Paramagnonlike excitations and spin diffusion in magnetic resonance studies of copper oxide superconductors

The relaxation function theory for a doped two-dimensional Heisenberg antiferromagnetic system in the paramagnetic state for all wave vectors through the Brillouin zone is presented in view of low frequency response of high-$T_c$ copper oxide superconductors. We deduced the regions of long lifetime [$T \lesssim 400(1-4x)$ K] and "overdamped" [$T \gtrsim 700(1-4x)$ K] paramagnonlike excitations in the temperature ($T$)-doping index ($x$) phase diagram from plane oxygen nuclear spin-lattice relaxation rate $^{17}(1/T_1)$ data in up to optimally doped La$_{2-x}$Sr$_{x}$CuO$_{4}$ thus providing the regimes for the spin wave concept and the ''overdamped'' mode.Comment: Physical Review B, accepted, in pres

Possibility of cold nuclear compression in antiproton-nucleus collisions

We study the dynamical response of the oxygen-16 nucleus to an incident antiproton using the Giessen Boltzmann-Uehling-Uhlenbeck microscopic transport model with relativistic mean fields. A special emphasis is put on the possibility of a dynamical compression of the nucleus induced by the moving antiproton. Realistic antibaryon coupling constants to the mean meson fields are chosen in accordance with empirical data. Our calculations show that an antiproton embedded in the nuclear interior with momentum less than the nucleon Fermi momentum may create a locally compressed zone in the nucleus with a maximum density of about twice the nuclear saturation density. To evaluate the probability of the nuclear compression in high-energy antiproton-nucleus collisions, we adopt a two-stage scheme. This scheme takes into account the antiproton deceleration due to the cascade of antiproton-nucleon rescatterings inside the nucleus (first stage) as well as the nuclear compression by the slow antiproton before its annihilation (second stage). With our standard model parameters, the fraction of antiproton annihilation events in the compressed zone is about $10^{-5}$ for $\bar p ^{16}$O collisions at $p_{\rm lab}=3-10$ GeV/c. Finally, possible experimental triggers aimed at selecting such events are discussed.Comment: 40 pages, 15 figures, new Sect. V on the in-medium modifications of annihilation, modified conclusions, added references, version accepted in Phys. Rev.