Orbital Mixing Research Articles

Reduction of Electron Repulsion in Highly Covalent Fe-Amido Complexes Counteracts the Impact of a Weak Ligand Field on Excited-State Ordering.

The ability to access panchromatic absorption and long-lived charge-transfer (CT) excited states is critical to the pursuit of abundant-metal molecular photosensitizers. Fe(II) complexes supported by benzannulated diarylamido ligands have been reported to broadly absorb visible light with nanosecond CT excited state lifetimes, but as amido donors exert a weak ligand field, this defies conventional photosensitizer design principles. Here, we report an aerobically stable Fe(II) complex of a phenanthridine/quinoline diarylamido ligand, Fe(ClL)2, with panchromatic absorption and a 3 ns excited-state lifetime. Using X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) at the Fe L-edge and N K-edge, we experimentally validate the strong Fe-Namido orbital mixing in Fe(ClL)2 responsible for the panchromatic absorption and demonstrate a previously unreported competition between ligand-field strength and metal-ligand (Fe-Namido) covalency that stabilizes the 3CT state over the lowest energy triplet metal-centered (3MC) state in the ground-state geometry. Single-crystal X-ray diffraction (XRD) and density functional theory (DFT) suggest that formation of this CT state depopulates an orbital with Fe-Namido antibonding character, causing metal-ligand bonds to contract and accentuating the geometric differences between CT and MC excited states. These effects diminish the driving force for electron transfer to metal-centered excited states and increase the intramolecular reorganization energy, critical properties for extending the lifetime of CT excited states. These findings highlight metal-ligand covalency as a novel design principle for elongating excited state lifetimes in abundant metal photosensitizers.

Induction of inverted morphology in brain organoids by vertical-mixing bioreactors

Organoid technology provides an opportunity to generate brain-like structures by recapitulating developmental steps in the manner of self-organization. Here we examined the vertical-mixing effect on brain organoid structures using bioreactors and established inverted brain organoids. The organoids generated by vertical mixing showed neurons that migrated from the outer periphery to the inner core of organoids, in contrast to orbital mixing. Computational analysis of flow dynamics clarified that, by comparison with orbital mixing, vertical mixing maintained the high turbulent energy around organoids, and continuously kept inter-organoid distances by dispersing and adding uniform rheological force on organoids. To uncover the mechanisms of the inverted structure, we investigated the direction of primary cilia, a cellular mechanosensor. Primary cilia of neural progenitors by vertical mixing were aligned in a multidirectional manner, and those by orbital mixing in a bidirectional manner. Single-cell RNA sequencing revealed that neurons of inverted brain organoids presented a GABAergic character of the ventral forebrain. These results suggest that controlling fluid dynamics by biomechanical engineering can direct stem cell differentiation of brain organoids, and that inverted brain organoids will be applicable for studying human brain development and disorders in the future.

Orbital mixing in few-layer graphene and non-Abelian Berry phase

In a magnetic field few-layer graphene supports, at the lowest Landau level, a multiplet of zero-mode levels nearly degenerate in orbitals as well as in spins and valleys. Those pseudo-zero-mode (PZM) levels are generally sensitive to interactions and external perturbations, and have a crossing among themselves or with other higher Landau levels when an external field is swept over a certain range. A close study is made of how such PZM levels evolve when they are gradually brought from empty to filled levels under many-body interactions. It is pointed out that the level spectra generally avoid a crossing via orbital level mixing and that orbital mixing is governed by a non-Abelian Berry phase that derives from an approximate degeneracy and interactions. A look is also taken into evolution/crossing of many-body ground states with increasing external bias in bilayer graphene.

Electronic structure, effective masses and g-factors and strain induced crystal field splitting of WZ-InP by method

In this work, we study the electronic structure of wurtzite (WZ) InP, following a procedure developed earlier by us for WZ-GaN [(2018) J. Phys. Chem. Solids 112, 280] and WZ-ZnO [(2016) Semicond. Sci. Technol. 31, 035018], where the Hamiltonian was diagonalised by taking the conduction band and one valence band at a time. We consider the effects of other valence bands on a given valence band through second order perturbation theory of the Hamiltonian. Valence band dispersions are compared with previous calculations. We notice satisfactory agreement between our results and the previous one. Valence band effective masses are calculated from the valence band dispersions and conduction band effective masses were calculated from standard effective mass formulas. While valence band effective masses compare well with earlier calculations the transverse electronic effective mass is slightly higher compared to the experimental value and other reported values. However, when we use a different set of momentum matrix elements the transverse effective mass agrees well with the experimental value but the longitudinal value becomes slightly less than the experimental value. Thus the calculations are quite sensitive to the change in momentum matrix elements. Additionally, we calculate the effective masses as functions of the wave vector along and Our results for the g-factors are the first of its kind for WZ-InP. These results are quite sensitive to the change of sign of the spin–orbit parameters. The changes are not significant in case of electronic g-factors. But these are quite significant for the transverse valence band effective g-factors. Even change of sign is noticed. Spin–orbit mixing is found to be important and responsible for the observed anisotropy. Electronic g-factors agree well with reported experimental values. We also study the effect of strain on the crystal field splitting energy at the point. Trends obtained as functions of strain match with a previous calculation.

Essential Electronic Properties of Silicon Nanotubes.

In this work, the various electronic properties of silicon nanotubes (SiNTs) were investigated by the density functional theory. The cooperative and competitive relationships between the chiral angle, periodic boundary conditions, and multi-orbital hybridizations create unusual narrow gaps and quasi-flat bands in the ultra-small armchair and zigzag tubes, respectively. The features varied dramatically with tube radii. Armchair SiNTs (aSiNTs) have an indirect-to-direct band gap transition as their radius is increased to a particular value, while zigzag SiNTs (zSiNTs) present a metal-semiconductor transition. The projected density of states was used to elucidate the critical transitions, and the evolution of p and s orbital mixing states during the process are discussed in detail. The information presented here provides a better understanding of the essential properties of SiNTs.

Energy-Level Alignment and Orbital-Selective Femtosecond Charge Transfer Dynamics of Redox-Active Molecules on Au, Ag, and Pt Metal Surfaces.

Charge transfer (CT) dynamics across metal–molecule interfaces has important implications for performance and function of molecular electronic devices. CT times, on the order of femtoseconds, can be precisely measured using synchrotron-based core-hole clock (CHC) spectroscopy, but little is known about the impact on CT times of the metal work function and the bond dipole created by metals and the anchoring group. To address this, here we measure CT dynamics across self-assembled monolayers bound by thiolate anchoring groups to Ag, Au, and Pt. The molecules have a terminal ferrocene (Fc) group connected by varying numbers of methylene units to a diphenylacetylene (DPA) wire. CT times measured using CHC with resonant photoemission spectroscopy (RPES) show that conjugated DPA wires conduct electricity faster than aliphatic carbon wires of a similar length. Shorter methylene connectors exhibit increased conjugation between Fc and DPA, facilitating CT by providing greater orbital mixing. We find nearly 10-fold increase in the CT time on Pt compared to Ag due to a larger bond dipole generated by partial electron transfer from the metal–sulfur bond to the carbon–sulfur bond, which creates an electrostatic field that impedes CT from the molecules. By fitting the RPES signal, we distinguish electrons coming from the Fe center and from cyclopentadienyl (Cp) rings. The latter shows faster CT rates because of the delocalized Cp orbitals. Our study demonstrates the fine tuning of CT rates across junctions by careful engineering of several parts of the molecule and the molecule–metal interface.

Unsupported Lanthanide-Transition Metal Bonds: Ionic vs Polar Covalent?

Lanthanide-transition metal complexes continue to be of interest, not only because of their synthetic challenge but also of their promising magnetic properties. Computational work examining the chemical bonding between lanthanides and transition metals in PyCp2Ln-TMCp(CO)2 (DyPyCp22- = [2,6-(CH2C5H3)2C5H3N]2-) reveals strong Ln-TM dative bonds. Gas-phase optimized geometries are in good agreement with experimental structures at the density functional theory (DFT) level with large-core pseudopotentials. From La to Lu, there is a small increase in the bond dissociation energy, as well as a decrease in Ln-Fe bond lengths. Energy decomposition analyses attribute this trend to an increase in the electrostatic contribution from the decreasing bond length and a modest increase in the orbital contribution. The natural bond orbital analysis clearly indicates that 3d6 "lone pairs" in the [FeCp(CO)2]- fragment act as a Lewis bases donating nearly 0.5 electron to Ln virtual orbitals of mainly d character. The interfragment bonding was also quantified by the quantum theory of atoms in molecules, which indicates that the Ln-Fe bond is more covalent than the Ca-Fe bond in the hypothetical CpCa-FeCp(CO)2 but less covalent than the Zn-Fe bond in the hypothetical CpZn-FeCp(CO)2. Further comparisons suggest that to the [PyCp2Ln]+ cation the [FeCp(CO)2]- anion appears much like a halide. Overall, these Ln-TM dative bonds appear to have strong electrostatic contributions as well as significant orbital mixing and dispersion contributions.

New insights in the electronic structure of doped graphene on adsorption with oxides of nitrogen

The electronic effects produced by molecular adsorption of some environmentally potent oxides of nitrogen, N2O, NO, and NO2 on chemically modified surfaces of graphene are investigated, employing spin-polarized density functional calculations. Graphene surface is modified chemically with substitutional doping through boron, nitrogen, and co-doping, with combinations of boron and nitrogen. In the case of adsorption of the paramagnetic species, NO and NO2, rather exotic effects on adsorption are observed, with unusual electronic structure leading to flat bands in the band structure. The significant alterations of the electronic structure are primarily due to the adsorbate-adsorbent orbital mixing. The adsorption-induced dispersion-less flat bands are produced, eventually turning some of the doped, semiconducting surfaces into a metallic one. These dispersion-less bands are due to stronger interactions of the gaseous molecule with the surface leading to the localization of the molecule over the surface. It is observed that with the adsorption of paramagnetic oxides of nitrogen, the flat bands are introduced between the valence and the conduction band resulting in the loss of dispersion. For some of the configurational patterns of the dopant, chemically driven adsorption of the molecules onto the doped graphene surface take place, characterized by strong adsorption energies and charge transfers.

Anisotropic quasiparticle coherence in nematic BaFe2As2 studied with strain-dependent ARPES

The hallmark of nematic order in iron-based superconductors is a resistivity anisotropy but it is unclear to which extent quasiparticle dispersions, lifetimes and coherence contribute. While the lifted degeneracy of the Fe $d_{xz}$ and $d_{yz}$ dispersions has been studied extensively, only little is known about the two other factors. Here, we combine in situ strain tuning with ARPES and study the nematic response of the spectral weight in BaFe$_2$As$_2$. The symmetry analysis of the ARPES spectra demonstrates that the $d_{xz}$ band gains quasiparticle spectral weight compared to the $d_{yz}$ band for negative antisymmetric strain $\Delta \epsilon_{yy}$ suggesting the same response inside the nematic phase. Our results are compatible with a different coherence of the $d_{xz}$ and $d_{yz}$ orbital within a Hund's metal picture. We also discuss the influence of orbital mixing.

Novel Ag decorated, BiOCl surface doped AgVO3 nanobelt ternary composite with Z-scheme homojunction-heterojunction interface for high prolific photo switching, quantum efficiency and hole mediated photocatalysis

A ternary composite Ag@AgVO3/BiOCl possessing homojunction-heterojunction interface with varied BiOCl concentration was obtained through a facile hydrothermal route. Ag deposition in Ag@AgVO3 nano-belt was achieved through in-situ generation and deposition of Ag forming a homojunction. Whereas Ag@AgVO3/BiOCl heterojunction was constructed through the surface doping. This unique two-way interaction and the interface formation were revealed through the electron microscope and crystallographic analysis. The localized surface plasmon resonance of Ag and the efficient orbital mixing with BiOCl of 1:1 B/Ag@A compound has imparted a remarkable light harvesting and photo switching ability in the ternary composite material. Conductive nature of the Ag nanoparticle has promoted light absorption in the visible region and reduced the charge transfer resistance. Reduction of width in space charge region along with the increase in the amount of surface charge carrier has resulted in an outstanding increment in photocurrent. The solar photocatalysis evaluated against the aqueous phase removal of methyl orange and Bisphenol A showed 100 % and 51 % removal in 50 and 240 min, respectively. Studies were extended to understand the sensitization effect of dye. Theoretical modelling of band structure drawn from charge trapping experiments showed the prevalence of Z-scheme mechanism with holes mediated catalytic degradation. Improved charge harvest, separation and transmission has inserted a higher quantum efficiency in the material. BPA removal was enhanced to 82 % after the peroxide activation.

Theoretical Study on the Degree of CO2 Activation in CO2-Coordinated Ni(0) Complexes.

The geometrical characteristic and the degree of CO2 activation of the CO2-coordinated Ni(0) complexes were investigated computationally by quantum chemical means for bidentate and tridentate ligands of PP, PPMeP, and PNP, and sometimes with co-complexing Fe(II) to differently coordinate CO2. We show that the coordination geometry of the central metal is determined by the ligand geometry. The charge and the energy decomposition analyses show that the charge transfer energy through orbital mixing has a strong correlation with CO2 net charge, while the binding energy cannot due to the lack of the coordination number and the deformation energy of the ligand. Among the examined ligands, PNP with negatively charged secondary amine makes Ni(0) an electron-rich atom, which results in an ∼20% higher CO2 activation than those of PP and PPMeP. In particular, Fe(II)-PNP in the CO2-bridged diatomic complex enhances CO2 activation by another ∼20%, partly through the inductive effect of Fe(II), which pulls electron density from Ni-PNP across the CO2-bridge and partly by the backward donation from Fe(II)-PNP. Therefore, the present study encourages us to design a strongly electron-donating ligand and a CO2-bridged diatomic complex to develop more efficient homogeneous catalyst.

Combined multiplet theory and experiment for the Fe 2p and 3p XPS of FeO and Fe2O3.

The Al K alpha, 1486.6 eV, based x-ray photoelectron spectroscopy (XPS) of Fe 2p and Fe 3p for Fe(III) in Fe2O3 and Fe(II) in FeO is compared with theoretical predictions based on ab initio wavefunctions that accurately treat the final, core-hole, multiplets. The principal objectives of this comparison are to understand the multiplet structure and to evaluate the use of both the 2p and 3p spectra in determining oxidation states. In order to properly interpret the features of these spectra and to use the XPS to provide atomistic insights as well as atomic composition, it is necessary to understand the origin of the multiplet energies and intensities. The theoretical treatment takes into account the ligand field and spin-orbit splittings, the covalent mixing of ligand and Fe 3d orbitals, and the angular momentum coupling of the open shell electrons. These effects lead to the distribution of XPS intensity into a large number of final, ionic, states that are only partly resolved with energies spread over a wide range of binding energies. For this reason, it is necessary to record the Fe 2p and 3p XPS spectra over a wide energy range, which includes all the multiplets in the theoretical treatment as well as additional shake satellites. We also evaluate the effects of differing assumptions concerning the extrinsic background subtraction, to make sure our experimental spectrum may be fairly compared to the theory. We conclude that the Fe 3p XPS provides an additional means for distinguishing Fe(III) and Fe(II) oxidation states beyond just using the Fe 2p spectrum. In particular, with the use of the Fe 3p XPS, the depth of the material probed is about 1.5 times greater than for the Fe 2p XPS. In addition, a new type of atomic many-body effect that involves excitations into orbitals that have Fe f,ℓ = 3, symmetry has been shown to be important for the Fe 3p XPS.

Relativistic fluid modelling of gamma-ray binaries

Context.Gamma-ray binaries are systems that radiate the dominant part of their non-thermal emission in the gamma-ray band. In a wind-driven scenario, these binaries are thought to consist of a pulsar orbiting a massive star, accelerating particles in the shock arising in the wind collision.Aims.We develop a comprehensive numerical model for the non-thermal emission of shock-accelerated particles including the dynamical effects of fluid instabilities and orbital motion. We demonstrate the model on a generic binary system.Methods.The model was built on a dedicated three-dimensional particle transport simulation for the accelerated particles that were dynamically coupled to a simultaneous relativistic hydrodynamic simulation of the wind interaction. In a post-processing step, a leptonic emission model involving synchrotron and inverse-Compton emission was evaluated based on resulting particle distributions and fluid solutions, consistently accounting for relativistic boosting andγγ-absorption in the stellar radiation field. The model was implemented as an extension to the CRONOScode.Results.In the generic binary, the wind interaction leads to the formation of an extended, asymmetric wind-collision region distorted by the effects of orbital motion, mixing, and turbulence. This gives rise to strong shocks terminating the pulsar wind and secondary shocks in the turbulent fluid flow. With our approach it is possible for the first time to consistently account for the dynamical shock structure in particle transport processes, which yields a complex distribution of accelerated particles. The predicted emission extends over a broad energy range, with significant orbital modulation in all bands.

Orbital rotations induced by charges of polarons and defects in doped vanadates

We explore the competiton of doped holes and defects that leads to the loss of orbital order in vanadate perovskites. In compounds such as La$_{1-{\sf x}}$Ca$_{\,\sf x}$VO$_3$ spin and orbital order result from super-exchange interactions described by an extended three-orbital degenerate Hubbard-Hund model for the vanadium $t_{2g}$ electrons. Long-range Coulomb potentials of charged Ca$^{2+}$ defects and $e$-$e$ interactions control the emergence of defect states inside the Mott gap. The quadrupolar components of the Coulomb fields of doped holes induce anisotropic orbital rotations of degenerate orbitals. These rotations modify the spin-orbital polaron clouds and compete with orbital rotations induced by defects. Both mechanisms lead to a mixing of orbitals, and cause the suppression of the asymmetry of kinetic energy in the $C$-type magnetic phase. We find that the gradual decline of orbital order with doping, a characteristic feature of the vanadates, however, has its origin not predominantly in the charge carriers, but in the off-diagonal couplings of orbital rotations induced by the charges of the doped ions.

Controlling ultralong room temperature phosphorescence in organic compounds with sulfur oxidation state.

Sulfur oxidation state is used to tune organic room temperature phosphorescence (RTP) of symmetric sulfur-bridged carbazole dimers. The sulfide-bridged compound exhibits a factor of 3 enhancement of the phosphorescence efficiency, compared to the sulfoxide and sulfone-bridged analogs, despite sulfone bridges being commonly used in RTP materials. In order to investigate the origin of this enhancement, temperature dependent spectroscopy measurements and theoretical calculations are used. The RTP lifetimes are similar due to similar crystal packing modes. Computational studies reveal that the lone pairs on the sulfur atom have a profound impact on enhancing intersystem crossing rate through orbital mixing and screening, which we hypothesize is the dominant factor responsible for increasing the phosphorescence efficiency. The ability to tune the electronic state without altering crystal packing modes allows the isolation of these effects. This work provides a new perspective on the design principles of organic phosphorescent materials, going beyond the rules established for conjugated ketone/sulfone-based organic molecules.

Reactivity of dicationic N-heterocyclic chalcogen carbene analogues with methane and ethene: a theoretical investigation.

The reactions of chalcogen N-heterocyclic carbenic (NHC) dications [(Dipp2DAB)M2+, M = O, S, Se, and Te, and Dipp2DAB = 1,4-(2,6-diisopropyl)phenyl-1,4-diaza-1,3-butadiene] with methane and ethene are investigated by the density functional theory. The activation energies and reaction enthalpies are analyzed by the energy decomposition analysis (EDA) to understand the reactivity of the reactions. Calculations show that the oxide carbenic dication (Dipp2DAB)O2+ reactant has a pair of rather unbalanced O-N bonds due to the combined effects of the smallness of the central O atom and the bulkiness of the two Dipp ligands surrounding the DAB ring, suggesting the oxide reactant is much less stable than the chalcogenide ones. Because of its unique characteristics, the oxide carbenic dication distinguishes itself from the rest of the chalcogen family. The differences between oxides and chalcogenides of the chalcogen family complexes well known in inorganic chemistry are also observed in this theoretical investigation. This work further reveals that the reasons for the distinctive characteristics between oxide and chalcogenide complexes are due to the fundamental fact that the 2s and 2p atomic orbitals are more amenable to hybridization than those of higher valences, and the mixing of the former valence orbitals results in much smaller atoms than the latter.

Aufbau vs. non-Aufbau ground states in two-coordinate d7 single-molecule magnets

Magnetic anisotropy is generated in two related d7 single-molecule magnets; (1) via 3d-4s orbital mixing in FeI; and (2) a non-Aufbau ground state in CoII, demonstrating that the electronic configurations are large retained independent of geometry.

Strong metal–metal Pauli repulsion leads to repulsive metallophilicity in closed-shell d8 and d10 organometallic complexes

Metallophilicity is defined as the interaction among closed-shell metal centers, the origin of which remains controversial, particularly for the roles of spd orbital hybridization (mixing of the spd atomic orbitals of the metal atom in the molecular orbitals of metal complex) and the relativistic effect. Our studies reveal that at close M-M' distances in the X-ray crystal structures of d8 and d10 organometallic complexes, M-M' closed-shell interactions are repulsive in nature due to strong M-M' Pauli repulsion. The relativistic effect facilitates (n + 1)s-nd and (n + 1)p-nd orbital hybridization of the metal atom, where (n + 1)s-nd hybridization induces strong M-M' Pauli repulsion and repulsive M-M' orbital interaction, and (n + 1)p-nd hybridization suppresses M-M' Pauli repulsion. This model is validated by both DFT (density functional theory) and high-level coupled-cluster singles and doubles with perturbative triples computations and is used to account for the fact that the intermolecular or intramolecular Ag-Ag' distance is shorter than the Au-Au' distance, where a weaker Ag-Ag' Pauli repulsion plays an important role. The experimental studies verify the importance of ligands in intermolecular interactions. Although the M-M' interaction is repulsive in nature, the linear coordination geometry of the d10 metal complex suppresses the L-L' (ligand-ligand) Pauli repulsion while retaining the strength of the attractive L-L' dispersion, leading to a close unsupported M-M' distance that is shorter than the sum of the van der Waals radius (rvdw) of the metal atoms.

Semirealistic tight-binding model for Dzyaloshinskii-Moriya interaction

In this work, we discuss the nature of Dzyaloshinskii-Moriya interaction (DMI) in transition metal heterostructures. We first derive the expression of DMI in the small-spatial-gradient limit using Keldysh formalism. This derivation provides us with a Green's function formula that is well adapted to tight-binding Hamiltonians. With this tool, we first uncover the role of orbital mixing: Using both a toy model and a realistic multiorbital Hamiltonian representing transition metal heterostructures, we show that symmetry breaking enables the onset of interfacial orbital momentum that is at the origin of the DMI. We then investigate the contribution of the different layers to the DMI and reveal that it can expand over several nonmagnetic metal layers depending on the Fermi energy, thereby revealing the complex orbital texture of the band structure. Finally, we examine the thickness dependence of DMI on both ferromagnetic and nonmagnetic metal thicknesses and we find that whereas the former remains very weak, the latter can be substantial.

Hidden anomalous Hall effect in Sr2RuO4 with chiral superconductivity dominated by the Ru dxy orbital

The polar Kerr effect in superconducting Sr$_2$RuO$_4$ implies finite ac anomalous Hall conductivity. Since intrinsic anomalous Hall effect (AHE) is not expected for a chiral superconducting pairing developed on the single Ru $d_{xy}$ orbital, multiorbital chiral pairing actively involving the Ru $d_{xz}$ and $d_{yz}$ orbitals has been proposed as a potential mechanism. Here we propose that AHE could still arise even if the chiral superconductivity is predominantly driven by the $d_{xy}$ orbital. This is demonstrated through two separate models which take into account subdominant orbitals in the Cooper pairing, one involving the oxygen $p_x$ and $p_y$ orbitals in the RuO$_2$ plane, and another the $d_{xz}$ and $d_{yz}$ orbitals. In both models, finite orbital mixing between the dominant $d_{xy}$ and the other orbitals may induce inter-orbital pairing between them, and the resultant states support intrinsic AHE, with Kerr rotation angles that could potentially reconcile with the experimental observation. Our proposal therefore sheds new light on the microscopic pairing in Sr$_2$RuO$_4$. We also show that intrinsic Hall effect is generally absent for non-chiral states such as $\mathcal{S}+i\mathcal{D}$, $\mathcal{D}+i\mathcal{P}$ and $\mathcal{D}+i\mathcal{G}$, which provides a clear constraint on the symmetry of the superconducting order in this material.