The Magnon Pairing Mechanism of Superconductivity in Cuprate Ceramics

Superconducting properties of copper oxide high-temperature superconductors

The equations for the magnon pairing theory of high-temperature copper-oxide-based superconductors are solved and used to calculate several properties, leading to results for specific heat and critical magnetic fields consistent with experimental results. In addition, the theory suggests an explanation of why there are two sets of transition temperatures (T(c) approximately 90 K and T(c) approximately 55 K) for the Y(1)Ba(2)Cu(3)O(6+x) class of superconductors. It also provides an explanation of why La(2-x)Sr(x)CuO(4) is a superconductor for only a small range of x (and suggests an experiment to independently test the theory). These results provide support for the magnon pairing theory of high-temperature superconductors. On the basis of the theory, some suggestions are made for improving these materials.

Experimental Constraints on Theories of High-Transition Temperature Superconductors

Recent experiments have revealed several key features of the unique nature of the new, high-transition temperature cuprate superconductors. These results provide an easily understandable, physical picture of the structure and behavior of the charge carriers in these materials, and point to the mechanism responsible for their existence. These experiments are now placing strong constraints on possible theoretical models of the phenomenon.

The Incommensurate Modulation of the 2212 Bi-Sr-Ca-Cu-O Superconductor

The incommensurate modulation evident in the diffraction pattern of the superconductor Bi(2)Sr3-xCa(x)Cu(2)O8+y consists of almost sinusoidally varying displacements of up to 0.4A of the Bi and Sr atoms in the a-and c-directions of the unit cell, and of up to 0.3 A of the Cu atoms in the c direction only. Thus, a newly discovered feature of the Bi(2)Sr3-xCaxCu(2)O8+y structure is sizable Cu displacement, which is related to static wave formation in the Cu-O sheets. Reported thermal parameters give evidence that similar distortions occur on cooling of the thallium-containing superconductors.

Electronic Structure and Valence-Bond Band Structure of Cuprate Superconducting Materials

From ab initio calculations on various clusters representing the La2-xSrxCu(1)O(4) and Y(1)Ba(2)Cu(3)O(7) classes of high-temperature superconductors, it is shown that (i) all copper sites have a Cu(II)(d(9))oxidation state with one unpaired spin that is coupled antiferromagnetically to the spins of adjacent Cu(II) sites; (ii) oxidation beyond the cupric (Cu(II)) state leads not to Cu(III) but rather to oxidized oxygen atoms, with an oxygen ppi hole bridging two Cu(II) sites; (iii) the oxygen ppihole at these oxidized sites is ferromagnetically coupled to the adjacent Cu(II)d electrons despite the fact that this is opposed by the direct dd exchange; and (iv) the hopping of these oxygen ppi holes (in CuO sheets or chains) from site to site is responsible for the conductivity in these systems (N-electron band structures are reported for the migration of these localized charges).