Active elastic dimers: Cells moving on rigid tracks

Experiments suggest that the migration of some cells in the three-dimensional extra cellular matrix bears strong resemblance to one-dimensional cell migration. Motivated by this observation, we construct and study a minimal one-dimensional model cell made of two beads and an active spring moving along a rigid track. The active spring models the stress fibers with their myosin-driven contractility and alpha-actinin-driven extendability, while the friction coefficients of the two beads describe the catch/slip bond behavior of the integrins in focal adhesions. In the absence of active noise, net motion arises from an interplay between active contractility (and passive extendability) of the stress fibers and an asymmetry between the front and back of the cell due to catch bond behavior of integrins at the front of the cell and slip bond behavior of integrins at the back. We obtain reasonable cell speeds with independently estimated parameters. We also study the effects of hysteresis in the active spring, due to catch bond behavior and the dynamics of cross-linking, and the addition of active noise on the motion of the cell. Our model highlights the role of alpha-actinin in three-dimensional cell motility and does not require Arp2/3 actin filament nucleation for net motion.Comment: 13 pages, 9 figure

Influence of Domain Wall on Magnetocaloric Effect in GdPt2_{2}

The resistivity, magnetoresistance and in-field heat capacity measurements were performed on GdPt$_{2}$ intermetallic compound. The magnetocaloric parameters $\Delta T_{ad}$ and $-\Delta S$ were derived from the in-field heat capacity data. Comparison has been made between the magnetocaloric effect $-\Delta S$ and difference in resistivity $-\Delta \rho$ $(=\rho(H)-\rho(0))$ as a function of temperature. There is distinct difference in the temperature dependence of $-\Delta S$ and $-\Delta \rho$ below the ferromagnetic transition temperature. However after removing the domain wall contribution from $-\Delta \rho$, the nature of $-\Delta S$ and $-\Delta \rho$ dependence as a function of temperature are similar. Our observation indicates that the domain wall contribution in magnetocaloric effect is negligible in spite of the fact that it has significant contribution in magnetotransport.Comment: RevTex 4 pages, 6 figure

Driven Heisenberg Magnets: Nonequilibrium Criticality, Spatiotemporal Chaos and Control

We drive a $d$-dimensional Heisenberg magnet using an anisotropic current. The continuum Langevin equation is analysed using a dynamical renormalization group and numerical simulations. We discover a rich steady-state phase diagram, including a critical point in a new nonequilibrium universality class, and a spatiotemporally chaotic phase. The latter may be `controlled' in a robust manner to target spatially periodic steady states with helical order.Comment: 7 pages, 2 figures. Published in Euro. Phys. Let