Symmetry of spin excitation spectra in the tetragonal paramagnetic and superconducting phases of 122-ferropnictides

摘要

We study the symmetry of spin excitation spectra in 122-ferropnictidesuperconductors by comparing the results of first-principles calculations withinelastic neutron scattering (INS) measurements on BaFe1.85Co0.15As2 andBaFe1.91Ni0.09As2 samples that exhibit neither static magnetic phases norstructural phase transitions. In both the normal and superconducting (SC)states, the spectrum lacks the 42/m screw symmetry around the (1/2 1/2 L) axisthat is implied by the I4/mmm space group. This is manifest both in thein-plane anisotropy of the normal- and SC-state spin dynamics and in theout-of-plane dispersion of the spin-resonance mode. We show that this effectoriginates from the higher symmetry of the magnetic Fe sublattice with respectto the crystal itself, hence the INS signal inherits the symmetry of theunfolded Brillouin zone (BZ) of the Fe sublattice. The in-plane anisotropy istemperature-independent and can be qualitatively reproduced in normal-statedensity-functional-theory calculations without invoking a symmetry-broken("nematic") ground state that was previously proposed as an explanation forthis effect. Below the SC transition, the energy of the magnetic resonant modeEr, as well as its intensity and the SC spin gap inherit the normal-stateintensity modulation along the out-of-plane direction L with a period twicelarger than expected from the body-centered-tetragonal BZ symmetry. Theamplitude of this modulation decreases at higher doping, providing an analogyto the splitting between even and odd resonant modes in bilayer cuprates.Combining our and previous data, we show that at odd L a universal linearrelationship Er=4.3*kB*Tc holds for all studied Fe-based superconductors,independent of their carrier type. Its validity down to the lowest dopinglevels is consistent with weaker electron correlations in ferropnictides ascompared to the underdoped cuprates.