NUM division - Publication Highlights

The neutron gyromagnetic ratio has been measured relative to that of the ¹⁹⁹Hg atom with an uncertainty of 0.8 ppm. We employed an apparatus where ultracold neutrons and mercury atoms are stored in the same volume and report the result γ_n/γ_Hg = 3.8424574(30).

The magnetic ground state of the J_eff = 1/2 hyperkagome lattice in Na₄Ir₃O₈ is explored via combined bulk magnetization, muon spin relaxation, and neutron scattering measurements. A short-range, frozen state comprised of quasistatic moments develops below a characteristic temperature of T_F = 6K, revealing an inhomogeneous distribution of spins occupying the entirety of the sample volume. Quasistatic, short- range spin correlations persist until at least 20 mK and differ substantially from the nominally dynamic response of a quantum spin liquid. Our data demonstrate that an inhomogeneous magnetic ground state arises in Na₄Ir₃O₈ driven either by disorder inherent to the creation of the hyperkagome lattice itself or stabilized via quantum fluctuations.

The muon decay parameter ξ″ has been determined in a measurement of the longitudinal polarization of positrons emitted from polarized and depolarized muons. The result, ξ″ = 0.981 ± 0.045_stat ± 0.003_syst, is consistent with the Standard Model prediction of unity, and provides an order of magnitude improvement in the relative precision of this parameter. This value sets new constraints on exotic couplings beyond the dominant V-A description of the leptonic weak interaction.

SmFeO₃ has attracted considerable attention very recently due to its reported multiferroic properties above room temperature. We have performed powder and single crystal neutron diffraction as well as complementary polarization dependent soft X-ray absorption spectroscopy measurements on floating-zone grown SmFeO₃ single crystals in order to determine its magnetic structure. We found a k=0 G-type collinear antiferromagnetic structure that is not compatible with inverse Dzyaloshinskii-Moriya interaction driven ferroelectricity. While the structural data reveal a clear sign for magneto-elastic coupling at the Néel-temperature of ∼675 K, the dielectric measurements remain silent as far as ferroelectricity is concerned.

The MuCap experiment at the Paul Scherrer Institute performed a high-precision measurement of the rate of the basic electroweak process of nuclear muon capture by the proton, μ^- + p → n + ν_μ. The experimental approach was based on the use of a time projection chamber (TPC) that operated in pure hydrogen gas at a pressure of 10 bar and functioned as an active muon stopping target. The TPC detected the tracks of individual muon arrivals in three dimensions, while the trajectories of outgoing decay (Michel) electrons were measured by two surrounding wire chambers and a plastic scintillation hodoscope.

We present neutron scattering data on the structure and dynamics of melts from polyethylene oxide rings with molecular weights up to ten times the entanglement mass of the linear counterpart. The data reveal a very compact conformation displaying a structure approaching a mass fractal, as hypothesized by recent simulation work. The dynamics is characterized by a fast Rouse relaxation of subunits (loops) and a slower dynamics displaying a lattice animal-like loop displacement.