R. O. Pohl

Normal modes of a Si(100) double-paddle oscillator

Six low-frequency eigenmodes of a double-paddle oscillator have been measured and have been identified with a finite-element model. The internal friction Q−1 of these modes has been measured in the range of 4–80 K. Only one of the oscillator’s modes has a Q−1<3×10−8 below 40 K, which is furthermore very reproducible. All other modes have a higher internal friction which is not as reproducible and also sometimes changes after thermal cycling. It is shown that the internal friction of the different modes is related to the restoring force needed to hold the oscillator in place. The finite-element model is used to predict the damping of the different modes.

Elastic properties of amorphous and crystalline ice films

Abstract Water is the basic ingredient of comets and interstellar dust. It is believed that the cometary ices were originally formed as amorphous ices that have since been reprocessed. In order to model cometary history it is therefore necessary to know the physical properties of amorphous water ice because it is well established that the low temperature properties of amorphous solids differ greatly from those of their crystalline counterparts. In a series of laboratory experiments the elastic properties (shear modulus and internal friction) of thin amorphous ice films are investigated. With the measurements of the elastic shear modulus it is shown that directly after evaporation some amorphous ice films are extremely porous with a porosity p of up to 0.6. The simultaneous measurement of the internal friction establishes the huge amount of local disorder which is present in these films. During annealing processes at temperatures below 100 K both the porosity and local disorder are greatly reduced. The impact of these findings is discussed on the thermal conductivity, which is the most important quantity needed for the thermal modelling of comets. It is concluded that the high porosity found in the ice films reduces the thermal conductivity by no more than two orders of magnitude.

Simple apparatus for the measurement of thermal diffusivity between 80–500 K using the modified Ångström method

An apparatus that is capable of measuring thermal diffusivity between 80–500 K using a modified Angstrom method is described. The main feature of the apparatus is its simplicity. A detailed description of the equipment and the experimental procedure as well as the difficulties that can be encountered are given. The results of measurements on samples covering a wide range of thermal diffusivities are also given.

Low temperature acoustic properties of amorphous silica and the tunneling model

Internal friction and speed of sound were measured on a-SiO2 above 6 mK using a torsional oscillator at 90 kHz, controlling for thermal decoupling, vibrational heating, background losses, and nonlinear effects. Strain amplitudes epsilon(A) = 10(-8) mark the transition between the linear and nonlinear regimes. In the former, agreement with the tunneling model was observed for both internal friction and speed of sound above 25 mK. The observed deviations in the speed of sound below 25 mK can be described with a cutoff energy of Delta(0, min)/k(B) = 6+/-0.5 mK. In the nonlinear regime, above 10 mK the behavior was typical for nonlinear harmonic oscillators, while below 10 mK different behavior was found.

Annealing and Sublimation of Noble Gas and Water Ice Films

Using the double–paddle oscillator, we have measured the mass and the shear modulus of thin films of water ice, argon and neon during annealing at cryogenic temperatures. From this, we have determined the vapor pressure of the three solids. They are in good agreement with published values for H2O and Ar; for Ne, they extend the vapor pressure to 10−11torr, ten orders of magnitude smaller than previous measurements. On the polycrystalline noble gas films condensed at low temperatures, the shear modulus was found to increase logarithmically with the annealing time. This stiffening is similar to that observed previously for amorphous water ice, and is hence not connected to the amorphous structure of the latter.

Lattice vibrations of disordered solids

The recent discovery of an amorphous solid without low-energy tunneling excitations (Two-Level-Systems) has shown that the amorphous structure is not their primary cause, as had previously been widely accepted. It has also been shown that these excitations arise as the crystalline order is gradually disturbed, and that their number densities saturate at their characteristic level regardless of whether the crystal amorphizes or not. It is concluded that the nature of the defects, and the cause for their saturation density, should be explored in disordered crystals, rather than in amorphous solids.

Phonon scattering and internal friction in dielectric and metallic films at low temperatures

We have measured the heat conduction between 0.05 K and 1.0 K of high purity silicon wafers carrying on their polished faces a variety of thin dielectric films and polycrystalline thin metallic films. Using a Monte Carlo simulation to analyze the conduction measurements, we have determined the phonon mean free path within the films, and found all of them to be much shorter even than in typical bulk amorphous solids, with no exceptions. We have also measured the internal friction of these films below 10 K and found, however, their internal friction at low temperatures strikingly close to that of amorphous solids, both in magnitude and in their temperature independence, with the exception of the MBE Si and alloy Al 5056, whose internal friction is even much smaller than that of amorphous solids. The internal friction results indicate the phonon scattering in these thin films is the same as, or even much less stronger than, in other amorphous solids, according to the Tunneling Model. Thus, we conclude that the heat conduction measurements do not support the picture that the lattice vibrations of these films are glasslike, as had been surmised earlier for thin metallic films, on the basis of low temperature internal friction measurements alone [Phys. Rev. B 59, 11767 (1999)]. At the least, the films must contain additional scattering centers which lead to the very small phonon mean free path. Most remarkably, the MBE Si shows the same strong scattering of thermal phonons as do the other films, while having the negligible internal friction expected for a perfect film. The disorder causing the strong scattering of the thermal phonons in this film is completely unknown. The non-glasslike phonon scattering phenomena observed here in thin dielectric and metallic films deserve further investigations.

Annealing of quench-condensed Argon films

When prepared by quench-condensation below 3K, Argon films of several hundred nanometers in thickness are found to be considerably softer than bulk Argon. The shear stiffness increases when the films are subsequently annealed at higher temperatures, with a time dependence described by stretched exponential functions. The magnitude of the cumulative annealing has an exponential, non-Arrhenius temperature dependence, and thinner films exhibit more stiffening than thicker films.

Coherent Phonon Scattering by Submicron Structures

Using densely packed monolayers of Latex spheres as masks, we have evaporated two-dimensional regular arrays of metal dots onto polished silicon wafers[1]. Performing thermal conduction experiments in the boundary scattering regime[2], we observe peak phonon scattering when the dominant phonon wavelength in the silicon equals the spacing of such a diffraction grating on the silicon surface[3].

New Method for Studying Phonon Thermalization

We perform thermal conduction experiments in the boundary scattering regime from 5 K down to 50 mK. The standard method, which uses do heater power, varies the dominant phonon frequency in the sample by varying the sample temperature. We decouple the dominant phonon frequency from the sample temperature by periodically pulsing the heater power. We vary the dominant phonon frequency injected into the sample by varying the height of the heat pulses. If the height of the pulses is increased, the width is decreased so that the average heat power flowing through the sample and, therefore, the average temperature of the sample as measured by the thermometers, both remain nearly constant.