The quantum spin Hall (QSH) state is a state of matter characterized by a non-trivial topology of its band structure, and associated conducting edge channels. The QSH state was predicted and experimentally demonstrated to be realized in HgTe quantum wells. The existence of the edge channels has been inferred from local and non-local transport measurements in sufficiently small devices. Here we directly confirm the existence of the edge channels by imaging the magnetic fields produced by current flowing in large Hall bars made from HgTe quantum wells.

Silicon is widely used for solar energy harvesting applications. Here we investigate the dynamics and transport properties of photoexcited carriers in silicon nanowires by ultrafast terahertz spectroscopy. The carrier lifetime was observed to approach 0.7 ns, and the carrier mobility to be ∼1000 cm 2/(Vs). We found that Silicon nanowire arrays fabricated by the metal-assisted chemical etching is better for solar cell application. © 2013 IEEE.

Two electrons on a string form a simple model system where Coulomb interactions are expected to play an interesting role. In the presence of strong interactions, these electrons are predicted to form a Wigner molecule, separating to the ends of the string. This spatial structure is believed to be clearly imprinted on the energy spectrum, yet so far a direct measurement of such a spectrum in a controllable one-dimensional setting is still missing. Here we use an ultraclean carbon nanotube to realize this system in a tunable potential.

We report the synthesis of a direct gap semiconductor, ZnSnN2, by a plasma-assisted vapor-liquid-solid technique. Powder X-ray diffraction measurements of polycrystalline material yielded lattice parameters in good agreement with predicted values. Photoluminescence efficiency at room temperature was observed to be independent of excitation intensity between 103 and 108 W/cm2. The band gap was measured by photoluminescence excitation spectroscopy to be 1.7 ± 0.1 eV.

Upon femtosecond laser pumping of a topological insulator Bi2Se3, we observed efficient THz generation from the surface electrons. By performing polarization-resolved studies on the emitted THz spectrum, two emission mechanisms are identified. THz emission spectroscopy provides a valuable spectroscopic tool for studies of the dynamics of the surface electrons in centrosymmetric topological insulators. © 2013 IEEE.

Precise vertical stacking and lateral stitching of two-dimensional (2D) materials, such as graphene and hexagonal boron nitride (h-BN), can be used to create ultrathin heterostructures with complex functionalities, but this diversity of behaviors also makes these new materials difficult to characterize. We report a DUV-vis-NIR hyperspectral microscope that provides imaging and spectroscopy at energies of up to 6.2 eV, allowing comprehensive, all-optical mapping of chemical composition in graphene/h-BN lateral heterojunctions and interlayer rotations in twisted bilayer graphene (tBLG).

We calculate the radio frequency (RF) spectrum of fermionic atoms near a narrow Feshbach resonance, explaining observations made in ultracold samples of 6Li. We use a two channel resonance model to show that the RF spectrum contains two peaks. In the wide-resonance limit, nearly all spectral weight lies in one of these peaks, and typically the second peak is very broad. We find strong temperature dependence, which can be traced to the energy dependence of the two-particle scattering.

Flying insects have evolved sophisticated sensory-motor systems, and here we argue that such systems are used to keep upright against intrinsic flight instabilities. We describe a theory that predicts the instability growth rate in body pitch from flapping-wing aerodynamics and reveals two ways of achieving balanced flight: active control with sufficiently rapid reactions and passive stabilization with high body drag.

Hierarchical porous polymer materials are of increasing importance because of their potential application in catalysis, separation technology, or bioengineering. Examples for their synthesis exist, but there is a need for a facile yet versatile conceptual approach to such hierarchical scaffolds and quantitative characterization of their nonperiodic pore systems.

The fabrication and testing of a prototype deep-depletion direct-conversion X-ray CCD detector are described. The device is fabricated on 600 μm-thick high-resistivity silicon, with 24 × 24 μm pixels in a 4k × 4k pixel format. Calibration measurements and the results of initial protein crystallography experiments at the Cornell High Energy Synchrotron Source (CHESS) F1 beamline are described, as well as suggested improvements for future versions of the detector. © 2013 International Union of Crystallography Printed in Singapore - all rights reserved.