Microscopic study of spin-orbit-induced Mott insulator in Ir oxides

Motivated by recent experiments of a novel 5$d$ Mott insulator in Sr$_2$IrO$_4$, we have studied the two-dimensional three-orbital Hubbard model with a spin-orbit coupling $\lambda$. The variational Monte Carlo method is used to obtain the ground state phase diagram with varying a on-site Coulomb interaction $U$ as well as $\lambda$. It is found that the transition from a paramagnetic metal to an antiferromagnetic (AF) insulator occurs at a finite $U=U_{\mathrm{MI}}$, which is greatly reduced by a large $\lambda$, characteristic of 5$d$ electrons, and leads to the "spin-orbit-induced" Mott insulator. It is also found that the Hund's coupling induces the anisotropic spin exchange and stabilizes the in-plane AF order. We have further studied the one-particle excitations using the variational cluster approximation, and revealed the internal electronic structure of this novel Mott insulator. These findings are in agreement with experimental observations on Sr$_2$IrO$_4$, and qualitatively different from those of extensively studied 3$d$ Mott insulators.Comment: 5 pages, 3 figures; accepted for publication in Phys. Rev. Let

Stress and crack monitoring during plasma spraying of TBC

Two types of process monitoring techniques are compared and discussed in this presentation. The first one is in-situ curvature monitoring, by which it was possible to evaluate the stress evolution during plasma spraying and separately identify the sources of stresses, i.e., the quenching stress and thermal stress as shown in Fig.1 (a). By changing the spraying parameters, it was possible to prepare specimens at largely different deposition temperatures, which resulted in significantly different levels of residual stresses. Also, it was found that the mechanical properties of the obtained YSZ coatings such as the elastic modulus are strongly dependent on the deposition temperature as shown in Fig.1 (b). Four-point bending test was conducted to these coatings, which clearly showed that the compressive residual stress effectively offset the applied tensile stress to initiate cracking in the YSZ coatings. Another method is based on acoustic emission (AE). Non-contacting laser AE sensors as shown in Fig.2 were used to detect cracking in YSZ coatings during spraying. Due to the intensive noise from the plasma spraying environment, extensive signal processing techniques have been developed to eliminate the noise in the frequency and time domains by using digital filtering and multi-threshold techniques. The obtained results so far indicate that the through thickness temperature gradient during spraying plays a major role in the formation of deep vertically segmentation cracks