On-site Coulomb Interaction Research Articles

Density-matrix renormalization group study of optical conductivity of the Mott insulator for two-dimensional clusters

The real part of optical conductivity $\text{Re}\sigma(\omega)$ of the Mott insulators has a large amount of information on how spin and charge degrees of freedom interact with each other. By using the time-dependent density-matrix renormalization group, we study $\text{Re}\sigma(\omega)$ of the two-dimensional Hubbard model on a square lattice at half filling. We find an excitonic peak at the Mott-gap edge of $\text{Re}\sigma(\omega)$ not only for the two-dimensional square lattice but also for two- and four-leg ladders. For the square lattice, however, we do not clearly find a gap between an excitonic peak and continuum band, which indicates that a bound state is not well defined. The emergence of an excitonic peak in $\text{Re}\sigma(\omega)$ implies the formation of a spin polaron. Examining the dependence of $\text{Re}\sigma(\omega)$ on the on-site Coulomb interaction and next-nearest neighbor hoppings, we confirm that an excitonic peak is generated from a magnetic effect. Electron scattering due to an electron-phonon interaction is expected to easily suppress an excitonic peak since spectral width of an excitonic peak is very narrow. Introducing a large broadening in $\text{Re}\sigma(\omega)$ by modeling the electron-phonon coupling present in La$_{2}$CuO$_{4}$ and Nd$_{2}$CuO$_{4}$, we obtain $\text{Re}\sigma(\omega)$ comparable with experiments.

Magnetic order, disorder, and excitations under pressure in the Mott insulator Sr2IrO4

Protected by the interplay of on-site Coulomb interactions and spin-orbit coupling, ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ at high pressure is a rare example of a Mott insulator with a paramagnetic ground state. Here, using optical Raman scattering, we measure both the phonon and magnon evolution in ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ under pressure and identify three different magnetically-ordered phases, culminating in a spin-disordered state beyond 18 GPa. A strong first-order structural phase transition drives the magnetic evolution at $\ensuremath{\sim}10$ GPa with reduced structural anisotropy in the ${\mathrm{IrO}}_{6}$ cages, leading to increasingly isotropic exchange interactions between the Heisenberg spins and a spin-flip transition to $c$-axis-aligned antiferromagnetic order. In the disordered phase of Heisenberg ${J}_{\mathrm{eff}}=1/2$ pseudospins, the spin excitations are quasielastic and continuous to 10 meV, potentially hosting a gapless quantum spin liquid in ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$.

Multipolar nematic state of nonmagnetic FeSe based on DFT+U

Clarifying the origin of nematic state in FeSe is one of urgent problems in the field of iron-based superconductivity. Motivated by the discovery of a nematic solution in the density-functional theory implemented by on-site Coulomb interaction ($\mathrm{DFT}+U$) [npj Quantum Mater. 5, 50 (2020)], we reexamine the $U$ dependence of electronic states in the nonmagnetic normal state of FeSe and perform full multipolar analyses for the nematic state. We find that with increasing $U$ the normal state experiences a topological change in the Fermi surfaces before the emergence of a nematic ground state. The resulting nematic ground state is a multipolar state having both antiferrohexadecapoles in the $E$ representation and ferromultipoles in the ${B}_{2}$ representation on each Fe site. Cooperative coupling between the $E$ and the ${B}_{2}$ multipoles in the local coordinate with the ${D}_{2d}$ point group will play an important role in the formation of the ${d}_{xz},{d}_{yz}$ orbital-splitting nematic state not only in FeSe, but also in other iron pnictides.

Effect of On-site Coulomb Repulsion on Ferromagnetic Fluctuations in Heavily Overdoped Cuprates

We theoretically study ferromagnetic (FM) fluctuations that are experimentally observed in the heavily overdoped region of cuprate superconductors. To explore the origin of FM fluctuations, we evaluate the spin susceptibilities of a single-band Hubbard model within the fluctuation exchange approximation. Model parameters are derived using the Wannierization technique and the constrained random phase approximation method based on the maximally localized Wannier functions. The constrained random phase approximation calculations reveal that the on-site Coulomb interaction decreases with an increase in hole doping. By taking this reduction of the on-site Coulomb interaction into account, the emergence of FM fluctuations in heavily overdoped cuprates can be explained.

First-principles investigation of the metal-insulator transition in rutile RuO2

The electronic and magnetic properties of rutile ruthenium dioxide (RuO2) are extensively investigated by employing density functional theory. The material RuO2 is observed as a nonmagnetic metal. The sharing of a single Ru 4d electron by the dyz/xz orbitals is responsible for the metallicity of RuO2. The ground-state properties of this material change significantly upon the application of on-site Coulomb interaction U. RuO2 preserves its nonmagnetic metallic character up to U=3 eV. Nevertheless, RuO2 encounters a first step transition from nonmagnetic metal to a ferromagnetic metal (FM) at U=4 eV. The present system remains in its FM phase till U=5 eV. It exhibits a second step transition from the FM to a half-metallic ferromagnetic phase at U=6 eV. Eventually, RuO2 encounters metal-insulator transition (MIT) at U=7 eV with a band gap of Eg ∼0.3 eV upholding ferromagnetism. It is revealed that the spin polarization arising from the strong dynamical electron correlation due to the application of U is responsible for the metal to half-metal and MIT in RuO2.

Multiband [formula omitted] model and the electronic structure of quasi-two dimensional Nd[formula omitted]Sr[formula omitted]NiO[formula omitted

We study the tight-binding model of NiO2 planes in novel superconductors that takes into account anisotropic nickel–oxygen d−p hopping, oxygen–oxygen p−p hoppings, realistic crystal-field splittings, and the on-site Coulomb interactions {U,JH} both at nickel and oxygen ions. Electronic structure of the NiO2 layer is investigated for doping x=0 and x=0.25. For an ideal undoped system NdNiO2 (x=0) we get robust Mott insulator and no magnetism as various magnetic ground states (ferromagnetic, C-type and G-type antiferromagnetic) have all the same energy. For self-doping x=0.25 the system becomes conducting (zero HOMO–LUMO gap), with nonmagnetic ground state due to competing degenerate magnetic meta-stable states. These findings correlate well with experimental and theoretical data. However, it seems that the two-dimensional model with p and d orbitals only is insufficient for a realistic description of Nd1−xSrxNiO2.

Electronic and magnetic properties of LaRuO3 and LaFeO3: Orbital order and canted antiferromagnetism

The electronic and magnetic structures of LaRuO3 has been attracted much attention. The remaining problem is to identify and understand the ground-state of LaRuO3. To clarify the situation we chose the LaFeO3 as a comparison and evaluated the ground-state properties by first-principles calculations. We found the LaRuO3 was GdFeO3-type Jahn-Teller distorted for the longer Ru-O bonds. The electronic structure calculations identified the LaRuO3 was a G-type antiferromagnetic insulator. The low spin eg0t2g5 population and the RuO6 octahedron distortion induced the LaRuO3 orbital ordered which was found from the spin charge density. We also found the onsite Coulomb interactions U regulated the ground-state and the spin-orbital coupling persisted the orbital order. The simulations of magnetic properties suggested the LaRuO3 was canted antiferromagnetic ordered with a lower Neel temperature of 90 K compared to the 802 K of LaFeO3. The difference between LaFeO3 and LaRuO3 revealed that the 4d electrons in LaRuO3 introduced an orbital freedom.

Sub-gap Fano resonances in a topological superconducting wire with on-site Coulomb interactions

We consider theoretically a 1D-semiconducting wire with strong Rashba interaction in proximity with s-wave superconductor, driven into topological phase by external magnetic field. Additionally, we take into account on-site Coulomb interactions inside the wire. The system is modelled by a tight binding Hamiltonian with Rashba hopping term and induced s-wave superconductivity. Calculations are performed utilizing recursive Green’s function method, and Coulomb interactions are treated selfconsistently within Hubbard I approximation. For the Hubbard levels residing within p-wave superconducting gap, particle–hole symmetric four-resonance structure develops in the density of states, apart from Majorana resonance. One pair of particle–hole symmetric resonances is created by the discrete II-Hubbard levels of the particular site, and the second pair of Hubbard sub-bands originates from recursive summation over the sites of the wire. Quantum interference between both types of pairs of states creates in-gap charge-conjugated Fano resonances with opposite asymmetry factors. We demonstrate that when quantum interference is dominated by two-particle tunneling, the Majorana resonance is strongly diminished, while it is not altered when single-particle tunneling dominates in interference process. We also discuss some consequences for experimental distinction of true Majorana states, and show that on-site Coulomb interactions support the appearance of topological phase.

Quantum interference effects in multi-channel correlated tunneling structures

In multi-channel tunneling systems quantum interference effects modify tunneling conductance spectra due to Fano effect. We investigated the impact of Hubbard type Coulomb interaction on tunneling conductance spectra for the system formed by several interacting impurity atoms or quantum dots localised between the contact leads. It was shown that the Fano shape of tunneling conductance spectra strongly changes in the presence of on-site Coulomb interaction between localised electrons in the intermediate system. The main effect which determines the shape of the tunneling peaks could be not Fano interference but mostly nonequilibrium dependence of the occupation numbers on bias voltage.

Magnetic ordering tendencies in hexagonal-boron-nitride–bilayer-graphene moiré structures

When hexagonal boron nitride (hBN) and graphene are aligned at zero or a small twist angle, a moir\'e structure is formed due to the small lattice constant mismatch between the two structures. In this paper, we analyze magnetic ordering tendencies, driven by on-site Coulomb interactions, of encapsulated bilayer graphene (BG) forming a moir\'e structure with one (hBN-BG) or both hBN layers (hBN-BG-hBN), using the random phase approximation. The calculations are performed in a fully atomistic Hubbard model that takes into account all $\ensuremath{\pi}$ electrons of the carbon atoms in one moir\'e unit cell. We analyze the charge neutral case and find that the dominant magnetic ordering instability is uniformly antiferromagnetic. Furthermore, at low temperatures, the critical Hubbard interaction ${U}_{c}$ required to induce magnetic order is slightly larger in those systems where the moir\'e structure has caused a band gap opening in the noninteracting picture, although the difference is less than 6%. Mean-field calculations are employed to estimate how such an interaction-induced magnetic order may change the observable single-particle gap sizes.

DFT study of structural, elastic, electronic, magnetic, thermal and transport properties of new multifunctional NiVSn half-Heusler for spintronic and thermoelectric applications

In this work, the first-principles density functional calculations of the structural, elastic, electronic, magnetic, thermal and thermoelectric properties of NiVSn half-Heusler compound are carried out. The exchange and correlation potential are treated by using Generalized Gradient approximation of Perdew, Burke and Ernzerhof (GGA-PBE), GGA plus Tran–Blaha-modified Becke–Johnson (mBJ-GGA) approach and mBJ-GGA+U where U is the Hubbard on-site Coulomb interaction correction (mBJ-GGA+U). Structural calculations revealed that NiVSn is stable in type 1 structure ferromagnetic state. Elastic properties show that our compound is mechanically stable, ductile and anisotropic. The results of the band structures and density of states display a half metallic behavior of NiVSn with an indirect bandgap of 0.476, 0.508 and 0.845 eV by using GGA-PBE, mBJ-GGA, and mBJ-GGA+U, respectively. The total magnetic moment calculated is integer of 1 [Formula: see text]B confirming a half metallic behavior of NiVSn and follows the well-known Slater–Pauling rule ([Formula: see text]); therefore, the studied compound is suitable for application in spintronic fields. The thermodynamic properties such as bulk modulus, the heat capacity, the Debye temperature, and the thermal expansion coefficient are investigated using quasi-harmonic Debye model (QHDM). The thermal results show that NiVSn can be applied in extreme temperature and pressure conditions. The thermoelectric properties are studied employing the BoltzTrap code. The calculated transport properties are very interesting for the spin-down channel with high electrical conductivity, high Seebeck coefficient, and figure of merit value approaching unity. As a result, the half-Heusler alloy NiVSn is a promoter for conventional thermoelectric materials.

Calculation of Band Offsets of Mg(OH)2-Based Heterostructures

The band alignment of Mg(OH)2-based heterostructures is investigated based on first-principles calculation. (111)-MgO/(0001)-Mg(OH)2 and (0001)-wurtzite ZnO/(0001)-Mg(OH)2 heterostructures are considered. The O 2s level energy is obtained for each O atom in the heterostructure supercell, and the band edge energies are evaluated following the procedure of the core-level spectroscopy. The calculation is based on the generalized gradient approximation with the on-site Coulomb interaction parameter U considered for Zn. For MgO/Mg(OH)2, the band alignment is of type II, and the valence band edge of MgO is higher by 1.6 eV than that of Mg(OH)2. For ZnO/Mg(OH)2, the band alignment is of type I, and the valence band edge of ZnO is higher by 0.5 eV than that of Mg(OH)2. Assuming the transitivity rule, it is expected that Mg(OH)2 can be used for certain types of heterostructure solar cells and dye-sensitized solar cells to improve the performance.

First-principles calculations on ferroelectricity and lattice dynamics of Type-II multiferroic SmMn2O5

In this work, we report on the structural, electronic, and ferroelectric properties of SmMn2O5 by using first-principles density functional theory plus on-site Coulomb interaction (DFT + U) calculations. A thorough analysis was preformed to reveal the competing characteristics of different high-temperature (T) phases and the polarization mechanism in the low-T multiferroic phase. We show that the structural characteristics of the high-T phases have a strong influence on the low-T multiferroicity. In addition to the spin-induced lattice distortion that reduces substantially the purely electronic ferroelectricity, the dominant polarization mechanism in low-T SmMn2O5 still originates from the electronic polarization. By performing mode decomposition of the Hellmann–Feynman forces and the lattice distortion induced by the q = (0.5, 0, 0) magnetic order, we find that the Raman-active Ag mode characterized by the Mn4+O6 octahedron distortion and synergistic displacement of Mn3+ and Sm ions is of primary importance, while the infrared (IR)-active B2u mode plays a secondary role. These findings provide a theoretical foundation for future studies concerning the enhanced magnetoelectric effects of SmMn2O5 due to its pure exchange–striction mechanism.

Modeling electrochemical properties of LiMn_{1-x}Co_{x}BO_3 for cathode materials in lithium-ion rechargeable batteries

In this work, we report first-principle calculations of the electrochemical properties of lithitated and delithiated LiMn_{1-x}Co_{x}BO_3 (x = 0, 0.25, 0.5, 0.75, 1) crystals based on the density functional theory (DFT) with the generalized gradient approximation (GGA) and also considering the on-site Coulomb interaction, the so-called Hubbard correction. We found that the top of the valence band and the bottom of the conduction band of these crystals are mainly formed by the hybridization of the 3d orbitals of mixed Mn_{1-x}Co_{x} ions and oxygen 2p orbitals. We observed a band gap narrowing with an increase of cobalt concentration and that the Hubbard correction implies a better theoretical description of their electronic structures. When considering the delithiated materials, our calculations show a metallic behavior for intermediate cobalt concentrations (x = 0.25, 0.5, 0.75), which is a good quality for cathodic materials, as it improves the battery discharge process. We also obtained high (4.14 V vs. Li^+/Li^0 and 4.16 V vs. Li^+/Li^0) open circuit voltage (OCV) values at cobalt concentrations of x = 0.5 and 0.75, where we believe that if these high OCV values are accompanied by a high charge storage capacity, these compounds can become promising and useful cathode materials. Finally, our results are in accordance with previous calculations and also with experimental results.

Local structure and electronic structure of LiFePO4 as a cathode for lithium-ion batteries

The local and electronic structure are significant to determine the performance of electrodes in lithium-ion batteries (LIB). Here we report the local structure of LiFePO4 as a cathode of LIB studied by Fe K-edge on X-ray absorption spectroscopy (XAS). The mixture of oxidation state of Fe ion (Fe2+/Fe3+) and the nearest neighbor of Fe atom were reported in this study. Furthermore, the preliminary study of electronic structure shown by the calculated density of states in antiferromagnetic spin configurations exhibited the prominent role of on-site Coulomb interactions in LiFePO4 material.

Doping-Enabled Reconfigurable Strongly Correlated Phase in a Quasi-2D Perovskite.

Highlighted by the discovery of high-temperature superconductivity, strongly correlated oxides with highly distorted perovskite structures serve as intriguing model systems for pursuing emerging materials physics and testing technological concepts. While 3d correlated oxides with a distorted perovskite structure are not uncommon, their 4d counterparts are unfortunately rare. In this work, we report the tuning of the electrical and optical properties of a quasi-2D perovskite niobate CsBiNb2O7 via hydrogenation. It is observed that hydrogenation induces drastic changes of lattice dynamics, optical transmission, and conductance. It is suggested that changing the orbital occupancy of Nb d orbitals could trigger the on-site Coulomb interaction in the NbO6 octahedron. The observed hydrogen doping-induced electrical plasticity is implemented for simulating neural synaptic activity. Our finding sheds light on the role of hydrogen in 4d transition metal oxides and suggests a new avenue for the design and development of novel electronic phases.

Room-temperature-superconducting Tc driven by electron correlation

Room-temperature-superconducting Tc measured by high pressure in hydrides can be theoretically explained by a Brinkman–Rice (BR)–Bardeen–Cooper–Schrieffer (BCS) Tc combining both the generalized BCS Tc and the diverging effective mass, m*/m = 1/(1 − (U/Uc)2), with the on-site Coulomb interaction U in the BR picture. A transition from U in a correlated metal of the normal state to Uc in the superconducting state can lead to superconductivity, which can be caused by volume contraction induced by high pressure or low temperature.

Mixed Mott–Hubbard and charge transfer nature of 4H–SrMnO3 thin film on Si (100)

Room temperature electronic structure of polycrystalline 4H–SrMnO3 thin film grown on Si (100) substrate has been studied using resonance photo emission spectroscopy and soft x-ray absorption spectroscopy measurements. Presence of charge transfer screen Mn 3d n L final state along with the 3d n-1 final state at the valence band edge of 4H–SrMnO3 thin film confirms that the ground state is strongly mixed between Mn 3d and O 2p states. The estimated equivalent values of on-site Coulomb interaction energy (U) and O 2p to Mn 3d- charge transfer energy (Δ) (U ≈ Δ ≈ 4.8 eV) from the combination of occupied and unoccupied spectra further confirm the intermediate Mott–Hubbard and charge transfer insulator nature of 4H–SrMnO3 film. Despite having similar Mn 4+ valence state in 4H–SrMnO3 and cubic SrMnO3, 4H phase is observed to reveal much higher band gap ∼1.5 eV than the cubic phase (0.3 eV), which arises due to different MnO6 octahedra environment.

Multiple spin-orbit excitons in \u03b1-RuCl3 from bulk to atomically thin layers

The van der Waals Kitaev magnet α-RuCl3 has recently garnered considerable attention due to its possible realization of topological spin liquids. Combining Raman spectroscopy with numerical calculations, we report here the thickness dependence of electronic structure and ensuing low-energy excitations for exfoliated α-RuCl3. We observe two pronounced peaks at A1 = 249 meV and A2 = 454 meV, which are assigned to single and double spin-orbit (SO) excitons, respectively. Our numerical calculations support this interpretation by reproducing their spectral energy and shape with the electronic parameters: SO coupling λ = 140 meV, Hund’s coupling JH = 350 meV, and on-site Coulomb interaction U = 2.35 eV. The multiple SO excitons persist down to a single layer, whereas their peaks shift slightly to lower energy. For frequencies below 350 cm−1, both a magnetic continuum and phonons show noticeable thickness dependence. These results demonstrate that a SO entangled jeff = 1/2 picture remains valid in a monolayer limit despite the presence of lattice distortions.

Influence of On-Site Coulomb Interaction on Parameters of Superconducting State in Two-Dimensional Lattices

Influence of On-Site Coulomb Interaction on Parameters of Superconducting State in Two-Dimensional Lattices