Kohn anomaly in metastable FCC lanthanum

The fcc phase of lanthanum has many rather intriguing properties. Its superconducting transition temperature at atmospheric pressure is approximately 1 K higher than that of the stable dhcp phase, increases with increasing pressure, and saturates to a value of nearly 13 K at approximately 200 kbar. The thermal expansion coefficient of this phase becomes negative at approximately 40 K and reaches its largest negative value in the vicinity of 18 K. A fundamental understanding of the rather unusual properties of fcc lanthanum has been hindered by a lack of information regarding the elastic and lattice dynamical properties of the metastable phase of this metal. Recently, the authors were able to grow a single crystal of the metastable fcc phase of sufficient size to study the lattice dynamics by inelastic scattering of neutrons. In this note, the authors present some of the results obtained in this study