Effective activation energy U(J,H) in epitaxial YBa2Cu3O7−δ thin films

We have measured the resistivity of epitaxial YBa₂Cu₃O_7?δ thin film as a function of temperature, current density and the magnetic field up to 8 T. The current density dependence of the effective activation energy exhibits a slight increase in the low current density range below 10³ A/cm² and a logarithmic decline in higher current density with increasing current density. The magnetic field dependence of the effective activation energy showed a crossover of vortex state from a quasi-2D for H//ab-plane to a 3D line liquid state for H//c-axis. The possible dissipation mechanisms responsible for the ln H dependence of the effective activation energy were discussed.     

The reaction of carbon and water as catalyzed by a nickel surface

Many of the individual steps which make up the reaction of carbon and water to produce CO and H₂ were studied on a nickel foil surface using temperature-programmed reaction spectroscopy (TPRS), Auger electron spectroscopy (AES), and ultraviolet photoelectron spectroscopy (UPS). Surface graphite and carbide, two metastable surface carbon forms, were prepared by dehydrogeneration of C₂H₂ and served as reactant carbon. UPS of the graphite monolayer in contact with the metal yielded a valence electronic structure that could be interpreted in terms of the bulk band structure of graphite. The fully carbided Ni surface was active for H₂O dissociation with an estimated activation energy ≤ 5 kcal/mol. The reaction of graphitic carbon in contact with the nickel surface and adsorbed oxygen occurs directly without isolated prior breaking of carbon-carbon bonds. The estimated activation energy for the direct reaction was 44 kcal/mol. A different catalytic reaction cycle involving carbon-carbon bond breaking followed by oxidation of the carbide is energetically more demanding. The activation energy for direct carbon-carbon bond breaking was estimated to be between 65 and 70 kcal/mol. Following this demanding step, the reaction between carbidic carbon and oxygen proceeded with estimated activation energy of 31 kcal/mol.     

Calculation of motional activation energies for interstitial ions in the rutile structure using a minimum energy path model

Motional activation enthalpies have been calculated for interstitial cations in the rutile structure, using a minimum energy path approach. Coulombic, overlap repulsion and polarization energy terms were included in the potential energy expression. Preexponential factors in the overlap repulsion terms were evaluated by minimizing of the total electrostatic energy, and permanent dipole moments of the anion by an iterative method. The most favorable diffusion path was found to be parallel to the c-axis in the $\text{1}\text{2}$, 0, z direction in both TiO₂ and MgF₂.Motional activation enthalpies increased with increasing interstitial cation radius and were larger in TiO₂ than in MgF₂. The path was independent of radius and material. Strongly anisotropic conductivity was predicted for both compounds.     

Memory switching of germanium tellurium amorphous semiconductor

The dc conductivity and switching properties of amorphous GeTe thin film of thickness 262 nm are investigated in the temperature range 303-373 K. The activation energy ΔE_σ, the room temperature electrical conductivity σ_RT and the pre-exponential factor σ₀ were measured and validated for the tested sample. The conduction activation energy ΔE_σ is calculated. The I-V characteristic curves of the thin film samples showing a memory switching at the turnover point (TOP) from high resistance state (OFF state) to the negative differential resistance state (NDRS) (ON state). It is found that the mean values of the threshold electrical field E_th decreased exponentially with increasing temperatures in the investigated range. The switching activation energy ΔE_th is calculated. Measurements of the dissipated threshold power P_th and the threshold resistance R_th were carried out at TOP point at different temperatures of the samples. The activation energies ΔE_R and ΔE_P caused by resistance and power respectively are deduced. The results obtained support thermal model for initiating switching process in this system.     

Electrical and thermal properties of a-(Se70Te30)100−x(Se98Bi2)x (0⩽x⩽20) alloys

In electrical properties, the dc conductivity and photoconductivity measurements have been made in vacuum evaporated thin films of a-(Se₇₀Te₃₀)_100−x(Se₉₈Bi₂)_x system, in the temperature range (308–355 K). It has been observed that dc conductivity and activation energy depend on the Bi concentration. Photocurrent dependence on incident radiation has also been observed which follow the power law (I_ph∝F^γ). Transient photocurrent exhibits the non-exponential decay time. All these parameters show that the recombination within the localized states is predominant. In crystallization kinetics, the heating rate dependence of glass transition and crystallization temperatures is studied to calculate the activation energy for thermal relaxation and activation energy for crystallization. The composition dependence of the activation energy for thermal relaxation and activation energy for crystallization is discussed in terms of the structure of Se–Te–Bi glassy system.     

The effective activation energy Ueff(T,B,J) in Hg-1223 single phase superconductors

We review the methods of calculating the effective activation energy U_eff(T,B,J) for both transport measurements and magnetic decay, together with some theoretical models. Then, we apply these methods to our Hg-1223 single-phase superconductor to obtain the activation energy. Transport results give that the magnetic field and temperature dependence of the U_eff can be well described as U₀B^−α(1−T/T_c)^m. Magnetic relaxation shows that the current density dependence of U(J) can be scaled onto a single curve, which can be considered as the activation energy at some temperature T₀. The pinning mechanism in the measured temperature range does not change, and the activation energy depends separately on the three variables: T, B, and J, are responsible for the magnetic decay data scaling onto a single curve at various temperatures. As temperatures close to zero and near T_c, thermally assisted flux motion model is no longer valid since other processes dominate.     

Photoconductivity of Se85−xTe15Hgx thin films

The photoconductive properties such as dark conductivity, steady state and transient characteristics of a-Se_85−xTe₁₅Hg_x thin films, prepared by thermal vacuum evaporation technique have been studied in the temperature range 312–380 K. Analysis of data shows that the activation energy of dark current is greater as compared to the activation energy of photocurrent. The activation energy increases at higher concentration of Hg which shows that the defect density of states decreases. Analysis of intensity dependent photoconductivity shows that the bimolecular recombination is predominant. The transient photoconductivity shows that the carrier lifetime decreases with the increase in Hg concentration and increases at higher concentration of Hg. This decrease is due to the transition trapping process. Further the photosensitivity and carrier lifetime increases at higher concentration of Hg which also confirms that the density of defect states decreases.     

Differential scanning calorimetric study of Ga5Se95 glass

Results of differential scanning calorimetry, at different heating rates, α, on Ga₅Se₉₅ glass are reported and discussed. From the heating rates dependence of values of T_g and T_p, the glass activation energy, E_g and the crystallization activation energy, E_c, are derived. The crystallization results are interpreted in terms of recent analyses developed for non-isothermal crystallization and also for the evaluation of E_c. The crystallization mechanism is then characterized. From the obtained results, the glassy Ga₅Se₉₅ has two-dimensional growth, the average value of the order of crystallization mechanism, n is 3. The average value of the glass activation energy, E_g and crystallization activation energy, E_c, for Ga₅Se₉₅ glass are 189±4 and 69±5 kJ/mol, respectively.     

Nuclear magnetic resonance study of HfV2Hx and ZrV2Hx (0 ⩽ x ⩽ 4.2) hydrides

A proton NMR spin echo study of the system HfV₂H_x and ZrV₂H_x (0 ? x ? 4.5) hydrides are reported. The T₁ spin lattice relaxation rate enables us to extract the activation energy for diffusion, E_a, as well as the attempt frequency v₀; both depend strongly on the hydrogen concentration. The existence of a correlation between E_a and v₀ in these and other hydride systems yield the temperature dependence of the activation energy.     

Ion beam induced atomic transport in bilayer systems

Atomic transport in ion beam mixed Co/Pt and Pd/Au bilayer systems have been studied from the shifts of maker layers in Rutherford backscattering spectroscopy. Thin layers (1 nm) of marker (Pd for Co/Pt and Ni for Pd/Au) were embedded as markers at each interfaces. 80 keV Ar⁺ was used to irradiate the marker samples at the temperature range between 90 and 600 K. The Co/Pt system shows isotropic atomic transport (J_Co/J_Pt∼1.1) at low temperatures and anisotropic atomic transport (J_Co/J_Pt∼5.0) at high temperatures. Meanwhile, the Pd/Au system shows near isotropic atomic transport (J_Pd/J_Au∼1.2) at all temperatures examined. These results were discussed in terms of the activation energies for the normal impurity diffusion, cohesive energy difference, and the vacancy migration energy. Atomic transport in thermal spike regime is closely related with the activation energy for normal impurity diffusion. In radiation enhanced diffusion regime, the cohesive energy and/or the vacancy migration energy plays a dominant role for the atomic transport.     

Magnetic viscosity in Nd-Fe-B sintered magnets

Magnetic viscosity in Nd-Fe-B sintered magnets was measured at different fields between 4.2 and 300 K. At a given temperature, the viscosity S(H) and the irreversible susceptibility χ^irr are found to be proportional. The temperature dependence of the coefficient S_v = S/_χ^irr is deduced and an expression for the activation energy is derived. It is found that the intrinsic activation energy E₀ is proportional to the domain wall energy, whilst the activation volume is proportional to the cube of the domain wall width.     

Unified model of the temperature quenching of narrow-line and broad-band emissions

Temperature quenchings of narrow-line and broad-band emissions are pictured through the quantum-mechanical single-configurational-coordinate model. The model is taken in the thermal-Condon approximation with the overlap integrals evaluated through the Manneback recursion formulas. The multiple activation energies m?ω_υ to the initial vibrational states produce an increase in the non-radiative rate poorly described, in general, by a single activation energy. The model applies to energy parabolas with large Franck- Condon offset and a consequent crossover and to energy parabolas with small Franck- Condon offset and no relevant crossover. For large offset, the model gives approximately Mott's single-activation-energy rate A_M exp(-E_X/kT) for upward transitions but faster rates poorly described by a single activation energy for downward transitions. For small offset, the model gives approximately Kiel's multiphonon-emission rate A_K?^p[1+〈m〉_υ]^p for downward transitions. A numerical matrix method is described which can handle all cases and which explicitly exhibits the multiple activation energies m?ω_υ in every case. This method is used to work out examples of the various types of quenchings which can occur: a fast bottom crossover, an outside crossover, small-offset multiphonon emission, a tunneling crossover, and two-step quenchings through a higher offset state.     

Crystallization behavior of GeSe2-Ga2Se3-CsI glasses studied by Differential Thermal Analysis

GeSe₂-Ga₂Se₃-CsI chalcohalide glasses had been prepared by the melt-quenching technique. With the addition of CsI, the short wavelength cut-off edge of the glasses shifts to the short wavelength gradually, while the long wavelength cut-off edge located at ∼16 μm is nearly unchanged. Thermal properties were measured by Differential Thermal Analysis (DTA). From the heating rate dependence of crystallization temperature, the activation energy for crystallization (E) and the order parameter (n) were calculated by the Kissinger equation. The results show that the activation energy of crystallization decreases dramatically with increasing of CsI content, and the most probable crystallization mechanism is volume controlled one-dimensional growth.     

Fluorescence-quenching studies and temperature dependence of fluorescence quantum yield, decay time and intersystem crossing activation energy of TPB

Fluorescence quenching of 1, 1, 4, 4-tetraphenyl-1, 3-butadiene (TPB) by aniline has been carried out at room temperature (298 K) to understand the role of quenching mechanisms. The study has been carried out by both steady state (in different solvents) and by transient method (in cyclohexane). The Stern-Volmer plot has been found to be linear for all the solvents studied. The probability of quenching per encounter ‘p’ is determined in all the solvents and is found to be less than unity. It is found that, the activation energy E_a (E′_a) is greater than the activation energy of diffusion, E_d. The results obtained by the transient method infer that the thermally assisted intersystem crossing, a non-radiative deactivation process from S₁ to T₂ is responsible for observed decrease in quantum yield and lifetime. Hence, from both the methods it can be concluded that quenching mechanism is not solely due to the material diffusion, but there is also contribution from the activation energy.     

Thermal activation measurement of positron binding energies at surfaces

The formation of positronium by low-energy positrons incident on “clean” metal surfaces is thermally activated by increasing temperature. The activation energy E_a has been measured for a number of surfaces. E_a is understood as the energy required to form positronium (binding energy 1/2Ry) from positrons bound at the surface by an energy E_b, E_a = E_b+φ_-? 1/2Ry, where φ_- is the electron work function. Representative values of E_b derived are Al(100):3.03(5) eV; Al(110):2.92(4) eV; Cu(111):2.80(5) eV.     

Crystallization kinetics of Li2O-PbO-V2O5 glasses

Lead vanadate glasses of the system 5Li₂O−(45−x) PbO−(50+x) V₂O₅, with x=0, 5, 10, and 15 mol% have been prepared and studied by differential scanning calorimetry (DSC). The crystallization kinetics of the glasses were investigated under non-isothermal conditions applying the formal theory of transformations for heterogeneous nucleation to the experimental data obtained by DSC using continuous-heating techniques. In addition, from dependence of the glass-transition temperature (T_g) on the heating rate, the activation energy for the glass transition was derived. Similarly the activation energy of the crystallization process was determined and the crystallization mechanism was characterized. The results reveal the increase of the activation energy for glass transition which was attributed to the increase in the rigidity, the cross-link density and the packing density of these glasses. The phases into which the glass crystallizes have been identified by X-ray diffraction. Diffractograms of the transformed material indicate the presence of microcrystallites of Li_0.30V₂O₅, Li_0.67O₅V₂, LiV₆O₁₅, Li₄O₄Pb, and O₇Pb₂V₂ in a remaining amorphous matrix.     

Change of conduction mechanism with gamma dose of irradiated borosilicate glasses containing copper ions

The d.c. electrical resistivity of sodium borosilicate glasses containing copper ions has been measured at the temperature range 300–623 K before and after gamma irradiation. The results obtained indicate that conduction may proceed ionically for the unirradiated glasses, with an activation energy in the range 23–24.2 Kcal/mol. Relatively low gamma doses (3.6–25 Mrads) induce two regions, separated at a critical temperature (T_c), of different activation energies. An activation energy E₁, 3.4 Kcal/mol, in the temperature range from 300–370 K, and E₂ 16.8–20.9 Kcal/mol, in the range of higher temperatures. The mechanism suggested is that the ionic conduction is replaced by an electronic one up to about 370 K then seems to be replaced at higher temperature region by ionic-like mechanism which can be expressed by exciton-like process. High gamma doses (28.33 M rads) cause the glasses to have activation energy ranges between 19.9–22.7 Kcal/mol, which may substantiate the predominance of the proposed ionic-like mechanism.The transition region around T_c seems to be divided into three regions, precritical, critical and post-critical.     

Optical properties of electron irradiation induced defects in InP

The optical properties of deep hole traps H₄ and H₅ in p type and of the deep electron trap E₁₁ in n type InP, introduced by electron irradiation, have been studied using deep level optical spectroscopy. Comparison of the optical threshold with the thermal activation energy of H₅ level shows that it is highly relaxed with a Frank-Condon shift d_Fc = 0.45 eV. The electron level E₁₁ is weakly relaxed and its optical cross section σ ⁰ is well accounted for by transitions to the Γ_6c minimum. The optical absorption σ_p⁰ associated to level H₄ shows two successive onsets at hν = 0.5 and hν = 1.2 eV which can be attributed to hole transitions to the Γ_7–8 and to the L_4–5 valence band extrema, respectively. The deduced Frank-Condon shift, d_Fc = 0.23 eV, agrees with the measured difference of 40 meV between its apparent activation energy E_a and its thermal activation energy E_T.     

Impurity centers in ZnSe:Na crystals

The influence of sodium impurity on photoluminescence (PL) spectra of ZnSe crystals doped in a growth process from a Se+Na melt is investigated. It is shown that the introduction of the impurity results in emergence of emission bands in the PL spectra due to the recombination of exciton impurity complexes associated with both donors and hydrogen-like acceptors. Apart from that, four bands generated by donor-acceptor pairs recombination and a band produced by electronic transitions from the conduction band to a shallow acceptor are discussed. As a result of the analysis it is concluded that Na impurity forms in ZnSe lattice Na_Zn hydrogen-like acceptors with activation energy of 105±3 meV, Na_i donor centers with activation energy of 18±3 meV, as well as Na_ZnV_Se and Na_iNa_Zn associative donors with activation energy of 35±3 and 52±9 meV, respectively.