Cuprate superconductors phases

In these and the other cuprate superconductors, the part of the structure that leads to superconductivity is the slab of Cu02 sheets. When more than one sheet is present, they are separated by cation layers, Q (usually Ca or Y) to give a sequence Cu02-(Q-Cu02) i, which forms the superconducting layer in the material. The index n is the total number of Cu02 layers in the phase, which is equal to the formula number of Cu atoms present (Fig. 8.5). 

Figure 8.9 Simplified generic phase diagram for cuprate superconductors.

The layer-type structures and chemical nature of the constituents of the bismuth and thallium-based cuprate superconductors - notably the lone-pair stereochemistry of Bis+, variable valence of copper, and considerable exchange among some of the cation sites - combine to make structural non-ideality, nonstoichiometry, and phase intergrowth the rule rather that the exception in these families of materials. These features, as well as the probable metastability of the phases (and possibly all high-temperature oxide superconductors), also contribute to the difficulties typically encountered in preparing single-phase samples with reproducible properties and compositions. 

This chapter presents an overview of our understanding of phase relationships and a summary of synthetic techniques for the synthesis of phase-pure superconducting samples in the bismuth-and thallium-based families of high Tc cuprate superconductors. 

A review of the synthetic methods used to prepare the bismuth and thallium families of cuprate superconductors has been presented. An overview of our current knowledge of phase relationships in the bismuth systems is also given such studies of... 

Figure 6.3. High Tq cuprate superconductors (HTSC) as catalysts (a) structural schematic diagram of YBa2Cu307 ( 123 ) HTSC with Cu02 sheets (b) HRTEM atomic image of the 123 phase in [010] projection with ED pattern. The image is recorded near the Scherzer defocus. The positions of the Y, Ba and Cu atom columns are indicated. The layer separation is "- 1.18 nm and the unit cell is outlined. (After Gai and Thomas 1991.)...

An unusual feature of the cuprate superconductors is the anomalous suppression of superconductivity in La2 Ba Cu04 and related phases when the hole concentration X is near 1/8. A possible explanation is a dynamical modulation of spin and charge giving antiferromagnetic stripes of copper spins periodically separated from the domains of holes. Neutron-diffraction evidence has been presented in the case of Laj g Nd() Sr CuO (x = 0.12) which is a static analogue of the dynamical stripe model (Tranquada et al., 1995). It appears that spatial modulations of spin and charge density are related to the superconductivity in these oxides. 

We have hitherto discussed cuprate superconductors with holes as charge carriers. Cuprates of T structure (Fig. 7.16) such as Nd2-jcCe Cu04 and Nd2Cu04 F are electron superconductors. There is however some symmetry in the phase diagrams of the electron and hole superconductors (Maple, 1990) as can be seen from Fig. 7.27. 

See, for instance, Phase separation in Cuprate Superconductors, edited by K.A. Muller and G. Benedek (World Scientific, Singapore, 1993). 

The problem of phase separation in cuprates superconductors has been longely debated [1-8], Recently, several experiments show the formation of electronic crystals at critical densities [9, 10], These results provide a strong experimental support for the scenario proposed some years ago (11-14) for the phase diagram of cuprate superconductors where generalized Wigner... 

Phase Separation in Cuprate Superconductors " edited by E. Sigmund and A. K. Muller (Springer Verlag, Berlin-Heidelberg, 1994), (proc. of the second int. workshop on Phase Separation held in Cottbus, Germany, Sept 4-10, 1993)... 

Rulfier-Albenque, F., Alloul, H., and Tourbot, R. (2003). Influence of pair breaking and phase fluctuations on disordered high tc cuprate superconductors. Phy. Rev. Lett., 91 047001. 

Sigmund, E. Muller, K. A. (eds) 1994. Phase separation in cuprate superconductors. Heidelberg, Germany Springer. 

The crystal structures (Section 8.2.2) and the phase-relations (Section 8.2.3) of the cuprate superconductors are considerably more complex than for the metallic superconductors. It is not surprising that, while there a common macrotheory, different microtheories are required. 

Cuprate superconductors exhibit complicated phase diagrams which are functions of the doping parameter, x which controls the amount of the electron-transfer into or out of the cuprate plane. See for example Fig. 8.2. 

In the Tl2 xBa2Ca2+ Cu3O10 system, Kaneko et al. [79] have postan-nealed the as-synthesized x = 0.3 sample in an evacuated quartz tube at 750°C for 250 h and obtained a Tc of 127 K. Similarly, Liu et al. [80] have done similar postannealing treatments on x = 0.4 samples and obtained a Tc value of 128 K. This is the highest Tc value known for thallium cuprate superconductors. Usually, starting with little excess calcium helps in the phase formation of Tl-2223. 

With the discovery of superconductivity (Tc = 15.5 K) in the Y-Ni-B-C system [6, 80], a new class of quaternary borocarbide superconductors has emerged. Superconductivity has been observed in several rare earth (Lu, Tm, Er and Ho) nickel borocarbides[80], and with transition metals such as Pd and Pt. The superconducting phase having the composition of YNi2B2C, crystallizes [81] in a tetragonal structure with alternating Y-C and Ni2B2 layers. Band structure calculations [82] indicate that these materials, unlike cuprate superconductors, are three-dimensional metals. 

J.-C. Grenier, A Wattiaux, M. Pouchard in Proc. 3rd Workshop on Phase Separation in Cuprate Superconductors, K. A. Muller, G. Benede (eds.). World Scientific, Singafore (1993). 

Vibronic Polarons and Electric Current Generation by a Berry Phase in Cuprate Superconductors... 

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