Spin dynamics near the critical doping in weakly superconducting underdopedYBa2Cu3O6.35(Tc=18K)

Abstract

From neutron inelastic and elastic scattering we have determined the magnetic structure and fluctuations in the $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.35}$ (YBCO) superconductor $({T}_{c}=18\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ near the boundary of the superconducting phase. The long-range ordered collinear spins of the insulating antiferromagnet are replaced by a commensurate central mode arising from slow, isotropically polarized, short-range spin correlations extending over four planar unit cells. The inelastic spectrum up to $30\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ is also broad in wave vector and commensurate. In contrast to the resonance peak of higher-${T}_{c}$ superconductors, the spins exhibit a single overdamped spectrum whose rate of relaxation $\ensuremath{\Gamma}$ decreases on cooling and saturates at $2\ensuremath{\Gamma}=5\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ below $\ensuremath{\sim}50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. As the relaxation rate saturates, the quasistatic spin correlations grow and become resolution limited in energy. The spin susceptibility above $\ensuremath{\sim}50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ follows the same $\ensuremath{\omega}∕T$ scaling relation found for the monolayer ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ system, indicating that the dominant energy scale is set by the temperature. Below $50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ the scale length is geometric and not linked by velocity to dynamic widths. Despite the large differences from an antiferromagnet, we show that integrated intensity conserves the total moment sum rule, and that on cooling the spectral weight transfers from the inelastic spin relaxation to the elastic central peak. There is no observable suppression of the spin fluctuations or central mode upon the onset of superconductivity. The spins respond not to coherent charge pairs but to hole doping, allowing coexistence of glassy short-range spin order with superconductivity. Since the physics of the weakly superconducting system ${\mathrm{YBCO}}_{6.35}$ must connect continuously with that in more strongly superconducting ${\mathrm{YBCO}}_{6.5}$, we find that neither incommensurate stripe-like spin modulations nor a well-defined neutron spin resonance are essential for the onset with doping of pairing in a high-temperature cuprate superconductor.