Transverse Depinning Research Articles

Dynamical commensuration effect in a two-dimensional Yukawa solid modulated by periodic substrates

Transverse depinning dynamics of a periodic-square-substrate modulated two-dimensional dusty plasma solid driven by a constant force in the longitudinal direction are investigated using Langevin dynamical simulations. When the commensuration ratio (the number ratio of particles to substrate potential wells) is increased, there is a nonmonotonic variation of the critical transverse depinning force, and the local maxima and minima of the critical transverse depinning force precisely correspond to the dynamical commensurate and incommensurate conditions, respectively. The dynamical commensuration effect is also clearly visible in the stable one-dimensional channel particle trajectories and the highly ordered structure, while both the particle trajectories and the structure are more disordered under the incommensurate conditions. The nonmonotonic variation of the critical transverse depinning force is attributed to the stability of the lattice structure at specific commensuration ratios.

Pinning Effect on Current-Induced Domain Wall Motion in Nanostrip

AbstractPinning effect on current-induced magnetic transverse domain wall dynamics in nanostrip is studied for its potential application to new magnetic memory devices. In this study, we carry out a series of calculations by solving generalized Landau-Lifshitz equation involving a current spin transfer torque in one and two dimensional models. The critical current for the transverse wall depinning in nanostrip depends on the size of artificial rectangular defects on the edges of nanostrip. We show that there is intrinsic pinning potential for a defect such that the transverse wall oscillates damply around the pinning site with an intrinsic frequency if the applied current is below critical value. The amplification of the transverse wall oscillation for both displacement and maximum value of m3 is significant by applying AC current and current pulses with appropriate frequency. We show that for given pinning potential, the oscillation amplitude as a function of the frequency of the AC current behaves like a Gaussian distribution in our numerical study, which is helpful to reduce strength of current to drive the transverse wall motion.

In Situ Observation of Dynamic Wetting Transition in Re-Entrant Microstructures.

Re-entrant microstructures exhibit excellent wetting stability under different pressure levels, but the underlying mechanism determined by wetting transition behavior at the microscale level remains unclear. We propose the "wetting chip" method for in situ assessment of the dynamic behavior of wetting transition in re-entrant microstructures. High sag and transverse depinning were observed in re-entrant microstructures. Analysis indicated that high sag and transverse depinning mainly influenced the stability of the structures. The threshold pressure and longevity of wetting transition were predicted and experimentally verified. The design criteria of wetting stability, including small geometry design, hydrophobic material selection, and sidewall condition, were also presented. The proposed method and model can be applied to different shapes and geometry microstructures to elucidate wetting stability.

Nonlinear driven response of a phase-field crystal in a periodic pinning potential

We study numerically the phase diagram and the response under a driving force of the phase field crystal model for pinned lattice systems introduced recently for both one- and two-dimensional systems. The model describes the lattice system as a continuous density field in the presence of a periodic pinning potential, allowing for both elastic and plastic deformations of the lattice. We first present results for phase diagrams of the model in the absence of a driving force. The nonlinear response to a driving force on an initially pinned commensurate phase is then studied via overdamped dynamic equations of motion for different values of mismatch and pinning strengths. For large pinning strength the driven depinning transitions are continuous, and the sliding velocity varies with the force from the threshold with power-law exponents in agreement with analytical predictions. Transverse depinning transitions in the moving state are also found in two dimensions. Surprisingly, for sufficiently weak pinning potential we find a discontinuous depinning transition with hysteresis even in one dimension under overdamped dynamics. We also characterize structural changes of the system in some detail close to the depinning transition.

Transverse commensurability effect for vortices in periodic pinning arrays

Using computer simulations, we demonstrate a type of commensurability that occurs for vortices moving longitudinally through periodic pinning arrays in the presence of an additional transverse driving force. As a function of vortex density, there is a series of broad maxima in the transverse critical depinning force that do not fall at the matching fields where the number of vortices equals an integer multiple of the number of pinning sites. The commensurability effects are associated with dynamical states in which evenly spaced structures consisting of one or more moving rows of vortices form between rows of pinning sites. Remarkably, the critical transverse depinning force can be more than an order of magnitude larger than the longitudinal depinning force.

Transverse depinning in weakly pinned vortices driven by crossed ac and dc currents

We present experiments on weakly pinned vortices, which exhibit a large critical transverse depinning force. These results are obtained in the superconducting metallic glasses ${\text{Fe}}_{x}{\text{Ni}}_{1\ensuremath{-}x}{\text{Zr}}_{2}$ using crossed ac and dc driving currents. We study the vortex depinning force due to the transverse ac drive as a function of a longitudinal dc drive; the ac and dc combination permits the separation of the transverse drive from the longitudinal one. We show that the force required for depinning in the transverse direction is greatly enhanced by the longitudinal drive, which demonstrates the existence of a large transverse critical force. The measurements are performed as a function of magnetic field and temperature and show that the transverse critical force exists in a large portion of the phase diagram. Hysteresis observed at the transverse depinning threshold is consistent with a first-order transverse depinning transition.

Vortex-lattice dynamics with channeled pinning potential landscapes

We have studied vortex-lattice dynamics as a function of driving force direction, in superconducting Nb films with periodic pinning arrays of magnetic dots. Square and rectangular symmetry arrays define channels that guide the vortex-lattice motion. We investigated the effect of the driving force direction on the commensurability between the vortex-lattice and dots array. We also studied the transverse depinning of the vortex-lattice as is moving longitudinally along channels. We found that transverse depinning forces are enhanced with respect to the static situation. The results are discussed in terms of the dynamical evolution of order in the vortex-lattice.

Nonlinear sliding friction of adsorbed overlayers on disordered substrates

We study the response of an adsorbed monolayer on a disordered substrate under a driving force using Brownian molecular-dynamics simulation. We find that the sharp longitudinal and transverse depinning transitions with hysteresis still persist in the presence of weak disorder. However, the transitions are smeared out in the strong disorder limit. The theoretical results here provide a natural explanation for the recent data for the depinning transition of Kr films on gold substrate.

Transverse pinning and vortex displacement fluctuations of moving vortex lattices interacting with periodic pinning

The dynamical behavior of driven vortex lattices commensurate with periodic pinning in a clean superconducting film is investigated theoretically. A mean-field model, previously introduced by us [Phys. Rev. B 66 (2002) 054514] and called dynamical cage model, is applied here to study transverse pinning and vortex displacement fluctuations. The critical force for transverse depinning and the mean-square fluctuations induced by the temperature and vortex motion are calculated as functions of the vortex lattice center of mass velocity (or driving force) and of the temperature. The critical force is found to be in good qualitative agreement with numerical simulation results. The displacement fluctuations induced by vortex motion are found to be anisotropic, with the largest ones occurring along the direction of motion. Application of the Lindemann criterion to these fluctuations suggests a transition from the commensurate vortex lattice to a smectic vortex phase, also in agreement with numerical results.

Transverse depinning threshold revealed by non linear Hall effect of Wigner solid in a disorder field

Both electrons and holes at a GaAs/GaAlAs heterojunction show a non linear Hall effect as a function of current in the magnetically induced Wigner solid phase. Arguing that this arises from a threshold Lorentz force transverse to the current lines, this threshold is found to be proportional to the longitudinal depinning threshold and about 10% of its value. This is to be compared with theoretical predictions by Giamarchi and LeDoussal and with numerical simulation results (Moon, Scalettar and Zimanyi).

Transverse depinning of a driven elastic string in a disordered media

We examine the dynamics of an elastic string interacting with quenched disorder driven perpendicular and parallel to the string. We show that the string is the most disordered at the depinning transition but with increasing drive partial ordering is regained. For low drives the noise power is high and we observe a 1/f^2 noise signature crossing over to a white noise character with low power at higher drives. For the parallel driven moving string there is a finite transverse critical depinning force with the depinning transition occuring by the formation of running kinks.

Melting and transverse depinning of driven vortex lattices in the periodic pinning of Josephson junction arrays

We study the nonequilibrium dynamical regimes of a moving vortex lattice in the periodic pinning of a Josephson junction array (JJA) for finite temperatures in the case of a fractional or submatching field. We obtain a phase diagram for the current driven JJA as a function of the driving current I and temperature T. We find that when the vortex lattice is driven by a current, the depinning transition at ${T}_{p}(I)$ and the melting transition at ${T}_{M}(I)$ become separated even for a field for which they coincide in equilibrium. We also distinguish between the depinning of the vortex lattice in the direction of the current drive, and the transverse depinning in the direction perpendicular to the drive. The transverse depinning corresponds to the onset of transverse resistance in a moving vortex lattice at a given temperature ${T}_{\mathrm{tr}}.$ For driving currents above the critical current we find that the moving vortex lattice has first a transverse depinning transition at low T, and later a melting transition at a higher temperature ${T}_{M}>{T}_{\mathrm{tr}}.$

Transverse depinning in strongly driven vortex lattices with disorder

Using numerical simulations we investigate the transverse depinning of moving vortex lattices interacting with random disorder. We observe a finite transverse depinning barrier for vortex lattices that are driven with high longitudinal drives, when the vortex lattice is defect free and moving in correlated 1D channels. The transverse barrier is reduced as the longitudinal drive is decreased and defects appear in the vortex lattice, and the barrier disappears in the plastic flow regime. At the transverse depinning transition, the vortex lattice moves in a staircase pattern with a clear transverse narrow-band voltage noise signature.

Transverse Depinning and Melting of a Moving Vortex Lattice in Driven Periodic Josephson Junction Arrays

We study the effect of thermal fluctuations in a vortex lattice driven in the periodic pinning of a Josephson junction array. The phase diagram current ($I$) vs. temperature ($T$) is studied. Above the critical current $I_c(T)$ we find a moving vortex lattice (MVL) with anisotropic Bragg peaks. For large currents $I\gg I_c(T)$, there is a melting transition of the MVL at $T_M(I)$. When applying a small transverse current to the MVL, there is no dissipation at low $T$. We find an onset of transverse vortex motion at a transverse depinning temperature $T_{tr}(I)<T_M(I)$.