Impurity-stabilized solid 4He below the solidification pressure of pure helium

The modification of melting temperatures and pressures by dissolved impurities is well known in classical fluids. However, to our knowledge such effects have never been studied in quantum solids because of the difficulties in introducing impurities into such crystals that exist only at cryogenic temperatures, and, in the case of He-4, at pressures exceeding 25 bar. Here, we present an effect that occurs during the melting of solid He-4 doped with nanoscopic impurities ( alkali atoms, clusters, ions and electrons): the doped part of the crystal remains solid under conditions at which pure helium is liquid. Using interferometry, we found that the density of the solid structure lies between the densities of pure liquid and pure solid helium. We tentatively interpret the solid structure as being an aggregation of positively charged particles and electron bubbles.