Consider first basic facts. The surprising discovery was that graphene becomes unconventional superconductor at temperature 1.7 K. It was already earlier discovered that the coupling of graphene to a superconductor can make also graphene superconducting.
- The system studied consists of two graphene (see this) layers twisted by angle θ with respect to each other (rotation of the second sheet by angle θ around the axis normal to sheets). For a generic value of θ the graphene layers behave as separate conductors. For certain critical twist angles below 1.1 degrees the two-layered system however behaves like single unit and Mott insulator (see this): this is due to the increase of the conduction band gap. In an applied electric field the system becomes a super conductor. The electric field provides the energy needed to kick the current carries to the conduction band, which for Mott insulators has higher energy than for the corresponding conductor: at the top of the band Cooper pairs are formed as in the case of ordinary superconductors.
- A kind of Moire effect (see this) is involved. The twist creates a superlattice with larger unit cell and the electrons associated with periodically occurring C-atom pairs above each other give rise to a narrow band where the superconducting electrons are. Electric field would kick the electrons to this band.
- There are intriguing analogies with high Tc superconductivity. Electron density as function of temperature has a pattern similar to that for cuprates. Superconductivity occurs at electron density, which is 10-4 times that for conventional superconductors at the same temperature. The pairing of electrons cannot be due to phonon exchange since the density is so low. Unidentified strong interaction between electrons is believed to be the reason.
The finding of Cao et al is believed to be highly significant concerning the understanding of high Tc super-conductivity and motivates the development of a model of Mott insulators based on TGD based views about valence bond inspired by the identification of dark matter as heff/h=n phases of ordinary matter emerging naturally in adelic physics (see this). Also a more detailed version about the model of high Tc superconductivity in TGD Universe developed earlier emerges.
The model starts from a model of elementary particles applied to electron.
- At space-time level elementary particles are identified as two-sheeted structures involving a pair of wormhole contacts connecting the space-time sheets and magnetic flux tubes connecting the wormhole throats at the two sheets. For the second sheet flux tubes are loop-like and define the magnetic body of the particle. These flux loops are associated with valence bonds and the value of heff/h=n can be large for them implying that the loops become long. In ohmic conductivity the reconnection of the valence loops would be the fundamental mechanism allowing to transfer conduction electrons between neighboring lattice sites.
- An essential role is played by TGD based model for valence bond predicting that the value of n increases along the row of the Periodic Table. For group 4 transition metals n for valence bond with O is largest for Ni (NiO is Mott insulator) and for Cu (copper oxides are high Tc superconductors) so that they are predicted to be excellent candidates for Mott insulators and even unconventional superconductors.
- TGD space-time time is many-sheeted and flux tubes form a hierarchy. In high Tc superconductivity also anti-ferromagnetic (AFM) flux loops with a shape of flattened and elongated rectangle would be present. The reconnection of valence flux loops with AFM flux loops would allow the transfer of the precursors of Cooper pairs - appearing already for Mott insulators but not yet giving rise to super-conductivity - to the AFM flux loops. In the phase transition leading to high Tc superconductivity in macroscopic scales the AFM flux loops would reconnect to longer flux loops making possible macroscopic supra currents.
For a summary of earlier postings see Latest progress in TGD.