University of Connecticut

Events Calendar

PhD Defense

Friday, May 11, 2018
10:30am – 12:30pm

Storrs Campus

Sahan Handunkanda, Department of Physics, University of Connecticut

Lattice dynamics studies of negative thermal expansion due to two low-temperature lattice instabilities

A material’s tendency to shrink upon heating is known as negative thermal expansion (NTE). A class of materials in which the NTE phenomenon arises from the lattice degrees of freedom has gained much attention over the past couple decades. Simple cubic ScF3 is a prominent candidate of this class of materials as it displays strong, isotropic NTE over a 1000 K temperature range. In addition, no structural phase transition has been reported above 0.4K and it retains the simple cubic structure up to its high melting point of 1800 K, which is unusual compared with other transition metal trifluorides.

Here I present a combined inelastic x-ray scattering (IXS), x-ray diffraction (XRD) and thermal diffuse scattering (TDS) study of ScF3, revealing some exciting features of this material such as clean-limit structural quantum phase transition (SQPT), evidence of essential dimensional reduction, disorder phase diagram and nanoscale correlation of NTE modes. Further investigation of the zone center (ZC) dynamics of ScF3 by using infrared (IR) reflectivity measurement reveals evidence of multi-phonon absorption involving low energy NTE modes resides at Brillouin zone boundary. The IR result motivates us to consider a new approach to study NTE dynamics by populating corresponding modes using optical pump. I then address whether the anomalously strong and thermally persistent NTE behavior of ScF3 is a consequence of the SQPT? We carried out an IXS study of a second system Hg2I2 also tuned near SQPT while retaining stoichiometric composition and high crystallinity. We find similar behavior and significant NTE below 100 K for dimensions along the body-centered tetragonal c axis, bolstering the connection between NTE and zero-temperature structural transitions.


Prof. J. Hancock

Physics Department (primary)

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