Annealing and measurement temperature dependence of W2B5-based rectifying contacts to n-GaN

The thermal stability and measurement temperature dependence of Schottky contact characteristics on n-GaN using a W₂B₅/Ti/Au metallization scheme was studied using current-voltage (I-V), scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) measurements. The elemental profile obtained from samples annealed at 350 °C showed some titanium diffusion into the gold layer but little other difference from the as-deposited wafer. Annealing at 700 °C produced significant diffusion of titanium. The Schottky barrier height increased with anneal temperature up to 200 °C, reaching a maximum value of 0.65 eV, but decreased at higher annealing temperatures. The reverse breakdown voltage from diodes fabricated using the W₂B₅-based contacts showed a similar dependence. The reverse current magnitude was larger than predicted by thermionic emission alone. The barrier height showed only minor changes with measurement temperature up to 150 °C.     

Rheological microscopy: local mechanical properties from microrheology

We demonstrate how tracer microrheology methods can be extended to study submicron scale variations in the viscoelastic response of soft materials; in particular, a semidilute solution of lambda-DNA. The polymer concentration is depleted near the surfaces of the tracer particles, within a distance comparable to the polymer correlation length. The rheology of this microscopic layer alters the tracers' motion and can be precisely quantified using one- and two-point microrheology. Interestingly, we found this mechanically distinct layer to be twice as thick as the layer of depleted concentration, likely due to solvent drainage through the locally perturbed polymer structure.     

Field theoretic calculation of energy cascade rates in non-helical magnetohydrodynamic turbulence

Energy cascade rates and Kolmogorov’s constant for non-helical steady magnetohydrodynamic turbulence have been calculated by solving the flux equations to the first order in perturbation. For zero cross helicity and space dimensiond = 3, magnetic energy cascades from large length-scales to small length-scales (forward cascade). In addition, there are energy fluxes from large-scale magnetic field to small-scale velocity field, large-scale velocity field to small-scale magnetic field, and large-scale velocity field to large-scale magnetic field. Kolmogorov’s constant for magnetohydrodynamics is approximately equal to that for fluid turbulence (≈ 1.6) for Alfvén ratio 05 ≤r _A ≤ ∞. For higher space-dimensions, the energy fluxes are qualitatively similar, and Kolmogorov’s constant varies asd ^1/3. For the normalized cross helicity σ_c →1, the cascade rates are proportional to (1 − σ_c)/(1 + σ_c , and the Kolmogorov’s constants vary significantly with σ_cc.     

Trap elimination and reduction of size dispersion due to aging in CdS<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> quantum dots

Quantum Dots of CdS _x Se_1−x embedded in borosilicate glass matrix have been grown using Double-Step annealing method. Optical characterization of the quantum dots has been done through the combinative analysis of optical absorption and photoluminescence spectroscopy at room temperature. Decreasing trend of photoluminescence intensity with aging has been observed and is attributed to trap elimination. The changes in particle size, size distribution, number of quantum dots, volume fraction, trap related phenomenon and Gibbs free energy of quantum dots, has been explained on the basis of the diffusion-controlled growth process, which continues with passage of time. For a typical case, it was found that after 24 months of aging, the average radii increased from 3.05 to 3.12 nm with the increase in number of quantum dots by 190% and the size-dispersion decreased from 10.8% to 9.9%. For this sample, the initial size range of the quantum dots was 2.85 to 3.18 nm. After that no significant change was found in these parameters for the next 12 months. This shows that the system attains almost a stable nature after 24 months of aging. It was also observed that the size-dispersion in quantum dots reduces with the increase in annealing duration, but at the cost of quantum confinement effect. Therefore, a trade off optimization has to be done between the size-dispersion and the quantum confinement.     

Spin-polarized first-principles study of ferromagnetism in zinc-blende In<Subscript>1−x</Subscript>Mn<Subscript>x</Subscript>Sb

First-principles calculations based on the density functional theory are performed to study the structural properties, spin-polarized electronic band structures, density of states and magnetic properties of the zinc blende In_{1− x} Mn _x Sb (x = 0.125, 0.25, 0.50, 0.75, 1.0). The calculated lattice constants of In_{1− x} Mn _x Sb obey the Vegard’s law with a marginal upward bowing. With the increase of Mn concentration in In_{1− x} Mn _x Sb, a transition from the semi-metallic to the half-metallic behavior happens such that the majority-spin valence states crosses the Fermi level and the minority-spin states have a gap at the Fermi level. A large exchange splitting (∼ 4 eV) is observed between Mn 3d states of the majority-spins and the minority-spins. The total magnetic moment of In_{1− x} Mn _x Sb half-metallic ferromagnets per Mn atom basis is 4μ _B. The total magnetic moment per Mn atom indicate that Mn atoms act as acceptors in InSb and contribute to holes in the lattice of InSb. Due to p-d hybridization, the free space charge of Mn reduces that results a loss in its magnetic moment. The loss in the magnetic moment of the Mn atoms is converted into a small local magnetic moments on the In and Sb sites.     

Thermal property of binary tetrahedral semiconductors

In this paper we present an expression relating the lattice thermal expansion coefficient α_L (10⁻⁶ K⁻¹) for the A^IIIB^V and A^IIB^VI semiconductors with the product of ionic charges (Z₁Z₂), melting temperature (T_m) and nearest neighbor distance d (Å). The lattice thermal expansion coefficient of these compounds exhibit a linear relationship when plotted on a log–log scale against the melting temperature (T_m), but fall on different straight lines according to the ionic charge product of the compounds. A good agreement has been found between the experimental and calculated values of the lattice thermal expansion coefficient for A^IIIB^V and A^IIB^VI semiconductors.     

Small angle neutron scattering study of doxorubicin-surfactant complexes encapsulated in block copolymer micelles

Self-assembling behaviour of block copolymers and their ability to evade the immune system through polyethylene oxide stealth makes it an attractive candidate for drug encapsulation. Micelles formed by polyethylene oxide-polypropylene oxide- polyethylene oxide triblock copolymers (PEO-PPO-PEO), pluronic P123, have been employed for encapsulating the anti-cancer drug doxorubicin hydrochloride. The binding affinity of doxorubicin within the micelle carrier is enhanced through complex formation of drug and anionic surfactant, aerosol OT (AOT). Electrostatic binding of doxorubicin with negatively charged surfactants leads to the formation of hydrophobic drug-surfactant complexes. Surfactant-induced partitioning of the anti-cancer drug into nonpolar solvents such as chloroform is investigated. SANS measurements were performed on pluronic P123 micelles in the presence of drug-surfactant complex. No significant changes in the structure of the micelles are observed upon drug encapsulation. This demonstrates that surfactant- drug complexes can be encapsulated in block copolymer micelles without disrupting the structure of aggregates.      

Structural investigation of viscoelastic micellar water/CTAB/NaNO<Subscript>3</Subscript> solutions

A highly viscoelastic worm-like micellar solution is formed in hexadecyltrimethylammonium bromide (CTAB) in the presence of sodium nitrate (NaNO₃). A gradual increase in micellar length with increasing NaNO₃ was assumed from the rheological measurements where the zero-shear viscosity (η ₀) versus NaNO₃ concentration curve exhibits a maximum. However, upon increase in temperature, the viscosity decreases. Changes in the structural parameters of the micelles with addition of NaNO₃ were inferred from small angle neutron scattering measurements (SANS). The intensity of scattered neutrons in the low q region was found to increase with increasing NaNO₃ concentration. This suggests an increase in the size of the micelles and/or decrease of intermicellar interaction with increasing salt concentration. Analysis of the SANS data using prolate ellipsoidal structure and Yukawa form of interaction potential between micelles indicate that addition of NaNO₃ leads to a decrease in the surface charge of the ellipsoidal micelles which induces micellar growth. Cryo-TEM measurements support the presence of thread-like micelles in CTAB and NaNO₃.      

Generalized structure of higher order nonclassicality

A generalized notion of higher order nonclassicality (in terms of higher order moments) is introduced. Under this generalized framework of higher order nonclassicality, conditions of higher order squeezing and higher order subpoissonian photon statistics are derived. A simpler form of the Hong-Mandel higher order squeezing criterion is derived under this framework by using an operator ordering theorem introduced by us in [A. Pathak, J. Phys. A 33 (2000) 5607]. It is also generalized for multi-photon Bose operators of Brandt and Greenberg. Similarly, condition for higher order subpoissonian photon statistics is derived by normal ordering of higher powers of number operator. Further, with the help of simple density matrices, it is shown that the higher order antibunching (HOA) and higher order subpoissonian photon statistics (HOSPS) are not the manifestation of the same phenomenon and consequently it is incorrect to use the condition of HOA as a test of HOSPS. It is also shown that the HOA and HOSPS may exist even in absence of the corresponding lower order phenomenon. Binomial state, nonlinear first order excited squeezed state (NLESS) and nonlinear vacuum squeezed state (NLVSS) are used as examples of quantum state and it is shown that these states may show higher order nonclassical characteristics. It is observed that the Binomial state which is always antibunched, is not always higher order squeezed and NLVSS which shows higher order squeezing does not show HOSPS and HOA. The opposite is observed in NLESS and consequently it is established that the HOSPS and HOS are two independent signatures of higher order nonclassicality.     

New massively parallel linear-response coupled-cluster module in ACES III: application to static polarisabilities of closed-shell molecules and oligomers and of open-shell radicals

ABSTRACT

Stemming from our implementation of parallel coupled-cluster (CC) capabilities for electron spin resonance properties [J. Chem. Phys. 139, 174103 (2013)], we present a new massively parallel linear response CC module within ACES III. Unlike alternative parallel CC modules, this general purpose module evaluates any type of first- and second-order CC properties of both closed- and open-shell molecules employing restricted, unrestricted and restricted-open-shell Hartree–Fock (HF) references. We demonstrate the accuracy and usefulness of this module through the calculation of static polarisabilities of large molecules. Closed-shell calculations are performed at the following levels: second-order many-body perturbation theory [MBPT(2)], CC with single- and double-excitations (CCSD), coupled-perturbed HF and density functional theory (DFT), and open-shell calculations at the unrestricted CCSD (UCSSD) one. Applications involve eight closed-shell organic-chemistry molecules (Set I), the first four members of the closed-shell thiophene oligomer series (Set II), and five open-shell radicals (Set III). In Set I, all calculated average polarisabilities agree reasonably well with experimental data. In Set II, all calculated average polarisabilities vs. the number of monomers show comparable values and saturation patterns and demonstrate that experimental polarisabilities may be inaccurate. In Set III, UCCSD perpendicular polarisabilities show a reasonable agreement with previous UCCSD(T) and restricted-open-shell-MBPT(2) values.