We present a review of the basic ideas and techniques of the spectral density functional theory which are currently used in electronic structure calculations of strongly{correlated materials where the one{electron description breaks down. We illustrate the method with several examples where interactions play a dominant role: systems near metal{insulator transition, systems near volume collapse transition, and systems with local moments.

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Electronic structure calculations with dynamical mean-field theory

Materials with correlated electrons exhibit some of the most intriguing phenomena in condensed matter physics. A new theoretical framework is now allowing theorists to calculate the electronic structure of these materials, which can exist in a rich variety of phases.

Strongly Correlated Materials: Insights From Dynamical Mean-Field Theory

Transition metal diselenides (MSe2, M=Ni, Co, Fe) were synthesized successfully through a simple solvothermal-reduction reaction at low temperatures. EDXA and XPS techniques confirmed that the compositions of these diselenides were in good agreement with their stoichiometries. With increasing temperature, the obtained NiSe2 transformed from initial filament nanocrystallites to final octahedral crystals. The studies on the thermal stability of the as-prepared NiSe2 showed that NiSe acted in an intermediate role in the pyrolysis process. The Raman spectrum of NiSe2 presented the peak of the Se−Se stretching mode at 216.5 cm-1. XRPD techniques revealed that the as-prepared CoSe2 at higher temperature had an orthorhombic structure similar to that of FeSe2. At low temperature this orthorhombic phase CoSe2, which displayed rodlike shape with the growth direction along 〈121〉 , coexisted with a trace of the cubic phase CoSe2.

Shape Control and Characterization of Transition Metal Diselenides MSe2 (M = Ni, Co, Fe) Prepared by a Solvothermal-Reduction Process

A series of nickel selenides (NiSe2 microcrystals, Ni(1-x)Se and Ni3Se2 microspheres) has been successfully synthesized through a convenient, low-temperature hydrothermal method. A good nucleation and growth environment has been created by forming a uniform and transparent solution reaction system. The compositions (including the x value of Ni(1-x)Se), phase structures, as well as the morphologies of nickel selenides, can be controlled by adjusting the Ni/Se ratio of the raw materials, the pH, the reaction temperatures and times, and so forth. The newly produced Se microspheres in the system have been used as both reactant and in situ template to the Ni(1-x)Se microspheres. It is found that Ni(1-x)Se microspheres act as the intermediate precursor during the formation of Ni3Se2 microspheres. Under certain conditions, hexagonal NiSe microspheres can be converted into rhombohedral NiSe nanowires in solution. The formation mechanisms of a series of nickel selenides has been investigated in detail by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. This work has provided a general, simple, and effective method to control the composition, phase structure, and morphology of metal selenides in aqueous solution, which will be important for inorganic synthesis methodology and further applications of selenides.

Controlled hydrothermal synthesis and structural characterization of a nickel selenide series.

Synthesis and characterization of a nickel selenide series via a hydrothermal process

We report a study of metal-insulator transition of a strongly correlated two-band Hubbard model using a dynamical mean-field theory approach. We find that the Mott transition appears at half filling even at $T=0$ in contrast to the one-band Hubbard model. The transition is characterized by the development of a ``Kondo-like'' peak near Fermi level. We also find a signature of the coexistence of metallic and antiferromagnetic phases from the study of the single-particle Green's function and the magnetic long-range order due to the superexchange coupling between the correlated electrons. We then suggest the relevance of our results to the metal-insulator transition and the recent angle-resolved photoemission measurements of ${\mathrm{NiS}}_{2\ensuremath{-}x}{\mathrm{Se}}_{x}.$ We also study the effect of carrier doping and the comparison of our findings with the experimental results suggests the possible importance of departures from stoichiometry associated with the Se substitution. The relevance of our results to high-temperature superconductivity is also discussed.

Metal-insulator transition of the two-band Hubbard model in infinite dimension and its relevance to a strongly correlated electron system: NiS 2 − x Se x

Angle-resolved photoemission spectroscopy (ARPES) measurements on ${\mathrm{NiS}}_{2\ensuremath{-}x}{\mathrm{Se}}_{x}$ single crystals show the evolution of a narrow band near the Fermi level, which develops in the vicinity of the correlation-driven metal-insulator transition. To model the metal-insulator transition, we use a two-band Hubbard model, which has been studied extensively to describe transition-metal compounds, and use a dynamical mean-field-theory approach. The ARPES data are in striking qualitative agreement with the predictions of the model. Comparison of our data to the calculations suggests a possible mechanism for the metal-insulator transition with Se substitution in which both charge-transfer and hole-doping effects are present.

Metal-insulator transition in NiS 2 − x Se x and the local impurity self-consistent approximation model

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Metal-insulator transition of the two-band Hubbard model in infinite dimension and its relevance to a strongly correlated electron system: NiS 2 − x Se x

Metal-insulator transition in NiS 2 − x Se x and the local impurity self-consistent approximation model