Quasiparticle Self-consistent GW Approximation Research Articles

Development of data-driven spd tight-binding models of Fe—parameterisation based on QSGW and DFT calculations including information about higher-order elastic constants

Quantum-mechanical (QM) simulations, thanks to their predictive power, can provide significant insights into the nature and dynamics of defects such as vacancies, dislocations and grain boundaries. These considerations are essential in the context of the development of reliable, inexpensive and environmentally friendly alloys. However, despite significant progress in computer performance, QM simulations of defects are still extremely time-consuming with ab-initio/non-parametric methods. The two-centre Slater–Koster (SK) tight-binding (TB) models can achieve significant computational efficiency and provide an interpretable picture of the electronic structure. In some cases, this makes TB a compelling alternative to models based on abstraction of the electronic structure, such as the embedded atom model. The biggest challenge in the implementation of the SK method is the estimation of the optimal and transferable parameters that are used to construct the Hamiltonian matrix. In this paper, we will present results of the development of a data-driven framework, following the classical approach of adjusting parameters in order to recreate properties that can be measured or estimated using ab-initio or non-parametric methods. Distinct features include incorporation of data from QSGW (quasi-particle self-consistent GW approximation) calculations, as well as consideration of higher-order elastic constants. Furthermore, we provide a description of the optimisation procedure, omitted in many publications, including the design stage. We also apply modern optimisation techniques that allow us to minimise constraints on the parameter space. In summary, this paper introduces some methodological improvements to the semi-empirical approach while addressing associated challenges and advantages.

Electronic Structure Correspondence of Singlet-Triplet Scale Separation in Strained Sr2RuO4

At a temperature of roughly 1 K, Sr2RuO4 undergoes a transition from a normal Fermi liquid to a superconducting phase. Even while the former is relatively simple and well understood, the superconducting state has not even been understood after 25 years of study. More recently, it has been found that critical temperatures can be enhanced by the application of uniaxial strain, up to a critical strain, after which it falls off. In this work, we take an “instability” approach and seek divergences in susceptibilities. This provides an unbiased way to distinguish tendencies to competing ground states. We show that in the unstrained compound, the singlet and triplet instabilities of the normal Fermi liquid phase are closely spaced. Under uniaxial strain, electrons residing on all orbitals contributing to the Fermiology become more coherent, while the electrons of the Ru-dxy character become heavier, and the electrons of the Ru-dxz,yz characters become lighter. In the process, Im χ(q,ω) increases rapidly around q = (0.3,0.3,0)2π/a and q = (0.5,0.25,0)2π/a, while it gets suppressed at all other commensurate vectors, in particular at q = 0, which is essential for spin-triplet superconductivity. We observe that the magnetic anisotropy under strain drops smoothly, which is concomitant with the increment in singlet instability. Thus, the triplet superconducting instability remains the lagging instability of the system, and the singlet instability enhances under strain, leading to a large energy-scale separation between these competing instabilities. However, since this happens even without spin-orbit coupling, we believe it is primarily the enhancement in the spin fluctuation glue around quasi-anti-ferromagnetic vectors that drives the Cooper pairing instead of the magnetic anisotropy. At large strain, an instability to a spin density wave overtakes the superconducting one. The analysis relies on a high-fidelity, ab initio description of the one-particle properties and two-particle susceptibilities, based on the quasiparticle self-consistent GW approximation augmented by dynamical mean field theory. This approach is described and its high fidelity confirmed by comparing to observed one- and two-particle properties.

Evening out the spin and charge parity to increase {T}_{c} in {{rm{Sr}}}_{2}{{rm{RuO}}}_{4}

Unconventional superconductivity in {{rm{Sr}}}_{2}{{rm{RuO}}}_{4} has been intensively studied for decades. However, the nature of pairing continues to be widely debated. Here we develop a detailed ab initio theory, coupling quasiparticle self-consistent GW approximation with dynamical mean field theory (DMFT), including both local and non-local correlations to address the subtle interplay among spin, charge and orbital degrees of freedom. We report that the superconducting instability has multiple triplet and singlet components. In the unstrained case the triplet eigenvalues are larger than the singlets. Under uniaxial strain, the triplet eigenvalues drop and the singlet components increase. This is concomitant with our observation of spin and charge fluctuations shifting closer to wave-vectors favoring singlet pairing. We identify a complex mechanism where charge fluctuations and spin fluctuations co-operate in the even-parity channel under strain leading to increment in critical temperature (Tc), thus proposing a novel mechanism for pushing the frontier of critical temperature (Tc) in unconventional ‘triplet’ superconductors.

Quasiparticle self-consistent GW band structures of Mg-IV-N2 compounds: The role of semicore d states

We present improved band structure calculations of the Mg-IV-N2 compounds in the quasiparticle self-consistent GW approximation. Compared to previous calculations (Phys. Rev. B 94, 125201 (2016)) we here include the effects of the Ge-3d and Sn-4d semicore states and find that these tend to reduce the band gap significantly. This places the band gap of MgSnN2 in the difficult to reach green region of the visible spectrum. The stability of the materials with respect to competing binary compounds is also evaluated and details of the valence band maximum manifold splitting and effective masses are provided.

Assessment of the Linearized GW Density Matrix for Molecules.

The GW approximation is well-known for the calculation of high-quality ionization potentials and electron affinities in solids and molecules. Recently, it has been identified that the density matrix that is obtained from the contraction of the GW Green's function allows one to include Feynman diagrams that are significant for the ionization potentials. However, the Green's function contains much more information than the mere quasi-particle energies. Here, we test and assess the quality of this so-called linearized GW density matrix for several molecular properties. First of all, we extend the original formulation to perform the linearization starting from any self-consistent mean-field approximation, and not only from Hartree-Fock. We then demonstrate the reliability and the stability of the linearized GW density matrix to evaluate the total energy out of a non-self-consistent GW calculation. Based on a comprehensive benchmark of 34 molecules, we compare the quality of the electronic density, Hartree energy, exchange energy, and the Fock operator expectation values against other well-established techniques. In particular, we show that the obtained linearized GW densities markedly differ from those calculated within the widespread quasi-particle self-consistent GW approximation.

First-principles treatment of Mott insulators: linearized QSGW+DMFT approach

AbstractThe theoretical understanding of emergent phenomena in quantum materials is one of the greatest challenges in condensed matter physics. In contrast to simple materials such as noble metals and semiconductors, macroscopic properties of quantum materials cannot be predicted by the properties of individual electrons. One of the examples of scientific importance is strongly correlated electron system. Neither localized nor itinerant behaviors of electrons in partially filled 3d, 4f, and 5f orbitals give rise to rich physics such as Mott insulators, high-temperature superconductors, and superior thermoelectricity, but hinder quantitative understanding of low-lying excitation spectrum. Here we present a new first-principles approach to strongly correlated solids. It is based on a combination of the quasiparticle self-consistent GW approximation and the dynamical mean-field theory. The sole input in this method is the projector to the set of correlated orbitals for which all local Feynman graphs are being evaluated. For that purpose, we choose very localized quasiatomic orbitals spanning large energy window, which contains most strongly hybridized bands, as well as upper and lower Hubbard bands. The self-consistency is carried out on the Matsubara axis. This method enables the first-principles study of Mott insulators in both their paramagnetic and antiferromagnetic phases. We illustrate the method on the archetypical charge transfer correlated insulators La2CuO4 and NiO, and obtain spectral properties and magnetic moments in good agreement with experiments.

Research Update: Relativistic origin of slow electron-hole recombination in hybrid halide perovskite solar cells

The hybrid perovskite CH3NH3PbI3 (MAPI) exhibits long minority-carrier lifetimes and diffusion lengths. We show that slow recombination originates from a spin-split indirect-gap. Large internal electric fields act on spin-orbit-coupled band extrema, shifting band-edges to inequivalent wavevectors, making the fundamental gap indirect. From a description of photoluminescence within the quasiparticle self-consistent GW approximation for MAPI, CdTe, and GaAs, we predict carrier lifetime as a function of light intensity and temperature. At operating conditions we find radiative recombination in MAPI is reduced by a factor of more than 350 compared to direct gap behavior. The indirect gap is retained with dynamic disorder.

Fermi surface symmetry and evolution of the electronic structure across the paramagnetic-helimagnetic transition in MnSi/Si(111)

MnSi has been extensively studied for five decades, nonetheless detailed information on the Fermi surface (FS) symmetry is still lacking. This missed information prevented from a comprehensive understanding the nature of the magnetic interaction in this material. Here, by performing angle-resolved photoemission spectroscopy on high-quality MnSi films epitaxially grown on Si(111), we unveil the FS symmetry and the evolution of the electronic structure across the paramagnetic-helimagnetic transition at T$_C$ $\sim$ 40 K, along with the appearance of sharp quasiparticle emission below T$_C$. The shape of the resulting FS is found to fulfill robust nesting effects. These effects can be at the origin of strong magnetic fluctuations not accounted for by state-of-art quasiparticle self-consistent GW approximation. From this perspective, the unforeseen quasiparticle damping detected in the paramagnetic phase and relaxing only below T$_C$, along with the persistence of the d-bands splitting well above T$_C$, at odds with a simple Stoner model for itinerant magnetism, open the search for exotic magnetic interactions favored by FS nesting and affecting the quasiparticles lifetime.

Electronic structure calculations of delafossite Cu-based transparent conducting oxidesCuMO2(M=B,Al,Ga,In)by quasiparticle self-consistentGWapproximation and Tran-Blaha's modified Becke-Johnson exchange potential

In this work, band gaps of the delafossite Cu-based transparent conducting oxides $\mathrm{Cu}M{\mathrm{O}}_{2}$ $(M=\mathrm{B},\mathrm{Al},\mathrm{Ga},\mathrm{In})$ are calculated by density functional theory (DFT) implemented with many-body perturbation theory (MBPT) based on quasiparticle self-consistent GW approximation (QPscGW) and with Tran-Blaha's modified Becke-Johnson functional (DFT-TB09). Their band gaps are explicitly improved from DFT within local density approximation (LDA). Their optical absorption spectra are also calculated by solving Bethe-Salpeter equation (BSE) that includes the electron-hole correlation effect; they show strong excitonic peaks.

Relativistic quasiparticle self-consistent electronic structure of hybrid halide perovskite photovoltaic absorbers

Solar cells based on a light absorbing layer of the organometal halide perovskite CH$_3$NH$_3$PbI$_3$ have recently reached 15% conversion efficiency, though how these materials work remains largely unknown. We analyse the electronic structure and optical properties within the quasiparticle self-consistent GW approximation. While this compound bears some similarity to conventional sp semiconductors, it also displays unique features. Quasiparticle self-consistency is essential for an accurate description of the band structure: bandgaps are much larger than what is predicted by the local density approximation (LDA) or GW based on the LDA. Valence band dispersions are modified in a very unusual manner. In addition, spin orbit coupling strongly modifies the band structure and gives rise to unconventional dispersion relations and a Dresselhaus splitting at the band edges. The average hole mass is small, which accounts for the long diffusion lengths recently observed. The surface ionisation potential (workfunction) is calculated to be 5.7 eV with respect to the vacuum level, explaining efficient carrier transfer to TiO$_2$ and Au electrical contacts.

Effect of correlations on electronic structure and transport across (001) Fe/MgO/Fe junctions

We developed a parametrization of transmission probability that reliably captures essential elements of the tunneling process in magnetic tunnel junctions. The electronic structure of the Fe/MgO/Fe junction is calculated within the quasiparticle self-consistent GW approximation and used to evaluate transmission probability across (001) Fe/MgO interfaces. The transmission has a peak at $+0.12$ V, in excellent agreement with recent differential conductance measurements for electrodes with antiparallel spin. These findings confirm that the observed current-voltage characteristics are intrinsic to well-defined (001) Fe/MgO interfaces, in contrast to previous predictions based on the local spin-density approximation, and also that many-body effects are important to realistically describe electron transport across well-defined metal-insulator interfaces.

Lattice dynamics and elastic properties of the4felectron system: CeN

The electronic structure, structural stability, and lattice dynamics of cerium mononitride are investigated using ab initiodensity-functional methods involving an effective potential derived from the generalized gradient approximation and without special treatment for the $4f$ states. The $4f$ states are hence allowed to hop from site to site, without an on-site Hubbard $U$, and contribute to the bonding, in a picture often referred to as itinerant. It is argued that this picture is appropriate for CeN at low temperatures, while the anomalous thermal expansion observed at elevated temperatures indicates entropy-driven localization of the Ce $f$ electrons, similar to the behavior of elemental cerium. The elastic constants are predicted from the total energy variation of strained crystals and are found to be large, typical for nitrides. The phonon dispersions are calculated showing no soft modes, and the Gr\"uneisen parameter behaves smoothly. The electronic structure is also calculated using the quasiparticle self-consistent GW approximation (where G denotes the Green's function and W denotes the screened interaction). The Fermi surface of CeN is dominated by large egg-shaped electron sheets centered on the $X$ points, which stem from the $p$-$f$ mixing around the $X$ point. In contrast, assuming localized $f$ electrons leads to a semimetallic picture with small band overlaps around $X$.

Quasiparticle band structure of Zn-IV-N2compounds

Electronic energy-band structures of the Zn-IV-N${}_{2}$ compounds, with IV equal to Si, Ge, and Sn calculated in the quasiparticle self-consistent GW approximation and using the full-potential linearized muffin-tin orbital approach, are presented. A comparison is made with local-density approximation results. The bands near the gap are fitted to an effective Kohn-Luttinger-type Hamiltonian appropriate for the orthorhombic symmetry, and conduction-band effective masses are presented. Exciton binding energies and zero-point motion corrections to the gaps are estimated. While ZnSiN${}_{2}$ is found to be an indirect gap semiconductor, ZnGeN${}_{2}$ and ZnSnN${}_{2}$ are direct gap semiconductors. The gaps range from the orange-red to deep UV. The valence-band maximum is split in three levels of different symmetry, even in the absence of spin-orbit coupling, and should show transitions to the conduction band, each for a separate polarization. Spin-orbit effects are found to be surprisingly small, indicating almost exact compensation of the N-$2p$ and Zn-$3d$ contributions.

Calculations of quasi-particle spectra of semiconductors under pressure

Different approximations in calculations of electronic quasi-particle states in semiconductors are compared and evaluated with respect to their validity in predictions of optical properties. The quasi-particle self-consistent GW (QSGW) approach yields values of the band gaps which are close to experiments and represents a significant improvement over single-shot GW calculations using local density approximation (LDA) start wavefunctions. The QSGW approximation is compared to LDA bands for a wide-gap material (CuAlO 2 ) and materials with very small gaps, PbX (X=S, Se, and Te). For wide-gap materials QSGW overestimates the gaps by 0.3―0.8 eV, an error which is ascribed to the omission of vertex corrections. This is confirmed by calculations of excitonic effects, by solving the Bethe-Salpeter equation. The LDA error in predicting the binding energy of the Cu-3d states is examined and the QSGW and LDA + U approximations are compared. For PbX the spinorbit coupling is included, and it is shown that although LDA gives a reasonable magnitude of the gap at L, only QSGW predicts the correct order of the L + 6 and L ― 6 states and thus the correct sign (negative) of the gap pressure coefficient. The pressure-induced gap closure leads to linear (Dirac-type) band dispersions around the L point.

Quasiparticle self-consistent GW theory of III-V nitride semiconductors: Bands, gap bowing, and effective masses

The electronic band structures of InN, GaN, and a hypothetical ordered ${\text{InGaN}}_{2}$ compound, all in the wurtzite crystal structure, are calculated using the quasiparticle self-consistent GW approximation. This approach leads to band gaps which are significantly improved compared to gaps calculated on the basis of the local approximation to density functional theory, although generally overestimated by 0.2--0.3 eV in comparison with experimental gap values. Details of the electronic energies and the effective masses including their pressure dependence are compared with available experimental information. The band gap of ${\text{InGaN}}_{2}$ is considerably smaller than what would be expected by linear interpolation implying a significant band gap bowing in InGaN alloys.

On the Chemical Origin of the Gap Bowing in (GaAs)1−x Ge2x Alloys: A Combined DFT–QSGW Study

Motivated by the research and analysis of new materials for photovoltaics and by the possibility of tailoring their optical properties for improved solar energy conversion, we have focused our attention on the (GaAs)1−xGe2x series of alloys. We have investigated the structural properties of some (GaAs)1−xGe2x compounds within the local-density approximation to density-functional theory, and their optical properties within the Quasiparticle Self-consistent GW approximation. The QSGW results confirm the experimental evidence of asymmetric bandgap bowing. It is explained in terms of violations of the octet rule, as well as in terms of the order–disorder phase transition.

Quasiparticle Self-ConsistentGWTheory

In past decades the scientific community has been looking for a reliable first-principles method to predict the electronic structure of solids with high accuracy. Here we present an approach which we call the quasiparticle self-consistent approximation. It is based on a kind of self-consistent perturbation theory, where the self-consistency is constructed to minimize the perturbation. We apply it to selections from different classes of materials, including alkali metals, semiconductors, wide band gap insulators, transition metals, transition metal oxides, magnetic insulators, and rare earth compounds. Apart from some mild exceptions, the properties are very well described, particularly in weakly correlated cases. Self-consistency dramatically improves agreement with experiment, and is sometimes essential. Discrepancies with experiment are systematic, and can be explained in terms of approximations made.