Condensed-Matter Physics & Materials Science Seminar

The importance of perovskite-structured materials to modern science cannot be understated, as they harbor diverse behavior and landscape of novel competing and intertwined phases. The structural phases of perovskites are critical to defining the framework of electronic conduction and magnetic exchange pathways in this interesting and technologically relevant class of materials. Large, isotropic negative thermal expansion is known to exist in only a handful of materials, beginning with the discovery of ZrW2O8 in the 1990s. In 2010, perovskite fluoride ScF3 was discovered to have a similarly profound negative thermal expansion (NTE) effect, shrinking in response to heat over a 1000 K temperature window with a linear thermal expansion coefficient lower than -10-5/K. Another curious property of this material is the structural stability â€" ScF3 retains a simple cubic structure and four atom unit cell from cryogenic temperature to its high melting point of 1800 K. ScF3 material does not feature the interesting phase competition of electrons and spin enjoyed by many of its oxide and fluoride cousins and can be classified as an ionic insulator. However the superlative nature of the NTE effect has motivated us to dive deeply into the lattice dynamics using high energy resolution inelastic X-ray scattering on strain-free single crystals. Surprisingly, we find that an entire optical mode branch circumscribing the Brillouin zone boundary softens to nearly zero frequency as the temperature T approaches T=0. ScF3 at T=0 thus sits in extreme proximity to a quantum phase transition. We interpret this result in the context of better studied trifluorides and examine in detail the disorder phase diagram. In addition, concomitant with softening of the optic branch, a quasielastic "central peak" (CP) emerges and strengthens toward low temperature, further bolstering the identification of a T=0 phase transition. The CP phe