Positron annihilation superconductivity

Figure 8.1 Angular correlation of positrons annihilating in normal and superconducting Pb. The arrow at approximately 6 mrad indicates the position of the Fermi cutoff for a free electron Fermi sphere, and no detectable smearing is seen in the superconducting state. The triangle indicates the angular resolution. From Briscoe et al. [17],...

Studies on other high-temperature superconductors Positron annihilation measurements across Tc, coupled with the calculations of PDD have been carried out in a variety of hole-doped superconductors that include YBa2Cu40g [48], Bi-Sr-Ca-Cu-0 [49], and Tl-Ba-Ca-Cu-0 [50, 51] systems. We will not labor with the details here, except to state that a variety of temperature dependencies are seen and these can be rationalized when the results are analysed in terms of positron density distribution and the electron-positron overlap function [39]. These calculations show that the positron s sensitivity to the superconducting transition arises primarily from the ability to probe the Cu-O network in the Cu-0 layer. The different temperature dependencies of lifetime, i.e., both the increase and decrease, can be understood in terms of a model of local electron transfer from the planar oxygen atom to the apical oxygen atom, after taking into account the correct positron density distribution within the unit cell of the cuprate superconductor. 

I peak-effects, physical and magnetic phase diagrams) -Raman, IR spectra -neutron diffraction -specific heat -thermal conductivity -positron annihilation (fundamental superconducting parameters, BCS theory)... 

Positron annihilation is widely applied to study high-rc superconductors. After presenting the basic concepts (sect. 2) and discussing some experimental aspects (sect. 3), there is an overview of the results obtained so far in YBa2Cu307 (sect. 4), in YBa2Cu30y 5 with substitutions (sect. 5), and in other high-temperature superconducting oxides (sect. 6). 

After investigating the positron behavior, the next step is to solve the band structure and to calculate the momentum density p (p). It is rather controversial to describe the electronic structure of high-Tc superconductors by band structure. But it is almost the only way to obtain a calculated momentum density in order to interpret the positron measurements. We refer to the chapter by Pickett and Mazin in this volume (ch. 193) for a general discussion of band-structure calculations in superconducting cuprates. Studies in relation with positron annihilation have been made using different methods FLAPW (Massidda 1990, Massidda et al. 1991), LMTO (Bansil et al. 1988, Barbiellini et al. 1992,... 

TTie superconducting state has not been yet observed directly by positron annihilation, and more theoretical investigations are required to state whether this will be possible or not. However, indirect effects have been observed, but the spread in the results prohibits a firm conclusion at the present time. These results are nevertheless stimulating for future research. 

Berko, S., 1989, Is there a Fermi surface in high T, superconductors A brief review of positron annihilation results, in Proc. High-T, Superconductivity Magnetic Interactions, eds L.H. Bermett, Y. Flow and G.C. Vezzoli (World Scientific, Singapore) pp. 196-205. 

While the above-mentioned physical picture may appear too simplistic, at least at the present stage of development in the theory of superconductivity in HTSC [4], we must mention that it provides a broad qualitative framework to account for the observed temperature dependence of annihilation parameters, even in other high-temperature superconductors [39], provided the correct positron distribution within the unit cell is taken... 

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