973 research outputs found
Berezinskii-Kosterlitz-Thouless transition in two-dimensional dipolar stripes
A two-dimensional quantum system of dipoles, with a polarization angle not perpendicular to the plane, shows a transition from a gas to a stripe phase. We have studied the thermal properties of these two phases using the path-integral Monte Carlo (PIMC) method. By simulating the thermal density matrix, PIMC provides exact results for magnitudes of interest such as the superfluid fraction and the one-body density matrix. As it is well known, in two dimensions the superfluid-to-normal phase transition follows the Berezinskii-Kosterlitz-Thouless (BKT) scenario. Our results show that both the anisotropic gas and the stripe phases follow the BKT scaling laws. At fixed density and increasing the tilting angle, the transition temperature decreases in going from the gas to the stripe phase. Superfluidity in the perpendicular direction to the stripes is rather small close to the critical temperature but it becomes larger at lower temperatures, mainly close to the transition to the gas. Our results are in qualitative agreement with the supersolidity observed recently in a quasi-one-dimensional array of dipolar droplets.Postprint (published version
Anisotropic Sliding Dynamics, Peak Effect, and Metastability in Stripe Systems
A variety of soft and hard condensed matter systems are known to form stripe
patterns. Here we use numerical simulations to analyze how such stripe states
depin and slide when interacting with a random substrate and with driving in
different directions with respect to the orientation of the stripes. Depending
on the strength and density of the substrate disorder, we find that there can
be pronounced anisotropy in the transport produced by different dynamical flow
phases. We also find a disorder-induced "peak effect" similar to that observed
for superconducting vortex systems, which is marked by a transition from
elastic depinning to a state where the stripe structure fragments or partially
disorders at depinning. Under the sudden application of a driving force, we
observe pronounced metastability effects similar to those found near the
order-disorder transition associated with the peak effect regime for
three-dimensional superconducting vortices. The characteristic transient time
required for the system to reach a steady state diverges in the region where
the flow changes from elastic to disordered. We also find that anisotropy of
the flow persists in the presence of thermal disorder when thermally-induced
particle hopping along the stripes dominates. The thermal effects can wash out
the effects of the quenched disorder, leading to a thermally-induced stripe
state. We map out the dynamical phase diagram for this system, and discuss how
our results could be explored in electron liquid crystal systems, type-1.5
superconductors, and pattern-forming colloidal assemblies.Comment: 18 pages, 22 postscript figure
Statistical properties of charged interfaces
We consider the equilibrium statistical properties of interfaces submitted to
competing interactions; a long-range repulsive Coulomb interaction inherent to
the charged interface and a short-range, anisotropic, attractive one due to
either elasticity or confinement. We focus on one-dimensional interfaces such
as strings. Model systems considered for applications are mainly aggregates of
solitons in polyacetylene and other charge density wave systems, domain lines
in uniaxial ferroelectrics and the stripe phase of oxides. At zero temperature,
we find a shape instability which lead, via phase transitions, to tilted
phases. Depending on the regime, elastic or confinement, the order of the
zero-temperature transition changes. Thermal fluctuations lead to a pure
Coulomb roughening of the string, in addition to the usual one, and to the
presence of angular kinks. We suggest that such instabilities might explain the
tilting of stripes in cuprate oxides. The 3D problem of the charged wall is
also analyzed. The latter experiences instabilities towards various tilted
phases separated by a tricritical point in the elastic regime. In the
confinement regime, the increase of dimensionality favors either the melting of
the wall into a Wigner crystal of its constituent charges or a strongly
inclined wall which might have been observed in nickelate oxides.Comment: 17 pages, 11 figure
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