Identifying the superconducting (SC) gap structure of the iron-based high-temperature superconductors (Fe-HTSs) remains a key issue for the understanding of superconductivity in these materials. In contrast to other unconventional superconductors, in
the Fe-HTSs both $d$-wave and extended s-wave pairing symmetries are close in energy, with the latter believed to be generally favored over the former. Probing the proximity between these very different SC states and identifying experimental parameters that can tune them, are of central interest. Here we report high-pressure muon spin rotation experiments on the temperature-dependent magnetic penetration depth (lambda) in the optimally doped Fe-HTS Ba_0.65Rb_0.35Fe_2As_2. At ambient pressure this material is known to be a nodeless s-wave superconductor. Upon pressure a strong decrease of (lambda) is observed, while the SC transition temperature remains nearly constant. More importantly, the low-temperature behavior of (1/lambda^{2}) changes from exponential saturation at zero pressure to a power-law with increasing pressure, providing unambiguous evidence that hydrostatic pressure promotes nodal SC gaps. Comparison to microscopic models favors a d-wave over a nodal s^{+-}-wave pairing as the origin of the nodes. Our results provide a new route of understanding the complex topology of the SC gap in Fe-HTSs.
EuTiO_3, which is a G-type antiferromagnet below T_N = 5.5 K, has some fascinating properties at high temperatures, suggesting that macroscopically hidden dynamically fluctuating weak magnetism exists at high temperatures. This conjecture is substant
iated by magnetic field dependent magnetization measurements, which exhibit pronounced anomalies below 200 K becoming more distinctive with increasing magnetic field strength. Additional results from muon spin rotation (${mu}$SR) experiments provide evidence for weak fluctuating bulk magnetism induced by spin-lattice coupling which is strongly supported in increasing magnetic field.
