Abstract
Experimental results for the isobaric-thermal-expansion coefficient of pressurized near the superfluid transition temperature are reported. Near , is an asymptotically linear function of the specific heat at constant pressure . Therefore these measurements yield some of the same critical-point parameters as those derivable from . The measurements were made with high-temperature resolution over the range , along nine isobars. They span the pressure interval bar. A new experimental technique was employed which yielded a temperature resolution of two parts in and a pressure stability of 1 × bar. The results for each isobar were fitted with the equation above , and with the same expression with primed coefficients below . When the amplitudes and of the confluent singularity are assumed to be equal to zero (i.e., the data are fitted with a pure power law), the leading exponents are pressure dependent and vary from 0.00 at low to 0.06 at high . This analysis also yields . The inequality between and , and the pressure dependence of and , are contrary to the predictions of the phenomenological and renormalization-group theories of critical phenomena. When and are permitted to assume nonzero values, it is statistically allowed by the data to impose the theoretically predicted relations , , and as constraints in the analysis. With these constraints, and the value of chosen to be equal to 0.5, we obtain pressureindependent (universal) amplitude ratios and leading exponents, as expected from theory. Their values are , , and . Similar results are obtained when is chosen to be equal to 0.4 or 0.6. The result for is consistent with that derived previously from specificheat measurements. The universal is contrary to the previous report of a pressure-dependent specificheat amplitude ratio. Using thermodynamic relations, we compare our results directly with the measurements. For bar the agreement is excellent; but at the higher pressures there are small but significant differences of unknown origin.
- Received 26 April 1976
DOI:https://doi.org/10.1103/PhysRevB.14.2096
©1976 American Physical Society