Metastable Solid and Liquid Helium-4

This manuscript presents the experiments done in the group "Quantum solids and fluids" of Laboratoire Kastler Brossel in Paris since I joined the group in 2009. This group was created in 2003 by Philippe Jacquier and Jacques Dupont-Roc joined him in 2004.We are interested in the metastable phases of condensed 4He.The fascinating phase diagram of 4He at low temperatures is shown in the figure below. Several features in this diagram are unique compared to that of all other simple elements. The most striking one is that there is no triple point (liquid-gas-solid coexistence point) in the phase diagram of 4He meaning that it is possible to cool a liquid-gas mixture of 4He all the way down to T ! 0 K. Below T = 2.17 K, liquid 4He is in its famous superfluid state which is understood as a mixture of two fluids: a superfluid part with zero entropy and viscosity and a normal fluid which carries the heat content. [...