Effect of spin fluctuations on quasiparticle excitations: First-principles theory and application to sodium and lithium

Abstract

We present first-principles calculations for quasiparticle excitations in sodium and lithium including the effects of charge and spin fluctuations. We employ the Overhauser-Kukkonen form for the electron self energy arising from spin fluctuations and demonstrate that the coupling of electrons to spin fluctuations gives an important contribution to the quasiparticle lifetime, but does not significantly reduce the occupied bandwidth. Including correlation effects beyond the random-phase approximation in the screening from charge fluctuations yields good agreement with experiment. PACS numbers: 71.20.Dg, 71.10.Ca, 71.15.Qe, 71.45.Gm Introduction.-The coupling of electrons to spin fluctuations causes many fascinating phenomena: for example, it has been proposed that spin fluctuations can "glue" electrons together to form Cooper pairs giving rise to unconventional high-temperature superconductivity [1][2][3][4]. In particular, spin fluctuations were invoked to explain superconductivity in the cuprates [1][2][3] and recently also in the iron pnictide and chalcogenide materials [5][6][7]. In addition, it is well known that the coupling of spin fluctuations to electrons can affect the electronic effective mass and consequently transport properties and the specific heat.Theoretically, the effect of spin fluctuations on quasiparticle excitations is usually calculated using model Hamiltonians. Early studies [8-10] constructed empirical theories including spin fluctuations based on the homogeneous electron gas and simple tight-binding models. More recently, many empirical theories involving spin fluctuations were constructed to investigate superconductivity in the cuprates and pnictides. In these theories the spin susceptibility is either parametrized using experimental neutron scattering and nuclear magnetic resonance data [1,3,11] or estimated by combining density-functional theory (DFT) with interaction parameters (such as the Hubbard U ) adjusted to reproduce experimental findings [6,7].While the aforementioned theories have been very instructive, their applications have been limited by the availability of concrete experimental data needed to determine their input parameters, supporting the need for a fully first-principles theory without empirical parameters. There have been several attempts to compute the spin fluctuation-electron coupling from first principles. Notably, Winter and coworkers [12,13] calculated the spin susceptibility and the spin fluctuation-electron self energy from DFT and evaluated the correction to the specific heat for palladium and vanadium. Later studies [14-16] employed a first-principles T-matrix approach to calculate satellites in the photoemission spectrum of nickel

Spectral functions of the uniform electron gas via coupled-cluster theory and comparison to theGWand related approximations

“…1 Interestingly, the experimental result agrees quite well with the bandwidth from a LDA+GW xc calculation, 11,31 which contains vertex corrections for the dielectric function; however, including vertex corrections also in the self-energy increases the bandwidth again. [54][55][56] As an alternative explanation, Shung and Mahan 57,58 suggested that the measured bandwidth results from many-body effects in combination with final-state effects and an interference between surface and bulk photoemission.…”

“…G 0 W 0 ) calculations 2,3 where screening is treated in the random-phase approximation, as well as G 0 W xc calculations where screening is treated with the DFT-LDA dielectric function. 11,31 We also evaluate spectral functions using the GW-plus-cumulant (henceforth GW+C) method. This approximation yields the exact solution for a dispersionless core electron interacting with plasmons 32 and noticeably improves the description of plasmon satellite properties compared to GW, while retaining the accuracy of GW for the quasiparticle energies.…”

First-principles theory of electron-spin fluctuation coupling and superconducting instabilities in iron selenide

“…Hence, there is a need for a fully first-principles theory without empirical parameters. We have recently developed such an ab initio theory [15] for the spin fluctuation-electron coupling based on the work of Overhauser and coworkers on the homogeneous electron gas [16,17].…”

“…The effective spin-fluctuation mediated interaction is obtained using a recently developed first-principles formalism [15,19] based on the work of Overhauser et al for the homogeneous electron gas [16,17]. In this approach, the effective interaction is expressed as the sum of a bare Coulomb interaction, a contribution arising from charge fluctuations and a contribution arising from spin fluctuations, which is given by…”

Vertex Corrections for Positive-Definite Spectral Functions of Simple Metals

“…In fact, in going from continuous to discrete translational symmetry the functions simply turn into matrix functions [e.g., Σðk; ωÞ → Σ GG 0 ðk; ωÞ], something that does not pose any conceptual difficulties for Monte Carlo momentum integration [85]. Alkali metals for which a vertex function has been partially included (typically using a model exchange-correlation kernel [87,88]) are a logical next step for our method. It is conceivable that higher order satellites can be accurately described by the combination of our approach with the cumulant expansion scheme outlined in Refs.…”