M. J. McKenna

A new resonant photoacoustic technique for measuring very low optical absorption in crystals and glasses

In this paper, a new photoacoustic technique is described for measuring very low optical absorption in crystals and glasses. The technique differs from the conventional photoacoustic technique in that it does not use a high‐power laser pulse to excite an acoustic signal in the sample, but instead uses a continuous laser with the beam modulated at an acoustic resonance frequency of the sample. By exciting the sample at resonance, the acoustic signal is enhanced by a factor proportional to the quality factor (Q) of the resonance. Because highly transparent crystals and glasses may have a Q on the order of 104 to 106, the technique is orders of magnitude more sensitive than conventional techniques. The Q is enhanced through the use of noncontact acoustic transducers, which also eliminate the problem of background signals due to scattered light. Measurements with various quartz samples have been used to verify the technique. Using a 10‐W continuous laser, the noise level of the resonant photoacoustic techniqu...

Second and fourth sound modes for superfluid helium in aerogel

Abstract Because of the interest in superfluid onset in porous media (high porosity aerogel in particular) one would like to measure the superfluid fraction as well as the heat capacity near T c . However, standard techniques such as torsional oscillators and fourth sound have encountered difficulties because the compliant aerogel allows the viscous normal fluid to move. Using data we obtained for fourth sound propagation below 1.8 K, we develop a theory which fits the data with no adjustible parameters, and show that the theory predicts another second-sound-like mode which may not be damped near T c .

Observation of soliton-like waves in adsorbed films of superfluid 4He

Abstract While there has been considerable theoretical attention given to nonlinear (soliton) effects for adsorbed films of superfluid 4He, there have been no systematic experimental studies. In this paper we report measurements of wave propagation in such films with drive levels varying over five orders of magnitude. In addition to the critical saturation of a third sound wave, we observe another wave whose velocity of propagation changes with drive amplitude (linearly over a limited range).This dependence agrees with theoretical predictions for soliton behavior.

Experiments on nonlinear wave propagation in disordered media

Abstract A fundamental question concerning systems which are both disordered and nonlinear is whether or not Anderson localization is weakened by the nonlinearity. Theory predicts that localized eigenstates will survive nonlinearity, whereas nonlinear pulses may or may not experience the effects of localization depending on the relative magnitude of the Anderson localization length and the characteristic width of the pulse. We have used nonlinear surface waves on a superfluid helium film to obtain results in agreement with the theoretical predictions.

Anderson localization of 3He with variable disorder provided by a 4He solid/liquid interface

Abstract When liquid helium is in contact with a graphite surface, and the pressure of the liquid is increased toward the bulk solidification pressure, the solid helium will pass through smooth and rough configurations during layering transitions. When 3He is added to the system, it preferentially resides at the solid/liquid interface. When the interface is atomically sharp, the 3He will diffuse easily on the surface, but when the interface is rough, the 3He, with a deBroglie wavelength of ∼1.5 nm, may experience Anderson localization. Currently we are using an NMR technique, which monitors the diffusion of the 3He, as a probe of this effect.

A comparison of elastic constants of the quasicrystalline and cubic approximant phases of AlCuLi, using resonant ultrasound spectroscopy

The anisotropy e=‖1−2c44/(c11−c12)‖ of a sample of quasicrystalline (T2‐phase) AlCuLi has been measured in our laboratory to be 0.0020±0.0004; since a perfect quasicrystal should be isotropic, this could signify deviation from perfect icosahedral order. A comparison with the closely related R‐phase cubic approximant, which is often present when macroscopic quasicrystals are grown, may provide a better basis for interpreting such measurements. Among the various methods of determining elastic properties, only resonant ultrasound spectroscopy, which uses the normal mode resonance frequencies to retreive the complete elastic tensor in one measurement, offers the necessary sensitivity to resolve such small anisotropies. Results on a recently received R‐phase sample, and on its neighboring icosahedral grains, will be compared with the results mentioned above. [Work supported by NSF Grant No. DMR‐9000549 and by the Office of Naval Research.]

Using piezoelectric film and resonant ultrasound to determine the elastic tensor of small, fragile samples

The ultrasonic and elastic properties of materials are conventionally measured using quartz or lithium niobate transducers in a pulse‐echo technique where the transducer is driven at resonance. Some problems include transducer ringing, transducer bonding, parallelism of the sample faces, beam diffraction, and the necessity of remounting transducers in order to measure all the elastic constants. Nearly all these problems disappear if a resonance technique is used, and all the elastic constants may be determined with a single measurement. For broadband response, minimum loading by the transducer required for a resonance measurement, and to gently contact a small fragile sample, polyvinylidene flouride (PVDF) piezoelectric film as thin as 9 μm is ideally suitable. Small active areas and leads are produced with metalization patterns on each side of the PVDF film. For resonance measurements, electrical crosstalk across the small sample is processed by frequency modulating the drive and using phase sensitive de...

Application of resonant ultrasound spectroscopy to the study of small single crystal wafers

The study of acoustic waves in a solid can reveal many of its fundamental properties, such as the Debye temperature, the second derivatives of the free energy, and the strength of the electron–phonon coupling−but only if a specimen can be considered a single crystal with a minimum of defects and impurities. Such specimens tend to be extremely small and fragile, especially for newly discovered materials. Often, as the case with the high‐temperature copper oxide superconductors, the samples can only be grown as wafers, typically 200×200×40 μm in size. These extremely small sizes and the ‘‘plate’’ proportions complicate the resonant ultrasound technique because stress on the sample from the transducers is more of a concern; second, only the ‘‘plate’’ modes, which have no dependence on the thin dimension, can be excited, and the overtones are widely spaced in frequency, so frequency‐dependent mechanisms have to be considered; for example, measurements of the attenuation are of particular interest in high Tc m...

Using acoustic resonators to study superfluid-filled silica aerogel, highT c superconductors, and quasicrystals

Investigations of many quantum and solid-state systems of interest require measurements on samples that are either very fragile, as in the aerogel-4 He system, very small, such as samples of high-Tc materials, or both small and fragile, such as a single grain quasicrystal. Many relevant properties of these systems can be deduced from sound-velocity measurements; we present here some techniques for making such measurements, using acoustic resonators and novel transduction mechanisms. The use of acoustic resonance, as opposed to pulses, for measuring sound velocity permits relatively low frequencies to be used; in addition, frequency becomes the primary quantity of interest, which can be measured with ease and precision.

Resonant photoacoustic measurements of very low optical absorption in piezoelectric and dielectric crystals

The photoacoustic effect is one of the most sensitive methods for measurements of the low optical absorption in glasses and crystals. Traditional methods using high‐powered pulse lasers and attached piezoelectric transducers are limited by noise and scattered light at the transducer. Recently, a new resonant photoacoustic technique has been developed for highly transparent solids, where a cw laser modulated at the acoustic resonant frequency of the sample generates an acoustic signal amplified by the quality factor (Q) of the resonance. This technique is several orders of magnitude more sensitive than conventional pulse techniques. Noncontact capacitive transducers eliminate the problem of scattered light. Two different transduction mechanisms are important in the detection of the acoustic wave at the sample surface. First, the displacement of the surface of the dielectric sample in the electric field of the capacitor results in small changes in the capacitance. Second, through the piezoelectric effect, t...