HELIUM , SOLID 1 Helium , Solid

Helium was first solidified at the famous Kamerlingh Onnes low-temperature physics laboratories in Leiden by W. H. Keesom [1] on June 25, 1926. The initial experiments by Sir Francis Simon at Oxford University and by Keesom and their collaborators focused on the melting curve, the specific heat, and the thermal conductivity of solid helium as a test of our early understanding of solids. These measurements showed, for example, that the Lindemann criterion of melting does not hold in solid helium. This pioneering work up to 1957 is elegantly and beautifully reviewed by Domb and Dugdale [2], a review that stands today as an excellent introduction to solid helium along with the books and reviews by Wilks [3], Keller [4], Wilks and Betts [5], Glyde [6], Dobbs [7] and Roger et al. [8]. The pair potential v(r) between helium atoms is precisely known [9,10]. It is weakly attractive at large separation, r & 3 Å, with a maximum well depth ǫ = 10.95 K. At close approach r ≤ σ = 2.63 Å, where hard-core radius σ defined by v(σ) = 0, v(r) becomes steeply repulsive. The potential parameters σ and ǫ of the rare gases are compared in Table 1. The potential seen by a helium atom lying between two atoms in a linear lattice is depicted in Fig. 1. The well shape, which is wide and anharmonic, is clearly dominated by the repulsive core of v(r).