Using first principles calculations, we show that $$\hbox {Li}_x\hbox {Al}_y\hbox {B}_{2(x+y)}$$ materials have strong electron-phonon coupling, with many having a superconducting critical temperature ( $$T_c$$ ) that exceeds that of the more familiar $$\hbox {MgB}_2$$ at ambient pressure. In particular, we find that $$\hbox {LiAlB}_4$$ is the most stable member of the family, with $$T_c > 44\,\hbox {K}$$ whilst the peak $$T_c$$ is with $$\hbox {Li}_3\hbox {AlB}_8$$ which has $$T_c > 77\,\hbox {K}$$ . Our results reveal that these materials are both thermodynamically and dynamically stable, with strong electron-phonon coupling, indicating significant potential for practical superconducting applications.

We present a methodology based on the calculation of the inelastic scattering from magnons via the spin scattering function in confined geometries such as thin films using a second quantization formalism, for both ferromagnetic and antiferromagnetic materials. The case studies are chosen with an aim to demonstrate the effects of film thickness and crystal orientation on magnon modes, using bcc Fe(100) and NiO with (100) and (111) crystallographic orientations as prototypical systems. Due to the quantization of the quasi-momentum we observe a granularity in the inelastic spectra in the reciprocal space path reflecting the orientation of the thin film. This approach also allows to capture softer modes that appear due to the partial interaction of magnetic moments close to the surface in a thin film geometry, in addition to bulk modes. The softer modes are also affected by crystallographic orientations as illustrated by the different surface-related peaks of NiO magnon density of states at approximately 65 meV for (100) and 42 meV for (111). Additionally, we explore the role of anisotropy on magnon modes, revealing that introducing anisotropy to both Fe and NiO films increases the overall hardness of the magnon modes. The introduction of a surface anisotropy produces a shift of the surface-related magnon DOS peak to higher energies with increased surface anisotropy, and in some cases leading to surface confined mode.

We explore the inelastic spectra of electrons impinging in a magnetic system. The methodology here presented is intended to highlight the charge-dependent interaction of the electron beam in a STEM-EELS experiment, and the local vector potential generated by the magnetic lattice. This interaction shows an intensity $10^{-2}$ smaller than the purely spin interaction, which is taken to be functionally the same as in the inelastic neutron experiment. On the other hand, it shows a strong scattering vector dependence ($\kappa^{-4}$) and a dependence with the relative orientation between the probe wavevector and the local magnetic moments of the solid. We present YIG as a case study due to its high interest by the community.

Abstract We have developed a high-throughput computational method to predict the superconducting transition temperature in stable hexagonal M2AX phases and applied it to all the known possible choices for M (M: Sc, Ti, V, Cr, Mn, Fe, Y, Zr, Nb, Mo, Lu, Hf and Ta). We combine this with the best candidates for A (A: Al, Cu, Ge and Sn) and X (X: C and N) from our previous work, and predict Tc for 60 M2AX-phase materials, 53 of which have never been studied before. From all of these, we identify Cr2AlN as the best candidate for the highest Tc, and confirm its high Tc with more detailed density functional theory electron-phonon coupling calculations. Our detailed calculations predict Tc = 14.8 K for Cr2AlN, which is significantly higher than any Tc value known or predicted for any material in the M2AX family to date.

We have developed a high-throughput computational method to predict the superconducting transition temperature in stable hexagonal M(2)AX phases, and applied it to all the known possible choices for M (M: Sc, Ti, V, Cr, Mn, Fe, Y, Zr, Nb, Mo, Lu, Hf and Ta). We combine this with the best candidates for A (A: Al, Cu, Ge and Sn ) and X (X: C and N) from our previous work, and predict Tc for 60 M2AX-phase materials, 53 of which have never been studied before. From all of these, we identify Cr2AlN as the best candidate for the highest T-c, and confirm its high T (c) with more detailed density functional theory electron-phonon coupling calculations. Our detailed calculations predict Tc = 14.8 K for Cr2AlN, which is significantly higher than any T-c value known or predicted for any material in the M2AX family to date.

We have developed a high-throughput computational method to predict the superconducting transition temperature in stable hexagonal M $$_2$$ AX phases, and applied it to all the known possible choices for M (M: Sc, Ti, V, Cr, Mn, Fe, Y, Zr, Nb, Mo, Lu, Hf and Ta). We combine this with the best candidates for A (A: Al, Cu, Ge and Sn ) and X (X: C and N) from our previous work, and predict T $$_c$$ for 60 M $$_2$$ AX-phase materials, 53 of which have never been studied before. From all of these, we identify Cr $$_2$$ AlN as the best candidate for the highest T $$_c$$ , and confirm its high T $$_c$$ with more detailed density functional theory electron-phonon coupling calculations. Our detailed calculations predict $$T_c$$ = 14.8 K for Cr $$_2$$ AlN, which is significantly higher than any $$T_c$$ value known or predicted for any material in the M $$_2$$ AX family to date.

Thermoelectric materials have the potential to convert waste heat into electricity, but their thermoelectric efficiency must be improved before they are effective and economically viable. One promising route to improving thermoelectric efficiency in thin-film thermoelectric materials is to reduce the material’s thermal conductivity through nanopatterning the surface. In this work nanoscale phononic resonators are introduced to the surface, and their potential to reduce thermal conductivity is explored via coupled experimental and theoretical techniques. Atomistic modelling is used to predict the dependence of the thermal conductivity on different design parameters and used to guide the design and fabrication of silicon fishbone nanostructures. The nanostructure design incorporates a variation on design parameters such as barb length, width and spacing along the shaft length to enable correlation with changes in thermal conductivity. The thermal characteristics of the nanostructures are investigated experimentally using the spatial resolution of scanning thermal microscopy to correlate changes in thermal conductivity with the changes in the structure parameters. The method developed uses a microheater to establish a temperature gradient along the structure which will be affected by any local variations in thermal conductivity. The impact on the thermal gradient and consequently on the tip temperature is modelled using finite element computer simulations. Experimental changes as small as 7.5% are shown to be detectable in this way. Despite the experimental technique being shown to be able to detect thermal changes far smaller than those predicted by the modelling, no modifications of the thermal conductivity are detected. It is concluded that in order to realise the effects of phononic resonators to reduce thermal conductivity, that much smaller structures with a greater ratio of resonator to shaft will be needed.

In this work we present first principles study of the effect of stoichiometric pairs of antisite defects, V occupying Al site (V-Al) and Al occupying V site (Al-V), on the electronic structure and Seebeck coefficient of the Fe(2)VAlHeusler alloy. We show that introduction of these defects opens the bandgap of Fe2VAl, changing it from semi-metal to semiconductor, which results in an increase of the Seebeck coefficient for a range of doping concentrations and temperatures. We calculated Seebeck coefficients at different doping concentrations and temperatures shows good agreement with experimental data.

1 SuperSTEM Laboratory, Sci-Tech Daresbury, Keckwick Lane, Daresbury WA4 4AD, UK 2 Scientific Computing Department, STFC Daresbury Laboratory, Keckwick Lane, Daresbury WA4 4AD, UK 3 York NanoCentre & Department of Physics, University of York, York YO10 5DD, UK. 4 Faculty of Technical Sciences, University Mother Theresa, Mirche Acev No. 4, Skopje 1000, Macedonia 5 Physics Department, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA. 6 School of Physics and Astronomy & School of Chemical and Process Engineering, University of Leeds, Leeds LS2 9JT, UK. *Corresponding author: psharp@superstem.org

The transition to exascale computing will make possible simulations of unprecedented accuracy and complexity. We focus on materials and molecular modeling (MMM) aspiring to high fidelity, in silico experiments on complex systems of technological interest. This progress will present unprecedented challenges to the software used, especially how to exploit the huge degree of parallelism and the associated problems of creating effective workflows and data management on such platforms. Within the U.K.’s ExCALIBUR computing initiative, our U.K.-led MMM Design and Development Working Group has worked with the broad MMM community to identify high-priority applications that will drive future exascale software developments. We present an overview of selected case studies that pose new methodological challenges on exascale platforms and discuss the requirements, software challenges, and impact of each application area.

Abstract A high-throughput computational method is used to predict 39 new superconductors in the Ti-based M $$_2$$ 2 AX phases, and the best candidates are then studied in more detail using density functional theory electron–phonon coupling calculations. The detailed calculations agree with the simple predictions, and Ti $$_2$$ 2 AlX (X: B, C and N) materials are predicted to have higher values of $$T_c$$ T c than any currently known hexagonal M $$_2$$ 2 AX phases. The electronic states at the Fermi level are dominated by the Ti 3d states. The choice of X (X: B, C and N) has a significant impact on the electronic density of states but not on the phonon characteristics. The electron–phonon coupling parameter for Ti $$_2$$ 2 AlX (X: B, C and N) was determined to be 0.685, 0.743 and 0.775 with a predicted $$T_c$$ T c of 7.8 K, 10.8 K and 13.0 K, respectively.

In this article, we present work to port the CASTEP first-principles materials modeling program to accelerators using open accelerator (OpenACC). We discuss the challenges and opportunities presented by graphical processing units (GPU) architectures in particular, and the approach taken in the CASTEP OpenACC port. Whilst the port is still under active development, early performance results show that significant speed-ups may be gained, particularly for materials simulations using so-called “nonlocal functionals,” where speed-ups can exceed a factor of ten.

The superconducting transition temperatures T c of hexagonal Nb2 AC (A: Al, S, Ge, As and Sn) are investigated using density functional perturbation theory to model the electron–phonon interaction. A critical assessment of the calculated electronic structure and density of states revealed that the electronic states near to the Fermi level are mostly composed of the Nb 4d states, which are responsible for the electrical conductivity. The theoretical T c data from electron–phonon calculations are in excellent agreement with the Fröhlich model, and this model was used as a computationally efficient screening method to identify promising Nb–C M 2 AX phase materials. For Nb2 AC (A: Zn, Cd, Al, Ga, In, Tl, Si, Pb and P), the model indicated that Nb2AlC should have the highest T c of this set, a little lower than Nb2GeC and comparable to Nb2SC and Nb2SnC. Superconductivity in Nb2AlC has not been studied experimentally, but this result was confirmed by full electron–phonon calculations, which also revealed that the mechanism for superconductivity is the interactions of Nb 4d-state electrons with low-frequency phonons (in particular, acoustic phonon and low-frequency optical phonons dominated by Nb and the A element). The average electron–phonon coupling parameter was found to be λ ∼ 0.646, 0.739, 0.685, 0.440 and 0.614 for Nb2 AC (A: Al, S, Ge, As and Sn), respectively, with a corresponding superconducting critical temperature T c ∼ 6.7 K, 7.7 K, 9.8 K, 2.1 K and 6.3 K, respectively.

We present a structural and density-functional theory study of the interface of the quasi-twin-free grown three-dimensional topological insulator Bi2Te3 on Ge(111). Aberration-corrected scanning transmission electron microscopy and electron energy-loss spectroscopy in combination with first-principles calculations show that the weak van der Waals adhesion between the Bi2Te3 quintuple layer and Ge can be overcome by forming an additional Te layer at their interface. The first-principles calculations of the formation energy of the additional Te layer show it to be energetically favorable as a result of the strong hybridization between the Te and Ge.

Organic molecular crystals contain long-range dispersion interactions that can be challenging for solid-state methods such as density functional theory (DFT) to capture, and in some industrial sectors are overlooked in favor of classical methods to calculate atomistic properties. Hence, this publication addresses the critical question of whether dispersion corrected DFT calculations for organic crystals can reproduce the structural and energetic trends seen from experiment, i.e., whether the calculations can now be said to be truly on-trend. In this work, we assess the performance of three of the latest dispersion-corrected DFT methods, in calculating the long-range, dispersion energy: the pairwise methods of D3(0) and D3(BJ) and the many-body dispersion method, MBD@rsSCS. We calculate the energetics and optimized structures of two homologous series of organic molecular crystals, namely, carboxylic acids and amino acids. We also use a classical force field method (using COMPASS II) and compare all results to experimental data where possible. The mean absolute error in lattice energies is 9.59 and 343.85 kJ/mol (COMPASS II), 10.17 and 16.23 kJ/mol (MBD@rsSCS), 10.57 and 18.76 kJ/mol [D3(0)], and 8.52 and 14.66 kJ/mol [D3(BJ)] for the carboxylic acids and amino acids, respectively. MBD@rsSCS produces structural and energetic trends that most closely match experimental trends, performing the most consistently across the two series and competing favorably with COMPASS II.

Previous first-principles calculations have failed to reproduce many of the key thermoelectric features of Fe2VAl, e.g. the maximum values of the Seebeck coefficient S and its asymmetry with respect to the chemical potential. Also, previous theoretical predictions suggested that the pseudo band gap of Fe2VAl switches from indirect to direct upon doping. In this work, we report first-principles calculations that correctly reproduce the experimentally measured thermoelectric properties of Fe2VAl. This is achieved by adding a larger Hubbard U term to V atoms than to Fe atoms and including a scissors operator afterwards. As a result, bulk Fe2VAl is modelled as a gapless semiconductor with maximum S values of 76 and  −158 V K−1 for p - and n-type, respectively, which agree well with the experimental measurements.