Role Of Electron Correlation Research Articles

The role of electron correlations in the electronic structure of putative Chern magnet TbMn6Sn6

A member of the RMn6Sn6 rare-earth family materials, TbMn6Sn6, recently showed experimental signatures of the realization of a quantum-limit Chern magnet. In this work, we use quantum Monte Carlo (QMC) and density functional theory with Hubbard U (DFT + U) calculations to examine the electronic structure of TbMn6Sn6. To do so, we optimize accurate, correlation-consistent pseudopotentials for Tb and Sn using coupled-cluster and configuration–interaction (CI) methods. We find that DFT + U and single-reference QMC calculations suffer from the same overestimation of the magnetic moments as meta-GGA and hybrid density functional approximations. Our findings point to the need for improved orbitals/wavefunctions for this class of materials, such as natural orbitals from CI, or for the inclusion of multi-reference effects that capture the static correlations for an accurate prediction of magnetic properties. DFT + U with Mn magnetic moments adjusted to the experiment predict the Dirac crossing in bulk to be close to the Fermi level, within ~120 meV, in agreement with the experiments. Our non-stoichiometric slab calculations show that the Dirac crossing approaches even closer to the Fermi level, suggesting the possible realization of Chern magnetism in this limit.

Superconductivity in black phosphorus and the role of dynamical screening

Simple cubic phosphorus exhibits superconductivity with a maximum $T_c$ of up to 12 K under pressure. The pressure dependence of $T_c$ cannot be consistently explained with a simple electron-phonon mechanism, which has stimulated investigations into the role of electronic correlations and plasmonic contributions. Here, we solve the gap equation of density functional theory for superconductors using different electron-electron and electron-phonon contributions to the kernel. We find that the phonon contribution alone yields an overestimation of $T_c$, while the addition of the static electronic contribution results in an underestimation. Taking into account the full frequency dependence of the screened interaction, the one-shot $GW$ approximation predicts $T_c$ values in good agreement with the experiments in the pressure range appropriate for the cubic phase. We also explore the use of quasi-particle bands in the calculation of the electronic and phononic kernels, and show that this modification significantly improves $T_c$ in the high-pressure region.

Investigation on the electronic structures of thiamine and related compounds: Free base or salt?

We report the simulated photoelectron spectra of thiamine free base, thiamine and thiamine diphosphate (ThDP) in the gas phase. The structures of eighty conformers of thiamine (vitamin B1) were optimized by DFT method at the B3LYP/6-31+G(d) level. Then population ratios (PRS) with Boltzmann distribution equation were calculated using Gibbs free energy and the stable conformers were obtained. The ionization energies of these conformers were determined by two high-level computational methods. General-R-symmetry adapted cluster-configuration interaction (general-R-SAC-CI) method and electron-propagator theory (EPT) method were used to obtain vertical valence ionization energies (IP). To characterize the nature of ionization processes about different spectral bands, the natural bonding orbital (NBO) method was used to obtain orbital electron densities. The electron correlation was taken into account with SAC-CI method and MOs shapes and ionization assignments given for each spectral band. Results for ThDP show that spectra of two conformers are different: oxygen lone pair ionizations appear at higher ionization energies in the spectrum of conformer II while giving rise to the first ionization band in conformer I. The validity of Koopmans’s approximation was considered for each ionization band and the role of electron correlation was attributed to breaking down of this approximation. The implications of electronic structure results for biological activity of thiamin are discussed. Comparison with the earlier work using aerosol thermodesorption method suggests that free base was present in their photoionization spectra in addition to the deprotonated cation.

Role of electron correlations in some Weyl systems

We are discussing a model to understand previously obtained results on Weyl semimetals as realized in MoTe2 using DFT and DFT+U calculations. The model is motivated from general principles and we use it to investigate the effects of Coulomb correlations originating from the localized nature of the Mo-d orbitals. We find that such correlations can eliminate or create pairs of Weyl points as the strength of the Coulomb interaction is varied. The effect of the spin-orbit coupling (SOC) is to split each Weyl point, which is assumed present in the absence of SOC, into pairs of spin-chiral partners.

Manipulation of Multielectron Dynamics of Molecules by Fourier-Synthesized Intense Laser Pulses: Effective Potential Analysis of CO

We present a theoretical investigation as to how multielectron dynamics of CO are manipulated by Fourier-synthesized intense laser pulses. The pulses used are assumed to be comprised of harmonics up to the fourth order. The multiconfiguration time-dependent (TD) Hartree-Fock (MCTDHF) method, where the multielectron wavefunction Ψ(t) is expressed as a linear combination of various electron configurations, is employed to simulate the dynamics of CO interacting with Fourier-synthesized pulses. The multielectron nature such as electron correlation is quantified by using our effective potential approach. To begin with, the time-dependent natural orbitals {ϕj(r,t)} which diagonalize the first order reduced density matrix are obtained from Ψ(t), where r is the one-electron coordinate. The effective potentials υjeff(r,t) that determine the dynamics of ϕj(r,t) are then derived from the equations of motion for {ϕj(r,t)}. υjeff(r,t) consists of the one-body part υ1(t) including the interaction with the laser electric field ε(t) and the two-body part υ2,j(t) originating from electron-electron interaction. In this way, the role of electron correlation can be quantified by comparing υjeff(r,t) with those obtained by the TDHF method, where Ψ(t) is approximated by a single Slater determinant. We found a very similar profile in υ5σeff(r,t) of the 5σ highest occupied molecular orbital for both near-infrared one-color (ω) and directionally asymmetric ω+2ω two-color pulses; when ε(t) points from the nucleus C to O, a hump appears in υ5σeff(r,t) only 2 bohrs outward from C. The hump formation, which originates from the field-induced change in υ2,5σ(t) (especially, due to electron correlation), is responsible for preferential electron ejection from the C atom side (experimentally observed anisotropic ionization). A coherent superposition of ω and 2ω fields with an appropriate relative phase thus works as a one-color pulse of which either positive or negative peaks are filtered out. More sophisticated manipulation is possible by adding higher harmonics to a synthesized field. We show that the 5σ orbital can be squeezed toward the inside of the potential valley in υ5σeff(r,t), which encloses the molecule at a radius of ∼7 bohrs (semicircle in the region of z <0), by adjusting the phases of a ω+2ω+3ω+4ω field. The hump and valley formation in υ5σeff(r,t) are closely correlated with domains of increasing and decreasing electron density, respectively.

Magnetic and f-electron effects in LaNiO2 and NdNiO2 nickelates with cuprate-like 3{d}_{{x}^{2}-{y}^{2}} band

Recent discovery of superconductivity in the doped infinite-layer nickelates has renewed interest in understanding the nature of high-temperature superconductivity more generally. The low-energy electronic structure of the parent compound NdNiO2, the role of electronic correlations in driving superconductivity, and the possible relationship between the cuprates and the nickelates are still open questions. Here, by comparing LaNiO2 and NdNiO2 systematically within a parameter-free, all-electron first-principles density-functional theory framework, we reveal the role of Nd 4f electrons in shaping the ground state of pristine NdNiO2. Strong similarities are found between the electronic structures of LaNiO2 and NdNiO2, except for the effects of the 4f electrons. Hybridization between the Nd 4f and Ni 3d orbitals is shown to significantly modify the Fermi surfaces of various magnetic states. In contrast, the competition between the magnetically ordered phases depends mainly on the gaps in the Ni 3{d}_{{x}^{2}-{y}^{2}} band. Our estimated value of the on-site Hubbard U in the nickelates is similar to that in the cuprates, but the value of the Hund’s coupling JH is found to be sensitive to the Nd magnetic moment. In contrast with the cuprates, NdNiO2 presents 3D magnetism with competing antiferromagnetic and (interlayer) ferromagnetic exchange, which may explain why the Tc is lower in the nickelates.

Electronic correlations and magnetic interactions in infinite-layer NdNiO2

The large antiferromagnetic exchange coupling in the parent high-$T_{\rm c}$ cuprate superconductors is believed to play a crucial role in pairing the superconducting carriers. The recent observation of superconductivity in hole-doped infinite-layer (IL-) NdNiO$_2$ brings to the fore the relevance of magnetic coupling in high-$T_{\rm c}$ superconductors, particularly because no magnetic ordering is observed in the undoped IL-NdNiO$_2$ unlike in parent copper oxides. Here, we investigate the electronic structure and the nature of magnetic exchange in IL-NdNiO$_2$ using state-of-the-art many-body quantum chemistry methods. From a systematic comparison of the electronic and magnetic properties with isostructural cuprate IL-CaCuO$_2$, we find that the on-site dynamical correlations are significantly stronger in IL-NdNiO$_2$ compared to the cuprate analog. These dynamical correlations play a critical role in the magnetic exchange resulting in an unexpectedly large antiferromagnetic nearest neighbor isotropic $J$ of 77 meV between the Ni$^{1+}$ ions within the $ab$-plane. While we find many similarities in the electronic structure between the nickelate and the cuprate, the role of electronic correlations is profoundly different in the two. We further discuss the implications of our findings in understanding the origin of superconductivity in nickelates.

Electronic and magnetic properties of carbide MXenes—the role of electron correlations

Transition metal compounds are known to be tricky for ab initio calculations mainly because of the strongly localized nature of transition metal d electrons. Nonetheless, the bulk of current theoretical studies of MXenes (transition metal carbide or nitride) relies on the density functional theory using a semilocal PBE functional, whose notorious self-interaction error grossly misrepresents electronic and magnetic properties of many well-known transition metal compounds. Although several studies have adopted Hubbard-U corrections to MXenes, the lack of systematic guidelines on how to determine the U parameters has led to a cornucopia of different results. To shed some light on the reliability of different methods (different functionals or different U parameters), we performed ab initio calculations for 22 carbide MXenes (M2CT2 with M= Sc, Y, La, Ti, Zr, Hf, V, Nb, Ta, Mo, W, and T= O, F) using density functional theory and four different methods: PBE, SCAN, HSE06, and PBE+U. In addition to trivial improvement of electronic structures of MXenes by SCAN, HSE06, and PBE+U, we found new ground-state structures for two MXenes (Hf2CF2 and Ta2CF2) and magnetic states for three MXenes (V2CO2, V2CF2, and Mo2CF2), which have not previously been reported. In addition, we found that SCAN, HSE06, and PBE+U dramatically improve the dynamical stability of V2CO2 and Mo2CF2 compared with PBE. This paper offers an overview of a broad range of MXenes with a systematic verification of various methods.

Metal-insulator transition in organic ion intercalated VSe2 induced by dimensional crossover

The charge-density wave (CDW) transition has been extensively studied in transition metal dichalcogenides (TMDs), the underlying mechanism and the related metal-insulator transition are not as simple as Peierls instability in one dimension and still under hot debate. Here, through electrochemical intercalation of organic ions, we have observed a dimensional crossover induced metal-insulator transition in an organic ion intercalated TMDs: ${(\mathrm{TBA})}_{0.3}{\mathrm{VSe}}_{2}$. In pristine ${\mathrm{VSe}}_{2}$, previous studies have revealed a three-dimensional CDW transition at ${T}_{\mathrm{CDW}}\ensuremath{\sim}110\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ with a metallic ground state. After intercalation of organic ions, the remarkable anisotropy of resistivity indicates a highly two-dimensional electronic state in ${(\mathrm{TBA})}_{0.3}{\mathrm{VSe}}_{2}$, which is consistent with our density functional theory (DFT) calculation. Interestingly, the dimensional crossover enhances the CDW transition with ${T}_{\mathrm{CDW}}$ of 165 K and leads to an insulating ground state in ${(\mathrm{TBA})}_{0.3}{\mathrm{VSe}}_{2}$. Moreover, a commensurate superstructure with $3a\ifmmode\times\else\texttimes\fi{}3a$ periodicity is also confirmed in this insulating CDW state. Although the DFT calculation suggests that the commensurate superstructure and the enhanced CDW temperature could be ascribed to the improved Fermi surface nesting, whether the metal-insulator transition is driven by a perfect Fermi surface nesting is still elusive at the present stage. The possible role of electronic correlation and electron-phonon coupling has also been discussed. Our work provides a different material platform to study the metal-insulator transition in TMDs.

Zero-bias anomaly and role of electronic correlations in a disordered metal film

Localization and electron correlation play significant roles in understanding the electronic states of low-dimensional systems. We carried out the tunneling spectroscopy measurements on a crystalline nano-sized island and a disordered two-dimensional metal film. The low temperature zero-bias anomaly was studied using theory and statistical analysis of the spatial distribution of the local density of states in both the systems. The effective capacitance and resistance of the tunnel junction extracted from theory gives the energy and temperature dependency of the measured ZBA. Statistical analysis reveals the electron correlation effect and the electron correlation length. By combining theory and the statistical analysis, we found that the microscopic origin of ZBA formation in the disordered two-dimensional film is strongly related to the electron localization and the correlations.

Ab initio study of the half-metallic full-Heusler compounds Co2ZAl [Z = Sc, Ti, V, Cr, Mn, Fe]; the role of electronic correlations

We study the structural, electronic, and magnetic properties of Co2ZAl compounds employing a pseudopotential electronic band structure method. The stability of the compounds is established through formation and cohesive energy calculations. The effect of the lattice parameter variation on the electronic and magnetic properties of the compounds is investigated and meticulous explanation is provided for the observed behavior. The variation of the individual spin magnetic moments and the stability of the total spin magnetic moment during the expansion and contraction of the lattice parameter is observed and an attempt is made to understand the obtained behavior. Finally, we implement DFT+U to examine its consequences on the electronic and magnetic properties of the Co2ZAl compounds. We find that the use of DFT+U is not justified for these compounds and in some cases like Co2MnAl it produces unrealistic properties. Exception is Co2FeAl where the desired half-metallicity is restored after the inclusion of on-site correlations. We explain why the on-site correlations might be important for Co2FeAl by comparing it with other Heusler alloys where the correlation was found to be meaningful to explain the observed magnetic moments.

Nanoscale phase separation and pseudogap in the hole-doped cuprates from fluctuating Cu-O-Cu bonds

The pseudogap phenomenology is one of the enigmas of the physics of high-Tc superconductors. Many members of the cuprate family have now been characterized with high resolution in both real and momentum space, which revealed highly anisotropic Fermi arcs and local domain which break rotational symmetry in the CuO2 plane at the intraunit cell level. While most theoretical approaches to date have focused on the role of electronic correlations and doping-induced disorder to explain these features, we show that many features of the pseudogap phase can be reproduced by considering the interplay between electronic and nonlinear electron-phonon interactions within a model of fluctuating Cu-O-Cu bonds. Remarkably, we find electronic segregation arises naturally without the need to explicitly include disorder. Our approach points not only to the key role played by the oxygen bond in the pseudogap phase, but opens different directions to explore how nonequilibrium lattice excitation can be used to control the properties of the pseudogap phase.

Chirality-Induced Spin Selectivity: The Role of Electron Correlations.

Chirality-induced spin selectivity, discovered about two decades ago in helical molecules, is a nonequilibrium effect that emerges from the interplay between geometrical helicity and spin-orbit interactions. Several model Hamiltonians building on this interplay have been proposed, and while these can yield spin-polarized transport properties that agree with experimental observations, they simultaneously depend on unrealistic values of the spin-orbit interaction parameters. It is likely, however, that a common deficit originates from the fact that all these models are uncorrelated or single-electron theories. Therefore, chirality-induced spin selectivity is here addressed using a many-body approach, which allows for nonequilibrium conditions and a systematic treatment of the correlated state. The intrinsic molecular spin polarization increases by 2 orders of magnitude, or more, compared to the corresponding result in the uncorrelated model. In addition, the electronic structure responds to varying external magnetic conditions which, therefore, enables comparisons of the currents provided for different spin polarizations in one or both of the leads between which the molecule is mounted. Using experimentally feasible parameters and room temperature, the obtained normalized difference between such currents may be as large as 5-10% for short molecular chains, clearly suggesting the vital importance of including electron correlations when searching for explanations of the phenomenon.

Time‐dependent simulation of ion stopping: Charge transfer and electronic excitations

The energy loss of charged particles in matter has been studied for many decades, both, analytically and via computer simulations. While the regime of high projectile energies is well understood, low energy stopping in solids is more challenging due to the importance of non‐adiabatic effects and electronic correlations. Here we consider two problems: the charge transfer between substrate and projectile and the role of electronic correlations, specifically formation of doubly occupied lattice sites in the material during the stopping process. The former problem is treated by time‐dependent density functional theory simulations and the latter by non‐equilibrium Green functions.

Magnetic structure and spin correlations in magnetoelectric honeycomb Mn4Ta2O9

The coupling between magnetic ordering and electric polarization in magnetoelectrics is technically relevant and not yet completely understood. The elucidation of this requires a comprehension of underlying magnetic interactions and the electronic structure. Here, the authors investigate the magnetoelectric honeycomb compound Mn${}_{4}$Ta${}_{2}$O${}_{9}$, by means of neutron scattering and electronic structure calculations, to reveal the ingredients that determine the ground state. For this compound, the ground state is determined by an electron correlation assisted magnetic dipole-dipole interaction. The investigation here offers detailed insight into the factors that determine the ground-state manifold and the role of electron correlation on the strength of anisotropy.

Correlation trends in the magnetic hyperfine structure of atoms: A relativistic coupled-cluster case study

The role of electron correlation in the hyperfine structure of alkali metals and alkaline earth metal monopositive ions in their ground electronic configuration is investigated using the Z-vector method in a relativistic coupled-cluster regime within the singles and doubles approximation. The systematic effects of core-correlating functions, polarization of core electrons, and high-lying virtual functions on core electrons correlation are studied. The study reveals that the core-correlating function plays a significant role in core polarization and thus is very important for precise calculation of the wave function near the nuclear region. The inner-core electrons (1s-2p) require very high virtual energy functions for proper correlation. Therefore, the all-electron correlation treatment and the inclusion of higher-energy virtual functions are the key factors for precise calculation of the hyperfine structure constant of atoms. Our calculated values are in excellent agreement with the available experimental values, which also implies that the wave function produced by the Z-vector method is accurate enough for further calculation of the parity- and time-reversal symmetry-violating properties in atoms and molecules.

Single- and double-electron transfer in low- and intermediate-energy C4+ + He collisions

The electron-capture processes in ${\mathrm{C}}^{4+}$ + He collisions have been studied theoretically using a two-active-electron semiclassical atomic-orbital close-coupling method in a wide energy domain. The results of the present calculations are compared with available theoretical predictions and experimental measurements: very good agreements are found for both total and state-selective single-electron-capture (SEC) and double-electron-capture (DEC) cross sections. We extend the understanding on that system to high energies for which only a single series of data exists. Furthermore, the mechanisms responsible for SEC and DEC processes have been investigated by additional restricted two-active-electron and single-active-electron calculations. The role of electronic correlations in the collisions is also discussed.

Structural stability and half-metallic ferromagnetism in PbMoO3: The role of electronic correlation

We theoretically investigate the electronic and magnetic properties of the recently reported cubic and orthorhombically (Ortho.) distorted phases of PbMoO3, a 4d transition-metal perovskite oxide with almost half-filled t2g states. Our spin-polarized bare generalized gradient approximation results exhibit that the cubic phase is of low energy structure than the Ortho. one. However, on-site Coulomb repulsion in the range of 3.3 eV ≤ Ueff ≤ 4.5 eV inclusion on Mo 4d orbitals reveals that at each value of Ueff, the Ortho. phase is more stable than the cubic one and with the increase in Ueff, both phases show more stability. We find a non-magnetic n-type conductivity with a high charge carrier density of ∼1022 cm−3 in both phases. Interestingly, a non-magnetic to magnetic phase transition occurs at Ueff = 3.8 eV and 3.5 eV for the cubic and Ortho. phases, respectively. Moreover, a half-metallic ferromagnetic state is obtained at Ueff = 4.1 eV and 4.3 eV for the cubic and Ortho. phases, respectively. Calculations also indicate a strong orbital hybridization between Pb 6p and Mo 4d, with a significant contribution of O 2p states. All findings are confirmed by the Yukawa screened Perdew-Burke-Ernzerhof 0 (YS-PBE0) hybrid functional. This work provokes further experimental investigations of magnetic properties of PbMoO3.

Theory of charge transport in molecular junctions: Role of electron correlation.

We extend the quasi-particle renormalized perturbation theory developed in our previous work [Y.-W. Chang and B.-Y. Jin, J. Chem. Phys. 141, 064111 (2014)] based on nonequilibrium Green's function techniques to study the effects of electron correlation on the charge transport process in molecular junctions. In this formalism, the single-impurity Anderson's model is used as the zeroth-order Hamiltonian of each channel orbital, and the inter-channel interactions are treated by perturbation corrections. Within this scheme, the on-channel Coulomb repulsion and the single-particle spectral line-broadening can be incorporated in the zeroth-order approximation, and thus the Coulomb blockade and coherent tunneling through individual channels can be described properly. Beyond the zeroth-order description, electron correlation can be included through the self-energy corrections in the forms of the second-Born approximation and the GW approximation. The effects of electron correlation on molecular junctions are manifested as the orbital energy correction, correlated transport process, and collisional line-broadening. As an application, we have applied the present formalism to phenyl-based molecular junctions described by the Pariser-Parr-Pople Hamiltonian. The signatures of electron correlation in the simulated current-voltage curves are identified and discussed.

Role of Electron Correlation along the Water Splitting Reaction.

Electron correlation plays a crucial role in the energetics of reactions catalyzed by transition metal complexes, such as water splitting. In the present work we exploit the performance of various methods to describe the thermodynamics of a simple but representative model of water splitting reaction, based on a single cobalt ion as catalyst. Density Functional Theory (DFT) calculations show a significant dependence on the adopted functional, and not negligible differences with respect to CCSD(T) findings are found along the reaction cycle. We performed quantum Monte Carlo calculations using an unrestricted single Slater determinant wave function multiplied by a Jastrow factor using both DFT and fully optimized orbitals. Variational and Lattice Regularized Diffusion Monte Carlo results are in overall agreement with the CCSD(T) free-energy profile, even though differences in the description of the thermodynamics of the reaction cycle are found. NEVPT2 calculations reveal that the role of the static correlation of the different reaction steps is not large, and it is limited to only a few intermediate structures. Finally, the free-energy difference of the overall water splitting reaction computed at the quantum Monte Carlo level shows an excellent match with the experimental value of 4.92 eV, underlining the capability of these techniques to properly describe the dynamical correlation of such reactions.