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Register a new userElectronic state unfolding for plane waves: energy bands, Fermi surfaces and spectral functions
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Modern computing facilities grant access to first-principles density-functional theory study of complex physical and chemical phenomena in materials, that require large supercell to properly model the system. However, supercells are associated to small Brillouin zones in the reciprocal space, leading to folded electronic eigenstates that make the analysis and interpretation extremely challenging. Various techniques have been proposed and developed in order to reconstruct the electronic band structures of super cells, unfolded into the reciprocal space of an ideal primitive cell. Here, we propose an efficient unfolding scheme embedded directly in the Vienna Ab-initio Simulation Package (VASP), that requires modest computational resources and allows for an automatized mapping from the reciprocal space of the supercell to primitive cell Brillouin zone. This algorithm can computes band structures, Fermi surfaces and spectral functions, by using an integrated post-processing tool (bands4vasp). The method is here applied to a selected variety of complex physical situations: the effect of doping on the band dispersion in the BaFe$_{rm 2(1-x)}$Ru$_{rm 2x}$As$_2$ superconductor, the interaction between adsorbates and polaronic states on the TiO$_2$(110) surface, and the band splitting induced by non-collinear spin fluctuations in EuCd$_2$As$_2$.
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We present Quantum Unfolding, a Fortran90 program for unfolding first-principles electronic energy bands. It unfolds energy bands accurately by handling the Fourier components of Bloch wavefunctions, which are reconstructed from Wannier functions from Wannier90. Due to the wide application of Wannier90 package and the possibility of focusing only on the most important energy bands, the present code works very conveniently.
We present an all-Heusler architecture which could be used as a rational design scheme for achieving high spin-filtering efficiency in the current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices. A Co2MnSi/Ni2NiSi/Co2MnSi trilayer stack is chosen as the prototype of such an architecture, of which the electronic structure and magnetotransport properties are systematically investigated by first principles approaches. Almost perfectly matched energy bands and Fermi surfaces between the all-Heusler electrode-spacer pair are found, indicating large interfacial spin-asymmetry, high spin-injection efficiency, and consequently high GMR ratio. Transport calculations further confirms the superiority of the all-Heusler architecture over the conventional Heusler/transition-metal(TM) structure by comparing their transmission coefficients and interfacial resistances of parallel conduction electrons, as well as the macroscopic current-voltage (I-V) characteristics. We suggest future theoretical and experimental efforts in developing novel all-Heusler GMR junctions for the read heads of the next generation high-density hard disk drives (HDDs).
We propose a novel periodicity-free unfolding method of the electronic energy spectra. Our new method solves a serious problem that calculated electronic band structure strongly depends on the choice of the simulation cell, i.e., primitive-cell or supercell. The present method projects the electronic states onto the free-electron states, giving rise to the {it plane-wave unfolded} spectra. Using the method, the energy spectra can be calculated as a completely independent quantity from the choice of the simulation cell. We have examined the unfolded energy spectra in detail for three models and clarified the validity of our method: One-dimensional interacting two chain model, monolayer graphene, and twisted bilayer graphene. Furthermore, we have discussed that our present method is directly related to the experimental ARPES (Angle-Resolved Photo-Emission Spectroscopy) spectra.
We present an implementaion of interface between the full-potential linearized augmented plane wave package Wien2k and the wannier90 code for the construction of maximally localized Wannier functions. The FORTRAN code and a documentation is made available and results are discussed for SrVO$_3$, Sr$_2$IrO$_4$ (including spin-orbit coupling), LaFeAsO, and FeSb$_2$.
Minimal surfaces in Euclidean space provide examples of possible non-compact horizon geometries and topologies in asymptotically flat space-time. On the other hand, the existence of limiting surfaces in the space-time provides a simple mechanism for making these configurations compact. Limiting surfaces appear naturally in a given space-time by making minimal surfaces rotate but they are also inherent to plane wave or de Sitter space-times in which case minimal surfaces can be static and compact. We use the blackfold approach in order to scan for possible black hole horizon geometries and topologies in asymptotically flat, plane wave and de Sitter space-times. In the process we uncover several new configurations, such as black helicoids and catenoids, some of which have an asymptotically flat counterpart. In particular, we find that the ultraspinning regime of singly-spinning Myers-Perry black holes, described in terms of the simplest minimal surface (the plane), can be obtained as a limit of a black helicoid, suggesting that these two families of black holes are connected. We also show that minimal surfaces embedded in spheres rather than Euclidean space can be used to construct static compact horizons in asymptotically de Sitter space-times.
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