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 ($\hslash \omega \approx 15$ 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.

arXiv comment9 pages, 7 figures, accepted by Phys. Rev. B

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• The effect of the pseudogap on suppressing high energy inelastic neutron scattering in superconducting YBa2Cu3O6.5
• Spin dynamics near the critical doping in weakly-superconducting underdoped YBa2Cu3O6.35 (Tc=18K)
• Magnetic field-enhanced spin freezing in YBa2Cu3O6.45 at the verge of the competition between superconductivity and charge order
• Glassy low-energy spin fluctuations and anisotropy gap in La1.88Sr0.12CuO4
• Electron-Spin Excitation Coupling in an Electron Doped Copper Oxide Superconductor
• Evidence of a critical hole concentration in underdoped YBa${}_2$Cu${}_3$O${}_y$ single crystals revealed by ${}^{63}$Cu NMR
• Cu-NMR study of oxygen disorder in ortho-II YBa2Cu3Oy
• Spin-roton excitations in the cuprate superconductors
• Magnetic field resonantly enhanced free spins in underdoped YBa${}_2$Cu${}_3$O${}_{6+x}$
• Separation of magnetic and superconducting behaviour in YBCO6.33 (Tc=8.4 K)
• Universality of the {\bf q}=1/2 Orbital Magnetism in the Pseudogap Phase of the High-$T_c$ superconductor $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{6+\mathrm{x}}$
• Gapless spin excitations in superconducting La${}_{2-x}$Ca${}_{1+x}$Cu${}_2$O${}_6$ with $T_c$ up to 55 K