W. Rüegg

The detection of muonium in water

Abstract We report the first direct observation of the muonium atom (μ + e − ) in a liquid sample. Using the transverse field μSR technique muonium spin precession signals were detected in water at six different fields between 4 and 80 G.

New determination of the magnetic moment of the positive muon from Larmor precession measurements in liquid bromine

Abstract The magnetic moment of positive muons stopped in liquid bromine has been measured by determining the precession frequency in a magnetic field of sub ppm homogeneity with a stroboscopic method. The diamagnetic shielding of the magnetic field was determined by NMR measurements on analogous chemical compounds. The present result is μ μ / μ p = 3.1833448 (29) (±0.9 ppm).

Muon knight shift in platinum and in β-PdHx (x=0.70, 0.75, 0.86)

Theμ+ Knight shift in platinum has been measured between 20 K and 785 K. It shows a strong temperature dependence and scales with the magnetic bulk susceptibility. A temperature independent contribution of +40 to +60 ppm and a d-electron induced hyperfine field per unpaired d-electron per atom ofBhf,dΩ=−5.03 kG (±8.5%) are obtained. Theμ+ Knight shift in PdH0.70 and PdH0.75 shows no dependence on temperature between 20 K andRT and increases fromKμ ppm forx=0.70 toKμ ppm forx=0.75, in good agreement with proton Knight shift measurements.

Anomalous temperature dependence in the depolarization rate of positive muons in pure niobium

Abstract The temperature dependence of the depolarization rate for positive muons implanted into a pure niobium crystal has a pronounced minimum in the vicinity of 20 K. The data show self-trapping of the muon at low temperature, and impurity limited diffusion at temperatures above 65 K.

Measurement of the field dependence (Knight shift) of the hyperfine field at a positive muon in ferromagnetic nickel

Abstract The dependence of the hyperfine field at an interstitial positive muon in ferromagnetic field has been measured at room temperature yielding a Knight Shift constant K = 0.0025 (3). This Knight shift is interpreted in terms of the Pauli spin paramagnetism of s-p band electrons.

Muon diffusion in the metal hydrides β-PdHx(x=0.70 and 0.75) and LaNi5H6

We report on zero and transverse fieldμSR measurements in LaNi5H6 and inβ-PdHx (x=0.70 and 0.75) between 16 K and room temperature. Theμ+-depolarization is predominantly caused by the spread in nuclear dipole fields from the protons. Motional averaging is caused by the combined motion of protons and theμ+. The results are quite unexpected and point toμ+-trapping within regions of largely immobile hydrogen configurations or to a highly correlatedμ+-proton diffusion. In LaNi5H6 a linear change of the second moment with temperature between 20 K and 120 K is indicated.

Muon spin rotation studies at SIN

Recently the accelerator and the muon channel at SIN became operational. We report here on some of the first positive muon spin rotation experiments at SIN. The first experiment we discuss concerns the chemistry of muonium. We have observed for the first time a muonium or radical signal in pure water. Next, we discuss the application of the positive muon for the study of ferromagnetic metals and alloys. First measurements on a single crystal of iron around liquid helium temperature seem to indicate that at low temperature the muon does not diffuse. We further report on the first stroboscopic observation of the muon spin rotation which will allow one to take full advantage of the high stopping density at SIN.

Detailed investigation of the “DIP” structure in the μ+-relaxation rate in Nb

The depolarization rate of positive muons implanted in a number of nominally pure, cylindrical Nb single crystals (maximal 250 ppm Ta, 100 ppm N + O) was investigated at two temperatures, viz. 14.0 and 36.8 K, in a high transverse field of 7.5 kG with the stroboscopicμSR technique in order to study the nature of the “dip” at 22 K. To determine the sites at which the muon is trapped on both sides of this dip, the full angular dependence of the depolarization rate was measured by rotating a large single crystal around its 〈110〉 cylinder axis in a transverse magnetic field. The resulting curves for both temperatures are quite different, reflecting clearly the different environment in which the muon is trapped above and below 22 K. The trapping site at 36.8 K was identified to be of tetrahedral symmetry, located near a Ta substitutional impurity and possibly associated with an interstitial impurity. Lattice distortions due to these impurities and radial relaxation around the muon,δR/R, were determined. The latter is +6.7(6)% for nearest neighbors and −6(2)% for next nearest neighbors. The 14.0 K angular dependence could not be fitted by considering distorted tetrahedral and octahedral sites and pointlike muons.

Anisotropic muon Knight shift in the HCP single crystals of Cd, Zn and Be

In single crystal samples of Zn, Cd and Be (hcp structure) stroboscopicμSR measurements successfully revealed anisotropies in the muon Knight shift (Kμ). An anisotropic Kμ can provide information on the amount of non s-electrons screening the charge of the muon implanted in these metals as a light hydrogen isotope. In Cd, the anisotropic part depends strongly on the temperature and shows a change in sign at roughly 110 K. In Zn, the anisotropic part below 10 K turns out to comprise 4th order contributions in the direction cosines of the external field. This can be understood on the basis of an anisotropicg-factor of the conduction electrons or spin-orbit coupling, respectively.

Possibility of a Lifshitz phase transition in Cd observed by a singularity in the muon Knight shift

During the past much effort has been devoted to a systematic study of the muon Knight shiftKμ in metallic environments and its implications on the local electronic structure of hydrogen in metals [1]. These measurements in simple metals were essentially all carried out in polycrystalline samples at room temperature. The present measurements in Cd in polycrystalline and single crystal samples cover a temperature range between 20 K and the melting point of this strongly anisotropic metal (hcp crystal structure,c/a ratio 1.89 — idealc/a ratio 1.63). These measurements add qualitatively new and interesting aspects and insights on the screening of a light hydrogen isotope in a metal as well as on certain properties of the host material itself. The outstanding features of the muon Knight shift in Cd are: (i) a strong intrinsic temperature dependence with an increase ofKμ of more than 100% between 20 K and the melting point (T=593 K), (ii) an anomaly at 110 K in the form of a singularity in the isotropic part ofKμ which is interpreted as a band structure effect, (iii) an anisotropic Knight shift contribution fitting the expressionK(T,θ)=Kiso(T)+Kax(T) * (3 · cos2θ−1)/2, where both, the isotropic and the axial contribution ofKμ, are strongly temperature dependent.