Positron states between positronium and “free positron” states in condensed matter

Positronium (Ps), the bound state of an electron and a positron, is formed in many insulating solids and liquids. Recently, several cases of a transition from a Ps state to a non-Ps state have been observed. This has been found for liquid CS2 if the temperature is decreased from about −40 °C to the melting point, −110.8 °C. In some solid aromatic hydrocarbons, such as naphthalene, biphenyl and p-terphenyl, similar transitions occur upon decreasing the temperature. These transitions are unique in physics since Ps is the only bound state of two light particles in normal low-energy physics. Measurements of the positron lifetime, angular correlation of the annihilation radiation, Ps chemical reactions, and Zeeman effect of Ps give information on the transitions.