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Register a new userPairing Glue Activation in Cuprates within the Quantum Critical Regime
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A grand challenge in many-body quantum physics is to explain the apparent connection between quantum criticality and high-temperature superconductivity in the cuprates and similar systems, such as the iron pnictides and chalcogenides. Here we argue that the quantum-critical regime plays an essential role in activating a strong-pairing mechanism: although pairing bosons create a symmetry-breaking instability which suppresses pairing, the combination of these broken-symmetry states within the critical regime can restore this symmetry for the paired quasiparticles. This condition is shown to be met within a large-U ansatz. A hidden quantum phase transition then arises between a Fermi-liquid and a non-Fermi-liquid broken-symmetry striped state, and a critical regime in which the broken-symmetry states are combined.
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The question of whether one should speak of a pairing glue in the Hubbard and t-J models is basically a question about the dynamics of the pairing interaction. If the dynamics of the pairing interaction arises from virtual states, whose energies correspond to the Mott gap, and give rise to the exchange coupling J, the interaction is instantaneous on the relative time scales of interest. In this case, while one might speak of an instantaneous glue, this interaction differs from the traditional picture of a retarded pairing interaction. However, if the energies correspond to the spectrum seen in the dynamic spin susceptibility, then the interaction is retarded and one speaks of a spin-fluctuation glue which mediates the d-wave pairing. Here we present results from numerical studies which provide insight into this question.
In strongly correlated materials the electronic and optical properties are significantly affected by the coupling of fermionic quasiparticles to different degrees of freedom, such as lattice vibrations and bosonic excitations of electronic origin. Broadband ultrafast spectroscopy is emerging as the premier technique to unravel the subtle interplay between quasiparticles and electronic or phononic collective excitations, by their different characteristic timescales and spectral responses. By investigating the femtosecond dynamics of the optical properties of Y-Bi2212 crystals over the 0.5-2 eV energy range, we disentangle the electronic and phononic contributions to the generalized electron-boson Eliashberg function, showing that the spectral distribution of the electronic excitations, such as spin fluctuations and current loops, and the strength of their interaction with quasiparticles can account for the high critical temperature of the superconducting phase transition. Finally, we discuss how the use of this technique can be extended to the underdoped region of the phase diagram of cuprates, in which a pseudogap in the quasiparticle density of states opens. The microscopic modeling of the interaction of ultrashort light pulses with unconventional superconductors will be one of the key challenges of the next-years materials science, eventually leading to the full understanding of the role of the electronic correlations in controlling the dynamics on the femtosecond timescale.
Along with some other researches we have realised that the true origin of high-temperature superconductivity should be found in the strong Coulomb repulsion combined with a significant electronphonon interaction. Both interactions are strong (on the order of 1 eV) compared with the low Fermi energy of doped carries which makes the conventional BCS-Eliashberg theory inapplicable in cuprates and related doped insulators. Based on our recent analytical and numerical results I argue that high-temperature superconductivity from repulsion is impossible for any strength of the Coulomb interaction. Major steps of our alternative polaronic theory are outlined starting from the generic Hamiltonian with the unscreened (bare) Coulomb and electron-phonon interactions accounting for critical temperatures of high-temperature superconductors without any adjustable parameters.
We propose a microscopical theory of superconductivity in CuO$_2$ layer within the effective two-band Hubbard model in the strong correlation limit. By applying a projection technique for the matrix Green function in terms of the Hubbard operators, the Dyson equation is derived. It is proved that in the mean-field approximation d-wave superconducting pairing mediated by the conventional exchange interaction occurs. Allowing for the self-energy corrections due to kinematic interaction, a spin-fluctuation d-wave pairing is also obtained. $Tsb{c}$ dependence on the hole concentration and $bf k$-dependence of the gap function are derived. The results show that the exchange interaction (which stems from the interband hopping) prevails over the kinematic interaction (which stems from the intraband hopping).
We study how superconducting Tc is affected as an electronic system in a tetragonal environment is tuned to a nematic quantum critical point (QCP). Including coupling of the electronic nematic variable to the relevant lattice strain restricts criticality only to certain high symmetry directions. This allows a weak-coupling treatment, even at the QCP. We develop a criterion distinguishing weak and strong Tc enhancements upon approaching the QCP. We show that negligible Tc enhancement occurs only if pairing is dominated by a non-nematic interaction away from the QCP, and simultaneously if the electron-strain coupling is sufficiently strong. We argue this is the case of the iron superconductors.
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