New high-$T_c$ Fe-based superconducting compounds, $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ with $A=\mathrm{K}$, Cs, $\mathrm{K}/\mathrm{Sr}$, and $\mathrm{Cs}/\mathrm{Sr}$, were synthesized. The $T_c$ of ${\mathrm{KFe}}_2{\mathrm{As}}_2$ and ${\mathrm{CsFe}}_2{\mathrm{As}}_2$ is 3.8 and 2.6 K, respectively, which rises with partial substitution of Sr for K and Cs and peaks at 37 K for 50%–60% Sr substitution, and the compounds enter a spin-density-wave state with increasing electron number (Sr content). The compounds represent $p$-type analogs of the $n$-doped rare-earth oxypnictide superconductors. Their electronic and structural behavior demonstrate the crucial role of the (${\mathrm{Fe}}_2{\mathrm{As}}_2$) layers in the superconductivity of the Fe-based layered systems, and the special feature of having elemental $A$ layers provides new avenues to superconductivity at higher $T_c$.

The synthesis and properties of LiFeAs, a high-$T_c$ Fe-based superconducting stoichiometric compound, are reported. Single crystal x-ray studies reveal that it crystallizes in the tetragonal PbFCl type (P4/nmm) with $a=3.7914\left(7\right)\text{Å}$ and $c=6.364\left(2\right)\text{Å}$. Unlike the known isoelectronic undoped intrinsic FeAs compounds, LiFeAs does not show any spin-density wave behavior but exhibits superconductivity at ambient pressures without chemical doping. It exhibits a respectable transition temperature of $T_c=18\text{K}$ with electronlike carriers and a very high critical field, ${\text{H}}_{c2}\left(0\right)>80\text{T}$. LiFeAs appears to be the chemical equivalent of the infinite layered compound of the high-$T_c$ cuprates.

Moving the heat aside with BAs: Thermal management becomes increasingly important as we decrease device size and increase computing power. Engineering materials with high thermal conductivity, such as boron arsenide (BAs), is hard because it is essential to avoid defects and impurities during synthesis, which would stop heat flow. Three different research groups have synthesized BAs with a thermal conductivity around 1000 watts per meter-kelvin: Kang et al. , Li et al. , and Tian et al. succeeded in synthesizing high-purity BAs with conductivities half that of diamond but more than double that of conventional metals (see the Perspective by Dames). The advance validates the search for high-thermal-conductivity materials and provides a new material that may be more easily integrated into semiconducting devices. Science , this issue p. 575 , p. 579 , p. 582 ; see also p. 549

A cool way to use isotopes: Thermal management of electronics requires materials that can efficiently remove heat. Several promising materials have been found recently, but diamond remains the bulk material with the highest thermal conductivity. Chen et al. found that isotopically pure cubic boron nitride has an ultrahigh thermal conductivity, 75% that of diamond. Using only boron-11 or boron-10 allows the crystal vibrations that carry heat to move more efficiently through the material. This property could be exploited for better regulating the temperature of high-power devices. Science , this issue p. 555

Using angle-resolved photoemission spectroscopy, we observe the low-temperature state of the ${\mathrm{A}}_x{\mathrm{Fe}}_{2\mathrm{\text{−}}y}{\mathrm{Se}}_2$ ($A=\mathrm{K}$, Rb) superconductors to exhibit an orbital-dependent renormalization of the bands near the Fermi level—the $d_{xy}$ bands heavily renormalized compared to the $d_{xz}/d_{yz}$ bands. Upon raising the temperature to above 150 K, the system evolves into a state in which the $d_{xy}$ bands have depleted spectral weight while the $d_{xz}/d_{yz}$ bands remain metallic. Combined with theoretical calculations, our observations can be consistently understood as a temperature-induced crossover from a metallic state at low temperatures to an orbital-selective Mott phase at high temperatures. Moreover, the fact that the superconducting state of ${\mathrm{A}}_x{\mathrm{Fe}}_{2\mathrm{\text{−}}y}{\mathrm{Se}}_2$ is near the boundary of such an orbital-selective Mott phase constrains the system to have sufficiently strong on-site Coulomb interactions and Hund’s coupling, highlighting the nontrivial role of electron correlation in this family of iron-based superconductors.

Establishing the appropriate theoretical framework for unconventional superconductivity in the iron-based materials requires correct understanding of both the electron correlation strength and the role of Fermi surfaces. This fundamental issue becomes especially relevant with the discovery of the iron chalcogenide superconductors. Here, we use angle-resolved photoemission spectroscopy to measure three representative iron chalcogenides, FeTe_0.56Se_0.44, monolayer FeSe grown on SrTiO₃ and K_0.76Fe_1.72Se₂. We show that these superconductors are all strongly correlated, with an orbital-selective strong renormalization in the d_xy bands despite having drastically different Fermi surface topologies. Furthermore, raising temperature brings all three compounds from a metallic state to a phase where the d_xy orbital loses all spectral weight while other orbitals remain itinerant. These observations establish that iron chalcogenides display universal orbital-selective strong correlations that are insensitive to the Fermi surface topology, and are close to an orbital-selective Mott phase, hence placing strong constraints for theoretical understanding of iron-based superconductors.