A dopon theory of high Tc superconductor

October 28, 2020

A dopon theory is an all-fermion slave-particle mean field theory for Cu high Tc superconductors, which was developed by Ribeiro and Wen in 2004 .

In such a theory, the spins form a spin liquid, which are described by the spinons (i.e. spin-1/2 charge-0 emergent fermions). At low doping, the spin liquid is described by a d-wave-paired (a staggered-flux) phase of spinons.  At high doping, the spin liquid is described by a Fermi liquid of the spinons with a large Fermi surface. The charges (i.e. the doping) are described by spin-1/2 charge-1 electrons (or holes), which are called dopons. The dopons and low energy spinons have a strong mixing.

The dopon theory is different from the more popular slave-boson mean field theory, where the charges are described by spin-0 charge-1 bosonic holons, and the spins are described by spin-1/2 charge-0 fermionic spinons.

The dopon theory produces a very reasonable phase diagram for Cu high Tc superconductors, for both hole doped (HD) and electron doped (ED) cases:

The following is the caluclated electron spectral function, from which one can see the evolution of Fermi surface from low doping to high doping. The white dashed line in (a,b,c) is roughly the spinon band. The dopon-spinon mixing gives some electron spectral weight to the spinon band. Also the electron spectral function from dopon theory is very different from that of slave-boson theory and BCS thoery,  which is a clear smoking-gun signature of the dopon theory.

In another paper, it was found that the quasiparticle currents from dopon theory (DC),  slave-boson (SB) theory, and BCS theory are also quite different. Here quasiparticle current describes how quasiparticle dispertion depends on the electromagnetic vector potential (which is proportional to supercurrent in superconductor): Ek(A) = Ek(0) - jk . A. In BCS theory jk = e vk, where vk is the quasiparticle velocity in normal state. This is quite amazing, since the quasiparticle current in BCS superconductor does not depend on the superconducting gap. The quasiparticle current in SB theory is similar to that in BCS theory, differing only by a constant factor. However, quasiparticle current from dopon theory has a very different k dependence, which is also a clear smoking-gun signature of the dopon theory (DC)