Coulomb coupling of like charges due to negative reduced effective mass

Pair Fermi contour and repulsion-induced superconductivity in cuprates

“…A character spatial scale of a pair both in QSS and SC states ͑the coherence length͒ is of the order of a few interatomic distances. 1 Thus we believe that the SC gap and the pseudogap have one and the same origin in the sense that they may be associated with pairs with large total momentum. Nevertheless, one can think that the pseudogap regime has a nonsuperconducting character because it is due to a rise of pairs with different total momenta, whereas the SC regime arises after the condensation of these pairs into a state with a definite total momentum and a rise of the phase coherence.…”

“…Indeed, a rise of K pairs results in their free in-plane motion without a change of charge density, whereas a character of the interaction of paired carriers may be changed essentially: this interaction, being inside antinodal regions, turns out to be more weak as compared to the interaction of unpaired carriers inside nodal regions. 1 The idea we use here is based, in the main, on the fact that the PFC should be ''opened''; that is, due to a rise of stripes and hyperbolic metrics of momentum space, some piece of the FC turns out to be the same as a line of zero kinetic energy of the relative motion of a hole pair with a large momentum. If such a line corresponding to a certain K is close enough to a rather large piece of the real FC ͑such a case may occur, for example, just at Ӷ1 and ͉2k F ϪK͉ Ӷk F ) the pairing mechanism discussed here may also be possible even without any hole redistribution both in momentum space ͑between the domains ⌶ K and ⌶ K Ј ) and in real space ͑that, is without a rise of a stripe structure͒.…”

“…Taking into account the explicit form of the Fourier transform of the screened Cou-lomb potential, one can write the coupling constant in the form u 0 ϭr 0 ⌶ K a 2 /a* where a*ϭប 2 /me 2 is an effective Bohr radius. 1 An increase of the carrier concentration due to doping leads to a decrease of the screening length and, as a result, to a decrease of the coupling constant. In Fig.…”

“…An evaluation of Coulomb repulsion in an electron system with and without QSS's indicates that, provided that the QSS concentration exceeds a certain value, an energy gain is possible, and thus incoherent QSS's with different total momenta may exist not only as excitations but in the ground state, resulting in some suppression of one-particle density of states. 1 Therefore, a rise of such QSS's may be directly related to the pseudogap state. A spatial separation of the domains in which one can observe either a SC gap or a pseudogap 20 can be interpreted as an alternation of the regions with increased and decreased doping, respectively.…”

“…[1][2][3][4] Pairs with large momentum K (K pairs͒ are considered; here KϷ2k F , and k F is the value of the Fermi momentum directed along K. It is well known 5 that the Cooper channel of pairing becomes inefficient when the pair momentum exceeds a value of the order of ⌬/v F ; here ⌬ is the SC gap at Kϭ0, and v F is the Fermi velocity. The same relatively small value of the pair momentum corresponds to the wavelength of a spatially inhomogeneous SC phase arising in a weakly ferromagnetic electron system 6 as well.…”