Inelastic neutron scattering experiments have been carried out on a YBa2(Cuo.98Zno.02)306.39 single crystal to investigate the spin-excitation spectrum in the hole-doped antiferromagnetic state. New excitations are found at low energy as a consequence of electron hole interactions, whereas standard magnons are recovered at sufficiently high energy.
The magnetic properties of cuprates have attracted considerable attention due to their presumed connection with high-T~ superconductivity. For both the YBa2Cu306+~ and La2-xSr~CuO4 systems, inelastic neutron scattering (INS) experiments have provided broad knowledge on the spin dynamics of hole-doped CuO2 planes in all typical regimes of the cuprate phase diagram [1~4]. In particular, the way the long-range antiferromagnetic-order (LRAFO) state is disturbed by adding approximatively 2% of holes is of great interest. This situation has been previously emphasized either in YBa2Cu306.3~ [2] or in lightly Sr doped La2CuO4 [3,4]. Through doping, the N6el temperature is reduced from that in the pure AF state, and the spin dynamics are also strongly changed as compared to that of the holefree parent compound. Roughly speaking, it has been shown that the low-energy part of the spin excitation is renormalized and the magnons become overdamped. The purpose of this paper is to demonstrate that, due to hole doping, the magnons are not only overdamped, but also that at low energy, the usual description of the spin * Corresponding author. dynamics in terms of magnons fails. Recent theoretical calculations in the framework of the t J model 1-5, 6] support this view.
We have investigated, in a YBa2(Cul yZny)306+x single crystal the magnetic excitation spectrum arising from the Cu 2+ spins with an oxygen content x = 0.39 and a zinc content of y = 2%. A small amount of zinc impurity is known to induce a dramatic disappearance of the superconducting state. In contrast, 3d-AF Bragg peaks are observed by neutron elastic measurements 1-7] in our sample (x = 0.39) with TN ~ 85 K. Then, the zinc impurities appear only to reduce the N6el temperature, with no change in the hole doping in the CuO2 plane as compared to zinc-free YBCO. Actually, assuming that Zn z + replaces Cu(2), the zinc substitution effect on the 3d-AF ordering is well described by a simple small-dilution effect of the AF lattice by a static non-magnetic impurity as is usually observed in quasi-2d Heisenberg magnets. So, in this hole-doped AF state, the main effect on the spin dynamics is expected from the hole doping. This point is in clear contrast with Zn-doped YBCO with no AF ground state, e.g. YBCO6.6 1-8] and 06.75 [9], where the Zn-doping of 3% dramatically changes the low-energy magnetic excitations.