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Anomalous electronic structure of the positive muon in antimony : evidence for an isolated kondo impurity Johnston, Thomas Michael Shaun


The anomalous electronic structure of a positive muon in the semimetal antimony wasinvestigated by the μSR (Muon Spin Rotation/Relaxation) technique. Precise measurements of the giant muon Knight shift (Kμ=+1.4%) were made as a function of temperature (2-20 K) and magnetic field (18-21 kG) with the applied magnetic field parallel to the c-axis of the antimony single crystal. No de Haas-van Alphen oscillations were observed at a temperature of 3 K, which indicates the electron spin density on the muon does not scale simply with the magnetic susceptibility of the conduction electrons. An upper limit of 0.2% of Kμ can be placed on the amplitude of the Kμ de Haas-van Alphen oscillations. The measured Kμ varies weakly with temperature below 10 K. The data were fit in three different ways. The best fit is to the form Kμ = 1/(a + bT² + cT4) with parameters a=7.167(3) x 10⁻³ b=6.4(7) x 1O⁻⁷K⁻²,and c=1.8(2) x 10⁻¹⁰K⁻⁴. A fit to an Arrhenius Law of the form 1— Ae^(Ea/kBT) yields a pre-exponential of 0.47(4) and an activation temperature Ea/kB of 40(2) K. If one fits to an expression for the Kondo susceptibility of the form Kμ = a(1/2πTK — 0.433T²/T³k) at low temperatures (2-10 K) one obtains a parameter a=14.0(0.3) K and a Kondo temperature TK=160(3) K. Together with existing measurements of Kμ at higher temperatures (90-180 K) we ob serve a crossover from a weakly temperature dependent Pauli paramagnetic behaviour below 10 K to a Curie-like behaviour above 90 K. This crossover is characteristic of a Kondo impurity indicating the anomalous electronic structure of a positive muon in antimony may be due to muonium formation, in which a local moment is centered on or near the muon. Within context of this model estimates of the hyperfine parameters (A=259.7(4) MHz and A±=129.9(2) MHz) for muonium in antimony are similar to those for Mu* in covalent semiconductor. The large anisotropy suggests a large spin density resides on the nearest neighbour antimony atom(s). Since the μSR technique only permits one muon in the sample at a time, the anomalous electronic structure of a muon in antimony may be a unique example of a truly isolated Kondo impurity in so far as other magnetic impurities in the sample can be neglected. A large Korringa-like relaxation of the muon in antimony was observed in longitudinal field implying the spin dynamics of a muon in antimony are also anomalous when compared to normal metals. The Korringa constant S/(K² μ T₁T) is temperature independent as expected for a Kondo impurity for T << TK. Above a temperature of 100 K, which is comparable to TK, a breakdown of the Korringa law is seen. In particular a peak in T₁⁻¹ at a temperature of 75 K is observed.

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