Search

Topological insulators are a new class of materials with an insulating bulk and topologically protected metallic surface states. Although it is widely assumed that these surface states display a Dirac-type dispersion that is symmetric above and below the Dirac point, this exact equivalence across the Fermi level has yet to be established experimentally. Here, we present a detailed transport study of the 3D topological insulator-strained HgTe that strongly challenges this prevailing viewpoint. First, we establish the existence of exclusively surface-dominated transport via the observation of an ambipolar surface quantum Hall effect and quantum oscillations in the Seebeck and Nernst effect. Second, we show that, whereas the thermopower is diffusion driven for surface electrons, both diffusion and phonon drag contributions are essential for the hole surface carriers. This distinct behavior in the thermoelectric response is explained by a strong deviation from the linear dispersion relation for the surface states, with a much flatter dispersion for holes compared with electrons. These findings show that the metallic surface states in topological insulators can exhibit both strong electron–hole asymmetry and a strong deviation from a linear dispersion but remain topologically protected. [ABSTRACT FROM AUTHOR]

The properties of InGaAsN V-groove QWRs are assessed here by polarization-dependent photoluminescence (PL) and micro-magneto-PL. Both the polarization anisotropy of the QWR emission and the strong dependence of the diamagnetic shift on the orientation of the applied magnetic field confirm the 1D nature of the QWR excitons. Further, the possibility of passivating N impurities by H irradiation is used to estimate the N content ( x) in the QWRs by turning off the effects of N incorporation. Both the H-induced blueshift of the QWR emission (70 meV) and the measured value of the electron effective mass are consistent with x ∼1%. Nitrogen is also found to enhance the In intake in the QWR, likely due to the strain reduction resulting from the smaller lattice parameter of the InGaAsN alloy. Such strain reduction is also responsible for the quick decay of the degree of linear polarization (ρ) of the QWR emission with increasing temperature, indicating a small splitting between the QWR valence-band levels. In fully hydrogenated samples, conversely, ρ remains roughly constant up to ∼240 K, suggesting the recovery of a larger energy separation between the QWR hole states upon N passivation. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

The puzzling electronic and transport properties of the Ga(AsBi) alloy are investigated for a wide range of Bi-concentrations ( x = 0-10.6%) by means of various experimental techniques in high magnetic fields ( B up to 30 T): magneto-photoluminescence spectroscopy, magneto-far-infrared (FIR) absorption spectroscopy, and Hall effect measurements. Our experimental findings suggest that the strength of hybridization of the continuum states of the valence and conduction bands with the Bi-related electronic levels depends on the Bi-concentration, thus leading to band edges with a localized (for x < 6%) or band-like character (for x > 8%). We report an unusual compositional-dependence of the exciton reduced mass ( µexc), whose value can be larger (for x < 6%) or smaller (for x > 8%) than in GaAs depending on the Bi-concentration. Correspondingly, the free-hole mobility ( µh) decreases with increasing Bi-concentration and eventually tends to increase for x > ∼8%. The incorporation of Bi in GaAs also induces the formation of acceptor levels, which we reveal by FIR absorption spectroscopy and Hall effect measurements. The Bi-induced acceptors are characterized by an exceedingly high value of the effective ground-state g-factor ( geff ∼ 15) and are responsible for the increasing p-type conductivity observed in nominally undoped Ga(AsBi) alloys with increasing Bi-concentration. [ABSTRACT FROM AUTHOR]

Most enzyme studies are carried out in bulk aqueous solution, at the so-called ensemble level, but more recently studies have appeared in which enzyme activity is measured at the level of a single molecule, revealing previously unseen properties. To this end, enzymes have been chemically or physically anchored to a surface, which is often disadvantageous because it may lead to denaturation. In a natural environment, enzymes are present in a confined reaction space, which inspired us to develop a generic method to carry out single-enzyme experiments in the restricted spatial environment of a virus capsid. We report here the incorporation of individual horseradish peroxidase enzymes in the inner cavity of a virus, and describe single-molecule studies on their enzymatic behaviour. These show that the virus capsid is permeable for substrate and product and that this permeability can be altered by changing pH. [ABSTRACT FROM AUTHOR]

Virus particles are probably the most precisely defined nanometre-sized objects that can be formed by protein self-assembly. Although their natural function is the storage and transport of genetic material, they have more recently been applied as scaffolds for mineralization and as containers for the encapsulation of inorganic compounds. The reproductive power of viruses has been used to develop versatile analytical methods, such as phage display, for the selection and identification of (bio)active compounds. To date, the combined use of self-assembly and reproduction has not been used for the construction of catalytic systems. Here we describe a self-assembled system based on a plant virus that has its coat protein genetically modified to provide it with a lipase enzyme. Using single-object and bulk catalytic studies, we prove that the virus-anchored lipase molecules are catalytically active. This anchored biocatalyst, unlike man-made supported catalysts, has the capability to reproduce itself in vivo, generating many independent catalytically active copies. [ABSTRACT FROM AUTHOR]

Magnetic birefringence was used for in situ monitoring of the morphological changes in diamagnetic polymersomes during shapetransformation by dialysis. The birefringence was found to be very sensitive to the polymersome morphology, as determined by electron microscopy. The deflation of polymersomes into disks was observed, followed by a bending and partial inflation into stomatocytes. [ABSTRACT FROM AUTHOR]