By measuring the pressure dependence of the velocity of sound, we have determined both the pressure dependence of the density and the Gruneisen constant u of liquid He. Measurements were made below 0.1 K and in the vicinity of 0.5 K. Our determinations of the pressure dependence of the density agree quite well with that determined by Boghosian and Meyer, who used a capacitance bridge. Since the latter results rely on the validity of the Clausius-Mossotti relation and a pressure-independent electric polarizability, the present work can be interpreted as supporting both of these assumptions.We found that u(pp) -= (p/c)dc/dp = 2.84 under the vapor pressure at 0.1 K. Using this value of u to calculate the attenuation of sound according to a three-phonon mechanism, we obtain an attenuation of less than half the measured value. Thus, the present theory of sound attenuation must be incomplete.

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We characterize the spontaneous magnetic field, and determine the associated temperature T(g), in the superconducting state of (Ca(x)La(1-x)) (Ba(1.75-x)La(0.25+x)) Cu(3)O(y) using zero and longitudinal field muon spin resonance measurements for various values of x and y. Our major findings are (i) T(g) and T(c) are controlled by the same energy scale, (ii) the phase separation between hole poor and hole rich regions is a microscopic one, and (iii) spontaneous magnetic fields appear gradually with no moment size evolution.

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