type-I Structure Research Articles

Optical characterization of a strain-compensated GaAs0.64Sb0.36/GaAs0.79P0.21 quantum well structure grown by metal organic vapor phase epitaxy

Optical characterization of a strained GaAs0.64Sb0.36/GaAs and a strain-compensated GaAs0.64Sb0.36/GaAs0.79P0.21 triple quantum well (TQW) structures grown by metal organic vapor phase epitaxy has been carried out by using photoreflectance (PR), surface photovoltage spectroscopy (SPS) and photoluminescence (PL) techniques. For the GaAs0.64Sb0.36/GaAs TQW, only a very weak PR feature was observed in the vicinity of fundamental transition. A large blueshift of the peak position of PL feature of the sample at low temperature with increasing of excitation power density has been attributed to a weakly type-II heterojunction formed between GaAs0.64Sb0.36 and GaAs. The PR and surface photovoltage spectra of GaAs0.64Sb0.36/GaAs0.79P0.21 TQW display a series of features originated from interband transitions which is a typical characteristic of type-I QW structure. In order to identify the interband transitions, a calculation was performed based on the envelope function approximation using the conduction band-offset Qc and strain compensation factor γ as adjustable parameters. Good agreement between experimental results and theoretical calculation is achieved by taking Qc = 0.30 ± 0.05 and γ = 0.60 ± 0.05. The results indicate that the energy band of the strain-compensated QW structure is significantly influenced by replacing GaAs barrier by GaAs0.79P0.21 layers, which changes the weakly type-II to a type-I structure. The strain-compensated GaAs0.64Sb0.36/GaAs0.79P0.21 TQW has a larger overlap integral and hence a higher transition probability, providing a possibility for fabricating high efficiency near infrared laser diodes.

Hypersialylated type-I lactosamine-containing N-glycans found in Artiodactyla sera are potential xenoantigens

There is increasing interest in biologics, i.e. human-originated biological pharmaceutics. Most of the protein drugs developed so far, such as immunoglobulins and erythropoietin, are secreted glycoproteins; as a result, any non-human-type glycans, such as αGal and NeuGc, derived from animal cells and sera must be removed to circumvent undesirable immunogenic reactions. In this study, we made an extensive search for potential xenoantigenic glycans among a panel of mammalian sera. As a result, sera belonging to the order Artiodactyla, i.e. bovine, lamb and goat sera, were found to contain substantial amounts of hypersialylated biantennary glycans closely associated with a type-I lactosamine structure containing a unique tetrasaccharide, Siaα2-3Galβ1-3(Siaα2-6)GlcNAc. In all three Artiodactyla sera, the most abundant structure was Siaα2-3Galβ1-3(Siaα2-6)GlcNAcβ1-2Manα1-3[Siaα2-6Galβ1-4GlcNAcβ1-2Manα1-6]Manβ1-4GlcNAcβ1-4GlcNAc. A dually hypersialylated biantennary structure, Siaα2-3Galβ1-3(Siaα2-6)GlcNAcβ1-2Manα1-3[Siaα2-3Galβ1-3(Siaα2-6)GlcNAcβ1-2Manα1-6]Manβ1-4GlcNAcβ1-4GlcNAc, was also abundant (10%) in bovine serum. The amount of hypersialylated glycans among total sialylated glycans was 46, 26 and 23% in bovine, lamb and goat sera, respectively. On the other hand, such structures could not be detected in the sera of other animals including human. The biological functions and the immunogenicity of the hypersialylated glycans in these animals remain to be elucidated; however, it is worth noting that glycoproteins biosynthesized from Artiodactyla cells and those contaminated with bovine serum might enhance undesirable antigenicity in human patients.

Lepton flavor violation in minimal flavor violation extensions of the seesaw

A minimal flavour violation hypothesis for leptons can be implemented essentially in two ways that are compatible with a type-I seesaw structure with three heavy singlet neutrinos $N$, and that satisfy the requirement of being predictive, in the sense that all lepton flavor violating (LFV) effects can be expressed in terms of low energy observables. The first realization is CP conserving and is based on the flavor group $SU(3)_e\times SU(3)_\ell\times O(3)_N$ (being $e$ and $\ell$ the SU(2) singlet and doublet leptons). The second realization allows for CP violation and is based on $SU(3)_e\times SU(3)_{\ell+N}$. I review the main features of the two schemes and point out their different implications for LFV observables.

Phase Range of the Type-I Clathrate Sr8AlxSi46–x and Crystal Structure of Sr8Al10Si36

Samples of the type-I clathrate Sr(8)Al(x)Si(46-x) have been prepared by direct reaction of the elements. The type-I clathrate structure (cubic space group Pm3n) which has an Al-Si framework with Sr(2+) guest atoms forms with a narrow composition range of 9.54(6) ≤ x ≤ 10.30(8). Single crystals with composition A(8)Al(10)Si(36) (A = Sr, Ba) have been synthesized. Differential scanning calorimetry (DSC) measurements provide evidence for a peritectic reaction and melting point at ∼1268 and ∼1421 K for Sr(8)Al(10)Si(36) and Ba(8)Al(10)Si(36), respectively. Comparison of the structures reveals a strong correlation between the 24k-24k framework sites distances and the size of the guest cation. Electronic structure calculation and bonding analysis were carried out for the ordered models with the compositions A(8)Al(6)Si(40) (6c site occupied completely by Al) and A(8)Al(16)Si(30) (16i site occupied completely with Al). Analysis of the distribution of the electron localizability indicator (ELI) confirms that the Si-Si bonds are covalent, the Al-Si bonds are polar covalent, and the guest and the framework bonds are ionic in nature. The Sr(8)Al(6)Si(40) phase has a very small band gap that is closed upon additional Al, as observed in Sr(8)Al(16)Si(30). An explanation for the absence of a semiconducting "Sr(8)Al(16)Si(30)" phase is suggested in light of these findings.

Enhancing the Conversion Efficiency of Semiconductor Sensitized Solar Cells via the Cosensitization of Dual-Sized Quantum Dots

Dual-sized CdS quantum dots were employed as light harvesters of solar cells to achieve high power conversion efficiency. Chemical bath deposition and annealing treatment were adopted to produce quantum dots with different sizes. The strategy of cosensitization by dual-sized CdS quantum dots was found effective to improve the performance of solar cells by forming a type-I band structure. Our work indicates that a reasonable configuration of quantum dots is crucial on boosting the performance of solar cells.

On the crystal structure of the germanium-based cationic clathrates [Ge38.3Sb7.7]I7.44, [Ge38.1P7.9]I8, and [Ge30.5Sn7.7P7.75]I7.88

Compounds [Ge38.3Sb7.7]I7.44, [Ge38.1P7.9]I8, and [Ge30.5Sn7.7P7.75]I7.88 with the clathrate type-I structure were synthesized. They crystallize in the cubic space group \(Pm\bar 3n\) with the unit cell parameter a = 10.8592(9), 10.4983(12), and 10.7210(10) A (Z = 1), respectively. Their crystal structure represents the germanium(tin)-pnictogen framework, capturing the guest iodine anions in its cavities. All compounds have no vacancies in the host substructure; however, two of them show vacancies in the guest positions. The atomic distribution over the framework sites is of the most interest as it follows trends associated with the relative electronegativities of the atoms composing the framework. The results of the band structure calculations and application of the Zintl counting scheme are also discussed in relation to potential thermoelectric properties.

Synthesis and clathrate-type crystal structure of a solid solution in the Sn-In-P-Br system

The solid solution of the general formula Sn24−x In x P19.2+0.2x Br8 (x ≤ 10) was synthesized in the Sn-In-P-Br system, and its crystal structure was determined for the composition with x = 7.8. This solid solution crystallizes in the cubic system, the space group Pm \(\bar 3\) n, and has a type-I clathrate structure, in which the clathrate framework is composed of tin, indium, and phosphorus atoms, whereas bromine atoms occupy two types of guest positions. The unit cell parameter a increases with increasing indium content, two linear regions with different increments being observed in the a(x) plot. The crystal structure of the solid solution is characterized by a complicated disorder manifested in the partial occupancy of three closely spaced positions by tin and indium atoms. The application of the Zintl formalism to the rationalization of the composition of the solid solution is discussed, and its structure is compared with the iodide analogs of similar composition but showing differences in structural details.

Minimal flavour violation extensions of the seesaw

We analyze the most natural formulations of the minimal lepton flavour violation hypothesis compatible with a type-I seesaw structure with three heavy singlet neutrinos N, and satisfying the requirement of being predictive, in the sense that all LFV effects can be expressed in terms of low energy observables. We find a new interesting realization based on the flavour group $SU(3)_e\times SU(3)_{\ell+N}$ (being $e$ and $\ell$ respectively the SU(2) singlet and doublet leptons). An intriguing feature of this realization is that, in the normal hierarchy scenario for neutrino masses, it allows for sizeable enhancements of $\mu \to e$ transitions with respect to LFV processes involving the $\tau$ lepton. We also discuss how the symmetries of the type-I seesaw allow for a strong suppression of the N mass scale with respect to the scale of lepton number breaking, without implying a similar suppression for possible mechanisms of N production

Phase stability and chemical composition dependence of the thermoelectric properties of the type-I clathrate Ba 8Al xSi 46− x (8≤ x≤15)

Phase stability of the type-I clathrate compound Ba 8Al x Si 46− x and the thermoelectric property dependence on chemical composition are presented. Polycrystalline samples were prepared by argon arc melting and annealing. Results of powder X-ray diffraction and electron microprobe analysis show that the type-I structure is formed without framework deficiency for 8≤ x≤15. Lattice constant a increases linearly with the increase of x. Thermoelectric properties were measured for x=12, 14 and 15. The Seebeck coefficients are negative, with the absolute values increasing with x. The highest figure of merit zT=0.24 was observed for x=15 at T=1000 K, with carrier electron density n=3×10 21 cm −3. A theoretical calculation based on the single parabolic band model reveals the optimum carrier concentration to be n∼4×10 20 cm −3, where zT∼0.7 at T=1000 K is predicted.

Crystal structure, characterization and thermoelectric properties of the type-I clathrate Ba 8− ySr yAl 14Si 32 (0.6≤ y≤1.3) prepared by aluminum flux

The title compound was prepared as single crystals using an aluminum flux technique. Single crystal and powder X-ray diffraction indicate that this composition crystallizes in the clathrate type-I structure, space group Pm3̄ n. Electron microprobe characterization indicates the composition to be Ba 8− y Sr y Al 14.2(2)Si 31.8(2) (0.77< y<1.3). Single-crystal X-ray diffraction data (90 and 12 K) were refined with the Al content fixed at the microprobe value (12 K data: R 1=0.0233, wR 2=0.0441) on a crystal of compositions Ba. The Sr atom preferentially occupies the 2 a position; mixed Al/Si occupancy was found on all framework sites. These refinements are consistent with a fully occupied framework and nearly fully occupied cation guest sites as found by microprobe analysis. Temperature dependent electrical resistivity and thermal conductivity have been measured from room temperature to 1200 K on a hot-pressed pellet. Electrical resistivity reveals metallic behavior. The negative Seebeck coefficient indicates transport processes dominated by electrons as carriers. Thermal conductivity is between 22 and 25 mW/cm K. The sample shows n-type conductivity with a maximum figure of merit, zT of 0.3 at 1200 K. A single parabolic band model predicts a five-fold increase in zT at 800 K if carrier concentration is lowered.

Substrate-modified ferrimagnetism in MnGa films

We report the substrate-modified magnetic properties of the CuAu type-I (L10) structure of MnxGa (1.2&amp;lt;x&amp;lt;1.5) films. The magnetic properties of the MnGa films differed greatly due to the influence of the substrate. The MnGa film is a hard ferrimagnet when grown on GaSb (111), becomes a soft ferrimagnet when grown on Al2O3 (0001), and exhibits an absence of a net magnetic moment when stabilized on a GaSb (100) substrate. This difference was attributed to the substrate, which forces MnGa film to be two-dimensionally stabilized in a different orientation and thus leads to the modified crystal symmetry and a change in the magnetic property. The results may be helpful for forming a comprehensive understanding of MnGa and for finding new applications in spintronic devices.

Demonstration of high-mobility electron and hole transport in a single InGaSb well for complementary circuits

Heterostructures consisting of an InGaSb quantum well situated between AlGaSb barriers were grown by molecular beam epitaxy. Calculations indicate a type-I band structure with substantial valence and conduction band offsets that can allow for the confinement of either electrons or holes in the InGaSb. Quantum wells with n-type conduction were achieved using modulation doping, with Te located in the barrier above the quantum well. A set of barrier layers was found which resulted in a sample with an In 0.2Ga 0.8Sb quantum well that exhibited an electron mobility of 3900 cm 2/V s as grown. After removal of upper barrier layers including the Te by selective etching, the conductivity switched to p-type, with hole mobilities near 800 cm 2/V s. This design could allow the integration of low-power n- and p-channel field-effect transistors for complementary logic applications.

Carrier lifetimes in type-II InAs quantum dots capped with a GaAsSb strain reducing layer

Carrier lifetimes have been measured for long-wavelength emitting InAs quantum dots (QDs) capped with a thin GaAsSb layer. Above a critical Sb composition, a type-II system is formed, resulting in an increase in the carrier lifetime. The carrier lifetime in a strongly type-II structure is increased by a factor ∼54 in comparison to the lifetime in a type-I structure. In addition, the type-II carrier lifetime varies across the inhomogeneously broadened ground-state emission, while the type-I QD lifetime is invariant.

Electronic structure and physical properties of Ba 8Ga 16Sn 30 clathrates with type-I and type-VIII structure

A joint experimental and theoretical investigation is performed for Ba–Sn–Ga type-I and type-VIII clathrates. The type-I sample shows a semiconductor characteristic which is accords with the first principle simulation. The type-I clathrate follows Zintl-like behaviour and the semiconducting gap is about 0.6 eV which is agreement with measurement. For the type-VIII clathrate, the simulation shows that a complicated band structure around Fermi level. For the density of states, a peak appears in the fundamental gap, which corresponds to hybridization of Ba bonding orbitals and Sn(Ga) network. A superconducting transition at T C=4 K is observed, which is close to the T C of Sn. An alternative explanation is superconductivity in type-VIII sample arises from increase of electronic density of states around Fermi level E F in the BCS theoretical frame.

The effect of packing structure of powder particles on warping during sintering

In this study, aqueous alumina suspension consisting of constant solid loading was chosen as a model system in order to observe the effect of particle packing structure in green tape on sintering deformation. The specific particle packing structure in consolidated green tape was intentionally formed during tape casting, for instance, a non-uniform particle packing structure (Type-I) and a relatively uniform particle packing structure (Type-II). The development of deformation in sintered specimens was explainable from anisotropic packing structure. Alumina tapes with Type-I structure particularly showed warping which was transverse to the casting direction. Tapes with Type-II structure appeared with irregular distortion which was expected as a result of the localized non-uniform particle packing structure.

Characterization of Hydrogen-Encapsulated Type-I Silicon Clathrate

This study describes a new stable inorganic clathrate compound encapsulating hydrogen in the Si cage structure. In this work, the characterization of the new type-I Si clathrate, Na6(H2)2Si46, is illustrated. Na6(H2)2Si46 was prepared from NaSi and NH4X (X = Cl and Br) under dynamic vacuum at 300 oC. Rietveld refinement of powder X-ray diffraction data was consistent with the clathrate type-I structure. Solid-state 1 H, 29 Si and 23 Na MAS NMR confirmed the presence of both hydrogen and sodium in the clatharte cages. 23 Na NMR indicated that the guest sites are not fully occupied by Na. In order to validate the structure and investigate the Na content, the synthesized particles were examined using spherical aberration (Cs) corrected VG HB501 scanning transmission electron microscope (STEM) operated at 100 kV. Fig. 2 shows a high-angle annular dark-field (HAADF) image taken along [100] with the convergence semi-angle of 20 mrad and collection semi-angle of 70-210 mrad. Image calculation was made with Kirkland’s code [3] with the thickness of 21 nm and defocus value of 22.5 nm. The calculated image inserted in Fig. 2 is in good agreement with the experimental one. The ring pattern and the center of the ring correspond to the Si24 cage and the 6d sites, respectively. When observed along [100], the 6d sites are aligned in an atomic column which is apart from the nearest columns by 0.23 nm so that the Na occupancy in the 6d sites can be determined by comparing the experimental intensity profiles with calculated ones. Fig. 3 shows an averaged experimental image using 2dx software [4] and calculated images with different Na occupancies in the 6d sites as well as intensity profiles along the line X-Y. The peak intensity at the

Structure and morphology of the tenfold surface of decagonalAl71.8Ni14.8Co13.4in its low-temperature random tiling type-I modification

We have investigated the structure and morphology of the tenfold surface of decagonal ${\mathrm{Al}}_{71.8}{\mathrm{Ni}}_{14.8}{\mathrm{Co}}_{13.4}$ by highly surface sensitive He atom scattering (HAS), high resolution low energy electron diffraction(SPA-LEED), and low temperature scanning tunneling microscopy (STM). The SPA-LEED patterns reveal more than 500 individual diffraction spots in the $\mathbf{k}$-vector range of $\ensuremath{\mid}{\mathbf{k}}_{\ensuremath{\Vert}}\ensuremath{\mid}l3\phantom{\rule{0.3em}{0ex}}{\mathrm{\AA{}}}^{\ensuremath{-}1}$. The positions of all observed diffraction spots agree with the surface projections of the reciprocal lattice structure of the type-I bulk phase. HAS shows identical spot positions as SPA-LEED, thus demonstrating a top surface layer with long range quasicrystalline order and a reciprocal lattice structure consistent with that of a bulk truncated surface. SPA-LEED peak widths are found to vary between different diffraction orders. Based on an analysis of a randomized Fibonacci sequence, this is linked to the random nature of the tiling of the type-I structure. STM measurements reveal a surface morphology characterized by rough single-height steps separating terraces with widths on the order of $100\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. Two different surface terminations are observed, a coarse and a fine one, frequently coexisting on single terraces. The fine structure termination directly reflects the atomic structure of a bulk truncated surface, allowing a random rhombic tiling to be identified. In order to compare diffraction, real-space data, and atomic structure models, the Patterson function and autocorrelation of the surface structure, respectively, are studied. This allows an understanding of the coarse structure termination as consisting of subunits of a few atoms each arranged statistically on sites defined by the atomic tiling of the bulk tenfold planes.

Power Control of Cellular Radio Systems Via Robust Smith Prediction Filter

In this paper, a robust Smith prediction filter is proposed for power control design of direct-sequence code-division multiple-access cellular mobile radio systems. Due to the type-I (integrator) structure in the close loop, we first show that zero steady-state tracking error of the averaged received signal-to-noise plus interference ratio (SINR) with respect to a constant target SINR can be ensured. Next, with the Smith predictor to compensate round trip delay, a fixed-order robust H/sub /spl infin// loop filter is developed by using the genetic algorithm to minimize the worst-case variance of the received SINR from the minimax perspective. Using the proposed robust control approach, the statistics of the fading and interference are not needed in the design procedure. To confirm the performance of the proposed method, several simulation results are given in comparison with other methods.

Hydrogen Bonding in ROH:R‘OH (R, R‘ = H, CH3, C2H5) Heterodimers: Matrix-Dependent Structure and Infrared-Induced Isomerization

The structure of ROH:R‘OH heterodimers (R, R‘ = H, CH3, C2H5) trapped in argon and nitrogen matrixes was examined through two approaches: 16O/18O isotopic substitution in water and methanol and infrared-induced isomerization. Isotopic substitution clearly shows that in N2 matrix the bigger molecule plays the role of proton donor (Type-I structure) and in Ar matrix that of proton acceptor (Type-II structure). This difference can be rationalized by considering the existence of weak OH···N2 hydrogen bonds stabilizing the less stable structure. Selective irradiations in the νOH region were carried out for the CH3OH:H2O and C2H5OH:H2O dimers. Type-I → Type-II interconversion was observed in N2 matrix while no effect was detected in Ar. Finally the hydrogen bond strength evolution within the nine ROH:R‘OH homo- and heterodimers was examined on both experimental and theoretical grounds. Experimentally this evolution was followed by considering the νOH frequency shifts of the proton donor subunit. Theoretically a...

Magnetic structure and electric-field gradients of uranium dioxide: Anab initiostudy

The magnetic structure of uranium dioxide has been investigated using linear augmented plane wave ab initio calculations, taking into account spin-orbit coupling, strong Coulomb correlation (using the $\mathrm{LDA}+U$ approach) and noncollinear magnetism. The collinear 1-k antiferromagnetic type-I structure and the noncollinear antiferromagnetic 2-k and 3-k orderings have been tested. The 1-k and 2-k structures can be excluded by a comparison between the calculated and experimental uranium electric field gradients (EFG). It is shown that the measured EFG agrees with theory only in the 3-k structure and an additional deformation of the oxygen cage with an oxygen displacement of ${\ensuremath{\Delta}}_{O}=0.16\mathrm{a}.\mathrm{u}.$ Also the magnetic moments and the corresponding hyperfine fields agree reasonably well with experiment.