The characteristics of shallow hydrogen-like muonium (Mu) states in nominally undoped ZnO and CdS (0001) crystals have been studied close to the surface at depths in the range of 10 nm - 180 nm by using low-energy muons, and in the bulk using convent
ional muSR. The muon implantation depths are adjusted by tuning the energy of the low-energy muons between 2.5 keV and 30 keV. We find that the bulk ionization energy of the shallow donor-like Mu state is lowered by about 10 meV at a depth of 100 nm, and continuously decreasing on approaching the surface. At a depth of about 10 nm the ionization energy is further reduced by 25-30 meV compared to its bulk value. We attribute this change to the presence of electric fields due to band bending close to the surface, and we determine the depth profile of the electric field within a simple one-dimensional model.
We present a low-energy muon-spin-rotation study of the magnetic and superconducting properties of YBa2Cu3O7/PrBa2Cu3O7 trilayer and bilayer heterostructures. By determining the magnetic-field profiles throughout these structures we show that a finit
e superfluid density can be induced in otherwise semiconducting PrBa2Cu3O7 layers when juxtaposed to YBa2Cu3O7 electrodes while the intrinsic antiferromagnetic order is unaffected.
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.
The formation of hydrogen-like muonium (Mu) has been studied as a function of implantation energy in intrinsic Si, thin films of condensed van der Waals gases (N2, Ne, Ar, Xe), fused and crystalline quartz and sapphire. By varying the initial energy
of positive muons (mu+) between 1 and 30 keV the number of electron-hole pairs generated in the ionization track of the mu+ can be tuned between a few and several thousand. The results show the strong suppression of the formation of those Mu states that depend on the availability of excess electrons. This indicates, that the role of H-impurity states in determining electric properties of semiconductors and insulators depends on the way how atomic H is introduced into the material.
Implanting fully polarized low energy muons on the nanometer scale beneath the surface of a superconductor in the Meissner state enabled us to probe the evanescent magnetic field profile B(z)(0<z<=200nm measured from the surface). All the investigate
d samples [Nb: kappa simeq 0.7(2), Pb: kappa simeq 0.6(1), Ta: kappa simeq 0.5(2)] show clear deviations from the simple exponential B(z) expected in the London limit, thus revealing the non-local response of these superconductors. From a quantitative analysis within the Pippard and BCS models the London penetration depth lambda_L is extracted. In the case of Pb also the clean limit coherence length xi0 is obtained. Furthermore we find that the temperature dependence of the magnetic penetration depth follows closely the two-fluid expectation 1/lambda^2 propto 1-(T/T_c)^4. While B(z) for Nb and Pb are rather well described within the Pippard and BCS models, for Ta this is only true to a lesser degree. We attribute this discrepancy to the fact that the superfluid density is decreased by approaching the surface on a length scale xi0. This effect, which is not taken self-consistently into account in the mentioned models, should be more pronounced in the lowest kappa regime consistently with our findings.
Muon-spin rotation (muSR) studies of the oxygen isotope (^{16}O/^{18}O) effect (OIE) on the in-plane magnetic field penetration depth lambda_{ab} in cuprate high-temperature superconductors (HTS) are presented. First, the doping dependence of the OIE
on the transition temperature T_c in various HTS is briefly discussed. It is observed that different cuprate families show a similar doping dependence of the OIE on T_c. Then, bulk muSR, low-energy muSR, and magnetization studies of the total and site-selective OIE on lambda_{ab} are described in some detail. A substantial OIE on lambda_{ab} was observed in various cuprate families at all doping levels, suggesting that cuprate HTS are non-adiabatic superconductors. The experiments clearly demonstrate that the total OIE on T_c and lambda_{ab} arise from the oxygen sites within the superconducting CuO_2 planes, demonstrating that the phonon modes involving the movement of planar oxygen are dominantly coupled to the supercarriers. Finally, it is shown that the OIE on T_c and lambda_{ab} exhibit a relation that appears to be generic for different families of cuprate HTS. The observation of these unusual isotope effects implies that lattice effects play an essential role in cuprate HTS and have to be considered in any realistic model of high-temperature superconductivity.
