Study of the positive muon Knight shift in : Evidence for a tetravalent -state and crystalline electric field splitting

We report on transverse field (TF) Muon Spin Rotation (μSR) measurements on a single crystal of the hexagonal heavy fermion superconductor UNi₂Al₃ between 5 K and 300 K. From the measured muon Knight shift (KS) in the easy ( a , b )-plane and along the c-axis we extracted the local magnetic susceptibility tensor [0pt] , which arises from the nearest U-neighbors. By comparison with the bulk susceptibility [0pt] it is found that [0pt] and [0pt] agree well above 150 K but deviate considerably in the basal plane below 150 K, due to the disturbance introduced by the . We succeed in reproducing both [0pt] and [0pt] on the basis of a crystalline electric field (CEF)-approach assuming U to be in the tetravalent state. The disturbance introduced by the affects the CEF-Hamiltonian in an expected manner, suggesting strongly that a CEF-picture implying a rather local 5 f-electron wave function is indeed valid. Reanalyzing older data on UPd₂Al₃ we arrive at the same conclusion. A necessary condition for extracting the local susceptibility was the knowledge of the -site, this information was derived from the analysis of the TF-relaxation rates. At low temperatures we found about 30% of the implanted at the d-site and none at this site above 200 K. The majority fraction was found to be in a tunneling state over six m (or k)-sites around the b-site. No long range diffusion was seen up to room temperature. Received 20 April 1999     

μSR studies of low charge carrier density heavy‐fermion systems

We report on μSR studies in low charge carrier density heavy‐fermion systems with cubic symmetry. Whereas for Ce₃Au₃Sb_4 and for Sm₃Te₄ the μSR data point to the presence of some kind of spin‐glass‐like magnetic states at low temperature, for Yb₄As₃ preliminary μSR studies just show a slight increase of the depolarization rate which indicates a slight slowing down of the Yb‐moment fluctuations. Consequently for this latter system, the large linear term of the specific heat seems indeed to be connected to electronic excitations. This revised version was published online in July 2006 with corrections to the Cover Date.     

First direct evidence for a magnetic phase transition in the heavy-electron system UCd11

Muon-spin-rotation and relaxation measurements performed on polycrystalline UCd₁₁ give first direct evidence for an antiferromagnetic phase transition at 5 K. As far as probed by the positive muon the behaviour close to the phase transition is clearly contrasted to that in other magnetic heavy-electron materials investigated by μ⁺SR, e.g. U₂Zn₁₇ and UCu₅, because in UCd₁₁ longitudinal field measurements reveal dynamic relaxation effects which are attributed to the critical slowing down of the uranium 5f-moments. No signal could be observed in the ordered regime because of a large dipolar field spread.     

Electric-field dependence of muonium formation in liquid helium

The influence of electric fields on the formation of muonium in liquid helium (⁴He,³He, and mixture of⁴He + 0.2%³He) has been studied. It was found that the relative distribution of muon-electron pairs is anisotropic. The maximum muon density is shifted with respect to the electrons in the direction of the initial muon momentum. Due to the anisotropy the muonium asymmetry in normal liquid helium is enhanced by a factor of 3 in an electric fieldE=1 kV/cm.     

μSR-Study of CeH2.7

Metallic CeH_2.7 shows a sizably increased electronic specific heat of γ=100 mJ/mol·K², compared to γ=0.04 mJ/mol·K² in LaH_2.7 at low tempratures, which could qualify it as another heavy electron system. μSR Knight shift and relaxation studies were started on a powderous CeH_2.7 sample in order to test this possibility further. It is found that the relaxation rate λ_T depends strongly on the applied field strength. These values indicate that the relaxation must be of electronic origin. Strong irreproducibilities at low temperature point to pronounced hysteretic features, which might be related to structural changes. The Knight shift shows a shallow peak around 150 K but no conspicious increase at low temperatures.     

Muon Knight shift and μ+ site in the monopnictide CeAs

The transverse field μSR-signal from μ⁺ implanted in a CeAs single crystal shows a splitting into two components, displaying a positive and a negative Knight shift, respectively which both scale reasonably well with the magnetic bulk susceptibility above 10 K. The orientation dependence of the frequency shifts, measured at 15 K, is fully accounted for by the demagnetization field, implying an isotropic Knight shift, consistent with the μ⁺ residing at the only interstitial site of high (cubic) symmetry, i.e., at the position (1/4,1/4,1/4). The occurrence of a split signal remains unexplained. This revised version was published online in July 2006 with corrections to the Cover Date.     

μ + SR studies of UM2Al3, M=Ni,Pd

Positive-muon spin rotation ( ⁺ SR) measurements have been carried out in the new heavy-fermion superconductors UM₂Al₃, M=Ni and Pd. In UNi₂Al₃ the observed ⁺ frequencies in zero applied field indicate commensurate antiferromagnetic (AF) ordering of U moments 0.1 _B /U atom. These moments coexist with superconductivity and have the highest values observed in an AF heavy-fermion superconductor. The absence of well-defined frequencies in zero-field ⁺ SR in the AF state of UPd₂Al₃ suggests symmetric ⁺ stopping sites. In this system ⁺ SR linewidths belowT _c yield a preliminary value of 8000 Å for the London penetration depth.     

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

The ⁺ 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 +53±15 ppm and a d-electron induced hyperfine field per unpaired d-electron per atom of B _hfd ^a =–5.03 kG(±8.5%) are obtained. The ⁺ Knight shift in PdH_0.70, PdH_0.75 and PdH_0.86 shows no dependence on temperature between 20 K and RT and increases from K=–(8±3) ppm for x=0.70 to K =+(6.5±3) ppm K=+(6.5±3) ppm for x=0.86, in good agreement with proton Knight shift measurements.