Anisotropy of the electron momentum distribution in Bi2Sr2CaCu2O8+δ superconductor studied by positron annihilation

The temperature dependent (30-300 K) Doppler broadening of the positron annihilated γ-radiation measurement has been investigated on single crystalline Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) high T_c superconducting samples along two different crystallographic orientations. It has been observed that throughout the temperature range the electron momentum distribution has a larger value along the crystallographic c-axis than in the a-b plane. The temperature dependent Doppler broadened positron annihilation γ-radiation lineshape analysis shows a step like increase of S-parameter at the temperature region 92-116 K.     

Homotopy classification of contact foliations on open contact manifolds

We give a homotopy classification of foliations on open contact manifolds whose leaves are contact submanifolds of the ambient space. The result is an extension of Haefliger’s classification of foliations on open manifold in the contact setting. While proving the main theorem, we also prove a result on equidimensional isocontact immersions on open contact manifolds.     

Homotopy Classification of Strict Contact Immersions

The purpose of this paper is to obtain a homotopy classification of a class of immersions in a contact manifold following $h$ -principle.     

A note on Pontrjagin forms

Let be a principal bundle over a manifold of dimension , and let be its -dimensional Pontrjagin class. In this paper, we aim at answering the following question: Which representatives of the class can be realised as the Pontrjagin form of some connection on ?

     

EXPERIMENTAL STUDY OF CONVECTIVE HEAT TRANSFER INSTABILITIES IN TRANSIENT RESPONSE ALONG A COUNTERCURRENT HEAT EXCHANGER

This article deals with experimental investigations to reveal the effect of combined natural and forced convection in a horizontal tubular countercurrent heat exchanger in both transient and steady states. In transient response, the temperature profiles of hot fluid in a cross section of the inner tube show the influence of buoyancy forces and radial conduction. These profiles are distorted in time and lead to the appearance of oscillating temperature along the inner tube of the heat exchanger. The transient Richardson dimensionless number is calculated to show these effects. Also, a spectral analysis using the discrete Fourier transform has been performed to characterize these temperature instabilities. Using this technique, the influence of flow rates of hot and cold fluids is investigated.     

Synthesis and optical properties of triphenylene-based dendritic donor perylene diimide acceptor systems

A donor-acceptor charge transfer system based on two discotic mesogens has been synthesized. The donor is either a triphenylene (POG0) or a triphenylene-based conjugated dendron (POG1), while the acceptor is a perylene diimide (PDI) core. The donors are covalently linked to the bay positions of the PDI core through an ether linkage. In chloroform, due to the short donor-acceptor distance and the matching frontier orbital levels, photoinduced charge transfer from either the donor excitation or the acceptor excitation are both thermodynamically and kinetically favored, resulting in efficient quenching of both donor and acceptor fluorescence. In a less polar solvent, hexane, while charge transfer is still the dominant mechanism for decay of the excited electronic state of POG1, photoinduced charge transfer is no longer energetically favorable for POG0 when the acceptor PDI core is excited, making the PDI core of POG0 weakly fluorescent in chloroform but strongly so in hexane. In solid film, POG0 is highly aggregated through both PDI-PDI and triphenylene-triphenylene homotopic stacking. POG1, on the other hand, aggregates through triphenylene dendrons with limited PDI-PDI core stacking, presumably due to the steric hindrance caused by bulky triphenylene moieties which block the access to the PDI core. The efficient photoinduced charge transfer, coupled with the homotopic stacking that forms separated electron-transporting PDI-stacked columns and hole transporting triphenylene-stacked columns, suggests that the reported donor-acceptor systems based on dual-discotic mesogens are potentially new efficient photovoltaic materials.     

Synthesis and optical properties of perylene diimide derivatives with triphenylene-based dendrons linked at the bay positions through a conjugated ethynyl linkage

A number of groups including trimethylsilyl, phenyl, triphenylene, and triphenylene-based dendron have been linked to the bay positions of a perylene diimide (PDI) core through an ethynyl bridge. The photophysical properties of the resulting bay-substituted PDI derivatives have been carefully studied in different solvents and as thin films. Without any capping group, the two ethynyl bay-substituted PDI derivates PAT and PRT both aggregate strongly even in dilute solutions but in different perylene-perylene π–π stacking modes; PRT aggregates through slipped (or longitudinal) stacking while PAT self-assembles by rotational (or cross) stacking. With capping groups, the perylene core stacking is completely blocked for PATS in both solution and solid film. For PRTS, the slipped stacking is observed only for its film sample, while for PTB, association only occurs after excitation (excimer formation). When triphenylene or triphenylene-based G1 dendron is attached to the acetylene bridge, the resulting donor–acceptor systems (PTG0 and PTG1) exhibit strong electronic coupling between the dendritic donors and the PDI acceptor, leading to significantly red-shifted absorption bands. The conjugated linkage also facilitates photoinduced electron transfer from the triphenylene or triphenylene dendron to the PDI core, effectively quenching fluorescence emissions of both the donor and the acceptor. The significantly red-shifted absorption bands and the efficient photoinduced electron transfer observed on PTG0 and PTG1 indicate that these new PDI derivatives may find applications in solar cells.     

Photoluminescence and positron annihilation spectroscopic investigation on a H(+) irradiated ZnO single crystal

Low temperature photoluminescence and room temperature positron annihilation spectroscopy have been employed to investigate the defects incorporated by 6?MeV H(+) ions in a hydrothermally grown ZnO single crystal. Prior to irradiation, the emission from donor bound excitons is at 3.378?eV (10?K). The irradiation creates an intense and narrow emission at 3.368?eV (10?K). The intensity of this peak is nearly four times that of the dominant near band edge peak of the pristine crystal. The characteristic features of the 3.368?eV emission indicate its origin as a 'hydrogen at oxygen vacancy' type defect. The positron annihilation lifetime measurement reveals a single component lifetime spectrum for both the unirradiated (164?±?1?ps) and irradiated crystal (175?±?1?ps). It reflects the fact that the positron lifetime and intensity of the new irradiation driven defect species are a little higher compared to those in the unirradiated crystal. However, the estimated defect concentration, even considering the high dynamic defect annihilation rate in ZnO, comes out to be ～4?×?10(17)?cm(-3) (using SRIM software). This is a very high defect concentration compared to the defect sensitivity of positron annihilation spectroscopy. A probable reason is the partial filling of the incorporated vacancies (positron traps), which in ZnO are zinc vacancies. The positron lifetime of ～175?ps (in irradiated ZnO) is consistent with recent theoretical calculations for partially hydrogen-filled zinc vacancies in ZnO. Passivation of oxygen vacancies by hydrogen is also reflected in the photoluminescence results. A possible reason for such vacancy filling (at both Zn and O sites) due to irradiation has also been discussed.     

Immersions in a symplectic manifold

In this paper we give a homotopy classification of symplectic isometric immersions following Gromov’sh-principle theorem.     

An Efficient Simultaneous Electrochemical Detection of Nanomolar Epinephrine and Uric Acid using Low Temperature Synthesized Nano-sized Copper Telluride

Herein we developed a simple, cost effective, electrochemical sensor based on nanosized copper telluride (nps-CuTe) for simultaneous detection of epinephrine (EP) and uric acid (UA). Voltammetric responses suggests dramatical improvement of electrocatalytic properties of both molecules by incorporating CuTe nps into unmodified graphite paste electrode (bare GP). Differential pulse voltammetric (DPV) measurement depicts large potential separation of 128 mV between EP and UA, allows their simultaneous determination from binary mixture. Under optimized condition, CuTe modified graphite paste electrode (CuTe/GP) manifested linear relationships of EP and UA in the range of 5–60 μM and 5–120 μM with detection limit (S/N=3) of 18 nM and 32 nM respectively. Moreover, CuTe/GP showed satisfactory response towards pharmaceutical and clinical samples for determining EP and UA concentrations.