Abstract
We use inelastic neutron scattering to study the temperature dependence of the spin excitations of a detwinned superconducting . In contrast to earlier work on , where the prominent features in the magnetic spectra consist of a sharp collective magnetic excitation termed “resonance” and a large superconducting spin gap, we find that the spin excitations in are gapless and have a much broader resonance. Our detailed mapping of magnetic scattering along the -axis directions at different energies reveals that spin excitations are unisotropic and consistent with the “hourglasslike” dispersion along the -axis direction near the resonance, but they are isotropic at lower energies. Since a fundamental change in the low-temperature normal state of when superconductivity is suppressed takes place at with a metal-to-insulator crossover (MIC), where the ground state transforms from a metallic to an insulatinglike phase, our results suggest a clear connection between the large change in spin excitations and the MIC. The resonance therefore is a fundamental feature of metallic ground state superconductors and a consequence of high- superconductivity.
- Received 2 November 2007
DOI:https://doi.org/10.1103/PhysRevB.77.014523
©2008 American Physical Society