Jess H. Brewer

First observation of an antiferromagnetic phase in the Y1Ba2Cu3Ox system

The magnetic properties of four different phases of the YBa2Cu3Ox system, including the famous 90 K superconductor, have been probed by the positive muon spin relaxation method (µ+SR). The oxygen-deficient tetragonal insulating phase (x~6.2) was found to be magnetically ordered, thus most likely antiferromagnetic, near room temperature.

Kinetic Isotope Effects for the Reactions of Muonic Helium and Muonium with H2

Calculated reaction rates for two hydrogen isotopes, one 36 times heavier than the other, agree with experiments at 500 kelvin. The neutral muonic helium atom may be regarded as the heaviest isotope of the hydrogen atom, with a mass of ~4.1 atomic mass units (4.1H), because the negative muon almost perfectly screens one proton charge. We report the reaction rate of 4.1H with 1H2 to produce 4.1H1H + 1H at 295 to 500 kelvin. The experimental rate constants are compared with the predictions of accurate quantum-mechanical dynamics calculations carried out on an accurate Born-Huang potential energy surface and with previously measured rate constants of 0.11H (where 0.11H is shorthand for muonium). Kinetic isotope effects can be compared for the unprecedentedly large mass ratio of 36. The agreement with accurate quantum dynamics is quantitative at 500 kelvin, and variational transition-state theory is used to interpret the extremely low (large inverse) kinetic isotope effects in the 10−4 to 10−2 range.

Muon level-crossing spectroscopy of organic free radicals

Abstract Muon level-crossing spectroscopy has been applied to the study of muonium-substituted radicals formed in liquid benzene, hexadeuterobenzene, furan, 2-methylpropene, 2,3-dimethyl-2-butene, and gaseous ethene. The magnitudes and signs of the proton and deuteron hyperfine constants are reported, and are discussed in terms of isotope effects and intramolecular motion.

Muonium chemistry in gases: Mu+Br2

We report a precise measurement of the rate of reaction of muonium atoms with bromine molecules in an argon moderator gas at a atm and 23 °C. The bimolecular rate constant is k= (2.4±0.3) ×1011 l/mole‐sec, ten times higher than that for the analogous reaction of hydrogen atoms. Since muonium can properly be treated as a light isotope of hydrogen, this comparison has potential significance for the theory of reaction rates. The technique is described and the results discussed.

μ+SR Studies of Magnetic Properties of the YBa2Cu3Ox System

Samples of YBa 2 Cu 3 O x in four different phases have been probed by positive muon spin relaxation (µ + SR) method. Each phase was found to show different magnetic behavior. In the famous orthorhombic 90K superconductor ( x ∼6.9), neither magnetic ordering nor magnetic fluctuations were observed. However, in another orthorhombic superconducting phase ( x ∼6.4, T c ≈60 K) a magnetic fluctuation was observed below 7 K via muon spin-nuclear spin double relaxation. Its magnetic ordering temperature was expected to be below 2.4 K. Similar behavior was observed in an oxygen-rich ( x ∼6.5) tetragonal insulating phase, whereas the oxygen-deficient ( x ∼6.2) tetragonal insulating phase was found to be magnetically ordered, most likely antiferromagnetic, near room temperature. This is the first observation of a magnetically ordered state in the YBa 2 Cu 3 O x system.

Probing magnetism and superconductivity of high-Tc systems with positive muons

Abstract Recent results from our μSR experiments on high- T c systems are presented. We describe studies of static magnetic order in hole-doped and electron-doped systems, correlations between T c and n s / m * (carrier density / effective mass) in hole-doped superconducting cuprates, the temperature dependence of the penetration depth in c-axis oriented sintered ceramic of Y Ba 2 Cu 3 O 7 , as well as the detection of flux pinning phenomena by μSR.

Magnetic Phase Transitions in 90 K Superconductors HoBa2Cu3O7-δ and GdBa2Cu3O7-δ Observed by ZF- µ+SR

Freezing of holmium magnetic moments was observed below 3 K in a 90 K superconductor HoBa2Cu3O7-δ by zero-field positive muon spin relaxation (ZF-µ+SR) measurements. The ZF-µ+SR function revealed a marked change of µ+ local field from a critically fluctuating regime (4–7 K) to a quasi-static regime below 3 K (of nearly Gaussian distribution). The observation indicates that holmium magnetic moments are short-range ordered around 2 K. In contrast, a 90 K superconductor GdBa2Cu3O7-δ showed a zero-field spin precession at 2 K, indicating long-range ordering of gadolinium magnetic moments.

Muonium quantum diffusion in cryocrystals

In this paper we review our recent study on muonium (Mu) diffusion in simplest solids — Van der Waals cryocrystals. We give experimental evidence for the quantum nature of Mu diffusion in these matters. The results are compared with the current theory of quantum diffusion in insulators. The first direct observation of theT7 power-law for the Mu hop rate (in solid nitrogen) is taken as confirmation of two-phonon scattering mechanism. In solid xenon the one-phonon interaction is shown to be dominant. Muonium diffusion in solid neon is discussed in terms of band-like motion. Finally, we report a dramatic effect of impurities on Mu quantum diffusion.

The magnetic structure of the zigzag chain family Na

We present muon-spin-rotation measurements on polycrystalline samples of the complete family of the antiferromagnetic AF zigzag chain compounds, NaxCa1�xV2O4. In this family, we explore the magnetic properties from the metallic NaV2O4 to the insulating CaV2O4. We find a critical xc0.833 which separates the low and high Na-concentration-dependent transition temperature and its magnetic ground state. In the xxc compounds, the magnetic ordered phase is characterized by a single homogenous phase and the formation of incommensurate spin-density-wave order. Whereas in the xxc compounds, multiple subphases appear with temperature and x. Based on the muon data obtained in zero external magnetic field, a careful dipolar field simulation was able to reproduce the muon behavior and indicates a modulated helical incommensurate spin structure of the metallic AF phase. The incommensurate modulation period obtained by the simulation agrees with that determined by neutron diffraction.

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Polarized negative muons were stopped in various materials containing nuclei with nonzero spin. The TF-μSR precession signal of theF+ hyperfine state (frequencyv+ > 0) was pronounced forμ−Li andμ−Be, faint forμ−Cl (in NiCl2), and undetectable forμ−F (in CaF2 or NiF2) andμ−P. TheF− signal (frequencyv− < 0) was observed clearly forμ−Be,μ−Al andμ−Na, marginally forμ−K,μ−V andμ−Nb, and not at all forμ−Ga (at 4 kOe) orμ−Co (ferrromagnetic, zero field). In the heavier elements theF− signal is “fed” by transitions from theF+ to theF− state at a rateR, as long asR≫(v+ −v−). [See separate paper onμ−Al in these Proceedings, p. 879.]