C. Laermans

Interaction of quasilocal harmonic modes and boson peak in glasses

The properties of harmonic vibrational excitations indisordered media and glasses become now a very activetopic of scientific research [1]. Contrary to the quite wellestablished behavior of electrons in disordered conduc-tors there is no consensus at all regarding the harmonicvibrations in ordinary glasses. The most common andchallenging of their signatures is the so-called boson peakobserved in numerous experiments in the low-frequencyRaman and inelastic neutron scattering. The physicalorigin of the peak is however still a matter of great de-bates. The common view is that the solution of this prob-lem is a corner-stone for our veritable understanding ofglassy vibrational dynamics.The main discussion in the literature involves now thequestion whether the harmonic vibrations responsible forthe boson peak are propagating plane waves (phononlike) [2] or localized because of disorder [3]. The thirdpossibility, which we share in this paper, is that theyare neither propagating waves nor localized but have adiffusive nature [4].For the solution of this crucial question quite power-ful and expensive experimental techniques are now in use.First ofall these are Raman experiments themselves. Butsince visible light due to energy and momentum conser-vation laws does not interact properly with sound-like ex-citations, one has to use two other possibilities, namelyinelastic X-ray and neutron scattering.The main difficulty in the inelastic X-ray scattering ex-periments is the very high incident photon energy (≃ 20KeV), and its relatively small change, of the order of10

Defects in neutron transmutation doped silicon studied by positron annihilation lifetime measurements

Neutron transmutation doping of n-type silicon was obtained by irradiating the samples with thermal neutrons in order to create P atoms. Positron lifetime measurements were carried out and the evolution of the induced defects was studied by thermal annealing of the samples. The annealing out of the divacancy was seen between 200 and 350 °C. Around 700 °C large vacancy clusters (containing about eight vacancies) are formed. All the defects are annealed above 800 °C.

Ultrasonic study of the tunneling states in quartz amorphized by neutron-irradiation

Abstract The ultrasonic absorption and velocity change is measured in neutron irradiated quartz, for a dose of 1 × 10 20 n/cm 2 . These experiments are carried out as a function of temperature at two different frequencies. The results are analysed using the tunneling model and the TS-parameters P and γ 1 are derived from numerical fits. A comparison with previous measurements reveals that the increase of P with the dose, observed so far, is no longer continued; instead a remarkable drop of the density of states is found in this highly irradiated sample. This confirms that a considerable part of the TS in samples that are only slightly damaged by irradiation must be attributed to the defective crystalline regions rather than to the radiation induced amorphous regions.

Velocity and attenuation of ultrasound in the Bi2Sr2CuO6 superconductor

Abstract We have measured the velocity and attenuation of longitudinal and transversal sound in ceramic Bi 2 Sr 2 CuO 6 samples. The velocities and most of the moduli do not exhibit significant unregularities in the temperature range investigated. Their absolute values are low in comparison with other oxide superconductors. The attenuation has a maximum near 250 K.

Study of fast-neutron irradiated vitreous silica by positron annihilation methods

Abstract The nature of the glass-like structure as obtained by fast neutron irradiation of vitreous silica has been investigated by positron annihilation lifetime and Doppler broadening experiments. Comparing the annihilation parameters obtained in the unirradiated sample to those obtained in irradiated ones we observed that in the irradiated SiO2 glasses the probability of positronium formation is reduced and the orthopositronium lifetime is increased. The decrease of the positronium formation probability is mainly attributed to the reduction of the amount of free-volume in compacted domains. The orthopositronium lifetime increase is explained by large free spaces generated along trajectories of fast knocked-on particles.

Longitudinal velocity and attenuation of sound in 80 K phase Bi-Sr-Ca-Cu-O superconductor

Abstract We have measured the velocity and the attenuation of longitudinal sound in polycrystalline Bi2Sr2CaCu2O8 sample. Both the velocity and the attenuation show unregularities near to 200 K and 260 K. The results are compared with other ultrasonic and related experiments.

Ultrasound versus thermal conductivity in Ge clathrates

In this paper, we compare the results of the thermal conductivity measurements in the polycrystal Sr 8 Ga 16 Ge 30 with those of the ultrasonic data in the single crystal at low temperatures (T < 1 K). The thermal conductivity calculated from the ultrasound data is one order of magnitude higher than the measured value in the polycrystal. There are several possible explanations: the anisotropy of the crystal, the tunneling properties of the Sr atom might depend on crystallographic direction, and the possible existence of two-level systems (TLSs) on the grain surfaces. Our estimation of TLSs density of states at the grain boundaries gives the value P surface = 10 28 erg -1 cm -2 , which is extremely high. More studies are necessary to discriminate between these possible explanations i.e. measurements of the thermal conductivity in a single crystal would be very interesting.

Measurements of the ultrasonic attenuation and velocity variation in neutron irradiated quartz for an intermediate dose of 2.6 × 1019 n/cm2

Abstract Ultrasonic measurements in neutron-irradiated quartz are carried out for an intermediate dose of 2.6 × 1019 n/cm2. The variation of the velocity of sound has been measured and previous attenuation measurements are extended to temperatures below 1.2 K. The TS-parameters P and 1 are calculated from numerical fittings to the tunneling model. The obtained values continue the tendency of previous measurements for lower neutron doses, where a linear increase of P with the dose was found. This behaviour, however, is not followed by a higher dose. situated near the “threshold regime”.

The shape of the boson peak in neutron-irradiated quartz crystals

We have studied the shape of the boson peak observed in neutron-irradiated crystalline quartz at different irradiation doses. This shape has a universal character and can be described by the soft potential model.

Density of states of interacting quasilocal harmonic modes in glasses

Abstract We have established for the first time that as a result of a bilinear coupling between quasilocal harmonic modes and phonons in glasses there should be always a correlation between the position of the boson peak and the Ioffe–Regel crossover frequency for phonons. Above this frequency, phonons (i.e. usual plane waves) cease to exist. At the same frequency the density of states of the quasilocal harmonic modes, as a result of their interaction (and strong level repulsion), transforms into a universal linear function of the frequency. As a result the new harmonic modes become completely delocalized and have a diffusive nature. We show that existing experimental data are in excellent agreement with these predictions.