For granular materials falling through a circular exit at the bottom of a silo, no continuous flow can be sustained when the diameter D of the exit is less than 5 times the characteristic size of the grains. If the bottom of the silo rotates horizontally with respect to the wall of the silo, finite flow rate can be sustained even at small D. We investigate the effect of bottom rotation to the flow rate of a cylindrical silo filled with mono-disperse plastic beads of d = 6 mm diameter. We find that the flow rate W follows Beverloo Law down to D = 1.2d and that W increases with the rotation rate ω in the small exit regime. If the exit is at an off-center distance R from the axis of the silo, W increases with rate of area swept by the exit. On the other hand, when the exit diameter is large, W decreases with rotation speed at small ω but increases with ω at large ω. Such non-monotonic behavior of W on rotation speed may be explained as a gradual change from funnel flow to mass flow due to the shear at the bottom of the silo. arXiv:1902.00393v1 [cond-mat.soft]

The ZrSiS-type compounds are Dirac semimetals and thus have been attracting considerable interest in recent years due to their topological electronic properties and possible technological applications. In particular, gapped Dirac nodes can possess large spin Berry curvatures and thus give rise to large spin Hall effect (SHE) and spin Nernst effect (SNE), which may be used to generate pure spin current for spintronics and spin caloritronics without applied magnetic field or magnetic material. In this paper we study both SHE and SNE in ZrXY (X = Si, Ge; Y = S, Se, Te) based on ab initio relativistic band structure calculations. Our theoretical calculations reveal that some of these compounds exhibit large intrinsic spin Hall conductivity (SHC) and spin Nernst conductivity (SNC). For example, the calculated SHC of ZrSiTe is as large as -755 ( /e)(S/cm). Since the electric conductivity of these Dirac semimetals are much smaller than that of platinum which has the largest intrinsic SHC of ∼2200 ( /e)(S/cm), this indicates that they will have a larger spin Hall angle (and hence a more efficient charge-spin current conversion) than that of platinum. Remarkably, we find that both the magnitude and sign of the SHE and SNE in these compounds can be significantly tuned by changing either the electric field direction or spin current direction and may also be optimized by slightly varying the Fermi level via chemical doping. Analysis of the calculated band-and k-resolved spin Berry curvatures show that the large SHE and SNE as well as their remarkable tunabilities originate from the presence of many slightly spin-orbit coupling-gapped Dirac nodal lines near the Fermi level in these Dirac semimetals. Our findings thus suggest that the ZrSiS-type compounds are promising candidates for spintronic and spin caloritronic devices, and will certainly stimulate further experiments on these Dirac semimetals.

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