Square-shaped Lattices Research Articles

Discrete solitons and vortices on two-dimensional lattices of PT-symmetric couplers.

We introduce a 2D network built of PT-symmetric dimers with on-site cubic nonlinearity, the gain and loss elements of the dimers being linked by parallel square-shaped lattices. The system may be realized as a set of PT-symmetric dual-core waveguides embedded into a photonic crystal. The system supports PT-symmetric and antisymmetric fundamental solitons (FSs) and on-site-centered solitary vortices (OnVs). Stability of these discrete solitons is the central topic of the consideration. Their stability regions in the underlying parameter space are identified through the computation of stability eigenvalues, and verified by direct simulations. Symmetric FSs represent the system's ground state, being stable at lowest values of the power, while anti-symmetric FSs and OnVs are stable at higher powers. Symmetric OnVs, which are also stable at lower powers, are remarkably robust modes: on the contrary to other PT-symmetric states, unstable OnVs do not blow up, but spontaneously rebuild themselves into stable FSs.

Interplay between elastic interactions in a core-shell model for spin-crossover nanoparticles

A coupled spin deformation model for spin-crossover (SC) materials, consisting of distortable 2D square-shaped lattices, whose sites may be occupied by high-spin $(\mathrm{HS})$ or low-spin $(\mathrm{LS})$ atoms, is studied by Monte Carlo simulations. To be consistent with the experimental studies, we have studied shell-free and core-shell nanoparticles. In the case of shell-free nanoparticles, we constrained the surface of the nanoparticle (one layer) to be in the $\mathrm{HS}$ state from the electronic and the elastic point of view because the surface atoms have weaker ligand-field energy than those of the bulk. We then investigated the size effects and found that the thermal hysteresis width $\ensuremath{\Delta}T$ and the transition temperature ${T}_{\mathrm{eq}}$ follow the respective universal laws, $\ensuremath{\Delta}T\ensuremath{\propto}\sqrt{(L\ensuremath{-}{L}_{0})}$ and ${T}_{\mathrm{eq}}\ensuremath{\propto}\sqrt{\mathrm{ln}(L\ensuremath{-}{L}_{c})}$, where $L$ is the nanoparticle size. These laws hold independently of the sweeping rate of temperature. In a second stage, we studied the effect of a soft shell on the thermal properties of the core shell nanoparticle for which we have investigated the thermodynamic properties at various sizes of the shell. We find that the thermal hysteresis shifts downwards and the corresponding width increases; a result that contrasts with that of shell-free nanoparticles. In addition, we have observed that large shell size widths hinder the domain formation upon the first-order transition, although the transition is still of first order. These behaviors originate from the elastic stress produced by the shell on the bulk of the nanoparticle, and are identified through the spatial distribution of the internal stress upon the thermal transition. Moreover, we studied the effect of the shell size on the relaxation of the photoinduced metastable HS fraction at low temperature. At this end, a preliminary optimization of the structure of the nanoparticle is performed. We then evidenced that increasing the size of the shell results in an acceleration of the relaxation process. This behavior is in excellent agreement with recent data reported on core shell nanoparticles of Prussian Blue analogs.

COMPACT SLOT ANTENNA WITH EBG FEEDING LINE FOR WLAN APPLICATIONS

A compact CPW-fed slot antenna is proposed for dual- band wireless local area network (WLAN) operations. In this paper, electromagnetic band gap (EBG) structures with square-shaped lattices have been incorporated into the feed network for harmonic suppression. Experimental results show that EBG structures not only exhibit well-behaved band stop characteristics, but also enhance the bandwidth of the proposed antennas. For the proposed antenna with square-shaped lattices, the i10dB return loss bandwidth could reach about 38.4% for the 2.4GHz band and 23.8% for the 5GHz band, which meet the required bandwidth speciflcation of 2.4/5GHz WLAN standard.

A broadband CPW-fed loop slot antenna with harmonic control

Photonic bandgap (PBG) structures with cross-shaped or square-shaped lattices have been incorporated into the antenna feed network for harmonic suppression. Experimental results show that PBG structures not only exhibit well-behaved bandstop characteristics, but also enhance the bandwidth of the proposed antennas. For the proposed antenna with square-shaped lattices, the 10 dB return loss bandwidth could reach 1541 MHz (1525-3066 MHz), which is about 70% with respect to the center frequency of 2200 MHz; for the antenna with cross-shaped lattices, the bandwidth could reach 1320 MHz (1560-2880 MHz), which is about 60% with respect to the center frequency of 2200 MHz.