Magnetic field resonantly enhanced free spins in underdoped YBa$_{2}$Cu$_{3}$O$_{6+x}$

TL;DR

This study reveals that in heavily underdoped YBa2Cu3O6+x, magnetic fields induce a resonant enhancement of spin fluctuations unrelated to superconductivity, likely due to free spins near hole-rich regions, contrasting with behavior in more doped cuprates.

Contribution

It demonstrates that magnetic fields cause a resonant enhancement of spin fluctuations in underdoped YBCO, independent of superconductivity, highlighting the role of free spins near hole-rich areas.

Findings

Magnetic field induces resonant enhancement of spin fluctuations.

Static spin correlations are unaffected by magnetic fields.

Enhancement matches Zeeman energy in both normal and vortex phases.

Abstract

Using neutron scattering, we investigate the effect of a magnetic field on the static and dynamic spin response in heavily underdoped superconducting YBa$_{2}$Cu$_{3}$O$_{6+x}$ (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 $(I/I_{0}-1) \sim 0.1$. We suggest that the enhancement is not directly correlated with superconductivity but is the result of almost free spins located near hole rich regions.