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The drag force exerted on an object intruding into granular media can depend on the object's velocity as well as the depth penetrated. We report on intrusion experiments at constant speed over four orders in magnitude together with systematic molecular dynamics simulations well beyond the quasi-static regime. We find that velocity dependence crosses over to depth dependence at a characteristic time after initial impact. This crossover time scale, which depends on penetration speed, depth, gravity and intruder geometry, challenges current models that assume additive contributions to the drag., 5 pages, 5 figures

Particle-based simulations of discontinuous shear thickening (DST) and shear jamming (SJ) suspensions are used to study the role of stress-activated constraints, with an emphasis on resistance to gear-like rolling. Rolling friction decreases the volume fraction required for DST and SJ, in quantitative agreement with real-life suspensions with adhesive surface chemistries and "rough" particle shapes. It sets a distinct structure of the frictional force network compared to only sliding friction, and from a dynamical perspective leads to an increase in the velocity correlation length, in part responsible for the increased viscosity. The physics of rolling friction is thus a key element in achieving a comprehensive understanding of strongly shear-thickening materials., 5 pages, 4 figures. +Supplemental Material--- --- Supplementary Material also consists of the following movies: ---https://youtu.be/wgrfN7jiu1g (Force network in 3D at volume fraction 0.45, at high stress limit, with and without rolling friction) --- https://youtu.be/SMzTB7CsRsY (Force network in 2D at packing fraction 0.7, at high stress limit, with and without rolling friction)

Dense suspensions of hard particles in a Newtonian liquid can be jammed by shear when the applied stress exceeds a certain threshold. However, this jamming transition from a fluid into a solidified state cannot be probed with conventional steady-state rheology because the stress distribution inside the material cannot be controlled with sufficient precision. Here we introduce and validate a method that overcomes this obstacle. Rapidly propagating shear fronts are generated and used to establish well-controlled local stress conditions that sweep across the material. Exploiting such transient flows, we are able to track how a dense suspension approaches its shear jammed state dynamically, and can quantitatively map out the onset stress for solidification in a state diagram., Comment: 12 pages, 12 figures

We use experiments and minimal numerical models to investigate the rapidly expanding monolayer formed by the impact of a dense suspension drop against a smooth solid surface. The expansion creates a lace-like pattern of particle clusters separated by particle-free regions. Both the expansion and the development of the spatial inhomogeneity are dominated by particle inertia, therefore robust and insensitive to details of the surface wetting, capillarity and viscous drag., 4 pages (5 with references), and a total of 4 figures

The capture cross section of the reaction ${}^{16}\mathrm{O}(p,\ensuremath{\gamma}{)}^{17}\mathrm{F}$ was measured in the energy range from ${E}_{\mathrm{c}.\mathrm{m}.}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}200--3750\mathrm{keV}$ using the windowless gas target facility Rhinoceros. The low-energy $S$ factor that is dominated by the transition to the $1/{2}^{+}$ first excited state in ${}^{17}\mathrm{F}$ increases strikingly with decreasing energy. This behavior is explained by the halo properties of this $1/{2}^{+}$ state within the framework of the direct capture model.

We investigate the impact of droplets of dense suspensions onto a solid substrate. We show that a global hydrodynamic balance is unable to predict the splash onset and propose to replace it by an energy balance at the level of the particles in the suspension. We experimentally verify that the resulting, particle-based Weber number gives a reliable, particle size and density dependent splash onset criterion. We further show that the same argument also explains why, in bimodal systems, smaller particles are more likely to escape than larger ones.

We have performed a high-resolution study of the temperature dependence of vortex dynamics for 5[times]10[sup [minus]4][lt][ital T]/[ital T][sub [ital c]][lt]10[sup [minus]2] in untwinned single crystals of the high-[ital T][sub [ital c]] superconductor, YBa[sub 2]Cu[sub 3]O[sub 7[minus][delta]]. Through the introduction of a bithermal'' magnetization relaxation technique, we have uncovered a previously unresolved, linear temperature contribution to the semiclassical action for vortex tunneling. Finally, we have compared the weak and strong pinning regimes through heavy ion irradiation of a series of crystals.

Random packings of granular chains are presented as a model polymer system to investigate the contribution of entanglements to strain-stiffening in the absence of Brownian motion. The chain packings are sheared in triaxial compression experiments. For short chain lengths, these packings yield when the shear stress exceeds a the scale of the confining pressure, similar to packings of spherical particles. In contrast, packings of chains which are long enough to form loops exhibit strain-stiffening, in which the effective stiffness of the material increases with strain, similar to many polymer materials. The latter packings can sustain stresses orders-of-magnitude greater than the confining pressure, and do not yield until the chain links break. X-ray tomography measurements reveal that the strain-stiffening packings contain system-spanning clusters of entangled chains., Comment: 4 pages, 4 figures. submitted to Physical Review Letters

We have investigated the rise of a single large glass bead through a vibrated cylindrical column of smaller particles and found that vibration-induced size separation in this geometry arises from convective processes rather than from local rearrangements as had been proposed previously. A convection cycle, rising in the middle and dropping in a thin stream along the walls of the cell, is responsible for all particle movement. Particles larger than the width of the thin downward convection zone are carried to the top of the column and then trapped, resulting in size segregation. For a variety of accelerations the position of the rising particle can be scaled onto a single curve.

Non-cohesive granular media exhibit complex responses to sudden impact that often differ from those of ordinary solids and liquids. We investigate how this response is mediated by the presence of interstitial gas between the grains. Using high-speed x-ray radiography we track the motion of a steel sphere through the interior of a bed of fine, loose granular material. We find a crossover from nearly incompressible, fluid-like behavior at atmospheric pressure to a highly compressible, dissipative response once most of the gas is evacuated. We discuss these results in light of recent proposals for the drag force in granular media., Comment: 4 pages, 4 figures, Accepted to PRL

We investigate the effects of inelastic cotunneling on the electronic transport properties of gold nanoparticle multilayers and thick films at low applied bias, inside the Coulomb blockade regime. We find that the zero-bias conductance, $g_0(T)$, in all systems exhibits Efros-Shklovskii-type variable range hopping transport. The resulting typical hopping distance, corresponding to the number of tunnel junctions participating in cotunneling events, is shown to be directly related to the power law exponent in the measured current-voltage characteristics. We discuss the implications of these findings in light of models on cotunneling and hopping transport in mesoscopic, granular conductors., Comment: 9 pages, 2 figures

The excitation function of the reaction (22)Ne(alpha,n)(25)Mg, the key neutron source in the astrophysical s process in massive stars, has been determined from threshold at E(alpha) = 570 up to 1450 keV with an experimental sensitivity of 10(-11) b. For all resonances in this energy range new resonance parameters have been measured. For a possible resonance at about 635 keV a new upper limit omega gamma < 60 neV for the strength was obtained. Based on the new data, improved reaction rates were calculated as a function of temperature. The new uncertainty limits are considerably smaller than in previous determinations, ruling out the large enhancement factors, up to 500, assumed in some stellar model calculations.

The excitation function of the reaction (22)Ne(alpha,n)(25)Mg, the key neutron source in the astrophysical s process in massive stars, has been determined from threshold at E(alpha) = 570 up to 1450 keV with an experimental sensitivity of 10(-11) b. For all resonances in this energy range new resonance parameters have been measured. For a possible resonance at about 635 keV a new upper limit omega gamma60 neV for the strength was obtained. Based on the new data, improved reaction rates were calculated as a function of temperature. The new uncertainty limits are considerably smaller than in previous determinations, ruling out the large enhancement factors, up to 500, assumed in some stellar model calculations.

We investigate the vortex phase diagram in untwinned YBaCuO single crystals with columnar defects. These randomly distributed defects, produced by heavy ion irradiation, are expected to induce a ``Bose Glass'' phase of localized vortices characterized by a vanishing resistance and a Meissner effect for magnetic fields transverse to the defect axis. We directly observe the transverse Meissner effect using an array of Hall probe magnetometers. As predicted, the Meissner state breaks down at temperatures Ts that decrease linearly with increasing transverse magnetic field. However, Ts falls well below the conventional melting temperature Tm determined by a vanishing resistivity, suggesting an intermediate regime where flux lines are effectively localized even when rotated off the columnar defects., Comment: 15 pages, 5 figures

Ultrathin films of Sn have been grown incrementally on substrates held at 15 K and then studied in situ at low temperatures. This process of repeatedly increasing the thickness of a film facilitates the investigation of thickness-dependent physical properties. The superconducting transition temperature has been found to oscillate with a period which was nominally about 4 \AA{}. The latter is approximately a factor of 2 larger than the prediction of simple theoretical models. Oscillations of the normal-state resistance with thickness have not been observed.

We have investigated the distribution of avalanches which occur when the slope of a sandpile exceeds the metastable slope given by the angle of repose. In contradiction to recent models by Bak, Tang, and Wiesenfeld for the dynamics of granular systems, we do not observe power-law distributions. We also find that the slope of a sandpile decays as $log(t)$ when vibrations are introduced. We propose a simple model for this effect.

Experimental evidence from studies of the onset of superconductivity in ultrathin Sn films is presented which implies that the sheet resistance is the only relevant variable in determining the onset of global phase coherence. This result is found by a theoretical argument involving both the phase-number and energy-time uncertainty relations.