The functional properties of many interesting materials are the best if the materials are dense and have ordered or partially ordered microstructure. Obtaining such materials most often requires very time and energy consuming technologies, like high temperature and high pressure synthesis. In this work the applicability of the molten salt synthesis method, which is a simple and cheap technology, to the synthesis of the selected high temperature superconductors, ionic conductors and dielectric materials have been discussed.
We consider the coexistent id-density wave order, at the antiferromagnetic wave vector Q =(π,π), representing the pseudo-gap state, and d-wave superconductivity, driven by an assumed attractive interaction, within the BCS framework for the two-dimensional fermion system on a square lattice starting with a mean-field Hamiltonian involving the singlet id-density wave and the d-wave superconductivity pairings. The second-neighbor hopping, which is known to be important for cuprates and frustrates the kinetic energy of electrons, leads to the Fermi surface sheets being not connected by Q. The signature of the particle-hole asymmetry in the single-particle excitation spectrum of the pure id-density wave state is reflected in the coexisting id-density wave and d-wave superconductivity states, though the latter is characterized by the Bogoliubov quasi-particle bands - a characteristic feature of superconducting state. Quite significantly, we find that the coexistence is possible due to the non-nesting property.
The normal-state energy spectrum of the two-dimensional t-J model in a homogeneous perpendicular magnetic field B is investigated using the Mori projection operator technique. The density of states at the Fermi level as a function of 1/B reveals both high- and low-frequency oscillations. The high-frequency oscillations correspond to large Fermi surfaces, while the low-frequency components are related to van Hove singularities in the Landau subbands, which stem from their bending due to strong electron correlations. Frequencies of the low-frequency components are of the same order of magnitude as those observed in underdoped cuprates. These components become dominant if smoothing processes are involved. It is shown that despite increased distances between subbands the pseudogap affects only slightly the frequency of density of states oscillations.
We discuss the t-J-U model in the mean-field approximation. The role of spin-exchange coupling J and the second nearest hopping t' are examined in the context of the coexistence of superconductivity and antiferromagnetism. Stability of the phases is studied with respect to temperature. The coexistence region exists for the sufficiently large Coulomb repulsion (U>U_{cr}), and in the vicinity of the half-filled band (hole doping δ < δ_{cr}). The critical hole doping is relatively small (δ_{cr} ≈ 0.006 for J/|t| = 1/3) and linear with respect to J. The decrease of U_{cr} is proportional to J, except the limit of small J (J/|t| < 0.03), where U_{cr} grows rapidly with decreasing J. The effect of the second nearest hopping is limited - the phase diagram does not change in a qualitative manner when the t' value is changed. In the limit of T → 0, SC phase is stable even for large hole-doping (such as δ = 0.5). Additional paramagnetic phase appears for large δ or small U at non-zero temperature. When temperature increases, both SC and AF+SC phase regions are reduced.
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