Superconductivity near the vibrational mode instability in MgCNi3

To understand the role of electron-phonon interaction in superconducting MgCNi$_{3}$ we have performed density functional based linear response calculations of its lattice dynamical properties. A large coupling constant $\lambda$= 1.51 is predicted and contributing phonons are identified as displacements of Ni atoms towards octahedral interstitials of the perovskite lattice. Instabilities found for some vibrational modes emphasize the role of anharmonic effects in resolving experimental controversies.Comment: 4 pages, 4 eps figures, replaces the older versio

Dimensional trend in CePt2In7, Ce-115 compounds, and CeIn3

We present realistic Kondo-lattice simulation results for the recently-discovered heavy-fermion antiferromagnet CePt2In7 comparing with its three-dimensional counterpart CeIn3 and the less two-dimensional ones, Ce-115's. We find that the distance to the magnetic quantum critical point is the largest for CeIn3 and the smallest for Ce-115's, and CePt2In7 falls in between. We argue that the trend in quasi-two-dimensional materials stems from the frequency dependence of the hybridization between Cerium 4f-electrons and the conduction bands.Comment: 4.8 pages, 5 figure

Linear-response theory and lattice dynamics: a muffin-tin orbital approach

A detailed description of a method for calculating static linear-response functions in the problem of lattice dynamics is presented. The method is based on density functional theory and it uses linear muffin-tin orbitals as a basis for representing first-order corrections to the one-electron wave functions. As an application we calculate phonon dispersions in Si and NbC and find good agreement with experiments.Comment: 18 pages, Revtex, 2 ps figures, uuencoded, gzip'ed, tar'ed fil

Acoustical-Mode-Driven Electron-Phonon Coupling in Transition-Metal Diborides

We show that the electron-phonon coupling in the transition-metal diborides NbB2 and TaB2 is dominated by the longitudinal acoustical (LA) mode, in contrast to the optical E_{2g} mode dominated coupling in MgB2. Our ab initio results, described in terms of phonon dispersion, linewidth, and partial electron-phonon coupling along Gamma to A, also show that (i) NbB2 and TaB2 have a relatively weak electron-phonon coupling, (ii) the E_{2g} linewidth is an order of magnitude larger in MgB2 than in NbB2 or TaB2, (iii) the E_{2g} frequency in NbB2 and TaB2 is considerably higher than in MgB2, and (iv) the LA frequency at A for TaB2 is almost half of that of MgB2 or NbB2.Comment: 4 pages, 4 figures, and 1 tabl

Ab initio calculations of the physical properties of transition metal carbides and nitrides and possible routes to high-Tc

Ab initio linear-response calculations are reported of the phonon spectra and the electron-phonon interaction for several transition metal carbides and nitrides in a NaCl-type structure. For NbC, the kinetic, optical, and superconducting properties are calculated in detail at various pressures and the normal-pressure results are found to well agree with the experiment. Factors accounting for the relatively low critical temperatures Tc in transition metal compounds with light elements are considered and the possible ways of increasing Tc are discussed.Comment: 19 pages, 7 figure

First- principle calculations of magnetic interactions in correlated systems

We present a novel approach to calculate the effective exchange interaction parameters based on the realistic electronic structure of correlated magnetic crystals in local approach with the frequency dependent self energy. The analog of ``local force theorem'' in the density functional theory is proven for highly correlated systems. The expressions for effective exchange parameters, Dzialoshinskii- Moriya interaction, and magnetic anisotropy are derived. The first-principle calculations of magnetic excitation spectrum for ferromagnetic iron, with the local correlation effects from the numerically exact QMC-scheme is presented.Comment: 17 pages, 3 Postscript figure

Phonons and related properties of extended systems from density-functional perturbation theory

This article reviews the current status of lattice-dynamical calculations in crystals, using density-functional perturbation theory, with emphasis on the plane-wave pseudo-potential method. Several specialized topics are treated, including the implementation for metals, the calculation of the response to macroscopic electric fields and their relevance to long wave-length vibrations in polar materials, the response to strain deformations, and higher-order responses. The success of this methodology is demonstrated with a number of applications existing in the literature.Comment: 52 pages, 14 figures, submitted to Review of Modern Physic

Semiempirical Hartree-Fock calculations for KNbO3

In applying the semiempirical intermediate neglect of differential overlap (INDO) method based on the Hartree-Fock formalism to a cubic perovskite-based ferroelectric material KNbO3, it was demonstrated that the accuracy of the method is sufficient for adequately describing the small energy differences related to the ferroelectric instability. The choice of INDO parameters has been done for a system containing Nb. Based on the parametrization proposed, the electronic structure, equilibrium ground state structure of the orthorhombic and rhombohedral phases, and Gamma-TO phonon frequencies in cubic and rhombohedral phases of KNbO3 were calculated and found to be in good agreement with the experimental data and with the first-principles calculations available.Comment: 7 pages, 2 Postscript figures, uses psfig.tex. To be published in Phys.Rev.B 54, No.4 (1996