Quantum spin excitations through the metal-to-insulator crossover in YBa₂Cu₃O_6+y

We use inelastic neutron scattering to study the temperature dependence of the spin excitations of a detwinned superconducting YBa₂Cu₃O_6.45 (T_c=48 K). In contrast to earlier work on YBa₂Cu₃O_6.5 (T_c=58 K), where the prominent features in the magnetic spectra consist of a sharp collective magnetic excitation termed “resonance” and a large (ℏω≈15 meV) superconducting spin gap, we find that the spin excitations in YBa₂Cu₃O_6.45 are gapless and have a much broader resonance. Our detailed mapping of magnetic scattering along the a^*∕b^*-axis directions at different energies reveals that spin excitations are unisotropic and consistent with the “hourglasslike” dispersion along the a^*-axis direction near the resonance, but they are isotropic at lower energies. Since a fundamental change in the low-temperature normal state of YBa₂Cu₃O_6+y when superconductivity is suppressed takes place at y∼0.5 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-T_c superconductivity.