I Why does doping make resonating valence bonds (singlet pairs) mobile?

[Faizan Samad]

TL;DR Summary

I'm a first year masters student so I'm looking for a qualitative, possibly intuitive explanation, rather than a math heavy quantitative one. This is for a presentation for my class.

I know in RVB theory that neighbouring Copper atoms form singlet pairs via the superexchange "force". Upon doping with holes, these neutral singlet RVB pairs become mobile and charged and are able to superconduct. I know that the resonating valence bonds are in the copper 3d(x^2-y^2) orbital and when hole doped, these are the electrons that are removed. My questions are, why are they called neutral singlets when they are undoped? And why does doping allow them to become mobile? I read that they separate into fractional quasiparticles as holon-antiholon pairs when they are hole doped. Are these mobile RVBs "traveling" through holes or am I completely missing the picture? Also I have read a lot of papers from PW Anderson and G Baskaran and I am still a little confused as to what makes RVBs mobile superconductors upon doping. Thank you. Any help is appreciated

 

[f95toli]

I am afraid that I can't help answer the specific questions; but it is perhaps worth pointing out that RVB is -most likely- not an actual explanation for HTS. which could be one reason for your confusion It was a very early theory, and does not -as far as I am aware- agree with experiments (at least in its "plain" form)
Hopefully, you are already aware of this and is studying this for "historical" reasons.

 

[trurle]

My questions are, why are they called neutral singlets when they are undoped?

The charge of electron pair (of Mott insulator in this case) is canceled by polarization of surrounding material when the pair is immobile. When pair is mobile, the lagging polarization can no longer fully cancel its charge.

I must also agree with @f95toli - the experimental proof for RVB (resonant valence bond) theory is substandard at the moment. The theory may be simply untrue.