Neutron scattering evidence for spin and charge inhomogeneity in cuprate superconductors

Neutron diffraction studies have provided clear evidence for charge and spin stripe order in La$_{2-x}$Ba$_{x}$CuO$_{4}$ and La$_{1.6-x}$Nd$_{0.4}$Sr$_{x}$CuO$_{4}$ for a range of $x$, with a maximum ordering temperature at $x$ = 1/8. The ordering of stripes competes with superconducting order. Recent measurements of the magnetic excitation spectrum in La$_{1.875}$Ba$_{0.125}$CuO$_{4}$ show that: 1) the energy scale corresponds to antiferromagnetic superexchange, 2) the qualitative features do not change when static stripe order disappears [1], and 3) the spectrum is very similar to that found in other cuprate superconductors. New measurements on optimally-doped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ [2] are consistent with the concept of a universal spectrum. Results on over-doped La$_{2-x}$Sr$_{x}$CuO$_{4}$ show that the magnetic spectral weight disappears as the superconductivity goes away [3]. These results suggest that slowly-fluctuating charge inhomogeneity is common to the cuprates and underlies the high-temperature superconductivity. \begin{enumerate} \item Guangyong Xu, J.M. Tranquada, T.G. Perring, G.D. Gu, M. Fujita, and K. Yamada, (unpublished). \item Guangyong Xu, J.M. Tranquada, B. Fauqu\'{e}, G.D. Gu, M. H\"{u}cker, T.G. Perring, L.-P. Regnault, and J.S. Wen, (unpublished). \item S. Wakimoto, K. Yamada, J.M. Tranquada, C.D. Frost, R.J. Birgeneau, and H. Zhang, cond-mat/0609155. \end{enumerate}