We measure the nuclear quadrupole resonance (NQR) signal on the Zn site in nearly optimally doped YBa$_2$Cu$_3$O$_{6.92}$, when Cu is substituted by 3% of isotopically pure $^{67}$Zn. We observe that Zn creates large insulating islands, confirming tw
o earlier conjectures: that doping provokes an orbital transition in the CuO$_2$ plane, which is locally reversed by Zn substitution, and that the islands are antiferromagnetic. Also, we find that the Zn impurity locally induces a breaking of the D$_4$ symmetry. Cluster and DFT calculations show that the D$_4$ symmetry breaking is due to the same partial lifting of degeneracy of the nearest-neighbor oxygen sites as in the LTT transition in La$_{2-x}$Ba$_x$CuO$_4$, similarly well-known to strongly suppress superconductivity. These results show that in-plane oxygen $2p^5$ orbital configurations are principally involved in the metallicity and superconductivity of all high-T$_c$ cuprates, and provide a qualitative symmetry-based constraint on the SC mechanism.
We have developed a system for contactless measurement of nonlinear conductivity in the radio-frequency band, and over a wide temperature range. A non-resonant circuit is used to electrically excite the sample, and the induced signal is detected by a
resonant circuit whose natural frequency matches higher harmonics of the excitation. A simple modification of the probe allows non-resonant detection suitable for stronger signals. Two measurement procedures are proposed that allow significant excitation power variation, up to 150 W. The apparatus has been validated trough the measurement of the nonlinear response at the superconducting transition of a high-Tc superconductor, and the nematic transition of an iron pnictide.
We present the observation of glass-like dynamic correlations of mobile mercury ions in the ionic conductor Cu2HgI4, detected in both NMR and nonlinear conductivity experiments. The results show that dynamic cooperativity appears in systems seemingly
unrelated to glassy and soft arrested materials. A simple kinetic two-component model is proposed, which seems to provide a good description of the cooperative ionic dynamics.
We present an improved approach to the impedance spectroscopy of conductive liquid samples using four-electrode measurements. Our method enables impedance measurements of conductive liquids down to the sub-Hertz frequencies, avoiding the electrode po
larization effects that usually cripple standard impedance analysers. We have successfully tested our apparatus with aqueous solutions of potassium chloride and gelatin. The first substance has shown flat spectra from $sim$100 kHz down to sub-Hz range, while the results on gelatin clearly show the existence of two distinct low frequency conductive relaxations.
