Spin dynamics near the critical doping in weakly-superconducting underdoped YBa2Cu3O6.35 (Tc=18K)

TL;DR

This study uses neutron scattering to explore the magnetic structure and fluctuations in underdoped YBa2Cu3O6.35, revealing that spin dynamics are unaffected by superconductivity and are dominated by hole doping, challenging previous notions of resonance and stripe order.

Contribution

It demonstrates that in weakly-superconducting YBCO6.35, spin fluctuations are unaffected by superconductivity and do not exhibit incommensurate modulations or a neutron resonance, suggesting a different mechanism for pairing.

Findings

Spin fluctuations are unaffected by superconductivity.

No incommensurate stripe order observed.

Resonance peak absent in low Tc superconductor.

Abstract

Using neutron scattering we have determined the magnetic structure and fluctuations in the YBa2Cu3O6.35 superconductor (Tc=18 K). 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. The inelastic spectrum up to 30 meV is broad in wave vector and commensurate. In contrast to the the resonance peak of higher Tc superconductors, the spins exhibit a single overdamped spectrum whose rate of relaxation decreases on cooling and saturates at 5 meV below 50 K. As the relaxation rate saturates the quasi-static spin correlations grow and become resolution limited in energy. The spin susceptibility at high temperatures illustrates that the dominant energy scale is set by the temperature. At low temperatures, the scale length is geometric and not linked by velocity to dynamic widths. 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 YBCO6.35 must connect continuously with that in more strongly superconducting YBCO6.5, 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.