42,806 research outputs found
Tuning the torque-speed characteristics of bacterial flagellar motor to enhance the swimming speed
In a classic paper, Edward Purcell analysed the dynamics of flagellated
bacterial swimmers and derived a geometrical relationship which optimizes the
propulsion efficiency. Experimental measurements for wild-type bacterial
species E. coli have revealed that they closely satisfy this geometric
optimality. However, the dependence of the flagellar motor speed on the load
and more generally the role of the torque-speed characteristics of the
flagellar motor is not considered in Purcell's original analysis. Here we
derive a tuned condition representing a match between the flagella geometry and
the torque-speed characteristics of the flagellar motor to maximize the
bacterial swimming speed for a given load. This condition is independent of the
geometric optimality condition derived by Purcell and interestingly this
condition is not satisfied by wild-type E. coli which swim 2-3 times slower
than the maximum possible speed given the amount of available motor torque. Our
analysis also reveals the existence of an anomalous propulsion regime, where
the swim speed increases with increasing load (drag). Finally, we present
experimental data which supports our analysis
Interface driven reentrant superconductivity in HoNi-NbN-HoNi nanostructures
Superconductivity (S) and ferromagnetism (F) are probed through transport and
magnetization measurements in nanometer scale HoNi-NbN (F-S) bilayers and
HoNi-NbN-HoNi (F-S-F) trilayers. The choice of materials has been made
on the basis of their comparable ordering temperatures and strong magnetic
anisotropy in HoNi. We observe the normal state reentrant behavior in
resistance vs. temperature plots of the F-S-F structures just below the
superconducting transition in the limited range of HoNi layer thickness
d (20 nm d 80 nm) when d is fixed at 10
nm. The reentrance is quenched by increasing the out-of-plane (H)
magnetic field and transport current where as in-plane (H) field
of 1500 Oe has no effect on the reentrance. The thermally activated flux
flow characteristics of the S, F-S and F-S-F layers reveal a transition from
collective pinning to single vortex pinning as we place F layers on both sides
of the S film. The origin of the reentrant behavior seen here in the range of
0.74 T/T 0.92 is attribute to a delicate balance
between the magnetic exchange energy and the condensation energy in the
interfacial regions of the trilayer.Comment: 13 pages and 5 figure
Feshbach resonances in ultracold 85Rb-87Rb and 6Li-87Rb mixtures
We present an analysis of experimentally accessible magnetic Feshbach
resonances in ultra-cold hetero-nuclear 85Rb-87Rb and 6Li-87Rb mixtures. Using
recent experimental measurements of the triplet scattering lengths for 6Li-87Rb
and 7Li-87Rb mixtures and Feshbach resonances for one combination of atomic
states, we create model potential curves and fine tune them to reproduce the
measured resonances and to predict the location of several experimentally
relevant resonances in Li-Rb collisions. To model 85Rb-87Rb collisions, we use
accurate Rb_2 potentials obtained previously from the analysis of experiments
on 87Rb-87Rb collisions. We find resonances that occur at very low magnetic
fields, below 10 G, which may be useful for entanglement generation in optical
lattices or atom chip magnetic traps.Comment: 8 pages, 5 figure
The second phase transition in the pyrochlore oxide Cd2Re2O7
Evidence for another phase transition at 120 K in the metallic pyrochlore
oxide Cd2Re2O7, following the structural transition at 200 K and followed by
the superconducting transition at 1.0 K, is given through resistivity,
magnetoresistance, specific heat, and X-ray diffraction measurements. The
results indicate unique successive structural and electronic transitions
occurring in the pyrochlore compound, revealing an interesting interplay
between the crystal and electronic structures on the itinerant electron system
in the pyrochlore lattice
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