Spinon Superconductivity and Superconductivities Mediated by Spin-Waves and Phonons in Cuprates

The disclosure of spinon superconductivity and superconductivity mediated by spin-waves in hole-doped Bi 2 Sr 2 CaCu 2 O 8+x (Bi2212) cuprate raises the question about the origin of the superconductivity in other cuprates and specially in an electron-doped Nd 2-x Ce x CuO 4 (NCCO) cuprate. The superconductivity (SC) in copper-oxide materials or the high-T c superconductivity (HTSC) is an extraordinary phenomenon. The cuprates is the first system with the coexistence of two different types of the SC with the different mechanisms [2]. The most astonishing fact is that the coherent state of the spinon SC in hole-doped cuprates is established via another type of the SC [2] mediated by spin-waves (magnons) [3]. The SC due to pairing of spinons we will call the spinon SC. Spinons are chargeless fermions which occur in one-dimensional physics [4]. The spinon SC occurs on charged stripes into CuO 2 planes [5]. The stripes are separated by antifferomagnetic (AF) isolating domains. The spinons create pairs on stripes because of the existence of a large spin-gap due to AF correlations [6]. The 2∆/k B T c value is a very important characteristic of the SC. It indicates the coupling strength of Copper pairs. In conventional superconductors, this value is about 3.5. In Bi2212, the 2∆/k B T c value for spinon SC can be incredibly high, up to 30 [2]. The coherence length of the spinon pairs can be very short since spinons have no problem with the Coloumb repulsive force. At the same time, it is a huge problem for the spinon SC in order to established the coherent-SC state because spinons are located on stripes which are separated. The distance between charged stripes is too large for the spinon SC in order to establish the coherent state. Nature had to look for another mechanism "to connect" the charged stripes. In hole-doped cuprates, it happened that spin-waves in AF regions [3] are the most suitable mechanism for it. Since electron-magnon interactions are much stronger than electron-phonon interactions in AF compounds [3], the T c can be much higher than in classical superconductors. At T c , the coherent state of the spinon SC into CuO 2 planes occurs due to and via the SC mediated by spin-waves, which can be considered as pairing of magnetic polarons [3]. The magnetic polaron (MP) can be pictured as the electron and a spin polarization around it …