Spin-polarized Generalized Gradient Approximation Research Articles

Hydrogen storage on MgO supported TiMgn (n = 2–6) clusters: A first principle investigation

The current study explores the potential of MgO-supported finite-sized TiMgn (n = 2–6) nanoclusters as hydrogen storage materials, employing density functional theory with a spin-polarized generalized gradient approximation (GGA). These systems' structural stability and electronic characteristics reveal that supported clusters offer superior hydrogen storage capabilities compared to their unassisted counterparts. Various parameters, including cluster-adsorption energy (Eads), hydrogen-adsorption energy in supported clusters (Eads-H), HOMO-LUMO gap, vertical ionization potential (VIP), vertical electron affinity (VEA), chemical potential (μ), and chemical hardness (ɳ) are computed. Substrate support notably enhances the thermodynamic stability and chemical reactivity of the TiMg5 cluster when contrasted with the bare TiMg5 cluster. Furthermore, a remarkable increase in the gravimetric hydrogen storage density, from 1.63 wt% for bare Mg5 clusters to 3.45 wt% for bare TiMg5 clusters, reaching 5.62 wt% in the supported TiMg5 cluster system is observed. These findings indicate the substrate-supported TiMg5 cluster as a promising candidate for hydrogen storage applications.

Coherent spin transport in a copper protein.

The coherent electron/spin transport in azurin, a species of copper protein, was calculated based on the Landauer model. The research is motivated by the fast electron transport and spin selectivity/polarization in azurin, which have been reported in relation to the chiral-induced spin selectivity of the peptide structure. The calculated spin polarization of copper proteins was large. This phenomenon was strongly influenced by the spin density of the atoms in the ligand group, whereas the contribution of copper was negligible. The results suggest that spin polarization in copper proteins is enhanced by that of the ligand groups. The predicted spin polarization aligns primarily with the scanning tunneling microscope-based break-junction technique to study the electronic properties of single-molecule junctions. Computational techniques employed in this study are nonequilibrium Green's functions (NEGF) and density functional theory (DFT) based on the Landauer model, implemented using the QuantumATK software (Synopsys Inc.). The Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional was adopted for spin-polarized generalized gradient approximation (SGGA). The valence atomic orbitals were constructed using the wavefunctions of the SIESTA package, which was based on the norm-conserving Troullier-Martins relativistic pseudopotentials for describing core electrons. The mesh used for real-space integration was 150 Ha.

Spin-Induced Switching of Electronic State Populations in Transition Metal Polyphthalocyanines.

Polyphthalocyanines (PPCs) are a new and promising class of two dimensional materials offering versatile avenues for next generation electronic devices. For organic spintronic devices, PPCs can be engineered to tailor the electric and magnetic properties. In this work, we investigate PPC's monolayers with embedded transition metal atoms (TM = Fe, Ni, Cu), utilizing first principle calculations based on spin-polarized generalized gradient approximation (SGGA). PPC sheets with central TM atoms are simulated for the dispersion curves, electronic density of states (DOS), and projected density of states (PDOS) using quantum atomistic toolkit (Quantum ATK) software. According to simulations, the FePPC supercell with four magnetic moments of Fe, aligned in a parallel ferromagnetic (FM) configuration, show the conductive FM state, while in the case of the anti-parallel antiferromagnetic (AFM) order of the magnetic moments, the material exhibits semiconducting non-magnetic behavior. FM-ordered NiPPC displays a metallic state, which is partly suppressed for AFM-ordered NiPPC. In contrast, non-magnetic CuPPC is found to be the best conductor due to its larger PDOS at the Fermi level among all considered systems.

Insights into the electronic structure and stability of TiMgn (n = 1–12) clusters: Validation of electron counting rule

The present study reports the investigation of the electronic structure and stability of TiMgn (n = 1–12) nanoclusters in the framework of the linear combination of atomic orbital density functional theory (DFT) together with a spin-polarized generalized gradient approximation (GGA). We have calculated different thermodynamic and chemical parameters, such as binding energy (BE), embedding energy (EE), fragmentation energy (FE), HOMO-LUMO gap, vertical ionization potential (VIP), vertical electron affinity (VEA), etc. during the growth process of the cluster to understand their stability. From the study, the TiMg8 cluster comes out to be the most stable cluster. Further, the positive charge (Mulliken charge) on the Ti in TiMg8 cluster indicates that the Mg8-cage behaves as an electron acceptor during hybridization with the Ti. The absence of any reactive site (obtained from NBO), minimum Mg-Ti bond length, and negative NICS in TiMg8 ground state cluster support it as a globally stable cluster. Low ELF index value revealing the non-reactivity of the TiMg8 cluster. Finally, orbital analysis shows that TiMg8 clusters follow closed-cell electronic orbital sequence 1S21P61D102S2 and satisfy the 20-electron counting rule. IR and Raman spectrum calculations are also presented to understand the vibrational nature of the clusters.

MnSnTeO6: A Chiral Antiferromagnet Prepared by a Two-Step Topotactic Transformation.

MnSnTeO6, a new chiral antiferromagnet, was prepared both by topotactic transformation of the metastable rosiaite-type polymorph and by direct synthesis from coprecipitated hydroxides. Its structure and its static and dynamic magnetic properties were studied comprehensively both experimentally (through X-ray and neutron powder diffraction, magnetization, specific heat, dielectric permittivity, and ESR techniques) and theoretically (by means of ab initio density functional theory (DFT) calculations within the spin-polarized generalized gradient approximation). MnSnTeO6 is isostructural with MnSb2O6 (space group P321) and does not show any structural transition between 3 and 300 K. The magnetic susceptibility and specific heat exhibit an antiferromagnetic ordering at TN ≈ 9.8 K, which is confirmed by low-temperature neutron data. At the same time, the thermodynamic parameters demonstrate an additional anomaly on the temperature dependences of magnetic susceptibility χ(T), specific heat Cp(T) and dielectric permittivity ε(T) at T* ≈ 4.9 K, which is characterized by significant temperature hysteresis. Clear enhancement of the dielectric permittivity at T* is most likely to reflect the coupling of dielectric and magnetic subsystems leading to development of electric polarization. It was established that the ground state of MnSnTeO6 is stabilized by seven exchange parameters, and neutron diffraction revealed incommensurate magnetic structure with propagation vector k = (0, 0, 0.183) analogous to that of MnSb2O6. Ab initio DFT calculations demonstrate that the strongest exchange coupling occurs between planes along diagonals. All exchange parameters are antiferromagnetic and reveal moderate frustration.

Magnetic phase transition from paramagnetic in Nb2AlC-MAX to superconductivity-like diamagnetic in Nb2C-MXene: an experimental and computational analysis.

Transition metal carbides (TMCs) have recently emerged as competent members among the family of two-dimensional (2D) materials, owing to their promising applications. There are many promising applications of MXenes; however, their magnetic properties lack a wide margin, both experimentally as well as theoretically, which needs to be investigated for potential use in spintronics. In this study, we carried out a comprehensive etching process via selective extraction of Al layers from Nb2AlC-MAX using a wet electrochemical route under well-optimized conditions to obtain fine 2D-Nb2C MXene sheets. Structural analysis using X-ray diffraction (XRD) confirms the effective removal of Al followed by confirmation of a 2D layered structure from morphological analysis using scanning electron microscopy (SEM). Zero-field-cooled (ZFC) and field-cooled (FC) measurements of MAX and MXene at different field strengths were performed using a superconducting quantum interference device (SQUID). Magnetic measurements reveal the paramagnetic nature of Nb2AlC-MAX measured under 5 mT; however, this changes to a clear superconductor-like diamagnetic behavior with a shift of the magnetization from positive to negative values at low temperatures when measured under 5 mT and 10 mT for Nb2C MXene. The diamagnetism, however, is changed to paramagnetism at 100 mT, which shows the existence of critical fields known typically for a type-II superconductor. To gain an insight into this unusual behavior in MXene, density functional theory (DFT) first-principles calculation was also performed in Wein2K software using spin-polarized generalized gradient approximation (sp-GGA). The magnetic moment of the compound is calculated to be negative, which corresponds well with the experimental finding and suggests that the negative magnetic moment originated from the d-orbital of Nb2C. The present report provides a pathway to deeply understanding the existence of superconductivity-like diamagnetic behavior in Nb2C MXene, which is useful for future magnetic applications.

Unexpected magnetic behavior of Ga doped CuFe1-xGaxO2 delafossite, x = 0.04: First principle calculation and Monte Carlo simulation

The structural electronic and magnetic properties of Ga doped delafossite CuFe0.96Ga0.04O2 are investigated using first principle calculations and Monte Carlo simulation. The calculations are based on the density functional theory using the Wien2k package within full potential linearized augmented plane wave method and spin-polarized generalized gradient approximation of the exchange-correlation functional. The simulated results show that an ideal Ga doped delafossite is an antiferromagnetic and the magnetic moments of the iron is about \( 3.91\mu_{B}\). Furthermore, we have explored the spin coupling interactions up to third nearest neighbors as well the coupling between adjacent layers in order to examine the magnetism and thermodynamical properties. In addition, we have reported the magnetic properties of this element using Monte Carlo simulation. The obtained values of the Neel temperature decrease as the absolute value of the single ion anisotropy \(| \Delta |\) increases. This result is in fair agreement with experiment.

Charge density wave and spin 1/2 insulating state in single layer 1T-NbS2

In bulk samples and few layer flakes, the transition metal dichalcogenides NbS2 and NbSe2 assume the H polytype structure with trigonal prismatic coordination of the Nb atom. Recently, however, single and few layers of 1T-NbSe2 with octahedral coordination around the transition metal ion were synthesized. Motivated by these experiments and by using first-principles calculations, we investigate the structural, electronic and dynamical properties of single layer 1T-NbS2. We find that single-layer 1T-NbS2 undergoes a star-of-David charge density wave. Within the generalized gradient approximation, the weak interaction between the stars leads to an ultraflat band that results to be located at the Fermi level and isolated from all other bands. The spin-polarized generalized gradient approximation stabilizes a total spin 1/2 magnetic state with a magnetic moment localized on the central Nb in the star and the opening of a 0.15 eV band gap. In the GGA + U framework, the magnetic moment on the central Nb is enhanced to and a larger gap occurs. Most important, this approximation gives a small energy difference between the 1T and 1H polytypes (only +0.5 mRy/Nb), suggesting that the 1T-polytype can be synthesized in a similar way as done for single layer 1T-NbSe2. Finally we compute first and second nearest neighbors magnetic inter-star exchange interactions finding J1 = 9.5 K and J2 = 0.4 K ferromagnetic coupling constants.

First principles study on new half-metallic ferromagnetic ternary zinc-based sulfide and telluride (Zn3VS4 and Zn3VTe4)

In this resear, we have investigated electronic and magnetic behavior and also some mechanical properties of ternary zinc-based chalcogenides Zn3VCh4 (Ch = S and Te) conform to space group with 215 space number, which have simple cubic (SC) crystal structure, by spin-polarized Generalized Gradient Approximation (GGA) within Density Functional Theory (DFT). To make extensive study within ab initio methods, after the optimization process to get optimal structural parameters in most suitable magnetic phase, the electronic band structures with the total density of electronic states and second order elastic constants to predict some thermal and elastic features have been calculated. The calculated negative formation enthalpy values indicate that, our materials are thermodynamically stable and structurally synthesizable. The observed minority (down) band gaps in the calculated electronic band structures, Eg = 2.62 eV for Zn3VS4 and Eg = 1.75 eV for Zn3VTe4, show that our crystal systems have half-metallic character. Also, the calculated elastic constants prove that our compounds are mechanically stable due to satisfy the Born-Huang criteria. Moreover, the anisotropic elastic properties have been visualized in two-dimensional (2D) and three-dimensional (3D) surfaces for Young’s modulus, linear compressibility, shear modulus and Poisson’s ratio as well as with the calculation of the anisotropic factors.

Magnetic Polymer Chains of Transition Metal Atoms and Zwitterionic Quinone

Ab initio calculations based on density functional theory (DFT) including an explicit treatment of the strong electron correlation in the d shell of the transition metal ions have been conducted using the spin-polarized generalized gradient approximation with Hubbard term U (SGGA+U) to investigate systematically the electronic and magnetic properties of a new material class representing one-dimensional transition metal zwitterionic quinone (TM-ZQ) polymers having many potential applications, especially in spintronics. The complete class of 3d transition metals (TMs) are investigated from Sc to Zn. Zn-ZQ is nonmagnetic, since it has a 3d10 configuration. All of the other TM-ZQ polymers are antiferromagnetic semiconductors with the exception of Mn-ZQ that is metallic for the ferromagnetic (FM) and the antiferromagnetic (AFM) spin arrangements and Sc-ZQ and Ti-ZQ which are FM semiconductors. All of these polymer chains have the potential to be produced by on-surface synthesis on metallic surfaces, as was recently shown for Fe-ZQ (Koudia, M.; et al. Nano Res. 2017, 10, 933–940).

Tunable induced magnetic moment and in-plane conductance of graphene in Ni/graphene/Ni nano-spin-valve-like structure: A first principles study

This study theoretically investigated the magnetic properties and electronic structure of a graphene-based nano-spin-valve-like structure. Magnetic nickel layers on both sides of the graphene were considered. A spin-polarized generalized-gradient approximation determined the electronic states. In an energetically stable stacking arrangement of graphene and two nickel layers, the anti-parallel spin configuration of the underlayer and overlayer magnetic moments had the lowest energy, which is in agreement with previous experimental studies. The spin density mapping and obtained band-structure results show that when the upper and lower Ni(111) slabs have an anti-parallel (parallel) magnetic-moment configuration, the carbon atoms of sublattices A and B will have an antiferromagnetic (ferromagnetic) spin configuration. A band gap at the Dirac cone was open when the alignment had an anti-parallel configuration and closed when the alignment had a parallel configuration. Therefore, the in-plane conductance of the graphene layer depends on the magnetic alignment of the two nickel slabs when the Fermi level is adjusted at the Dirac point. Both the magnetic properties and electronic structures of the Ni/graphene/Ni nanostructure cause the system to be a new prospective spintronic device showing controllable in-plane magnetoresistance.

Study of Structural, Electronic, and Magnetic Properties of Cubic Lanthanide Based on Oxide Perovskite-Type NdGaO3

Based on the density functional theory (DFT), the full-potential linearized augmented plane wave with local orbital (FP-L/APW+lo) technique is now employed in this approach to understand the structural, electronic, and magnetic properties of simple cubic oxide perovskite NdGaO3 compound. In all this investigation, the exchange-correlation (XC) energy is selected in the framework of spin-polarized generalized gradient approximation (spin-GGA). The structural analysis unveils that the ferromagnetic (FM) phase is the stable ground state of the cubic NdGaO3 compound, where the equilibrium lattice parameters (lattice constant (a0), bulk modulus (B0), and its first pressure derivative (B′)) are determined in both FM and paramagnetic (PM) phases. The spin-polarized electronic properties (band structure and density of states) of the cubic NdGaO3 oxide perovskite are studied under the platform of equilibrium lattice parameters; this investigation demonstrates the half-metallic behavior of the studied cubic NdGaO3 compound because the spin-up case displays the metallic nature, whereas the semiconducting character is observed in spin-down case. The magnetic properties reveal that the total magnetic moment of the cubic NdGaO3 compound is equal to 3 μB and its contribution is mostly generated by Nd atoms, whereas feeble local magnetic moments are installed in non-magnetic Ga and O sites. Through the electronic and magnetic results, we conclude that the cubic perovskite NdGaO3 compound is classified as a half-metallic ferromagnetic material.

Charge density wave phase, Mottness, and ferromagnetism in monolayer 1T−NbSe2

The recently investigated $1T$-polymorph of monolayer NbSe$_2$ revealed an insulating behaviour suggesting a star-of-David phase with $\sqrt{13}\,\times\sqrt{13}$ periodicity associated with a Mott insulator, reminiscent of $1T$-TaS$_2$. In this work, we examine this novel two-dimensional material from first principles. We find an instability towards the formation of an incommensurate charge-density-wave (CDW) and establish the star-of-David phase as the most stable commensurate CDW. The mottness in the star-of-David phase is confirmed and studied at various levels of theory: the spin-polarized generalized gradient approximation (GGA) and its extension involving the on-site Coulomb repulsion (GGA+$U$), as well as the dynamical mean-field theory (DMFT). Finally, we estimate Heisenberg exchange couplings in this material and find a weak nearest-neighbour ferromagnetic coupling, at odds with most Mott insulators. We point out the close resemblance between this star-of-David phase and flat-band ferromagnetism models.

On-Surface Synthesis of Spin Crossover Polymeric Chains

We report on the successful on-surface polymerization reaction of codeposited zwitterionic quinones with Fe atoms on Au(110) at an appropriate temperature. The resulting covalent one-dimensional polymer chains arrange in a well-ordered two-dimensional (2D) structure as proven by scanning tunneling microscopy and low-energy electron diffraction. The ordered regions can reach the micrometer size. Furthermore, the electronic and magnetic properties of the zwitterionic quinoidal polymers on Au(110) were investigated using the density functional theory with an explicit inclusion of the Hubbard U term. The spin-polarized generalized gradient approximation plus U method (SGGA + U) has been used, and the freestanding isolated polymer chain, the 2D arrangement, and the adsorbed polymers have been calculated. From ab initio calculations, we predict the zwitterionic quinoidal polymer chains to be a one-dimensional spin crossover compound. For the freestanding chains, we find two local minima with comparable energies...

Possibility of a ferromagnetic and conducting metal-organic network

In this paper, we present first principles calculations based on the spin-polarized generalized gradient approximation with on-site Coulomb repulsion term (SGGA + U), to explore the electronic and magnetic properties of the novel planar metal-organic networks TM-Pc and TM-TCNB (where TM means a transition metal of the 3d series: Ti, V, Cr, …, or Zn, Pc - Phthalocyanine, and TCNB - Tetracyanobenzene) as free-standing sheets. This work is an extension of two earlier research works dealing with the Mn (Mabrouk et al., 2015) and Fe (Mabrouk et al., 2017) cases. Our theoretical investigations demonstrate that TM-Pc are more stable than TM-TCNB. Our results unveil that all the TM-Pc frameworks have an insulating behavior with the exception of Mn-Pc which is half-metallic and favor antiferromagnetic order in the case of our magnetic systems except for V-Pc which is ferromagnetic. In contrast, the TM-TCNB networks are metallic at least in one spin direction and exhibit long-range ferromagnetic coupling in case for magnetic structures, which represent ideal candidates and an interesting prospect of unprecedented applications in spintronics. In addition, these results may shed light to achieve a new pathway on further experimental research in molecular spintronics.

Investigation of the structural, electronic, elastic and thermodynamic properties of Curium Monopnictides: An ab initio study

The structural, electronic, elastic and thermodynamic properties of Curium Monopnictides CmX (X = N, P, As, Sb and Bi) are investigated using first-principles calculations based on the density functional theory (DFT) and full potential linearized augmented plane wave (FP-LAPW) method under ambient condition and high pressure. The exchange-correlation term is treated using two approximations spin-polarized local density approximation (LSDA) and spin-polarized generalized gradient approximation generalized (GGA). The structural parameters such as the equilibrium lattice parameters, bulk modulus and the total energies are calculated in two phases: namely NaCl (B1) and CsCl (B2). The obtained results are compared with the previous theoretical and experimental results. A structural phase transition from B1 phase to B2 phase for Curium pnictides has been obtained. The highest transition pressure is 122 GPa for CmN and the lowest one is 10.0 GPa for CmBi compound. The electronic properties show that these materials exhibit half-metallic behavior in both phases. The magnetic moment is found to be around 7.0 [Formula: see text]B. The mechanical properties of CmX (X = N, P, As, Sb and Bi) are predicted from the calculated elastic constants. Our calculated results are in good agreement with the theoretical results in literature. The effect of pressure and temperature on the thermodynamic properties like the cell volume, bulk modulus and the specific heats C[Formula: see text] and C[Formula: see text], the entropy [Formula: see text] and the Grüneisen parameter [Formula: see text] have been foreseen at expanded pressure and temperature ranges.

First-principles investigation of graphitic carbon nitride monolayer with embedded Fe atom

Density-functional theory (DFT) calculations with spin-polarized generalized gradient approximation and Hubbard U correction are carried out to investigate the mechanical, structural, electronic and magnetic properties of graphitic heptazine with embedded Fe atom under bi-axial tensile strain and applied perpendicular electric field. It was found that the binding energy of heptazine with embedded Fe atom system decreases as larger tensile strain is applied, while it increases as larger electric field strength is applied. Our calculations also predict a band gap at a peak value of 5% tensile strain but at expense of the structural stability of the system. The band gap open up at 5% tensile strain is due to distortion in the structure caused by the repulsive effect in the cavity between the lone pairs of the edge nitrogen atoms and dxy/dx2−y2 orbital of Fe atom, forcing the unoccupied pz- orbital is forced to shift toward higher energy. The electronic and magnetic properties of the heptazine with embedded Fe system under perpendicular electric field up to a peak value of 8 V/nm is also well preserved despite an obvious buckled structure. Such properties are desirable for diluted magnetic semiconductors, spintronics, and sensing devices.

Electronic Structure Calculations of Hydrogen Storage in Lithium-Decorated Metal-Graphyne Framework.

Porous metal-graphyne framework (MGF) made up of graphyne linker decorated with lithium has been investigated for hydrogen storage. Applying density functional theory spin-polarized generalized gradient approximation with the Perdew-Burke-Ernzerhof functional containing Grimme's diffusion parameter with double numeric polarization basis set, the structural stability, and physicochemical properties have been analyzed. Each linker binds two Li atoms over the surface of the graphyne linker forming MGF-Li8 by Dewar coordination. On saturation with hydrogen, each Li atom physisorbs three H2 molecules resulting in MGF-Li8-H24. H2 and Li interact by charge polarization mechanism leading to elongation in average H-H bond length indicating physisorption. Sorption energy decreases gradually from ≈0.4 to 0.20 eV on H2 loading. Molecular dynamics simulations and computed sorption energy range indicate the high reversibility of H2 in the MGF-Li8 framework with the hydrogen storage capacity of 6.4 wt %. The calculated thermodynamic practical hydrogen storage at room temperature makes the Li-decorated MGF system a promising hydrogen storage material.

Fundamental Study of Reversible Hydrogen Storage in Titanium- and Lithium-Functionalized Calix[4]arene

Hydrogen is the most promising candidate for a sustainable energy source in the transport sector. However, the storage of hydrogen is a major problem. Calix[4]arene (CX) is functionalized with Ti and Li metals on the delocalized π electrons of benzene rings, and the metal-functionalized system is studied for hydrogen storage efficiency by applying density functional theory using the M06 hybrid functional and 6-311G(d,p) basis set. The calculated binding energy indicates Ti coordinates with CX strongly while Li coordinates weakly and the binding of CX and metal is through Dewar mechanism. On saturation with hydrogen, each Ti atom traps four H2 molecules while each Li atom traps three H2 molecules on CX. Hydrogen molecules are adsorbed on the metal atoms by Kubas–Niu–Rao–Jena interaction. The global reactivity index obtained for the system obeys the maximum hardness and minimum electrophilicity principle. Molecular dynamics simulations are performed using spin-polarized generalized gradient approximation wi...

Magnetic gap opening in rhombohedral-stacked multilayer graphene from first principles

We investigate the occurrence of magnetic and charge density wave instabilities in rhombohedral-stacked multilayer (three to eight layers) graphene by first principles calculations including exact exchange. Neglecting spin-polarization, an extremely flat surface band centered at the special point ${\bf K}$ of the Brillouin zone occurs at the Fermi level. Spin polarization opens a gap in the surface state by stabilizing an antiferromagnetic state. The top and the bottom surface layers are weakly ferrimagnetic in-plane (net magnetization smaller than $10^{-3}\mu_B$), and are antiferromagnetic coupled to each other. This coupling is propagated by the out-of-plane antiferromagnetic coupling between the nearest neighbors. The gap is very small in a spin-polarized generalized gradient approximation, while it is proportional to the amount of exact exchange in hybrid functionals. For trilayer rhombohedral graphene it is $38.6$ meV in PBE0, in agreement with the $42$ meV gap found in experiments. We study the temperature and doping dependence of the magnetic gap. At electron doping of $n \sim 7 \times 10^{11}$ cm$^{-2}$ the gap closes. Charge density wave instabilities with $\sqrt{3}\times\sqrt{3}$ periodicity do not occur.