Cuprate superconductors

Details of the nomenclature and the importance of the charge reservoir block, as weU as the doping level, are explained below for the Y-Ba-Cu-O system. 

To comply with health and environmental regulations, neither T1 nor Hg can be considered for widespread application in superconductors, owing to the risk of their accidental toxic release. Even though T1 in superconducting cuprates is in a 3-i-valence state (and is therefore less toxic), its easy reduction to the l-i- oxidation state and its high cationic mobility are dangerous features. Tl can readily replace K due 

2) Atoms with repulsive interactions in a periodic potential possess two possible ground states of the many body system. If the repulsive interactions are weak, the ground state is a superfluid Bose-Einstein condensate. However, if the interactions 

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CUO2 layers appear in all cuprate superconductors and appear to be a necessary but not sufficient condition for high temperature superconduction. The La2SrCu20g 2 compound has CUO2 layers but does not superconduct. Experiments also indicate that T is proportional to the carrier density in the CUO2 layer but not to the volume carrier density, which is further evidence that the YBa2Cu202 is a two-dimensional superconductor. 

Karpfen A (2007) Theoretical Characterization of the Trends in Halogen Bonding. 126 1-15 Keller H (2005) Unconventional Isotope Effects in Cuprate Superconductors 114 143-169 Keller H, see Bussmann-Holder A (2005) 114 367-386 Khan AI, see Williams GR (2005) 119 161-192... 

Muller KA (2005) Essential Heterogeneities in Hole-Doped Cuprate Superconductors 114 1-11... 

For e > 0.1,there is a possibility to adjust e to the recent experimental data on k(T) (Brandstatter,1994) for high — Tc cuprate superconductor TI2CC12(7 — 2223). Our calculations show that,the best choice of e is found to be e = 0.21.The appropriate k(t) is presented in Fig.4 (solid line). The dashed line in this figure shows k(t) for D = 3. This fitting process allows us to get an estimation on the effective dimensionality of the high — Tc superconducting materials. 

Apart from structures that are built of slabs, modular structures that can be constructed of columns in a jigsawlike assembly are well known. In the complex chemistry of the cuprate superconductors and related inorganic oxides, series of structures that are described as tubular, stairlike, and so on have been characterized. Alloy structures that are built of columns of intersecting structures are also well known. Structures built of linked columns, tunnels, and intersecting slabs are also found in minerals. Only one of these more complex structure types will be described, the niobium oxide block structures, chosen as they played a significant role in the history of nonstoichiometry. 

The important and widely studied copper-oxide-derived high-temperature superconductors, known as cuprate superconductors, are basically insulators. Doping converts these into metallic materials, many of which are superconductors over rather more restricted composition ranges. Several of these materials have already been discussed La2Cu04 and Sr2Cu02F2 (Section 4.3.3), La2 A.SrxCu04 (Section 8.5.1), and Nd2, Ce,Cu04 (Section 8.5.2). 

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 8.5 Superconducting planes found in cuprate superconductors (a) a single Cu02 sheet and (b) a Cu02 (Q — Cu02) i superconducting layer.

Figure 8.6 Schematic depiction of a cuprate superconductor. Doping into the charge reservoir layers results in the transfer of holes to the superconducting layers.

Figure 8.9 Simplified generic phase diagram for cuprate superconductors.

Twin boundaries are frequently encountered in cuprate superconductors. There are a number of ways in which twins might form in a crystal, one of which is... 

The charge reservoir layer in the cuprate superconductor HgBa2CaCu206+s is ... 

Cuprammonium rayon, 11 263—265 Cuprate oxides, 23 838-839 Cuprate superconductors, 23 837 Cupric bromide, physical properties of,... 

Essential Heterogeneities in Hole-Doped Cuprate Superconductors... 

A number of chemical reviews of cuprate superconductors have included the bismuth and thallium families (11)-(14). Reviews focussing on the structural chemistry of these two series are also available (15),(16). On the thallium cuprates, an overview of structural studies has appeared (17), and a detailed review of... 

Figure 2 Schematic representation of the ideal structures of the thallium monolayer cuprate superconductors.

Two series of thallium-containing cuprate superconductors have been synthesized with the following ideal general formulas (10) ... 

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. 

The primary difficulty in preparing the thallium-based cuprate superconductors lies in the toxicity and volatility of the reactant Tl2Os and its decomposition products. Above 600°C, the following redox-vaporization process is well under way and would lead to substantial loss of reactant in an open system, although the... 

The following sections will outline specific methods for the synthesis of Bi- and Tl-based cuprate superconductors. Because the synthetic methods and historical evolution of the compounds are different, the bismuth and thallium families are described separately. 

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