Deviations from Matthiessen’s rule and resistivity saturation effects in Gd and Fe from first principles

According to earlier first-principles calculations, the spin-disorder contribution to the resistivity of rare-earth metals in the paramagnetic state is strongly underestimated if Matthiessen’s rule is assumed to hold. To understand this discrepancy, the resistivity of paramagnetic Fe and Gd is evaluated by taking into account both spin and phonon disorder. Calculations are performed using the supercell approach within the linear muffin-tin orbital method. Phonon disorder is modeled by introducing random displacements of the atomic nuclei, and the results are compared with the case of fictitious Anderson disorder. In both cases, the resistivity shows a nonlinear dependence on the square of the disorder potential, which is interpreted as a resistivity saturation effect. This effect is much stronger in Gd than in Fe. The nonlinearity makes the phonon and spin-disorder contributions to the resistivity nonadditive, and the standard procedure of extracting the spin-disorder resistivity by extrapolation from high temperatures becomes ambiguous. An “apparent” spin-disorder resistivity obtained through such extrapolation is in much better agreement with experiment compared to the results obtained by considering only spin disorder. By analyzing the spectral function of the paramagnetic Gd in the presence of Anderson disorder, the resistivity saturation is explained by the collapse of a large area of the Fermi surface due to the disorder-induced mixing between the electron and hole sheets

On the relative abundances of Cavansite and Pentagonite

Cavansite is a visually stunning blue vanadosilicate mineral with limited occurrences worldwide, whereas Pentagonite is a closely related dimorph with similar physical and chemical properties, yet is extremely rare. The reasons behind Pentagonite's exceptional rarity remain largely unknown. In this study, we utilize density functional theory (DFT) to investigate the electronic structures of Cavansite and Pentagonite at ground state and finite pressures. We then employ the Boltzmann probability model to construct a comprehensive phase diagram that reveals the abundance of each species across a wide range of pressure and temperature conditions. Our analysis reveals the key factors that contribute to the relative scarcity of Pentagonite, including differences in structural arrangement and electronic configurations between the two minerals. Specifically, because of the peculiar arrangements of SiO4 polyhedra, Cavansite forms a compact structure (about 2.7% less in volume) resulting in lower energy. We also show that at a temperature of about 600K only about 1% Pentagonite can be formed. This probability is only slightly enhanced within a pressure range of up to about 3GPa. We also find that vanadium induces a highly localized state in both of these otherwise large band gap insulators resulting in extremely weak magnetic phase that is unlikely to be observed at any reasonable finite temperature. Finally, our dehydration studies reveal that water molecules are loosely bound inside the microporous crystals of Cavansite and Pentagonite, suggesting potential applications of these minerals in various technological fields

Density functional investigations of defect induced mid-gap states in graphane

We have carried out ab initio electronic structure calculations on graphane (hydrogenated graphene) with single and double vacancy defects. Our analysis of the density of states reveal that such vacancies induce the mid gap states and modify the band gap. The induced states are due to the unpaired electrons on carbon atoms. Interestingly the placement and the number of such states is found to be sensitive to the distance between the vacancies. Furthermore we also found that in most of the cases the vacancies induce a local magnetic moment.Comment: 15 page

Exploiting Grids for Applications in Condensed Matter Physics

Abstract Grids -the collection of heterogeneous computers spread across the globe -present a new paradigm for the large scale problems in variety of fields. We discuss two representative cases in the area of condensed matter physics outlining the widespread applications of the Grids. Both the problems involve calculations based on commonly used Density Functional Theory and hence can be considered to be of general interest. We demonstrate the suitability of Grids for the problems discussed and provide a general algorithm to implement and manage such large scale problems

Efficient treatment of solvation shells in 3D molecular theory of solvation

We developed a technique to decrease memory requirements when solving the integral equations of three-dimensional (3D) molecular theory of solvation, a.k.a. 3D reference interaction site model (3D-RISM), using the modified direct inversion in the iterative subspace (MDIIS) numerical method of generalized minimal residual type. The latter provides robust convergence, in particular, for charged systems and electrolyte solutions with strong associative effects for which damped iterations do not converge. The MDIIS solver (typically, with 2 7 10 iterative vectors of argument and residual for fast convergence) treats the solute excluded volume (core), while handling the solvation shells in the 3D box with two vectors coupled with MDIIS iteratively and incorporating the electrostatic asymptotics outside the box analytically. For solvated systems from small to large macromolecules and solid\u2013liquid interfaces, this results in 6- to 16-fold memory reduction and corresponding CPU load decrease in MDIIS. We illustrated the new technique on solvated systems of chemical and biomolecular relevance with different dimensionality, both in ambient water and aqueous electrolyte solution, by solving the 3D-RISM equations with the Kovalenko\u2013Hirata (KH) closure, and the hypernetted chain (HNC) closure where convergent. This core\u2013shell-asymptotics technique coupling MDIIS for the excluded volume core with iteration of the solvation shells converges as efficiently as MDIIS for the whole 3D box and yields the solvation structure and thermodynamics without loss of accuracy. Although being of benefit for solutes of any size, this memory reduction becomes critical in 3D-RISM calculations for large solvated systems, such as macromolecules in solution with ions, ligands, and other cofactors.Peer reviewed: YesNRC publication: Ye

\u3ci\u3eAb Initio\u3c/i\u3e Construction of Magnetic Phase Diagrams in Alloys: The Case of Fe\u3csub\u3e1−x\u3c/sub\u3eMn\u3csub\u3ex\u3c/sub\u3ePt

A first-principles approach to the construction of concentration-temperature magnetic phase diagrams of metallic alloys is presented. The method employs self-consistent total energy calculations based on the coherent potential approximation for partially ordered and noncollinear magnetic states and is able to account for competing interactions and multiple magnetic phases. Application to the Fe1−xMnxPt “magnetic chameleon” system yields the sequence of magnetic phases at T=0 and the c − T magnetic phase diagram in good agreement with experiment, and a new low-temperature phase is predicted at the Mn-rich end. The importance of non-Heisenberg interactions for the description of the magnetic phase diagram is demonstrated