Delta Ea Research Articles

Effect of Cu(2+)-ions on semiconductor properties of synthetic DOPA melanin polymer.

The purpose of the present study was to examine semiconductor properties of synthetic DOPA melanin, which are basic for future biological applications. DC conductivity, electron spin resonance (ESR), and atomic absorption spectroscopy (AAS) measurements have been performed to investigate the effect of Cu(2+)-ions on the semiconductor properties of melanin polymer synthesized from DOPA (3,4-dihydroxyphenylalanine). DOPA melanin - Cu2+ complexes examined show the decrease of both thermal activation energy delta Ea and pre-exponential factor sigma o values upon doping. At the same time no substantial changes in conductivity at 293 K have been observed. Formation of bipolaron states due to chelation of copper ions by melanin orthosemiquinones has been postulated. The Meyer-Neldel rule with a characteristic temperature T0 equal to 298 K and possible physiological implication of this fact are discussed. These data suggest, that DOPA melanin polymer could be useful as a type of culture substratum.

InGaAs Schottky contacts with an iron-doped InP enhancement layer

The effective Schottky barrier height of n-In0.53Ga0.47As is increased by a 200 nm thick Fe-doped InP layer grown by metal-organic vapour-phase epitaxy. Schottky diodes fabricated by this method exhibit an effective barrier height of 0.57 eV and a very low reverse-current density of 5*10-4 A cm-2 at 10 V. Current in the reverse direction is caused by thermionic emission, whereas a complicated transport mechanism, with generation-recombination and tunnelling components, determines the current when the structure is forward biased. Besides peaks clearly related to Fe levels in InP and In0.53Ga0.47As, deep level transient spectroscopy also shows a donor-like level with Delta Ea=0.77 eV of an origin that has so far remained unresolved.

Pressure and temperature studies of Raman scattering of the molecular crystal sym-C6Cl3F3

Earlier experimental results have been considerably strengthened by Raman scattering from crystalline sym-C6Cl3F3. Measurements have been made as a function of temperature at ambient pressure, and as a function of pressure at room temperature. Apart from a new first-order phase transition observed at 62 kbar but as yet uncharacterised, the crystal structure maintains the P63/m space group symmetry. Anomalies in phonon frequencies and line-widths at 296 K indicate a phase transition, which is 'isostructural' order-disorder. Similar features are seen at 15 kbar, suggesting that the phase transition is of the same nature as that observed at 296 K. The half-width of the phonon associated with dynamic disorder is found by fitting to be Gamma 12/(T)=0.032+0.0061 T+3400 exp(- Delta Ea/kT)cm-1, with Delta Ea=35 kJ mol-1. Although in-plane 120 degrees molecular reorientational jumps may play a role, this may not be a major role at room temperature due to a rather high activation energy. The full understanding of the results requires there to be another minor activation process that also obeys Ag symmetry. The variation in the crystal structure parameters and the external mode frequencies with pressure up to 60 kbar is calculated using a 6-exp atom-atom potential with the rigid-body approximation. The molecular orientation changes by only 0.5 degrees between 0 and 60 kbar. The calculated external mode frequencies agree with Raman experiment up to 60 kbar within 5-15%. A molecular dynamics simulation for 300 K indicates a residence time between reorientations greater than 500 ps, giving a quasi-elastic Lorentzian broadening of less than 0.01 cm-1.

Phenomenological model for the frequency dependence of the susceptibility of spin glasses and related compounds

A phenomenological model is introduced to describe the frequency dependence of the AC susceptibility, chi ( omega ), of random magnetic materials. The model is applied to CuMn, (Eu, Sr)S, holmium borate glass and aluminosilicate glass. On the basis of the model criterion the various systems are classified, and it is possible to distinguish between spin glasses and superparamagnets. The authors' approach is to divide the spin system into two temperature-dependent parts: one with correlated spins (clusters) and one with non-correlated (free) spins. Thereby they obtained meaningful values for the activation energy Delta Ea and the intrinsic time constant tau 0 which follows from an Arrhenius law. In those cases where the authors were able to combine susceptibilities over twelve decades in frequency their analysis showed an enormous evolution towards long times for the cluster relaxation times near and below Tf.

Electric and photoelectric properties of polycrystalline copper sulphide

Polycrystalline samples of copper sulphide having a predominantly tetragonal phase have been investigated. The thermal activation energy of the electric conductivity Delta Ea=0.15 eV associated with the activation energy of an acceptor level has been determined. Low-temperature photoconductivity has been observed for the first time. The photoconductivity spectra measured at 80-120K reveal an appreciable surface recombination rate. From the analysis of these spectra, the minority carrier diffusion length (3.3*10-5-2.5*10-4 cm) and the possible lower limit of the band gap ( Delta Eg approximately=1.2 eV) have been evaluated.

Purification and characterization of bovine brain 5'-nucleotidase.

The 5'-nucleotidase located in the cytoplasmic fraction of bovine brain cortex was purified to electrophoretic homogeneity. The molecular weight was 134,000 daltons in the presence of sodium deoxycholate, whereas the enzyme formed high molecular weight aggregates in the absence of detergent. The purified enzyme showed the same kinetic and electrophoretic behaviour as the enzyme present in the original cytoplasmic fraction, and the presence of surfactants did not change the Km and Vm values. The nucleotidase from this source was a phosphohydrolase of 5'-mononucleotides acting on the deoxyribonucleotides and ribonucleotides of purines and pyrimidines. 5'-IMP was the preferred substrate; the optimum pH was 7.5. The study of the influence of the temperature on the initial reaction rates allowed calculation of the delta Ea and delta H degrees values. The variation of Vm and Km with a change in pH suggests the existence of a sulfhydryl group and an imidazole group in the enzyme-substrate complex.

Electron transport in direct-gap III-V ternary alloys

Electron drift, Hall mobility and thermoelectric power in InPxAs1-x, GaxIn1-xAs and InAsxSb1-x have been calculated by an iterative solution of the Boltzmann equation. A Kane-type band structure corrected for the lattice disorder is assumed. All the relevant scattering mechanisms are included together with band non-parabolicity, wavefunction admixture, degeneracy of the distribution function and screening of the scattering probabilities due to free carriers. Only the ionised impurity concentration (Ni) and the alloy scattering potential ( Delta Ea) have been treated as adjustable parameters to be determined from a fit with the experimental data. Variations of the other different material parameters across the alloy system have been taken from different sources, or reasonable assumptions are made regarding such variations. A large number of experimental results on both mobility and thermoelectric power are considered and good agreement is obtained with Delta Ea larger than that given by the difference in electron affinity between the constituent compounds.