The temperature dependence of the in-plane magnetic penetration depth, ${\lambda }_{ab}(T)$, has been measured in a $c$-axis oriented polycrystalline ${\mathrm{CaC}}_6$ bulk sample using a high-resolution mutual inductance technique. A clear exponential behavior of ${\lambda }_{ab}(T)$ has been observed at low temperatures, strongly suggesting isotropic $s$-wave pairing. Data fit using the standard BCS theory yields ${\lambda }_{ab}(0)=(720\pm 80)\AA$ and $\Delta (0)=(1.79\pm 0.08)\mathrm{meV}$. The ratio $2\Delta (0)/k_B{T}_c=(3.6\pm 0.2)$ gives indication for a weakly coupled superconductor.

Thanks to high resolution and polarization analysis, resonant inelastic x-ray scattering (RIXS) magnetic spectra of ${\mathrm{La}}_2{\mathrm{CuO}}_4$, ${\mathrm{Sr}}_2{\mathrm{CuO}}_2{\mathrm{C}}_{l2}$ and ${\mathrm{CaCuO}}_2$ reveal a rich set of properties of the spin-$1/2$ antiferromagnetic square lattice of cuprates. The leading single-magnon peak energy dispersion is in excellent agreement with the corresponding inelastic neutron scattering measurements. However, the RIXS data unveil an asymmetric line shape possibly due to odd higher order terms. Moreover, a sharp bimagnon feature emerges from the continuum at $(1/2,0)$, coincident in energy with the bimagnon peak detected in optical spectroscopy. These findings show that the inherently complex spin spectra of cuprates, an exquisite manifestation of quantum magnetism, can be effectively explored by exploiting the richness of RIXS cross sections.

A new Selective Solar Absorber, designed to improve the Sun-to-thermal conversion efficiency at mid temperatures in high vacuum flat thermal collectors, is presented. Efficiency has been evaluated by using analytical formulas and a numerical thermal model. Both results have been experimentally validated using a commercial absorber in a custom experimental set-up. The optimization procedure aimed at obtaining Selective Solar Absorber is presented and discussed in the case of a metal dielectric multilayer based on Cr₂O₃ and Ti. The importance of adopting a real spectral emissivity curve to estimate high thermal efficiency at high temperatures in a selective solar absorber is outlined. Optimized absorber multilayers can be 10% more efficient than the commercial alternative at 250 $°$C operating temperatures, reaching 400 $°$C stagnation temperature without Sun concentration confirming that high vacuum flat thermal collectors can give important contribution to the energy transition from fossil fuels to renewable energy for efficient heat production.

We report the relationship between epitaxial strain and the crystallographic orientation of the in-phase rotation axis and $A$-site displacements in Pbnm-type perovskite films. Synchrotron diffraction measurements of ${\mathrm{EuFeO}}_3$ films under strain states ranging from 2% compressive to 0.9% tensile on cubic or rhombohedral substrates exhibit a combination of $a^{-}{a}^{+}{c}^{-}$ and $a^{+}{a}^{-}{c}^{-}$ rotational patterns. We compare the ${\mathrm{EuFeO}}_3$ behavior with previously reported experimental and theoretical work on strained Pbnm-type films on nonorthorhombic substrates, as well as additional measurements from ${\mathrm{LaGaO}}_3,{\mathrm{LaFeO}}_3$, and ${\mathrm{Eu}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ films on ${\mathrm{SrTiO}}_3$. Compiling the results from various material systems reveals a general strain dependence in which compressive strain strongly favors $a^{-}{a}^{+}{c}^{-}$ and $a^{+}{a}^{-}{c}^{-}$ rotation patterns and tensile strain weakly favors $a^{-}{a}^{-}{c}^{+}$ structures. In contrast, ${\mathrm{EuFeO}}_3$ films grown on Pbnm-type ${\mathrm{GdScO}}_3$ under 2.3% tensile strain take on a uniform $a^{-}{a}^{+}{c}^{-}$ rotation pattern imprinted from the substrate, despite strain considerations that favor the $a^{-}{a}^{-}{c}^{+}$ pattern. These results point to the use of substrate imprinting as a more robust route than strain for tuning the crystallographic orientations of the octahedral rotations and $A$-site displacements needed to realize rotation-induced hybrid improper ferroelectricity in oxide heterostructures.