Phonon Pulses Research Articles

Kapitza transmission through atomically clean silicon (111) surfaces

The phonon transmission from atomic clean Si(111) surfaces into liquid helium was studied by the phonon pulse technique. Reflection experiments and angular resolving transmission experiments were performed to measure the absolute value of the transmission coefficient and its dependence on the phonon emission angle into the solid. A coefficient of the order of 1% was obtained by a chemical preparation method. We found that the observed anomalous transmission is caused by mass defects. Local inhomogeneities caused by the deposition of water molecules on the surface could be visualised by an enhanced transmission.

A spatially resolving superconducting bolometer for phonon imaging

In this paper we describe a highly sensitive spatially resolving bolometer for heat pulse detection. The bolometer is based on a continuous strip of superconducting aluminium deposited on a sample by vacuum evaporation. In operation, the sample is cooled to just below the transition temperature of the aluminium and then a small part of the strip is biased onto its superconducting transition by heating it with a focused laser beam. The resistance of the strip is then very sensitive to the small changes in temperature brought about by phonon pulses incident on the biased section. By scanning the laser along the strip the position of the sensitive region can be moved. We have tested this bolometer by using it to measure the spatial dependence of the anisotropic phonon flux emanating from a heater on the opposite surface of a GaAs crystal. We show that the results are in good agreement with Monte Carlo simulations of the phonon focusing patterns. We also demonstrate that the bolometer can be biased using thin-film heaters so that, in systems without optical access, multiple bolometers can be used in close proximity without the need for a separate co-axial line to each one.

Scattering of acoustic phonons by rare earth substitutional atoms in yttrium aluminum garnets

The diffusive maxima of phonon signals, and in particular their arrival timestm are examined for a number of solid solutions of rare earth atoms in yttrium aluminum garnets. The phonon pulses are generated by metallic films of the characteristic lengthlh heated by current pulses to the temperatureTh slightly higher than the ambient temperatureT. The injected phonons travel in wafers of the thicknessLz. They are scattered by substitutional atoms of rare earth occupying the yttrium dodecahedral sites, rare earth and yttrium atoms occupying the aluminum octahedral sites and by another lattice imperfections generated in the process of sample growing. The qualitative analysis based on our exact formula for the diffusion coefficientD allows us to extract the contribution of rare earth atoms substituting the Y atoms toD. Considering the dependence oftm/Lz2 on the temperature and the ratiolh/Lz we conclude thatD∼Th−4 and that the energy of phonons forming the diffusive maxima ranges from 3.2kBTH to 4.2kBTH, which is in reasonable agreement with the existing estimates.

Scattering of thermal phonon pulses by isotopes in silicon

The pure ballistic propagation of acoustic phonons in crystals at low temperatures can be described within anisotropic continuum acoustics. One needs only the elastic constants and mass densities to calculate the time-dependent spectral phonon irradition of the bolometer for a given radiator pulse power and detector/radiator geometry. We extend this treatment by including single isotope-scattering events for the phonons in a (111)-cut silicon disk on their flight from the radiator to the detector. Using the earlier experimentally determined polarization and frequency dependent phonon absorption in the bolometer metal, the instantaneous temperature of the bolometer can be calculated from this irradiation. This allows a direct comparison with measured bolometer temperatures using exactly the same transmission or reflection arrangement as in calculation. A very satisfying agreement is observed in the expected range of single phonon scattering.

Kapitza phonon transmission from clean H‐silicon(111) surfaces into liquid helium

AbstractWe have studied the phonon transmission from ultraclean Si(111) surfaces into liquid helium by the phonon pulse technique. Reflection experiments and angular resolving transmission experiments were performed to measure the absolute value of the transmission coefficient and its dependence on the phonon emission angle into the solid. Clean samples etched conventionally in dilute HF show a significant anomalous transmission. An additional etch step in highly buffered HF (i.e., NH4F) with a pH‐value of 7.8 reduces the transmission coefficient drastically. Local inhomogeneities of the transmission coefficient caused by the deposition of water molecules on the surface could be visualized by the enhanced transmission. We found that the observed anomalous transmission is caused by mass defects.

Mean free path of THz phonons in aluminum films

We have measured the rise of temperature in superconducting aluminum bolometers under irradiation of thermal phonon pulses. Using experimentally confirmed model calculations, the frequency dependent phonon absorption can be derived as a fit parameter. A comparison is made to the earlier determined phonon absorption in tin and lead.

Pulse-shape distortion in transmission experiments with incoherent phonons in isotropic solids due to elastic scattering

Assuming homogeneously distributed elastic scattering centers in an isotropic dielectric plate, the transmission and distortion of phonon pulses, that are transmitted from a radiator element to a centered detector element at the opposite face, are investigated theoretically. Analytic expressions are derived for the singlescattered phonon signal due to three different phenomenological scattering characteristics: isotropic scattering, preferential forward scattering, and preferential forward/backward scattering. In particular, we find that the asymptotic decay of the scattered phonon signal depends on the associated scattering characteristic. Numerical results are given for the excitation by a delta pulse using all three scattering characteristics. As a special case, the pulse-shape distortion of a rectangular pulse of 100 ns duration is treated in more detail. Parameter is the ratio of phonon mean free path to plate thickness.

Measurement of electron-phonon decoupling time in neutron-transmutation doped germanium at 20 mK

We have studied the electron-phonon decoupling in Neutron-Transmutation Doped (NTD) Germanium thermistors below 50 mK, and measured a characteristic time constant of this phenomenon. We will discuss how our decoupling model accounts for observed non-linearities in I–V characteristics of NTD Ge and for the time structure of phonon pulses detected in a Ge crystal operated at 20 mK.

Phonon Images of Crystals for Different Sources

In our previous papers, we have considered the energy and quasi-momentum focusing patterns [1] (hereafter referred to as I) and the image of crystalline GaAs obtained by the phonon-drag effect in GaAs—AlGaAs heterostuctures [2]. All of these patterns were obtained for sources of pulses of monochromatic phonons. Τhe duration time of these pulses was taken as the shortest of the characteristic times • in the considered problem. However, most experiments are performed not for pulses of monochromatic phonons but for heat pulses. Additionally, the time of duration of pulses may not be the shortest time characterizing the generation, propagation and detection of pulsed phonon beams. Therefore our approach should be generalized. In this paper we shall show that the kinetic approach, based on the use of the Boltzmann equation, can be used for the description of experiments with the use of heat pulses. Since mechanisms for the generation of phonon beams seem not yet properly understood we construct the source terms of the Boltzmann equation using arguments based on dimensional analysis and imposing suitable normalization conditions [1, 2, 3]. Let us briefly describe the experiment performed by Huebener and collaboration [4, 5] and Dietsche et al. [6]. All of these experiments were performed at an ambient temperature Τ much lower than the Debye temperature θD. Thus the thermal phonons were frozen out. Huebener et al. studied the energy focusing patterns of [001] oriented single crystalline Ge [4] and GaAs samples in the form of wafer discs. The diameters of these discs were several times greater than their thickness (1-2 mm). Τhe top

Imaging nonequilibrium phonon-induced backscattering in the quantum Hall regime.

Nonequilibrium phonon pulses, generated by laser heating a metal film, have been used to probe small areas of an extended two-dimensional electron gas (2DEG) in a GaAs-AlGaAs heterojunction, at low temperature and in a quantizing magnetic field. The electron-phonon interaction was studied via the change in the two-terminal resistance of the 2D electron system. By scanning the phonon beam over the entire area of the 2DEG, images of the effects were made. We show that our images are in qualitative agreement with theoretical descriptions of the quantum Hall effect which include electron states confined near the edges of the 2DEG.

The ultimate solar neutrino detector: Simultaneous detection of light and phonons in a fast scintillator made of single crystals of an indium compound and cooled to very low temperature

Real time detection of low energy solar neutrinos remains a technical challenge. Progress has been made on scintillating single crystals of indium compounds. Cerium doping has been successfully implemented by a French group (Talence-Lyon-Annecy Collaboration). Since Ce 3+ fluorescence is expected to remain fast at very low temperature, luminescence can be combined with phonon detection to improve energy resolution and localization, which are crucial for background rejection. Devices based on superconducting tunnel junctions may provide a read-out sensitive to the light strobe followed by the delayed pulse of phonons. Such a detector would operate at 3He temperatures ( T ap; 350 mK). Alternatives to develop solar neutrino detectors, as well as other applications of low temperature scintillation, are briefly discussed.

Optical studies of high-frequency nonequilibrium phonons in noncrystalline solids

The first results of optical (flourescent) technique application to the study of nonequilibrium terahertz range phonons in noncrystalline solids are reported. 1) The propagation of heat pulses through thin amorphous semiconductor film was studied by phonon detection via phonon-induced effects in exciton luminescence of a crystalline c-Si substrate. The effective frequency down-conversion of THz phonons in a-Si: H films was observed depending on H content. In a-Ge films a time delay in THz phonon pulses propagation was detected which is due to multiple phonon scattering in the films. 2) In Cr3+ -doped bulk oxide glasses the relaxation of optically generated THz phonons was studied by observation of phonon-induced anti-Stokes FLN R-spectra. It was found that fully trapped phonon distribution in the excited glass volume does not thermalize even in a millisecond time scale. The possible role of configurational “softness” and TLS in the observed specific behaviour of THz phonons in amorphous and glassy materials is discussed.

Phonon images of crystals

The kinetic description of the phonon pulses propagation and the phonon focusing phenomena is presented. Besides the familiar energy focusing we consider a novel phenomenon — the quasimomentum focusing. The density of energy falling onto a bolometer is related to the focusing factor. A component of the quasimomentum along the direction of the detector of an elongated form is related to the focusing factor and the suitable component of the slowness vector. Using the above mentioned analytic formulae and the data corresponding to the phonon-drag experiment in GaAs-GaAlAs heterostructures of Dietsche et al., we calculated the corresponding energy and quasimomentum focusing patterns.

Nonequilibrium dynamics in thermal desorption

The model for thermal desorption proposed by Efrima et al. is used to follow the time evolution or the bound states and to compute the desorption constant or the Ar/W system. The vibrational relaxation is studied, and the validity or the mean first passage time approximation, which is used in the calculation or the desorption constants, is examined. Finally, the question or whether or not the adatom can be desorbed by a phonon pulse is addressed

Propagation and filtering of point-contact injected phonon pulses in ruby

The 29-cm -1 component of intense phonon pulses injected into ruby at 1.5 K by a whiskerfilm point contact is studied via the phonon-induced R 2 luminescence at distances from tens of μm up to mm. Ballistic propagation is found at the Debye velocity of TA phonons. Additional experiments with a phonon filter of optically excited Cr 3+, when compared to Monte Carlo simulations, show that inelastic scattering of phonons by weakly exchange-coupled pairs reduces the absorption by the filter.

Phonon images of crystals

The kinetic description of the phonon pulses propagation and the phonon focusing phenomena is presented. Besides the familiar energy focusing we consider a novel phenomenon - the quasimomentum focusing. The density of energy falling onto a bolometer is related to the focusing factor. A component of the quasimomentum along the direction of the detector of an elongated from is related to the focusing factor and the suitable component of the slowness vector

Resonant scattering of phonons by adsorbates on clean silicon surfaces

We used Phonon Surface Scattering Spectroscopy (PSSS) to study excitations of adsorbates in the far infrared region from 100 μeV up to 3 meV. We investigated the scattering of pulses of monochromatic phonons by thin films of H2O, D2O and Ne deposited in situ onto laserannealed Si(100)-surfaces. For all substances we observed a broad scattering at coverages below one monolayer and a strong resonant scattering at about 3 monolayers. The resonance frequency decreased with increasing coverage. The reasons for the resonances are discussed. Because of the lack of any observable isotope effect between H2O and D2O, the possibility of tunneling states in these layers can be excluded. Instead, we conclude that a lateral vibrational mode of the adlayer is responsible for the resonance effect. Corresponding calculations by means of continuum elasticity theory describe the scattering behaviour very well if similar elastic constants are used for all film substances.

Quantitative investigation of thermal phonon pulses in sapphire, silicon, and quartz

In this paper we present reflection experiments with thermal phonon pulses at bare crystal surfaces of a-cut sapphire, [111]-cut silicon, X-cut quartz and Z-cut quartz. The observed bolometer signals are compared with an adequate rigorous model calculation. An attempt is made to investigate also for the first time quantitatively the strong background signal which is mainly due to scattering from isotopes and other crystal defects but contains also contributions from phonon-phonon scattering as is shown by a detailed investigation

Quantitative investigation of thermal phonon pulses in sapphire, silicon, and quartz

Using pulse-heated constantan films as a thermal phonon radiator and superconducting tin bolometer as a phonon detector, we present for the first time a full quantitative comparison between observed bolometer signals and adequate rigorous model calculations for transmission experiments ina-cut sapphire, [111]-cut silicon, as well asX-cut quartz andZ-cut quartz. Details of the observed phonon signals are explained and understood. From these experiments, we are also able to extract information about the phonon absorption coefficient in the normal state of the polycrystalline tin bolometer for longitudinal and transverse polarized phonons in quantitative agreement with an earlier experiment ina-cut sapphire which was performed with a superconducting tunnel junction as a detector. The observed transmission signals can be explained for sapphire and silicon by ballistic propagation with additional small angle scattering, but for quartz strong frequency downconversion occurs for phonons with frequencies above half a Terahertz.

Spontaneous emission by ballistic electronics in semiconducting heterostructures : M. Botton and A. Ron. Solid-St. Commun.71(12), 1131 (1989)

We have used nonequilibrium phonon-induced conductivity (phonoconductivity) measurements to probe the electronic states in semiconductor quantum wire devices. The devices were based on high mobility two-dimensional electron systems (2DESs) in GaAs/Al0.3Ga0.7As heterostructures and quantum wires formed using the well-known split-gate technique. Short (20 ns-long) pulses of nonequilibrium acoustic phonons were generated by heating a metal film on the back surface of the substrate. These phonons propagated ballistically across the substrate and were incident on the quantum wire. The electron–phonon interaction was detected via the phonon-induced change in electrical conductance of the device. We observed giant oscillations of the phonoconductivity with increasing (negative) gate bias. Maxima occurred when the Fermi energy was coincident with the bottom of any one-dimensional electronic subband. In this paper we argue that the phonoconductance is due to phonon-induced backscattering of the electrons in the quantum wire and present evidence that the strength of the phonon signal is proportional to the density of electronic states in the quantum wire.