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We conduct a theoretical study in which we determine the zero-point vacancyconcentration in solid 4He at T=0 K. To this end, we employ thequantum-classical isomorphism, by which the quantum-mechanical probabilitydensity function of a system composed of bosons at T=0K can be interpreted interms of a Boltzmann factor of a classical system at finite temperature. Byusing this classical isomorph we apply the methods of classical statisticalmechanics to compute the vacancy formation free energy and the vacancyconcentration in the associated quantum system at T=0. In this context, wefocus specifically on the role of anharmonic effects that are expected to benon-negligible due to the significant zero-point motion. For this purpose, wecompute the formation free energies using both the harmonic approximation (HA)as well as reversible-work (RW) method, in which all anharmonic effects aretaken into account. The results suggest that anharmonic effects indeed play asignificant role, lowering the classical formation free energy by ~25% andincreasing the zero-point vacancy concentration by more than an order ofmagnitude compared to the HA.

Influence of Anharmonic Effects on the Zero-point Vacancy Concentration in Solid 4He