Quantum magnetic excitations from stripes in copper-oxide superconductors

Recent inelastic neutron scattering studies show that the magnetic excitation spectra of two well-studied families of cuprate superconductors are much more similar than previously believed. In particular, I will present results we have obtained on La$_{2-x}$Ba$_{x}$CuO$_{4}$ (LBCO) with $x $= 0.125 [1,2]. Using very large single crystals grown at Brookhaven, we were able to measure the magnetic excitations up to 200 meV using the MAPS time-of-flight spectrometer at the ISIS spallation source. While the lowest energy excitations are split incommensurately, these disperse inwards towards the antiferromagnetic wave vector with increasing energy, merging at $\sim $50 meV. At higher energies the excitations disperse outwards again. There is a significant enhancement of the \textbf{Q}-integrated magnetic scattering near $\sim $50 meV compared to lower energies, suggestive of quantum correlations and distinct from spin-wave predictions. Many features of the spectrum are quite similar to those found in YBa$_{2}$Cu$_{3}$O$_{6.6}$ [3]. One can qualitatively characterize the results with a universal excitation spectrum, together with a material-dependent spin gap in the superconducting state. It is important to note that the LBCO sample exhibits static stripe order [2], as this has significant implications for the origin of the magnetic excitations in superconducting cuprates. \begin{enumerate} \item J. M. Tranquada, H. Woo, T. G. Perring, H. Goka, G. D. Gu, G. Xu, M. Fujita, and K. Yamada, Nature \textbf{429}, 534 (2004). \item M. Fujita, H. Goka, K. Yamada, J. M. Tranquada, and L.-P. Regnault, Phys. Rev. B \textbf{70}, 104517 (2004). \item S. M. Hayden, H. A. Mook, P. C. Dai, T. G. Perring, and F. Dogan, Nature \textbf{429}, 531 (2004). \end{enumerate}