418 research outputs found
Time-Delay Polaritonics
Non-linearity and finite signal propagation speeds are omnipresent in nature,
technologies, and real-world problems, where efficient ways of describing and
predicting the effects of these elements are in high demand. Advances in
engineering condensed matter systems, such as lattices of trapped condensates,
have enabled studies on non-linear effects in many-body systems where exchange
of particles between lattice nodes is effectively instantaneous. Here, we
demonstrate a regime of macroscopic matter-wave systems, in which ballistically
expanding condensates of microcavity exciton-polaritons act as picosecond,
microscale non-linear oscillators subject to time-delayed interaction. The ease
of optical control and readout of polariton condensates enables us to explore
the phase space of two interacting condensates up to macroscopic distances
highlighting its potential in extended configurations. We demonstrate
deterministic tuning of the coupled-condensate system between fixed point and
limit cycle regimes, which is fully reproduced by time-delayed coupled
equations of motion similar to the Lang-Kobayashi equation
Directional planar antennae in polariton condensates
We report on the realization of all-optical planar microlensing for
exciton-polariton condensates in semiconductor microcavities. We utilize
spatial light modulators to structure a nonresonant pumping beam into a
planoconcave lens-shape focused onto the microcavity plane. When pumped above
condensation threshold, the system effectively becomes a directional polariton
antenna, generating an intense focused beam of coherent polaritons away from
the pump region. The effects of pump intensity, which regulates the interplay
between gain and blueshift of polaritons, as well as the geometry of
lens-shaped pump are studied and a strategy to optimize the focusing of the
condensate is proposed. Our work underpins the feasibility to guide nonlinear
light in microcavities using nonresonant excitation schemes, offering
perspectives on optically reprogrammable on-chip polariton circuitry
Realizing the classical XY Hamiltonian in polariton simulators.
The vast majority of real-life optimization problems with a large number of degrees of freedom are intractable by classical computers, since their complexity grows exponentially fast with the number of variables. Many of these problems can be mapped into classical spin models, such as the Ising, the XY or the Heisenberg models, so that optimization problems are reduced to finding the global minimum of spin models. Here, we propose and investigate the potential of polariton graphs as an efficient analogue simulator for finding the global minimum of the XY model. By imprinting polariton condensate lattices of bespoke geometries we show that we can engineer various coupling strengths between the lattice sites and read out the result of the global minimization through the relative phases. Besides solving optimization problems, polariton graphs can simulate a large variety of systems undergoing the U(1) symmetry-breaking transition. We realize various magnetic phases, such as ferromagnetic, anti-ferromagnetic, and frustrated spin configurations on a linear chain, the unit cells of square and triangular lattices, a disordered graph, and demonstrate the potential for size scalability on an extended square lattice of 45 coherently coupled polariton condensates. Our results provide a route to study unconventional superfluids, spin liquids, Berezinskii-Kosterlitz-Thouless phase transition, and classical magnetism, among the many systems that are described by the XY Hamiltonian
Geometric frustration in polygons of polariton condensates creating vortices of varying topological charge.
Vorticity is a key ingredient to a broad variety of fluid phenomena, and its quantised version is considered to be the hallmark of superfluidity. Circulating flows that correspond to vortices of a large topological charge, termed giant vortices, are notoriously difficult to realise and even when externally imprinted, they are unstable, breaking into many vortices of a single charge. In spite of many theoretical proposals on the formation and stabilisation of giant vortices in ultra-cold atomic Bose-Einstein condensates and other superfluid systems, their experimental realisation remains elusive. Polariton condensates stand out from other superfluid systems due to their particularly strong interparticle interactions combined with their non-equilibrium nature, and as such provide an alternative testbed for the study of vortices. Here, we non-resonantly excite an odd number of polariton condensates at the vertices of a regular polygon and we observe the formation of a stable discrete vortex state with a large topological charge as a consequence of antibonding frustration between nearest neighbouring condensates
Percolative phase separation induced by nonuniformly distributed excess oxygens
The zero-field La and Mn nuclear magnetic resonances were
studied in with different oxygen
stoichiometry . The signal intensity, peak frequency and line
broadening of the La NMR spectrum show that excess oxygens have a
tendency to concentrate and establish local ferromagnetic ordering around
themselves. These connect the previously existed ferromagnetic clusters
embedded in the antiferromagnetic host, resulting in percolative conduction
paths. This phase separation is not a charge segregation type, but a
electroneutral type. The magnetoresistance peak at the temperature where
percolative paths start to form provides a direct evidence that phase
separation is one source of colossal magnetoresistance effect.Comment: 4 pages, 5 figure
Inferring the Scale of OpenStreetMap Features
International audienceTraditionally, national mapping agencies produced datasets and map products for a low number of specified and internally consistent scales, i.e. at a common level of detail (LoD). With the advent of projects like OpenStreetMap, data users are increasingly confronted with the task of dealing with heterogeneously detailed and scaled geodata. Knowing the scale of geodata is very important for mapping processes such as for generalization of label placement or land-cover studies for instance. In the following chapter, we review and compare two concurrent approaches at automatically assigning scale to OSM objects. The first approach is based on a multi-criteria decision making model, with a rationalist approach for defining and parameterizing the respective criteria, yielding five broad LoD classes. The second approach attempts to identify a single metric from an analysis process, which is then used to interpolate a scale equivalence. Both approaches are combined and tested against well-known Corine data, resulting in an improvement of the scale inference process. The chapter closes with a presentation of the most pressing open problem
Shaping potential landscape for organic polariton condensates in double-dye cavities
We investigate active spatial control of polariton condensates independently
of the polariton-, gain-inducing excitation profile. This is achieved by
introducing an extra intracavity semiconductor layer, non-resonant to the
cavity mode. Saturation of the optical absorption in the uncoupled layer
enables the ultra-fast modulation of the effective refractive index and,
through excited-state absorption, the polariton dissipation. Utilising these
mechanisms, we demonstrate control over the spatial profile and density of a
polariton condensate at room temperature
Zero Order Estimates for Analytic Functions
The primary goal of this paper is to provide a general multiplicity estimate.
Our main theorem allows to reduce a proof of multiplicity lemma to the study of
ideals stable under some appropriate transformation of a polynomial ring. In
particular, this result leads to a new link between the theory of polarized
algebraic dynamical systems and transcendental number theory. On the other
hand, it allows to establish an improvement of Nesterenko's conditional result
on solutions of systems of differential equations. We also deduce, under some
condition on stable varieties, the optimal multiplicity estimate in the case of
generalized Mahler's functional equations, previously studied by Mahler,
Nishioka, Topfer and others. Further, analyzing stable ideals we prove the
unconditional optimal result in the case of linear functional systems of
generalized Mahler's type. The latter result generalizes a famous theorem of
Nishioka (1986) previously conjectured by Mahler (1969), and simultaneously it
gives a counterpart in the case of functional systems for an important
unconditional result of Nesterenko (1977) concerning linear differential
systems. In summary, we provide a new universal tool for transcendental number
theory, applicable with fields of any characteristic. It opens the way to new
results on algebraic independence, as shown in Zorin (2010).Comment: 42 page
- …