Computational predictions of energy materials using density functional theory

In the search for new functional materials, quantum mechanics is an exciting starting point. The fundamental laws that govern the behaviour of electrons have the possibility, at the other end of the scale, to predict the performance of a material for a targeted application. In some cases, this is achievable using density functional theory (DFT). In this Review, we highlight DFT studies predicting energy-related materials that were subsequently confirmed experimentally. The attributes and limitations of DFT for the computational design of materials for lithium-ion batteries, hydrogen production and storage materials, superconductors, photovoltaics and thermoelectric materials are discussed. In the future, we expect that the accuracy of DFT-based methods will continue to improve and that growth in computing power will enable millions of materials to be virtually screened for specific applications. Thus, these examples represent a first glimpse of what may become a routine and integral step in materials discovery

Non-Perturbative Theory of Dispersion Interactions

Some open questions exist with fluctuation-induced forces between extended dipoles. Conventional intuition derives from large-separation perturbative approximations to dispersion force theory. Here we present a full non-perturbative theory. In addition we discuss how one can take into account finite dipole size corrections. It is of fundamental value to investigate the limits of validity of the perturbative dispersion force theory.Comment: 9 pages, no figure

Dynamical transitions and sliding friction in the two-dimensional Frenkel-Kontorova model

The nonlinear response of an adsorbed layer on a periodic substrate to an external force is studied via a two dimensional uniaxial Frenkel-Kontorova model. The nonequlibrium properties of the model are simulated by Brownian molecular dynamics. Dynamical phase transitions between pinned solid, sliding commensurate and incommensurate solids and hysteresis effects are found that are qualitatively similar to the results for a Lennard-Jones model, thus demonstrating the universal nature of these features.Comment: 11 pages, 12 figures, to appear in Phys. Rev.

Transverse thermal depinning and nonlinear sliding friction of an adsorbed monolayer

We study the response of an adsorbed monolayer under a driving force as a model of sliding friction phenomena between two crystalline surfaces with a boundary lubrication layer. Using Langevin-dynamics simulation, we determine the nonlinear response in the direction transverse to a high symmetry direction along which the layer is already sliding. We find that below a finite transition temperature, there exist a critical depinning force and hysteresis effects in the transverse response in the dynamical state when the adlayer is sliding smoothly along the longitudinal direction.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let

Neutron diffraction study on phase transition and thermal expansion of SrFeAsF

The magnetic ordering and crystal structure of iron pnictide SrFeAsF was investigated by using neutron powder diffraction method. With decreasing temperature, the tetragonal to orthorhombic phase transition is found at 180 K, while the paramagnetic to antiferromagnetic phase transition set in at 133 K. Similar to the parent compound of other iron pnictide system, the striped Fe magnetism is confirmed in antiferromagnetic phase and the Fe moment of 0.58(6) uB aligned along long a axis. The thermal expansion of orthorhombic phase of SrFeAsF is also investigated. Based on the Grueneisen approximation and Debye approximation for internal energy, the volume of SrFeAsF can be well fitted with Debye temperature of 347(5) K. The experimental atomic displacement parameters for different crystallographic sites in SrFeAsF are analyzed with Debye model. The results suggested that the expansion of FeAs layers plays an important role in determining the thermal expansion coefficient.Comment: 4 pages, 3 figure

Magnetic structure of EuFe2As2 determined by single crystal neutron diffraction

Among various parent compounds of iron pnictide superconductors, EuFe2As2 stands out due to the presence of both spin density wave of Fe and antiferromagnetic ordering (AFM) of the localized Eu2+ moment. Single crystal neutron diffraction studies have been carried out to determine the magnetic structure of this compound and to investigate the coupling of two magnetic sublattices. Long range AFM ordering of Fe and Eu spins was observed below 190 K and 19 K, respectively. The ordering of Fe2+ moments is associated with the wave vector k = (1,0,1) and it takes place at the same temperature as the tetragonal to orthorhombic structural phase transition, which indicates the strong coupling between structural and magnetic components. The ordering of Eu moment is associated with the wave vector k = (0,0,1). While both Fe and Eu spins are aligned along the long a axis as experimentally determined, our studies suggest a weak coupling between the Fe and Eu magnetism.Comment: 7 pages, 7 figure

Electronic structures of free-standing nanowires made from indirect bandgap semiconductor gallium phosphide

We present a theoretical study of the electronic structures of freestanding nanowires made from gallium phosphide (GaP)--a III-V semiconductor with an indirect bulk bandgap. We consider [001]-oriented GaP nanowires with square and rectangular cross sections, and [111]-oriented GaP nanowires with hexagonal cross sections. Based on tight binding models, both the band structures and wave functions of the nanowires are calculated. For the [001]-oriented GaP nanowires, the bands show anti-crossing structures, while the bands of the [111]-oriented nanowires display crossing structures. Two minima are observed in the conduction bands, while the maximum of the valence bands is always at the $\Gamma$-point. Using double group theory, we analyze the symmetry properties of the lowest conduction band states and highest valence band states of GaP nanowires with different sizes and directions. The band state wave functions of the lowest conduction bands and the highest valence bands of the nanowires are evaluated by spatial probability distributions. For practical use, we fit the confinement energies of the electrons and holes in the nanowires to obtain an empirical formula.Comment: 19 pages, 10 figure

“I Don’t Mind Being Ugly but I Don’t Wanna Have Skin Cancer”: A Qualitative Study of Attitudes to UV Exposure and a Facial Morphing Intervention in Men 35 Years and Older

Objectives: Skin cancer is a growing problem globally. Older men have been largely ignored in previous research, even though men may generally be prone to riskier exposure to the sun than women. Past research suggests that appearance-focused techniques such as facial morphing can increase motivations to use sun protection among women of all ages, and younger men. Design: was qualitative, consisting of individual interviews. Method: was semi-structured interviews with 25 older men, to examine attitudes to UV exposure and reactions to a facial morphing intervention. Interviews were subjected to thematic analysis. Results: Three themes were constructed: masculine UV exposure; appearance that’s accepted; personal responsibility. The men did not identify gaps in their sun protection behaviour, and demonstrated a lack of concern about ageing and appearance. These attitudes translated into a lack of motivation for behaviour change due to ageing acceptance. However, motivation to change behaviour came from health concerns resulting from the intervention. Conclusions: The men experienced different motivations and barriers for sun protection use than women of similar ages. Facial morphing can be effective with older men, but may need to be reframed to focus on health implications and personal choice

Rubber friction: role of the flash temperature

When a rubber block is sliding on a hard rough substrate, the substrate asperities will exert time-dependent deformations of the rubber surface resulting in viscoelastic energy dissipation in the rubber, which gives a contribution to the sliding friction. Most surfaces of solids have roughness on many different length scales, and when calculating the friction force it is necessary to include the viscoelastic deformations on all length scales. The energy dissipation will result in local heating of the rubber. Since the viscoelastic properties of rubber-like materials are extremely strongly temperature dependent, it is necessary to include the local temperature increase in the analysis. At very low sliding velocity the temperature increase is negligible because of heat diffusion, but already for velocities of order 0.01 m/s the local heating may be very important. Here I study the influence of the local heating on the rubber friction, and I show that in a typical case the temperature increase results in a decrease in rubber friction with increasing sliding velocity for v > 0.01 m/s. This may result in stick-slip instabilities, and is of crucial importance in many practical applications, e.g., for the tire-road friction, and in particular for ABS-breaking systems.Comment: 22 pages, 27 figure