We introduce a new method to measure Lagrangian vorticity and the rotational dynamics of anisotropic particles in a turbulent fluid flow. We use 3D printing technology to fabricate crosses (two perpendicular rods) and jacks (three mutually perpendicular rods). Time-resolved measurements of their orientation and solid-body rotation rate are obtained from four video images of their motion in a turbulent flow between oscillating grids with R λ = 91. The advected particles have a largest dimension of 6 times the Kolmogorov length, making them a good approximation to anisotropic tracer particles. Crosses rotate like disks and jacks rotate like spheres, so these measurements, combined with previous measurements of tracer rods, allow experimental study of axisymmetric ellipsoids across the full range of aspect ratios. The measured mean square tumbling rate, p ṗi i 〈 〉, confirms previous direct numerical simulations that indicate that disks tumble much more rapidly than rods. Measurements of the alignment of a unit vector defining the orientation of crosses with the direction of their solid-body rotation rate vector provide the first direct observation of the alignment of anisotropic particles by the velocity gradients in a turbulent flow.

No abstract

We report on the observation of magnon thermal conductivity κm ∼ 70 W/mK near 5 K in the helimagnetic insulator Cu2OSeO3, exceeding that measured in any other ferromagnet by almost two orders of magnitude. Ballistic, boundary-limited transport for both magnons and phonons is established below 1 K, and Poiseuille flow of magnons is proposed to explain a magnon mean-free path substantially exceeding the specimen width for the least defective specimens in the range 2K < T < 10 K. These observations establish Cu2OSeO3 as a model system for studying longwavelength magnon dynamics.

We report the discovery of a field driven transition from a single-q to multi-q spin density wave (SDW) in the tetragonal heavy fermion compound CeAuSb_{2}. Polarized along c, the sinusoidal SDW amplitude is 1.8(2)μ_{B}/Ce for T≪T_{N}=6.25(10)  K with a wave vector q_{1}=(η,η,1/2) [η=0.136(2)]. For H∥c, harmonics appearing at 2q_{1} evidence a striped magnetic texture below μ_{∘}H_{1}=2.78(1)  T. Above H_{1}, these are replaced by coupled harmonics at q_{1}+q_{2}=(2η,0,0)+c^{*} until μ_{∘}H_{2}=5.42(5)  T, where satellites vanish and magnetization nonlinearly approaches saturation at 1.64(2)μ_{B}/Ce for μ_{∘}H≈7  T.