Showing papers by "Ove Jepsen published in 2005"
TL;DR: It is illustrated that it is possible to obtain Wannier-like functions for only the occupied states in a metallic system by generating NMTOs for cesium, and the pressure-induced s-->d transition is visualize.
Abstract: Herein, we outline a method that is able to generate truly minimal basis sets that accurately describe either a group of bands, a band, or even just the occupied part of a band. These basis sets are the so-called NMTOs, muffin-tin orbitals of order N. For an isolated set of bands, symmetrical orthonormalization of the NMTOs yields a set of Wannier functions that are atom-centered and localized by construction. They are not necessarily maximally localized, but may be transformed into those Wannier functions. For bands that overlap others, Wannier-like functions can be generated. It is shown that NMTOs give a chemical understanding of an extended system. In particular, orbitals for the pi and sigma bands in an insulator, boron nitride, and a semimetal, graphite, will be considered. In addition, we illustrate that it is possible to obtain Wannier-like functions for only the occupied states in a metallic system by generating NMTOs for cesium. Finally, we visualize the pressure-induced s-->d transition.
42 citations
TL;DR: In this article, the authors examined the pressure dependence of superconductivity in the two metallic phases of boron (B), body-centered-tetragonal (bct) and fcc, and found that the transition from the fcc to bct phase, as a result of increasing volume or decreasing pressure, is caused by softening of the transverse phonons.
Abstract: Using the full-potential linear muffin-tin orbitals (FP-LMTO) method we examine the pressure dependence of superconductivity in the two metallic phases of boron (B), body-centered-tetragonal (bct) and fcc. Linear response calculations are carried out to examine the phonon frequencies and electron-phonon coupling for various lattice parameters, and superconducting transition temperatures are obtained from the isotropic Eliashberg equation. The fcc phase is found to be stable only at very high pressure $(\text{volume per atom}l21.3\phantom{\rule{0.3em}{0ex}}{\text{bohrs}}^{3})$, estimated to be in excess of 360 GPa. The bct phase $(\text{volume per atom}g21.3\phantom{\rule{0.3em}{0ex}}{\text{bohrs}}^{3})$ is stable at lower pressures in the range 210--360 GPa. In both bct and fcc phases the superconducting transition temperature ${T}_{c}$ is found to decrease with increasing pressure, due to the stiffening of phonons with an accompanying decrease in electron-phonon coupling. This is in contrast to a recent report, where ${T}_{c}$ is found to increase with pressure. Even more drastic is the difference between the measured ${T}_{c}$, in the range 4--11 K, and the calculated values for both bct and fcc phases, in the range 60--100 K. The calculation reveals that the transition from the fcc to bct phase, as a result of increasing volume or decreasing pressure, is caused by the softening of the $X$-point transverse phonons. This phonon softening also causes large electron-phonon coupling for high volumes in the fcc phase, resulting in coupling constants in excess of 2.5 and ${T}_{c}$ nearing 100 K. Although it is possible that the method used somewhat overestimates the electron-phonon coupling, its success in studying several other systems, including ${\mathrm{MgB}}_{2}$, clearly suggests that the experimental work should be reinvestigated. We discuss possible causes as to why the experiment might have revealed ${T}_{c}$'s much lower than what is suggested by the present study. The main assertion of this paper is that the possibility of high ${T}_{c}$, in excess of 50 K, in high pressure pure metallic phases of B cannot be ruled out, thus pointing to (substantiating) the need for further experimental investigations of the superconducting properties of high pressure pure phases of B.
22 citations
TL;DR: In this article, a reciprocal space formulation of the augmented space recursion is presented, which uses the lattice translation symmetry in the full augmented space to produce configuration averaged quantities, such as spectral functions and complex band structures.
Abstract: We present here a reciprocal space formulation of the Augmented space recursion (ASR) which uses the lattice translation symmetry in the full augmented space to produce configuration averaged quantities, such as spectral functions and complex band structures. Since the real space part is taken into account {\\sl exactly} and there is no truncation of this in the recursion, the results are more accurate than recursions in real space. We have also described the Brillouin zone integration procedure to obtain the configuration averaged density of states. We apply the technique to Ni$_{50}$Pt$_{50}$ alloy in conjunction with the tight-binding linearized muffin-tin orbital basis. These developments in the theoretical basis were necessitated by our future application to obtain optical conductivity in random systems.
11 citations
Journal Article•
1 citations
Posted Content•
TL;DR: In this paper, the Muffin Tin Orbitals of order N (NMTOs) are used to describe a group of bands, a band, or even the occupied part of a band accurately.
Abstract: Within this paper we outline a method able to generate truly minimal basis sets which describe either a group of bands, a band, or even just the occupied part of a band accurately. These basis sets are the so-called NMTOs, Muffin Tin Orbitals of order N. For an isolated set of bands, symmetrical orthonormalization of the NMTOs yields a set of Wannier functions which are atom-centered and localized by construction. They are not necessarily maximally localized, but may be transformed into those Wannier functions. For bands which overlap others, Wannier-like functions can be generated. It is shown that NMTOs give a chemical understanding of an extended system. In particular, orbitals for the pi and sigma bands in an insulator, boron nitride, and a semi-metal, graphite, will be considered. In addition, we illustrate that it is possible to obtain Wannier-like functions for only the occupied states in a metallic system by generating NMTOs for cesium. Finally, we visualize the pressure-induced s to d transition.
TL;DR: In this article, the NMTO method was generalized to arbitrary order in the energy expansion of the partial wave and discrete energy meshes, with controlled errors and without increasing the size of the basis set.
Abstract: The muffin-tin orbital (MTO) method has been generalized to arbitrary $(N\text{th})$ order in the energy expansion of the partial waves and discrete energy meshes. This so-called NMTO method can provide energies and wave functions in a broad energy window, with controlled errors and without increasing the size of the basis set. Here we present the fully relativistic version of the NMTO method. Several tests of the applicability of the method are provided for both nonmagnetic and magnetic solids.