Motivated by recent experiments on electric transport through single molecules and quantum dots, we investigate a model for transport that allows for significant coupling between the electrons and a boson mode isolated on the molecule or dot. We focus our attention on the temperature-dependent properties of the transport. In the Holstein picture for polaronic transport in molecular crystals the temperature dependence of the conductivity exhibits a crossover from coherent ͑band͒ to incoherent ͑hopping͒ transport. Here, the temperature dependence of the differential conductance on resonance does not show such a crossover, but is mostly determined by the lifetime of the resonant level on the molecule or dot.

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Recently it has been reported that a high-pressure cubic phase of ruthenium dioxide has an unusually large bulk modulus, and consequently is a most interesting candidate as a very hard material. Based on ab initio calculations it is shown that the high bulk modulus is a result of a strong covalent bonding between ruthenium d states and oxygen p states in combination with the favorable geometry of the orbitals in the fluorite structure. In addition an even higher bulk modulus is predicted for the isoelectronic and isostructural compound OsO 2 .Hard materials have been studied for a long time both experimentally and theoretically due to their wide range of important technological applications. 1 Although hardness is a macroscopic phenomena, the microstructure, such as bonds and atomic size, is very important for the measured strength of a material. As an example, the general explanation for why diamond is the hardest material observed today is that directed covalent sp 3 bonds between the carbon atoms give rise to a very rigid and hard structure. 2 Even a small deformation requires a lot of energy due to the strength of the carbon-carbon bond. Although diamond is not especially close packed it is, due to the rigidity of its covalent bonds, harder than all other presently known materials. Other examples of hard materials are the cubic BN and the hexagonal WC compounds. 1 The hardness is measured by making an indent in the material and measure its size. The difficulty in calculating hardness directly has led scientists to look for an additional measure of hardness. A good and frequently used candidate for this is the bulk modulus, which reflects the volume stiffness of the lattice. 1-3 In a plot of hardness versus bulk modulus one sees a strong correlation between the two. 4 For noncubic systems the shear modulus may be a more precise measure of the hardness. However, for cubic systems the bulk modulus should be a quite good indicator of hardness.In this paper we investigate a class of candidate hard materials; transition-metal dioxides in the cubic-CaF 2 structure. According to experiments 5 there is a structural transition in RuO 2 from the ground state rutile phase to an orthorombic distortion of the rutile phase at a pressure of 8 GPa and a second transition to the cubic fluorite phase at 12 GPa. In a later paper 6 this structure was reported as a distorted fluorite phase, where the Ru-O distance (ϭu), is different from the ideal value 1/4. We have investigated also this phase, and it will be discussed below. This high-pressure phase is metastable at ambient conditions, with a very high bulk modulus 399 GPa ͑Refs. 4 and 5͒ at zero pressure. The possibility to stabilize cubic RuO 2 at ambient conditions, for instance by epitaxial growth on a cubic substrate, could open avenues for ultrahard materials to be used in applications. We have investigated this experimental finding on RuO 2 by means of first-principles calculations, which confirm the experimentally found extreme behavior of RuO 2 . Further, we analyze t...

There is a general need to change hydropower operational regimes to balance the growing contribution of variable renewable energy sources in power systems. Quantifying the burden on generation equipment is increasingly uncertain and difficult. Here, we propose a framework combining technical and economic indicators to analyze primary frequency control (PFC) on a timescale of seconds. We develop a model integrating hydraulic, mechanical, and electrical subsystems to characterize efficiency loss, wear and fatigue, regulation mileage, and frequency quality. We evaluate burden relief strategies under three idealized remuneration schemes for PFC, inspired by those used in Sweden, the USA, and China, respectively. We show how burden and compensation vary under future scenarios of renewable power systems. Our framework can be used by producers to develop favorable operation strategies that reduce burden and increase economic value, and by transmission system operators to provide insights on the relation between incentive structures and regulating performance.

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