Magnetic Dirac semimetal state of (Mn,Ge)Bi2_2Te4_4

For quantum electronics, the possibility to finely tune the properties of magnetic topological insulators (TIs) is a key issue. We studied solid solutions between two isostructural Z$_2$ TIs, magnetic MnBi$_2$Te$_4$ and nonmagnetic GeBi$_2$Te$_4$, with Z$_2$ invariants of 1;000 and 1;001, respectively. For high-quality, large mixed crystals of Ge$_x$Mn$_{1-x}$Bi$_2$Te$_4$, we observed linear x-dependent magnetic properties, composition-independent pairwise exchange interactions along with an easy magnetization axis. The bulk band gap gradually decreases to zero for $x$ from 0 to 0.4, before reopening for $x>0.6$, evidencing topological phase transitions (TPTs) between topologically nontrivial phases and the semimetal state. The TPTs are driven purely by the variation of orbital contributions. By tracing the x-dependent $6p$ contribution to the states near the fundamental gap, the effective spin-orbit coupling variation is extracted. As $x$ varies, the maximum of this contribution switches from the valence to the conduction band, thereby driving two TPTs. The gapless state observed at $x=0.42$ closely resembles a Dirac semimetal above the Neel temperature and shows a magnetic gap below, which is clearly visible in raw photoemission data. The observed behavior of the Ge$_x$Mn$_{1-x}$Bi$_2$Te$_4$ system thereby demonstrates an ability to precisely control topological and magnetic properties of TIs

Ultralight vector dark matter search using data from the KAGRA O3GK run

Among the various candidates for dark matter (DM), ultralight vector DM can be probed by laser interferometric gravitational wave detectors through the measurement of oscillating length changes in the arm cavities. In this context, KAGRA has a unique feature due to differing compositions of its mirrors, enhancing the signal of vector DM in the length change in the auxiliary channels. Here we present the result of a search for U(1)B−L gauge boson DM using the KAGRA data from auxiliary length channels during the first joint observation run together with GEO600. By applying our search pipeline, which takes into account the stochastic nature of ultralight DM, upper bounds on the coupling strength between the U(1)B−L gauge boson and ordinary matter are obtained for a range of DM masses. While our constraints are less stringent than those derived from previous experiments, this study demonstrates the applicability of our method to the lower-mass vector DM search, which is made difficult in this measurement by the short observation time compared to the auto-correlation time scale of DM

Charge Transfer Kinetics of Redox-Active Microgels

Polymer microgel particles decorated with redox-active functional groups are a new and promising object for electrochemical applications. However, the process of charge exchange between an electrode and a microgel particle carrying numerous redox-active centers differs fundamentally from charge exchange involving only molecular species. A single act of contact between the microgel and the electrode surface may not be enough to fully discharge the microgel, and partial charge states are to be expected. Understanding the specifics of this process is crucial for the correct analysis of the data obtained from electrochemical experiments with redox-active microgel solutions. In this study, we employed coarse-grained molecular dynamics to investigate in detail the act of charge transfer from a microgel particle to a flat electrode. The simulations take into account both the mobility of functional groups carrying the charge, which depend on the microgel architecture and the charge exchange between the groups, which can accelerate the propagation of charge within the microgel volume. A set of different microgel systems were simulated in order to reveal the impact of their characteristics: fraction of redox-active groups, microgel molecular mass, cross-linker content, cross-linking topology, and solvent quality. We have found trends in microgel composition leading to the most efficient charge transfer kinetics. The obtained results would be useful for understanding experimental results and for optimizing the design of redox-active microgel particles aimed at faster discharge rates