Kagome Arrays Research Articles

Investigation of the heat transfer enhancement and deterioration induced by vortex generators in low Prandtl number sodium-potassium alloy liquid

Liquid metals, being characterized by low molecular viscosity and high molecular thermal conductivity, appear to be a stupendous choice for heat transfer fluids in deep-space nuclear reactors. Nevertheless, because of space nuclear reactor weight and volume constraints, the heat transfer capability of the heat exchanger must be substantially improved for a deeper space exploration mission. In this study, the influence of the vortex generator applied to the liquid metal was investigated to further promote the heat transfer capability of the liquid metal. And the thermo-fluid behaviors of the sodium-potassium alloy (NaK-78) are compared among the channel with the Body-Centered Cubic (BCC) lattice array, the rectangular ring rib array, and the Kagome lattice array under identical porosity. The numerical results indicate that the heat transfer deterioration phenomena occurred in the case of rectangular ring rib channel, which is a relatively rare occurrence in conventional fluids with the local Nu number even being lower than that of the smooth channel at the same location. Therefore, the rectangular ring rib should be carefully considered and evaluated when it is applied to heat transfer enhancement in NaK alloy fluid. The Kagome structure and the BCC structure could effectively promote the heat transfer performance of the NaK alloy fluid by means of the induced composite vortex system. In comparison to the smooth channel, the average Nu of the cases with the Kagome array and the BCC array yield 17.26–37.32% and 51.98–78.85% higher, respectively.

Topological Edge and Corner States Designed via Meta‐Atoms Orientation

AbstractRapid development of topological concepts in photonics unveils exotic phenomena such as unidirectional propagation of electromagnetic waves resilient to backscattering at sharp bends and disorder‐immune localization of light at stable frequencies. Recently introduced higher‐order topological insulators (HOTIs) bring in additional degrees of control over light confinement and steering. However, designs of photonic HOTIs reported so far are solely exploiting lattice geometries which are hard to reconfigure thus limiting tunability. This article reports a conceptually new mechanism to engineer topological edge and corner states including higher‐order topological phases which exploits both electric and magnetic responses of the meta‐atoms. Hybridization between these responses gives rise to the difference in the effective coupling which is controlled by the meta‐atoms mutual orientations. This feature allows to tailor photonic band topology exclusively via particle alignment and flexibly reconfigure the topological phase. Focusing on the kagome array of split‐ring resonators, the topological edge and corner states are experimentally demonstrated in the microwave domain. To highlight the generality of this proposal, the formation of higher‐order topological phase is also predicted numerically in a C6‐symmetric lattice of split‐ring resonators. These findings provide a new promising route to induce and control higher‐order topological phases and states.

Topological dipole Floquet solitons

We theoretically introduce a new type of topological dipole solitons propagating in a Floquet topological insulator based on a kagome array of helical waveguides. Such solitons bifurcate from two edge states belonging to different topological gaps and have bright envelopes of different symmetries: fundamental for one component, and dipole for the other. The formation of dipole solitons is enabled by unique spectral features of the kagome array which allow the simultaneous coexistence of two topological edge states from different gaps at the same boundary. Notably, these states have equal and nearly vanishing group velocities as well as the same sign of the effective dispersion coefficients. We derive envelope equations describing components of dipole solitons and demonstrate in full continuous simulations that such states indeed can survive over hundreds of helix periods without any noticeable radiation into the bulk.

Comparing endwall heat transfer among staggered pin fin, Kagome and body centered cubic arrays

Truss-type lattices present good mechanical and thermal properties and are regarded as promising structures for internal cooling. This study presents a systematic comparison of thermo-fluid behaviors among staggered pin fin array, Kagome lattice array and Body Centered Cubic (BCC) lattice array in a rectangular channel under an identical porosity. Transient liquid crystals (TLC) and numerical methods were utilized to explore the flow and endwall heat transfer characteristics in structures. The results reveal that, due to stronger flow mixing and more irregular vortices, the endwall-averaged Nusselt number of the Kagome array and the BCC array is 17–24% and 26–41% higher than that of the pin fin array, respectively. The similar topology results in similar heat transfer characteristics between the Kagome array and the BCC array, and an arc-shaped high heat transfer wake region outside the vertex is induced by the horseshoe vortex and the secondary flow washing the endwall. Meanwhile, for the Kagome array, a strong counter rotating vortex is formed near the bottom endwall, while in contrast, there is no obvious similar flow behavior near the top endwall. This leads to a 46–54% difference in the Nusselt number between the bottom endwall and the top endwall of the Kagome. In addition, the relationship between the element-averaged Nusselt number and Reynolds number of each array is obtained to guide practical applications. This research indicates that BCC or Kagome lattices may become a potential alternative to conventional pin fins due to their higher thermal efficiency.

Nonlinear higher-order polariton topological insulator.

We address the resonant response and bistability of the exciton-polariton corner states in a higher-order nonlinear topological insulator realized with a kagome arrangement of microcavity pillars. Such states are resonantly excited and exist due to the balance between pump and losses, on one hand, and between nonlinearity and dispersion in inhomogeneous potential landscape, on the other hand, for pump energy around eigen-energies of corresponding linear localized modes. Localization of the nonlinear corner states in a higher-order topological insulator can be efficiently controlled by tuning pump energy. We link the mechanism of corner state formation with symmetry of the truncated kagome array. Corner states coexist with densely packed edge states but are well isolated from them in energy. Nonlinear corner states persist even in the presence of perturbations in a corner microcavity pillar.

Designing Kagome Lattice from Potassium Atoms on Phosphorus-Gold Surface Alloy.

Materials with flat bands are considered as ideal platforms to explore strongly correlated physics such as the fractional quantum hall effect, high-temperature superconductivity, and more. In theory, a Kagome lattice with only nearest-neighbor hopping can give rise to a flat band. However, the successful fabrication of Kagome lattices is still very limited. Here, we provide a new design principle to construct the Kagome lattice by trapping atoms into Kagome arrays of potential valleys, which can be realized on a potassium-decorated phosphorus-gold surface alloy. Theoretical calculations show that the flat band is less correlated with the neighboring trivial electronic bands, which can be further isolated and dominate around the Fermi energy with increased Kagome lattice parameters of potassium atoms. Our results provide a new strategy for constructing Kagome lattices, which serve as an ideal platform to study topological and more general flat band phenomena.

PT-symmetric phase in kagome-based photonic lattices

The kagome lattice is a two-dimensional network of corner-sharing triangles and is often associated with geometrical frustration. In particular, the frustrated coupling between waveguide modes in a kagome array leads to a dispersionless flat band consisting of spatially localized modes. Here we propose a complex photonic lattice by placing PT-symmetric dimers at the kagome lattice points. Each dimer corresponds to a pair of strongly coupled waveguides. With balanced arrangement of gain and loss on individual dimers, the system exhibits a PT-symmetric phase for finite gain/loss parameter up to a critical value. The beam evolution in this complex kagome waveguide array exhibits a novel oscillatory rotation of optical power along the propagation distance. Long-lived local chiral structures originating from the nearly flat bands of the kagome structure are observed when the lattice is subject to a narrow beam excitation.

Nonuniversality of artificial frustrated spin systems

Magnetic frustration effects in artificial kagome arrays of nanomagnets with out-of-plane magnetization are investigated using magnetic force microscopy and Monte Carlo simulations. Experimental and theoretical results are compared to those found for the artificial kagome spin ice in which the nanomagnets have in-plane magnetization. In contrast with what has been recently reported, we demonstrate that long-range (i.e., beyond nearest-neighbor) dipolar interactions between the nanomagnets cannot be neglected when describing the magnetic configurations observed after demagnetizing the arrays using a field protocol. As a consequence, there are clear limits to any universality in the behavior of these two artificial frustrated spin systems. We provide arguments to explain why these two systems show striking similarities at first sight in the development of pairwise spin correlations.

Uncontrolled disorder effects in fabricating photonic quantum simulators on a kagome geometry: A projected-entangled-pair-state versus exact-diagonalization analysis

We propose a flexible numerical framework for extracting the energy spectra and photon transfer dynamics of a unit kagome cell with disordered cavity-cavity couplings under realistic experimental conditions. A projected-entangled pair state (PEPS) ansatz to the many-photon wavefunction allows to gain a detailed understanding of the effects of undesirable disorder in fabricating well-controlled and scalable photonic quantum simulators. The correlation functions associated with the propagation of two-photon excitations reveal intriguing interference patterns peculiar to the kagome geometry and promise at the same time a highly tunable quantum interferometry device with a signature for the formation of resonant or Fabry-Pe\'rot-like transmission of photons. Our results justify the use of the proposed PEPS technique for addressing the role of disorder in such quantum simulators in the microwave regime and promises a sophisticated numerical machinery for yet further explorations of the scalability of the resulting kagome arrays. The introduced methodology and the physical results may also pave the way for unraveling exotic phases of correlated light on a kagome geometry.

Perpendicular Magnetization and Generic Realization of the Ising Model in Artificial Spin Ice

We have studied frustrated kagome arrays and unfrustrated honeycomb arrays of magnetostatically interacting single-domain ferromagnetic islands with magnetization normal to the plane. The measured pairwise spin correlations of both lattices can be reproduced by models based solely on nearest-neighbor correlations. The kagome array has qualitatively different magnetostatics but identical lattice topology to previously studied artificial spin ice systems composed of in-plane moments. The two systems show striking similarities in the development of moment pair correlations, demonstrating a universality in artificial spin ice behavior independent of specific realization in a particular material system.

Artificial Kagome Arrays of Nanomagnets: A Frozen Dipolar Spin Ice

Magnetic frustration effects in artificial kagome arrays of nanomagnets are investigated using x-ray photoemission electron microscopy and MonteCarlo simulations. Spin configurations of demagnetized networks reveal unambiguous signatures of long range, dipolar interaction between the nanomagnets. As soon as the system enters the spin ice manifold, the kagome dipolar spin ice model captures the observed physics, while the short range kagome spin ice model fails.

Vortex Dynamics in a Superconducting Film with a Kagome and a Honeycomb Pinning Landscape

The vortex dynamics in a superconducting thin Al film with a periodic Honeycomb or Kagome array of antidots has been investigated by electrical transport measurements. The large values of the superconducting coherence length and penetration depth of the Al films guarantee a maximum of one flux quantum trapped per pinning site. This allows us to directly compare the experimental results with previous theoretical investigations based on molecular dynamics simulations. For the Kagome lattice, two submatching features not anticipated theoretically at H/H1=1/3 and 2/3, where H1 is the field at which the number of vortices coincides with the number of pinning sites, are observed. Possible corresponding stable vortex patterns are suggested. For the Honeycomb pinning landscape, the commensurability effects are in agreement with the theoretical expectations. A preliminary analysis of the vortex mobility in this lattice shows the presence of a weak vortex guidance.

Modulated multipolar structures in magnetic arrays

Electric and magnetic multipole moments are important quantities in studies of intermolecular forces, and electrostatic and magnetostatic potentials. The experimental determination of multipole moments in multipole–multipole coupling is difficult and therefore the theoretical prediction of these quantities is important. The aim of the present review is to give a general theoretical description of multipolar ordering on two-dimensional periodic and aperiodic lattices. After an introduction to the role of multipolar interactions in magnetic nanoarrays in the first part of this article, the static multipole expansions in Cartesian and spherical coordinates are outlined. Next, the established numerical approach for the calculation of multipolar ground states, i.e. Monte Carlo simulations, are summarized. Special emphasis is put on the review of ground states in multipolar systems consisting of moments of odd parity relevant for the magnetized or polarized particle ensembles. We demonstrate that higher-order interactions considerably change the dipolar ground states of in-plane magnetized arrays. While in periodic triangular, square and kagome arrays the higher-order interactions induce three- or four-fold in-plane anisotropy, on a Penrose tiling with ten-fold symmetry the multipolar terms do not seriously affect the dipolar order.