https://repository.library.northeastern.edu/files/neu:331186/fulltext.pdf

A survey of the Van Hove scenario for high-tc superconductivity with special emphasis on pseudogaps and striped phases

Single crystals of the layered superconductor Bi 2 Sr 2 (Ca 1− x La x )Cu 2 O 8+ δ were successfully grown out by a self-flux method. Upon La-doping, the c -axis contracts but the structure remains stable up to the critical concentration x c , above which the crystals contain the pure Bi 2 (Sr,La) 2 CuO 6 phase. The reduction of the superconducting transition temperature follows a linear relation, T c ( x )= T c (0)(1− x / x c ), with x c =0.3 being consistent with the structure data. This, and a comparison with literature data on Pr 3+ - ( x c ≈0.65) and Y 3+ -doping ( x c =1), indicate an intimate relation between structural and superconducting stability in the 2212 phase. Besides the widely discussed hole-filling effect, the structural instability due to the substitution of large La 3+ for Ca 2+ is considered to be another contribution to T c suppression in this system.

Effect of La-doping on growth and superconductivity of Bi-2212 crystals

Trial of intercalation of Br and Li into Bi2Sr2Can−1CunO2n+4 (n = 1, 2, 3)

Single crystals of the layered superconductor Bi2Sr2(Ca1-xLax)Cu2O8+d were successfully grown out by a self-flux method. Upon La-doping, the c-axis contracts but the structure remains stable up to the critical concentration xc, above which the crystals contain the pure Bi2(Sr,La)2CuO6 phase. The reduction of the superconducting transition temperature follows a linear relation, Tc(x)=Tc(0) (1-x/xc), with xc=0.3 being consistent with the structure data. This and a comparison with literature data on Pr3+- (xc>>0.65) and Y3+-doping (xc=1) indicate an intimate relation between structural and superconducting stability in the 2212 phase. Besides the widely discussed hole-filling effect, the structural instability due to the substitution of large La3+ for Ca2+ is considered to be another contribution to Tc suppression in this system.

http://arxiv.org/pdf/cond-mat/9909077.pdf

The quantum-lattice-gas model has been introduced by Matsubara and Matsuda to describe the superfluid properties of4He. It relates the Hamiltonian of a system of hard-core bosonic quasi-particles, hopping on a lattice with nearest-neighbour transfer integral t and interaction v, to that of a spin S = 12, Heisenberg antiferromagnet with uniaxial anisotropy. The quasi-particle density in the boson problem maps on the magnetization in an applied magnetic field in the antiferromagnetic problem. Interest in the charged-boson version of this model has recently been revived by its application to the superconductivity (superfluidity) of local pairs of carriers (onsite and/or intersite bipolarons) by Ranninger, Micnas, Alexandrov and Robaszkiewicz.

In this paper we investigate some applications of this model to the observed properties of the superconducting oxides. Here we profit from the wealth of knowledge that is available on the antiferromagnetic Heisenberg model with uniaxial anisotropy in a magnetic field. Once the idea is accepted that the superconductivity in the oxides is due to local (real-space pairs), one may directly transpose this knowledge of the magnetic analogues to the boson problem. In particular the comparison between the cubic, BaBiO3-based superconductors and the quasi 2-dimensional copper-oxide superconductors appears to be quite rewarding. We discuss account of the phase diagrams of Tc versus carrier density, the behaviour of the specific heat, the fluctuation effects, the field-dependent effects and the variation of Tc with the number of sheets in the superconducting multilayer compounds.

Some applications of the quantum-lattice-gas model to high-Tc superconductivity

Correlation between superconducting T c and hole concentration was investigated in the Bi 2 Sr 2 CaCu 2 O 8+δ system. The hole concentration was varied by substituting cations with different valences for Sr or Ca. In order to increase the hole concentration, some samples were annealed at 430°C in 250 bar of oxygen pressure. We find that T c increases, takes a maximum at the hole concentration corresponding to 0.2–0.3 holes per CuO 2 unit, and then decreases, with increasing hole concentration. This correlation is markedly similar to those in the La 2− x Sr x CuO 4 and YBa 2 Cu 3 O 7− δ systems.

Correlation between Tc and hole concentration in the cation-substituted Bi2Sr2CaCu2O8+δ system

The mode of the incommensurate modulation in the {ital b} direction of the Bi{sub 1.8}Sr{sub 2}(Ca{sub 1{minus}{ital x}}Y{sub {ital x}})Cu{sub 2.2}O{sub {ital z}} (0.05{lt}{ital x}{lt}0.75) system is investigated by means of electron diffraction and high-resolution lattice imaging. The change of period of the long-period structure with {ital x} is found to be basically due to the mixing ratio of domains of two modulation periods with {ital b}=4.5{ital b}{sub 0} and 5{ital b}{sub 0} or 4.5{ital b}{sub 0} and 4{ital b}{sub 0}, thus creating periods of {ital b}=4.75{ital b}{sub 0--}4.0{ital b}{sub 0}. The fundamental orthorhombic lattice has dimensions of {ital a}{congruent}{ital b}{congruent}{ital b}{sub 0} (0.54 nm) and {ital c}{congruent}{ital c}{sub 0} (3.1 nm). The change of the mixing mode from one to the other mentioned above occurs just in the yttrium concentration range, 0.45{lt}{ital x}{lt}0.65, which also corresponds to the superconductor (metallic)-to-semiconductor transition boundary. The mixing modes of the domains are directly recorded as a contrast modulation with half periods, 4.5{ital b}{sub 0}/2 and 5{ital b}{sub 0}/2 or 4.5{ital b}{sub 0}/2 and 4{ital b}{sub 0}/2 in high-resolution lattice images. These images are reproduced well by a multislice computer-simulation technique.

Yoshihiro Iwabuchi

Papers

Correlation between Tc and hole concentration in the cation-substituted Bi2Sr2CaCu2O8+δ system

Structural changes of Bi1.8Sr2(Ca1-xYx)Cu2.2Oz ceramics with yttrium content studied by electron diffraction and high-resolution electron microscopy