The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

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A comprehensive review of zno materials and devices

Every few years, a new material with unique properties emerges and fascinates the scientific community, typical recent examples being high-temperature superconductors and carbon nanotubes. Graphene is the latest sensation with unusual properties, such as half-integer quantum Hall effect and ballistic electron transport. This two-dimensional material which is the parent of all graphitic carbon forms is strictly expected to comprise a single layer, but there is considerable interest in investigating two-layer and few-layer graphenes as well. Synthesis and characterization of graphenes pose challenges, but there has been considerable progress in the last year or so. Herein, we present the status of graphene research which includes aspects related to synthesis, characterization, structure, and properties.

Graphene: The New Two-Dimensional Nanomaterial

Topological insulators are materials with a bulk excitation gap generated by the spin-orbit interaction that are different from conventional insulators. This distinction is characterized by ${Z}_{2}$ topological invariants, which characterize the ground state. In two dimensions, there is a single ${Z}_{2}$ invariant that distinguishes the ordinary insulator from the quantum spin-Hall phase. In three dimensions, there are four ${Z}_{2}$ invariants that distinguish the ordinary insulator from ``weak'' and ``strong'' topological insulators. These phases are characterized by the presence of gapless surface (or edge) states. In the two-dimensional quantum spin-Hall phase and the three-dimensional strong topological insulator, these states are robust and are insensitive to weak disorder and interactions. In this paper, we show that the presence of inversion symmetry greatly simplifies the problem of evaluating the ${Z}_{2}$ invariants. We show that the invariants can be determined from the knowledge of the parity of the occupied Bloch wave functions at the time-reversal invariant points in the Brillouin zone. Using this approach, we predict a number of specific materials that are strong topological insulators, including the semiconducting alloy ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$ as well as $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Sn}$ and HgTe under uniaxial strain. This paper also includes an expanded discussion of our formulation of the topological insulators in both two and three dimensions, as well as implications for experiments.

/pdf/topological-insulators-with-inversion-symmetry-1vaykhe1u1.pdf

Topological insulators with inversion symmetry

We have used the pH-induced self-assembly of a peptide-amphiphile to make a nanostructured fibrous scaffold reminiscent of extracellular matrix. The design of this peptide-amphiphile allows the nanofibers to be reversibly cross-linked to enhance or decrease their structural integrity. After cross-linking, the fibers are able to direct mineralization of hydroxyapatite to form a composite material in which the crystallographic c axes of hydroxyapatite are aligned with the long axes of the fibers. This alignment is the same as that observed between collagen fibrils and hydroxyapatite crystals in bone.

http://science.sciencemag.org/content/sci/294/5547/1684.full.pdf

Self-assembly and mineralization of peptide-amphiphile nanofibers

Nano-optics of surface plasmon polaritons

The momentum-dependent electron-hole exchange interaction splits the triply degenerate orthoexcitons into the bright and dark states in ${\mathrm{Cu}}_{2}\mathrm{O}$, depending on the direction of $\mathbf{k}$ inside the crystal. Employing resonant two-photon excitation along a (100) direction at $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, we selectively create the dark orthoexcitons that do not couple to the radiation field. However, these excitons can radiatively recombine due to subsequent conversion into the bright states. By measuring the polarization dependence, we find that the two bright states are equally populated due to cross relaxation. We also find that this bright orthoexciton luminescence is highly directional and well aligned with the initial laser propagating direction. This strongly indicates that propagating quadrupole orthoexciton-polaritons, which are the quantum superposition of bright orthoexcitons and photons, are indirectly generated. We find that the dark-to-bright relaxation mechanism can be significantly affected in the presence of hot excitons due to density-dependent scattering.

Indirect generation of quadrupole polaritons from dark excitons in Cu 2 O

We have measured the complex acoustic shear impedance of liquid 3He by observing the decay of a transiently excited AC cut quartz transducer. Comparison with recent theories suggests that our results at low temperatures can only be due to the excitation of transverse zero sound. We have also observed the direct transmission of transverse excitations between two closely spaced transducers in the vicinity of 3 mK. These results are analyzed in terms of the propagation of transverse zero sound although theory suggests that a single-particle contribution must also be present.

Transverse Zero Sound in Normal 3He

A review of ultrasonic measurements on the three heavy fermion compounds UPt3, URu2Si2 and CeCu6 is given. These compounds show one or more of the three types of co-operative behavior; Kondo-like effect, antiferromagnetism and superconductivity; all these features are observed as ultrasonic anomalies. Both UPt3 and CeCu6 show a strong coherence effect which is akin to a Kondo-like transition in the electronic system. Velocity anomalies are found at the antiferromagnetic transition for URu2Si2. UPt3 also has an antiferromagnetic transition which is believed to influence the subsequent superconducting ordering. For UPt3, the superconductivity is of an unconventional nature; multiple superconducting phases have been observed and a complete phase diagram has been obtained from high resolution velocity measurements.

Ultrasonic studies of the heavy fermion compounds

Anisotropic ultrasound propagation was studied in a magnetically aligned monodomain smectic-$A$ liquid crystal (di-$n$-decyl azoxy methyl cinnamate). A large attenuation was observed near the isotropic-smectic-$A$ phase transition. The anisotropy in the sound velocity was found to be in agreement with the predictions of hydrodynamic theory, and no dispersion was found in the frequency range used (4-12 MHz). The temperature dependence of the physically meaningful elastic constants was determined. The anisotropy in the attenuation was also measured. Unlike the velocity anisotropy, no intermediate minimum was observed.

John B Ketterson

Papers

Indirect generation of quadrupole polaritons from dark excitons in Cu 2 O

Transverse Zero Sound in Normal 3He

Ultrasonic studies of the heavy fermion compounds

Anisotropic ultrasound propagation in a smectic-A liquid crystal

New Relaxation Effect in the Sound Attenuation in Liquid 4 He Under Pressure