In high-TC cuprates, superposition to a localized low-energy antiferromagnetic background of a delocalized high-energy assembly of exciton-solvated doping holes (ESH) accounts for singlet state hole pairing in real space, above TC. Below TC, pairs can glide in coherent motion along one-dimensional spines, formed by aligned charge transfer excitons, and serving as rails. Most of old and new experimental observations agree with this model.
In high-TC cuprates, Coulomb interactions between exciton-solvated doping holes (ESH) can result in real-space pairing of holes.
Keywords: Superconductivity; Cuprates; Pairing mechanism; Charge-transfer excitons
- 1. Introduction
- 2. A short reminder
- 3. An excitonic glue for real space pairing
- 4. From dimers to infinite chains?
- 5. Superconductivity and phase coherence
- 6. Superposition of antiferromagnetic and excitonic state
- 7. An excitonic model
- 8. Isotope effects
- 9. Conclusions
- Supplementary material
- 2. A short reminder
Fig. 1. Charge transfer state (CTS) resulting from the addition of an exciton (EXC) pseudoparticle (charge Q = 0, spin S = −1) to the normal state (NS). μE is the effective electrical dipole.
Fig. 2. Smallest I1+–ESH associating one doping hole (hD+) to an exciton as in Fig. 1, and its schematic representation with Q = 1 (in |e| unit) and spin S = −1/2, antiparallel to Cu2+ spin, leading locally to Stot = 0.
Fig. 3. Approach and linking, with a strong negative stabilizing energy ++, of two L12+ and L13+ ESHs, forming thus a stable dimer.
Fig. 4. Coulomb interaction between two L12+ and L13+ monomers, as a function of their separation N (in units of the a cell parameter) along . An activation energy (+310 meV), larger than the pair binding energy, is required to break the dimer.
Fig. 5. Influence of the Cu2+ antiferromagnetic background (α) on the spin direction of the doping holes hD+. Note the 1D antiferromagnetic character of O− (S = 1/2) spins in the spine (shaded), replacing Cu2+ spins.
Fig. 6. (a) L11+ and (b) L12+ sequences where odd (Δn = 5) and even (Δn = 6) numbers of unit cells separate α from β in the former and the latter, respectively. The respective spin states are S = 0 and S = ±1.
Fig. 7. Two types of  spine succession along  for a 4 × a superlattice with (a) one and (b) two spine(s) per stripe, prefiguring the SP1 and SP2 ladders (see Fig. 8).
Fig. 8. Two types of ladder: (a) single-leg ladder with rungs of I1+ type on both sides, and (b) double-leg ladder with rungs of I2+ type.
Fig. 9. Sum of attractive (spine–rung) and repulsive (rung–rung) interaction energies, as a function of the total number N of rungs introduced in both SP1 (triangles) and SP2 (squares) ladders, presenting a sharp minimum for N 9 (SP1) and N 13 (SP2).
Fig. 10. (a) L13+–L12+ sequence corresponding to the maximum stability for SP1, with a composition x = 0.1, for an infinite ladder. (b) U(12)2+–U(11)2+ sequence corresponding to the maximum stability for SP2, with a composition x = 0.167, for an infinite ladder. Composition x = 0.222 corresponds to a 3 × a stripe with 2D spine–rung interactions.
Fig. 11. TC variation with composition x, for the La2−xBaxCuO4 system (adapted from ), showing the correspondence between the two TC maxima and x values of 0.1 and 0.167.
Fig. 12. Antiferromagnetic coupling between the two doping holes located on the A and B oxygen atoms of a U(12)2+–U(11)2+ ladder, through the oxygen spins of the O10 cycle (Δn = 5). Note that two Cu2+ only are present in the 4 × 3 SP2 supercell (see also Fig. 10b).
Fig. 13. Three possible oxygen displacement patterns (arrows) in the CuO2 layers, associated to various phonon modes: (a) ferrodistorsive 1D mode, (b) breathing mode with two inequivalent copper atoms, (c) antiferrodistorsive mode retained in  and .
Fig. 14. Potential as a function of the distortion parameter x for the Cu–O–Cu bonds, showing the most stabilized (x = xmax) and destabilized (x = 0) situations for the antibonding b1g(O–Cu)* electron (see ), and the charge transfer hindrance by phonons.
Table S1. Interaction energies (in meV) vs. separation length N (in units of cell parameter a).
Solid State Sciences
Volume 12, Issue 5, May 2010, Pages 691-698
International Symposium on Structure-Property Relationships in Solid-State Materials