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.
We report on muon spin rotation/relaxation and $^{119}$Sn nuclear magnetic resonance (NMR) measurements to study the microscopic superconducting and magnetic properties of the Heusler compound with the highest superconducting transition temperature,
ypd ($T_c=5.4$ K). Measurements in the vortex state provide the temperature dependence of the effective magnetic penetration depth $lambda(T)$ and the field dependence of the superconducting gap $Delta(0)$. The results are consistent with a very dirty s-wave BCS superconductor with a gap $Delta(0)=0.85(3)$ meV, $lambda(0)= 212(1)$ nm, and a Ginzburg-Landau coherence length $xi_{mathrm{GL}}(0)cong 23$ nm. In spite of its very dirty character, the effective density of condensed charge carriers is high compared to the normal state. The mSR data in a broad range of applied fields are well reproduced by taking into account a field-related reduction of the effective superconducting gap. Zero-field mSR measurements, sensitive to the possible presence of very small magnetic moments, do not show any indications of magnetism in this compound.
We report beta-NMR investigations of polarized 8Li implanted in thin Pb and Ag/Nb films. At the critical superconducting temperature, we observe a singular peak in the spin relaxation rate in small longitudinal magnetic fields, which is attributed to
fluctuations in the superconducting order parameter. However, the peak is more than an order of magnitude larger than the prediction based on the enhancement of the dynamic electron spin susceptibility by superconducting fluctuations and reflects the presence of unexpected slow fluctuations. Furthermore the fluctuations are rapidly suppressed in a small magnetic field, which may explain why they have not been observed previously with conventional NMR or NQR.
We have performed depth dependent muon spin rotation/relaxation studies of the dynamics of single layer films of {it Au}Fe and {it Cu}Mn spin glasses as a function of thickness and of its behavior as a function of distance from the vacuum interface (
5-70 nm). A significant reduction in the muon spin relaxation rate as a function of temperature with respect to the bulk material is observed when the muons are stopped near (5-10 nm) the surface of the sample. A similar reduction is observed for the whole sample if the thickness is reduced to e.g. 20 nm and less. This reflects an increased impurity spin dynamics (incomplete freezing) close to the surface although the freezing temperature is only modestly affected by the dimensional reduction.
