Paramagnetic colloidal particles move in the potential energy landscape of a magnetically modulated bubble lattice of a magnetic garnet film. The modulation causes the energy minima to alternate between positions above the centres of the bubbles and interstitial positions. The particles deterministically follow the time-dependent positions of the energy minima until the minima become unstable in one or several directions and allow the particles to hop to a new minimum. We control the time delay between instabilities of the minima in alternative directions by the angle of the external magnetic field with the crystallographic directions of the bubble lattice. When the time delay is large, the particles deterministically hop to the next minimum along the direction that becomes unstable first. When the time delay is short, diffusion of the particle in the marginal potential randomizes the choice of the hopping directions or the choice of the transport network. Gradual changes of the external field direction from 0 • to 30 • lead to a continuous crossover from a deterministic to a fully stochastic path of the colloids.
An optimal track format for a holographic read-only-memory (ROM) disk is proposed that minimizes the intertrack crosstalk and stores conventional optical disk pit patterns with a density equivalent to that of digital versatile discs (DVDs) in a single holographic layer. To quantify the effect of intertrack crosstalk for the cases of variations in track width and pitch, we defined signal-to-crosstalk ratio (SCR) as a criterion to find the optimal track format. A numerical simulation is used to calculate SCR as a function of track width and pitch. The simulation results show that SCR varies with track width, pitch and pit length. When the track width, pitch, and minimum pit length are 0.5, 0.74, and 0.4 m, respectively, we can set a minimum pit length with a maximum SCR so that the storage density of each layer of the holographic ROM is equivalent to that of DVDs.
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