No abstract

Topological insulators, a new quantum state of matter, create exciting opportunities for studying topological quantum physics and for exploring spintronic applications due to their gapless helical metallic surface states. Here, we report the observation of weak anti-localization and quantum oscillations originated from surface states in Bi2Se2Te crystals. Angle-resolved photoemission spectroscopy measurements on cleaved Bi2Se2Te crystals show a well-defined linear dispersion without intersection of the conduction band. The measured weak anti-localization effect agrees well with the Hikami-Larkin-Nagaoka model and the extracted phase coherent length shows a power-law dependence with temperature (∼T−0.44), indicating the presence of the surface states. More importantly, the analysis of a Landau-level fan diagram of Shubnikov-de Hass oscillations yields a finite Berry phase of ∼0.42π, suggesting the Dirac nature of the surface states. Our results demonstrate that Bi2Se2Te can serve as a suitable topological insulator candidate for achieving intrinsic quantum transport of surface Dirac fermions.

We introduce an improved and simplified structure made of periodic arrays of pairs of H-shaped metallic wires that offer a potentially simpler approach in building negative-index materials. Using simulations and microwave experiments, we have investigated the negative-index n properties of these structures. We have measured experimentally both the transmittance and the reflectance properties and found unambiguously that a negative refractive index with Re(n)<0 and Im(n)

We investigate the radiation properties of an antenna that was formed by a hybrid combination of a monopole radiation source and a cavity built around a dielectric layer-by-layer three-dimensional photonic crystal. We measured a maximum directivity of 310, and a power enhancement of 180 at the resonant frequency of the cavity. We observed that the antenna has a narrow bandwidth determined by the cavity, where the resonant frequency can be tuned within the band gap of the photonic crystal. The measured radiation patterns agree well with our theoretical results. © 2000 American Institute of Physics. ͓S0021-8979͑00͒03601-X͔ Photonic crystals, in which electromagnetic ͑EM͒ wave propagation is forbidden in all directions for a certain range of frequencies, have a wide range of applications extending from microwave to optical frequencies. 1-3 However, the fabrication of photonic crystals at optical frequencies was a major challenge since the invention of these materials nearly a decade ago. Recently, a photonic crystal with a full threedimensional ͑3D͒ band gap at 1.55 m wavelength was reported by Fleming and Lin. 4 In addition to this major breakthrough, the same structure was previously fabricated at millimeter wave and microwave frequencies, 5,6 where a number of photonic crystal based applications were demonstrated. 7 Among these applications, there is a great deal of growing interest for photonic crystal-based antennas. 8,9 The reported experimental and theoretical studies on the antenna applications mostly made use of the total reflection property of photonic crystals. The antennas mounted on photonic crystal substrate surfaces exhibited high efficiency and directivity compared to conventional antennas on dielectric substrates. 10 Although high directivities which could be achieved using array antennas on photonic crystals were suggested, 11 the maximum directivity that was demonstrated by Brown and McMahon using a photonic crystal-based single dipole antenna was 10, along with a radiative gain of 8. 10 One other important property of photonic crystals is that by breaking the periodicity of the crystal, one can create resonant cavities. Resonant cavity enhanced detectors 12 and waveguide applications 13 were recently demonstrated using localized modes of the cavities built around photonic crystals. In this letter, we report a photonic crystal-based resonant antenna with a very high directivity and gain. The antenna was formed by a hybrid combination of a monopole radiation source and a cavity built around a dielectric 3D layer-by-layer photonic crystal.The layer-by-layer dielectric photonic crystal we used in our experiments was designed to have a three-dimensional band gap with a midgap frequency around 12 GHz. 13 We used the output port of a microwave network analyzer and a monopole antenna to obtain EM waves. The monopole antenna was constructed by removing the shield around one end of a microwave coaxial cable. The cleaved center conductor, which also acted as the radiation source, was 6 mm long. The chosen ...