Competing electronic states are found in the large majority of unconventional superconductors, including high-Tc cuprates, iron based superconductors and many heavy fermion systems. The complex interplay is reflected in phase diagrams as a function o
f doping or other tuning parameters involving besides superconducting other phases (often magnetic) and quantum critical points. Superconductivity is also found in the vicinity of charge density wave (CDW) order in phase diagrams reminiscent of superconductivity mediated by magnetic fluctuations. There is however less knowledge about the interplay of superconductivity and CDW compared to the magnetic analogon. Here we report about microscopic studies by muon spin rotation as a function of pressure of the Ca_3Ir_4Sn_13 and Sr_3Ir_4Sn_13 cubic compounds, which display superconductivity and a structural phase transition associated with the formation of a CDW. We find a strong enhancement of the superfluid density and of the coupling strength above a pressure of about 1.6 GPa giving direct evidence of the presence of a quantum critical point separating a superconducting phase coexisting with CDW from a pure superconducting phase. The superconducting order parameter in both phases are found to have the same s-wave symmetry. In spite of the conventional phonon-mediated BCS character of this compound, the dependence of the effective superfluid density on the critical temperature puts this system in the Uemura plot close to unconventional superconductors.
The magnetic penetration depth ($lambda$) as a function of applied magnetic field and temperature in SrPt$_3$P($T_csimeq8.4$ K) was studied by means of muon-spin rotation ($mu$SR). The dependence of $lambda^{-2}$ on temperature suggests the existence
of a single $s-$wave energy gap with the zero-temperature value $Delta=1.58(2)$ meV. At the same time $lambda$ was found to be strongly field dependent which is the characteristic feature of the nodal gap and/or multi-gap systems. The multi-gap nature of the superconduicting state is further confirmed by observation of an upward curvature of the upper critical field. This apparent contradiction would be resolved with SrPt$_3$P being a two-band superconductor with equal gaps but different coherence lengths within the two Fermi surface sheets.
Two superconducting phases of Re3W have been found with different physical properties. One phase crystallizes in a non-centrosymmetric cubic (alpha-Mn) structure and has a superconducting transition temperature, Tc, of 7.8 K. The other phase has a he
xagonal centrosymmetric structure and is superconducting with a Tc of 9.4 K. Switching between the two phases is possible by annealing the sample or remelting it. The properties of both phases of Re3W have been characterized by powder neutron diffraction, magnetization, and resistivity measurements. The temperature dependence of the lower and the upper critical fields have been measured for both phases. These are used to determine the penetration depths and the coherence lengths for these systems.
