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The strongly correlated electron system (SCES) insulators promise a wide range of intriguing physical phenomena arising from the exotic charge gap depending on the strength of the Coulombic repulsion between d and f orbital cations. The SCES exhibits polaron hopping conduction, which requires a higher activation energy for carrier conduction between nearest cations, indicating a strong insulating character with relatively high permittivity. Here, we demonstrate that the NiWO 4 SCES insulator, which is formed by Coulombic repulsion between the nearest d orbital of Ni, proposes a suitable triboelectric material unlisted system from conventional metal oxides. The higher degree of freedom in NiWO 4 by the lower symmetry of monoclinic structure and an additional selection of composition exhibits high relative permittivity (∼13.8) in a wide range of temperatures (up to 180 ℃). The optimized simple mixture of NiWO 4 and PDMS (polydimethylsiloxane) for the negatively charged material to triboelectric nanogenerator (TENG) presents 1.78 times higher output compared to the pristine PDMS film-based TENG. NiWO 4 , considering the physical properties of the SCES determined by Coulombic repulsion strength, paves the way for a new option of TENG materials following the high permittivity. [Display omitted] • Strongly correlated electron system insulator NiWO 4 exhibits an inherently high relative permittivity of 13.8. • A triboelectric nanogenerator is fabricated using a composite film of polydimethylsiloxane with NiWO 4 particles. • Instantaneous power of 1.15 mW is obtained with the optimized NiWO 4 concentration. • Strongly correlated electron system insulators with low symmetry might be an option of triboelectric materials. [ABSTRACT FROM AUTHOR]

The persistent challenge of NO x selective catalytic reduction degradation in the presence of SO x has remained unresolved for decades. In this study, we report a paradoxical boosted denitration performance more than 20% under the SO x environment through controlling the charge state of V utilizing strongly correlated electron systems (SCES) in Ni 1−x V x WO 4. Substitutional doped V at the Ni site, and the computed charge gap from DFT confirms the SCES. Through the manipulation of the strong correlation between V and Ni, V preserves the between + 2–3 charge state in the presence of SO x. Furthermore, we achieved a stable oxidation/reduction cycle and higher denitration efficiency by altering the oxidation point of (NH 4)HSO 4 -contaminated Ni 1−x V x WO 4. This results from deliberate decrease in Coulombic repulsion between Ni and V, improving electron transfer for catalytic performance. This revelation offers a resolution of exceptional denitration performance within SO x in industrial exhaust systems, exploiting the foundational principles of SCES. [Display omitted] • Improved denitration performance of V containing NiWO 4 in the presence of SO x. • Persistent + 2–3 valence state of V by strong electron correlation with Ni in NiWO 4. • Acidified V improve NH 3 absorption by higher electron transfer from weak correlation. [ABSTRACT FROM AUTHOR]

This short review article provides the reader with a summary of the history of organic conductors. To retain a neutral and objective point of view regarding the history, background, novelty, and details of each research subject within this field, a thousand references have been cited with full titles and arranged in chronological order. Among the research conducted over ~70 years, topics from the last two decades are discussed in more detail than the rest. Unlike other papers in this issue, this review will help readers to understand the origin of each topic within the field of organic conductors and how they have evolved. Due to the advancements achieved over these 70 years, the field is nearing new horizons. As history is often a reflection of the future, this review is expected to show the future directions of this research field.

Abstract We report the results of our investigation of magnetization and heat capacity on a series of compounds Ce 1 − x Y x NiGe 2 (x = 0.1 , 0.2 and 0.4) under the influence of external magnetic field. Our studies of the thermodynamic quantity − d M / d T on these compounds indicate that magnetic frustration persists in Ce 0.9 Y 0.1 NiGe 2 , as also reported for the parent compound CeNiGe 2. The weak signature of this frustration is also noted in Ce 0.8 Y 0.2 NiGe 2 , whereas, it is suppressed in Ce 0.6 Y 0.4 NiGe 2. Heat capacity studies on Ce 0.9 Y 0.1 NiGe 2 and Ce 0.8 Y 0.2 NiGe 2 indicate the presence of a new magnetic anomaly at high field which indicates that quantum criticality is absent in these compounds. However, for Ce 0.6 Y 0.4 NiGe 2 such an anomaly is not noted. For this later compound, the magnetic field (H) and temperature (T) dependence of heat capacity and magnetization obey H / T scaling above critical fields. However, the obtained scaling critical parameter (δ) is 1.6, which is away from mean field value of 3. This deviation suggests the presence of unusual fluctuations and anomalous quantum criticality in these compounds. This unusual fluctuation may arise from disorderness induced by Y-substitution. Highlights • Field induced magnetic frustration. • Suppression of magnetic frustration. • H / T scaling of quantum criticality. • Observation of field induced quantum criticality. [ABSTRACT FROM AUTHOR]

Investigation of broken symmetry phases with long range order in strongly correlated electron systems is among subjects that have always been of interest to condensed matter scientists. In this paper we tried to study the existence of the 120 degrees magnetic spiral order, based on anisotropy in geometrically frustrated triangular lattices, using variational cluster approximation. We observed that by increasing the anisotropy in the system, the spiral order can be found for U≥7.5t and for t'

Abstract: Elucidating the mechanism that differentiates the oxygen‐evolving center of photosystem II with its inorganic counterpart is crucial to develop efficient catalysts for the oxygen evolution reaction (OER). Previous studies have suggested that the larger overpotential for MnO2 catalysts under neutral conditions may result from the instability of the Mn3+ intermediate to charge disproportionation. Here, by monitoring the surface intermediates of electrochemical OER on rutile MnO2 with different facet orientations, a correlation between the stability of the intermediate species and crystal facets is confirmed explicitly for the first time. The coverage of the Mn3+ intermediate is found to be 11‐fold higher on the metastable (101) surfaces compared to (110) surfaces, leading to the superior OER activity of (101) surfaces. The difference in OER activity may result from the difference in surface electronic states of Mn3+, where interlayer charge comproportionation of Mn2+ and Mn4+ to generate two Mn3+ species is favored on (101) facets. Considering the fact that the OER enzyme accommodates Mn3+ stably during the Kok cycle, the enhanced OER activity of the rutile MnO2 catalyst with a metastable surface highlights the importance of mimicking not only the crystal structure but also the electronic structure of the targeted natural enzyme. [ABSTRACT FROM AUTHOR]

A new superconducting field-effect transistor (FET) in the vicinity of bandwidth-controlled Mott transition was developed using molecular strongly correlated system κ-(BEDT-TTF)2Cu[N(CN)2]Br [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene] laminated on CaF2 substrate. This device exhibited significant cooling-rate dependence of resistance below about 80 K, associated with glass transition of terminal ethylene group of BEDT-TTF molecule, where more rapid cooling through glass transition temperature leads to the decrease in bandwidth. We demonstrated that the FET properties such as ON/OFF ratio and polarity can be controlled by utilizing cooling rate. Our result may give a novel insight into the design of molecule-based functional devices.

Recently, FeSe$_{1-x}$S$_x$ systems have received much attention because of the unique pressure-temperature phase diagram. We performed $^{77}$Se-NMR measurements on a single crystal of FeSe$_{0.88}$S$_{0.12}$ to investigate its microscopic properties. The shift of $^{77}$Se spectra exhibits anomalous enhancement at $1.0~\mathrm{GPa}$, suggesting a topological change in the Fermi surfaces, so-called Lifshitz transition, occurs at $1.0~\mathrm{GPa}$. The magnetic fluctuation simultaneously changes its properties, which implies a change in the dominant nesting vector. Edited by: A. Goñi, A. Cantarero, J. S. Reparaz

Here, we present the structural studies on MnTi 1-x Ru x O 3 (x = 0, 0.2) compounds. The role of Ti ions in the magnetism of MnTiO 3 was unclear so here this issue has been tried to address. The magnetic susceptibility (χ) data shows that the 3-dimensional magnetic character has been improved in the doped MnTiO 3 . The x = 0 compound goes paramagnetic to antiferromagnetic phase at a temperature ∼64 K followed by a broad peak at a temperature ∼100 K. But in x = 0.2 compound, the antiferromagnetic transition temperature has been shifted towards the lower temperature to ∼47.5 K with a broad peak at temperature ∼79 K. On the doping of Ru at Ti site a sharp anomaly in the temperature dependent magnetic susceptibility (χ) measurement is observed ∼47.5 K in the case of x = 0.2 compound. This sharp anomaly is attributed to the improved 3D character of magnetism which arises because of the direct exchange interaction between Mn-Ti/Ru ions in addition to the inter-layer super superexchange interaction between Mn-Mn ions via two intervening oxygen ions. It is interesting to note that even though the T N and T 2D are at low temperatures compared to the x = 0 compound, the onset temperature of these two temperatures occurs at much higher temperature than as observed in x = 0 compound. [ABSTRACT FROM AUTHOR]

We present properties of single crystals of the iron pnictide superconductor RbFe2As2 grown using a self-flux method. A bulk superconducting transition at [ABSTRACT FROM AUTHOR]

Organic molecular conductors with a strongly correlated electron system, in which the itinerancy of electrons (or holes) and the electron correlation (U/W, U, the on-site Coulomb repulsion, W, the bandwidth) compete with each other, are promising candidates for achieving superconductivity and also for exploring remarkable physical properties induced by external stimuli such as pressure, light, voltage and current. Our synthetic approach to the construction of strongly correlated organic electron systems is based on chemical modifications to the donor molecule BDH-TTP [2,5-bis(1,3-dithiolan-2-ylidene)-1,3,4,6-tetrathiapentalene] capable of producing metallic CT (charge-transfer) salts stable down to low temperatures (4.2–1.5 K). This aims at enhancing the electron correlation in the itinerant electron system by decreasing the bandwidth. Chemical modifications of BDH-TTP such as ring expansion of two outer dithiolane rings, replacement of one sulfur atom in an outer dithiolane ring with an oxygen atom and introduction of two methyl substituents into an outer ditiolane ring led to BDA-TTP [2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene], DHOT-TTP [2-(1,3-dithiolan-2-ylidene)-5-(1,3-oxathiolan-2-ylidene)-1,3,4,6-tetrathiapentalene] and DMDH-TTP [2-(4,5-dimethyl-1,3-dithiolan-2-ylidene)-5-(1,3-dithiolan-2-ylidene)-1,3,4,6-tetrathiapentalene], respectively. In this review, the physical properties and the crystal and electronic structures of molecular conductors derived from these donor molecules will be described.

An isostructural series of anion radical salts, β'-(EtxMe4−xZ)[Pd(dmit)2]2 (x = 0–2, Z = P, As, Sb), with a quasi-triangular lattice comprising the dimer unit [Pd(dmit)2]2− belong to a strongly correlated electron system with geometrical frustration. Intra and interdimer transfer integrals between the frontier molecular orbitals, which characterize the strength of electron correlation and degree of frustration, can be tuned by selection of the counter cation. We have systematically analyzed the crystal structure with X-ray diffraction method and intermolecular transfer integrals using extended Hückel molecular orbital calculations based on structural data. The variation in the cation affects the unit cell in a manner equivalent to an anisotropic pressure. Increasing the covalent radius of the central atom Z and the number of ethyl groups (x) in the cation leads to slight arching of the Pd(dmit)2 molecule. This arch-shaped distortion of the Pd(dmit)2 molecule modifies the interdimer transfer integrals in formation of the regular triangular dimer lattice. On the other hand, the intradimer transfer integral, which is correlated with the effective on-site Coulomb interaction of the dimer, is weakly dependent on the type of cation.

Emergence of anomalous weak ferromagnetism in geometrically frustrated magnetic system of R 1− x Y x B 4 ( R = Tb and Dy) is quite interesting [Phys. Rev. B 91 , 024414 (2015)]. In order to study the weak ferromagnetism in other compounds, the magnetic properties of R 1− x Y x B 4 ( R = Sm, Gd, and Er) were investigated. The anomalous weak ferromagnetism was also found to occur in all compounds, R 1− x Y x B 4 ( R = Sm, Gd, and Er). The weak ferromagnetism is revealed in the magnetic easy plane, (001), for R 1− x Y x B 4 ( R = Sm and Gd) and along the magnetic easy axis, [001], for R 1− x Y x B 4 ( R = Er). The magnitude of saturated magnetic moment at T = 2 K shows a strong dependence on Y-concentration, i.e., the maximum magnitude occurs at x ≈ 0.3 for all compounds. The general magnetic properties of weak ferromagnetism is quite similar to the weak ferromagnetism in R 1− x Y x B 4 ( R = Tb, Dy). Therefore, it is experimentally demonstrated that the emergence of weak ferromagnetism, induced by non-magnetic substitution in a magnetically frustrated system, is the universal feature in R B 4 compounds. [ABSTRACT FROM AUTHOR]

We have obtained the exact expressions for the thermoelectric power and the Lorenz number for the infinite U Hubbard model using orthofermion approach. It is found that in one dimension, our results coincide with that of known exact results. In limiting cases, our exact expressions reduce to the known exact results at low and high temperature limits. We present our calculations for one and two dimensions for square as well as triangular lattices. A comparison between the thermoelectric power and Lorenz number for a free Fermi gas and noninteracting orthofermions has also been provided. [ABSTRACT FROM AUTHOR]

Competitions between different interactions in strongly correlated electron systems often lead to exotic phases. Renormalization group is one of the powerful techniques to analyze the competing interactions without presumed bias. It was recently shown that the renormalization group transformations to the one-loop order in many correlated electron systems are described by potential flows. Here we prove several rigorous theorems in the presence of renormalization-group potential and find the complete classification for the potential ows. In addition, we show that the relevant interactions blow up at the maximal scaling exponent of unity, explaining the puzzling power-law Ansatz found in previous studies. The above findings are of great importance in building up the hierarchy for relevant couplings and the complete classification for correlated ground states in the presence of generic interactions. [ABSTRACT FROM AUTHOR]

We report magnetic properties of single-crystal Ca 5 Ir 3 O 12 having a hexagonal noncentrosymmetric structure. We measured the anisotropy of the magnetic phase transition at 7.8 K up to 70 kOe and obtained magnetic phase diagrams parallel and perpendicular to the c -axis. The magnetic phase diagrams showed that the transition temperature was gradually suppressed on application of a magnetic field to perpendicular to the c -axis. In contrast, the transition temperature was increased when a magnetic field of up to 70 kOe was applied to the c -axis. We examined this enhancement in the magnetic transition temperature along the c -axis. From these features of the observed magnetic properties, possibility of multipole magnetic ordering was examined. • Magnetic moments of Ir4+ are aligned in the c -plane of its triangular sublattice. • Van Vleck susceptibility derived from Ir5+ is highly anisotropic. • The magnetic ordered temperature along the c -axis is enhanced. • A multipole magnetic order is considered from the magnetic phase diagram. [ABSTRACT FROM AUTHOR]

We present the exact solution of a two-orbital Hubbard model on a two-site molecule for arbitrary electron filling and arbitrary interaction couplings. The knowledge of the many-particle spectrum, determined via a diagonalization procedure performed by fully taking into account the symmetry properties of the model, has been used to investigate the temperature dependence of charge, spin and orbital response functions as well as of the intra- and inter-orbital on-site occupations. We point out that this study may allow easy access to many interesting features of the model and may serve as a reference tool for various numerical or perturbation methods dealing with complex correlated electron models defined on a lattice, in particular in the case in which strong local interactions dominate over kinetic effects. [ABSTRACT FROM AUTHOR]

We obtain that when the spinless model is off the half-filling regime (μ ≠ V), the Helmholtz free energy (HFE) can be written as two β-expansions: one expansion comes from the half-filling configuration and another one that depends on the parameter x = μ - V. We show numerically that the chemical potential as a function of temperature satisfies a relation similar to one derived from the particle-hole symmetry of the fermionic spinless model. We extend the β-expansion of the HFE of the one-dimensional fermionic spinless Hubbard model up to order β8. [ABSTRACT FROM AUTHOR]

Superconductivity in absence of inversion symmetry of the crystal structure is basically controlled by a Rashba-like antisymmetric spin orbit coupling which splits the Fermi surface and removes the spin degeneracy of electrons. The Fermi surface splitting can originate a mixing of spin-singlet and spin-triplet states in the superconducting condensate. The presence of spin-triplet states is expected to be responsible for various uncommon features of the superconducting ground state. Experimentally, distinct deviations from the expectations of the BCS theory are found, in general, only in those systems where beside the missing of inversion symmetry strong correlations among electrons are present. Materials of this group are primarily based on Ce, Yb or U. For the much larger group of materials without substantial electronic correlations, BCS-like superconductivity was observed in the overwhelming number of known examples. Hence, unconventional superconductivity requires, in general, the mutual presence of electronic correlations and non-centrosymmetric crystal structures. [ABSTRACT FROM AUTHOR]

We present a detailed study on the temperature-dependence of the Sm 4 f states in mixed-valent samarium hexaboride (SmB 6 ) which, as a consequence of the hybridization with the conduction band states, largely determine the physical properties of the system. By comparing soft X-ray photoemission spectra of SmB 6 with those of Sm 0.35 La 0.65 B 6 , we discuss the apparent differences and similarities of the two systems. Based on our analyses we show that the 4 f spectra of SmB 6 can be separated into two independent spectral contributions with the 4 f peak maxima at different binding energies, representing two different electronic 4 f configurations existing in the system. [ABSTRACT FROM AUTHOR]

We have performed a variational Monte Carlo simulation to study the ground state of a two-dimensional Hubbard model on a square lattice in the strong coupling region. The energy gain of possible inhomogeneous electron states are computed as a function of U when the hole density ϵ = 1 / 8 and next nearest-neighbor hopping t ′ / t = − 0.30. The bond-centered ferromagnetic diagonal stripe state is stabilized in the strong coupling region (U / t ≥ 16), which is due to the gain of both kinetic energy and on-site Coulomb interaction energy due to the holon moving over the ferromagnetic domain and the gain of kinetic-exchange-interaction energy at the antiferromagnetic domain wall. • The variational Monte Carlo method was used to investigate the ground state of two-dimensional square lattice Hubbard model with U ≲ ∞. • At 1/8 doping, a ferromagnetic (FM) diagonal stripe state consisting of a FM domain and an antiferromagnetic domain wall is stable in U / t ≥ 16. • In the strongly correlated region, the FM stripe state has a large kinetic energy gain of holes compared to the uniform electronic state. • The spatial distribution of the staggered magnetization, the charge density, and the doublon density in the FM diagonal stripe state is shown. [ABSTRACT FROM AUTHOR]

We investigate the Anderson-Holstein model with strong repulsion of impurity electrons using a strongcoupling approximation. The strong electron-phonon interaction determines the formation of polarons with heavy clouds of phonons surrounding the impurity electrons. We describe the collective mode of phonon clouds and the relation between electron propagators and correlation functions. We obtain the dependence of the collective mode energy on the hybridization parameter in the case T = 0 and prove that it is suppressed by increasing hybridization. [ABSTRACT FROM AUTHOR]

We revisit the particle-hole symmetry of the one-dimensional (D = 1) fermionic spinless Hubbard model, associating that symmetry to the invariance of the Helmholtz free energy of the one-dimensional spin-1/2 X X Z Heisenberg model, under reversal of the longitudinal magnetic 1eld and at any 1nite temperature. Upon comparing two regimes of that chain model so that the number of particles in one regime equals the number of holes in the other, one 1nds that, in general, their thermodynamics is similar, but not identical: both models share the speci1c heat and entropy functions, but not the internal energy per site, the 1rst-neighbor correlation functions, and the number of particles per site. Due to that symmetry, the difference between the 1rst-neighbor correlation functions is proportional to the z-component of magnetization of the X X Z Heisenberg model. The results presented in this paper are valid for any value of the interaction strength parameter V, which describes the attractive/null/repulsive interaction of neighboring fermions. [ABSTRACT FROM AUTHOR]

We obtain that when the spinless model is off the half-filling regime (μ ≠ V), the Helmholtz free energy (HFE) can be written as two β-expansions: one expansion comes from the half-filling configuration and another one that depends on the parameter x = μ - V. We show numerically that the chemical potential as a function of temperature satisfies a relation similar to one derived from the particle-hole symmetry of the fermionic spinless model. We extend the β-expansion of the HFE of the one-dimensional fermionic spinless Hubbard model up to order β8.

Spectroscopic ellipsometry and Raman scattering measurements of single-crystal Li _x CoO _2 ( x = 0.33, 0.43, 0.50, 0.53, 0.72, and 0.87) are reported. The room temperature optical absorption spectra for x values in the range of 0.33–0.72 exhibit three bands near 1.60, 3.35, and 5.20 eV. On the basis of first-principles calculations, the observed optical excitations were appropriately assigned. The charge-transfer absorption bands shift to higher energies in Li _0.87 CoO _2 because of symmetry-breaking-induced distortions of the hybridized Co-O orbitals with shifted oxygen 2 p states. Furthermore, two Raman-active phonon modes, which display E _g and A _1 _g symmetry, are sensitive to lithium doping. Upon cooling across 200 K, which is the antiferromagnetic phase transition temperature of Li _0.50 CoO _2 , large splitting of the E _g mode and a discontinuous change in the frequency of the A _1 _g mode were observed. These results highlight the importance of spin-phonon coupling in Li _0.50 CoO _2 .

Abstract: The magnetic properties of polycrystalline (x=0.2,0.4,0.6,0.8) samples were probed by magnetization, specific heat, ac susceptibility, and resistivity techniques. For , the curves obtained are consistent with the features expected for the corresponding magnetic modes, namely at x=0; at and 0.4; at , and at x=0.8 and 1. For each of these compositions, we thoroughly discuss the analyses leading to the determination of the magnetic critical temperature through separate independent measurements. For x=0.4, even though neutron diffraction indicates a mode (but with a reduced magnetic moment) the magnetization, the ac susceptibility, and resistivity indicate two magnetic events, in sharp contrast with other concentrations, for which a single critical event is observed; furthermore, deviation from Curie–Weiss behavior is observed below 150K for this sample. These features, together with the evolution of both magnetic moment and critical temperature, are attributed to a combination of crystalline electric field effects and an interplay between competing magnetic couplings. [Copyright &y& Elsevier]

Pu can be loaded with H forming complicated continuous solid solutions and compounds, and causing remarkable electronic and structural changes. Full potential linearized augmented plane wave methods combined with Hubbard parameter U and the spin-orbit effects are employed to investigate the electronic and structural properties of stoichiometric and non-stoichiometric face-centered cubic Pu hydrides (PuHχ, χ = 2, 2.25, 2.5, 2.75, 3). The decreasing trend with increasing χ of the calculated lattice parameters is in reasonable agreement with the experimental findings. A comparative analysis of the electronic-structure results for a series of PuHχ compositions reveals that the lattice contraction results from the associated effects of the enhanced chemical bonding and the size effects involving the interstitial atoms. We find that the size effects are the driving force for the abnormal lattice contraction. [ABSTRACT FROM AUTHOR]

An isostructural series of anion radical salts, β'-(EtxMe4-xZ)[Pd(dmit)2]2 (x = 0-2, Z = P, As, Sb), with a quasi-triangular lattice comprising the dimer unit [Pd(dmit)2]2 belong to a strongly correlated electron system with geometrical frustration. Intra and interdimer transfer integrals between the frontier molecular orbitals, which characterize the strength of electron correlation and degree of frustration, can be tuned by selection of the counter cation. We have systematically analyzed the crystal structure with X-ray diffraction method and intermolecular transfer integrals using extended Hückel molecular orbital calculations based on structural data. The variation in the cation affects the unit cell in a manner equivalent to an anisotropic pressure. Increasing the covalent radius of the central atom Z and the number of ethyl groups (x) in the cation leads to slight arching of the Pd(dmit)2 molecule. This arch-shaped distortion of the Pd(dmit)2 molecule modifies the interdimer transfer integrals in formation of the regular triangular dimer lattice. On the other hand, the intradimer transfer integral, which is correlated with the effective on-site Coulomb interaction of the dimer, is weakly dependent on the type of cation. [ABSTRACT FROM AUTHOR]

Organic molecular conductors with a strongly correlated electron system, in which the itinerancy of electrons (or holes) and the electron correlation (U/W, U, the on-site Coulomb repulsion, W, the bandwidth) compete with each other, are promising candidates for achieving superconductivity and also for exploring remarkable physical properties induced by external stimuli such as pressure, light, voltage and current. Our synthetic approach to the construction of strongly correlated organic electron systems is based on chemical modifications to the donor molecule BDH-TTP [2,5-bis(1,3-dithiolan-2-ylidene)-1,3,4,6-tetrathiapentalene] capable of producing metallic CT (charge-transfer) salts stable down to low temperatures (4.2-1.5 K). This aims at enhancing the electron correlation in the itinerant electron system by decreasing the bandwidth. Chemical modifications of BDH-TTP such as ring expansion of two outer dithiolane rings, replacement of one sulfur atom in an outer dithiolane ring with an oxygen atom and introduction of two methyl substituents into an outer ditiolane ring led to BDA-TTP [2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene], DHOT-TTP [2-(1,3-dithiolan-2-ylidene)-5-(1,3-oxathiolan-2-ylidene)-1,3,4,6-tetrathiapentalene] and DMDH-TTP [2-(4,5-dimethyl-1,3-dithiolan-2-ylidene)-5-(1,3-dithiolan-2-ylidene)-1,3,4,6-tetrathiapentalene], respectively. In this review, the physical properties and the crystal and electronic structures of molecular conductors derived from these donor molecules will be described. [ABSTRACT FROM AUTHOR]

We investigate the minimal model that takes orbital degrees of freedom into account: the degenerate two-orbital Hubbard model. Our consideration includes the intraatomic Coulomb interaction of two electrons with opposite spins on the same orbital and on different orbitals and interorbital hopping of tunneling electrons. We take the influence of states caused by Hund's rule coupling on the metal-insulator phase transition into account. We generalize the diagram theory developed for strongly correlated orbitally nondegenerate systems to the case of orbital degeneration. For the one-particle renormalized Green's function, we establish an equation of Dyson type for calculating the system spectral function using a simple approximation based on summing chain diagrams. [ABSTRACT FROM AUTHOR]

We develop a diagram theory for the periodic Anderson model assuming that the Coulomb repulsion of localized f electrons is the main parameter of the theory. The f electrons are strongly correlated and the c conduction electrons are uncorrelated. We determine the f-electron correlation function and the c-electron mass operator. We formulate the Dyson equation for c electrons and a Dyson-type equation for f electrons and their propagators. We define the skeleton diagrams for the correlation function and the thermodynamic functional. We establish the stationarity of the renormalized thermodynamic potential under variation of the mass operator. The obtained results are applicable to both the normal and the superconducting system states. [ABSTRACT FROM AUTHOR]

We propose a diagrammatic theory around the atomic limit for the normal state of the Anderson impurity model. The new diagram method is based on Wick’s theorem for conduction electrons and a generalized Wick’s theorem for strongly correlated impurity electrons, which coincides with the definition of the Kubo cumulant. We prove a linked-cluster theorem for the mean of the evolution operator and obtain Dysontype equations for the one-particle propagators. The main element in these equations is the impurity electron correlation function, which contains the spin, charge, and pairing fluctuations of the system. We express the system thermodynamic potential in terms of the full propagator of conduction electrons and the correlation function. We establish that the thermodynamic potential is stationary under changes of the correlation function. [ABSTRACT FROM AUTHOR]

Abstract: The interplay between Coulomb interactions and randomness has been a long-standing problem in condensed matter physics. Recent thermodynamic and transport experiments have shown that in clean two-dimensional electron systems, strong interactions between carriers lead to Pauli spin susceptibility growing critically at low electron densities. In the immediate vicinity of the metal–insulator transition (MIT), both the resistance and the effective interactions become temperature dependent and exhibit a fan-like spread as the MIT is crossed. A resistance-interaction flow diagram clearly reveals a quantum critical point. [Copyright &y& Elsevier]

We consider the four-band model of high-temperature superconductors with copper $$3d_{x^2 - y^2 } $$ and 4 s electrons and oxygen 2 p x and 2 p y electrons. We assume that the Coulomb repulsion between the copper 3 d electrons is strong. We diagonalize the oxygen subsystem and pass to the Wannier representation for the CuO 2 cluster. We study the properties of a local cell model and find its low-energy states such as the one-particle doublet and the two-particle singlet and triplet states. We consider the cell fermion delocalization because of their tunneling between the copper sites. We obtain a system of Dyson-type equations for renormalized one-particle Green’s functions and discuss whether it can be simplified. [ABSTRACT FROM AUTHOR]

This work provides a brief survey of the Hall effect data collected for some strongly correlated electron systems. The experimental results illustrate the behaviour of the heavy-fermion system UCu5Al, heavy-fermion semiconductor U2Ru2Sn, ferromagnetic superconductor under pressure UGe2, and the ferromagnet with a weak 2D localization effect UCo0.5Sb2. [ABSTRACT FROM AUTHOR]

In view of a contrasting behaviour of Pd-containing and Rh-containing systems, we investigate solid solutions CeRh1-xPdxAl in order to determine the dependence of their electronic properties on the number of the conduction electrons. We present structural and X-ray photoemission spectroscopy (XPS) data for CeR1-xPdxAl. We also discuss the influence of the number of free electrons in the conduction band on the stability of the crystallographic structure and the occupation number of the f-shell. [ABSTRACT FROM AUTHOR]

We have investigated the XPS spectra of Ce2Rh3Al9 and calculated the DOS by using the full potential linear augmented plane wave (FP-LAPW) and tight binding linear muffin tin orbitals (TB-LMTO-ASA) methods. The overall agreement between the calculated and measured XPS valence band spectra is good. The analysis of the 3d XPS (X-ray Photoemission) spectra, using Gunnarson-Schonhammer theory, suggests a mixed valence behaviour of Ce. FP-LAPW calculations show a half-metallic behaviour of Ce2Rh3Al9, whereas LMTO calculations result in a semiconducting and nonmagnetic ground state. This result is, however, in contradiction to the experimental observation of resistivity ρ(T), which does not show an activated behaviour. We attribute the possible appearance of a semiconducting/half-metallic gap for Ce2Rh3Al9 to atomic disorder. [ABSTRACT FROM AUTHOR]

We have investigated the XPS spectra of Ce2Rh3Al9 and calculated the DOS by using the full potential linear augmented plane wave (FP-LAPW) and tight binding linear muffin tin orbitals (TB-LMTO-ASA) methods. The overall agreement between the calculated and measured XPS valence band spectra is good. The analysis of the 3d XPS (X-ray Photoemission) spectra, using Gunnarson-Schonhammer theory, suggests a mixed valence behaviour of Ce. FP-LAPW calculations show a half-metallic behaviour of Ce2Rh3Al9, whereas LMTO calculations result in a semiconducting and nonmagnetic ground state. This result is, however, in contradiction to the experimental observation of resistivity ρ(T), which does not show an activated behaviour. We attribute the possible appearance of a semiconducting/half-metallic gap for Ce2Rh3Al9 to atomic disorder. [ABSTRACT FROM AUTHOR]

Abstract: Soft and hard X-ray photoelectron spectroscopy (SX- and HAXPES) has been performed for strongly correlated transition-metal oxides and rare-earth compounds in order to reveal the bulk electronic structures and/or the bulk Fermi surfaces. The variation of the bulk V 3 spectrum for SrCaVO depending on and the feature of the Fermi surfaces for SrCaRuO is consistent with other bulk-sensitive measurements and band structure calculations, respectively. It has been demonstrated for rare-earth compounds that the application of the HAXPES enables us to obtain more reliable information on the bulk electronic structures and valences. [Copyright &y& Elsevier]

Abstract: We investigated a one-dimensional strongly correlated electron system [NiBr(chxn)2]Br2 by time-resolved luminescence spectroscopy in the picosecond region and found that the luminescence was constructed from two components distinguished by energy and lifetime. One of these had a small Stokes shift and the other had a large Stokes shift of approximately 0.2eV. These results suggest the coexistence of excited states with large and small lattice relaxations. We propose a model to interpret the dynamics of luminescence spectra. [Copyright &y& Elsevier]

We revisit the particle-hole symmetry of the one-dimensional (D=1) fermionic spinless Hubbard model, associating that symmetry to the invariance of the Helmholtz free energy of the one-dimensional spin-1/2 XXZ Heisenberg model, under reversal of the longitudinal magnetic field and at any finite temperature. Upon comparing two regimes of that chain model so that the number of particles in one regime equals the number of holes in the other, one finds that, in general, their thermodynamics is similar, but not identical: both models share the specific heat and entropy functions, but not the internal energy per site, the first-neighbor correlation functions, and the number of particles per site. Due to that symmetry, the difference between the first-neighbor correlation functions is proportional to the z-component of magnetization of the XXZ Heisenberg model. The results presented in this paper are valid for any value of the interaction strength parameter V, which describes the attractive/null/repulsive interaction of neighboring fermions.

• We investigated kinetic properties of correlated pairing states in strongly correlated phase of the Hubbard model in two dimensions, employing an optimization variational Monte Carlo method. • The Gutzwiller-BCS state is stabilized as the potential energy driven superconductivity because the Coulomb interaction energy is lowered while the kinetic energy increases in this state. • In contrast, in the ψ λ -BCS wave function ψ λ − B C S = e − λ K P G ψ B C S , the Coulomb energy increases and instead the kinetic energy is lowered in the strongly correlated phase. • The correlated superconducting state is realized as a kinetic energy driven pairing state and this indicates the enhancement of superconductivity due to kinetic-energy effect. We investigated kinetic properties of correlated pairing states in strongly correlated phase of the Hubbard model in two space dimensions. We employ an optimization variational Monte Carlo method, where we use the improved wave function ψ λ = e − λ K ψ G for the Gutzwiller wave function ψ G with K being the kinetic part of the Hamiltonian. The Gutzwiller-BCS state is stabilized as the potential energy driven superconductivity because the Coulomb interaction energy is lowered while the kinetic energy increases in this state. In contrast, we show that in the ψ λ -BCS wave function ψ λ − B C S = e − λ K P G ψ B C S , the Coulomb energy increases and instead the kinetic energy is lowered in the strongly correlated phase where the Coulomb repulsive interaction U is large. The correlated superconducting state is realized as a kinetic energy driven pairing state and this indicates the enhancement of superconductivity due to kinetic-energy effect. [ABSTRACT FROM AUTHOR]