Indian Association for the Cultivation of Science

Periodically driven closed quantum systems are expected to eventually heat up to infinite temperature; reaching a steady state described by a circular orthogonal ensemble. However, such finite driven systems may exhibit sufficiently long prethermal regimes; their properties in these regimes are qualitatively different from that of their corresponding infinite temperature steady states. These, often experimentally relevant, prethermal regimes host a wide range of phenomena; they may exhibit dynamical localization and freezing, host Floquet scars, display signatures of Hilbert space fragmentation, and exhibit time crystalline phases. Such phenomena are often accompanied by emergent approximate dynamical symmetries which have no analogue in equilibrium systems. In this review, we provide a pedagogical introduction to the origin and nature of these symmetries and discuss their role in shaping the prethermal phases of a class of periodically driven closed quantum systems.

An exotic quantum mechanical ground state, i.e. the non-magnetic Jeff = 0 state, has been predicted for higher transition metal t2g4 systems, due to the influence of strong spin–orbit coupling (SOC) or in other words, due to unquenched orbital moment contribution. However, previous attempts to experimentally realize such a state in 5d4 systems had mostly been clouded by solid-state effects or the reduced strength of the renormalized SOC that might allow significant triplon condensation. Interestingly, a recent study on vacancy ordered double perovskite compound K2RuCl6 by Takahashi et al (2021 Phys. Rev. Lett.127 227201) concluded that even within LS coupling regime the Ru4+ 4d4 ions, within isolated RuCl6 octahedra, strongly accommodate J multiplets having Jeff = 0 as the ground state with weakly interacting Jeff = 1 excitation, due to large unquenced Ru orbital angular momentum in the system. In the present report, we show results from the double perovskite La2ZnRuO6, where Ru4+ ions form isolated RuO6 octahedra but unlike K2RuCl6, they remain chemically connected via corner-sharing with nonmagnetic ZnO6 octahedra. Next, we move on to separate out the RuO6 octahedra further by doping the Ru-site with Ti4+, in order to probe the character of the Ru4+ ions within a different structural background. We find that the system stabilizes in P21/n space group with tilted octahedra without distortion as has been confirmed by the x-ray powder diffraction and x-ray absorption spectroscopic studies. Interestingly, the x-ray photoelectron spectroscopic valance band spectra indicated certain inhomogeneity around the half-doping region, while confirming insulating ground state for all. Moreover, unlike the vacancy ordered double perovskite cases, it is observed that here the Ru orbital angular momentum gets substantially quenched and only the Ru spin magnetic moments are realized.

Blood coagulation, the tightly regulated biological process prevents bleeding upon injury to the blood vessels. Vessel injury exposes the sub-endothelial tissue factor (TF) to the blood stream, thereby leading to the binding of coagulation protease, factor VII/activated VII with TF, and thus initiating the extrinsic pathway of blood coagulation. Apart from coagulation, FVIIa also promotes intracellular signaling via the activation of a unique class of G-protein-coupled receptor (GPCR) family protein, protease-activated receptor 1 (PAR1), thereby promoting anti-inflammation and endothelial barrier protection. Blood coagulation and inflammation are intrinsically connected, the activation of one process often leads to the activation of the other. The present review highlights the mechanisms by which FVIIa contributes to cytoprotective responses, either by direct action or through the release of extracellular vesicles (EVs) from vascular endothelium. FVIIa, due to its well-known ability to promote coagulation, is also used as a hemostatic agent in the treatment of several hyper bleeding disorders like hemophilia, thrombocytopenia etc. In addition to its hemostatic role, the topics discussed in the present review open a new therapeutic off-label effect of FVIIa, i.e., providing anti-inflammatory and vascular protective responses in several bleeding disorders and beyond.