A two-phase charge-density real-space-pairing model of high-T_c superconductivity

Abstract

It is usually assumed that high-T c superconductors have a periodic band structure and a periodic charge density, although amorphous low-T c superconductors are known. In this paper, it is suggested that the CuO 2 conduction planes of cuprate superconductors consist of regions of two different charge densities which do not normally repeat periodically. It is suggested that the pairing of holes occurs in real space in cuprate superconductors. It is proposed that the hole-pairing mechanism is magnetic exchange coupling and the pairing force is strong, the pairing energy being greater than kT at room temperature. The bound hole pair is essentially a bipolaron. A real-space model is very tentatively suggested in which the CuO 2 planes of YBa 2 Cu 3 O 7 contain nanodomains of a 3 Â 3 hole lattice surrounded by interfaces one unit cell wide in which the holes are paired. In the superconducting state in this model, the existing hole pairs condense and move coherently and collectively around the insulating nanodomains, like trams running around blocks of houses, with one hole on each tramline. The hole pairs move in an elegant manner with hole pairs hopping from oxygen to oxygen via adjacent copper sites. The model explains the superconducting current being in the ab plane and it also explains the very short coherence lengths. Because the pairing force is strong, the model suggests that room-temperature superconductivity might be possible in carefully designed new oxide materials.

A nanometre‐scale non‐periodic structural variation in high temperature superconducting ceramics and the implications for properties

“…Recent work suggests that the electronic structure of HTS cuprates has ‘two components’ spatially separated as stripes (Tranquada et al ., 1995; Bianconi et al ., 1996; Salkola et al ., 1996; Ostenson et al ., 1997; Mook & Dogan, 1999). In addition, there have been several theoretical studies showing how charge might phase separate in these structures (Carter et al ., 1996; Bianconi et al ., 1996; Emery et al ., 1997; Goss Levi, 1998; Humphreys, 1999). Müller et al .…”

Electrons seen in orbit

“…Although plant defences may be induced simply because herbivores are chewing the foliage, many organisms have sophisticated early-warning systems. Some plants, for example, switch on their defence mechanisms as soon as they detect volatile chemicals released by damaged plants close by 5 . Some of these volatiles also directly lure predators to attack planteating insects or mites 6 .…”