Hubbard Approximation Research Articles

Exploring spin photovoltaics in defective armchair phosphorene nanoribbons

This study explores the spin photovoltaic potential within armchair phosphorene nanoribbons (APNRs) that feature a periodic distribution of monovacancies (MVs) under the influence of light radiation. We investigate spin-semiconducting behavior induced by MV defects by utilizing both the mean-field Hubbard approximation and the self-consistent non-equilibrium Green's function model. This behavior is characterized by localized and anisotropic band structures around the Fermi energy, particularly within the antiferromagnetic phase. The existence of spin-splitting band gaps in defective APNRs not only enables the crafting of spin-optoelectronic nanodevices but also allows for the manipulation of electronic structure behavior with applied electric fields in both the vertical and transverse directions. Notably, the implementation of electric fields, offering tunability in electronic structure, results in varied spin photovoltaic responses encompassing a broad spectrum of photon energies from visible to ultraviolet. This research reveals promising avenues for advancing the field of spin-optoelectronic devices by MVs in APNRs.

Understanding the structural, electro-magnetic, mechanical, thermal and transport properties of Strontium based perovskite (SrPrO3)

With the help of density functional theory, we have explored the structural, electro-magnetic, mechanical, thermal and transport properties of SrPrO3 perovskite. The optimization of energy reveals that the alloy is stable in ferromagnetic phase. The electronic and magnetic properties were calculated by generalized gradient approximation (GGA), Hubbard approximation (GGA + U) and Tran–Blaha modified Becke–Johnson (GGA+mBJ) potentials methods. The electronic structure depicts the half-metallic nature for the material in both the GGA and GGA+U approximations. The semiconducting nature is revealed by mBJ approximation. The elastic parameters reveal the ductile nature of the material. The response towards temperature and pressure of the different thermal parameters have also been evaluated with the help of quasi-harmonic Debye model. The thermoelectric potency of the material is measured in terms of Seebeck coefficient (S), electrical conductivity (σ/τ), thermal conductivity (κ), power factor and figure of merit (ZT). On the whole, these properties pose the possible applications of this material towards solid state device and thermoelectric sphere.

Multiphoton excitation and high harmonic generation in rectangular graphene quantum dot

The multiphoton excitation and high harmonic generation (HHG) processes are considered using the microscopic quantum theory of nonlinear interaction of strong coherent electromagnetic (EM) radiation with rectangular graphene quantum dot (RGQD). The dynamic Hartree–Fock approximation is developed for the consideration of the quantum dot-laser field nonlinear interaction at the nonadiabatic multiphoton excitation regime. The many-body Coulomb interaction is described in the extended Hubbard approximation. By numerical results, we show the significance of the RGQD lateral size, shape, and EM wavefield orientation in RGQD of the zigzag edge compear to the armchair edge in the HHG process allowing for increasing the cutoff photon energy and the quantum yield of higher harmonics. The differences via edge on the elongated side of the RGQD have been explained by the investigation of the dipole momentum in both cases. Numerical results have shown that the HHG spectra have a strong anisotropy depending on the orientation of the laser wave, and the cutoff photon energy shifts toward blue with an increase in the transverse size of the RGQD.

Magnetism in diamond graphene nanoflakes

We numerically study in this work the magnetic properties induced by the on-site electron-electron interaction in graphene nanoflakes shaped diamond with a variety of sizes. By the mean-field Hubbard approximation, a phase transition in analogy to infinite graphene from non-magnetism to antiferromagnetism is observed. A very weak interaction U, approximately zero, is reported to be able to trigger magnetic ordering in a finite nanoflake compared to infinite structure. Furthermore, the investigation also indicates the edge and size dependence of magnetism. The antiferromagnetic ground state is of robust stability to larger zigzag nanoflake size and stronger interaction. The phase transition point, Uc, is found to be sensitive to the size denoted by means of the reduction of Uc as the size increases. The important role of edge effect causing the spin polarization along zigzag termination is confirmed for the diamond nanoflakes.

On Laser-Induced High-Order Wave Mixing and Harmonic Generation in a Graphene Quantum Dot

We present results of numerical investigations of high-order wave mixing/harmonic generation (HWM/HHG) with many-body interaction processes derived by a strong two-frequency circular laser field in a graphene quantum dot (GQD). The influence of the relative phase of such a laser field on the spectrum of high-order harmonics generated in the GQD is shown. This may allow controlling the polarization of the generated harmonics. The GQD is described by the nearest neighbor tight-binding (TB) model. The multiparticle interaction is considered in an extended Hubbard approximation. We solve quantum kinetic equations in on-site representations for charged carriers by method have already applied by us, and obtain a general formula for high-order wave mixing/harmonic generation processes in GQD. The considerable enhancement of the HWM/HHG yield due to the matching of the symmetries of the light-wave–dot system takes place for the GQD with the particular group symmetry. The analysis of obtained results confirms enough efficiency of HWM/HHG in GQDs of the triangular and hexagonal shapes with zigzag edges at certain phases of a two-frequency circular laser field.

High harmonic generation with many-body Coulomb interaction in rectangular graphene quantum dots of armchair edge

High harmonic generation (HHG) in graphene quantum dots of armchair edge by strong coherent electromagnetic radiation taking into account collective electron-electron scattering as well in such nanosystems is considered. The microscopic quantum theory describing the plane quantum dot-laser field nonlinear interaction within the dynamic Hartree–Fock approximation is developed. The many-body Coulomb interaction is considered in the extended Hubbard approximation. The obtained results indicate the significance of the nanostructure lateral size, shape, and bandgap width in HHG process that allows to increase in the quantum yield of high-order harmonics and the cutoff photon energy.

Atomic collapse in graphene quantum dots in a magnetic field

We investigate finite size and external magnetic field effects on the atomic collapse due to a Coulomb impurity placed at the center of a hexagonal graphene quantum dot within tight binding and mean-field Hubbard approaches. For large quantum dots, the atomic collapse effect persists when the magnetic field is present, characterized by a series of Landau level crossings and anticrossings, in agreement with previous bulk graphene results. However, we show that a new regime arises if the size of the quantum dot is comparable to or smaller than the magnetic length: While the lowest bound states cross the Fermi level at a lower value of coupling constant β<0.5 , a size independent critical coupling constant βc∗>0.5 emerges in the local density of states spectrum, which increases with the applied magnetic field. These effects are found to be persistent in the presence of electron–electron interactions within mean-field Hubbard approximation.

Structural, magnetic and polar effects in R-doped ZnO (R = Co, Cr, Cu and V): Study of first principles

This work reports the first-principles calculations, which describe the structural, magnetic and polar properties of different supercells of ZnO and R-doped ZnO (R = Co2+, Cr2+, Cu2+ and V2+). Here the Hubbard approximation was applied in 3d orbitals of some transition metals and in the O-2p, allowing a more realistic approximation of the bandgap and the magnetic properties. The results show a reduction in the bandgap for all doping, from 3.07 eV for ZnO to 1.94 when ZnO is doped with Cu. In turn, the density of states is calculated and ferromagnetic states are induced by unpaired electrons in the transition metals. On the other hand, the ZnO polar state is caused by the relative displacement of ions along the c-axis, and the maximum polarization is observed in V-doped ZnO, which is associated with the decrease in the degree of covalence of ZnO. Thus, simple doping can produce magnetic and polar states, opening up a range of possibilities in the field of multifunctional materials.

Study of coulomb drag with the effect of local field correction and dielectric medium

We present a numerical calculation for the drag resistivity of charge carriers interaction in coupled layer system separated by a barrier. Analytical expressions of non-linear susceptibility function and effective interlayer Coulomb interaction have been shown for non homogeneous dielectric environment at low temperature and low frequency regime. The system is studied within the model of random phase approximation (RPA) and taking into account the static local field correction (LFC). We employ Hubbard approximations (HA) and self consistent STLS approximations based LFC for obtaining the results, where LFC includes the exchange and correlation effects. Exchange and correlation effects enhanced the drag resistivity and the net screening. Another important quantity is the local form factors (LFF), which is the function of the bare inter- and intra-layer potential of the dielectric media. LFF for a multi-layer system is derived from the Poisson equation solution.

Spin-thermoelectric transport in nonuniform strained zigzag graphene nanoribbons

We study a nonuniform strain in zigzag graphene nanoribbons for producing the spin-thermoelectric effects, using the mean-field Hubbard approximation and a Green's function approach. Our theoretical results show that a sinusoidal-shaped inhomogeneous strain with electron-electron interaction could induce a different effect on each edge of zigzag nanoribbons and finally generate a spin semiconductor with a tunable spin-dependent band gap. The strength of strain also controls the magnitude of magnetization in each edge. Interestingly, pure spin current and a giant spin Seebeck coefficient can be produced even at low values of strain by applying a thermal gradient and without magnetic elements. These results pave a practical way toward improved design for spin-thermoelectric applications through strain engineering.

Magnetocaloric effect, electronic and magnetic properties of Ba1-xSrxFeO3 barium-strontium ferrites: Monte Carlo simulations and comparative study between TB-mBJ and GGA+U

The magnetocaloric effect, electronic and magnetic properties of cubic perovskite oxide Ba1-xSrxFeO3 (BSFO) with x = 0 and 0.2 have been investigated using the method of linear augmented plane wave (FPLAPW) based on density functional theory (DFT) and Monte Carlo simulations. Spin polarization calculations has been studied using the full potential linearized augmented plane wave (FP-LAPW) method within generalized gradient approximation (GGA), Hubbard approximation (GGA + U), and modified Becke-Johnson potential (TB-mBJ). The total magnetic moment, Curie temperature, and exchange integrals were found to decrease with Sr substitution. The obtained results were in good agreement with the experimental ones. The spin polarization for Ba0.8Sr0.2FeO3 shows half-metallic ground state with the ferromagnetic coupling of Fe spin. The computed results are in good agreement with available theoretical results. The total magnetic moment and the exchange couplings are deduced from Ab initio calculations, using Monte Carlo simulations, to a quantitative agreement with the experimental transition temperatures. The magnetic entropy change decreased from 5.5 J.K−1. kg−1 to 4 J.K−1. kg−1 with substitution by Sr. This value of maximum magnetic entropy reaches the experimental value. Our results could show that BSFO has desirable magnetic properties that meet the needs of magnetic / spintronic devices.

Spin photocurrents in zigzag phosphorene nanoribbons: From infrared to ultraviolet

Using the self-consistent non-equilibrium Green’s function model and the mean-field Hubbard approximation, we studied the possibility of inducing the spin-photovoltaic effects in zigzag phosphorene nanoribbons. We numerically showed that an applied electric field could induce a spin-semiconducting behavior with anisotropic and localized band structures around the Fermi energy in the antiferromagnetic zigzag phosphorene nanoribbons. Moreover, a tunable energy gap with an electric field could induce a spin photocurrent in a wide range of photon energies. Interestingly, increasing the electric field strength induces the spin-valve effect from terahertz to infrared irradiation. Furthermore, ferromagnetic zigzag phosphorene nanoribbons reveal a spin-dependent photoresponsivity, which is induced by infrared to ultraviolet frequencies. These results could enhance photovoltaic effects with a generation of the spin photocurrent in phosphorene junctions.

DFT study on the electronic and magnetic properties of the Sr2FeNbO6 compound

The Sr2FeNbO6 (SFNO) compound is a material with potential applications to be used as an electrode in solid oxide fuel cells, as well as a photocatalyst material for hydrogen generation. However, there is much controversy on the crystal structure related to this compound. Establishing a crystal structure that correctly describes the electronic and magnetic properties presented by this compound is important to understand the origin of its properties. Theoretical DFT calculations were performed on the cubic, tetragonal, and orthorhombic structure based on the generalized gradient approximation (GGA) and the corrected local density of Hubbard approximation (LDA + U). Our results reveal that the orthorhombic and tetragonal structures are suitable models for reproducing an A-type AFM arrangement and an indirect energy band gap for SFNO in agreement with the reported experimental results if UFe = 10 and 9 eV are used respectively. Meanwhile, the cubic structure does not reproduce the SFNO properties since this modeled structure exhibits a metallic and ferromagnetic behavior.

Cation effect on electronic, optical and thermoelectric properties of perovskite oxynitrides: Density functional theory

Perovskite oxynitride are of great interest due to their use as photo-catalysts. We investigated the electronic, thermoelectric and optical properties of ATaO2N (A = Ba, Sr, Ra) with generalized gradient plus Hubbard approximation (GGA + U). In electronic properties, we have computed, for each compound, densities of states (total and partial), band structure to extract their band gaps, nature and transition probability when the cation changes. Optical properties were explored by calculating dielectric constant ε (ω), refractive index n (ω), reflectivity R (ω), optical loss L (ω), and absorption coefficient I (ω) response against different photon energies. In order to examine the possibility of using these compounds in thermoelectric devices, we calculated their transport properties from Boltzman transport theory. We examined their thermal conductivity κ, electrical conductivity σ, seebeck coefficient S, power factor PF and dimensionless figure of merit ZT.

Carbon atomic chains in a spin thermoelectric device

We investigated spin-dependent thermoelectric properties of a carbon atomic chain (CAC) between two zigzag-edged triangular graphene nanoflakes (GNFs), with two semi-infinite armchair graphene nanoribbons (AGNRs) as the electrodes. Using the non-equilibrium Green’s function method combined with the effective self-consistent mean-field Hubbard approximation, we found that the designed configuration shows large thermo-spin effects that could be tuned by the gate voltage, chemical potential as well as temperature. Our results show that the non-magnetic AGNRs with the edge magnetism of the GNFs and CAC located between them induces a notable spin Seebeck coefficient (SSC), comparable to or more than that in zigzag graphene nanoribbon junctions, for all sizes of CAC. Finally, applying a gate voltage and a temperature gradient across the chain-based caloritronic device could induce a pure spin current, a perfect spin filtering effect, and an unrivaled negative differential thermal resistance in a broad range of temperatures close to room temperature.

Transport of Strongly Correlated Bosons in an Optical Lattice

The transport of strongly correlated bosons in a three‐dimensional optical lattice is studied within the Bose–Hubbard approximation. The transport is induced by a small displacement of the overall harmonic trapping potential. The subsequent relaxation dynamics is monitored by high precision density measurements with the help of scanning electron microscopy. Good agreement with a real space time‐dependent Gutzwiller mean‐field description is found.

Structural, elastic, mechanical, electronic, magnetic, thermoelectric and thermodynamic investigation of half metallic double perovskite oxide Sr2MnTaO6

Abstract Experimental lattice constant has been used for theoretical predictions on structural, elastic, mechanical, electronic magnetic thermoelectric and thermodynamic properties of Sr2MnTaO6 double perovskite oxide within well-known ab-initio density functional theory. Optimized lattice constant has been employed for obtaining spin involved electronic structure results within generalized gradient approximation (GGA), Hubbard approximation (GGA + U) and modified Becke Johnson approximation (mBJ). Electronic results show half-metallic nature with majority spin channels as metallic and minority spin channels as semi-conducting. Magnetic moment calculated within GGA + U was found equal to 4 µb. Calculated large value of Bulks modulus (B) and Young modulus (Y) characterize it as hard and stiffer material. Pugh ratio (B/G) and Cauchy pressure (C12–C44) portray its brittle nature. Using Boltztrap code we have calculated the variation of electrical conductivity (σ/τ), Seebeck coefficient (S), electronic thermal conductivity (k/τ) and Power factor (PF) in both spin channels. (σ/τ) is found to have decreasing nature in spin up states and increasing nature in spin down states, hence confirms the metallic nature in spin up states and semi-conducting in spin down states. Seebeck coefficients (S) reveal the presence of positive charge carriers in spin up states and negative in spin down states. The computed value of total power factor was found to be 1.99 × 1012 WK−2 m−1 s−1 at 1000 K. Furthermore, we have computed pressure and temperature dependent thermodynamic parameters for this compound in the temperature range of 0 K to 1000 K and pressure range of 0 GPa to 18 GPa with a step size of 3 GPa.

Half-metallicity and onsite Hubbard interaction on d-electronic states: a case study of Fe2NiZ (Z = Al, Ga, Si, Ge) Heusler systems

ABSTRACT DFT-based structural optimisations of Fe2NiZ (Z = Al, Ga, Si, Ge) Heusler compounds confirm the stability of these alloys in F-43m phase. While defining the electronic structure, onsite Hubbard approximation scheme for exchange correlations predicted better results than the generalised gradient approximation. Calculated band structure and densities of states together with spin magnetic moments designate the half-metallic character of these alloys. Indirect band gaps, 1.2 eV for Fe2NiAl, 0.98 eV for Fe2NiGa, 1.3 eV for Fe2NiSi and 1.1 eV for Fe2NiGe in spin-down states are observed. The ferromagnetic spin moments amount to an integral value of 5μB for (Al, Ga) and 6μB for (Si, Ge) systems with a maximum contribution from transition metal atom (Fe). To forecast the possible turnout of the thermopower, Seebeck coefficients, electrical and thermal conductivities are calculated, which directly hints the thermoelectric response of these materials. This study creates a possibility of these alloys in thermoelectrics and spintronics.

Full Heusler alloys (Co2TaSi and Co2TaGe) as potential spintronic materials with tunable band profiles

DFT based structural optimizations and cohesive energies of novel Co-based Heusler compounds confirm the stability of these alloys in L21 phase (Cu2MnAl prototype). Later, the equilibrium lattice constants are predicted. While defining the electronic structure, the modified Beckhe Johnson scheme for exchange correlations predicted more efficient results than the generalized gradient approximation and onsite Hubbard approximation. Calculated band structure and densities of states together with spin magnetic moments designate the half-metallic character of these alloys. The elastic constants are calculated to define the mechanical stability and ductile nature of these alloys. The ferromagnetic spin moments amount to an integral value of 3μB for each system with a maximum contribution from transition metal cobalt atom. Present study opens a way out for the potential application of these alloys as spintronic materials.

A combined DFT, DFT + U and mBJ investigation on electronic structure, magnetic, mechanical and thermodynamics of double perovskite Ba2ZnOsO6

First successful ab initio calculations on electronic structure, magnetic, elasto-mechanical and thermodynamic properties of cubic double perovskite oxide Ba2ZnOsO6 has been effectively calculated within density functional theory via full potential linearized augmented plane wave (FP-LAPW) method. The structural investigation exposes the ferromagnetic phase stability of the compound. The spin polarized electronic and magnetic properties were calculated within generalized gradient approximation (GGA), Hubbard approximation (GGA + U), mBJ (modified Becke-Johnson approximation). The electronic profile establishes half-metallic nature for the compound. The calculated total spin magnetic moment was found equal to 2 µB. The elastic constants have been calculated and used to predict mechanical stuffs like Shear modulus (G) Poisson ratio (ν) and anisotropic factor (A). The calculated B/G and Cauchy pressure (C12-C44) both characterize the material as brittle. The thermodynamic parameters like heat capacity and Debye temperature have also been predicted in the temperature range of 0 K–1000 K.