DMFT Method Research Articles

Mixed valence nature of the Ce4fstate inCeCo5based on spin-polarizedDFT+DMFTcalculations

Cerium-based intermetallics are currently attracting much attention as highly promising alternatives to conventional permanent magnets that contain a scarce rare earth element like neodymium. In the present work we apply a charge fully self-consistent approach combining density functional theory and dynamical mean-field theory (DFT $+$ DMFT) to investigate the magnetization and electronic structure of the ${\mathrm{CeCo}}_{5}$ system. We treat simultaneously the correlation effects in Ce-$4f$ and Co-$3d$ orbitals while taking the spin polarization into account. The calculated magnetic moment corresponding to the Ce-$4f$ shell using the DFT $+$ DMFT method is found to be drastically reduced as compared to the DFT results. Moreover, the Ce-$4f$ valence state fluctuations are evaluated and compared within ${\mathrm{CeCo}}_{5}$ and $\mathrm{Ce}{({\mathrm{Co}}_{0.8}{\mathrm{Cu}}_{0.2})}_{5}$ on account of the trivalent Ce in ${\mathrm{CeCu}}_{5}$. Regarding the Cu substitutions at two Wyckoff positions of Co (2c and 3g), the substitution at 3g sites slightly enhances the magnetic moment of Ce while the substitution at 2c sites leads to nearly vanishing Ce and Co moments. Such a contrast may contribute to the experimentally reported nonmonotonic change of the magnetic anisotropy with increasing Cu alloying content in $\mathrm{Ce}{({\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x})}_{5}$ alloys.

RTGW2020: An efficient implementation of the multi-orbital Gutzwiller method with general local interactions

In the present paper, we propose an efficient numerical scheme for Gutzwiller method for multi-band Hubbard models with general onsite Coulomb interaction. Following the basic idea of Deng et al. (2009) [26] and extensions by Lanata et al. (2012) [28], the ground state is variationally determined through optimizing the total energy with respect to the variational single particle density matrix (n0), which is called “outer loop”. In the corresponding “inner loop” where n0 is fixed, the non-interacting wave function and the parameters contained in the Gutzwiller projector are determined by a two-step iterative approach. All derivatives of the implementation process have been analytically derived, which allows us to apply some advanced minimization or root-searching algorithms for both the inner and outer loops leading to the highly efficient convergence. In addition, an atomic diagonalization method taking the point group symmetry into account has been developed for the customized design of the Gutzwiller projector, making it convenient to explore many interesting orders at a lower cost of computation. As benchmarks, several different types of correlated models have been studied utilizing the proposed method, which are in perfect agreement with the previous results by DMFT and multi-orbital slave-boson mean field method. Compared with the linear mixing method, Newton's method with analytical derivatives shows much faster convergence for the inner loop. As for the outer loop, the minimization using analytical derivatives also shows much better stability and efficiency compared with that using numerical derivatives.

Spatial non-locality of electronic correlations beyond GW approximation

The question of spatial locality of electronic correlations beyond GW approximation is one of the central issues of the famous combination of GW and dynamical mean field theory, GW + DMFT. In this work, the above question is addressed directly (for the first time) by performing calculations with and without assumption of locality of the corresponding diagrams. For this purpose we use sc(GW + G3W2) approach where the higher order part (G3W2) is evaluated with fully momentum dependent Green’s function G and screened interaction W and with ‘local’ variant, where the single site approximation is assumed for both G and W. For all three materials studied in this work (NiO, α-Ce, LiFeAs), we have found the spatial non-locality effects to be strong. For NiO and LiFeAs they, in fact, are decisive for the proper evaluation of vertex corrections. The results of this study have direct impact on our understanding of approximations made in practical implementations of GW + DMFT method, where all diagrams beyond GW (DMFT part) are assumed to be local. Taking into account the fact that the first diagrams beyond GW represent the most important contribution also in GW + DMFT calculations, we conclude that the basic assumption of GW + DMFT, namely the locality of diagrams evaluated in the DMFT part, is not as good as it is believed to be.

Diagnostic Value of Tele-dentistry in Decay Detection by DMFT Method

Diagnostic Value of Tele-dentistry in Decay Detection by DMFT Method

Weak Coulomb correlations stabilize the electride high-pressure phase of elemental calcium

Theoretical studies using the state-of-the-art density functional theory and dynamicalmean-field theory (DFT + DMFT) method show that weak electronic correlation effects are crucial for reproducing the experimentally observed pressure-induced phase transitions of calcium from β-tin to Cmmm and then to the simple cubic structure. The formation of an electride state in calcium leads to the emergence of partially filled and localized electronic states under compression. The electride state was described using a basis containing molecular orbitals centered on the interstitial site and Ca-d states. We investigate the influence of Coulomb correlations on the structural properties of elemental Ca, noting that approaches based on the Hartree–Fock method (DFT + U or hybrid functional schemes) are poorly suited for describing correlated metals. We find that only the DFT + DMFT method reproduces the correct sequence of high-pressure phase transitions of Ca at low temperatures.

A real-space Green’s function approach for disordered Hubbard model

Based on the equation-of-motion approach, a real-space Green’s function method is proposed, which allows us to fully consider the disorder effect in Hubbard model. Meanwhile, we improved the decoupling scheme to obtain the quasi particle resonance peak on the Fermi energy, consistent with the DMFT method with Quantum Monte Carlo solver. After introducing the Anderson disorder into the system, the three peak structure in spectrum density still exists at intermediate regime of Coulomb interaction. However, the electronic state tends to be restricted in some small puddles in presence of on-site disorder, suggesting the Anderson localization has been introduced in the Kondo peak.

Studying the occupied and unoccupied electronic structure of LaCoO3 by using DFT+embedded DMFT method with the calculated value of U

In this work, we present a systematic study of the occupied and unoccupied electronic states of LaCoO$_{3}$ compound using DFT, DFT+$\textit{U}$ and DFT+embedded DMFT methods. The value of $\textit{U}$ used here is evaluated by using constrained DFT method and found to be $ \backsim $ 6.9 eV. It is found that DFT result has limitations with energy positions of PDOS peaks due to its inability of creating a hard gap although the DOS distribution appears to be fine with experimental attributes. The calculated value of $\textit{U}$ is not an appropriate value for carrying out DFT+$\textit{U}$ calculations as it has created an insulating gap of $ \backsim $ 1.8 eV with limitations in redistribution of DOS which is inconsistent with experimental spectral behaviour for the occupied states mainly. However, this value of $\textit{U}$ is found to be an appropriate one for DFT+embedded DMFT method which creates a gap of $\backsim $ 1.1 eV. The calculated PDOS of Co 3$\textit{d}$, La 5$\textit{d}$, La 4$\textit{f}$ and O 2$\textit{p}$ states are giving a remarkably good explanation for the occupied and unoccupied states of the experimental spectra in the energy range $\backsim $ -9.0 eV to $\backsim $ 12.0 eV.

Spin–Orbit and Coulomb Effects in Single‐Layered Ruthenates

Single‐layered ruthenates continue to surprise us decades after their discovery. Here we briefly review some recent advances in the understanding of their electronic properties. They have been obtained via the LDA + DMFT method, the state‐of‐the‐art approach for strongly correlated systems. In particular, we discuss the effects of the spin–orbit interaction and its interplay with the crystal field in the presence of electron–electron repulsion. We focus on the Fermi surface of Sr2RuO4 and the metal–insulator transition in Ca2−xSrxRuO4.

Fluctuating local field method probed for a description of small classical correlated lattices.

Thermal-equilibrated finite classical lattices are considered as a minimal model of the systems showing an interplay between low-energy collective fluctuations and single-site degrees of freedom. Standard local field approach, as well as classical limit of the bosonic DMFT method, do not provide a satisfactory description of Ising and Heisenberg small lattices subjected to an external polarizing field. We show that a dramatic improvement can be achieved within a simple approach, in which the local field appears to be a fluctuating quantity related to the low-energy degree(s) of freedom.

Phase transitions in FeBO3 under pressure: DFT + DMFT study

We present a theoretical study of spectral, magnetic, and structural properties of the iron borate FeBO3. Within the DFT + DMFT method combining density functional theory with dynamical mean-field theory FeBO3 was investigated under pressures up to 70 GPa at 300 K. We found that FeBO3 is an insulator with a gap of 2.0 eV with antiferromagnetic ordering at ambient pressure in agreement with experiments. In our calculations, we showed that Fe ions in FeBO3 undergo a high-spin to low-spin transition under pressure with change from antiferromagnetic to paramagnetic state, and demonstrate that the spin and magnetic transitions occur simultaneously with an isostructural transition at 50.4 GPa with the volume collapse of 13%.

Phase stability and large in-plane resistivity anisotropy in the 112-type iron-based superconductor Ca1−xLaxFeAs2

The recently discovered high-T$_c$ superconductor Ca$_{1-x}$La$_{x}$FeAs$_{2}$ is a unique compound not only because of its low symmetry crystal structure, but also because of its electronic structure which hosts Dirac-like metallic bands resulting from (spacer) zig-zag As chains. We present a comprehensive first principles theoretical study of the electronic and crystal structures of Ca$_{1-x}$La$_{x}$FeAs$_{2}$. After discussing the connection between the crystal structure of the 112 family, which Ca$_{1-x}$La$_{x}$FeAs$_{2}$ is a member of, with the other known structures of Fe pnictide superconductors, we check the thermodynamic phase stability of CaFeAs$_{2}$, and similar hyphothetical compounds SrFeAs$_{2}$ and BaFeAs$_{2}$ which, we find, are slightly higher in energy. We calculate the optical conductivity of Ca$_{1-x}$La$_{x}$FeAs$_{2}$ using the DFT + DMFT method, and predict a large in-plane resistivity anisotropy in the normal phase, which does not originate from electronic nematicity, but is enhanced by the electronic correlations. In particular, we predict a 0.34 eV peak in the $yy$ component of the optical conductivity of the 30\% La doped compound, which correponds to coherent interband transitions within a fast-dispersing band arising from the zig-zag As-chains which are unique to this compound. We also study the Landau free energy for Ca$_{1-x}$La$_{x}$FeAs$_{2}$ including the order parameter relevant for the nematic transition and find that the free energy does not have any extra terms that could induce ferro-orbital order. This explains why the presence of As chains does not broaden the nematic transition in Ca$_{1-x}$La$_{x}$FeAs$_{2}$.

Fermi surface evolution andd-wave superconductivity inCeCoIn5: Analysis based on LDA+DMFT method

Based on the advanced first-principles theoretical approach, we investigate the superconducting gap structure and the pairing glue in the heavy-fermion superconductor ${\mathrm{CeCoIn}}_{5}$. Unexpectedly, the nesting function in the original GGA-based band structure, which is considered to be consistent with the dHvA measurement, shows a $Q$ structure incompatible with experimental observations. Instead we find the importance of the temperature-dependent Fermi surface evolution driven by electron correlations, which has been calculated by the DMFT method. Considering this effect, we obtain reasonable antiferromagnetic correlation, which can also induce the expected $d$-wave superconductivity. The system encounters the superconducting transition, before a part of the Fermi surface is formed. Similar effects can be expected in generic heavy-fermion superconductors.

Electronic structure of SrVO3withinGW+DMFT

We present a detailed calculation of the electronic structure of SrVO3 based on the GW + DMFT method. We show that a proper inclusion of the frequency-dependent Hubbard U and the nonlocal self-energy via the GW approximation, as well as a careful treatment of the Fermi level, are crucial for obtaining an accurate and coherent picture of the quasiparticle band structure and satellite features of SrVO3. The GW + DMFT results for SrVO3 are not attainable within the GW approximation or the LDA + DMFT scheme. We also compare the results of GW + DMFT to DMFT calculations based on the GW quasiparticle bands.

Correlated band structure of superconducting NdFeAsO0.9F0.1: Dynamical mean-field study

We report the LDA + DMFT (method combining local density approximation with dynamical mean-field theory) results for spectral properties of the superconductor NdFeAsO0.9F0.1 in the paramagnetic phase. The calculated momentum-resolved spectral functions are in good agreement with angle-resolved photoemission spectra (ARPES). The obtained effective quasiparticle mass enhancement (m*/m = 1.4) is smaller than the one in isostructural parent compound LaFeAsO which critical temperature under the same fluorine doping (LaFeAsO0.9F0.1) is two times lower. Our results demonstrate that in quaternary FeAs-based superconductors of the same class, changes of the crystal structure caused by substitution of one rare-earth atom, implicitly result in reduction of the electronic correlation strength.

Nonequilibrium dynamical mean-field simulation of inhomogeneous systems

We extend the nonequilibrium dynamical mean field (DMFT) formalism to inhomogeneous systems by adapting the ``real-space'' DMFT method to Keldysh Green's functions. Solving the coupled impurity problems using strong-coupling perturbation theory, we apply the formalism to homogeneous and inhomogeneous layered systems with strong local interactions and up to 39 layers. We study the diffusion of doublons and holes created by photoexcitation in a Mott insulating system, the time-dependent build-up of the polarization and the current induced by a linear voltage bias across a multilayer structure, and the photoinduced current in a Mott insulator under bias.

Equation of motion solutions to Hubbard model retaining Kondo effect

We propose a new way of analyzing the Hubbard model using equations of motion (EOM) for the higher-order Green's functions approach within the DMFT scheme. In calculating the higher order Green function we will differentiate over both times (t) and (t′). This allows us to obtain the metallic Fermi liquid at nonzero Coulomb interaction, where the three center DOS structure with two Hubbard bands and the quasiparticle resonance peak is obtained. At small Coulomb interactions and zero temperature the height of the quasiparticle resonance peak on the Fermi energy is constant similarly as in the full DMFT method with numerical (Quantum Monte Carlo) or with analytical (e.g. iterative perturbation theory) calculations.

Consistent LDA’ + DMFT approach to the electronic structure of transition metal oxides: Charge transfer insulators and correlated metals

We discuss the recently proposed LDA’ + DMFT approach providing a consistent parameter-free treatment of the so-called double counting problem arising within the LDA + DMFT hybrid computational method for realistic strongly correlated materials. In this approach, the local exchange-correlation portion of the electron-electron interaction is excluded from self-consistent LDA calculations for strongly correlated electronic shells, e.g., d-states of transition metal compounds. Then, the corresponding double-counting term in the LDA’ + DMFT Hamiltonian is consistently set in the local Hartree (fully localized limit, FLL) form of the Hubbard model interaction term. We present the results of extensive LDA’ + DMFT calculations of densities of states, spectral densities, and optical conductivity for most typical representatives of two wide classes of strongly correlated systems in the paramagnetic phase: charge transfer insulators (MnO, CoO, and NiO) and strongly correlated metals (SrVO3 and Sr2RuO4). It is shown that for NiO and CoO systems, the LDA’ + DMFT approach qualitatively improves the conventional LDA + DMFT results with the FLL type of double counting, where CoO and NiO were obtained to be metals. Our calculations also include transition-metal 4s-states located near the Fermi level, missed in previous LDA + DMFT studies of these monoxides. General agreement with optical and the X-ray experiments is obtained. For strongly correlated metals, the LDA’ + DMFT results agree well with the earlier LDA + DMFT calculations and existing experiments. However, in general, LDA’ + DMFT results give better quantitative agreement with experimental data for band gap sizes and oxygen-state positions compared to the conventional LDA + DMFT method.

Charge self-consistent dynamical mean-field theory based on the full-potential linear muffin-tin orbital method: Methodology and applications

Full charge self-consistence (CSC) over the electron density has been implemented into the local density approximation plus dynamical mean-field theory (LDA+DMFT) scheme based on a full-potential linear muffin-tin orbital method (FP-LMTO). Computational details on the construction of the electron density from the density matrix are provided. The method is tested on the prototypical charge-transfer insulator NiO using a simple static Hartree–Fock approximation as impurity solver. The spectral and ground state properties of bcc Fe are then addressed, by means of the spin-polarized T-matrix fluctuation exchange solver (SPTF). Finally the permanent magnet SmCo5 is studied using multiple impurity solvers, SPTF and Hubbard I, as the strength of the local Coulomb interaction on the Sm and Co sites are drastically different. The developed CSC–DMFT method is shown to in general improve on materials properties like magnetic moments, electronic structure and the materials density.

Evidence for strong Coulomb correlations in the metallic phase of vanadium dioxide

The influence of Coulomb correlations on magnetic and spectral properties in the metallic rutile phase of vanadium dioxide is studied by state of the art LDA + DMFT method. Calculation results in strongly correlated metallic state with an effective mass renormalization m*/m ≈ 2. Uniform magnetic susceptibility shows Curie-Weiss temperature dependence with effective magnetic moment, p theoreff = 1.54μB, in a good agreement with the experimental value p expeff = 1.53μB that is close to ideal value for V4+ ion with the spin S = 1/2, p eff = 1.73μB. Calculated spectral function shows well developed Hubbard bands observable in the recent experimental photoemission spectra. We conclude that VO2 in rutile phase is strongly correlated metal with local magnetic moments formed by vanadium d-electrons.

Coulomb correlation effects in LaFeAsO: An LDA + DMFT(QMC) study

Effects of Coulomb correlation on the LaFeAsO electronic structure are investigated by the LDA + DMFT(QMC) method (combination of the local density approximation with the dynamic mean-field theory; impurity solver is a quantum Monte Carlo algorithm). The calculation results show that LaFeAsO is in the regime of intermediate correlation strength with a significant part of the spectral density moved from the Fermi energy to the Hubbard bands and far from the edge of the metal-insulator transition. Correlations affect iron d-orbitals differently. The t 2g states (xz, yz and x 2 − y 2 orbitals) have a higher energy due to crystal field splitting and are nearly half-filled. Their spectral functions have a pseudogap with the Fermi level position on the higher subband slope. The lower energy e g set (xy and 3z 2 − r 2 orbitals) have occupancies significantly larger than 1/2 with typically metallic spectral functions.