Evidence for the formation of magnetic moments in the cuprate superconductor Hg$_{0.8}$Cu$_{0.2}$Ba$_2$Ca$_2$Cu$_3$O$_{8+\delta}$ below $T_c$ seen by NQR

TL;DR

This study provides evidence of spontaneous magnetic moment formation in a high-temperature cuprate superconductor below its critical temperature, using zero-field NQR measurements to reveal internal magnetic fields associated with magnetic ordering.

Contribution

The paper demonstrates the formation of magnetic moments in Hg-based cuprate superconductors below Tc through NQR spectral analysis, revealing internal magnetic fields and magnetic ordering.

Findings

Magnetic moments form below Tc, causing Zeeman splitting of Cu NQR lines.

Internal magnetic fields of about 1000 G are detected within CuO layers.

Magnetic moments are oriented parallel to the electric field gradient axis.

Abstract

We report pure zero field nuclear magnetic resonance (NQR) measurements on the optimally doped three layer high-$ T_{c} $-compounds HgBaCaCuO and HgBaCaCuO(F) with $T_c$ 134 K. Above $ T_{c}$ two Cu NQR line pairs are observed in the spectra corresponding to the two inequivalent Cu lattice sites. Below $ T_{c}$ the Cu NQR spectra show additional lines leading to the extreme broadened Cu NQR spectra at 4.2 K well known for the HgBaCaCuO compounds. The spin-lattice relaxation curves follow a triple exponential function with coefficients depend onto the saturation time (number of saturation pulses), whereas the spin-spin relaxation curve is described by a single exponential function. From the spin-lattice relaxation we deduced a complete removal of the Kramers degeneracy of the Cu quadrupole indicating that the additional lines are due to a Zeemann splitting of the $^{63/65}$Cu lines due to the spontaneous formation of magnetic moments within the CuO layers. Below 140 K, the spectra are well fitted by a number of 6 $^{63/65}$Cu line pairs. From the number of the Cu lines, the position of the lines relative to each other and the complete removal of the Kramers degeneracy we deduced an orientation of the magnetic moments parallel to the symmetry axis of the electric field gradient tensor with magnitudes of the order of 1000 G. We also discuss the possible microscopic origin of the observed internal magnetic fields.