Another simple and universal role in high Tc ?

These authors presented a very simple rule that seems validated by their analysis of experimental data [J. Phys.: Condens. Matter 23 (2011) 295701 (17pp)]. In this rule, the Tc of optimal compounds is essentially set by two length scales and the electron charge, i.e., Tc~e^2/l\times l'. What is striking is that, this rue was argued to cover a wide range of materials, including cuprates, pnictides and ruthenates. They proposed a paring mechanism via Compton scattering: e.g., the holes in the conducting layer is scattered by the electrons in the charge reservoir layer. Instead of forming excitons, superfluid forms. The following is a brief sojourn over this work [http://iopscience.iop.org/0953-8984/labtalk-article/46706]:

High-T_C superconductors have layered crystal structures, where T_C depends on bond lengths, ionic valences, and Coulomb coupling between electronic bands in adjacent, spatially separated layers. Analysis of 31 high-T_C materials—cuprates, ruthenates, rutheno-cuprates, iron pnictides and organics—has revealed that the optimal transition temperature T_CO is given by the universal expression k_B^-1e²Λ / ℓζ. Here, ℓ is the spacing between interacting charges within the layers, ζ is the distance between interacting layers, Λ is a universal constant, equal to about twice the reduced electron Compton wavelength, k_B is Boltzmann's constant and e is the elementary charge. Non-optimum compounds in which sample degradation is evident typically exhibit T_C below T_CO. Figure 1 shows T_CO versus (ση/A)^1/2/ζ—a theoretical expression determining 1 / ℓζ, where σ is the charge fraction, η is the layer number count and A is the formulaic area. The diagonal black line represents the theoretical T_CO. Coloured data points falling within ± 1.4 K of the line constitute validation of the theory.

Figure 1. Experimental test of theory.

Figure 2. Proposed pairing interaction.

The elemental building block of high-T_C superconductors comprises two adjacent and spatially separated charge layers. The factor e² / ℓζ, determining T_CO arises from Coulomb forces between them. Remarkably an explicit dependence on phonons, plasmons, magnetism, spins, band structure, effective masses, Fermi-surface topologies and pairing-state symmetries in high-T_C materials is absent. The magnitude of Λ suggests a universal role of Compton scattering in high-T_C superconductivity, as illustrated in figure 2 that considers pairing of carriers (h) mediated by electronic excitation (e) via virtual photons (ν). Several other important predictions are given. A conducting charge sheet is non-superconducting without a second mediating charge layer next to it, and a charge structure representing a room-temperature superconductor yet to be discovered is presented.

Manifesto for higher Tc

A perspective by A.V.Chubukov, who is an enthusiast of spin-fluctuation type gluon. He compared the iron-based and the copper-based superconductors and surmise a higher Tc, observing
(1)Kexp/KLDA for existing high Tc SCs is around 0.5;
(2)superfluid density ~ dc conductivity x Tc.
Point (1) indicates that, high Tc is likely with a mixture of mobility and localization [Nature Physics, 7:272(2011)].

double exchange relevant for High Tc superconductivity

Last year there published a paper [Hai-Yao Deng 2009 J. Phys.: Condens. Matter 21 075702] in which a model was proposed to address the issues regarding the coexistence of Zhang-Rice singlet formation and the spin glass at very low doping in cuprates. This model contains not only itinerant charges but also localized spins. There is strong on-site anti-ferromagnetic coupling between them. This coupling is held responsible for the formation of ZRS and the formation of spin glass, as a result of double exchange. Now there came very latest studies [Phys. Rev. B 82, 045125 (2010);Phys. Rev. Lett. 105, 107004 (2010)] that held similar ideas in solving puzzles residing in iron-based pnictides. This should not be regarded as a simple coincidence. It may strengthen the idea that, high Tc SC is intimately connected with magnetic fluctuations. More likely, this model may serve as a unification for High Tc superconductors of both iron-based and copper-based.