Bose-Einstein Condensation Picture of Superconductivity in YBa2 Cu3 O7 (91 K) and YBa2 Cu3Se7 (371 K). (Dilute metals)

A metal dilution degree in the compounds YBa2 Cu3 O7 and YBa2 Cu3Se7 is defined as z=(rMe/rO2-;Se2-)3. A substitution of oxygen by selenium changes z by 8 times and the Bose-Einstein condensation temperature equals TcSe = TcO*8 2/3 = 364 K. The "active" electron pairs density in YBa2 Cu3 O7 is about 1.7 1020 cm-3. The electron effective mass is about 5me and is proportional to the dielectric constant.Comment: 1 page

Chemical Bond Type, Atom Packing, and Superconductivity of YBa2Cu3O7 and NaxWO3

The YBa2Cu3O8-x and NaxWO3 superconductors are treated not as an ionic compounds but rather as an arrays of atoms featuring both covalent and metallic bonding. We accept as particle radii the positions of the principal maxima in the radial distribution functions of the charge density, which are very close to the known covalent and metallic radii of atoms while differing strongly from the traditional ionic radii. The graphic pattern of atomic packing in the lattices reveals a number of features inherent in nonstoichiometric compounds. The network of atoms Y and Ba in medium Cu-O and of atoms Na in medium WO3 may be considered not as a dopant but rather as the second component of the nanocomposite. The scheme of the onset of the superconducting state and possible methods of its verification are discussed.Comment: 6 pages, 3 figure

Dilute Metals

Superconductivity (Tc), like any other property of a condensate, depends critically on the concentration of atoms. "Physical" dilution of metals exists in nonstoichiometric compounds. In such stoichiometric compounds as oxides, oxygen initiates "chemical" dilution of the metal, but its efficiency can be estimated only if the real radius of the oxygen ion, r0 ~ 0.5 A, is used in the calculation. The ground-state radii of metal atoms rm ~ (1.3--2.0 A) >> r0, so that atoms of metals occupy in the lattice ~90% of the total volume. Therefore, the lattice parameter and the electronic properties are determined by the metal-atom ground states. (For TiO2, the parameter c = 2.95 A ~ 2rTi = 2.94 A; for TiO, a*sqrt(2) = 5.99 A ~ 4rTi = 5.90 A; for BaTiO3, c = 4.05 A ~ 2rBa = 4.12 A; for Y-Ba-Cu-O, c = 11.68 A ~ (4rBa + 2rY) = 11.63 A, and so on.) Each atomic quantum state can be identified with a specific physical property of the condensate. As a result of superposition of the ground and ionic states of each metal atom, the fraction of the ground states decreases at the expense of the oxygen-excited ionic ones (thus reducing the effective concentration of atoms). The bands narrow down (in the limit, to the electron localization length), with the metal becoming an insulator, as is the case, for instance, with YBa2Cu3O8-x as x decreases from xM to 0. Conceivably, reducing the lattice deficiency in oxygen by saturating part of its valence bonds with H, Li, or B atoms could increase the Tc (YBa2Cu3O7-x ~ YBa2Cu3O7-x(1 + x)H2O). "Self-dilution" of lattices of excited atoms by ground-state atoms was observed to occur in Pd and inert gases.Comment: 7 pages, 3 figure

Bose-Einstein Condensation Picture of Superconductivity in Ag2 (Ag3Pb2H2O6), Na0.05WO3 and Na0.041NH3 composites. (Dilute metals)

Traditionally, when one describes the crystallographic structure of oxides, the oxygen ion radius r02 is assumed to be approximately equal to 1.4A. The oxygen ions occupy in this case 80-90% of the crystal volume. Metal atoms are considered then as ions playing a role of donors with rather small radius of (0.5 - 0.8) A. However, the atomic packing picture and, therefore, physical properties such as electric conductivity and superconductivity of oxides will be essentially different, if we assume r02- ~ 0.56 A. Such magnitude of the radius is known from the quantum mechanics calculations [2]. According to this picture, 80-90 % of the crystal volume is occupied by the metal atom orbitals with radius (1.3-1.9) A, while the oxygen ions play a role of acceptors, which reduce occupancy of these orbitals ("indirect" dilution of the metal). A "direct" dilution of metals takes place in stoichiometric matrices. When r02-~0.56 A, channels with diameter 3.6 A present in the hexagonal matrix Ag3Pb2H2O6 directed along the "c" axis. The channels are filled by chains of the Ag2 molecules with atomic diameter of 2.6 A and the molecule concentration n_B = 25.6 10^20 cm^-3. The Bose-Einstein condensation (BEC) temperature T_cB ~ 400K is calculated for the electron effective mass value m*=7.5m_e, where m_e is the isolated electron mass. Three-dimensional networks of Na2 chains form in Na solutions in NH3 and WO3 as well (respectively, with n_B=4.71 10^20 cm^-3; T_c sim 180K; m*=5.0m_e and n_B=4.74 10^20 cm^-3; T_c sim 91K; m*=10m_e). Close magnitudes of the Ag2 and Na2 chains parameters respectively in NH3 and WO3 favors the opinion that all these structures have a composite structure and similar mechanisms of the high temperature superconductivity.Comment: 3 pages, 1 figur

Metallic xenon. Polarizability. Equation of state

It is shown that some of the physical properties of inert gas (IG) condensates (polarizability, compressibility, metallization under pressure, equation of state) may be well described taking into account the first excited state of atoms only. The Herzfeld criterion of metallization well corresponds to the Mott transition criterion and to the percolation threshold. For metallic xenon concentration of the molecular type excitations corresponds to the Bose condensation temperature $T_C\sim 4000K$. The BCS formula gives $T_c\sim 5000K$. If phonons are changed by fluctuations of interatomic interaction energy. A simple relations between the parameters of atoms at metallization has been found.Comment: 2 pages, RevTe

The Two-Component Model and Metallization of Van der Waals Crystals

The paper discusses a model of Van der Waals crystals in which band-gap structures do not form. An effect of strong and chaotic electron-electron repulsion, which was excluded from consideration in the traditional approach, is taken into account. A condensate exists as a result of a dynamic equilibrium among atoms acted upon by constant Van der Waals forces and periodically forming and disappearing covalent bonding. One part of atoms is, on the average, in the ground, and the other, in excited state, to form diatomic virtual molecules. Treated in terms of this pseudoclassical model, the interatomic distances, binding energies, volumes, and pressures at which metallization, for instance, of inert gases and hydrogen, sets in is described by simple relations involving only two spectroscopic parameters of atoms (molecules). Applying pressure to a VdW crystals transfers it from the insulator first to a Bose superconductor, and after that, to a Fermi metal. An empirical relation $T_c \sim N^{2/3}$ between the superconductivity transition temperature $T_c$ and the particles concentration $N$ in chalcogens under pressure is considered as an example of such situation.Comment: 3 pages, RevTeX

Dilute Metals: Superconductivity, Critical Currents, Magnetic Properties

Properties of oxides are interpreted as a result of existence of the virtual sublattices formed by the atomic quantum states. An infinite cluster with the superconductivity of the Bose-Einstein condensate kind can be formed in the ground state sublattice at certain oxigen atoms concentration in the effectively diluted system of metal atoms (above the percolation threshold). Then the electron pairs concentration n/2 can be much less than the metal atoms concentration N in the oxide. The similar situation takes place in metals with superconductivity of the BCS type. Above the percolation threshold the superconductivity Tc may be limited by the magnetic properties of the oxigen 2p4quantum state sublattice. Data on the critical current density allow us to estimate the electronic pair density n/2 and to obtain an information concerning the superconductivity nature.Comment: 3 pages,1 figure

Metallic sulphur. "Electronic" mechanism of superconductivity?

It is shown that the rapid increase of the superconducting transition temperature $T_c$ of sulphur with increasing pressure above 93 GPa does not contradict with some hypothetical ``electronic'' mechanism of superconductivity with participation of the electron interaction energy fluctuations. Such ``electronic'' mechanism is supposed to be intrinsic property of the molecular condensates and corresponds to very high $T_c$. The low $T_c$ of sulphur (10 -17)K is likely connected with the magnetic properties of the sulphur atoms and molecules. The equation of state for sulphur is obtained. The molar volume of sulphur at metallization is 10 cm}$^3${/mol. The principal difference between the ''physical'' and the ''chemical'' type bonds are discussed. Under some pressure one bond type is changed by another and}${T}_c${may have an extremum (transition from the Bose condensation to the BCS superconductivity).Comment: RevTeX, 7 pages, no figure

Superconductivity of the two-component non-stoichiometric compounds with incommensurate sublattices

There exists a class of non-stoichiometric materials (berthollides) that can be considered as constituted by two sublattices, which have specific physicochemical properties. These properties can be essentially modified by even rather weak interaction between these components. One of them can be regarded as a rigid matrix, while another one as a filling in the form of isolated atoms, molecules or clusters. Structures containing voids of the diameter up to D ~ (1 - 2)nm in diameter in the stoichiometric sublattice belong to this class of compounds. These voids are filled by the second component (of diameter d_{0}), which can be compressed or stretched because of the sublattice parameters misfit. A stretched matter (D - d_{0} = h > 0) can exist in a unique intermediate state between the metal and the dielectric; this state cannot be implemented by another way. The period doubling occurs and a weak modulation of the metal lattice constant leads to forming not only the energy gap, but the bound electronic states of the molecular type with two paired electrons as well. Validity of this model with the Peierls-type lattice instability for explanation of the well known experimental data on superconducting transition temperature (T_{c}) in such systems (fullerides, perovskite-type compounds like Na-WO_{3}, high temperature superconductors) is considered in this work. The transition temperature T_{c} of fullerides is proportional to h/D; for the tungsten-bronzes with Na, Rb, or Cs, T_{c} > 0 for h > 0, and T_{c} ~ 0 for h < 0.Comment: 3 pages, RevTeX

Bose-Einstein Condensation Picture of Superconductivity in High Temperature Superconductors (Dilute Metals)

Structures and parameters of some high and low temperature superconductors (HTSC, LTSC) are considered basing on the alternative estimate of the O2- ion radius magnitude (0.5-0.6) A. Phase transitions into the superconducting state are considered as the Bose-Einstein condensation (BEC). The super HTSC with Tc = 371 K (YBa2Cu3Se7) and TC ~ 400K (Ag2(Ag3Pb2H2O6)) and LTSC with Tc~0.3K (SrNbxTi(1-x)O3) are shown to be of the BEC type. Instability of the structure of the first one results from higher magnitude of the Se2- ion radius in comparison with the O2- radius. The second one forms quasi one-dimensional structures and is rather inpractical. The electron density and the effective mass are estimated for some stoichiometric and non-stoichiometric (nano-composite) high temperature superconductors, which have some peculiar features. Large effective masses can indicate existance of polarons (bipolarons) in such systems. Some new superconductors MgxWO3.Comment: 5 pages, 5 figure