Pressure Effects on the Magnetic Phase Diagram of the CeNMSb2 (NM: Au and Ag): A DFT Study.

We explore the influence of pressure on the magnetic ground state of the heavy-fermion antiferromagnet (ferromagnet) CeAuSb 2 (CeAgSb 2 ) using first-principles calculations. The total-energy differences obtained by including the spin-orbit interactions and the on-site Coulomb potential for the Ce-derived 4f-orbitals are necessary to realize the accurate magnetic ground state of CeNMSb 2 (NM: Au and Ag). According to our results, the appearance of a new magnetic phase of CeAuSb 2 (CeAgSb 2 ) at the pressure of 2.1 GPa (3.5 GPa) is due to the rotation of the magnetic easy axis from the <001> to the <100> direction. Additionally, our data confirm that CeAgSb 2 is antiferromagnetic (AFM) above a critical pressure P c , and such a tendency is expected for CeAuSb 2 and remains to be seen. Through the spin-orbit-coupling Hamiltonian and detailed information on the occupation of individual 4f-orbitals of the Ce atom obtained by the electronic-structure calculations, we can deduce the rotation of the magnetic easy axis upon the application of pressure. According to the present and previous studies, the differences among the magnetic properties of CeNMSb 2 (NM: Cu, Ag and Au) compounds are not due to the different noble metals, but due to the subtle differences in the relative position of Ce atoms and, in turn, different occupations of Ce 4f-orbitals.

Origin of extremely large magnetoresistance in the candidate type-II Weyl semimetal MoTe2-x.

The recent observation of extremely large magnetoresistance (MR) in the transition-metal dichalcogenide MoTe2 has attracted considerable interest due to its potential technological applications as well as its relationship with novel electronic states predicted for a candidate type-II Weyl semimetal. In order to understand the origin of the MR, the electronic structure of MoTe2-x (x = 0.08) is systematically tuned by application of pressure and probed via its Hall and longitudinal conductivities. With increasing pressure, a monoclinic-to-orthorhombic (1 T' to Td) structural phase transition temperature (T*) gradually decreases from 210 K at 1 bar to 58 K at 1.1 GPa, and there is no anomaly associated with the phase transition at 1.4 GPa, indicating that a T = 0 K quantum phase transition occurs at a critical pressure (Pc) between 1.1 and 1.4 GPa. The large MR observed at 1 bar is suppressed with increasing pressure and is almost saturated at 100% for P > Pc. The dependence on magnetic field of the Hall and longitudinal conductivities of MoTe2-x shows that a pair of electron and hole bands are important in the low-pressure Td phase, while another pair of electron and hole bands are additionally required in the high-pressure 1 T' phase. The MR peaks at a characteristic hole-to-electron concentration ratio (nc) and is sharply suppressed when the ratio deviates from nc within the Td phase. These results establish the comprehensive temperature-pressure phase diagram of MoTe2-x and underscore that its MR originates from balanced electron-hole carrier concentrations.

Ultra-subwavelength thickness for dual/triple-band metamaterial absorber at very low frequency.

An integrated model utilizing external parasitic capacitors for a dual-band metamaterial perfect absorber (DMPA) is proposed and demonstrated in the UHF radio band. By adjusting the lumped capacitors on a simple meta-surface, the thickness of absorber is reduced to be only 1/378 and 1/320 with respect to the operating wavelength at 305 and 360.5 MHz, respectively. The simulations and the experiments confirm that the DMPA can maintain an absorption over 91% in a wide range of incident angle (up to 55°) and independent of the polarization of incident radiation. Additionally, we examine the integrated model for smaller dual-band absorber and absorption performance at higher frequencies (LTE band). Finally, we consolidate our approach by fabricating an ultrathin triple-band perfect absorber miniaturized to be only 1/591 of the longest operating wavelength. Our work is expected to contribute to the actualization of metamaterial-based devices working at radio frequency.

Analysis of a systematic error appearing as a periodic fluctuation in the frequency-domain absorption spectra of metamaterial absorbers.

A metamaterial is a periodic structure which can be considered as an effective medium that exhibits an atypical response for a corresponding range of electromagnetic (EM) waves. For metamaterial absorbers (MMA) working in the GHz regime, the frequency-domain-measurement setup consisting of two horn antennas connected to a network analyzer is widely used. The absorption spectra obtained with this setup usually exhibits some periodic fluctuation (PF) across the entire range of measurement. Typically, this PF in the frequency-domain spectrum is ignored because it is distinct from major features and has a small amplitude. We have examined the PF through a finite-difference time-domain (FDTD) simulation in order to verify the validity of such assumption. We concluded that the PF is caused by a part of EM wave radiated by an antenna (source), and directly propagating to another (detector), resulting in an interference between this leaked EM wave and the one reflected from the sample. We have successfully reproduced the periodic fluctuation of the experimental spectra by FDTD simulation. We examined the angle of incidence dependence. The amplitude increases because the amount of leaked EM waves increases, while the periodicity increases because the distance between the two antennas increases, resulting in a shortened path difference. We conclude that the PF is a systematic error which can be safely ignored.

Experimental Realization of Tunable Metamaterial Hyper-transmitter.

We realized the tunable metamaterial hyper-transmitter in the microwave range utilizing simple planar meta-structure. The single-layer metamaterial hyper-transmitter shows that the transmission peak occurs at 14 GHz. In case of the dual-layer one, it is possible to control the transmission peak from 5 to 10 GHz. Moreover, all the transmission peaks reveal transmission over 100%. We experimentally and theoretically investigated these phenomena through 3-dimensional simulation and measurement. The reason for being over 100% is also elucidated. The suggested hyper-transmitter can be used, for example, in enhancing the operating distance of the electromagnetic wave in Wi-Fi, military radar, wireless power transfer and self-driving car.

Metamaterial Absorber for Electromagnetic Waves in Periodic Water Droplets.

Perfect metamaterial absorber (PMA) can intercept electromagnetic wave harmful for body in Wi-Fi, cell phones and home appliances that we are daily using and provide stealth function that military fighter, tank and warship can avoid radar detection. We reported new concept of water droplet-based PMA absorbing perfectly electromagnetic wave with water, an eco-friendly material which is very plentiful on the earth. If arranging water droplets with particular height and diameter on material surface through the wettability of material surface, meta-properties absorbing electromagnetic wave perfectly in GHz wide-band were shown. It was possible to control absorption ratio and absorption wavelength band of electromagnetic wave according to the shape of water droplet-height and diameter- and apply to various flexible and/or transparent substrates such as plastic, glass and paper. In addition, this research examined how electromagnetic wave can be well absorbed in water droplets with low electrical conductivity unlike metal-based metamaterials inquiring highly electrical conductivity. Those results are judged to lead broad applications to variously civilian and military products in the future by providing perfect absorber of broadband in all products including transparent and bendable materials.

Dual broadband metamaterial absorber.

We propose polarization-independent and dual-broadband metamaterial absorbers at microwave frequencies. This is a periodic meta-atom array consisting of metal-dielectric-multilayer truncated cones. We demonstrate not only one broadband absorption from the fundamental magnetic resonances but additional broadband absorption in high-frequency range using the third-harmonic resonance, by both simulation and experiment. In simulation, the absorption was over 90% in 3.93-6.05 GHz, and 11.64-14.55 GHz. The corresponding experimental absorption bands over 90% were 3.88-6.08 GHz, 9.95-10.46 GHz and 11.86-13.84 GHz, respectively. The origin of absorption bands was elucidated. Furthermore, it is independent of polarization angle owing to the multilayered circular structures. The design is scalable to smaller size for the infrared and the visible ranges.

Multifunctional antireflection coatings based on novel hollow silica-silica nanocomposites.

Antireflection (AR) coatings that exhibit multifunctional characteristics, including high transparency, robust resistance to moisture, high hardness, and antifogging properties, were developed based on hollow silica-silica nanocomposites. These novel nanocomposite coatings with a closed-pore structure, consisting of hollow silica nanospheres (HSNs) infiltrated with an acid-catalyzed silica sol (ACSS), were fabricated using a low-cost sol-gel dip-coating method. The refractive index of the nanocomposite coatings was tailored by controlling the amount of ACSS infiltrated into the HSNs during synthesis. Photovoltaic transmittance (TPV) values of 96.86-97.34% were obtained over a broad range of wavelengths, from 300 to 1200 nm; these values were close to the theoretical limit for a lossy single-layered AR coating (97.72%). The nanocomposite coatings displayed a stable TPV, with degradation values of less than 4% and 0.1% after highly accelerated temperature and humidity stress tests, and abrasion tests, respectively. In addition, the nanocomposite coatings had a hardness of approximately 1.6 GPa, while the porous silica coatings with an open-pore structure showed more severe degradation and had a lower hardness. The void fraction and surface roughness of the nanocomposite coatings could be controlled, which gave rise to near-superhydrophilic and antifogging characteristics. The promising results obtained in this study suggest that the nanocomposite coatings have the potential to be of benefit for the design, fabrication, and development of multifunctional AR coatings with both omnidirectional broadband transmission and long-term durability that are required for demanding outdoor applications in energy harvesting and optical instrumentation in extreme climates or humid conditions.

Multi-band metamaterial absorber based on the arrangement of donut-type resonators.

We propose multi-band metamaterial absorbers at microwave frequencies. The design, the analysis, the fabrication, and the measurement of the absorbers working in multiple bands are presented. The numerical simulations and the experiments in the microwave anechoic chamber were performed. The metamaterial absorbers consist of an delicate arrangement of donut-shape resonators with different sizes and a metallic background plane, separated by a dielectric. The near-perfect absorptions of dual, triple and quad peaks are persistent with polarization independence, and the effect of angle of incidence for both TE and TM modes was also elucidated. It was also found that the multiple-reflection theory was not suitable for explaining the absorption mechanism of our investigated structures. The results of this study are promising for the practical applications.

Polarization-independent dual-band perfect absorber utilizing multiple magnetic resonances.

We propose a dual-band metamaterial perfect absorber at microwave frequencies. Using a planar metamaterial, which consists of periodic metallic donut-shape meta-atoms at the front separated from the metallic plane at the back by a dielectric layer, we demonstrate the multi-plasmonic high-frequency perfect absorptions induced by the third-harmonic as well as the fundamental magnetic resonances. The origin of the induced multi-plasmonic perfect absorption was elucidated. It was also found that the perfect absorptions at dual peaks are persistent with varying polarization.

Tunable dual-band perfect absorbers based on extraordinary optical transmission and Fabry-Perot cavity resonance.

Magnetic resonance is considered to be a necessary condition for metamaterial perfect absorbers, and dual-band absorbers can be composed of a pair of metallic layers with anti-parallel surface currents. We designed and fabricated a tunable dual-band perfect absorber based on extraordinary-optical-transmission (EOT) effect and Fabry-Perot cavity resonance. The idea and the mechanism are completely different from the absorber based on the near-field interaction. The important advantage of our structure is that we can switch a single-band absorber to a dual-band absorber by changing the distance between two metallic layers and/or incident angle. The peak originating from the EOT effect becomes significantly narrower, resulting in an increase of the Q-factor from 16.88 to 49. The dual-band absorber can be optimized to be insensitive to the polarization of the incident electromagnetic wave by slightly modifying the absorber structure.

Highly-dispersive transparency at optical frequencies in planar metamaterials based on two-bright-mode coupling.

Using a planar metamaterial, which consists of two silver strips, we theoretically demonstrate the plasmonic electromagnetically-induced transparency (EIT)-like spectral response at optical frequencies. The two silver strips serve as the bright modes, and are excited strongly by the incident wave. Based on the weak hybridization between the two bright modes, a highly-dispersive plasmonic EIT-like spectral response appears in our scheme. Moreover, the group index is higher than that of another scheme which utilizes the strong coupling between the bright and dark modes.

Active manipulation of plasmonic electromagnetically-induced transparency based on magnetic plasmon resonance.

Plasmonic electromagnetically-induced transparency (EIT) can be excited by a single optical field unlike EIT in atom system, since the coupling between the bright and the dark modes is inherently induced through the near-field interaction in metamaterials. As a result, the complexity of the experimental realization can be reduced significantly, while the tunability is lost inevitably.We suggest a scheme that the plasmonic EIT is possible to be actively manipulated even by the single optical field. The bright and the dark modes are selective to be either coupled or uncoupled, depending on the angle of incidence. Even though the mechanical control has the disadvantage for high-speed applications, it paves the way for active manipulation of plasmonic EIT and benefits the clarification of its origin.

Plasmonic electromagnetically-induced transparency in symmetric structures.

A broken symmetry is generally believed to be a prerequisite for plasmonic electromagnetically-induced transparency (EIT), since the asymmetry allows the excitation of the otherwise forbidden dark mode. Nevertheless, according to the picture of magnetic plasmon resonance (MPR)-mediated plasmonic EIT, we show that plasmonic EIT can be achieved even in symmetric structures based on the second-order MPR. This not only sharpens our understanding of the existing concept, but also provides a profound insight into the plasmonic coherent interference in the near-field zone.

Rigorous approach on diffracted magneto-optical effects from polar and longitudinal gyrotropic gratings.

The rigorous coupled-wave analysis with Airy-like internal-reflection series and Fourier-factorization for the calculation of the diffracted magneto-optical (MO) effects from polar and longitudinally magnetized gyrotropic gratings are fully described. For both gratings the numerical and experimental results are in good agreement, and the enhancement of Kerr rotation in higher orders compared to that of the 0th order diffraction is calculated as a function of grating depth. At last, this numerical method can be applied to many other applications such as extraordinary optical transmission from metallic gratings either through surface plasmon or cavity mode, and MO hysteresis loops.

Direct observation on the temperature-dependent change of magnetic domains in epitaxial MnAs film on GaAs (001).

We present a systematic change of the magnetic domain structure with temperature in epitaxial ferromagnetic MnAs film on GaAs (001), observed in a wide temperature range of 15-45 degrees C by magnetic force microscopy. Interestingly, it is found that, as temperature increases, the domain structure within the ferromagnetic alpha-MnAs stripes shows a mixture of head-on and simple domains at 15 degrees C and then, takes a complete transition to simple ones above 15 degrees C. This change could be understood by change in the demagnetizing factor of the cross-section of the ferromagnetic stripes with temperature.