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Using ab initio calculations we have investigated the thermal conductivity (κ) of diamond nanowires, unveiling unusual features unique to this system. In sharp contrast with Si, κ(T ) of diamond nanowires as thick as 400 nm still increase monotonically with temperature up to 300K, and room temperature size effects are stronger than for Si. A marked dependence of κ on the crystallographic orientation is predicted, which is apparent even at room temperature.[001] growth direction always possesses the largest κ in diamond nanowires. The predicted features point to a potential use of diamond nanowires for the precise control of thermal flow in nanoscale devices.

Solid state electrolytes could address the current safety concerns of lithium-ion batteries as well as provide higher electrochemical stability and energy density.

It has been highly debated whether the thermal conductivity κ of a disordered SiGe alloy can be lowered by redistributing its constituent species so as to form an ordered superlattice. By ab initio calculations backed by systematic experiments, we show that Ge segregation occurring during epitaxial growth can lead to κ values not only lower than the alloy's, but also lower than the perfect superlattice values. Thus we theoretically demonstrate that κ does not monotonically decrease as the Si- and Ge-rich regions become more sharply defined. Instead, an intermediate concentration profile is able to lower κ below both the alloy limit (total intermixing) and also the abrupt interface limit (zero intermixing). This unexpected result is attributed to the peculiar behavior of the phonon mean free path in realistic Si/Ge superlattices, which shows a crossover from abrupt-interface- to alloylike values at intermediate phonon frequencies of ∼3  THz. Our calculated κ's quantitatively agree with the measurements when the realistic, partially intermixed profiles produced by segregation are considered.

Transition metal layered oxides are promising cathode materials for sodium‐ion batteries. Phase transitions involving different stacking sequences of the oxide layers often plague the electrochemistry of these materials during cycling, which strongly impacts in their electrochemical performance. However, the underlying mechanisms of these processes remain elusive. Interestingly, P2‐ and O3‐Na2/3Fe2/3Mn1/3O2 phases are the first transition metal layered oxide polymorphs that have been synthesized with exactly the same composition. This offers unprecedented access to the study of bistability in these systems as well as isolates the effect of local structure on Na ion mobility. Here, first‐principles calculations and experiments are combined to unveil the physical origin of such bistability and identify important differences in Na ion diffusion between these two phases. It has been found that electrostatic interactions between oxide layers control the bistable nature of P2 and O3 phases. It is also put forward that the interlayer distance between oxide layers may be a useful descriptor to rationalize the relative stability of other P and O phases in general. Furthermore, this study tracks down to the molecular level the differences regarding Na ion mobility in P2‐ and O3‐Na2/3Fe2/3Mn1/3O2 by computing activation energies and estimating diffusion coefficients.

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Our understanding of both structure and dynamics of adsorbed liquids heavily relies on the capillary wave Hamiltonian, but a thorough test of this model is still lacking. Here we study the capillary wave fluctuations of a liquid film with short-range forces adsorbed on a solid exhibiting van der Waals interactions. We show for the first time that the measured capillary wave spectrum right above the first order wetting transition provides an interface potential consistent with independent calculations from thermodynamic integration. However, the surface tension exhibits an oscillatory film thick dependence which reveals a hitherto unnoticed capillary wave broadening mechanism beyond mere interfacial displacements.

Using a first principles theoretical approach, we show that vacancies give anomalously strong suppression of the lattice thermal conductivity, κ, of cubic Boron arsenide (BAs), which has recently been predicted to have an exceptionally high κ. This effect is tied to the unusually large phonon lifetimes in BAs and results in a stronger reduction in the BAs κ than occurs in diamond. The large bond distortions around vacancies cannot be accurately captured using standard perturbative methods and are instead treated here using an ab initio Green function approach. As and B vacancies are found to have similar effects on κ. In contrast, we show that commonly used mass disorder models for vacancies fail for large mass ratio compounds such as BAs, incorrectly predicting much stronger (weaker) phonon scattering when the vacancy is on the heavy (light) atom site. The quantitative treatment given here contributes to fundamental understanding of the effect of point defects on thermal transport in solids and provides guidance to synthesis efforts to grow high quality BAs.