Broken Gap Research Articles

Estimation of Fracture Toughness of Fiber/Matrix Interface

To estimate the level of interfacial adhesion between the fiber and polymer-matrix in fiber/matrix composites, interfacial fracture toughness was derived based on an energy balance scheme. A break gap occurring at the initial interface debond during fragmentation test was taken into account. The external work and work done against friction slip at debonded interface were included in energy balance scheme. The interfacial fracture toughness obtained is a function of fiber and matrix material properties and critical debond strain which is the external strain applied on the specimen to initiate interfacial debonding. The break gap calculated increases with increasing applied load and decreasing friction coefficient. The proportion of each energy terms in energy balance equation was calculated. Comparison between the predicted interfacial fracture toughness and that numerical result was done.

Carrier control and transport modulation in GaSb/InAsSb core/shell nanowires

We report transport studies of GaSb/InAs core/shell nanowires. It is shown that with increasing InAs shell thickness, it is possible to tune the carrier concentrations and transport in the structures from p-type (core-dominated) to n-type (shell dominated). For nanowires with an intermediate shell thickness (5–7 nm), we show that the transport is ambipolar, such that an applied top-gate potential can provide further control of carrier type and transport path. In this range, the nature of the GaSb-InAs junction also changes from broken gap (semimetal) to staggered (narrow bandgap) with a small decrease in shell thickness. From a device point of view, we demonstrate that the presence of a thin (<3 nm) InAs shell improves p-type GaSb nanowire transistor characteristics.

Magneto-optical and optical probes of gapped ground states of bilayer graphene

We study the influence of different kinds of gaps in a quasiparticle spectrum on longitudinal and transverse optical conductivities of bilayer graphene. An exact analytical expression for magneto-optical conductivity is derived using a low-energy two-band Hamiltonian. We consider how the layer asymmetry gap caused by a bias electric field and a time-reversal symmetry breaking gap affect the absorption lines. The limit of zero magnetic field is then analyzed for an arbitrary carrier density in the two-band model. For a neutral bilayer graphene, the optical Hall and longitudinal conductivities are calculated exactly in the four-band model with four different gaps and zero magnetic field. It is shown that two different time-reversal symmetry breaking states can be distinguished by analyzing the dependence of the optical Hall conductivity on the energy of photon. These time-reversal symmetry breaking states are expected to be observed experimentally via optical polarization rotation either in the Faraday or Kerr effects. We analyze a possibility of such an experiment for a free-standing graphene, graphene on a thick substrate, and graphene on a double-layer substrate.

Ultra-low resistive GaSb/InAs tunnel junctions

The GaSb and InAs(Sb) material combination results in a type-III (broken gap) band alignment and is of particular interest for use as an ohmic, low-resistive intra-cavity contact in complex optoelectronic devices, such as buried-tunnel-junction vertical-cavity surface-emitting lasers. In this work, we report electrical characteristics of MBE-grown p+-GaSb/n+-InAs tunnel junctions. The investigated structures exhibit ultra-low resistive behavior, yielding specific resistivity values below 2.8 × 10−7Ω cm2. This value is nearly ten times better than previously reported best values.

Influences of Rail Expansion Joint on Jointless Turnout on Ballasted Continuous Beam Bridge

Based on turnout/bridge interaction principle and finite element method (FEM), an integrated turnout/beam/pier model of jointless turnout on ballasted track was established to analyze the influences of expansion joint on stress and deformation of the turnout. The results are concluded as follows: whether the rail expansion joint is set in front of or behind the turnout, expansion additional force of stock rail will be reduced greatly at the end of the beam; so do the expansion displacement of switch rail and nose rail, the stress of displacement restrictor and spacer block, rail break gap and the longitudinal force of one rail after the other rail broke. But when the expansion joint is set in front of the turnout, the pier’s longitudinal force of continuous beam bridge and the simply supported beam bridge (within the expansion range of the device) increase greatly. When the device is close to the turnout, longitudinal relative displacement of the stock rail to the girder also increases a lot. By comparison, it is more favorable to set the expansion joint behind the turnout, or to set the device at the both ends of the continuous beam, or to set the device at the ends of continuous beam rather than in the center of the span.

Band structure of core-shell semiconductor nanowires

We have calculated band structures for strained core-shell nanowires involving all combinations of AlN, GaN, and InN, as well as all combinations of AlP, GaP, AlAs, GaAs, InP, InAs, AlSb, GaSb, and InSb, as functions of core and shell radii. This gives 78 combinations, most of which have not been experimentally realized, and provides a quite complete overview of which interesting structures can be realized in core-shell zinc-blende III-V nanowires. Both the Gamma-and the X-conduction-band minima were included in the calculations in addition to the valence-band maximum. The calculations were performed using continuum elasticity theory for the strain, eight-band strain-dependent k center dot p theory for the Gamma-point energies, and a single-band approximation for the X-point conduction minima. All combinations of materials having type-I, type-II, and type-III (broken gap) band alignments have been identified, as well as all combinations for which one material becomes metallic due to a negative band gap. We identify structures that may support exciton crystals, excitonic superconductivity, and biomolecular detection. We have also computed the effective masses from which the confinement energy may be estimated. While graphical presentation of the results helps identify trends, all the numerical results are also available online. (Less)

Infrared imaging with InAs/GaSb type‐II superlattices

AbstractInAs/GaSb short period superlattices (SLs) with broken gap typ‐II band alignment are very well suited for the fabrication of high performance infrared camera systems for the second atmospheric window (3–5 µm). High resolution focal plane arrays for single‐ and dual‐colour applications based on InAs/GaSb SLs have been fabricated. Those camera systems reveal excellent performance data which are comparable and even superior to state of the art thermal imaging systems. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

256×256 focal plane array midwavelength infrared camera based on InAs/GaSb short-period superlattices

An infrared camera based on a 256×256 focal plane array (FPA) for the second atmospheric window (3–5 µm) has been realized for the first time with InAs/GaSb short period superlattices (SLs). The SL detector structure with a broken gap type-II band alignment was grown by molecular beam epitaxy on GaSb substrates. Effective bandgap and strain in the superlattice were adjusted by varying the thickness of the InAs and GaSb layers and the controlled formation of InSb-like bonds at the interfaces. The FPAs were processed in a full wafer process using optical lithography, chemical-assisted ion beam etching, and conventional metallization technology. The FPAs were flip-chip bonded using indium solder bumps with a read-out integrated circuit and mounted into an integrated detector cooler assembly. The FPAs with a cut-off wavelength of 5.4 µm exhibit quantum efficiencies of 30% and detectivity values exceeding 1013 Jones at T=77 K. A noise equivalent temperature difference (NETD) of 11.1 mK was measured for an integration time of 5 ms using f/2 optics. The NETD scales inversely proportional to the square root of the integration time between 5 ms and 1 ms, revealing background limited performance. Excellent thermal images with low NETD values and a very good modulation transfer function demonstrate the high potential of this material system for the fabrication of future thermal imaging systems.

New developments in mid-infrared Sb-based lasers

A number of different approaches are being investigated to obtain high-performance mid-infrared (2-5 μm) diode lasers for applications such as infrared lidar, remote sensing and environmental monitoring. They include laser heterostructures based on interband transitions in quantum-well or superlattice active region, and quantum cascade structures based on unipolar intersubband transitions. The Sb-containing structures, employing GaSb, InAs, AlSb and related alloys, are focusing actually much attention. Significant improvements in the molecular beam epitaxy of these alloys make possible now the growth of laser antimonide structures of high structural quality. Excellent performances have been reported at = 2 μm from GaInAsSb /AlGaAsSb and at = 3.5 μm from InAsSb/InAlAsSb type-I quantum-well diode lasers. Type-II (staggered alignment) GaInAsSb/GaSb and type-III (broken gap alignment) InAs/Ga(In)Sb strained multiquantum-well lasers are promising material systems for mid-infrared sources, due to their large conduction and valence band offsets, the potential of Auger process suppression and the enhancement of the electron-hole optical coupling by wave function engineering. Besides high performance interband quantum cascade lasers operating at room temperature with negligible current leakage and high output power can be designed from Sb-containing type-III structures.

Minigaps and the quantum Hall effect in broken gap InAs/GaSb heterostructures

InAs/GaSb heterojunctions offer the unique situation in a semiconductor of overlapping conduction and valence bands. A wavevector dependent minigap occurs due to the anticrossing of the electron and hole dispersion relations. As a result the resistivity increases strongly in very pure intrinsic structures due to the reduction in the electron group velocity. Applying a magnetic field parallel to the layers shifts the conduction and valence band relative to each other in k-space and the structure turns back to an indirect semimetal. As a result we can observe a giant negative magnetoresistance. Interband absorption is observed across the minigap, and this can also be removed by the parallel field which transforms the system to an indirect band gap alignment. In high perpendicular magnetic fields strong oscillations occur in the Hall coefficient, and at low temperatures a series of zero Hall resistance states are found in which a divergence of the diagonal resistivity occurs at the same point as a zero in the Hall resistance.

Current transport in multiple superlattice structures

We report on transport measurements in `broken gap' superlattices. A three terminal device is used to inject a tunable electron beam via a tunneling barrier into a multiple superlattice and to collect the transmitted current as a function of the injector energy. Significant increase of the derivative of the collector current is observed due to miniband conduction in the superlattice. The transconductance can be used to probe miniband positions, miniband width, and interminiband transition probabilities as a function of miniband gaps and electrical field.

Silastic rod pseudosheaths and tendon graft healing.

By examining the breaking strength and gap formation of tendon graft junction sites covered with pseudosheath at various times of immobilization, no significant increase was found compared to junction sites not covered by the pseudosheath. The optimum time for immobilization following tendon grafting is between 4 and 5 weeks in the chicken. Total coverage of the junction sites by pseudosheath did not affect gap formation.