Single-phase insulating BiFeO3 ceramics have been synthesized by a simple but effective method that conventional solid state reaction is followed immediately by quenching processing. At room temperature, the ceramics show a metastable, distorted rhombohedral phase and the refined structure parameters are presented based on x-ray diffraction. It is revealed that the formations of Fe2+ and oxygen deficiency are greatly suppressed by the quenching processing. Awell-saturated ferroelectric hysteresis loop with a large remnant polarization (2Pr=23.5μC/cm2) is observed with an applied field of 155kV/cm. Temperature-dependent magnetic property is investigated and weak ferromagnetism with a remnant magnetization of 4×10−6μB/Fe at 10K is established. These results may have implications for further studies on multiferroics.

In this paper, a two-dimensional square photonic crystal sPCd with superconductor cylinders is proposed to realize tunable negative refraction. Based on the dependence of the superconductors' permittivity on temperatures, photonic band structures thus negative refraction could be tuned by temperatures, whereby the refractive angle could be scanned from positive to negative. The feasibility of the PC operating in infrared and visible regions was discussed. The tunability resulted from the lattice, superconductors, operating frequency, and incident angle may lead the PC to great promise in photoelectronics and superconductor electronic applications.

The phonon-polariton dispersion relation is calculated in the piezoelectric superlattice (PSL). In this specific case, the coupling could take place not only between the electromagnetic (EM) waves (photons) and the transverse superlattice vibrations (transverse phonons), but also between the EM waves and the longitudinal superlattice vibrations (longitudinal phonons), which gives rise to two types of phonon polaritons. It differs with the case in an ionic crystal, where the phonon polariton could only originate from the coupling between transverse lattice vibrations (transverse optical phonons) and EM waves. The two types of phonon polaritons are experimentally observed in the one-dimensional PSL with a periodicity of micrometer scale, corresponding to the frequency region of microwave. The PSL is based on a congruent lithium tantalite (LiTaO3) crystal and fabricated by the method of electric field poling at room temperature. Both microwave absorption and dielectric abnormality are observed and the sizes of band gaps are measured for the two types of phonon polaritons.

Acoustic negative refractions with backward-wave (BW) effects were both theoretically and experimentally established in the second band of a two-dimensional (2D) triangular sonic crystal (SC). Intense Bragg scatterings result in the extreme deformation of the second band equifrequency surface (EFS) into two classes: one around the K point and the other around the Г point of the reduced Brillouin zone. The two classes can lead to BW negative refractions (BWNRs) but with reverse negative refraction dependences on frequencies and incident angles. Not only BWNR but BW positive refraction can be present at EFSs around the K point, so it is possible to enhance the resolution of acoustic waves with a subdiffraction limit regardless of refractions, which is no analogy in both left-handed material and SCs' first band. These abundant characters make refractions in the second band distinguished.

A two-dimensional square photonic crystal sPCd with magnetically active semiconductor constituents, for example GaAs, is proposed to realize tunable negative refractions. By the Voigt effect, the magnetic-dependent permittivity of the semiconductor, then PC's band structure and equifrequency surface could be tuned by an external magnetic field. Taking a real model into consideration with absorption, the calculation reveals that the refraction could be manipulated from positive to negative, and then to positive by the applied fields. With the tunable negative refraction, tunable superlenses with all angle negative refraction could be established by a relatively small magnetic field s0.262-0.315Td, which might lead to great potential applications in photoelectronic devices.