Investigation of the effect of Zn impurities and magnetic field on the spin dynamics in underdoped YBa2(Cu1-yZny)3Ox

This work employs neutron scattering to examine the changes in the spin excitation spectrum of the underdoped high-temperature superconductor YBa2Cu3Ox which is doped with zinc or subject to a strong magnetic field. Doping levels of x=6.6 and x=6.45 are used with an optional zinc substitution of 2% zinc per copper atom. A detwinning procedure is an important factor in order to reorient all twinning domains of the orthorhombic crystal cell in the same direction. Numerous single crystals were oriented and assembled to create a sample of total mass 2 g neccessary for neutron scattering experiments. The experiments were performed on triple axis neutron scattering spectrometers to measure spin excitations around the antiferromagnetic wave vector at energies up to 50 meV. Zinc substitution and magnetic field influence the spin excitations, which in the pure compound feature the resonance mode, a spin gap and an hour-glass dispersion. Upon zinc substitution the spectrum of YBa2(Cu0.98Zn0.02)3O6.6 is quantitatively unchanged around the resonance energy, however its marked temperature onset disappears. At lower energies new induced quasi-static excitations emerge. Similar finding have been observed in other cuprate compounds. The induced excitation exhibit a strong anisotropy in the copper planes. All these results are compared to other experimental techniques which provide comparable experimental evidence. Another compounds examined in this work is the more strongly oxygen underdoped YBa2(Cu0.98Zn0.02)3O6.45. The common bilayer related signal remains unchanged with zinc substitution, however a new inter-plane correlated signal is induced indicating ordering between copper planes. A comparison with other experimental techniques indicated that the superconducting critical temperature is the sole determining factor of the qualitative spin excitation spectrum, no matter what factor are responsible for it. Additional examinations have been devoted to the magnetic field effect on YBa2Cu3O6.45. Similar to other cuprate families, a magnetic-field induced enhancement of the low-energy excitations has been observed. Both the static and the inelastic excitations around 4 meV are enhanced. The latter could indicate remains of the resonance mode, which is not detected by common techniques otherwise. Finally, at results on the zinc substitution and magnetic field effect are compared with complementary experimental measurements to deduce many similarities. (orig.)