Double negativity in acoustic waves

It has been demonstrated that light bends into the “wrong” quadrant when impinging from a normal medium to a double negative (DNG) medium. A DNG medium in electromagnetism is regarded as an effectively homogeneous medium having both negative permittivity and negative permeability. Here, we prove that such a concept can apply also to other kinds of classical waves. In particular, for acoustic waves, if we want sound to bend into the “wrong” quadrant after hitting an interface between air and the DNG medium, it turns out that we need a double negativity in both density and bulk modulus for the DNG medium.

Continue reading Double negativity in acoustic waves

Electrorheological (ER) fluids

Electrorheological (ER) fluids denote a class of materials consisting of nanometer to micrometer sized solid particles dispersed in a liquid, whose rheological (i.e., deformation and flow) properties are controllable by an external electric field. In particular, they can reversibly transform from a liquid to a solid within one hundredth of a second. While in the solid state (with the electric field applied), the strength of that solid, measured by the yield stress, is the critical parameter that governs the application potential of the ER fluid. Since their discovery some sixty years ago, there has been much effort in searching for ER fluids with high yield stress, mainly due to their applications in vibration damping, clutches, and practically all mechanical devices that involve motion transmission. In particular, the automotive applications, as a potential replacement for gears, have attracted sustained research at all the research laboratories of major automotive companies. In the late 1980s General Motors carried out a study on the application potential of ER fluids, and concluded that a major hurdle was the low yield stress of the ER fluids.

Continue reading Electrorheological (ER) fluids

Carbon Nanotubes: fabrication of world’s smallest single-walled carbon nanotubes

Using microporous zeolite single crystals as hosts, Dr Zikang Tang and Dr Ning Wang from the Physics Department succeeded in fabricating the world’s smallest single-walled carbon nanotubes (SWNTs) that are periodically aligned in the crystal channels. The diameter of the single-walled carbon nanotube is only 0.4nm (nanometer).

Carbon nanotubes are long, thin cylinders of carbon. Single-walled carbon nanotubes are formed by rolling single-atomic graphite layer into a cylinder. They are extremely small in size – a bundle of 1,000,000 carbon nanotubes equals the size of a hair. Their unique physical structures, electronic properties, and their intriguing potentials for wide applications have sparked an explosion of research into their understanding since they were discovered in 1991.

Continue reading Carbon Nanotubes: fabrication of world’s smallest single-walled carbon nanotubes

Discovery of superconductivity in 4-Angstrom single-walled carbon nanotubes

Following on the success of Zikang Tang in fabricating 4-Angstrom single-walled carbon nanotubes (which are at or close to the theoretical limit of nanotubes’ diameter) and Ning Wang’s direct TEM visualization of these nanotubes, superconductivity has been discovered in these nanotubes. This discovery followed a rather tortuous path, somewhat interesting in its own right. Hence I record it here. The related work was started in 1997-98, when measurement of low temperature electrical transport through the nanotubes, by Jiannong Wang, uncovered a rather sudden disappearance of charge carriers below 12 kelvin. Recognizing the importance of this result, Ping organized efforts to elucidate the underlying cause of this intriguing phenomenon. A casual visit by Steve Louie (UC Berkeley) has brought the attention to a 1995 paper by himself, Marvin Cohen, and their collaborators, predicting superconductivity in carbon nanotubes with increased transition temperature as the diameter decreases. With this jolt, Ping persuaded Xixiang Zhang and his student to measure the Meissner effect. The positive result on the Meissner effect clarified, for Ping at least, the earlier observed phenomenon of the disappearance of the charge carriers (caused by the opening of the superconducting gap at the Fermi level). Continue reading Discovery of superconductivity in 4-Angstrom single-walled carbon nanotubes