Theory of ferromagnetism in planar heterostructures of (Mn,III)-V semiconductors

A density-functional theory of ferromagnetism in heterostructures of compound semiconductors doped with magnetic impurities is presented. The variable functions in the density-functional theory are the charge and spin densities of the itinerant carriers and the charge and localized spins of the impurities. The theory is applied to study the Curie temperature of planar heterostructures of III-V semiconductors doped with manganese atoms. The mean-field, virtual-crystal and effective-mass approximations are adopted to calculate the electronic structure, including the spin-orbit interaction, and the magnetic susceptibilities, leading to the Curie temperature. By means of these results, we attempt to understand the observed dependence of the Curie temperature of planar δ-doped ferromagnetic structures on variation of their properties. We predict a large increase of the Curie temperature by additional confinement of the holes in a δ-doped layer of Mn by a quantum well.

Coherent transport in graphene nanoconstrictions

We study the effect of a structural nanoconstriction on the coherent transport properties of otherwise ideal zigzag-edged infinitely long graphene ribbons. The electronic structure is calculated with the standard one-orbital tight-binding model and the linear conductance is obtained using the Landauer formula. We find that, since the zero-bias current is carried in the bulk of the ribbon, this is very robust with respect to a variety of constriction geometries and edge defects. In contrast, the curve of zero-bias conductance versus gate voltage departs from the (2n+1)e2∕h staircase of the ideal case as soon as a single atom is removed from the sample. We also find that wedge-shaped constrictions can present nonconducting states fully localized in the constriction close to the Fermi energy. The interest of these localized states in regards to the formation of quantum dots in graphene is discussed.

Prediction of hidden multiferroic order in graphene zigzag ribbons

An electronic phase with coexisting magnetic and ferroelectric order is predicted for graphene ribbons with zigzag edges. The electronic structure of the system is described with a mean-field Hubbard model that yields results very similar to those of density functional calculations. Without further approximations, the mean-field theory is recasted in terms of a BCS wave function for electron-hole pairs in the edge bands. The BCS coherence present in each spin channel is related to spin-resolved electric polarization. Although the total electric polarization vanishes, due to an internal phase locking of the BCS state, strong magnetoelectric effects are expected in this system. The formulation naturally accounts for the two gaps in the quasiparticle spectrun, Δ0 and Δ1, and relates them to the intraband and interband self-energies.

Vacancy-induced magnetism in graphene and graphene ribbons

We address the electronic structure and magnetic properties of vacancies and voids both in graphene and graphene ribbons. By using a mean-field Hubbard model, we study the appearance of magnetic textures associated with removing a single atom (vacancy) and multiple adjacent atoms (voids) as well as the magnetic interactions between them. A simple set of rules, based on the Lieb theorem, link the atomic structure and the spatial arrangement of the defects to the emerging magnetic order. The total spin S of a given defect depends on its sublattice imbalance, but some defects with S=0 can still have local magnetic moments. The sublattice imbalance also determines whether the defects interact ferromagnetically or antiferromagnetically with one another and the range of these magnetic interactions is studied in some simple cases. We find that in semiconducting armchair ribbons and two-dimensional graphene without global sublattice imbalance, there is a maximum defect density above which local magnetization disappears. Interestingly, the electronic properties of semiconducting graphene ribbons with uncoupled local moments are very similar to those of diluted magnetic semiconductors, presenting giant Zeeman splitting.

Mn-doped II-VI quantum dots: artificial molecular magnets

The notion of artificial atom relies on the capability to change the number of carriers one by one in semiconductor quantum dots, and the resulting changes in their electronic structure. Organic molecules with transition metal atoms that have a net magnetic moment and display hysteretic behaviour are known as single molecule magnets (SMM). The fabrication of CdTe quantum dots chemically doped with a controlled number of Mn atoms and with a number of carriers controlled either electrically or optically paves the way towards a new concept in nanomagnetism: the artificial single molecule magnet. Here we study the magnetic properties of a Mn-doped CdTe quantum dot for different charge states and show to what extent they behave like a single molecule magnet.

Graphene single-electron transistor as a spin sensor for magnetic adsorbates

We study single-electron transport through a graphene quantum dot with magnetic adsorbates. We focus on the relation between the spin order of the adsorbates and the linear conductance of the device. The electronic structure of the graphene dot with magnetic adsorbates is modeled through numerical diagonalization of a tight-binding model with an exchange potential. We consider several mechanisms by which the adsorbate magnetic state can influence transport in a single-electron transistor: tuning the addition energy, changing the tunneling rate, and in the case of spin-polarized electrodes, through magnetoresistive effects. Whereas the first mechanism is always present, the others require that the electrode has to have either an energy- or spin-dependent density of states. We find that graphene dots are optimal systems to detect the spin state of a few magnetic centers.

Role of potassium orbitals in the metallic behavior of K3picene

Detailed electronic structure calculations of picene clusters doped by potassium modeling the crystalline K3picene structure show that while two electrons are completely transferred from potassium atoms to the lowest-energy unoccupied molecular orbital of pristine picene, the third one remains closely attached to both material components. Multiconfigurational analysis is necessary to show that many structures of almost degenerate total energies compete to define the cluster ground state. Our results prove that the 4s orbital of potassium should be included in any interaction model describing the material. We propose a quarter-filled two-orbital model as the most simple model capable of describing the electronic structure of K-intercalated picene. Precise solutions obtained by a development of the Lanczos method show low-energy electronic excitations involving orbitals located at different positions. Consequently, metallic transport is possible in spite of the clear dominance of interaction over hopping.

On the forbidden gap of finite graphene nanoribbons

The electronic structure of isolated finite graphene nanoribbons is investigated by solving, at the Hartree-Fock (HF) level, the Pariser, Parr and Pople (PPP) many-body Hamiltonian. The study is mainly focused on 7-AGNR and 13-AGNR (Armchair Graphene Nano-Ribbons), whose electronic structures have been recently experimentally investigated. Only paramagnetic solutions are considered. The characteristics of the forbidden gap are studied as a function of the ribbon length. For a 7-AGNR, the gap monotonically decreases from a maximum value of ~6.5 eV for short nanoribbons to a very small value of ~0.12 eV for the longer calculated systems. Gap edges are defined by molecular orbitals that are spatially localized near the nanoribbon extremes, that is, near both zig-zag edges. On the other hand, two delocalized orbitals define a much larger gap of about 5 eV. Conductance measurements report a somewhat smaller gap of ~3 eV. The small real gap lies in the middle of the one given by extended states and has been observed by STM and reproduced by DFT calculations. On the other hand, the length dependence of the gap is not monotonous for a 13-AGNR. It decreases initially but sharply increases for lengths beyond 30 Å remaining almost constant thereafter at a value of ~2.1 eV. Two additional states localized at the nanoribbon extremes show up at energies 0.31 eV below the HOMO (Highest Occupied Molecular Orbital) and above the LUMO (Lowest Unoccupied Molecular Orbital). These numbers compare favorably with those recently obtained by means of STS for a 13-AGNR sustained by a gold surface, namely 1.4 eV for the energy gap and 0.4 eV for the position of localized band edges. We show that the important differences between 7- and 13-AGNR should be ascribed to the charge rearrangement near the zig-zag edges obtained in our calculations for ribbons longer than 30 Å, a feature that does not show up for a 7-AGNR no matter its length.

Can model Hamiltonians describe the electron–electron interaction in π-conjugated systems?: PAH and graphene

Model Hamiltonians have been, and still are, a valuable tool for investigating the electronic structure of systems for which mean field theories work poorly. This review will concentrate on the application of Pariser–Parr–Pople (PPP) and Hubbard Hamiltonians to investigate some relevant properties of polycyclic aromatic hydrocarbons (PAH) and graphene. When presenting these two Hamiltonians we will resort to second quantisation which, although not the way chosen in its original proposal of the former, is much clearer. We will not attempt to be comprehensive, but rather our objective will be to try to provide the reader with information on what kinds of problems they will encounter and what tools they will need to solve them. One of the key issues concerning model Hamiltonians that will be treated in detail is the choice of model parameters. Although model Hamiltonians reduce the complexity of the original Hamiltonian, they cannot be solved in most cases exactly. So, we shall first consider the Hartree–Fock approximation, still the only tool for handling large systems, besides density functional theory (DFT) approaches. We proceed by discussing to what extent one may exactly solve model Hamiltonians and the Lanczos approach. We shall describe the configuration interaction (CI) method, a common technology in quantum chemistry but one rarely used to solve model Hamiltonians. In particular, we propose a variant of the Lanczos method, inspired by CI, that has the novelty of using as the seed of the Lanczos process a mean field (Hartree–Fock) determinant (the method will be named LCI). Two questions of interest related to model Hamiltonians will be discussed: (i) when including long-range interactions, how crucial is including in the Hamiltonian the electronic charge that compensates ion charges? (ii) Is it possible to reduce a Hamiltonian incorporating Coulomb interactions (PPP) to an 'effective' Hamiltonian including only on-site interactions (Hubbard)? The performance of CI will be checked on small molecules. The electronic structure of azulene and fused azulene will be used to illustrate several aspects of the method. As regards graphene, several questions will be considered: (i) paramagnetic versus antiferromagnetic solutions, (ii) forbidden gap versus dot size, (iii) graphene nano-ribbons, and (iv) optical properties.

Enzyme mediated synthesis of polypyrrole in the presence of chondroitin sulfate and redox mediators of natural origin

Polypyrrole (PPy) was synthesized by enzyme mediated oxidation of pyrrole using naturally occurring compounds as redox mediators. The catalytic mechanism is an enzymatic cascade reaction in which hydrogen peroxide is the oxidizer and soybean peroxidase, in the presence of acetosyringone, syringaldehyde or vanillin, acts as a natural catalysts. The effect of the initial reaction composition on the polymerization yield and electrical conductivity of PPy was analyzed. Morphology of the PPy particles was studied by scanning electron microscopy and transmission electron microscopy whereas the chemical structure was studied by X-ray photoelectron and Fourier transformed infrared spectroscopic techniques. The redox mediators increased the polymerization yield without a significant modification of the electronic structure of PPy. The highest conductivity of PPy was reached when chondroitin sulfate was used simultaneously as dopant and template during pyrrole polymerization. Electroactive properties of PPy obtained from natural precursors were successfully used in the amperometric quantification of uric acid concentrations. PPy increases the amperometric sensitivity of carbon nanotube screen-printed electrodes toward uric acid detection.

Derivation of the spin Hamiltonians for Fe in MgO

A method to calculate the effective spin Hamiltonian for a transition metal impurity in a non-magnetic insulating host is presented and applied to the paradigmatic case of Fe in MgO. In the first step we calculate the electronic structure employing standard density functional theory (DFT), based on generalized gradient approximation (GGA), using plane waves as a basis set. The corresponding basis of atomic-like maximally localized Wannier functions is derived and used to represent the DFT Hamiltonian, resulting in a tight-binding model for the atomic orbitals of the magnetic impurity. The third step is to solve, by exact numerical diagonalization, the N electron problem in the open shell of the magnetic atom, including both effects of spin–orbit and Coulomb repulsion. Finally, the low energy sector of this multi-electron Hamiltonian is mapped into effective spin models that, in addition to the spin matrices S, can also include the orbital angular momentum L when appropriate. We successfully apply the method to Fe in MgO, considering both the undistorted and Jahn–Teller (JT) distorted cases. Implications for the influence of Fe impurities on the performance of magnetic tunnel junctions based on MgO are discussed.

Oxidation of mixed-valence CoIII/FeII complexes reversed at high pH: A kinetico-mechanistic study of water oxidation

The outer-sphere oxidation of Fell in the mixed-valence complex trans-[(LCoNCFeII)-Co-14S-N-III(CN)(6)](-), being L-14S an N3S2 macrocylic donor set on the cobalt(III) center, has been studied. The comparison with the known processes of N-5 macrocycle complexes has been carried out in view of the important differences occurring on the redox potential of the cobalt center. The results indicate that the outer-sphere oxidation reactions with S2O82- and [Co(ox)(3)](3-) involve a great amount of solvent-assisted hydrogen bonding that, as a consequence from the change from two amines to sulfur donors, are more restricted. This is shown by the more positive values found for DeltaS(double dagger) and DeltaV(double dagger). The X-ray structure of the oxidized complex has been determined, and it is clearly indicative of the above-mentioned solvent-assisted hydrogen bonding between nitrogen and cyanide donors on the cobalt and iron centers, respectively. trans-[(LCoNCFeIII)-Co-14S-N-III(CN)(6)], as well as the analogous N-5 systems trans-[(LCoNCFeIII)-Co-14-N-III(CN)(6)], trans-[(LCoNCFeIII)-Co-15-N-III-(CN)(6)], and cis-[(LCoNCFeIII)-Co-n-N-III(CN)(6)], Oxidize water to hydrogen peroxide at pH > 10 with a rather simple stoichiometry, i.e., [(LCoNCFeIII)-Co-n-N-III(CN)(5)] + OH- - [(LCoNCFeII)-Co-n-N-III(CN)(5)](-) + 1/2H(2)O(2). In this way, the reversibility of the iron oxidation process is achieved. The determination of kinetic and thermal and pressure activation parameters for this water to hydrogen peroxide oxidation leads to the kinetic determination of a cyanide based OH- adduct of the complex. A second-order dependence on the base concentration is associated with deprotonation of this adduct to produce the final inner-sphere reduction process. The activation enthalpies are found to be extremely low (15 to 35 kJ mol(-1)) and responsible for the very fast reaction observed. The values of DeltaS(double dagger) and DeltaV(double dagger) (-76 to -113 J K-1 mol(-1) and -5.5 to -8.9 cm(3) mol(-1), respectively) indicate a highly organized but not very compressed transition state in agreement with the inner-sphere one-electron transfer from O2- to Fe-III.

Transition-state-theory calculations for reactions of O(3P) with halogenated olefins

The potential energy surfaces for the reactions of atomic oxygen in its ground electronic state, O(P-3), with the olefins: CF2=CCl2 and CF2=CF - CF3, have been characterized using ab initio molecular orbital calculations. Geometry optimization and vibrational frequency calculations were performed for reactants, transition states and products at the MP2 and QCISD levels of theory using the 6-31G(d) basis set. This database was then used to calculate the rate constants by means of Transition-State-Theory. To obtain a better reference and to test the reliability of the activation barriers we have also carried out computations using the CCSD(T)(fc)/6-311Gdagger, MP4(SDQ)(fc)/CBSB4 and MP2(fc)/CBSB3 single point energy calculations at both of the above levels of theory, as well as with the composite CBS-RAD procedure ( P. M. Mayer, C. J. Parkinson, D. M. Smith and L. Radom, J. Chem. Phys., 1998, 108, 604) and a modi. cation of this approach, called: CBS-RAD( MP2, MP2). It was found that the kinetic parameters obtained in this work particularly with the CBS-RAD ( MP2, MP2) procedure are in reasonable agreement with the experimental values. For both reactions it is found that the channels leading to the olefin double-bond addition predominates with respect to any other reaction pathway. However, on account of the different substituents in the alkenes we have located, at all levels of theory, two transition states for each reaction. Moreover, we have found that, for the reactions studied, a correlation exists between the activation energies and the electronic structure of the transition states which can explain the influence of the substituent effect on the reactivity of the halo-olefins.

Evidence for energy relaxation via a radiative cascade in surface-passivated PbS quantum dots

Multiple emission peaks have been observed from surface passivated PbS nanocrystals displaying strong quantum confinement. The emission spectra are shown to be strongly dependent on the excited-state parity. We also find that intraband energy relaxation from initial states excited far above the band-edge is nearly three orders of magnitude slower than that found in other nanocrystal quantum dots, providing evidence of inefficient energy relaxation via phonon emission. The initial-state parity dependence of the photoluminescent emission properties suggests that energy relaxation from the higher excited states occurs via a radiative cascade, analogous to energy relaxation in atomic systems. Such radiative cascade emission is possible from ideal zero-dimensional semiconductors, where electronic transitions can be decoupled from phonon modes.

Direct observation of mixed-parity excited states in surface-passivated PbS nanocrystals

Photoluminescent emission is observed from surface-passivated PbS nanocrystals following the two-photon excitation of high-energy excitonic states. The emission appears directly at the excitation energy with no detectable Stokes-shift for a wide range of excitation energies. The observation of direct emission from states excited by two-photon absorption indicates that the parity of the excited states of surface-passivated PbS nanocrystals is partially mixed.