Shear-induced anisotropic plastic flow from body-centred-cubic tantalum before melting.

There are many structural and optical similarities between a liquid and a plastic flow. Thus, it is non-trivial to distinguish between them at high pressures and temperatures, and a detailed description of the transformation between these phenomena is crucial to our understanding of the melting of metals at high pressures. Here we report a shear-induced, partially disordered viscous plastic flow from body-centred-cubic tantalum under heating before it melts into a liquid. This thermally activated structural transformation produces a unique, one-dimensional structure analogous to a liquid crystal with the rheological characteristics of Bingham plastics. This mechanism is not specific to Ta and is expected to hold more generally for other metals. Remarkably, this transition is fully consistent with the previously reported anomalously low-temperature melting curve and thus offers a plausible resolution to a long-standing controversy about melting of metals under high pressures.

Using alloying to promote the subtle rhombohedral phase transition in vanadium.

Recently it has been suggested theoretically and discovered experimentally that pressure can induce body-centered cubic vanadium to transition to a rhombohedral phase. Here we show using density functional theory calculations that alloying can affect the same transition, and in particular alloying can increase the stability of the rhombohedral phase, reducing the pressure needed to induce the transition. These calculations are full supercell calculations, as opposed to the virtual crystal approximation and other approximate schemes that neglect atomic relaxation and local bonding effects. These results suggest a way in which alloying provides a means of designing this class of exotic phases to be more robust.

Experimental observation of quantum confinement in the conduction band of CdSe quantum dots.

X-ray absorption spectroscopy has been used to characterize the evolution in the conduction band (CB) density of states of CdSe quantum dots (QDs) as a function of particle size. We have unambiguously witnessed the CdSe QD CB minimum (CBM) shift to higher energy with decreasing particle size, consistent with quantum confinement effects, and have directly compared our results with recent theoretical calculations. At the smallest particle size, evidence for a pinning of the CBM is presented. Our observations can be explained by considering a size-dependent change in the angular-momentum-resolved states at the CBM.

Osmium has the lowest experimentally determined compressibility.

On the basis of high pressure diamond-anvil compression studies for the precious metals Ru, Ir, and Os we report the surprising discovery that metallic osmium has a lower compressibility than covalently bonded diamond. We also find that Ir and Ru are as incompressible as Re. In addition, we have performed first principles calculations that confirm the trend in the measured transition metal compressibilities.