cooled down to 90 K reveal a clear endothermic feature before crystallization, which is compatible with a glass-to-liquid transition. The onset temperature is located at 174 K at a heating rate of 2.4 × 10(4) K/s and is independent of film thickness in the range of 16-150 nm. Comparison of our data with other calorimetric measurements at various heating rates suggests that water is a strong glass former in the deeply supercooled state.

Structural and magnetic characterization of FeNbBCu alloys as a function of Nb content

In this work we describe the changes in the crystallization behaviour and the magnetic properties with variation of the Nb content in Fe79−xNb5+xB15Cu1 (x = 0,2,4) alloys. The microstructure of the samples, as-quenched and after several heat treatments, is analysed by transmission Mossbauer spectroscopy (TMS), x-ray diffraction, differential scanning calorimetry and vibrating sample magnetometry. The saturation polarization, Curie temperature, magnetic entropy change and the maximum value of the hyperfine field distribution of the amorphous phase are composition dependent and are enhanced with a reduction in Nb content. Devitrification is produced by the nanocrystallization of bcc-Fe followed by the precipitation of iron borides. The calorimetric analysis indicates that Nb stabilizes the alloy against nanocrystallization. Both the amount of bcc-Fe precipitates and their mean grain size decrease significantly with increasing Nb content, suggesting a higher level of disorder at the interface between the amorphous matrix and the nanocrystal that reduces grain growth. The variation in the Fe environments after crystallization events deduced from TMS measurements are analysed in terms of the amount of Nb in the alloy. Both amorphous and nanocrystallized alloys show a soft magnetic behaviour with coercivity values in the range 4–10 A m−1.

Tailoring thermal conductivity by engineering compositional gradients in Si1-xGex superlattices

The transport properties of artificially engineered superlattices (SLs) can be tailored by incorporating a high density of interfaces in them. Specifically, SiGe SLs with low thermal conductivity values have great potential for thermoelectric generation and nano-cooling of Si-based devices. Here, we present a novel approach for customizing thermal transport across nanostructures by fabricating Si/Si1−xGex SLs with well-defined compositional gradients across the SiGe layer from x = 0 to 0.60. We demonstrate that the spatial inhomogeneity of the structure has a remarkable effect on the heat-flow propagation, reducing the thermal conductivity to ∼2.2 W·m−1·K−1, which is significantly less than the values achieved previously with non-optimized long-period SLs. This approach offers further possibilities for future applications in thermoelectricity.

Growth morphology of low-pressure metalorganic chemical vapor deposition silicon carbide on a-SiO2Si(100) substrates

Abstract Silicon carbide films were deposited on Si(100) wafers in the temperature range 950–1150°C, from a mixture of Si(CH 3 ) 4 + H 2 in a hot-wall low-pressure chemical vapor deposition (LPCVD) reactor. A thin a-SiO 2 layer of 60 A was grown on the Si substrate prior to deposition. The total pressure, H 2 flow rate and H 2 Si(Ch 3 ) 4 ratio were fixed at 0.65 Torr, 1300 cm 3 /min and 130, respectively. The microstructure of the as-grown films was investigated by X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and electron microprobe analysis (EPMA). The deposition reaction of SiC is thermally activated with an apparent activation energy of 150 kJ/mol. It was found that a mixture of micro-crystalline and amorphous phases is formed with an overall C content of 60 at% at the deposition temperature of 950°C. Crystallinity is improved and microstructure becomes columnar as growth proceeds on films deposited at 1000°C. Films grown at temperatures between 1050 and 1105°C are stoichiometric β-SiC polycrystals, with a columnar morphology and well developed (111) preferential orientation. In agreement with a columnar grains development, surface roughness increases dramatically as deposition temperature changes from 950 to 1150°C. At T = 1105°C cylindrical crystallites with diameters ∼ 150 nm are observed in films ∼ 20 μ m thickness. Planar defects perpendicular to the axis of the crystallites are microtwins generated by the rapid incorporation of a high number of atoms with low adatom mobilities and are responsible for the fast growth along the [111] direction.

Probing equilibrium glass flow up to exapoise viscosities

Significance “Does the glass cease to flow at some finite temperature?” Answering this question––of pivotal importance for glass formation theories––would require ridiculously long observation times. We circumvent this infeasibility relating the (directly inaccessible) ultraviscous flow of a liquid to the elastic properties of the corresponding glass, which we measure as a function of its age. The older the glass, the lower the temperature at which viscosity can be determined. Taking advantage of physical vapor deposition, we rapidly obtain a wide spectrum of ages rivaling those of millenary ambers, enabling viscosity determinations at values as large as those pertaining to the asthenosphere. Our result ultimately rules out the finite-temperature divergence of the molecular diffusion timescale in a glass. Glasses are out-of-equilibrium systems aging under the crystallization threat. During ordinary glass formation, the atomic diffusion slows down, rendering its experimental investigation impractically long, to the extent that a timescale divergence is taken for granted by many. We circumvent these limitations here, taking advantage of a wide family of glasses rapidly obtained by physical vapor deposition directly into the solid state, endowed with different “ages” rivaling those reached by standard cooling and waiting for millennia. Isothermally probing the mechanical response of each of these glasses, we infer a correspondence with viscosity along the equilibrium line, up to exapoise values. We find a dependence of the elastic modulus on the glass age, which, traced back to the temperature steepness index of the viscosity, tears down one of the cornerstones of several glass transition theories: the dynamical divergence. Critically, our results suggest that the conventional wisdom picture of a glass ceasing to flow at finite temperature could be wrong.

Interfacial effects on the thermal conductivity of a-Ge thin films grown on Si substrates

We use the 3ω method to measure the effective thermal conductivity of thin films of a-Ge with thicknesses of 20–150 nm in the temperature range of 30–300 K. By using a moving shadow mask, the films are grown on the same Si (001) substrate in a single deposition run to minimize changes in the microstructure. We observe a reduction in the effective conductivity of the films with the decreasing layer thickness. From the measured data we estimate values for both the film thermal conductivity and the thermal boundary resistance (TBR) between SiO2/a-Ge/Si at the different temperatures. An experimental value of the interface resistance of 2×10−8 m2 K/W is obtained at 300 K. The temperature dependence of the TBR differs appreciably from calculations based on the diffusive mismatch model. The values derived for the intrinsic thermal conductivity of the films, kfilm(300 K)=0.64 W/mK, agree with predictions from the minimum thermal conductivity model and with values measured by Cahill and Pohl [Phys. Rev. B 37, 8773...