Fermi surfaces and Phase Stability of Ba(Fe1−x_{1-x}Mx_x)2_2As2_2 (M=Co, Ni, Cu, Zn)

BaFe$_2$As$_2$ with transition-metal doping exhibits a variety of rich phenomenon from coupling of structure, magnetism, and superconductivity. Using density functional theory, we systematically compare the Fermi surfaces (FS), formation energies ($\Delta E_f$), and density of states (DOS) of electron-doped Ba(Fe$_{1-x}$M$_x$)$_2$As$_2$ with M={Co, Ni, Cu, Zn} in tetragonal (I$4/mmm$) and orthorhombic (F$mmm$) structures in nonmagnetic (NM), antiferromagnetic (AFM), and paramagnetic (PM, disordered local moment) states. We explain changes to phase stability ($\Delta E_f$) and Fermi surfaces (and nesting) due to chemical and magnetic disorder, and compare to observed/assessed properties and contrast alloy theory with that expected from rigid-band model. With alloying, the DOS changes from common-band (Co,Ni) to split-band (Cu,Zn), which dictates $\Delta E_f$ and can overwhelm FS-nesting instabilities, as for Cu,Zn cases

Electronic structure and energetics of Fe-based superconductors: Effects of chemical and magnetic disorder in (Ba-X)(Fe-Y)2As2

The Fe-based superconductors (Fe-SCs) have reinvigorated the community of high-Tc researchers worldwide. Like copper-oxide superconductors (Cu-SCs), they are layered compounds in proximity to an antiferromagnetic (AFM) state. The Fe-SC systems are amenable to controlled chemical doping/alloying, exhibiting coexistence of AFM and SC, as well as structural transformations versus temperature and pressure that lead to competing magnetic and structural defects. Because of this, exploration of the electronic structure of the normal (non-SC) state, and its related properties, may reveal key physics underlying connections between structure, magnetism and SC. In this thesis we study the phase stability, electronic structure, and magnetism of alloyed (Ba-Am)(Fe-Tm)2As2 superconductors involving transition metals (Tm=Co, Ni, Cu, Zn) and alkali metals (Am = K, Na) in nonmagnetic, paramagnetic, and antiferromagnetic states for the competing tetragonal and orthorhombic structures. These cover prominent electron (Tm) and hole (Am) doping scenarios studied experimentally. To accomplish this in a unified way, we utilize a Green's function approach based upon the all-electron, Korringa-Kohn-Rostoker (KKR) multiple scattering theory in combination with the coherent-potential approximation (CPA) to handle chemical (alloying) and magnetic (orientational) disorder, all implemented within a self-consistent-field, density functional theory (DFT). For $Tm$ doping, we detail the Fermi-surface evolution and nesting that dictate instabilities to the observed spin-density wave (SDW) state. For Am doping we track topological changes in the Fermi surface and connect these to transitions between SC phases. For K-doping, dissolution of electron cylinders occurs near 90%K with a Lifshitz (topological) transition, as observed, which reduces key inter-band interactions. This result reveals a transition that influences s+/- to d-like SC and suggests the origin for the deviations for the empirically identified Bud'ko-Ni-Canfield scaling. Formation energies indicate alloying at 35%K, as observed, but a tendency for segregation on the K-rich (>= 60%K) side, explaining the difficulty of controlling sample quality and conflicting results between characterized electronic structures. In addition, due to the observed proliferation of twins and magnetic twin boundaries in BaFe2As2 (and possible other operative magnetic planar defects) with temperature and pressure, we study the stability and magnetic properties of competing antiphase and domain boundaries, twins and isolated $nano$twins (twin nuclei). These nanoscale defects have very low surface energy (22-210 m Jm^-2), with twins favorable to the mesoscale. The nano-twins explain features in measured pair distribution functions obtained from neutron diffraction. Notably, these low-energy defects are tied to the magneto-structural transition whose fluctuations are widely expected to drive SC

Green's function multiple-scattering theory with a truncated basis set: An Augmented-KKR formalism

Korringa-Kohn-Rostoker (KKR) Green's function, multiple-scattering theory is an efficient site-centered, electronic-structure technique for addressing an assembly of $N$ scatterers. Wave-functions are expanded in a spherical-wave basis on each scattering center and indexed up to a maximum orbital and azimuthal number $L_{max}=(l,m)_{max}$, while scattering matrices, which determine spectral properties, are truncated at $L_{tr}=(l,m)_{tr}$ where phase shifts $\delta_{l>l_{tr}}$ are negligible. Historically, $L_{max}$ is set equal to $L_{tr}$; however, a more proper procedure retains free-electron and single-site contributions for $L_{max}>L_{tr}$ with $\delta_{l>l_{tr}}$ set to zero [Zhang and Butler, Phys. Rev. B {\bf 46}, 7433]. We present a numerically efficient and accurate \emph{augmented}-KKR Green's function formalism that solves the KKR secular equations by matrix inversion [$\mathcal{R}^3$ process with rank $N(l_{tr}+1)^2$] and includes higher-order $L$ contributions via linear algebra [$\mathcal{R}^2$ process with rank $N(l_{max}+1)^2$]. Augmented-KKR yields properly normalized wave-functions, numerically cheaper basis-set convergence, and a total charge density and electron count that agrees with Lloyd's formula. For fcc Cu, bcc Fe and L$1_0$ CoPt, we present the formalism and numerical results for accuracy and for the convergence of the total energies, Fermi energies, and magnetic moments versus $L_{max}$ for a given $L_{tr}$.Comment: 7 pages, 5 figure

Discovering Lexical Similarity Using Articulatory Feature-Based Phonetic Edit Distance

Lexical Similarity (LS) between two languages uncovers many interesting linguistic insights such as phylogenetic relationship, mutual intelligibility, common etymology, and loan words. There are various methods through which LS is evaluated. This paper presents a method of Phonetic Edit Distance (PED) that uses a soft comparison of letters using the articulatory features associated with their International Phonetic Alphabet (IPA) transcription. In particular, the comparison between the articulatory features of two letters taken from words belonging to different languages is used to compute the cost of replacement in the inner loop of edit distance computation. As an example, PED gives edit distance of 0.82 between German word ‘vater’ ([fa:tər]) and Persian word ‘ ’ ([pedær]), meaning ‘father,’ and, similarly, PED of 0.93 between Hebrew word ‘ ’ ([ʃəɭam]) and Arabic word ‘ ’ ([səɭa:m], meaning ‘peace,’ whereas classical edit distances would be 4 and 2, respectively. We report the results of systematic experiments conducted on six languages: Arabic, Hindi, Marathi, Persian, Sanskrit, and Urdu. Universal Dependencies (UD) corpora were used to restrict the comparison to lists of words belonging to the same part of speech. The LS based on the average PED between pair of words was then computed for each pair of languages, unveiling similarities otherwise masked by the adoption of different alphabets, grammars, and pronunciations rules

Effects of transition metal substitutions on the incommensurability and spin fluctuations in BaFe2As2 by elastic and inelastic neutron scattering

The spin fluctuation spectra from nonsuperconducting Cu-substituted, and superconducting Co-substituted, BaFe2As2 are compared quantitatively by inelastic neutron scattering measurements and are found to be indis- tinguishable. Whereas diffraction studies show the appearance of incommensurate spin-density wave order in Co and Ni substituted samples, the magnetic phase diagram for Cu substitution does not display incommensu- rate order, demonstrating that simple electron counting based on rigid-band concepts is invalid. These results, supported by theoretical calculations, suggest that substitutional impurity effects in the Fe plane play a signifi- cant role in controlling magnetism and the appearance of superconductivity, with Cu distinguished by enhanced impurity scattering and split-band behavior.Comment: 5 pages, 5 figures, Major change in the manuscrip