No abstract

We report a comprehensive polarized and unpolarized neutron scattering study of the evolution of the dynamical spin susceptibility with temperature and doping in three underdoped single crystals of the YBa2Cu3O6+x high temperature superconductor: YBa2Cu3O6.5 (Tc = 52 K), YBa2Cu3O6.7 (Tc = 67 K), and YBa2Cu3O6.85 (Tc = 87 K). The spin susceptibility is determined in absolute units at excitation energies between 1 and 140 meV and temperatures between 1.5 and 300 K. Polarization analysis is used extensively at low energies. Transitional matrix elements, including those between spin states, in a bilayer system such as YBa2Cu3O6+x can be generally classified into even and odd, according to the sign change under a symmetry operation that exchanges the layers, and both even and odd excitations are detected in YBa2Cu3O6.5 and YBa2Cu3O6.7. While the even spin excitations show a true gap which depends on doping, the odd spectrum is characterized by a weakly dopingdependent pseudogap. Both even and odd components are substantially enhanced upon lowering the temperature from 300 K. The even excitations evolve smoothly through the superconducting transition temperature Tc, but the odd excitations develop a true gap below Tc. At the same time, the odd spin susceptibility is sharply enhanced below Tc around an energy that increases with doping. This anomaly in the magnetic spectrum is closely related to the magnetic resonance peak that appears at 40 meV in the superconducting state of the optimally doped compound (Tc = 93 K). From these data we extract the energy and the energy-integrated spectral weight of the resonance peak in absolute units as a function of doping level. Theoretical implications of these measurements are discussed, and a critique of recent attempts to relate the spin excitations to the thermodynamics of high temperature superconductors is given.

The pseudogap is one of the most pervasive phenomena of high temperature superconductors [1]. It is attributed either to incoherent Cooper pairing setting in above the superconducting transition temperature Tc, or to a hidden order parameter competing with superconductivity. Here we use inelastic neutron scattering from underdoped YBa2Cu3O6.6 to show that the dispersion relations of spin excitations in the superconducting and pseudogap states are qualitatively different. Specifically, the extensively studied "hour glass" shape of the magnetic dispersions in the superconducting state [2,3,4] is no longer discernible in the pseudogap state and we observe an unusual "vertical" dispersion with pronounced in-plane anisotropy. The differences between superconducting and pseudogap states are thus more profound than generally believed, suggesting a competition between these two states. Whereas the high-energy excitations are common to both states and obey the symmetry of the copper oxide square lattice, the low-energy excitations in the pseudogap state may be indicative of collective fluctuations towards a state with broken orientational symmetry predicted in theoretical work [5,6,7,8].

A detailed inelastic neutron scattering study of the high temperature superconductor YBa2Cu3O6.85 provides evidence of new resonant magnetic features, in addition to the well-known resonant mode at 41 meV: (i) a commensurate magnetic resonance peak at 53 meV with an even symmetry under exchange of two adjacent CuO2 layers, and (ii) high-energy incommensurate resonant spin excitations whose spectral weight is around 54 meV. The locus and the spectral weight of these modes provides unrevealed insight about the momentum shape of the electron-hole spin-flip continuum of d-wave superconductors.

We study the symmetry of spin excitation spectra in 122-ferropnictide superconductors by comparing the results of first-principles calculations with inelastic neutron scattering (INS) measurements on BaFe 1.85 Co 0.15 As 2 and BaFe 1.91 Ni 0.09 As 2 samples that exhibit neither static magnetic phases nor structural phase transitions. In both the normal and superconducting (SC) states, the spectrum lacks the threedimensional (3D) 4 2 /m screw symmetry around the ( 1 2 1 2 L) axis that is implied by the I4/mmm space group. This is manifest both in the in-plane anisotropy of the normal-and SC-state spin dynamics and in the out-ofplane dispersion of the spin-resonance mode. We show that this effect originates from the higher symmetry of the magnetic Fe-sublattice with respect to the crystal itself, hence the INS signal inherits the symmetry of the unfolded Brillouin zone (BZ) of the Fe-sublattice. The in-plane anisotropy is temperature-independent and can be qualitatively reproduced in normal-state density-functional-theory calculations without invoking a symmetry-broken ("nematic") ground state that was previously proposed as an explanation for this effect. Below the SC transition, the energy of the magnetic resonant mode ω res , as well as its intensity and the SC spin gap inherit the normal-state intensity modulation along the out-of-plane direction L with a period twice larger than expected from the body-centered-tetragonal BZ symmetry. The amplitude of this modulation decreases at higher doping, providing an analogy to the splitting between even and odd resonant modes in bilayer cuprates. Combining our and previous data, we show that at odd L a universal linear relationship ħ hω res ≈ 4.3 k B T c holds for all the studied Fe-based superconductors, independent of their carrier type. Its validity down to the lowest doping levels is consistent with weaker electron correlations in ferropnictides as compared to the underdoped cuprates.