Emergence of superconductivity in the cuprates via a universal percolation process

A pivotal step toward understanding unconventional superconductors would be to decipher how superconductivity emerges from the unusual normal state upon cooling. In the cuprates, traces of superconducting pairing appear above the macroscopic transition temperature $T_c$, yet extensive investigation has led to disparate conclusions. The main difficulty has been the separation of superconducting contributions from complex normal state behaviour. Here we avoid this problem by measuring the nonlinear conductivity, an observable that is zero in the normal state. We uncover for several representative cuprates that the nonlinear conductivity vanishes exponentially above $T_c$, both with temperature and magnetic field, and exhibits temperature-scaling characterized by a nearly universal scale $T_0$. Attempts to model the response with the frequently evoked Ginzburg-Landau theory are unsuccessful. Instead, our findings are captured by a simple percolation model that can also explain other properties of the cuprates. We thus resolve a long-standing conundrum by showing that the emergence of superconductivity in the cuprates is dominated by their inherent inhomogeneity

Ultrafast Dynamics of Vibrational Symmetry Breaking in a Charge-ordered Nickelate

The ability to probe symmetry breaking transitions on their natural time scales is one of the key challenges in nonequilibrium physics. Stripe ordering represents an intriguing type of broken symmetry, where complex interactions result in atomic-scale lines of charge and spin density. Although phonon anomalies and periodic distortions attest the importance of electron-phonon coupling in the formation of stripe phases, a direct time-domain view of vibrational symmetry breaking is lacking. We report experiments that track the transient multi-THz response of the model stripe compound La$_{1.75}$Sr$_{0.25}$NiO$_{4}$, yielding novel insight into its electronic and structural dynamics following an ultrafast optical quench. We find that although electronic carriers are immediately delocalized, the crystal symmetry remains initially frozen - as witnessed by time-delayed suppression of zone-folded Ni-O bending modes acting as a fingerprint of lattice symmetry. Longitudinal and transverse vibrations react with different speeds, indicating a strong directionality and an important role of polar interactions. The hidden complexity of electronic and structural coupling during stripe melting and formation, captured here within a single terahertz spectrum, opens new paths to understanding symmetry breaking dynamics in solids.Comment: 21 pages, 4 figures; updated version with journal re

Preparation of SiO 2

The effect of SiO2 capping on the optical properties of nanoparticles was investigated. The photoluminescence (PL) intensity was successfully improved by SiO2-capping. Sr2MgSi2O7:Eu,Dy nanoparticles were prepared by laser ablation in liquid. The SiO2 capping was performed using the Stöber method with ultrasonication. The TEM images indicated that the Sr2MgSi2O7:Eu,Dy nanocrystal was capped with amorphous SiO2, and the shape of the completely capped nanoparticle was an elliptical nanorod, which aggregated after a long SiO2 capping reaction time. The peak wavelength and the shape of the PL spectra were not changed by the pelletization and the laser ablation in liquid. The PL intensity of SiO2 capped nanoparticles was significantly increased. Nonradiative relaxation via surface defects and energy transfer to water molecules decrease the PL intensity. These phenomena accelerate in the case of nanoparticles. SiO2 capping would prevent these phenomena and improve the optical properties of nanoparticles. The combination of laser ablation in liquid and the chemical reaction is important to expand the applications of this method in various research fields