From Cooper-pair glass to unconventional superconductivity: a unified approach to cuprates and pnictides

Abstract

-We report a microscopic model wherein the unconventional superconductivity emerges from an incoherent 'Cooper-pair glass' state. Driven by the pair-pair interaction, a new type of quasi-Bose phase transition is at work. The interaction leads to the unconventional coupling of the quasiparticles to excited pair states, or 'super-quasiparticles', with a non-retarded energydependent gap. The model describes quantitatively the quasiparticle excitation spectra of both cuprates and pnictides, including the universal 'peak-dip-hump' signatures, and for the pseudogap phase above Tc. The results show that instantaneous pair-pair interactions account for the SC condensation without a collective mode.Despite its wide applications, the BCS theory [1] fails to account for the physical properties of a large variety of high-T c superconductors (SC), the cuprate family, but also the more recent iron-based superconductors. A striking feature of these materials is the proximity to an insulating phase, whether anti-ferromagnetic (cuprates), spin density wave (iron based SC, Bechgard salts) or localization (ultra-thin films). Just beyond the insulating phase, the SC dome appears in the phase diagram as a function of carrier concentration between two critical points. Understanding the transition from such an insulating to SC state is still a major challenge.Microscopic measurements reveal an unconventional quasiparticle (QP) dispersion, the 'peak-dip-hump' structure [2], often attributed to the coupling to a collective mode [3][4][5][6][7][8]. Although the peak to dip energy follows both the neutron resonance and T c as a function of doping [6,9,10], the finer shape of the QP spectra and their temperature dependence remain a challenge. Moreover, in the temperature range [T c , T * ] a pseudogap (PG) state persists, having a Fermi-level gap ∆ p much larger than the critical energy scale k B T c in cuprates (see [11] and ref. therein) and also in iron-based SC [12][13][14].In this letter, these questions are addressed within the pair-pair interaction model (PPI). We show that the main unconventional features of high-T c SC can be understood p-1 arXiv:1612.02268v2 [cond-mat.supr-con]

Magnon-mediated attraction between two holes doped in a CuO2 layer

“…[29][30][31][32][33][34] More recently, several groups have suggested that various unusual properties on the underdoped side can be explained as being due to the scattering of fermionic carriers on a bosonic liquid of preformed pairs. [35][36][37] Our work seems to be consistent with these scenarios.…”

Cooper pairs without “glue” in high-T c superconductors: A universal phase diagram

“…Using the parameters of table 1, the theoretical curves fit accurately the experimental spectra for both classes of materials [32]. The peak-dip structure present at low temperature is thus a clear signature of long-range SC coherence, while the mere presence of a spectral gap is not.…”

“…The resolution of the microscopic Hamiltonian [16,32] leads to the alternative conclusion that, due to the PPI, the quasiparticles are now coupled to excited pairon states (see fig. 4, right panel).…”

Origin of the Fermi arcs in cuprates: a dual role of quasiparticle and pair excitations

“…However in our case, the i-index represents pairons of differing binding energies and the Hamiltonian (4) represents a non-superconducting state of independent Cooper pairs. In order to establish a macroscopic coherent state, a coupling between pairs of different energies is necessary [28][29][30]. Without the interaction term between pairons, no long range order is possible.…”

“…In the absence of pairon-pairon interactions, the system is in an incoherent state, the 'Cooper glass' [28][29][30]. The pairon energies are distributed with a pair density of states P 0 (∆ i ), characterized by the mean value ∆ 0 and dispersion σ 0 .…”