34 research outputs found
Vortex configurations and dynamics in elliptical pinning sites for high matching fields
Using numerical simulations we study the configurations, dynamics, and
melting properties of vortex lattices interacting with elliptical pinning sites
at integer matching fields with as many as 27 vortices per pin. Our pinning
model is based on a recently produced experimental system [G. Karapetrov et
al., Phys. Rev. Lett. 95, 167002 (2005)], and the vortex configurations we
obtain match well with experimental vortex images from the same system. We find
that the strong pinning sites capture more than one vortex each, and that the
saturation number of vortices residing in a pin increases with applied field
due to the pressure from the surrounding vortices. At high matching fields, the
vortices in the intestitial regions form a disordered triangular lattice. We
measure the depinning thresholds for both the x and y directions, and find
distinctive dynamical responses along with highly anisotropic thresholds. For
melting of the vortex configurations under zero applied current, we find
multi-step melting transitions in which the interstitial vortices melt at a
much lower temperature than the pinned vortices. We associate this with
signatures in the specific heat.Comment: 11 pages, 13 postscript figure
Dynamics, Rectification, and Fractionalization for Colloids on Flashing Substrates
We show that a rich variety of dynamic phases can be realized for mono- and
bidisperse mixtures of interacting colloids under the influence of a symmetric
flashing periodic substrate. With the addition of dc or ac drives, phase
locking, jamming, and new types of ratchet effects occur. In some regimes we
find that the addition of a non-ratcheting species increases the velocity of
the ratcheting particles. We show that these effects occur due to the
collective interactions of the colloids.Comment: 4 pages, 4 postscript figures. Version to appear in Phys. Rev. Let
Point Defect Dynamics in Two-Dimensional Colloidal Crystals
We study the topological configurations and dynamics of individual point
defect vacancies and interstitials in a two-dimensional colloidal crystal. Our
Brownian dynamics simulations show that the diffusion mechanism for vacancy
defects occurs in two phases. The defect can glide along the crystal lattice
directions, and it can rotate during an excited topological transition
configuration to assume a different direction for the next period of gliding.
The results for the vacancy defects are in good agreement with recent
experiments. For the interstitial point defects, which were not studied in the
experiments, we find several of the same modes of motion as in the vacancy
defect case along with two additional diffusion pathways. The interstitial
defects are more mobile than the vacancy defects due to the more
two-dimensional nature of the diffusion of the interstitial defects.Comment: 8 pages, 9 postscript figures. Version to appear in Phys. Rev.
Guided nucleation of superconductivity on a graded magnetic substrate
We demonstrate the controlled spatial nucleation of superconductivity in a
thin film deposited on periodic arrays of ferromagnetic dots with gradually
increasing diameter. The perpendicular magnetization of the dots induces
vortex-antivortex molecules in the sample, with the number of (anti)vortices
increasing with magnet size. The resulting gradient of antivortex density
between the dots predetermines local nucleation of superconductivity in the
sample as a function of the applied external field and temperature. In
addition, the compensation between the applied magnetic field and the
antivortices results in an unprecedented enhancement of the critical
temperature
Realizing Colloidal Artificial Ice on Arrays of Optical Traps
We demonstrate how a colloidal version of artificial ice can be realized on
optical trap lattices. Using numerical simulations, we show that this system
obeys the ice rules and that for strong colloid-colloid interactions, an
ordered ground state appears. We show that the ice rule ordering can occur for
systems with as few as twenty-four traps and that the ordering transition can
be observed at constant temperature by varying the barrier strength of the
traps.Comment: 4 pages, 3 postscript figures; version to appear in Phys. Rev. Let
Zero- and one-dimensional magnetic traps for quasi-particles
We investigate the possibility of trapping quasi-particles possessing spin
degree of freedom in hybrid structures. The hybrid system we are considering
here is composed of a semi-magnetic quantum well placed a few nanometers below
a ferromagnetic micromagnet. We are interested in two different micromagnet
shapes: cylindrical (micro-disk) and rectangular geometry. We show that in the
case of a micro-disk, the spin object is localized in all three directions and
therefore zero-dimensional states are created, and in the case of an elongated
rectangular micromagnet, the quasi-particles can move freely in one direction,
hence one-dimensional states are formed. After calculating profiles of the
magnetic field produced by the micromagnets, we analyze in detail the possible
light absorption spectrum for different micromagnet thicknesses, and different
distances between the micromagnet and the semimagnetic quantum well. We find
that the discrete spectrum of the localized states can be detected via
spatially-resolved low temperature optical measurement.Comment: 15 pages, 9 figure
A network model for field and quenched disorder effects in artificial spin ice
We have performed a systematic study of the effects of field strength and
quenched disorder on the driven dynamics of square artificial spin ice. We
construct a network representation of the configurational phase space, where
nodes represent the microscopic configurations and a directed link between node
i and node j means that the field may induce a transition between the
corresponding configurations. In this way, we are able to quantitatively
describe how the field and the disorder affect the connectedness of states and
the reversibility of dynamics. In particular, we have shown that for optimal
field strengths, a substantial fraction of all states can be accessed using
external driving fields, and this fraction is increased by disorder. We discuss
how this relates to control and potential information storage applications for
artificial spin ices
Topology by Design in Magnetic nano-Materials: Artificial Spin Ice
Artificial Spin Ices are two dimensional arrays of magnetic, interacting
nano-structures whose geometry can be chosen at will, and whose elementary
degrees of freedom can be characterized directly. They were introduced at first
to study frustration in a controllable setting, to mimic the behavior of spin
ice rare earth pyrochlores, but at more useful temperature and field ranges and
with direct characterization, and to provide practical implementation to
celebrated, exactly solvable models of statistical mechanics previously devised
to gain an understanding of degenerate ensembles with residual entropy. With
the evolution of nano--fabrication and of experimental protocols it is now
possible to characterize the material in real-time, real-space, and to realize
virtually any geometry, for direct control over the collective dynamics. This
has recently opened a path toward the deliberate design of novel, exotic
states, not found in natural materials, and often characterized by topological
properties. Without any pretense of exhaustiveness, we will provide an
introduction to the material, the early works, and then, by reporting on more
recent results, we will proceed to describe the new direction, which includes
the design of desired topological states and their implications to kinetics.Comment: 29 pages, 13 figures, 116 references, Book Chapte
Multi-Step Ordering in Kagome and Square Artificial Spin Ice
We show that in colloidal models of artificial kagome and modified square ice
systems, a variety of ordering and disordering regimes occur as a function of
biasing field, temperature, and colloid-colloid interaction strength, including
ordered monopole crystals, biased ice rule states, thermally induced ice rule
ground states, biased triple states, and disordered states. We describe the
lattice geometries and biasing field protocols that create the different states
and explain the formation of the states in terms of sublattice switching
thresholds. For a system prepared in a monopole lattice state, we show that a
sequence of different orderings occurs for increasing temperature. Our results
also explain several features observed in nanomagnetic artificial ice systems
under an applied field.Comment: 16 pages, 11 postscript figure
Thermodynamics of elementary excitations in artificial magnetic square ice
We investigate the thermodynamics of artificial square spin ice systems
assuming only dipolar interactions among the islands that compose the array.
The emphasis is given on the effects of the temperature on the elementary
excitations (magnetic monopoles and their Dirac strings). By using Monte Carlo
techniques we calculate the specific heat, the density of poles and their
average separation as functions of temperature. The specific heat and average
separation between monopoles and antimonopoles exhibit a sharp peak and a local
maximum, respectively, at the same temperature,
(here, is the strength of the dipolar interaction and is the
Boltzmann constant). As the lattice size is increased, the amplitude of these
features also increases but very slowly. Really, the specific heat and the
maximum in the average separation between oppositely charged
monopoles increase logarithmically with the system size, indicating that
completely isolated charges could be found only at the thermodynamic limit. In
general, the results obtained here suggest that, for temperatures , these systems may exhibit a phase with separated monopoles, although
the quantity should not be larger than a few lattice spacings for
viable artificial materials.Comment: 7 pages, 9 figure