Magnetic field resonantly enhanced free spins in heavily underdoped YBa₂Cu₃O_6+x

Using neutron scattering, we investigate the effect of a magnetic field on the static and dynamic spin response in heavily underdoped superconducting YBa₂Cu₃O_6+x (YBCO_6+x) with x=0.33 (T_c=8 K) and 0.35 (T_c=18 K). In contrast to the heavily doped and superconducting monolayer cuprates, the elastic central peak characterizing static spin correlations does not respond observably to a magnetic field which suppresses superconductivity. Instead, we find a magnetic-field-induced resonant enhancement of the spin fluctuations. The energy scale of the enhanced fluctuations matches the Zeeman energy within both the normal and vortex phases, while the momentum dependence is the same as the zero-field bilayer response. The magnitude of the enhancement is very similar in both phases with a fractional intensity change of (I/I₀−1)∼0.1. We suggest that the enhancement is not directly correlated with superconductivity but is the result of almost free spins located near hole-rich regions.