Thermodynamic properties of the Dicke model in the strong-coupling regime

We discuss the problem of a N two-level systems interacting with a single radiation mode in the strong-coupling regime. The thermodynamic properties of Dicke model are analyzed developing a perturbative expansion of the partition function in the high-temperature limit and we use this method to investigate the connections between the Dicke and the collective one-dimensional Ising modelComment: 7 pages, accepted for publication in EPJ

Wakefield excitation via metasurface-loaded waveguide

A metallic waveguide loaded with layers of complementary split ring resonator (CSRR) based metasurface is presented for accelerator and coherent source applications. This structure presents left handed behaviour arising from the strong electrical response of CSRRs which form the metasurface and the transverse field confined between the closely positioned metasurface layers. The loaded waveguide structure is known to have a TM-like mode at 5.47GHz suitable for acceleration. In this paper, the results of wakefield simulations are presented and a narrow band excitation identified around the frequency of the TM-like mode, indicating strong coupling between the beam and the field of this mode

Design considerations on complementary split ring resonator-loaded waveguides for Wakefield generation

In this paper, we present design considerations of metamaterial (MTM)-based structures to address practical issues relating to operation in particle accelerators and at high power in general. To the best of our knowledge, this is the first time these problems have been addressed. We focus on the structure proposed in our earlier paper based on a complementary split ring resonator (CSRR)-loaded waveguide for applications in accelerator science, as a backward propagating Cherenkov detector. Through the modification of the metasurface thickness, CSRR ring width and ring curvature, a number of possible waveguides are investigated to increase structural integrity and robustness, maintain the dispersion relation, and reduce the number of hybrid modes. Accurate numerical simulations are performed to compare the different setups with the original structure. Both the longitudinal and transverse wakefield excitations are analyzed to investigate how these modifications affect beam coupling. A suitable design is successfully identified and offers a promising opportunity for the development of MTMs for high-power beam-based applications

Electron beam excitation of CSRR loaded waveguide for Cherenkov radiation

An electron beam is used to excite a unique electromagnetic response from a complementary split ring resonator (CSRR) metasurface loaded waveguide and investigate it as a backward propagating Cherenkov radiation source. This novel structure comprises of a metallic WR-284 waveguide loaded with four layers of CSRR-metasurface 1 mm thick, with sufficient spacing between the metasurface layers for electron beam propagation. The loaded waveguide exhibits left handed behaviour around 5.86 GHz where a TM-like mode exists. The transverse confinement of this mode between the closely lying metasurface layers and the strong electrical response of the CSRRs leads permeability and permittivity to be simultaneously negative. The structure has been optimised to reduce the surface current on the metasurface, improve the fabrication suitability and minimise the effect of hybrid modes. The structure is suitable for beam-based applications as it exhibits strong beam coupling parameters and excitation of longitudinal wake impedance at the frequency of the TM-like mode. The beam coupling parameters exhibited are high with R/Q of 36 Ω and shunt impedance of 177 kΩ. Strong excitation of the longitudinal wake impedance of 10 kΩ, with minimal transverse wake impedance and minimal beam disruption, has been observed for this mode. Results from particle-in-cell simulations will be shown to verify that backward propagating Cherenkov is radiated when a suitable electron beam propagates between the central layers of the waveguide. This investigation can lead to new solutions for non-destructive beam detection and wakefield acceleration which can potentially be scaled to higher frequency ranges

Perturbative study of the Kitaev model with spontaneous time-reversal symmetry breaking

We analyze the Kitaev model on the triangle-honeycomb lattice whose ground state has recently been shown to be a chiral spin liquid. We consider two perturbative expansions: the isolated-dimer limit containing Abelian anyons and the isolated-triangle limit. In the former case, we derive the low-energy effective theory and discuss the role played by multi-plaquette interactions. In this phase, we also compute the spin-spin correlation functions for any vortex configuration. In the isolated-triangle limit, we show that the effective theory is, at lowest nontrivial order, the Kitaev honeycomb model at the isotropic point. We also compute the next-order correction which opens a gap and yields non-Abelian anyons.Comment: 7 pages, 4 figures, published versio

Photonic band gap corrugated slow wave structure for THz sheet-beam vacuum electron devices

The use of photonic band gap (PBG) technology is investigated to alleviate some of the typical issues of vacuum electron devices at terahertz and is shown as particularly suitable for the use of large sheet beams. A full interaction structure including the slow wave structure and the coupler based on a tapered PBG corrugated waveguide is proposed for sheet beam backward wave oscillators (BWO). The case of a 346 GHz BWO is considered

Photonic crystal-coupler for sheet beam THz vacuum electron tubes

Photonic Crystal (PhC) technology was recently proposed as a compact and effective solution to improve the performance and ease the fabrication of terahertz vacuum electron devices. In particular, the introduction of defects in a bi-dimensional, all metallic PhC provides very effective transmission of the useful signal in a compact input/output coupler where a novel tunnel for sheet-beam injection/collection is realized. Simulation results are here experimentally validated via measurements on a Ku-band scaled model of the PhC-coupler which confirm the validity of the concept

Optimization of PBG-Waveguides for Terahertz-Driven Electron Acceleration

The properties of 2-D photonic bandgap dielectric structures, also called photonic crystals, are numerically investigated to assist the design of waveguides for terahertz (THz)-driven linear electron acceleration. Given the broadband nature of the driving pulses in THz acceleration regimes, one design aim is to maximize the photonic bandgap width to allow propagation of the relevant frequencies within the photonic crystal linear defect waveguide. The proposed design is optimized to provide the best compromise between effective acceleration bandwidth and strong beam–wave interaction at the synchronism central frequency. Considerations on achieved acceleration bandwidth, accelerating voltage, and surface magnetic field are given to compare the proposed geometry to one of the main counterparts in the literature—the dielectric-lined waveguide

Missing upper incisors: A retrospective study of orthodontic space closure versus implant

Background: The aim of this retrospective study was to compare the esthetic, periodontal, and functional outcomes of orthodontic space closure versus implant substitution in patients with missing maxillary incisors 5 years after completion of treatment. Methods: The study group consisted of ten patients treated with orthodontic space closure (six males, four females, mean age 19 ± 2.1 years at the completion of treatment) and ten patients treated with implant insertion (five males, five females, mean age 20 ± 1.4 years at the time of implant insertion). Tooth mobility, plaque index, probing depth, infraocclusion, open gingival embrasure (black triangle), and temporomandibular joint function were recorded at the 5.6 years follow-up. Self-perceived dental esthetic appearance was also evaluated through a visual analog scale (VAS) questionnaire. T-test was used to evaluate the data. Results: All patients were equally satisfied with the appearance of their teeth 5.6 ± 0.4 years after the completion of treatment. No statistically significant differences were found in relation to the VAS scores of the subjects (P < 0.857). No significant differences were found in tooth mobility, plaque index (P < 0.632), and the prevalence of signs and symptoms of temporomandibular disorders. However, significant infraocclusion was noticed in all implant patients (P < 0.001). Probing depth was also significantly higher in implant patients (P < 0.001). Conclusions: Orthodontic space closure and implant of missing maxillary incisors produced similar, well-accepted esthetic results. None of the treatments impaired temporomandibular joint function. Nevertheless, infraocclusion was evident in implant patients. Space closure patients also showed better periodontal health in comparison with implant patients