Thermal Excitations in LiquidHe3

Abstract

Using recently obtained data on various thermal properties of liquid ${\mathrm{He}}^{3}$, the constant-volume heat capacity of the saturated liquid has been evaluated. This elementary heat capacity ${C}_{v}(T)$ was found to exhibit at least three inflection points over the temperature interval extending to the critical temperature. Of these, the one at the lowest temperature was shown to originate with the partial heat capacity of spin disorder. The inflection point at the approach of the critical temperature ${T}_{c}$ is imposed by the thermodynamic result that ${C}_{v}(T)$ should reach its finite limit ${C}_{v}({T}_{c})$ from below with positively infinite first and second temperature derivatives. The laws governing the pressure dependence of the partial spin heat capacity over the liquid region of the ${\mathrm{He}}^{3}$ phase diagram have been deduced. The remarkable pressure effects displayed by the spin system of liquid ${\mathrm{He}}^{3}$ through its heat capacity may be expected to manifest themselves through the observable pressure dependence of the total liquid heat capacity at low enough temperatures where the spin heat capacity is dominant. A purely heuristic and indirect approach toward the problem of the existence of phonon type of partial excitations in liquid ${\mathrm{He}}^{3}$ yields a negative answer, without, however, a definite exclusion of such symmetrical excitations in this antisymmetric liquid.