On-site Coulomb Correlation Research Articles

Partial molecular orbitals in face-sharing 3d manganese trimer: Comparative studies on Ba4TaMn3O12 and Ba4NbMn3O12

We present a molecular orbital candidate Ba4TaMn3O12 with a face-sharing octahedra trimer, by comparing it with a related compound Ba4NbMn3O12. The synthesis of the polycrystalline powder is optimized by suppressing the secondary impurity phase via x-ray diffraction. Magnetic susceptibility measurements on the optimized samples reveal a weak magnetic hysteresis with magnetic transitions consistent with heat capacity results. The effective magnetic moments from susceptibility indicate a strongly coupled S=2 antiferromagnetic trimer at around room temperature, whereas the estimated magnetic entropy from heat capacity suggests the localized S=3/2 timer. These results can be explainable by a partial molecular orbital state, in which three t2g electrons are localized in each Mn ion and one eg electron is delocalized over two-end Mn ions of the trimer based on density functional theory calculations. This unconventional 3d orbital state is comprehended as a consequence of competition between the hybrid interatomic orbitals within the Mn trimer and the local moment formation by on-site Coulomb correlations. Published by the American Physical Society 2024

Oxygen on-site Coulomb energy in Pr1.3−xLa0.7CexCuO4 and Bi2Sr2CaCu2O8+δ and its relation with Heisenberg exchange

We study the electronic structure of electron-doped Pr$_{1.3-x}$La$_{0.7}$Ce$_{x}$CuO$_{4}$ (PLCCO ; $T_{c}$ = 27 K, x = 0.1) and hole-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212 ; $T_{c}$ = 90 K) cuprate superconductors using x-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (Res-PES). From Res-PES across the O K-edge and Cu L-edge, we identify the O 2p and Cu 3d partial density of states (PDOS) and their correlation satellites which originate in two-hole Auger final states. Using the Cini-Sawatzky method, analysis of the experimental O 2p PDOS shows an oxygen on-site Coulomb energy for PLCCO to be $U_{p}$ = 3.3$\pm$0.5 eV and for Bi2212, $U_{p}$ = 5.6$\pm$0.5 eV, while the copper on-site Coulomb correlation energy, $U_{d}$ = 6.5$\pm$0.5 eV for Bi2212. The expression for the Heisenberg exchange interaction $J$ in terms of the electronic parameters $U_{d}$, $U_{p}$, charge-transfer energy $\Delta$ and Cu-O hopping $t_{pd}$ obtained from a simple Cu$_2$O cluster model is used to carry out an optimization analysis consistent with $J$ known from scattering experiments. The analysis also provides the effective one band on-site Coulomb correlation energy $\tilde{U}$ and the effective hopping $\tilde{t}$. PLCCO and Bi2212 are shown to exhibit very similar values of $\tilde{U}$/$\tilde{t}$ $\sim$9-10, confirming the strongly correlated nature of the singlet ground state in the effective one-band model for both the materials.

Magneto-strain effects in 2D ferromagnetic van der Waal material CrGeTe_3

The idea of strain based manipulation of spins in magnetic two-dimensional (2D) van der Waal (vdW) materials leads to the development of new generation spintronic devices. Magneto-strain arises in these materials due to the thermal fluctuations and magnetic interactions which influences both the lattice dynamics and the electronic bands. Here, we report the mechanism of magneto-strain effects in a vdW material CrGeTe_3 across the ferromagnetic (FM) transition. We find an isostructural transition in CrGeTe_3 across the FM ordering with first order type lattice modulation. Larger in-plane lattice contraction than out-of-plane give rise to magnetocrystalline anisotropy. The signature of magneto-strain effects in the electronic structure are shift of the bands away from the Fermi level, band broadening and the twinned bands in the FM phase. We find that the in-plane lattice contraction increases the on-site Coulomb correlation (U_{eff}) between Cr atoms resulting in the band shift. Out-of-plane lattice contraction enhances the d-p hybridization between Cr–Ge and Cr–Te atoms which lead to band broadening and strong spin-orbit coupling (SOC) in FM phase. The interplay between U_{eff} and SOC out-of-plane gives rise to the twinned bands associated with the interlayer interactions while the in-plane interactions gives rise to the 2D spin polarized states in the FM phase.

Unraveling the strain and inherent onsite-correlation effect on the electronic structure of pure and iso-electronic Ag doped copper nitride

Promising optoelectronic properties of semiconducting copper nitride (Cu3N), have brought the material into limelight for numerous device applications. As band gap (Eg), undoubtedly is a key parameter in determining proficiency of a semiconductor in optoelectronics, hence its modulation needs clear understanding both from the fundamental and application perspectives. The present study discusses the impact of inherent onsite Coulomb-correlation (through Hubbard U) and strain on the electronic properties of pure and iso-electronic element (Ag) doped Cu3N using density functional calculations. A slow rate of increase in Eg with U recognizes Cu3N a weakly correlated system. Alternately, U for the doped system (Cu2AgN) is discovered to have further weaker effect on the Eg, nearly independent for U > 2 eV. However, doping of the 4d electronic element in the 3d transition metal nitride predicts an increased carrier mobility which will subsequently improve electrical conductivity. While examining the effect of strain in pristine Cu3N, the lowest conduction band is found to have both approaching and avoiding feature with respect to the Fermi level at two different high symmetry k-points. Consequently, Eg decreases (increases) for the compressive (tensile) strain which is attributed to the higher (lower) dispersive nature of the valence bands due to stronger (weaker) p-d hybridization. In case of Cu2AgN, though a similar shifting in the lower conduction band is observed, the variation of Eg with strain is quite different to that of Cu3N. While Eg increases (decreases) for the uniaxial compressive (tensile) strain, it decreases both for tensile and compressive strains corresponding to the isotropic deformations.

Theoretical and experimental study of effects of Co2+ doping on structural and electronic properties of ZnO

In this study, we theoretically investigated the electronic structure of Zn1-xCoxO using different approaches for the exchange-correlation potential comprising GGA with the on-site Coulomb correlation interaction U to the Zn d orbital (GGA + UZn), GGA with modified Becke–Johnson exchange potential (GGA + mBJ), and (GGA + mBJ + UZn). To support the theoretical results, a Co2+-doped ZnO sample was obtained experimentally by using the microwave–hydrothermal method. Changes in the structural, vibrational, electronic, and magnetic properties induced by the insertion of the Co2+ impurities in the ZnO lattice were determined based on first principles calculations. The theoretical results showed that the 3d orbitals derived from Co2+ appear in the deep region of the band gap. These orbitals are responsible for the magnetic behavior of cobalt doped materials. The energy levels introduced by the Co2+ dopant ions reduced the theoretical band gap value, which was also observed experimentally. The addition of Co2+ ions weakened the Raman mode E2H intensity, which was attributed to the increasing distortion induced by doping. Photoluminescence spectroscopy results indicated a reduction in the visible emission region after adding Co2+ ions, thereby indicating the formation of alternative pathways for the recombination process.

Effect of finite temperature and external magnetic field on non-equilibrium transport in a single molecular transistor with quantum dissipation: Anderson-Holstein-Caldeira-Leggett model

A magnetic field and temperature dependent quantum transport is investigated in a single molecular transistor. Our system is modelled by an onsite electron-electron correlation, polaronic interaction in the quantum dot and the coupling between quantum dot local phonon and the substrate phonon which produces a damping effect on the dynamics of the quantum dot. We use the Anderson-Holstein Hamiltonian along with the Caldeira-Leggett model to tackle the dissipation and polaronic effects and mean-field Hartree-Fock approximation to treat the onsite Coulomb correlation. We calculate transport properties such as spectral density function, tunnelling current, differential conductance employing finite-temperature Keldysh non-equilibrium Green function method under the influence of a finite temperature and an external magnetic field. It is shown that temperature has an intervening effect on the spin-resolved transport properties which can be used as a tuneable spin-filter.

Metal-insulator transition and band magnetism in the spin-1∕2 Falicov-Kimball model on a triangular lattice with external magnetic field

Ground state properties of the spin$-1/2$ Falicov-Kimball model on a triangular lattice in the presence of uniform external magnetic field are explored. Both the orbital and the Zeeman field-induced effects are taken into account and in each unit cell only rational flux fractions are considered. Numerical results, obtained with the help of Monte Carlo simulation algorithm, reveal that the ground state properties strongly depend on the onsite Coulomb correlation between itinerant and localized electrons, orbital magnetic field as well as the Zeeman splitting. Strikingly, for the on-site Coulomb correlation $U/t \approx 1$, the Zeeman splitting produces a phase transition from paramagnetic metal/insulator to ferromagnetic insulator/metal transition in the itinerant electron subsystem accompanied by the phase segregation to the bounded/regular phase in the localized electrons subsystem. For the onsite Coulomb correlation $U/t \approx 5$, although no metal to insulator transition is observed but a magnetic phase transition from paramagnetic phase to ferromagnetic phase in the itinerant electron subsystem is observed with the Zeeman splitting. These results are applicable to the layered systems e.g. cobaltates, rare earth and transition metal dichalcogenides, $GdI_{2}$, $NaTiO_{2}$, $NaVO_{2}$ and $Be_{x}Zn_{1-x}O$ etc. It has been also proposed that the results can be realized in the optical lattices with mixtures of light atoms and heavy atoms using the cold atomic techniques.

Antiferromagnetism-induced second-order nonlinear optical responses of centrosymmetric bilayer CrI3

Antiferromagnetism (AF) in AB′-stacked centrosymmetric bilayer (BL) CrI3 breaks both spatial inversion (P) and time-reversal (T) symmetries but maintains the combined PTsymmetry, thus inducing novel second-order nonlinear optical (NLO) responses such as second-harmonic generation (SHG), linear electric-optic effect (LEO) and bulk photovoltaic effect (BPVE). In this work, we calculate AF-induced NLO responses of the BL CrI3 based on the density functional theory with the generalized gradient approximation (GGA) plus onsite Coulomb correlation (U), i.e., the GGA+U method. Interestingly, we find that the magnetic SHG, LEO and photocurrent in the AF BL CrI3 are huge, being comparable or even larger than that of the well-known nonmagnetic noncentrosymmetric semiconductors. For example, the calculated SHG coefficients are in the same order of magnitude as that of MoS2 monolayer (ML), the most promising 2D material for NLO devices. The calculated LEO coefficients are almost three times larger than that of MoS2 ML. The calculated NLO photocurrent in the CrI3BL is among the largest values predicted so far for the BPVE materials. On the other hand, unlike nonmagnetic semiconductors, the NLO responses in the AF BL CrI3 are nonreciprocal and also switchable by rotating magnetization direction. Therefore, our interesting findings indicate that the AF BL CrI3 will not only provide a valuable platform for exploring new physics of low-dimensional magnetism but also have promising applications in magnetic NLO and LEO devices such as frequency conversion, electro-optical switches, and light signal modulators as well as high energy conversion efficiency photovoltaic solar cells.

Persistent charge and spin currents in the 1D Holstein-Hubbard ring at half filling and at away from half filling by Bethe-ansatz approach

The persistent charge and spin currents are studied by considering the Holstein-Hubbard model for one dimensional mesoscopic ring. Conventional Lang-Firsov transformation is used to eliminate the phonon coordinates and then the zero phonon state is applied to obtain the effective electronic Hamiltonian which is then treated exactly by the Bethe-Ansatz technique. The coupled transcendental equations that arise in this method are solved numerically and the persistent charge and spin currents are studied with respect to the on-site electron-electron Coulomb correlation (U), on-site electron-phonon interaction strength (g0) and inter-site electron-phonon interaction strength (g1) at half-filling and away from half-filling.

Role of hybridization and on-site correlations in generating plasmons in strongly correlated La2CuO4

Electronic correlation has been shown to play an important role in generating plasmons unconventionally in strongly correlated electron systems. In this work, we calculate the band structure, complex dielectric function, reflectivity, and loss function of ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$ in its insulating and antiferromagnetic phase through first-principle calculations. We find strong evidence of unconventional plasmons that are generated by hybridizations and enhanced by on-site Coulomb correlations. Interestingly, these unconventional plasmons, predominantly located in the Cu-O planes, are driven by hybridizations between $\mathrm{La}\phantom{\rule{0.16em}{0ex}}5p$/$\mathrm{Cu}\phantom{\rule{0.16em}{0ex}}3d$ and out-of-plane ${\mathrm{O}}_{\mathrm{z}}\phantom{\rule{0.16em}{0ex}}2p$. On the other hand, in-plane oxygen ${\mathrm{O}}_{\mathrm{xy}}\phantom{\rule{0.16em}{0ex}}2p$ induces conventional plasmons. This nonuniform hybridization scheme creates an anisotropic complex dielectric function and loss function, which can be influenced by conventional plasmons, unconventional plasmons, or a mixed state between both. Our result shows that the interplay of hybridizations, particularly involving oxygen, and on-site Coulomb correlations plays an important role in determining the properties of unconventional plasmons in strongly correlated ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$.

Orbital ordering and magnetism in layered Perovskite Ruthenate Sr2RuO4

Local density approximation plus on-site Coulomb interaction U electronic structure calculations reveal that layered perovskite oxide Sr2RuO4 exhibits the ferromagnetic (FM) half-metallic ground state, which is nearly degenerate with the antiferromagnetic (AFM) phase with a slightly higher total energy. The nearly degenerate FM/AFM total energies provide a reasonable explanation for the experimentally observed spin-fluctuation. In addition, a dumbbell-shape 4d − t2g recombined dxz − dyz orbital ordering on the Ru sublattice is obtained owing to the on-site Coulomb interaction U associated with the elongated RuO6 octahedron local structure. The discovered orbital ordering is robust against the spin-orbit interaction as well as the surface terminations. Our findings unravel the on-site Coulomb correlation as the driving force of the Ru-4d orbital ordering as well as the inherent magnetic degeneracy.

Evidence for undoped Weyl semimetal charge transport in Y2Ir2O7

Weyl fermions scattering from a random Coulomb potential are predicted to exhibit resistivity versus temperature in a single particle model. Here we show that, in closed-environment-grown polycrystalline samples of Y2Ir2O7, over four orders of magnitude in . While the measured prefactor, , is obtained from the model using reasonable materials parameters, the behavior extends far beyond the model’s range of applicability. In particular, the behavior extends into the low-temperature, high-resistivity region where the Ioffe-Regel parameter, . Strong on-site Coulomb correlations, instrumental for predicting a Weyl semimetal state in Y2Ir2O7, are the possible origin of such ‘bad’ Weyl semimetal behavior.

Correlation effect on the electronic properties of pair-checkerboard AFM monolayer FeSe: a first-principles study

We presented a study of the influence of on-site Coulomb correlation U and Hund coupling J on the electronic properties of monolayer FeSe in a pair-checkerboard antiferromagnetic (PAFM) state based on Hubbard-corrected density functional theory (DFT + U). The results demonstrated the Hubbard-U have a strong influence on the electronic bands and lattice structure of monolayer FeSe in PAFM state. The in-plane lattice constant changes under U > 1.85 eV has been identified under full structure optimization. Furthermore, the Hund coupling J has significant influence on the electronic bands and exhibits a remarkable orbital selective behavior at U = 0. Additionally, our results also demonstrated the Hund coupling J has a weaker effect on electronic properties of FeSe when U gets larger. These results are useful for elucidating the possible role of correlation strength on the electronics band and magnetic ground state of monolayer FeSe.

Tight-Binding Model Study of the Tunable Anti-ferromagnetism and Electron specific heat of AA-Staked bi-layer Graphene in a Transverse Electric Field

We communicate a microscopic theoretical model study of electronic and thermal properties of AA-stacked bi-layer graphene in a transverse electric field. In order to describe the system, we have considered a Hamiltonian consisting of nearest-neighbor electron hopping in both the layers in presence of a transverse electric field between the two layers created by a gate electrode. The inter-layer electron hopping from A1 atom of the first layer to A2 atom of the second layer is considered with a hopping energy εk,⊥=t⊥γ⊥(k). Due to strong on-site Coulomb correlation between the electrons of sub-lattices of both the layers, anti-ferromagnetic order is developed in the system. The electron Green's functions of both sub-lattices in the two layers are solved by Zubarev’s Green's function technique. The anti-ferromagnetic (AFM) gap equation is found from the electron correlations of the corresponding Green’s functions. It is assumed that the A-site magnetization dominates over the B-site magnetization giving rise to the anti-ferromagnetic order. The temperature dependent AFM gap exhibits high Neel’s temperature where electronic specific heat shows a sharp jump. Besides this, the specific heat exhibits another flat peak arising due to the onset of AFM order in A and B sub-lattices of the layer. The evolution of the gap equation and specific heat are reported by varying the model parameters like Coulomb energy, electric field, transverse electron hopping integral and chemical potential.

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.

Phase Transitions in Spin-1/2 Falicov–Kimball Model on a Two-dimensional Triangular Lattice

Phase transitions from low-temperature (ordered) phases to high-temperature (disordered/homogeneous) phases for different fillings are studied on a triangular lattice using the spin-dependent Falicov–Kimball model. Numerical diagonalization and Monte Carlo simulation methods are used to study thermodynamic properties of the system. It has been observed that low-temperature ordered phases persist up to a finite temperature and after reaching a critical temperature ( $$T_c$$ ), homogeneous phases are observed for all parameter space. We have also calculated the temperature dependence of specific heat and observed a sharp jump at $$T_c$$ indicating the phase transition, and this $$T_c$$ increases with increase in on-site Coulomb correlation U and electron fillings.

High-pressure studies on the properties of FeGa3 : Role of on-site Coulomb correlation

High pressure X-ray diffraction measurements have been carried out on the intermetallic semiconductor FeGa$_3$ and the equation of state for FeGa$_3$ has been determined. First principles based DFT calculations within the GGA approximation indicate that although the unit cell volume matches well with the experimentally obtained value at ambient pressure, it is significantly underestimated at high pressures and the difference between them increases as pressure increases. GGA + U calculations with increasing values of U$_{Fe(3d)}$ (on-site Coulomb repulsion between the Fe 3d electrons) at high pressures, correct this discrepancy. Further, the GGA+U calculations also show that along with U$_{Fe(3d)}$, the Fe 3d band width also increases with pressure and around a pressure of 4 GPa, a small density of states appear at the Fermi level. High pressure resistance measurements carried out on FeGa$_3$ also clearly show a signature of an electronic transition. Beyond the pressure of 19.7 GPa, the diffraction peaks reduce in intensity and are not observable beyond $\sim$ 26 GPa, leading to an amorphous state.

Sr(Ti<sub>0.875</sub>Fe<sub>0.125</sub>)O<sub>3</sub>薄膜能带结构的垂直应变调控效应研究

本文采用密度泛函理论框架下考虑强关联效应的广义梯度近似方法(GGA + U)研究了SrTi1-xFexO3(x = 0.125)薄膜的能带结构，又利用c轴晶格常数拉伸或压缩模拟了SrTi0.875Fe0.125O3薄膜中的垂直张应变和压应变。在基态晶胞结构下，Fe掺杂离子导致了晶胞内空间电荷密度重新排布，并使得非磁性离子周围出现了磁化密度分布。同时借助不同强度的垂直应变作用，实现了SrTi0.875Fe0.125O3薄膜能带结构的连续变化。进而发现垂直张应变能够在很大程度上改善SrTi0.875Fe0.125O3薄膜的半金属特性。 The calculations were performed by means of the generalized gradient approximation with on-site coulomb correlation corrections (GGA + U) within the framework of density functional theory (DFT) to study the band structures of SrTi1-xFexO3(x = 0.125) thin films, and the vertical tensile and compressive strain were simulated by the expansion of the out-of-plane crystal lattice in the SrTi0.875Fe0.125O3 films. In the stable cell of SrTi0.875Fe0.125O3, the introduction of doped Fe ions led to the re-arrangement of the charge density in the supercell, and the magnetization density distribution occurred around the non-magnetic ions. Meanwhile, the band structures could be continuously tuned by the different values of vertical strain. Furthermore, the vertical tensile strain can largely improve the half-Metallic behavior of the SrTi0.875Fe0.125O3 films.

Covalency effects reflected in the magnetic form factor of low-dimensional cuprates

We analyze the magnetic form factor of Cu$^{2+}$ in low-dimensional quantum magnets by taking the metal-ligand hybridization into account explicitly. In this analysis we use the form of magnetic Wannier orbitals, derived from the first-principles calculations, and identify the contributions of different atomic sites. Having performed local density approximation calculations for cuprates with different types of ligand atoms, we discuss the influence of the on-site Coulomb correlations on the structure of the magnetic orbital. The typical composition of Wannier functions for copper oxides, chlorides and bromides is defined and related to features of the magnetic form factor. We propose easy-to-use approximations of the partial orbital contributions to the magnetic form factor in order to give a microscopic explanation for the results obtained in previous first-principles studies.

Study of electronic structure of Co2MnSn Heusler alloy by resonant photoemission spectroscopy and ab initio calculations

The electronic structure of a ferromagnetic Heusler alloy Co2MnSn has been investigated using resonant photoemission spectroscopy to probe the Mn 3d and Co 3d states in the valence band region. The motivation of the present work is to understand the low value of spin polarization reported for this system, which mainly depends on the density of states present at the Fermi level. Valence band of Co2MnSn shows a large density of delocalized Co 3d states present near the Fermi level. The Mn 3d states are more localized and exist below the Fermi level. First principles calculations using density functional theory with GGA and GGA+U formalisms have been performed to understand and analyze the experimental results. The experimental density of states and magnetic moment show a good matching with the calculation carried out with on-site Coulomb correlation U of ∼3eV on the Mn 3d states. The presence of U on the Mn 3d states also reduces the spin polarization at the Fermi level, which is one of the reasons for the observed low value of spin polarization reported for this system.