Degree Of Electron Correlation Research Articles

Iron telluride ladder compounds: Predicting the structural and magnetic properties of BaFe2Te3

Since the discovery of pressure-induced superconductivity in the two-leg ladder system BaFe$_2X_3$ ($X$=S, Se), with the 3$d$ iron electronic density $n = 6$, the quasi-one-dimensional iron-based ladders have attracted considerable attention. Here, we use Density Functional Theory (DFT) to predict that the novel $n = 6$ iron ladder BaFe$_2$Te$_3$ could be stable with a similar crystal structure as BaFe$_2$Se$_3$. Our results also indicate that BaFe$_2$Te$_3$ will display the complex 2$\times$2 Block-type magnetic order. Due to the magnetic striction effects of this Block order, BaFe$_2$Te$_3$ should be a magnetic noncollinear ferrielectric system with a net polarization $0.31$ $\mu$C/cm$^2$. Compared with the S- or Se-based iron ladders, the electrons of the Te-based ladders are more localized, implying that the degree of electronic correlation is enhanced for the Te case which may induce additional interesting properties. The physical and structural similarity with BaFe$_2$Se$_3$ also suggests that BaFe$_2$Te$_3$ could become superconducting under high pressure.

Magnetic states of iron-based two-leg ladder tellurides

The recent discovery of superconductivity at high pressure in the two-leg ladder compounds BaFe$_2X_3$ ($X$=S, Se) started the novel field of quasi-one-dimensional iron-based superconductors. In this publication, we use Density Functional Theory (DFT) to predict that the previously barely explored ladder compound RbFe$_2$Te$_3$ should be magnetic with a CX-type arrangement involving ferromagnetic rungs and antiferromagnetic legs, at the realistic density of $n=5.5$ electrons per iron. The magnetic state similarity with BaFe$_2$S$_3$ suggests that RbFe$_2$Te$_3$ could also become superconducting under pressure. Moreover, at $n=6.0$ our DFT phase diagrams (with and without lattice tetramerization) reveal that the stable magnetic states could be either a 2$\times$2 magnetic Block-type, as for $X$=Se, or a previously never observed before CY-type state, with ferromagnetic legs and antiferromagnetic rungs. In the Te-based studies, electrons are more localized than in S, implying that the degree of electronic correlation is enhanced for the Te case.

Two-dimensional electron gas and its electric control at the interface between ferroelectric and antiferromagnetic insulator studied from first principles.

A multiferroic interface between the antiferromagnetic Slater insulator SrTcO3 and ferroelectric BaTiO3 (BTO) is studied from first principles. Although the interfacial magnetoelectric coupling of SrTcO3(001) is relatively small, we found that a two-dimensional electron gas (2DEG) appears for both BaO/TcO2 and TiO2/SrO terminations. The charge character of the carriers, induced in the band gap due to polar BTO, can be switched from electrons to holes by the reversal of the electric polarization in BTO. The 2DEG is robust and stable against the degree of electronic correlations, whereas the paraelectric state of BTO suppresses the 2DEG. The origin of the 2DEG at the BTO/SrTcO3 interface and its key factors are discussed.

Electron correlations in the quasi-two-dimensional organic conductorθ-(BEDT-TTF)2I3investigated by13C NMR

We report a ${}^{13}$C-NMR study on the ambient-pressure metallic phase of the layered organic conductor $\ensuremath{\theta}$-(BEDT-TTF)${}_{2}$I${}_{3}$ (BEDT-TTF: bisethylenedithio-tetrathiafulvalene), which is expected to connect the physics of correlated electrons and Dirac electrons under pressure. The orientation dependence of the NMR spectra shows that all BEDT-TTF molecules in the unit cell are to be seen equivalent from a microscopic point of view. This feature is consistent with the orthorhombic symmetry of the BEDT-TTF sublattice and also indicates that the monoclinic ${I}_{3}$ sublattice, which should make three molecules in the unit cell nonequivalent, is not practically influential on the electronic state in the conducting BEDT-TTF layers at ambient pressure. There is no signature of charge disproportionation in opposition to most of the $\ensuremath{\theta}$-type BEDT-TTF salts. The analyses of NMR Knight shift $K$ and the nuclear-spin-lattice relaxation rate $1/{T}_{1}$ revealed that the degree of electron correlation, evaluated by the Korringa ratio [$\ensuremath{\propto}1/({T}_{1}T{K}^{2}$)], is in an intermediate regime. However, NMR relaxation rate $1/{T}_{1}$ is enhanced above $\ensuremath{\sim}$ 200 K, which possibly indicates that the system enters into a quantum critical regime of charge-order fluctuations as suggested theoretically.

Quantum oscillations probe the normal electronic states of novel superconductors

In 2008, new classes of high-temperature superconductors containing iron have been discovered. These iron pnictides offer a new area of exploration and understanding of superconductivity. Quantum oscillations is a bulk probe that allows us to map out the full Fermi surface of a superconducting system in its normal metallic state. These oscillations are determined by the Landau quantization in high magnetic fields and are usually observed at very low temperatures and in very clean samples. By knowing the exact nature of the quasi-particles in the normal state and the degree of electronic correlations, one can simplify and restrict theoretical models required to understand the pairing mechanism in superconductors. I will discuss the current understanding of the Fermi surface studies in iron-based superconductors as determined from quantum oscillations.

Electron-correlation dynamics of a one-dimensional H 2 model in a quantized photon field

We investigate the dynamics of the degree of electron correlation for a one-dimensional H 2 model in the presence of a one-mode coherent field with a small average photon number. The degree of electron correlation is examined through the information entropy. It is found that the interaction with such a quantized photon field presents an unusual case where by only the degree of electron correlation changes with a constant charge density distribution.

Correlation Effects in the NiS2−xSex System

Electrical and selected spectroscopic measurements on single crystal NiS2−xSex (0≤x≤0.71) samples are interpreted in terms of a qualitative band structure scheme that resolves several unusual experimental observations. The guiding principle is that the degree of electron correlation can be controlled by varying the degree of substitution of Se for S.

Two-electron defect systems in ionic crystals: application to F' centres in alkali halides

The extended-ion method for the study of excited states in ionic crystals is modified for such two-electron defect systems as the F' centre and positronium in alkali halides. Exclusive use of 1s floating Gaussian functions as the basis is the main feature of the method. Some degree of electron correlation is built-in in this method. Applied to F' centres, it is found that in NaI the spin-singlet ground state is deeply bound, and there is also a bound triplet state. Comparison with experimental data regarding the optical and thermal dissociation energies in several alkali halides gives reasonable agreement.

Crystal structures, electrical conductivity and band-structure calculations of three new [cation][Ni(C3S5)2]2compounds

The mixed-valence radical salts [smdt][Ni(C3S5)2]21, [dmp][Ni(C3S5)2]22 and [dmm][Ni(C3S5)2]23(C3S52–= 1,3-dithiole-2-thione-4,5-dithiolate, smdt =S-methyl-1,3-dithianium, dmp =N,N-dimethylpyrrolidinium and dmm =N,N-dimethylmorpholinium) have been obtained by electrochemical oxidation of the corresponding [cation][Ni(C3S5)2] complexes in acetonitrile. Single-crystal X-ray studies revealed that all three crystallise in essentially the same packing mode. Their structure consists of anionic diads stacked face-to-face along one direction. The stacks form layers of Ni(C3S5)2 units which are separated by the cations. In the structure of 3 two types of Ni(C3S5)2 layers exist, exhibiting different structural and electrical properties. The acceptor molecules neighbouring along the longest axis make an angle of 135 (1), 139 (2) and 152°(3) with each other, thereby forming a herringbone motif. In all three structures, short intermolecular S ⋯ S contacts can be found predominantly between Ni(C3S5)2 units lying side-by-side to each other, but also within the stacks in 3. At ambient pressure, all three Ni(C3S5)2 compounds have room-temperature conductivities ranging between 1 and 70 S cm–1. Their thermal behaviour is consistent with semiconductors having activation energies of about 0.14 eV. Band-structure calculations were carried out and found to be in agreement with the observed conductivities when some degree of electron correlation is taken into account. Calculation of molecular orbital overlaps indicated dimerisation of the anions, with a weak, essentially two-dimensional conduction pathway for salts 1 and 2. In the crystal lattice of 3 one Ni(C3S5)2 layer can be considered as conducting along the stacking direction, while in the second type of layer a two-dimensional conduction pathway appears to be present.

Systematic interpretation of experimentally measured and theoretically calculated spectra of transition-metal compounds by an optimized, linearized crystal field fitting procedure

Linearized, least-squares fitting of experimental and theoretical spectra of transition-metal clusters by the crystal field (CF) model yields not only the usual systematically optimized CF orbital splitting and Racah parameters but also a convenient and direct measure of the sensitivity of the transition energies to those various quantities. Specifically considered are the transitions observed for Cr 3+ and Ni 2+ ions in octahedral fluoride clusters. The quality of the fit is less influenced by the Racah C B ratio than by the scaling of the metal d orbitals (i.e., the nephelauxetic effect). Consistent fittings of observed and SCF-MO-calculated spectra of NiF 6 4− and CrF 6 3− show very close agreement in values of, and sensitivities to, the CF parameter 10 Dq, the nephelauxetic factor, and the effective orbital population, the last being a direct measure of breakdown of the orbital approximation to the wavefunction. In comparing Racah parameters for free and complexed ions it is essential to choose values such that the same degree of electron correlation is encompassed in both cases.

Model of Electron Correlation in Solids

The usual Hartree—Fock model (energy-band theory) does not always give an adequate description of electronic structure in a solid, because it ignores the effects of electron correlation. It was shown first by Wigner that such a situation always develops in an electron ``gas'' at sufficiently low density; a solid structure described by ``resonance'' of Heitler—London pair bonds between electrons localized on neighboring atoms is then a good model of the system. The transition from a Bloch-type state to such a highly correlated state as a function of electron density (lattice parameter) is a problem of considerable interest for the theory of solids, particularly those with tight binding and high electron correlation. This work is an extension of the ``alternant molecular orbital'' (AMO) model for molecules to a simple infinite lattice structure (one atom per lattice point), for which the lattice itself is composed of two interpenetrating and equivalent sublattices; a bcc lattice is an example. Electron correlation is treated by a variable parameter which controls alternation of electron density for a given spin between the sublattices. The ground state of this model dissociates into neutral atoms as the lattice parameter a→∞; the energy-band model ground state does not. des Cloiseaux seems to have been the first to consider a wavefunction of the AMO form for a solid; in his work, however, no real calculation is carried out, the second quantization formalism being employed and some drastic approximations made to obtain a semiquantitative description of electron behavior. In this work an explicit energy expression is obtained which is practical for exact calculations; the energy expression is also a variational form. This is important because in our opinion the model may not show all the properties ascribed to it by des Cloiseaux, and accurate calculation can establish this. This work also differs in some respects from the method of ``different bands for different spins'' of Calais. While both are extensions of the AMO method to infinite lattices, there are certain incorrect assumptions in Calais' treatment which lead to errors in the case of two- and three-dimensional lattices. An approach differing from those of des Cloiseaux and Calais is used here, employing a transformed set of basis functions localized in r space, the localized alternant orbitals (LAO's). The LAO description explicitly shows aspects of the model which are not obvious in the k-space basis set, particularly suggesting the relation to a ``resonating'' Heitler—London model. In addition, the LAO basis set makes it easy to obtain practical energy expressions valid to higher order in N−1 (2N electrons), for spin eigenstates projected from the AMO single-determinant wavefunction, for spin s«N½ (and for the ferromagnetically ordered state, s=N). des Cloiseaux and Calais considered only the single determinant. The density of states of spin s in the vicinity of the 1Γ1 state varies strongly with the degree of electron correlation. In this model either the 1Γ1, or the ferromagnetic state (s=N) lies lowest; it is also conjectured though not definitely established that the 1Γ1 state is always in fact the ground state, with the state s=N separated from it by terms of order 1 in the energy per particle. All the energy coefficients determining these splittings are obtained as interactions of a single site with its local environment. The spin correlation between two lattice sites is computed and it is found that for all states with spin s«N½, the AMO correlation is antiferromagnetic (i.e., long-range order exists), a result agreeing with the obvious character of the single-determinant wavefunction from which spin eigenstates are projected. For s=N, the correlation is of course completely ferromagnetic. Though the model is not a good one for metals, the type of electron correlation it considers closely resembles the spin-density waves (SDW) recently used by Overhauser to discuss the alkali metals. By contrast, though, the conditions of primary validity of this model are not those of the ``electron gas'' but the very low density limit where correlation effects are dominant.