Solid metal has “structural memory” of its liquid state

Caption: This recovered bismuth sample has a rhombohedral structure and contains liquid structural motifs after deep melting at high pressures. The surprising structural memory effect in the molten state is responsible for the unexpected ferromagnetism in bismuth. Image courtesy of Yu Shu and Guoyin Shen.
Caption: This recovered bismuth sample has a rhombohedral structure and contains liquid structural motifs after deep melting at high pressures. The surprising structural memory effect in the molten state is responsible for the unexpected ferromagnetism in bismuth. Image courtesy of Yu Shu and Guoyin Shen.

As an archetypal semimetal with complex and anisotropic Fermi surface and unusual electric properties (e.g., high electrical resistance, large magnetoresistance, and giant Hall effect), bismuth (Bi) has played a critical role in metal physics. For over a century, Bi has been known to be diamagnetic. A research team observed unusual ferromagnetism in bulk Bi samples recovered from a molten state at pressures of 1.4–2.5 GPa and temperatures above 1,250 K. Using HPCAT facility, the ferromagnetism is found to be associated with a surprising structural memory effect in the molten state. The ability for solid Bi to remember liquid structure motifs seems to be related to structural similarities between the solid and liquid around 2 GPa, where melting temperature is pressure-independent (i.e., no discontinuity in specific volume or density across the melting line). On heating at ~2 GPa, low-temperature Bi liquid (L) transforms to a more randomly disordered high-temperature liquid (L’) around 1,250 K. By cooling from above 1,250 K, certain structural characteristics of liquid L’ are preserved in L. Bi clusters with characteristics of the liquid L’ motifs are further preserved through solidification into the Bi-II phase across the pressure-independent melting curve, which may be responsible for the observed anomalous ferromagnetism. More in Shu et al, Deep melting reveals liquid structural memory and anomalous ferromagnetism in bismuth PNAS 2017 ; doi:10.1073/pnas.1615874114