We report on measurements of the in-plane magnetic penetration depth (${\lambda }_{\mathrm{ab}}$) in single crystals of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ down to $\simeq 0.015\mathrm{K}$ by means of muon-spin rotation-relaxation. The linear temperature dependence of ${\lambda }_{\mathrm{ab}}^{-2}$ for $T\lesssim 0.7\mathrm{K}$ suggests the presence of nodes in the superconducting gap. This statement is further substantiated by observation of the Volovik effect, i.e., the reduction of ${\lambda }_{ab}^{-2}$ as a function of the applied magnetic field. The experimental zero-field and zero-temperature value of ${\lambda }_{\mathrm{ab}}=124(3)\mathrm{nm}$ agrees with ${\lambda }_{\mathrm{ab}}\simeq 130\mathrm{nm}$, calculated based on results of electronic structure measurements reported in A. Tamai et al. [High-resolution photoemission on ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ reveals correlation-enhanced effective spin-orbit coupling and dominantly local self-energies, Phys. Rev. X 9, 021048 (2019)]. Our analysis reveals that a simple nodal superconducting energy gap, described by the lowest possible harmonic of a gap function, does not capture the dependence of ${\lambda }_{\mathrm{ab}}^{-2}$ on $T$, so the higher angular harmonics of the energy gap function need to be introduced.

Muon spin rotation/relaxation ($\mu \mathrm{SR}$) and polar Kerr effect measurements provide evidence for a time-reversal symmetry breaking (TRSB) superconducting state in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$. However, the absence of a cusp in the superconducting transition temperature ($T_{\mathrm{c}}$) vs stress and the absence of a resolvable specific heat anomaly at TRSB transition temperature ($T_{\mathrm{TRSB}}$) under uniaxial stress challenge a hypothesis of TRSB superconductivity. Recent $\mu \mathrm{SR}$ studies under pressure and with disorder indicate that the splitting between $T_{\mathrm{c}}$ and $T_{\mathrm{TRSB}}$ occurs only when the structural tetragonal symmetry is broken. To further test such behavior, we measured $T_{\text{c}}$ through susceptibility measurements and $T_{\text{TRSB}}$ through $\mu \mathrm{SR}$, under uniaxial stress applied along a $\langle 110\rangle$ lattice direction. We have obtained preliminary evidence for suppression of $T_{\text{TRSB}}$ below $T_{\text{c}}$, at a rate much higher than the suppression rate of $T_{\text{c}}$.

Iron-based superconductors are a popular candidate in the search for affordable and simple superconductors for high-field applications. In particular, the relaxed texture requirements fuel hope that films deposited on RABiTS with simple buffer layer architectures could enable cheap coated conductors. We find that a single Yttrium oxide buffer layer can act as a suitable diffusion barrier and epitaxial Fe(Se,Te) thin films were successfully grown by pulsed laser deposition. An analysis of the local current distribution by means of scanning Hall probe microscopy reveals current densities exceeding 1 MA cm⁻², however, granularity still seems to be an issue. Transmission electron microscopy images and analysis by transmission Kikuchi diffraction show that the out-of-plane orientation of underlying Ni-W grains in the substrate has a severe impact on the growth of Fe(Se,Te) films.

We report the local structural and superconducting properties of undoped and Ag-doped YBa₂Cu₃O_6+x (YBCO) films with a thickness of up to 1 µm prepared by pulsed laser deposition on SrTiO₃ (STO) single crystals and on ion-beam-assisted deposition (IBAD) and rolling-assisted biaxially textured substrate (RABiTS)-based metal templates. X-ray diffraction demonstrates the high crystalline quality of the films on both single crystalline substrates and metal-based templates, respectively. Although there was only a slight decrease in T_c of up to 1.5 K for the Ag-doped YBCO films on all substrates, we found significant changes in their transport characteristics. The effect of the silver doping mainly depended on the concentration of silver, the type of substrate, and the temperature and magnetic field. In general, the greatest improvement in J_c over a wide range of magnetic fields and temperatures was observed for the 5%Ag-doped YBCO films on STO substrates, showing a significant increase compared to undoped films. Furthermore, a slight J_c improvement was observed for the 2%Ag-doped YBCO films on the RABiTS templates at temperatures below 65 K, whereas J_c decreased for the Ag-doped films on IBAD-MgO-based templates compared to undoped YBCO films. Using detailed electron microscopy studies, small changes in the local microstructure of the Ag-doped YBCO films were revealed; however, no clear correlation was found with the transport properties of the films.

REBCO based coated conductors (CCs) are a viable alternative to conventional superconductors for many applications, therefore the optimization of their current carrying capacity is an ongoing process. A promising route for the increase in performance is the introduction of artificial pinning centers such as BaHfO₃ (BHO) nanoparticles. However, granularity still imposes a substantial performance limitation, especially in REBCO CCs deposited on RABiTS based templates, as the critical current density is severely reduced by moderate misalignment angles of adjacent grains. A combined study of scanning Hall probe microscopy and electron microscopy of undoped and BHO-doped YBa₂Cu₃O_7-δ (YBCO) films on technical templates shows that BHO-doping leads to a denser microstructure of the superconducting layer and higher global and local critical current densities. The statistical evaluation of local current maps allows for a quantification of the magnetic granularity where a reduction of granularity with increasing film thickness, doping and increasing temperature is found. In particular, the dependence of granularity on the film thickness and enhanced film growth through BHO-doping shows the potential for further optimization of YBCO films on RABiTS based templates.