Coulomb Correlation Effects Research Articles

Coulomb correlations in a germanide iron-based superconductor: The example of YFe2Ge2

We report results of a detailed theoretical study of the effect of Coulomb correlations on the structural, electronic, and magnetic properties of the non-pnictide iron-based superconductor ${\mathrm{YFe}}_{2}{\mathrm{Ge}}_{2}$. In particular, by employing the density functional theory plus dynamical mean-field theory method ($\mathrm{DFT}+\mathrm{DMFT}$) we perform a structural optimization of the unit-cell volume and the internal coordinate ${z}_{\mathrm{Ge}}$ of Ge. Our results for the lattice parameters obtained by $\mathrm{DFT}+\mathrm{DMFT}$ show a much better agreement with experiment as compared to those calculated by DFT implying the importance of correlation effects. We show that ${\mathrm{YFe}}_{2}{\mathrm{Ge}}_{2}$ is a moderately correlated system with electron-electron correlations being strong enough to renormalize the low-energy band structure but not sufficient to form Hubbard bands, in close analogy to the iron-based pnictides and chalcogenides. The analysis of the local spin-spin correlation function and momentum-dependent spin susceptibility $\ensuremath{\chi}(\mathbf{q})$ indicates the correlation induced formation of short-lived local moments in the Fe ${t}_{2g}$ states and the presence of competing spin fluctuations, in agreement with experiment.

First Principles Calculations of the Optical Response of LiNiO2

We discuss optical properties of layered Lithium Nickel oxide (LiNiO2), which is an attractive cathode material for realizing cobalt-free lithium-ion batteries, within the first-principles density functional theory (DFT) framework. Exchange correlation effects are treated using the generalized gradient approximation (GGA) and the strongly-constrained-and-appropriately-normed (SCAN) meta-GGA schemes. A Hubbard parameter (U) is used to model Coulomb correlation effects on Ni 3d electrons. The GGA+U is shown to correctly predict an indirect (system wide) band gap of 0.46 eV in LiNiO2, while the GGA yields a bandgap of only 0.08 eV. The calculated refractive index and its energy dependence is found to be in good agreement with the corresponding experimental results. Finally, our computed optical energy loss function yields insight into the results of recent RIXS experiments on LiNiO2.

Modeling of Multiresonant Thermally Activated Delayed Fluorescence Emitters─Properly Accounting for Electron Correlation Is Key!

With the surge of interest in multiresonant thermally activated delayed fluorescent (MR-TADF) materials, it is important that there exist computational methods to accurately model their excited states. Here, building on our previous work, we demonstrate how the spin-component scaling second-order approximate coupled-cluster (SCS-CC2), a wavefunction-based method, is robust at predicting the ΔEST (i.e., the energy difference between the lowest singlet S1 and triplet T1 excited states) of a large number of MR-TADF materials, with a mean average deviation (MAD) of 0.04 eV compared to experimental data. Time-dependent density functional theory calculations with the most common DFT functionals as well as the consideration of the Tamm-Dancoff approximation (TDA) consistently predict a much larger ΔEST as a result of a poorer account of Coulomb correlation as compared to SCS-CC2. Very interestingly, the use of a metric to assess the importance of higher order excitations in the SCS-CC2 wavefunctions shows that Coulomb correlation effects are substantially larger in the lowest singlet compared to the corresponding triplet and need to be accounted for a balanced description of the relevant electronic excited states. This is further highlighted with coupled cluster singles-only calculations, which predict very different S1 energies as compared to SCS-CC2 while T1 energies remain similar, leading to very large ΔEST, in complete disagreement with the experiments. We compared our SCS-CC2/cc-pVDZ with other wavefunction approaches, namely, CC2/cc-pVDZ and SOS-CC2/cc-pVDZ leading to similar performances. Using SCS-CC2, we investigate the excited-state properties of MR-TADF emitters showcasing large ΔET2T1 for the majority of emitters, while π-electron extension emerges as the best strategy to minimize ΔEST. We also employed SCS-CC2 to evaluate donor-acceptor systems that contain a MR-TADF moiety acting as the acceptor and show that the broad emission observed for some of these compounds arises from the solvent-promoted stabilization of a higher-lying charge-transfer singlet state (S2). This work highlights the importance of using wavefunction methods in relation to MR-TADF emitter design and associated photophysics.

Disentangling Many-Body Effects in the Coherent Optical Response of 2D Semiconductors.

In single-layer (1L) transition metal dichalcogenides, the reduced Coulomb screening results in strongly bound excitons which dominate the linear and the nonlinear optical response. Despite the large number of studies, a clear understanding on how many-body and Coulomb correlation effects affect the excitonic resonances on a femtosecond time scale is still lacking. Here, we use ultrashort laser pulses to measure the transient optical response of 1L-WS2. In order to disentangle many-body effects, we perform exciton line-shape analysis, and we study its temporal dynamics as a function of the excitation photon energy and fluence. We find that resonant photoexcitation produces a blue shift of the A exciton, while for above-resonance photoexcitation the transient response at the optical bandgap is largely determined by a reduction of the exciton oscillator strength. Microscopic calculations based on excitonic Heisenberg equations of motion quantitatively reproduce the nonlinear absorption of the material and its dependence on excitation conditions.

Resonant inelastic x-ray scattering in Ca3LiOsO6 from first-principles

We have investigated the electronic and magnetic properties of the transition metal oxide (TMO) Ca3LiOsO6 within the density functional theory (DFT) using the generalized gradient approximation while taking into account strong Coulomb correlations (GGA+U) in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. Ca3LiOsO6 is an insulator and forms the K4CdCl6-type crystal structure with relatively high Néel temperature TN = 117 K. Despite of the large strength of spin–orbit coupling, it has only a small effect on the electronic and magnetic properties of Ca3LiOsO6. We have also investigated theoretically the x-ray absorption spectra (XAS), x-ray magnetic circular dichroism (XMCD) and resonant inelastic x-ray scattering (RIXS) spectra at the Os L3 edge. The experimentally measured RIXS spectrum of Ca3LiOsO6 in addition to the elastic scattering peak at 0 eV possesses a sharp feature below 2 eV corresponding to transitions within the Os t2g levels. The excitation located at 4.5 eV is due to t2g→eg transitions. The third wide structure situated at 6–8 eV appears due to ligand-to-metal charge transfer excitations: d−d transitions to the Os t2g manifold from the Os 5dO states derived from the ”tails” of oxygen 2p states. The high energy structure at 10–11.5 eV is due to Os 5dO→eg interband transitions. At higher energy above 11 eV transitions from the Os 5dO states to the empty Ca 5d states are located. We have investigated the effect of SO coupling, Coulomb correlations and core-hole effect on the energy band structure, XAS, XMCD, and RIXS spectra in Ca3LiOsO6.

Interplay between multipole expansion of exchange interaction and Coulomb correlation of exciton in colloidal II–VI quantum dots

We study the effect of Coulomb correlation on the emission properties of the ground state exciton in zincblende CdSe/ZnS core–shell and in wurtzite ZnO quantum dots (QDs). We validate our theory model by comparing results of computed exciton energies of CdSe/ZnS QDs to photoluminescence and scanning near-field optical microscopy measurements. We use that to estimate the diameter of the QDs using a simple model based on infinitely deep quantum well and compare the results with the statistics of the atomic force microscopy scans of CdSe/ZnS dots, obtaining excellent agreement. Thereafter, we compute the energy fine structure of exciton, finding striking difference between properties of zincblende CdSe/ZnS and wurtzite ZnO dots. While in the former the fine structure is dominated by the dipole terms of the multipole expansion of the exchange interaction, in the latter system that is mostly influenced by Coulomb correlation. Furthermore, the correlation sizeably influences also the exciton binding energy and emission radiative rate in ZnO dots.

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.

Metal-insulator transition in Cr-doped hollandite vanadate K2V8O16

The hollandite K2V8O16 is a metal with ferromagnetic spin fluctuations in the high temperature (tetragonal I4/m) phase and encounters metal-insulator transition (MIT) at 170 K. The mechanism behind the MIT is still controversial due to inadequate information on the electronic structure. Furthermore, substitution of Rb for K or Ti for V increases the transition temperature. We have investigated whether break in the mirror symmetry is responsible for the insulating ground state of K2V8O16 and since the Cr counterpart is a robust ferromagnet, whether Cr doping which leads to break in mirror symmetry can help in driving a gap. We find that both 25% and 75% Cr doping break the mirror symmetry but only the former leads to an insulating ground state. Ferromagnetism (FM) remains intact in the insulating phase of K2V6Cr2O16. The structural, electronic and magnetic properties of pure and doped K2V8O16 were investigated within first-principles calculations using density functional theory (DFT). Electron correlation suppresses orbital fluctuations between the partially occupied Cr and V-3dt2g states. Consequently, transfer of charge (electron) from V-3d to Cr-3dt2g states is observed which facilitates Cr3+-V4+ charge ordering. Furthermore, Peierls like structural distortion is associated to the breaking of mirror symmetry in the Cr-V rectangular four chain columns within the crystal. Therefore, the simultaneous effect of Peierls instability, charge ordering and Coulomb correlation is responsible for the MIT in K2V6Cr2O16. Besides, Cr-3d O-2p hybridization and Cr-O coupling increases with Cr doping. These two effects are cooperatively responsible for the observed FM in insulating K2V6Cr2O16. More interestingly, the strength of FM is augmented with Cr concentration in K2V8O16.

Coulomb correlation and information entropies in confined helium-like atoms

The present work studies aspects of the electronic correlation in confined H$^{-}$, He and Li$^+$ atoms in their ground states using the informational entropies. In this way, different variational wavefunctions are employed in order of better take account of Coulomb correlation. The obtained values for the $S_r$, $S_p$ and $S_t$ entropies are sensitive in relation to Coulomb correlation effects. In the strong confinement regime, the effects of the Coulomb correlation are negligible and the employment of the models of independent particle and two non-interacting electrons confined by a impenetrable spherical cage gains importance in this regime. Lastly, energy values are obtained in good agreement with the results available in the literature.

Measuring correlated electron motion in atoms with the momentum-balance density.

Three new measures of relative electron motion are introduced: equimomentum, antimomentum, and momentum-balance. The equimomentum is the probability that two electrons have the exact same momentum, whereas the antimomentum is the probability that their momenta are the exact opposite. Momentum-balance (MB) is the difference between the equimomentum and antimomentum and, therefore, indicates if equal or opposite momentum is more probable in a system of electrons. The equimomentum, antimomentum, and MB densities are also introduced, which are the local contribution to each quantity. The MB and MB density of the extrapolated-full configuration interaction wave functions of atoms of the first three rows of the periodic table are analyzed, with a particular focus on contrasting the correlated motion of electrons with opposite-spin and parallel-spin. Coulomb correlation between opposite-spin electrons leads to a higher probability of equimomentum, whereas Fermi correlation between parallel-spin electrons leads to a higher probability of antimomentum. The local contribution to MB, given an electron is present, is a minimum at the nucleus and generally increases as the distance from the nucleus increases. There are also interesting similarities between the effects of Fermi correlation and Coulomb correlation (of opposite-spin electrons) on MB.

Experimental and theoretical study of magnetization and magnetocaloric effect in Nd1−xSmxFeO3

We have experimentally and theoretically investigated the magnetic properties and magnetocaloric effect in doped orthoferrite series Nd1−xSmxFeO3 (x = 0, 0.1, and 0.4). Our magnetization measurements reveal that the spin-reorientation transition temperatures of the Fe3+ moments increase as a function of Sm doping. Our measurements on the magnetocaloric effect indicate that the effect [measured by the maximum change in magnetic entropy (−ΔSM)] reduces with Sm doping. Non-collinear calculations including the effect of Coulomb correlation (U) and spin–orbit interaction (SO) were performed within the GGA + U + SO approximation using the first-principles density functional theory (DFT) to establish the magnetic structure above and below the spin-reorientation transition. Calculations of the exchange interaction strengths using DFT reveal a higher strength of Sm–Fe exchange interaction than that of Nd–Fe interaction, which explains the higher spin-reorientation transition temperatures observed in the doped compounds. Using Monte Carlo calculations on the Ising model with large moments of Nd/Sm, we have extracted the effective nearest neighbor exchange interaction values between rare-earth moments, which provide good agreement with the corresponding experimental results on −ΔSM around and below the transition temperature.

First-principles study of the electronic and magnetic properties of Ti-substituted K2Cr8O16

The hollandite K2Cr8O16 is a paramagnetic metal (PM) at the high temperature (HT) tetragonal I4/m structure. This material encounters a first step transition from PM to ferromagnetic metal at Tc = 180 K and finally a metal–insulator transition (MIT) at 95 K. It is well known that the formation of tetramers, accompanied by dimerization in the Cr-Cr double chains is responsible for the MIT in K2Cr8O16. Nevertheless, the substitution effect has a marked impact on the MIT of other hollandite compounds. For example, Rb substitution for K or Ti substitution for V in K2V8O16 significantly increases the transition temperature. There is no available report on the electronic and magnetic properties of Ti-substituted K2Cr8O16. In this spirit, Ti-substitution effect on the electronic and magnetic properties of K2Cr8O16 are extensively investigated for its HT structure. It is revealed in this study that this compound preserves its ferromagnetism at 25% (the corresponding compound is K2Cr6Ti2O16), 50% (the corresponding compound is K2Cr4Ti4O16) and 75% (the corresponding compound is K2Cr6Ti4O16) Ti substitution for Cr. We observed that K2Cr8O16 undergoes MIT for 75% Ti substitution. The cooperative effect of Coulomb correlation and Cr3+-Ti4+ and Ti4+-Ti4+ charge ordering is responsible for the observed MIT in K2Cr2Ti6O16. The ferromagnetism in all the Ti-substituted compounds arises from Cr-O coupling. The strength of ferromagnetism decreases with Ti concentration due to the weakening in the Cr-O coupling. More interestingly, Tc reduces significantly by 45 K, 76 K and 81 K respectively for 25%, 50% and 75% Ti-substitution.

Charge density wave instability and pressure-induced superconductivity in bulk 1T−NbS2

Charge-density-wave (CDW) instability and pressure-induced superconductivity in bulk 1T-NbS2 are predicted theoretically by first-principles calculations. We reveal a CDW instability towards the formation of a stable commensurate CDW order, resulting in a sqart(13)*sqart(13) structural reconstruction featured with star-of-David clusters. The CDW phase exhibits one-dimensional metallic behavior with in-plane flat-band characteristics, and coexists with an orbital-density-wave order predominantly contributed by 4d_(z^2-r^2 ) orbital from the inner Nb atoms of the star-of-David cluster. By doubling the cell of the CCDW phase along the layer stacking direction, a metal-insulator transition may be realized in the CDW phase in case the interlayer antiferromagnetic ordering and Coulomb correlation effect have been considered simultaneously. Bare electron susceptibility, phonon linewidth and electron-phonon coupling calculations suggest that the CDW instability is driven by softened phonon modes due to the strong electron-phonon coupling interactions. CDW order can be suppressed by pressure, concomitant with appearance of superconductivity. Our theoretical predictions call for experimental investigations to further clarify the transport and magnetic properties of 1T-NbS2. Furthermore, it would also be very interesting to explore the possibility to realize the CDW order coexisting with the superconductivity in bulk 1T-NbS2.

Exciton-Scattering-Induced Dephasing in Two-Dimensional Semiconductors.

Enhanced Coulomb interactions in monolayer transition metal dichalcogenides cause tightly bound electron-hole pairs (excitons) that dominate their linear and nonlinear optical response. The latter includes bleaching, energy renormalizations, and higher-order Coulomb correlation effects like biexcitons and excitation-induced dephasing. While the first three are extensively studied, no theoretical footing for excitation-induced dephasing in exciton-dominated semiconductors is available so far. In this Letter, we present microscopic calculations based on excitonic Heisenberg equations of motion and identify the coupling of optically pumped excitons to exciton-exciton scattering continua as the leading mechanism responsible for an optical-power-dependent linewidth broadening (excitation-induced dephasing) and sideband formation. Performing time-, momentum-, and energy-resolved simulations, we quantitatively evaluate the exciton-induced dephasing for the most common monolayer transition metal dichalcogenides and find an excellent agreement with recent experiments.

Exchange-correlation corrections for electronic properties of half-metallic Co2FeSi and nonmagnetic semiconductor CoFeTiAl

We consider two cobalt-based full-Heusler compounds, CoFeTiAl and Co2FeSi, for which Coulomb correlation effects play an important role. Since the standard GGA scheme does not provide a precise description of the electronic properties near the Fermi level, we use a meta-GGA functional capable to improve the description of the electronic properties of CoFeTiAl and Co2FeSi. In particular, we find a better agreement with the experiment for the magnetic moment and the energy bandgap. Moreover, our calculations show that pressure enhances the insulating properties of Co2FeSi and CoTiFeAl.

Magnetic fluctuations and superconducting pairing in ε -iron

We study Coulomb correlation effects and their role in superconductivity of $\varepsilon$-iron under pressure from 12 to 33 GPa by using a combination of density functional and dynamical mean-field theory. Our results indicate a persistence of the Fermi-liquid behavior below the temperature $\sim$1000 K. The Coulomb correlations are found to substantially renormalize the density of states, reducing the distance from the peak to the Fermi level to 0.4 eV compared to 0.75 eV obtained in DFT calculations. We find significant antiferromagnetic correlations, which are accompanied by the formation of short-lived local magnetic moments. We use the obtained results as a starting point for construction of the multi-band Bethe-Salpeter equation, which eigenvalues indicate that antiferromagnetic spin fluctuations may result in the superconducting pairing in $\varepsilon$-Fe. Moreover, the tendency to superconducting instability becomes weaker with the increase of pressure, which may explain the disappearance of superconductivity at $\sim$30 GPa.

Valence transition in topological Kondo insulator

We investigate the valence transition in three-dimensional topological Kondo insulator through slave-boson analysis of periodic Anderson model. By including the effect of intra-atomic Coulomb correlation between conduction and local electrons, we find a first-order valence transition from Kondo region to mixed valence state upon ascending of local f- level above a critical , and this valence transition usually occurs very close to or simultaneously with a topological transition. Near the parameter region of zero-temperature valence transition, rise of temperature can generate a thermal valence transition from mixed valence to Kondo region, accompanied by a first-order topological transition. Remarkably, above a critical which is considerably smaller than that generating paramagnetic valence transition, the original continuous antiferromagnetic transition is shifted to first order one, at which a discontinuous valence shift takes place. Upon increasing , the paramagnetic valence transition approaches then converges with the first-order antiferromagnetic transition, leaving a significant valence shift on the magnetic boundary. The continuous antiferromagnetic transition, first-order antiferromagnetic transition, paramagnetic valence transition and topological transitions are all summarized in a global phase diagram. Our proposed exotic transition processes can help to understand the thermal valence variation as well as the valence shift around the pressure-induced magnetic transition in topological Kondo insulator candidates and in other heavy-fermion systems.

Effect of phonon induced spin-flip processes on correlated quantum dot kinetics

We present a theoretical analysis of relaxation processes in a correlated quantum dot (QD) coupled to a reservoir in the presence of electron-phonon interaction. The effect of both electron-phonon interaction and Coulomb correlations on system's kinetics is analyzed by means of equations of motion for generalized non-equilibrium Green's functions. It is shown that phonon-assisted tunneling enhances the relaxation processes in a correlated quantum dot. Moreover, electron-phonon interaction changes stationary values and typical relaxation rates of the second-order correlation functions for localized electrons. In spite of the fact that electron-phonon interaction is spin independent, it strongly influences spin correlations in the presence of Coulomb interactions between localized electrons. This effect appears to be due to phonon-induced spin-flip processes of localized electrons.

Electronic structure and magnetic properties of Ca2IrO4, using first principles

A comprehensive set of electronic structure calculations are performed to understand the origin of the insulating gap and magnetic properties of hexagonal Ca2IrO4. Although, isoelectronic with Sr2IrO4, the spin-orbit coupling driven Jeff model is anticipated to be less appropriate for Ca2IrO4 following its structural considerations. We find that the local density approximation (LDA), and those including the effects of Coulomb correlations and spin-orbit coupling fail to reproduce the experimental results. Moreover, the calculations employing the modified Becke-Johnson formalism seems to provide sufficiently good results, by predicting Ca2IrO4 to be an antiferromagnetic insulator. The origin of electronic gap is attributed to the antiferromagnetic ordering of Ir spins and the effects of spin-orbit coupling is found marginal. However, due to the anisotropy in the IrO bonding within the distorted IrO6 octahedra we deduce large magneto-anisotropic energy in Ca2IrO4. Further, our analysis shows that Ca2IrO4 is an itinerant material, suggesting that band structure effects play an important role in determining the ground state properties of iridates.

Effect of Coulomb Correlation on the Magnetic Properties of Mn Clusters.

In spite of decades of research, a fundamental understanding of the unusual magnetic behavior of small Mn clusters remains a challenge. Experiments show that Mn2 is antiferromagnetic while small clusters containing up to five Mn atoms are ferromagnetic with magnetic moments of 5 μB/atom and become ferrimagnetic as they grow further. Theoretical studies based on density functional theory (DFT), however, find Mn2 to be ferromagnetic, with ferrimagnetic order setting in at different sizes that depend upon the computational methods used. While quantum chemical techniques correctly account for the antiferromagnetic ground state of Mn2, they are computationally too demanding to treat larger clusters, making it difficult to understand the evolution of magnetism. These studies clearly point to the importance of correlation and the need to find ways to treat it effectively for larger clusters and nanostructures. Here, we show that the DFT+ U method can be used to account for strong correlation. We determine the on-site Coulomb correlation, Hubbard U self-consistently by using the linear response theory and study its effect on the magnetic coupling of Mn clusters containing up to five atoms. With a calculated U value of 4.8 eV, we show that the ground state of Mn2 is antiferromagnetic with a Mn-Mn distance of 3.34 Å, which agrees well with the electron spin resonance experiment. Equally important, we show that on-site Coulomb correlation also plays an important role in the evolution of magnetic coupling in larger clusters, as the results differ significantly from standard DFT calculations. We conclude that for a proper understanding of magnetism of Mn nanostructures (clusters, chains, and layers) one must take into account the effect of strong correlation.