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Based on the 3D Dirac semimetals (DSM) supported tilted double elliptical resonators, the tunable propagation properties of quasi-bound in continuum (BIC) resonance have been investigated in the THz regime, including the effects of rotation angles, DSM Fermi level, and the configuration of resonators. The results manifest that by altering the rotation angle of elliptical resonator, an obvious sharp BIC transmission dip is observed with the Q-factor of more than 60. The DSM Fermi level affects the BIC resonance significantly, a sharp resonant dip is observed if Fermi level is larger than 0.05 eV, resulting from the contributions of reflection and absorption. If Fermi level changes in the range of 0.01–0.15 eV, the amplitude and frequency modulation depths are 92.75 and 44.99%, respectively. Additionally, with the modified configurations of elliptical resonators, e.g. inserting a dielectric hole into the elliptical resonator, another transmission dip resonance is excited and indicates a red shift with the increase of the permittivity of the dielectric filling material. The results are very helpful to understand the mechanisms of DSM plasmonic structures and develop novel tunable THz devices, such as modulators, filters, and sensors in the future.

Based on the proposed elliptical dielectric fiber-polyethylene gap-3D Dirac semimetal (DSM) hybrid plasmonic waveguide structure, the tunable propagation characteristics have been systematically investigated in the terahertz region, taking into account the influences of the structural parameters, the modified dielectric fiber, and the 3D DSM Fermi levels. The results show that as the ratio of the elliptical semi-axis along the y -direction a y and the x -direction a x ( a y / a x ) increases, the hybrid mode confinement increases. The real part of the effective mode index and propagation length increase with increasing the refractive index of the elliptical fiber. The propagation length and figure of merit of the hybrid modes reach 1.56 × 10 4 µ m and 300, respectively. In addition, by changing the Fermi level of the 3D DSM layer, the propagation properties of the hybrid modes can also be modulated in a wide range, e.g., the modulation depth of the propagation length reaches about 71.53% if the Fermi level varies in the range of 0.03–0.15 eV. The propagation properties of the hybrid modes are enhanced significantly by utilizing the modified three elliptical fiber structures, the real part of the effective mode index, and the propagation length of the modified structure are enhanced simultaneously. The results are very helpful for understanding the tunable mechanism of the 3D DSM devices and aids the design of novel plasmonic devices, e.g., lasers, modulators, and resonators.

By depositing the trapezoidal dielectric stripe on top of the 3D Dirac semimetal (DSM) hybrid plasmonic waveguide, the thermal tunable propagation properties have been systematically investigated in the terahertz regime, taking into account the influences of the structure of the dielectric stripe, temperature and frequency. The results manifest that as the upper side width of the trapezoidal stripe increases, the propagation length and figure of merit (FOM) both decrease. The propagation properties of hybrid modes are closely associated with temperature, in that when the temperature changes in the scope of 3-600 K, the modulation depth of propagation length is more than 96%. Additionally, at the balance point of plasmonic and dielectric modes, the propagation length and FOM manifest strong peaks and indicate an obvious blue shift with the increase of temperature. Furthermore, the propagation properties can be improved significantly with a Si-SiO2 hybrid dielectric stripe structure, e.g., on the condition that the Si layer width is 5 µm, the maximum value of the propagation length reaches more than 6.46 × 105 µm, which is tens of times larger than those pure SiO2 (4.67 × 104 µm) and Si (1.15 × 104 µm) stripe. The results are very helpful for the design of novel plasmonic devices, such as cutting-edge modulator, lasers and filters.

The tunable propagation properties of 3D Dirac semimetal (DSM)-supported dielectric-loaded surface plasmons structures have been investigated in the THz regime, including the influences of the Fermi level of 3D DSM layer, the fiber shape and operation frequencies. The results indicate that the shape of dielectric fiber affects the hybrid mode significantly, on the condition that if a x (the semi-minor axis length of the dielectric semi-ellipse) is relatively small, the fiber shows good mode confinement and low loss simultaneously, and the figure of merit reaches more than 200. The propagation property can be manipulated in a wide range by changing the Fermi level of 3D DSM, e.g. if the Fermi level varies in the range of 0.05 eV–0.15 eV, the propagation length changes in the range of 9.073 × 103–2.715 × 104 μm, and the corresponding modulation depth is 66.5%. These results are very helpful to understand the tunable mechanisms of the 3D DSM plasmonic devices, such as switchers, modulators, and sensors.

Based on 3D Dirac-semimetal (DSM) modified hybrid waveguides, tunable propagation properties have been investigated, including the effects of Fermi levels, structural parameters, and operation frequency. The results show that if the operation frequency is smaller (larger) than the transition frequency ( ℏ ω ≈ 2 | μ c | ), the proposed hybrid waveguides indicate strong (weak) confinement because the DSM layer manifests a high plasmonic (dielectric low) loss property. The dielectric fiber shape affects the propagation property obviously, as the elliptical parameter decreases, the confinement and figure of merit increase, and the loss reduces. With the increase in Fermi level, the propagation constant increases, and the frequency (amplitude) modulation depth is 32.31% (12.93%) if the Fermi level changes in the range of 0.01–0.15 eV. The results are very helpful in understanding the tunable mechanisms of hybrid waveguides and designing novel plasmonic devices in the future, e.g., modulators, filters, lasers, and resonators.

We investigated the propagation properties of all-dielectric metamaterials (ADMs) based on a SiO2-Si asymmetric hybrid block, including the effects of structural parameters, asymmetrical degrees, carrier doping concentrations, and graphene Fermi levels. The Q-factor of Fano resonance reaches more than 270, and the amplitude modulation depth (MD) is about 75% if the asymmetric degree changes in the range of 2–10 μm. The carrier concentration of silicon significantly affects the intensity of excited Fano resonance. When the carrier concentration of Si is 1 × 1014 cm−3, the excited Fano resonance is the strongest, and the transmission peak is about 0.92. With the help of a uniform graphene layer, the Fano resonance can be effectively modulated, the frequency MD is about 20% if the Fermi level changes in the scope of 0.01–0.3 eV. These results can help us to design THz high Q-factor devices, such as sensors, filters, and modulators.

Based on the strontium titanate ( S r T i O 3 , STO) spherical shell all-dielectric metamaterials (ADMs) structure, the tunable propagation characteristics have been systematically studied, taking into account the effects of structural geometrical parameter, temperature, and graphene Fermi level. The results indicate that STO-based ADMs exhibit excellent thermally and electrically tunable performances, e.g., when the temperature rises from 77 to 400 K, the modulation depths (MDs) of resonant frequency and amplitude of the reflection dip reach 62% and 93%, and the absorption is tunable in the range of 0.7995–0.9017. Additionally, when the Fermi level of graphene changes from 0.1 to 1.0 eV, the amplitude MD of the reflection dip reaches 99.89%, and the absorption of the STO-based ADM increases from 0.8352 to 0.9953. These results provide a guide for understanding the resonant mechanism of STO-based ADMs and designing multifunctional terahertz devices.

Based on asymmetric graphene ellipses, the tunable propagation characteristics of metamaterial absorber (MMA) have been investigated in the THz region. Two distinct absorption peaks of 84% and 90% are observed at 1.06 THz and 1.67 THz, respectively. Besides a high Q factor exceeding 20, the Fano resonance can also be modulated in a wide range (e.g., the frequency modulation depth reaches more than 43.8% if the Fermi energy level changes in the range of 0.2–1.0 eV). Additionally, a bidirectional THz MMA is achieved by replacing the metal substrate with a uniform graphene layer. If the terahertz wave is incident in the forward direction, the proposed graphene double stripe microstructure shows a typical MMA with its absorption reaching 88%. On the other hand, if the terahertz wave is incident in the reverse direction, the graphene double stripe microstructure behaves as a reflective modulator, and its amplitude and frequency MD will reach 60% and 85%. These results contribute to the design of tunable THz devices, such as filters, absorbers, and modulators.

BiFeO3 (BFO) and Bi1−x La x Fe0.9Cr0.1O3 (x = 0.1–0.2, BLFC x=0.1–0.2) ceramics were prepared by modified solid-state reaction. The influence of La/Cr codoping on the microstructure, leakage, dielectric and magnetic properties of BFO was studied. It is found that the La/Cr codoping can induce a structural transformation from a rhombohedral (R3c) phase to a triclinic (P1) phase, suppress the formation of oxygen vacancies and modulate the spiral spin structure. With the increasing La concentration, the leakage current density can be diminished by over two orders of magnitude compared with undoped BFO, in a reasonable range for device applications. The dielectric loss follows almost the same trend as the leakage current, while the dielectric constant is raised to 400.8 at 100 kHz for x = 0.2, over eight times larger than that of undoped BFO. The substantially enhanced magnetization can be mainly attributed to the suppression of the spiral spin structure and a new ferromagnetic Fe3+–O2−–Cr3+ super-exchange interaction. More interestingly, abrupt changes are observed around x = 0.15, not only in microstructure but also in electrical and magnetic properties, which may indirectly reflect the coupling effect among them. The BLFC x=0.2 ceramic shows the highest magnetization and dielectric constant together with the lowest leakage current and dielectric loss. The results indicate that the La/Cr codoping can effectively improve the magnetic and electrical properties of BFO ceramic for potential applications in magnetoelectric devices.

The influences of different kinds of phonon scatterings (i.e., acoustic (AC) phonon, impurity, and longitudinal optical (LO) phonon scatterings) on the tunable propagation properties of graphene metamaterials structures have been investigated, also including the effects of graphene pattern structures, Fermi levels, and operation frequencies. The results manifested that, at room temperature, AC phonon scattering dominated, while with the increase in temperature, the LO phonon scattering increased significantly and played a dominate role if temperature goes beyond 600 K. Due to the phonon scatterings, the resonant properties of the graphene metamaterial structure indicated an optimum value (about 0.5&ndash, 0.8 eV) with the increase in Fermi level, which were different from the existing results. The results are very helpful to understand the tunable mechanisms of graphene functional devices, sensors, modulators, and antennas.

Recently, deep learning-based models have exhibited remarkable performance for image manipulation detection. However, most of them suffer from poor universality of handcrafted or predetermined features. Meanwhile, they only focus on manipulation localization and overlook manipulation classification. To address these issues, we propose a coarse-to-fine architecture named Constrained R-CNN for complete and accurate image forensics. First, the learnable manipulation feature extractor learns a unified feature representation directly from data. Second, the attention region proposal network effectively discriminates manipulated regions for the next manipulation classification and coarse localization. Then, the skip structure fuses low-level and high-level information to refine the global manipulation features. Finally, the coarse localization information guides the model to further learn the finer local features and segment out the tampered region. Experimental results show that our model achieves state-of-the-art performance. Especially, the F1 score is increased by 28.4%, 73.2%, 13.3% on the NIST16, COVERAGE, and Columbia dataset., Accepted to IEEE International Conference on Multimedia and Expo (ICME2020)

The tunable propagation properties of 3D Dirac semimetal (DSM) patterned metamaterial (MM) structures have been symmetrically investigated in the terahertz (THz) regime. The results demonstrate that the resonant properties are very sensitive to the thicknesses of DSM MMs, and hundreds of nanometers are required to excite strong resonant curves. The DSM MMs support both strong LC and dipolar resonances, quite different from graphene MM patterns which mainly depend on dipolar resonance. As the Fermi level increases, the resonant strength becomes stronger, and significant modulation can be achieved, e.g. the amplitude and frequency modulation depths of transmission curves are more than 99% and 80%, respectively. In addition, by utilizing asymmetrical resonators, a very sharp Fano resonant peak is achieved with a large Q-factor of more than 25, for which the figure of merit is about 20. The results are very helpful to understand the tunable mechanisms of DSM devices and design novel THz plasmonic components, such as modulators, filters, and sensors.

Cu(Cr0.96−xMg0.04Mnx)O2−δ ceramics (0 ≤ x ≤ 15 at.%) were prepared by solid-state reaction. The influence of Mg acceptor codoping on microstructure, magnetic, and optical properties was studied. The codoping with Mg acceptors and the resulting lattice expansion are both favorable for the enhancement of hole density. Furthermore, the lower valence of Mg2+ compared with Cr3+ enables the mixed valence state of Mn ions: Mn3+ and Mn4+, and hence induces the hole-mediated double-exchange mechanism not only between Mn3+ and Cr3+ but also between Mn3+ and Mn4+. The coexistence of two ferromagnetic interactions and the high hole density are responsible for the improved saturation magnetization and Curie temperature. For photoluminescence, an unusual near-infrared emission at 1.5 eV is observed in the Mn-contained samples and can be attributed to the internal transition between the e and t2 states of the Mn3+ impurity band. The emission intensity is primarily affected by two factors: the Mn3+ concentration and the position of the Fermi level. The results show that codoping with Mg acceptors is an effective way to strengthen the hole-mediated ferromagnetic interactions in CuCrO2 delafossite.

Based on a graphene asymmetric sdouble ellipse (ADE) structure, the tunable Fano resonance characteristics in the terahertz range have been symmetrically studied, including the effects of graphene Fermi levels, asymmetrical degrees, operation frequencies and structural parameters. The results show that due to the strong coupling between the incident THz waves and graphene ADE structures, a strong Fano peak can be observed by arranging double ellipse asymmetrically. The maximum peak of Fano resonance reaches 0.918, and its Q-factor is about 20. As the asymmetrical degree of the ADE structure increases, the Fano peak value increases, and the amplitude modulation depth reaches about 75%. As graphene level increases, the amplitude and frequency modulation depths reaches more than 31% and 11%, respectively. These results are very helpful to design novel tunable graphene devices with high Q-factor devices in the future, such as sensors, modulators, and filters.

Based on SrTiO3 (STO) elliptical metamaterials (MMs) structures, the tunable propagation properties have been systematically investigated in the THz regime, including the effects of temperatures, incident angles, operation frequencies, and graphene Fermi levels. The results reveal that STO MMs manifest excellent thermally tunable properties, e.g. the amplitude and frequencies modulation depths are larger than 95.3% and 21.8% if temperature varies in the range of 70-320 K. The incident angles affect the resonant curves significantly, the amplitude modulation depth approximates 90% by changing the polarization angles, and the Q-factor reaches more than 20. The tunable Fano resonances can be achieved by utilizing a symmetrical STO-graphene structure. If the graphene Fermi level changes in the range of 0.3-1.0 eV, the amplitude modulation depth of transmission curve is more than 80%. The results are very helpful to understand the resonant mechanisms of ferroelectric metamaterials and design novel high Q-factor devices in the future, e.g. sensors, modulators, and filters.

The tunable propagation properties of MoS

The tunable plasmonic induced transparency has been theoretically investigated based on graphene patterns/SiO2/Si/polymer multilayer structure in the terahertz regime, including the effects of graphene Fermi level, structural parameters and operation frequency. The results manifest that obvious Fano peak can be observed and efficiently modulated because of the strong coupling between incident light and graphene pattern structures. As Fermi level increases, the peak amplitude of Fano resonance increases, and the resonant peak position shifts to high frequency. The amplitude modulation depth of Fano curves is about 40% on condition that the Fermi level changes in the scope of 0.2-1.0 eV. With the distance between cut wire and double semi-circular patterns increases, the peak amplitude and figure of merit increases. The results are very helpful to develop novel graphene plasmonic devices (e.g. sensors, modulators, and antenna) and find potential applications in the fields of biomedical sensing and wireless communications.

Cu(Cr1−xMnx)O2 delafossite thin films (0 ⩽ x ⩽ 15 at.%), with a c-axis quasi-epitaxial orientation, were prepared by pulsed laser deposition. The effects of Mn content on microstructure and physical properties were investigated. Both the variation of lattice constant c and X-ray photoelectron spectroscopy reveal that the Mn ions substitute for Cr3+ as Mn3+ and Mn4+. The proportion of Mn4+ is gradually increased with the Mn doping, leading to the changes of hole density and mobility. As a result, the p-type conductivity is decreased at first and then increased. The conduction mechanism is thermal activation between 130 and 300 K. It is indicated that the PLD method and the Mn3+–O–Mn4+ and Mn3+–O–Cr3+ double-exchange interactions are favorable to the high hole density and mobility, respectively. Meanwhile, the double-exchange interactions produce the near-room-temperature ferromagnetism. The saturation magnetization and Curie temperature are gradually increased with the Mn content. For optical properties, all thin films have a comparatively high transmittance for visible light, with the highest value 70% for x = 15 at.% at the wavelength of 750 nm. As increasing the Mn content, the transparency and direct optical bandgap exhibit the similar trend as the hole density. With the relatively balanced electrical, magnetic and optical properties, these pulsed-laser-deposition-grown Cu(Cr1−xMnx)O2 thin films are expected to become a promising p-type transparent diluted magnetic semiconductor material.

In this work, the solid solutions of 0.8Pb(Zr 0.52 Ti 0.48 )O 3 −(0.2− x )Pb(Zn 1/3 Nb 2/3 )O 3 − x Pb(Al 1/2 Nb 1/2 )O 3 (PZT–PZN– x PAN) were designed and fabricated via the traditional solid state reaction method. The dielectric, ferroelectric, piezoelectric properties of PZT–PZN– x PAN solid solutions were investigated in detail. The optimum electric properties of P m = 46.3 μC/cm 2 , P r = 42 μC/cm 2 , P r / P m = 90.5%, S total = 0.48%, d 33 = 410 pC/N, and T c = 283 °C were achieved in PZT–PZN– x PAN ceramics with x = 0.02. A large strain response of ∼0.24% with normalized strain S max / E max as high as 767 pm/V was obtained for the PZT–PZN–0.1PAN under a low electric field of ∼3 kV/mm at room-temperature.

The tunable propagation properties of MoS2 supported hybrid surface plasmon waveguides based on dielectric fiber-gap-metal substrate structures have been investigated by using the finite element method, including the effects of structural parameters, the dielectric fiber shape and carrier concentration of the MoS2 layer. The results reveal that as the dielectric fiber radius increases, the confinement of the hybrid mode increases, and the losses show a peak. The shape of the dielectric fiber affects the propagation properties obviously, with an optimum structural parameter (a large value of the elliptical parameter) the confinement and figure of merits increase, and the dissipation decreases simultaneously. In addition, as the carrier concentration of the MoS2 layer increases, the modulation depth of absorption reaches more than 40%, and the propagation constants manifest obvious double peaks at wavelengths of 610 nm (2.03 eV) and 660 nm (1.88 eV), coming from the excitons' absorption of the MoS2 layer. The results are very useful in helping one to understand the tunable mechanisms of hybrid mode waveguide structures and for the design of novel surface plasmonic devices in the future, e.g. absorbers, modulators, lasers, and resonators.

Fe-doped CuCrO 2 delafossite thin films, with a c -axis quasi-epitaxial orientation, were prepared by pulsed laser deposition, exhibiting reasonably good p-type conductivity, ferromagnetism with T C well above room temperature, and comparatively high transparency in the visible range. The influence of Fe concentration (0–15 at%) on microstructure and physical properties was investigated. The substitution of Fe 3+ for Cr 3+ induces the lattice expansion, which is favorable for the oxygen insertion and thus produces the higher hole concentration. Accordingly the p-type conductivity is remarkably improved to 28 S cm −1 . Moreover, the introduction of Fe 3+ leads to the hole-mediated Fe 3+ –Cr 3+ super-exchange coupling responsible for the room-temperature ferromagnetism. The saturation magnetization is enhanced as the Fe concentration increases, with Curie temperature ⩾550 K. Nevertheless, the transparency in the visible range does not profit from the Fe doping. The transmittance is diminished from 75% for x = 0 to 65% for x = 15 at% at the wavelength of 750 nm, which can be interpreted as the strengthened scattering of photons by charge carriers. The direct optical bandgap is modulated by 3.8% via the Fe addition. The results show that the substitution of iron for chromium is an effective way to achieve high-performance CuCrO 2 thin films which are supposed to become a promising p-type transparent diluted magnetic semiconductor material.

The preferred c-oriented Cu(Cr1−xFex)O2 ceramics (0 ⩽ x ⩽ 15 at.%) were prepared by solid-state reaction. The effects of Fe concentration on microstructure, morphology and magnetism were investigated. All the samples have a pure 3R-CuCrO2 delafossite structure, with a stress-induced preferential orientation. The lattice expansion supports the Fe entrance into the Cr sublattice in the form of Fe3+, which is further certified by the X-ray photoelectron spectra and the Mossbauer spectra. The Mossbauer study also reveals a preferential growth orientation and an increase of Fe–O bond lengths with the Fe addition, matching well with the X-ray diffraction analysis. Ferromagnetism is achieved in the Fe-doped samples with Curie temperatures higher than 246 K, which is expected to originate from the hole-mediated Fe3+–Cr3+ super-exchange interaction. The saturation magnetization of this CuMO2 delafossite (M = Cr, Fe) is decreased almost linearly with the increasing Fe concentration, due to the combined influence of the number of the M–M pairs, the M–M distances and the hole concentration. The present work indicates that Fe-doped CuCrO2 is a dilute magnetic semiconductor and Curie temperature can be remarkably enhanced to close to room temperature by Fe doping.

By depositing graphene circular double rings (DR) on a SiO2/Si/polymer substrate, the tunable Fano resonance has been theoretically investigated in the terahertz regime, including the effects of the graphene Fermi level, structural parameters and operation frequency. The results demonstrate that the obvious Fano peak can be efficiently modulated because of strong coupling between the incident waves and graphene ribbons. As the Fermi level increases, the peak amplitude of the Fano curve increases, and the resonant peak position shifts to a high frequency. The amplitude modulation depth of the Fano curves is about 30% if the Fermi level changes in the scope of 0.1-1.0 eV. The optimum gap distance between the DR is about 8-12 μm, where the value of the figure of merit shows a peak. The results are very helpful in order to develop novel graphene plasmonic devices, e.g. sensors and modulators.

Using p-type transparent conducting CuCrO 2 as a host material, (Cu 1− x Fe x )CrO 2 (0 ≤ x ≤ 6 at.%) and Cu(Cr 1− y Fe y )O 2 (0 ≤ y ≤ 6 at.%) nanocrystals were prepared by combining solid-state reaction and ball milling. The microstructure, morphology, optical and magnetic properties were characterized by X-ray diffraction, atomic force microscopy, diffuse reflectance spectroscopy and vibrating sample magnetometer. It is found that all the samples have a single CuCrO 2 delafossite structure. The variations in lattice constants support the Fe entrance into the Cu and Cr sublattices, respectively, in (Cu 1− x Fe x )CrO 2 and Cu(Cr 1− y Fe y )O 2 . The gradual increase of optical bandgap with the Fe content and the low-temperature ferromagnetism are observed in two series of samples. The present work indicates that Fe-doped CuCrO 2 is a dilute magnetic semiconductor and the Curie temperature can be expected to enhance by the optical bandgap modulation.

The A-site substituted BaTiO 3 ceramics were prepared by solid-state reaction via partial substitution of Fe for Ba 2+ . By comparison with the B-site substituted sample made under similar conditions, the effect of Fe doping site on microstructure and magnetism was investigated using X-ray diffraction, Mossbauer spectroscopy and vibrating sample magnetometer. It is found that A-site substitution can be realized to a certain extent at 7 at% Fe addition, whereas impurities are observed at higher Fe concentrations. In the nominal (Ba 0.93 Fe 0.07 )TiO 3 sample, the Fe ions are present as Fe 2+ and Fe 3+ , respectively, replacing A-site Ba 2+ and octahedral B-site Ti 4+ in hexagonal perovskite lattice. The double-exchange Fe 2+ –O 2− –Fe 3+ interactions produce ferromagnetism well above room temperature, but the saturation magnetization and the Curie temperature are both obviously lower than those for B-site substitution due to different magnetic exchange mechanisms. In the B-site substituted sample Ba(Ti 0.93 Fe 0.07 )O 3 , the super-exchange interactions between Fe 3+ on pentahedral and octahedral Ti 4+ sites are responsible for ferromagnetism. These results mean that B-site substitution is a better way for Fe-doped BaTiO 3 system to obtain high-Curie-temperature ferromagnetism. Moreover, increasing pre-sintering time can further improve the magnetism of B-site substituted samples, through which the saturation magnetization for Ba(Ti 0.93 Fe 0.07 )O 3 is enhanced ∼6 times.

Tunable propagation properties of graphene asymmetrical bowtie metamaterials (MMs) structures have been investigated in the terahertz regime, including the effects of graphene Fermi levels, structural parameters and operation frequencies. Because of its thin film thickness, the strong resonant curves of the proposed graphene MMs structures are dominated by the plasmonic mode instead of the Fabry-Perot mode for the metal structures. Compared with existing tunable graphene devices, the sharp Fano resonant curve manifests a large Q-factor of more than 40. In addition, as the width of graphene bowtie aperture increases, the resonant frequency dip shifts low frequency, and the resonant amplitude and figure of merit increase. The results are very helpful in order to understand the tunable mechanisms of graphene components and design high sensitivity functional devices, sensors, modulators, and antenna.

Based on the complementary graphene asymmetric double bars patterns, the tunable Fano resonances with large Q-factors have been investigated in the terahertz regime, including the effects of Fermi levels, structural parameters and operation frequency. The results reveal that compared with existed graphene tunable devices, the Fano resonant curve is very narrow and indicates a large Q-factor of about 60. The strong Fano resonant curves can be convenient tailored. As Fermi level increases, the amplitude of the Fano dip decreases, and the resonant peak position shifts to high frequency. The amplitude modulation depth (MD) of the Fano curve is more, about 90%, if the Fermi level changes in the scope of 0.2-1.0 eV. With the increase of the sample refractive index, the resonant Fano dip shifts low frequency, and the dip amplitude MD can reach more than 40%. The results are very helpful to understand the tunable mechanisms of graphene based Fano systems and to design high sensitivity functional devices, e.g. sensors, modulators, and antenna.

Fe-doped (Ba 1− x Sr x )TiO 3 ceramics were prepared by solid-state reaction, and ferromagnetism was realized at room temperature. The microstructure and magnetism were modified by the Sr concentration control (0≤ x ≤75 at%) at a fixed Fe concentration, and the relevant magnetic exchange mechanism was discussed. All the samples are shown to have a single perovskite structure. When increasing the Sr concentration, the phase structure is transformed from a hexagonal perovskite into a cubic perovskite, with a monotonic decrease in lattice parameters induced by ionic size effect. The room-temperature ferromagnetism is expected to originate from the super-exchange interactions between Fe 3+ on pentahedral and octahedral Ti sites mediated by the O 2− ions. The increase in Sr addition modifies two main influencing factors in magnetic properties: the ratio of pentahedral to octahedral Fe 3+ and the concentration of oxygen vacancies, leading to a gradually enhanced saturation magnetization. The highest value, obtained for Fe-doped (Ba 0.25 Sr 0.75 )TiO 3 , is an order of magnitude higher than that of the Fe-doped BaTiO 3 system with similar Fe concentration and preparation conditions, which may indicate (Ba 1− x Sr x )TiO 3 as a more suitable matrix material for multiferroic research.

The (Ba 0.8 K 0.2 )(Ti 0.3 Fe 0.7 )O 3 ceramic was prepared by solid-state reaction, and post-annealed in oxygen ambient. By comparison with Ba(Ti 0.3 Fe 0.7 )O 3 made under identical conditions, the effect of non-isovalent A-site substitution of K + on microstructure and magnetism of as-prepared and annealed Ba(Ti 0.3 Fe 0.7 )O 3 samples was investigated using X-ray diffraction, Mossbauer spectroscopy, vibrating sample magnetometer and iodometric titration. It is found that all samples have a single 6H-BaTiO 3 -type hexagonal perovskite structure without any impurities detected, regardless of A-site K + substitution or annealing. In the as-prepared state, non-isovalent A-site substitution of K + induces the variation in Fe occupational site, resulting in the disappearance of room-temperature ferromagnetism. The super-exchange interactions of Fe 3+ at tetrahedral and octahedral Ti sites determine the paramagnetism of (Ba 0.8 K 0.2 )(Ti 0.3 Fe 0.7 )O 3 . During the O 2 annealing process, the presence of Fe 4+ , an unusual valence for iron, besides Fe 3+ is observed, both distributed over octahedral Ti site. By A-site substitution of K + with a lower valence than Ba 2+ , the charge compensation mechanism is further enhanced, and thus more Fe 3+ ions are oxidized to Fe 4+ in annealed (Ba 0.8 K 0.2 )(Ti 0.3 Fe 0.7 )O 3 . Consequently, the ferromagnetic Fe 4+ –O 2− –Fe 4+ super-exchange interactions are strengthened, which leads not only to a paramagnetism–ferromagnetism transition but also to a higher saturation magnetization compared with annealed Ba(Ti 0.3 Fe 0.7 )O 3 .

Fe-doped (Ba0.8Sr0.2)TiO3 ceramic was prepared by solid-state reaction, and room-temperature ferromagnetism was achieved via vacuum annealing. The effect of annealing on microstructure, magnetic properties and exchange mechanism was investigated using X-ray diffraction, Mossbauer spectroscopy, and vibrating sample magnetometer. The as-prepared and annealed samples are shown to be monophasic, crystallizing in a 6H-BaTiO3-type hexagonal perovskite structure. The paramagnetism is exhibited in the as-prepared state, arising from the super-exchange interactions of Fe3+ on tetrahedral and octahedral Ti sites. On the other hand, vacuum annealing is proved to be a feasible way for Fe-doped (Ba1−xSrx)TiO3 to obtain ferromagnetism via modifying the Fe occupational distribution. The super-exchange interactions between Fe3+ on pentahedral and octahedral Ti sites lead to the room-temperature ferromagnetism observed in the annealed sample.

By integrating a graphene layer with asymmetric split-ring metamaterial (MM) metal resonators, we investigated tunable propagation properties in the terahertz regime, including the effects of graphene Fermi levels, structural parameters, and operation frequencies. The results reveal that a sharp inductor-capacitor (LC) resonance can be observed at low frequency for the asymmetric MM structure, and its Q factor can reach more than 17.5. With the help of a graphene layer, the optical response is modulated efficiently. For instance, if the Fermi level changes in the range of 0.01–0.3 eV, for the semiconductor MM structure, the modulation depths (MDs) of amplitude and frequency are 27.0% and 43.4%, respectively. In addition, the resonant curves of indium antimonide (InSb) MMs can be modulated by changing the temperature; the amplitude MD is 56.2% as the temperature changes in the range of 350–800 K. The Q factor of the InSb MM structure is about 44.6. The results are helpful for designing novel graphene-based tunable terahertz devices, e.g., filters and modulators.

Ba(Ti 0.65 M 0.05 Fe 0.3 )O 3 ( M = Cr, Mn and Ni) ceramics were prepared by solid-state reaction, and post-annealed in vacuum and oxygen, respectively. Mossbauer spectroscopy was used to investigate the local environments of Fe atoms. Combined with the configuration of the other transition metal M , the mechanisms responsible for magnetism were discussed for different samples. The results show that all samples are single-phase with a 6H–BaTiO 3 -type hexagonal perovskite structure. The as-prepared ceramics are all paramagnetic, while the annealed ones are all ferromagnetic at room temperature. The origin of ferromagnetism changes with doping element and annealing atmosphere, including the double-exchange mechanism of transition metal M with mixed valences, the super-exchange mechanism of Fe 3+ in different occupational sites, and the super-exchange mechanism of Fe 4+ –O 2− –Fe 4+ .

Ba(Ti 0.3 Fe 0.7 )O 3 ceramic was prepared by solid-state reaction and post-annealed in vacuum and oxygen, respectively. The as-prepared and annealed samples are all single-phase, crystallizing in a 6H–BaTiO 3 -type hexagonal perovskite structure. Room-temperature ferromagnetism is exhibited in all ceramics. For the as-prepared sample, the super-exchange interactions of Fe 3+ in different occupational sites (pentahedral and octahedral sites) are expected to produce the ferromagnetism observed. After annealing in vacuum, the magnetization is reduced while the exchange mechanism remains unchanged. On the contrary, O 2 annealing can effectively enhance the magnetization due to the presence of Fe 4+ , an unusual valence for iron. The simultaneous presence of Fe 3+ and Fe 4+ allows new exchange mechanism responsible for the ferromagnetic interaction. The exchange couplings of Fe ions with mixed valences (Fe 3+ and Fe 4+ ) determine the magnetic behavior.

Microstructure and magnetic properties of crystalline Ce 1 Y 2 Fe 5 O 12 thin films prepared on GGG and on SiO 2 /Si substrates by pulsed laser deposition were studied. The results show that highly textured Ce 1 Y 2 Fe 5 O 12 film with (4 4 4) preferred orientation prepared on GGG (1 1 1) shows strong paramagnetism superimposed by a weak ferromagnetism. However, polycrystalline Ce 1 Y 2 Fe 5 O 12 thin films on SiO 2 /Si, which can only be obtained after post-annealing, show strong ferromagnetism with easy axis of magnetization lying in the plane of the film. With post-annealing temperature increasing, CeO 2 segregates from Ce 1 Y 2 Fe 5 O 12 ; then YIG continues to be decomposed, forming Fe 2 O 3 . Consequently, the saturation magnetization of Ce 1 Y 2 Fe 5 O 12 films decreases first and then increases correspondingly, which indicates that the magnetic properties of Ce 1 Y 2 Fe 5 O 12 films are mainly related to the microstructure.

Ba(Ti 1− x Fe x )O 3 ceramics ( x =7, 30 and 70 at%) were prepared by solid-state reaction. All samples are single-phase with 6H-BaTiO 3 -type hexagonal perovskite structure. Mossbauer spectra show all Fe atoms to be present as Fe 3+ in BaTiO 3 lattice, occupying M 1 octahedral and pentahedral sites. Room-temperature ferromagnetism is exhibited and saturation magnetization gradually decreases with increasing Fe content. The observed ferromagnetism is considered to be an intrinsic property of Ba(Ti 1− x Fe x )O 3 , originating from super-exchange interactions between Fe 3+ in different occupational sites associated with oxygen vacancies. The variation in magnetization with Fe content is related to the ratio of pentahedral to octahedral sites and oxygen vacancies.

Hf x Zn 1− x O thin films ( x =3, 7, 10 and 15 mol%) were deposited on Si (1 0 0) using pulsed laser deposition. The influence of the Hf concentration on the microstructure and optical properties of the films was studied. It is found that Hf ions can be effectively doped into ZnO and all films crystallize in the hexagonal wurtzite structure with a preferred c -axis orientation. The lattice constants of Hf x Zn 1− x O films increase with the Hf contents. Two ultraviolet peaks centered at about 364 and 380 nm coexist in the fluorescent spectra. With increasing the Hf contents, the intensity of fluorescent peaks enhances remarkably. At the same time the energy gaps gradually increase, while the positions of ultraviolet peaks remain unchanged. The mechanism of luminescent emission for Hf x Zn 1− x O films was discussed.

The single-phase Fe3Si thin films with preferred (220) growth orientation were prepared on Si(100) substrates by pulsed-laser deposition. Different states of order were developed by changing the substrate temperature from room temperature to 500 °C. X-ray diffraction, Mossbauer spectroscopy and macroscopic magnetic measurements were used to analyze changes of structural order and magnetic properties with the substrate temperature. The results show that over the whole range of substrate temperatures considered all films are of Fe3Si single phase, highly oriented along the (220) plane. With increasing the substrate temperature, the structural order type changes from A2 through B2 to DO3 and the order degree gradually increases. Meanwhile, the saturation magnetization remarkably decreases with the increase of the substrate temperature, induced by Si segregation from the substrate and embedment into the film probably as the amorphous phase. The room temperature grown film has a high saturation magnetization of 917 kA m− 1, which nearly equals that of bulk DO3-Fe3Si.

Amorphous Ce1Y2Fe5O12 (Ce:YIG) thin films deposited on single crystal Si(1 0 0) and thermally oxidized Si(1 0 0) substrates by pulsed laser deposition were annealed in the temperature range of 700–1000 °C in air. The annealing temperature dependence of microstructure and magnetic properties of Ce:YIG films was studied using X-ray diffraction combined with vibrating sample magnetometer. The results show that single phase of polycrystalline Ce:YIG thin films can be obtained by the post-annealing of as-deposited films at the temperature of 700 °C. However, two steps of phase segregation of Ce:YIG occur as the post-annealing temperature increases: at first, Ce:YIG is decomposed into YIG and non-magnetic CeO2 when annealed at 800 °C; then YIG continues to be decomposed forming Fe2O3 when the temperature is increased up to 900 °C. Consequently, the saturation magnetization of Ce:YIG films decreases first and then increases with the post-annealing temperature going up, which indicates that the saturation magnetization of Ce:YIG films is mainly related to the phase composition of the films. Meanwhile, the presence of SiO2 buffer layer can significantly enhance the saturation magnetization of Ce:YIG films.

Highly c-axis oriented ZnO thin films were deposited by laser ablating Zn target in oxygen atmosphere at room temperature. Changes of c-axis lattice constant and optical energy gap caused by oxygen defects were studied via the control of oxygen vacancy concentration afforded by changing oxygen pressure in the deposition process. The results show that ZnO thin films prepared at room temperature have a very strong preferred c-axis growth orientation. As oxygen pressure increases, the lattice constant c decreases, and optical energy gap shifts towards higher energy values.

FeGeNi ternary alloy has been prepared by mechanical alloying followed by thermal treatment. Microstructure of the as-milled and annealed samples as well as thermally induced phase transition and variations in degree of order were investigated through a combination of X-ray diffraction and Mossbauer spectroscopy. The results show that alloying in the (Fe 0.81 Ni 0.19 ) 3.94 Ge 2 ternary system initiates during ball milling, with the appearance of partially D0 3 -ordered A2 solid solution phase and paramagnetic B8 2 FeGeNi ternary phase. A subsequent thermal treatment of 2 h at 800 K produces a remarkable increase in the degree of the D0 3 order, whilst ferromagnetic phase with amorphous character in the as-milled sample is transformed into paramagnetic B8 2 FeGeNi ternary phase where almost all Ni atoms occupy the vacant 2d site. Therefore in the annealed system coexist two phases, namely the D0 3 phase and paramagnetic B8 2 FeGeNi ternary phase.

Structural properties of Fe60Cr40 powders prepared by mechanical alloying in different atmosphere were investigated by using X-ray diffraction and 57Fe Mossbauer spectrum. The results show the formation of BCC paramagnetic structure when the sample sealed under Argon milled for 85 h. While in vacuum of 6×10−2 Pa, the BCC magnetic disordered solid solution is obtained for milled 45 h and more. The hyperfine filed distributions of Fe60Cr40 milled for 85 h in Argon is comprised of the crystalline grains and the disordered grain boundaries. Those of Fe60Cr40 milled for 85 h in vacuum indicate the formation of disordered alloys with nanoscale heterogeneous structures and the homogenization still proceed for a long milling time.

X-ray diffraction (XRD), 57Fe Mossbauer and magnetic measurements were performed on mechanically alloyed FexCr90−xMn10 (x=18 and 35) alloys. XRD and 57Fe Mossbauer spectra show the systems are non-magnetic BCC phase. The irreversibility between field-cooled and zero-field-cooled magnetization curves at relative high temperature is due to the blocking of some single particles. A correlation between the Fe concentration and Tmax of ZFC curves can be explained by the fine Fe-rich clusters which are embedded in a paramagnetic solid solution. The Fe35Cr55Mn10 alloy exhibits two type of blocking/freezing in an ensemble of nanoparticles. With the decreasing of temperature, the Fe18Cr72Mn10 alloy shows spin-glass-like behavior and the Fe35Cr55Mn10 alloy goes through two magnetic transitions from paramagnetic phase to ferromagnetic phase and then to re-entrant spin-glass state.

We have prepared FeSiBNbCu thin films by radio frequency (RF) magnetron sputtering and investigated the influence of the sputtering power on the microstructures of FeSiBNbCu thin films through analyzing their microstructural configurations by X-ray diffraction (XRD) and Mossbauer spectroscopy. The results showed that FeSiBNbCu thin films were amorphous when deposited with low sputtering power density and exhibited the mixed structure of crystalline and amorphous components with increasing the sputtering power density, without any heat treatment. The crystallites contained nanosized α-Fe(Si) and α-Fe(B) solid solutions of which the volume fractions and the microstructural configurations changed with the sputtering power.

Ge 99.04 Mn 0.96 thin film was fabricated by thermal evaporation of Mn doped GeO 2 ceramic film under hydrogen atmosphere. Secondary phases were not detected by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) analyses. The film is p type. Room-temperature ferromagnetism was detected in the film. The ferromagnetic behavior may arise from alignment of the bound magnetic polarons (BMP) mediated by the localized holes in the system as well as ultra small secondary phases unable to be detected by XRD and HRTEM analyses.