Quantum Spin Excitations through the metal-to-insulator crossover in $YB{a}_{2}C{u}_3{O}_{6+y}$

Abstract

We use inelastic neutron scattering to study the temperature dependence of the spin excitations of a detwinned superconducting YBa${}_2$Cu${}_3$O${}_{6.45}$ ($T_c=48$ K). In contrast to earlier work on YBa${}_2$Cu${}_3$O${}_{6.5}$ ($T_c=58$ K), where the prominent features in the magnetic spectra consist of a sharp collective magnetic excitation termed ``resonance'' and a large ($\hslash \ddot{I}‰\approx 15$ meV) superconducting spin gap, we find that the spin excitations in YBa${}_2$Cu${}_3$O${}_{6.45}$ are gapless and have a much broader resonance. Our detailed mapping of magnetic scattering along the $a^{\ast }$/$b^{\ast }$-axis directions at different energies reveals that spin excitations are unisotropic and consistent with the ``hourglass''-like dispersion along the $a^{\ast }$-axis direction near the resonance, but they are isotropic at lower energies. Since a fundamental change in the low-temperature normal state of YBa${}_2$Cu${}_3$O${}_{6+y}$ when superconductivity is suppressed takes place at $y\sim 0.5$ with a metal-to-insulator crossover (MIC), where the ground state transforms from a metallic to an insulating-like 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.

9 pages, 7 figures, accepted by Phys. Rev. B