The effect of collective spin-1 excitations on electronic spectra in high-T_csuperconductors

Abstract

We review recent experimental and theoretical results on the interaction between single-particle excitations and collective spin excitations in the superconducting state of high-Tc cuprates. We concentrate on the traces, that sharp features in the magnetic-excitation spectrum (measured by inelastic neutron scattering) imprint in the spectra of single-particle excitations (measured e.g. by angle-resolved photoemission spectroscopy, tunneling spectroscopy, and indirectly also by optical spectroscopy). The ideal object to obtain a quantitative picture for these interaction effects is a spin-1 excitation around 40 meV, termed 'resonance mode'. Although the total weight of this spin-1 excitation is small, the confinement of its weight to a rather narrow momentum region around the antiferromagnetic wavevector makes it possible to observe strong self-energy effects in parts of the electronic Brillouin zone. Notably the sharpness of the magnetic excitation in energy has allowed to trace these self-energy effects in the single-particle spectrum rather precisely. Namely, the doping-and temperature dependence together with the characteristic energy-and momentum behavior of the resonance mode has been used as a tool to examine the corresponding self-energy effects in the dispersion and in the spectral lineshape of the single-particle spectra, and to separate them from similar effects due to electron-phonon interaction. This leads to the unique possibility to single out the self-energy effects due to the spin-fermion interaction and to directly determine the strength of this interaction in high-Tc cuprate superconductors. The knowledge of this interaction is important for the interpretation of other experimental results as well as for the quest for the still unknown pairing mechanism in these interesting superconducting materials.

Quantum spin excitations through the metal-to-insulator crossover inYBa2Cu3O6+y

“…Alternatively, the resonance and incommensurate spin excitations around it can be explained by quasiparticle scattering across a nested Fermi surface as in conventional metals and superconductors 43,44 . Based on this approach, a rapid reduction in the magnitude of the spin gap and the near-disappearance of the resonance in the YBa 2 Cu 3 O 6.45 suggests a sudden change in the Fermi surface topology across the MIC in YBCO 45 .…”

Anisotropic neutron spin resonance in superconductingBaFe1.9Ni0.1As2

“…5 Although the resonance appears to be a ubiquitous property of unconventional superconductors, [1][2][3][4][5][6][7][8][9][10][11][12] its microscopic origin and relationship with superconductivity are still debated. 14 In all these materials, the resonance occurs at the antiferromagnetic ͑AF͒ wave vector Q of the parent compound. It is thought to be a triplet excitation of the singlet Cooper pairs, 14,15 implying a superconducting order parameter that satisfies ⌬ k+Q =−⌬ k .…”

“…14 In all these materials, the resonance occurs at the antiferromagnetic ͑AF͒ wave vector Q of the parent compound. It is thought to be a triplet excitation of the singlet Cooper pairs, 14,15 implying a superconducting order parameter that satisfies ⌬ k+Q =−⌬ k . In the iron arsenide superconductors, this condition is satisfied by an order parameter whose sign reverses between the electron and hole pockets.…”

Polarized neutron scattering studies of magnetic excitations in electron-overdoped superconducting BaFe1.85Ni0.15As

“…Assuming that the resonance is a spin-1 singlet-to-triplet excitation of the Cooper pairs 29 , it should be possible to split it into three peaks under the influence of a magnetic field via the Zeeman effect by an amount ∆E = ±gµ B B, where g = 2 is the Lande factor and B is the magnitude of the field [30][31][32][33] . Although there have been attempts to split the resonance for copper oxide 30 and iron-based superconductors 31,32 in this way, the results are inconclusive and it has not been possible determine if the mode is indeed a singlet-to-triplet excitation.…”