The connection between superconducting phase correlations and spin excitations in YBa$_2$Cu$_3$O$_{6.6}$: A magnetic field study

TL;DR

This study investigates how magnetic fields affect spin excitations in underdoped YBa$_2$Cu$_3$O$_{6.6}$, revealing that magnetic resonance correlates with superconducting pairing and phase coherence, thus highlighting magnetism's role in high-$T_c$ superconductivity.

Contribution

The paper demonstrates that magnetic fields significantly suppress the magnetic resonance in underdoped YBa$_2$Cu$_3$O$_{6.6}$, linking the resonance to pairing and phase coherence in cuprates.

Findings

Magnetic fields suppress the resonance more when perpendicular to CuO$_2$ planes.

Resonance suppression correlates with reduced superconducting phase coherence.

Persistence of field effects above $T_c$ suggests preformed pairs in the normal state.

Abstract

One of the most striking universal properties of the high-transition-temperature (high-$T_c$) superconductors is that they are all derived from the hole-doping of their insulating antiferromagnetic (AF) parent compounds. From the outset, the intimate relationship between magnetism and superconductivity in these copper-oxides has intrigued researchers. Evidence for this link comes from neutron scattering experiments that show the unambiguous presence of short-range AF correlations (excitations) in cuprate superconductors. Even so, the role of such excitations in the pairing mechanism and superconductivity is still a subject of controversy. For YBa$_2$Cu$_3$O$_{6+x}$, where $x$ controls the hole-doping level, the most prominent feature in the magnetic excitations spectra is the ``resonance''. Here we show that for underdoped YBa$_2$Cu$_3$O$_{6.6}$, where $x$ and $T_c$ are below the optimal values, modest magnetic fields suppress the resonance significantly, much more so for fields approximately perpendicular rather than parallel to the CuO$_2$ planes. Our results indicate that the resonance measures pairing and phase coherence, suggesting that magnetism plays an important role in the superconductivity of cuprates. The persistence of a field effect above $T_c$ favors mechanisms with preformed pairs in the normal state of underdoped cuprates.