Date of Completion

Spring 5-8-2018

Thesis Advisor(s)

Jason N. Hancock

Honors Major

Physics

Disciplines

Condensed Matter Physics

Abstract

Most materials, upon heating, expand into a larger volume through an effect known as thermal expansion. However, some materials do not obey this intuitive phenomenon and instead exhibit negative thermal expansion (NTE), which describes the anomalous propensity of these materials to shrink when warmed. Since its discovery, NTE has been found in a wide variety of materials with an array of magnetic, electronic and structural properties. In some cases, the NTE originates from the electronic degrees of freedom in the system or through magnetic phase competition, but we here focus on a particular class of NTE which originates solely from the temperature-dependence of the vibrational modes in the lattice and structural phase competition in the absence of magnetic contributions, a property termed structural negative thermal expansion (SNTE). The primary subject of the study herein is scandium trifluoride (ScF3), a material exhibiting exceptionally strong NTE that persists over a broad temperature range which was identified nearly a decade ago. We present recent experimental and theoretical work which attempts to elucidate the lattice dynamical origins of the observed NTE, in particular through a connection to the low temperature soft-mode dynamics arising from phase competition near a structural quantum phase transition (SQPT). To further bolster the hypothesized connection between the vibrational dynamics near the SQPT and the development of NTE, we present new experimental evidence in mercurous iodide (Hg2I2), which - in parallel with studies of ScF3 - is suggestive of a generic mechanism for realizing NTE in certain material classes.

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