High frequency critical attenuation of sound in He3-He4 mixtures near T

2014 We have measured the critical attenuation of sound at 1.1 GHz in He3-He4 mixtures of 0.06, 0.20 and 0.24 molar He3 concentrations. The results above the superfluid-normal transition are scaled with the same scaling function used in the kHz frequency range for pure He4. LE JOURNAL DE PHYSIQUE LETTRES TOME 40, 15 SEPTEMBRE 1979, Classification Physics Abstracts 62.60 67.20 67.60 Recent experimental results on the critical attenuation of sound in pure helium 4 around 7B, at both low frequencies [1, 2] (1) (v ~ 1 MHz) and high frequencies [3] ( ~ 1 GHz) have helped specify the different contributions to the attenuation. At GHz frequencies the main contribution comes from the order parameter fluctuations. On both sides of the transition, these fluctuations contribute to the attenuation in a form which is not symmetric in temperature relative to 7~. This contribution takes the form : with a characteristic time T=To~~==!l2013 TIT, I and c is a constant. Such an expression fits the experimental data from 2.3 kHz to 1.2 GHz, for T > 7B. In He3-He4 mixtures the He3 concentration acts as an inert variable (as the pressure along the À line), thus it becomes interesting to test the universality of the scaling function (1) for different mixtures. This was done recently at low frequencies [1, 4] (v 45 MHz). The purpose of this paper is to extend these measurements up to 1.1 GHz; this high frequency offers an especially good opportunity to study the contribution of the fluctuation mechanism to the attenuation [3]. (*) Associated with the Centre National de la Recherche Scientifique. (1) References [1] and [2] correspond respectively to results on He3-He4 mixtures and pressurized He4. Each of these papers enables results for pure He4 at P = 0 to be obtained. The acoustical transmission technique used and the method by which we relate the transmitted signal to the attenuation, have already been described in previous articles [3]. Because we use thin samples ( ~ 5 to 10 ~), no gravity corrections are needed. The first results of our measurements concern the dispersion of sound velocity : within the accuracy of our measurements ( ~ 0.2 m/s), we see no critical dispersion at any concentration; this result is in agreement with our previous result in pure He4 [3] and with the fact that the critical dispersion is expected to decrease with increasing concentration [1, 4]. Before analysing the critical attenuation Ctc, we have to subtract the non critical attenuation aB from the measured attenuation. For T > T~, we take into account the contribution of the shear viscosity and of the thermal conductivity; the increase of aB with the He3 concentration is connected with the decrease of the density and of the velocity of sound. For T T~, the problem is more complicated : the elementary excitations (phonons and rotons) contribute to the attenuation; this contribution is well known in pure He4 [4], it peaks at 1.5 K for 1 GHz. In He3-He4, this contribution is completely unknown at GHz frequencies. Therefore it is not possible for us to extract the critical attenuation for T T~ at this frequency. The A transition temperature 7~(~) is deduced from the He3 molar concentration (see table) with a maximum error of 10 mK. The accuracy of our temperature measurement is better than 1 mK; but due to the error in the determination of 7~(A"), our total accuracy is worse. Also shown in the table is the Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyslet:019790040018047700 L-478 JOURNAL DE PHYSIQUE LETTRES temperature TmaX of the small apparent maximum of the attenuation just below the transition. Table. X is the molar He 3 concentration of the mixture ; T~, the temperature of the normal superfluid transition [1, 4, 6] ; TmaX, the temperature of the small maximum of the attenuation below the transition temperature T~(X) ; zo(X) is the value which gives the best fit to the data (see text).