Novel neutron resonance mode indx2−y2-wave superconductors

Abstract

We show that a new resonant magnetic excitation at incommensurate momenta, observed recently by inelastic neutron scattering experiments on YBa2Cu3O6.85 and YBa2Cu3O6.6, is a spin exciton. Its location in the Brillouin zone and its frequency are determined by the momentum dependence of the particle-hole continuum. We identify several features that distinguish this novel mode from the previous resonance mode observed near Q=(pi,pi).

Dynamical spin susceptibility and the resonance peak in the pseudogap region of the underdoped cuprate superconductors

“…In contrast, in the superconducting state the susceptibility is gapped up to an energy Ω DSC cr = min k (|∆ k | + |∆ k+Q |) where ∆ k is the superconducting gap and both k and k + Q lie on the Fermi surface. Due to the symmetry of the superconducting gap, one finds ∆ k = −∆ k+Q , resulting in a discontinuous jump of Imχ 0 at Ω DSC cr 16,17 . In order to discuss the behavior of Imχ 0 in the DDWstate, we first note that the expression for χ 0 in Eq.…”

“…16,17,18 ). In the normal state Imχ 0 increases linearly at low frequencies with a slope determined by the Landau damping rate, while, at higher energies its behavior is determined by the presence of the van Hove singularity.…”

“…(2) 19 . In order to directly compare the dynamical spin susceptibility in the DDW state with that in the DSC state, we take the DDW order parameter, W 0 = 42meV, to be equal to that in the DSC state 17 . We note here, that the above Hamiltonian can be obtained from a microscopic Hamiltonian with short-range repulsion or superexchange interactions 20,21 .…”

“…In the optimally doped cuprates, the resonance's frequency is ω res ≈ 41 meV 7,11 , a frequency which decreases with increasing underdoping 12,13 . A number of theoretical scenarios have been suggested for the appearance of the resonance peak in a superconducting state with d x 2 −y 2 -wave symmetry 16,17,18 . In one of them, the so-called 'spin exciton' scenario 16,17 , the resonance peak is attributed to the formation of a particlehole bound state below the spin gap (a spin exciton), which is made possible by the specific momentum dependence of the d x 2 −y 2 -wave gap.…”

“…A number of theoretical scenarios have been suggested for the appearance of the resonance peak in a superconducting state with d x 2 −y 2 -wave symmetry 16,17,18 . In one of them, the so-called 'spin exciton' scenario 16,17 , the resonance peak is attributed to the formation of a particlehole bound state below the spin gap (a spin exciton), which is made possible by the specific momentum dependence of the d x 2 −y 2 -wave gap. Within this scenario, the structure of spin excitations in the superconducting state as a function of momentum and frequency can be directly related to the topology of the Fermi surface and the phase of the superconducting order parameter.…”