Program LMTART for Electronic Structure Calculations
A computer program LMTART for electronic structure calculations using full potential linear muffin-tin orbital method is described
cond-mat.mtrl-sci↗
arXiv subjects
S. Y. Savrasov
Publications and source records attributed to S. Y. Savrasov.
Calculations of Optical Properties in Strongly Correlated Materials
We present a new method to calculate optical properties of strongly correlated systems. It is based on dynamical mean field theory and it uses as an input realistic electronic structure obtained by local density functional calculations. Numerically tractable equations for optical conductivity, which show a correct non-interacting limit, are derived. Illustration of the method is given by computing optical properties of the doped Mott insulator La_{1-x}Sr_xTiO_3.
cond-mat.str-el↗
Towards realistic electronic structure calculations of strongly correlated electron systems
We review some aspects of the realistic implementation of the dynamical mean-field method. We extend the techniques introduced in Ref.[A. Georges, G. Kotliar, W. Krauth, and M.J. Rozenberg, Rev. Mod. Phys. 68, 13 (1996)] to include the calculations of transport coefficients. The approach is illustrated on La_{1-x}Sr_xTiO_3 material undergoing a density driven Mott transition.
cond-mat.str-el↗
Electronic structure and magnetic anisotropy of CrO_2
The problem of importance of strong correlations for the electronic structure, transport and magnetic properties of half--metallic ferromagnetic CrO_2 is addressed by performing density functional electronic structure calculations in the local spin density approximation (LSDA) as well as using the LSDA+U method. It is shown that the corresponding low--temperature experimental data are best fitted without accounting for the Hubbard U corrections. We conclude that the ordered phase of CrO$_2 is weakly correlated.
cond-mat.str-el↗
Interpolative Approach for Solving Quantum Impurity Model Based on the Slave--Boson Mean--Field Approximation
A rational representation for the self-energy is explored to interpolate the solution of the Anderson impurity model in general orbitally degenerate case. Several constrains such as the Friedel's sum rule, high--frequency moments and the value of quasiparticle residue are used to establish the equations for the coefficients of the interpolation. We test two fast techniques, the slave--boson mean--field and the Hubbard I approximation to determine the coefficients. The obtained self--energies are compared with the results of numerically exact Quantum Monte Carlo method. We find that using the slave--boson mean--field approach we can construct an accurate self--energy for all frequencies via the proposed interpolation procedure.
cond-mat.str-el↗
Superconductivity near the vibrational mode instability in MgCNi3
To understand the role of electron-phonon interaction in superconducting MgCNi$_{3}$ we have performed density functional based linear response calculations of its lattice dynamical properties. A large coupling constant $% λ$= 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.
cond-mat.supr-con↗
Spectral Density Functionals for Electronic Structure Calculations
We introduce a spectral density functional theory which can be used to compute energetics and spectra of real strongly--correlated materials using methods, algorithms and computer programs of the electronic structure theory of solids. The approach considers the total free energy of a system as a functional of a local electronic Green function which is probed in the region of interest. Since we have a variety of notions of locality in our formulation, our method is manifestly basis--set dependent. However, it produces the exact total energy and local excitational spectrum provided that the exact functional is extremized. The self--energy of the theory appears as an auxiliary mass operator similar to the introduction of the ground--state Kohn--Sham potential in density functional theory. It is automatically short--ranged in the same region of Hilbert space which defines the local Green function. We exploit this property to find good approximations to the functional. For example, if electronic self--energy is known to be local in some portion of Hilbert space, a good approximation to the functional is provided by the corresponding local dynamical mean--field theory. A simplified implementation of the theory is described based on the linear muffin--tin orbital method widely used in electronic strucure calculations. We demonstrate the power of the approach on the long--standing problem of the anomalous volume expansion of metallic plutonium.
cond-mat.str-el↗
A density functional study of pressure induced superconductivity in P and its implication for spintronics
The stability of high-pressure phases of P has been studied using density functional theory and the local density approximation. Using a linear response technique, we have calculated the phonon spectrum and electron-phonon interaction for bcc P and predict it to be superconducting with $T_c$ of 19 K. We propose that this phase might be realized in epitaxial thin films using templates such as V(100), Fe(100) or Cr(100) relevant to spintronics applications.
cond-mat.supr-con↗
Vibrational Modes in LiBC: Theory Compared with Experiment
The search for other superconductors in the MgB2 class currently is focussed on Li{1-x}BC, which when hole-doped (concentration x) should be a metal with the potential to be a better superconductor than MgB2. Here we present the calculated phonon spectrum of the parent semiconductor LiBC. The calculated Raman-active modes are in excellent agreement with a recent observation, and comparison of calculated IR-active modes with a recent report provides a prediction of the LO--TO splitting for these four modes, which is small for the B-C bond stretching mode at ~1200 cm^{-1}, but large for clearly resolved modes at 540 cm^{-1} and 620 cm^{-1}.
cond-mat.supr-con↗
Extreme Electron-Phonon Coupling in Boron-based Layered Superconductors
The phonon-mode decomposition of the electron-phonon coupling in the MgB2-like system Li_{1-x}BC is explored using first principles calculations. It is found that the high temperature superconductivity of such systems results from extremely strong coupling to only ~2% of the phonon modes. Novel characteristics of E_2g branches include (1) ``mode lambda'' values of 25 and greater compared to a mean of $\sim 0.4$ for other modes, (2) a precipitous Kohn anomaly, and (3) E_2g phonon linewidths within a factor of ~2 of the frequency itself, indicating impending breakdown of linear electron-phonon theory. This behavior in borne out by recent inelastic x-ray scattering studies of MgB2 by Shukla et al.
cond-mat.supr-con↗
Correlation Effects on Magnetic Anisotropy in Fe and Ni
We calculate magnetic anisotropy energy of Fe and Ni by taking into account the effects of strong electronic correlations, spin-orbit coupling, and non-collinearity of intra-atomic magnetization. The LDA+U method is used and its equivalence to dynamical mean-field theory in the static limit is derived. The effects of strong correlations are studied along several paths in $(U,J)$ parameter space. Both experimental magnitude of MAE and direction of magnetization are predicted correctly near $U=1.9 eV$, $J=1.2 eV$ for Ni and $U=1.2 eV$, $J=0.8 eV$ for Fe. The modified one-electron spectra by strong correlations are emphasized in conjunction with magnetic anisotropy.
cond-mat.mtrl-sci↗
Dynamical Mean Field Theory, Model Hamiltonians and First Principles Electronic Structure Calculations
We review the basic ideas of the dynamical mean field theory (DMFT) and some of the insights into the electronic structure of strongly correlated electrons obtained by this method in the context of model Hamiltonians. We then discuss the perspectives for carrying out more realistic DMFT studies of strongly correlated electron systems and we compare it with existent methods, LDA and LDA+U. We stress the existence of new functionals for electronic structure calculations which allow us to treat situations where the single--particle description breaks down such as the vicinity of the Mott transition.
cond-mat.str-el↗
Linear Response Calculations of Lattice Dynamics in Strongly Correlated Systems
We introduce a new linear response method to study the lattice dynamics of materials with strong correlations. It is based on a combination of dynamical mean field theory of strongly correlated electrons and the local density functional theory of electronic structure of solids. We apply the method to study the phonon dispersions of a prototype Mott insulator NiO. Our results show overall much better agreement with experiment than the corresponding local density predictions.
cond-mat.str-el↗
Metal-insulator and magnetic collapse transitions in FeO at high pressure: predictions of the LDA+U method
The accuracy of LDA + U total energies is tested by predicting the high pressure behavior of rocksalt-structured FeO, including metal-insulator and magnetic transitions, equation of state, and lattice strain, as functions of the Coulomb repulsion U. For U = 4.6 eV, 6.0 eV and 8.0 eV, two self-consistent solutions, one with rhombohedral and one with monoclinic electronic symmetry, are found, and for U = 4.6 eV the monoclinic solution becomes more stable than the rhombohedral solution at 100 GPa, leading to an insulator-metal transition. With increasing U, metallization occurs at higher pressures. Despite the insulator-metal transition, the high-spin magnetic state persists to pressures greater than 300 GPa for U > 0. The LDA + U method also gives improved agreement with experiments for ground state properties as compared to LDA and GGA methods.
cond-mat.str-el↗
Spectral Density Functionals for Electronic Structure Calculations
We introduce a functional of the local spectral electron density which can be used to to compute the total energy and the local spectral function of strongly-correlated materials. We illustrate the applicability of the method by using as an example the long-standing problem of the electronic structure of metallic plutonium.
cond-mat.str-el↗
Correlation effects in the ground state charge density of Mott-insulating NiO: a comparison of ab-initio calculations and high-energy electron diffraction measurements
Accurate high-energy electron diffraction measurements of structure factors of NiO have been carried out to investigate how strong correlations in the Ni 3d shell affect electron charge density in the interior area of nickel ions and whether the new ab-initio approaches to the electronic structure of strongly correlated metal oxides are in accord with experimental observations. The generalized gradient approximation (GGA) and the local spin density approximation corrected by the Hubbard U term (LSDA+U) are found to provide the closest match to experimental measurements. The comparison of calculated and observed electron charge densities shows that correlations in the Ni 3d shell suppress covalent bonding between the oxygen and nickel sublattices.
cond-mat.str-el↗
Linear Response Calculations of Spin Fluctuations
A variational formulation of the time--dependent linear response based on the Sternheimer method is developed in order to make practical ab initio calculations of dynamical spin susceptibilities of solids. Using gradient density functional and a muffin-tin-orbital representation, the efficiency of the approach is demonstrated by applications to selected magnetic and strongly paramagnetic metals. The results are found to be consistent with experiment and are compared with previous theoretical calculations.
cond-mat.str-el↗
Is doped BaBiO$_3$ a conventional superconductor?
We report density functional calculations based on local density approximation (LDA) of the properties of doped barium bismuthates. Using linear-response approach developed in the framework of the linear muffin-tin-orbital method the phonon spectrum of the Ba$_{0.6}$K$_{0.4}$BiO$% _3$ system is calculated and is compared with the results of the neutron diffraction measurements. The effect of doping in the calculation is modelled by the virtual crystal and mass approximations. The electron-phonon coupling constant $λ$ is then evaluated for a grid of phonon wave-vectors using the change in the potential due to phonon distortion found self-consistently. A large coupling of the electrons to the bond-stretching oxygen vibrations and especially to the breathing-like vibrations is established. Also, a strongly anharmonic potential well is found for tilting-like motions of the oxygen octahedra. This mode is not coupled to the electrons to linear order in the displacements, therefore an anharmonic contribution to $λ$ is estimated using frozen-phonon method. Our total (harmonic plus anharmonic) $λ$ is found to be 0.34. This is too small to explain high-temperature superconductivity in Ba$_{0.6}$K$_{0.4}$BiO% $_3$ within the standard mechanism. Finally, based on standard LDA and LDA+U like calculations, a number of properties of pure BaBiO$_3$ such as tilting of the octahedra, breathing distortion, charge disproportionation and semiconducting energy gap value is evaluated and discussed in connection with the negative $U$ extended Hubbard model frequently applied to this compound.
cond-mat.supr-con↗