We present measurements of how a single vortex line in superfluid helium interacts with a macroscopic bump on the chamber wall. Rather than observing a unique pin location, we find that a given applied velocity field can support pinning at multiple sites along the bump, both near its apex and near its edge. We also find that a vortex can pass near or even traverse the bump itself with or without pinning, depending on its path of approach to the bump. We discuss our results in light of past computational work on vortex pinning by a hemispherical bump, as well as our own simulations that incorporate additional aspects of the experimental setup. © 2014 American Physical Society.

We study vortex motion using a Hopf flow on a 3-sphere, in place of the standard 3-torus defined by periodic boundary conditions. Although both of these underlying manifolds look locally like R3, the vortex behavior differs significantly. This raises the question of how well calculations on the 3-torus correspond to behavior in R3, for this problem or others. We find that, on both the 3-sphere and 3-torus, vortex tangles appear, with their line density related to the strength of the driving velocity field. However, tangles on the 3-sphere are highly anisotropic, with vortices tending to align along the flow direction. Standard power-law dependences change accordingly from their values in more isotropic tangles. The line length density (L) is proportional to vns1.28, where vns is the drive velocity, and the reconnection rate depends roughly on (L)2. These compare to vns2 and (L)2.5 for the more isotropic tangles on the 3-torus. In addition, in simulations using the local induction approximation rather than the full Biot-Savart law, the tangle collapses so that all vortices are nearly aligned with a single flow line. By contrast, vortices on the 3-torus become perpendicular to the driving velocity.

Simulating superfluid turbulence using the localized induction approximation allows neighboring parallel vortices to proliferate. In many circumstances a turbulent tangle becomes unsustainable, degenerating into a series of parallel, noninteracting vortex lines. Calculating with the fully nonlocal Biot-Savart law prevents this difficulty but also increases computation time. Here we use a truncated Biot-Savart integral to investigate the effects of nonlocality on homogeneous turbulence. We find that including the nonlocal interaction up to roughly the spacing between nearest-neighbor vortex segments prevents the parallel alignment from developing, yielding an accurate model of homogeneous superfluid turbulence with less computation time.

We describe a simple manganin foil resistance manometer for uniaxial stress measurements. The manometer functions at low pressures and over a range of temperatures. In this design, no temperature seasoning is necessary although the manometer must be prestressed to the upper end of the desired pressure range. The prestress pressure cannot be increased arbitrarily; irreversibility arising from shear stress limits its range. Attempting larger pressures yields irreproducible resistance measurements.

We apply the first-order reversal curve (FORC) method, adapted from studies of ferromagnetic materials, to the magnetostructural phase transition of Fe1+yTe. FORC measurements reveal two features in the hysteretic phase transition, even in samples where traditional temperature measurements display only a single transition. For Fe1.13Te, the influence of magnetic field suggests that the main feature is primarily structural while a smaller, slightly higher-temperature transition is magnetic in origin. By contrast, Fe1.03Te has a single transition which shows a uniform response to magnetic field, indicating a stronger coupling of the magnetic and structural phase transitions. We also introduce uniaxial stress, which spreads the distribution width without changing the underlying energy barrier of the transformation. The work shows how FORC can help disentangle the roles of the magnetic and structural phase transitions in FeTe.

We have measured the room-temperature phonon spectrum of Mo-stabilized γ-U. The dispersion curves show unusual softening near the H point, q = [1/2, 1/2, 1/2], which may derive from the metastability of the γ-U phase or from strong electron-phonon coupling. Near the zone center, the dispersion curves agree well with theory, though significant differences are observed away from the zone center. The experimental phonon density of states is shifted to higher energy compared to theory and high-temperature neutron scattering. The elastic constants of γ-UMo are similar to those of body-centered cubic elemental metals.

We have studied the Ce valence as a function of pressure in CeRhIn5 at 300 K and at 22 K using x-ray absorption spectroscopy in partial fluorescent yield mode. At room temperature, we found no detectable change in Ce valence greater than 0.01 up to a pressure of 5.5 GPa. At 22 K, the valence remains robust against pressure below 6 GPa, in contrast to the predicted valence crossover at P  =  2.35 GPa. This work yields an upper limit for the change in Ce-valence and suggests that the critical valence fluctuation scenario, in its current form, is unlikely.

We have performed pressure-dependent X-ray diffraction and resonant X-ray emission spectroscopy experiments on USb2 to further characterize the antiferromagnetic-ferromagnetic transition occurring near 8 GPa. We have found the magnetic transition coincides with a tetragonal to orthorhombic transition resulting in a 17% volume collapse as well as a transient f-occupation enhancement. Compared to UAs2 and UAsS, USb2 shows a reduced bulk modulus and transition pressure and an increased volume collapse at the structural transition. Except for an enhancement across the transition region, the f occupancy decreases steadily from 1.96 to 1.75.

We have performed an extensive pressure-dependent structural, spectroscopic, and electrical transport study of LaCrSb3. The ferromagnetic phase (TC=120 K at p = 0 GPa) is fully suppressed by p = 26.5 GPa and the Cr moment decreases steadily with increasing pressure. The unit-cell volume decreases smoothly up to p = 55 GPa. We find that the bulk modulus and suppression of the magnetism are in good agreement with theoretical predictions, but the Cr moment decreases smoothly with pressure, in contrast to steplike drops predicted by theory. The ferromagnetic ordering temperature appears to be driven by the Cr moment.

We report a comprehensive structural and valence study of the intermediate valent materials YbNiGa4 and YbNiIn4 under pressures up to 60 GPa. YbNiGa4 undergoes a smooth volume contraction and shows steady increase in Yb valence with pressure, though the Yb valence reaches saturation around 25 GPa. In YbNiIn4, a change in pressure dependence of the volume and a peak in Yb valence suggest that a pressure-induced electronic topological transition occurs around 10-14 GPa. In the pressure region where YbNiIn4 and YbNiGa4 possess similar Yb-Yb spacings, the Yb valence reveals a precipitous drop. This drop is not captured by density functional theory calculations and implies that both the lattice degrees of freedom and the chemical environment play an important role in establishing the valence of Yb.