Ruqing Xu

Phonons of the anomalous element cerium

Many physical and chemical properties of the light rare-earths and actinides are governed by the active role of f electrons, and despite intensive efforts the details of the mechanisms of phase stability and transformation are not fully understood. A prominent example which has attracted a lot of interest, both experimentally and theoretically over the years is the isostructural γ - α transition in cerium. We have determined by inelastic X-ray scattering, the complete phonon dispersion scheme of elemental cerium across the γ → α transition, and compared it with theoretical results using ab initio lattice dynamics. Several phonon branches show strong changes in the dispersion shape, indicating large modifications in the interactions between phonons and conduction electrons. This is reflected as well by the lattice Grüneisen parameters, particularly around the X point. We derive a vibrational entropy change , illustrating the importance of the lattice contribution to the transition. Additionally, we compare first principles calculations with the experiments to shed light on the mechanism underlying the isostructural volume collapse in cerium under pressure.

Imaging phonons in a fcc Pu-Ga alloy by thermal diffuse x-ray scattering

X-ray thermal diffuse scattering intensity patterns from phonons in a fcc δ-Pu–Ga alloy have been recorded using an 18 keV undulator x-ray beam with a beam diameter of 25 μm. The results are consistent with patterns calculated using the Born–von Karman force constant model of lattice dynamics, and support the pronounced softening of the transverse acoustic branch along the [111] direction observed from inelastic x-ray scattering measurements. This work demonstrates the feasibility of using a “large-grain, small beam” approach to study lattice properties, such as phonon dispersion curves, of materials not readily available in the form of large single crystals.

Pressure-induced antiferrodistortive phase transition in SrTiO3: Common scaling of soft-mode with pressure and temperature

We report a study of the pressure-induced antiferrodistortive cubic-to-tetragonal phase transition in strontium titanate (SrTiO3) at ambient temperature. High-resolution inelastic x-ray scattering measurements reveal the softening of a phonon mode (R25) at the zone boundary; a static distortion sets in at a critical pressure of 9.5 GPa, which corresponds to a critical volume reduction of 5.3%. Prior studies have shown that similar phonon softening and ensuing lattice distortion can be induced under ambient pressure by lowering the sample temperature through a critical temperature of 105 K. The relationship between the two phase transitions is clarified by comparing the power laws of the pressure and temperature dependences of the softening behavior. The results are explained in terms of the analytic properties of the soft mode. Copyright c EPLA, 2014

Studying structural phase transitions with X-ray thermal diffuse scattering

X-ray thermal diffuse scattering is a powerful probe of lattice dynamics. It is especially effective in the investigation of structural phase transitions. In this article, we review and compare recent experimental studies of the behaviour of phonons in connection with the structural phase transitions in TiSe2, SrTiO3 and Pu–Ga. A complete phonon softening is observed for the second-order transitions in TiSe2 and SrTiO3. By contrast, no phonon softening is detected for the first-order transition in Pu–Ga, which is unusual in comparison to other related systems.

Quantum growth of a metal/insulator system: Lead on sapphire

We report the observation of quantum growth behavior in a metal-on-insulator system. Using insulating substrates, with their large band gaps, should maximize quantum confinement effects. In a study of Pb film growth and thermal processing on sapphire, we have observed robust preferred island height selection over a wide thickness range—a hallmark of quantum confinement effects—up to 250 °C. By contrast, room temperature is the limit for Pb films prepared on Si(111). These results provide the evidence connecting the quantum growth behavior of overlayers with the substrate band gap.