Vortex ratchet reversal at fractional matching fields in kagomélike array with symmetric pinning centers

Abstract

Arrays of Ni nanodots embedded in Nb superconducting films have been fabricated by sputtering and electron-beam lithography techniques. The arrays are periodic triangular lattices of circular Ni dots arranged in a kagomélike pattern with broken reflection symmetry. Relevant behaviors are found in the vortex lattice dynamics: ͑i͒ at values lower than the first integer matching field, several fractional matching fields are present when the vortex lattice moves parallel or perpendicular to the reflection symmetry axis of the array showing a clear anisotropic character in the magnetoresistance curves, ͑ii͒ injecting an ac perpendicular to the reflection symmetry axis of the array yields an unidirectional motion of the vortex lattice ͑ratchet effect͒ as a result of the interaction between the whole vortex lattice and the asymmetric lattice of dots, ͑iii͒ increasing the input current amplitudes the ratchet effect changes polarity independently of matching field values. These experimental results can be explained taking into account the vortex lattice density.

Vortex ratchet reversal: Role of interstitial vortices

“…15,16 Figure 2 shows the magnetoresistance with the current applied parallel and perpendicular to the symmetry axis of the triangles, corresponding to vortex-lattice motion perpendicular and parallel to the triangle symmetry axis, respectively. In both cases resistance minima appear at H = 36 Oe (see inset, Fig.…”

“…To the best of our knowledge, such an unusual phenomenon in a simple electronic system has been reported only once. 16 The dc rectified amplitude develops in a very systematic way as a function of temperature and field in this kind of ratchet, denoted as a "rocking" ratchet. Figure 4 shows the evolution of the ratchet reversal effect with various parameters that characterize it: minimum negative voltage (red down triangles) and maximum positive voltage (black up triangles) highlighted by arrows in the inset of Fig.…”

Experimental realization of smectic phase in vortex matter induced by symmetric potentials arranged in two-fold symmetry arrays

“…Arrays of non-superconducting centers embedded in the superconducting films are a suitable way to accomplish this aim. Many researchers have studied vortices in superconducting films with artificially periodic pinning centers [28][29][30][31][32][33]. Superconducting films with periodic pinning nanocenter arrays show noteworthy effects when matching between the vortex lattice and the array unit cell occurs.…”

Electrical transport and pinning properties of Nb thin films patterned with focused ion beam-milled washboard nanostructures

“…Various techniques have been used to provide different pinning sites in the form of dots [26,27], (blind) antidots [22][23][24][25], stripes [28] and more complex nonmagnetic [16][17][18][19] and magnetic [29][30][31] (nano)structures. All these approaches share the problem that a full and exact theoretical description of the two-dimensional nonlinear vortex dynamics in these systems has not been available so far.…”