Emergent Magnetic Degeneracy in Iron Pnictides due to the Interplay between Spin-Orbit Coupling and Quantum Fluctuations

Electronic, optical, and thermoelectric characteristics of (Ae)xFBiS2 (Ae=Sr, Ba, and x=1.7) layered materials useful in optical modulator devices

The recent discovery of superconductivity behavior in the mother BiS2-layered compounds has captivated the attention of several physicists. The crystal structure of superconductors with alternate layers of BiS2 is homologous to that of cuprates and Fe-based superconductors. The full-potential linearized augmented plane-wave (FP-LAPW) technique was utilized to investigate the electronic structures and density of states in the vicinity of the Fermi energy of SrFBiS2 and BaFBiS2 compounds under the electron carriers doping. The introduction of electron doping (carries doping) reveals that the host compounds SrFBiS2 and BaFBiS2 exhibit features indicative of superconductivity. This carrier doping of SrFBiS2 and BaFBiS2 compounds (electron-doped) has a significant impact on the lowest conduction states near the Fermi level for the emergence of the superconducting aspect. The electron doping modifies and induces changes in the electronic structures with superconducting behavior in (Ae)1.7FBiS2(Ae=Sr,Ba) compounds. A Fermi surface nesting occurred under the modification of electrons (carriers) doping in the host compounds SrFBiS2 and BaFBiS2. Furthermore, the optical characteristics of the carrier-doped SrFBiS2 and BaFBiS2 compounds are simulated. Due to the anisotropic behavior, the optical properties of these materials based on BiS2 demonstrate a pronounced polarization dependency. The starting point at zero photon energy in the infrared region is elucidated by considering the Drude features in the optical conductivity spectra of SrFBiS2 and BaFBiS2 compounds, when the electron carriers doping is applied. It was clearly noticed that the spin-orbit coupling (SOC) influences the electronic band structures, density of states, Femi surface, and optical features because of the heavy Bismuth atom, which may disclose fascinating aspects. Further, we conducted simulations to assess the thermoelectric properties of these mother compounds. The two BiS2-layered compounds could be suitable for practical thermoelectric purposes and are highlighted through assessment of electrical conductivity, thermal conductivity, Seebeck coefficient, and power factor. As a result, we propose that the mechanisms of superconducting behavior in BiS2 family may pave new avenues for investigating the field of unconventional superconductivity. It may also provide new insights into the origin of high-Tc superconductivity nature.

Pressure-Induced Large Volume Collapse, Plane-to-Chain, Insulator to Metal Transition in CaMn2Bi2

In situ high pressure single crystal X-ray diffraction study reveals that the quantum material CaMn₂Bi₂ undergoes a unique plane to chain structural transition between 2 and 3 GPa, accompanied by a large volume collapse. Puckered Mn-Mn honeycomb layer converts to quasi-one-dimensional (1D) zigzag chains above the phase transition pressure. Single crystal measurements reveal that the pressure-induced structural transformation is accompanied by a dramatic 2 orders of magnitude drop of resistivity. Although the ambient pressure phase displays semiconducting behavior at low temperatures, metallic temperature dependent resistivity is observed for the high pressure phase, as surprisingly, are two resistivity anomalies with opposite pressure dependences, while one of them could be a magnetic transition and the other originates from Fermi surface instability. Assessment of the total energies for hypothetical magnetic structures for high pressure CaMn₂Bi₂ indicates that ferrimagnetism is thermodynamically favored.

Preferred Magnetic Excitations in the Iron-Based Sr_{1-x}Na_{x}Fe_{2}As_{2} Superconductor

With spin-orbit coupling, both local-moment magnetism and itinerant electrons are expected to behave anisotropically in spin space, but such effects' influence on the formation of unconventional superconductivity has been hitherto unexplored. Here, in an iron-based superconductor, Sr_{1-x}Na_{x}Fe_{2}As_{2}, we report spectroscopic evidence that itinerant electrons "prefer" to be assisted by c-axis polarized magnetic excitations in their formation of superconducting Cooper pairs, against the polarization of the local-moment excitations. Our result naturally explains why the superconductivity competes strongly with the tetragonal magnetic phase in this material, and provides a fresh view on how to make a good superconductor out of a magnetic "Hund's metal."