Nominal Barrier Research Articles

Fusion Dynamics of 27Al + 74Ge Nuclear Reaction

The experimental data for the nuclear fusion of 27Al with 74Ge are examined within the context of symmetric-asymmetric Gaussian barrier distribution (SAGBD) model in sub-barrier domain. For bombarding energies smaller than the Coulomb barrier, predictions owing to simple Wong formula are found to be much smaller with reference to experimental outcomes. On other side, the SAGBD method is used, wherein impacts of inherent channels as a result of its multidimensional aspect of quantum mechanical tunneling arises because of weighted Gaussian function, to get desired outcomes. Larger fusion enhancement appears at lower energy region, which signifies the effective participation of inherent channels originated due to structure of participants. In present approach, the impacts of various inherent channels are cumulatively determined by calculating values of channel couplings parameter (λ) & VCBRED . Present calculations due to the SAGBD method suggests that the theoretical fusion excitation functions for 27Al + 74Ge reaction are in close resemblance with experimental findings around the nominal barrier.

Fusion of Stable Odd-A isotope targets with Doubly Magic 16O Projectile

Fusion excitation functions of stable odd-A targets 147,149Sm with 16O projectile are theoretically analyzed within view of the symmetric-asymmetric Gaussian barrier distribution (SAGBD) formalism. For the purposes of this study, we assumed that bombardment energies of the 16O + 147,149Sm reactions would be around nominal barrier. For these reactions, Wong based computations fails to retrace the fusion data whereas SAGBD predictions fairly retrace the fusion data in entire bombarding energy range. The evaluated values of channel coupling parameter (λ) and VCBRED from SAGBD outcomes are found larger for heavier (16O + 149Sm) over lighter (16O + 149Sm) system, which suggests that heavier system possess extra fusion enhancement in sub-barrier domain. Present theoretical investigation highlights the significant contributions of intrinsic channels that emerges due to structure of reacting nuclei and such effects are empirically included in SAGBD method.

Comprehensive analysis of heavy-ion fusion reactions around the Coulomb barrier

We have investigated the fusion dynamics of 12C + 92Zr, 16O + 92Zr, 28Si + 92Zr, 35Cl + 92Zr, 16O + 60Ni, 18O + 58Ni, 12C + 194,198Pt, 16O + 144,148,154Sm and 17O + 144Sm reactions in the close vicinity of the Coulomb barrier. For this purpose, we have opted one-dimensional Wong formula, coupled channel approach, and symmetric-asymmetric Gaussian barrier distribution (SAGBD) model to analyze the fusion data for above mentioned reactions. For all studied systems, the theoretical results obtained from the one-dimensional Wong formula are unable to reproduce the fusion data especially at incident energies lying below the nominal barrier. This appeals for the inclusion of intrinsic degrees of freedom which have been originated from the nuclear structure of fusing nuclei, and thus essentially required to explain the experimental data at energies lying in the sub-barrier realm. For the chosen reactions, the coupled channel analysis suggests that the coupling of relative motion with the inelastic surface excitations and/or nucleon transfer channels associated with the reaction partners have produced a remarkable fusion enhancement at energies lying near and below the nominal barrier. It is particularly intriguing that the SAGBD model can replicate the impacts of dominant intrinsic reaction channels such as multi-phonon quantum states and/or nucleon transfer channels via Gaussian type of weight function. The predictions of the SAGBD approach appropriately reproduce the fusion cross-sections data and related barrier distributions data for the selected reactions due the fact that the multi-dimensional nature induces the barrier lowering effects and subsequently reduces the effective fusion barrier between the participants. In the SAGBD approach, the channel coupling effects are quantitively measured in terms of channel coupling parameter and percentage decrement in height of effective fusion barrier with respect to the Coulomb barrier Furthermore, the recovered -values for selected reactions are found to be smaller and hence clearly demonstrate the applicability of the model outcomes to explore the fusion dynamics of the studied systems.

Influence of the MgO barrier thickness on the lifetime characteristics of magnetic tunnelling junctions for sensors

Magnetic tunnelling junctions increasingly enter the market for magnetic sensor applications. Thus, technological parameters such as the lifetime characteristics become more and more important. Here, an analysis of the lifetime characteristics of magnetic tunnelling junctions using the Weibull statistical distribution for CoFeB/MgO/CoFeB junctions is presented. The Weibull distribution is governed by two parameters, the characteristic lifetime η of the population and the shape parameter β, which gives information about the presence of an infant mortality. The suitability of the Weibull distribution is demonstrated for the description of dielectric breakdown processes in MgO-based tunnelling junctions at different voltages. A study of the dependence of the characteristic lifetime extrapolated to the low voltage regime, and the β parameter on the nominal barrier thickness and the resistance × area product of the MgO barrier is shown. The influence of the RF deposition power for the MgO barrier and an annealing step on the Weibull parameters is also discussed.

Interaction spatial barrier distributions for the proton-halo system 8B+58Ni

Radial barrier distributions for the proton-halo system 8B+58Ni are obtained by the use of Woods-Saxon polarization potentials calculated from a fit to elastic scattering angular distribution data. The strength, diffuseness and reduced radii of the polarization potentials, besides fitting the data, are such that reproduce the barrier height and position as extracted from a Wong's fit to the universal behavior of reduced interaction cross sections for halo systems. It is found that, for the proton-halo system 8B+58Ni, the interaction barrier height VI and the interaction barrier radius RI result correspondingly lower and larger than the Coulomb nominal barrier height VB and barrier position RB as expected from Barrier Penetration Models and double folding density dependent interactions such as the São Paulo Potential. It is also shown that, the determined energy dependent polarization potentials, besides satisfying the dispersion relation, present a behavior consistent with the Breakup Threshold Anomaly. Keywords. Nuclear Reactions, weakly bound nuclei, threshold anomaly.

Electronic structures at organic heterojunctions of N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamin (NPB)-based organic light emitting diodes

Interface electronic structures of four-kinds of electron transporting or hole blocking organic materials (n-type) on a widely-used hole transporting material (p-type) in organic light emitting diodes (OLEDs), N,N′-bis (1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamin (NPB), were investigated by means of photoelectron spectroscopy (PES). 1,3-bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene (OXD-7) and 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) overlayers show continuous energy shift of each overlayer-derived spectral components and the vacuum level proportional to the thickness. This energy shift is ascribed to a spontaneous building up of the electrostatic potential within the organic layers (giant surface potential; GSP). The energy shift of the overlayers induced by GSP as well as the interface vacuum level shift are adequately taken into account to determine the actual energy barrier heights of the hole conduction levels at the heterojunctions. 4,4′-bis(9-carbazolyl)biphenyl (CBP) and p-bis(triphenylsilyl) benzene (UGH2) induce band bending in the NPB film which presumably results from charge transfer (CT) to the n-type materials from NPB. Despite absence of a practical vacuum level shift and thickness dependent shift of the overlayer-derived electronic states, the CT-derived energy shift of NPB reduces the actual energy barrier height with respect to the nominal barrier height being simply interpreted from PES spectra of a thick overlayer of each material. The energy level diagrams across these ‘n-on-p’ organic–organic heterojunctions were finely determined based on the above interpretation of the PES spectra.

Different behavior in transport measurements on planar-type YBa2Cu3O7−δ/La1/3Ca2/3MnO3/La2/3Ca1/3MnO3 heterostructures

YBa2Cu3O7−δ(YBCO)/La1/3Ca2/3MnO3(LCMO-AF)/La2/3Ca1/3MnO3(LCMO-F) junctions with nominal layer thicknesses of 100, 7, and 80nm, respectively, were fabricated using a dc sputtering technique. The junctions were structured to transistor dimensions by standard UV-photolithography and chemical etching process. Electrical measurements were carried out both on the superconducting micro-bridges and the antiferromagnetic (AF) barriers. The AF barriers displayed different electrical behavior for the same nominal barrier thickness. Thus, for junctions with a barrier resistance of around 60Ω (V=0), a clear superconducting gap-like structure was observed. On the contrary, junctions with a barrier resistance of around 30Ω (V=0) featured a pronounced, temperature dependent zero-bias conductance peak G(V=0). The value of G(V=0) decreased as the temperature was increased, which was an indication of metallic-like behavior associated to the presence of pin-holes in the junction area. On the other hand, spin-polarized carrier injection from the ferromagnetic layer into the strip-like YBa2Cu3O7−δ film provoked a strong reduction of IC with an efficiency K(K=ΔIC/ΔIg) as large as 1.4 at 15K.

An Experimental and Theoretical Investigation of the Photophysics of 1-Hydroxy-2-naphthoic Acid

Photophysical and photochemical properties of 1-hydroxy-2-naphthoic acid (1,2-HNA) have been investigated experimentally by steady state and time domain fluorescence measurements and theoretically by Hartree-Fock (HF), configuration interaction at the single excitation (CIS) level, density functional theoretic (DFT), and semiempirical (AM1) methods. 1,2-HNA exhibits normal fluorescence that depends on its concentration, nature of the solvent, pH, temperature, and wavelength of excitation. It seems to form different emitting species in different media, akin to 3-hydroxy-2-naphthoic acid (3,2-HNA). The large Stokes shifted emission observed at pH 13 is attributed to species undergoing excited-state intramolecular proton transfer. Nonradiative transition seems to increase on protonation and decrease on deprotonation. AM1(PECI=8) calculations predict the absorption maximum (lambda(max) = 335.9 nm) in reasonable agreement with experiment (lambda(max) = 352 nm) for the neutral 1,2-HNA. They also predict a red shift in absorption on protonation and a blue shift on deprotonation as observed experimentally. CIS calculations tend to overestimate the energy gap and hence underestimate the absorption maxima between the ground and the excited electronic states of 1,2-HNA and its protonated and deprotonated forms. However, they do predict correctly that the excited state intramolecular proton transfer is likely to occur in the deprotonated form of 1,2-HNA and not in the neutral and the protonated forms. A single minimum is found in the potential energy profile for the ground state as well as the first excited state of 1,2-HNA and its protonated species. In contrast, a double minimum with a nominal barrier in between is predicted for the ground state and also the first three excited states of the deprotonated species. The keto form of the deprotonated species is found to be slightly less stable than the enol form in all the states investigated.

Direct versus resonances mediated F+OH collisions on a new 3A" potential energy surface.

A theoretical study of the F(2P) + OH(2Pi) --> HF(1Sigma+) + O(3P) reactive collisions is carried out on a new global potential energy surface (PES) of the ground 3A" adiabatic electronic state. The ab initio calculations are based on multireference configuration interaction calculations, using the aug-cc-pVTZ extended basis sets of Dunning et al. A functional representation of the PES shows no nominal barrier to reaction, contrary to previous results by others. Wave packet and quasiclassical trajectory calculations have been performed for this PES to study the F + OH(v = 0,j) reactive collision. The comparison was performed at fixed and constant values of the total angular momentum from 0 to 110 and relative translational energy up to 0.8 eV. The reaction presents a dynamical barrier, essentially due to the zero-point energy for the bending vibration near the saddle point. This determines two different reaction mechanisms. At energies higher than approximately 0.125 eV the reaction is direct, while below that value it is indirect and mediated by heavy-light-heavy resonances. Such resonances, also found in the simulations of the photodetachment spectrum of the triatomic anion, manifest themselves in the quasiclassical simulations, too, where they are associated to periodic orbits.

Transport properties of ultrathin SrTiO3 barriers for high-temperature superconductor electronics applications

Current transport through ultrathin SrTiO3 (STO) barriers has been studied systematically with respect to its dependence on barrier thickness d=2–30 nm, temperature T, and voltage V in state-of-the-art planar YBa2Cu3O7−x/SrTiO3/Au (YBCO/STO/Au) heterojunctions with c-axis oriented YBCO layer. We identified different transport regimes: Elastic tunneling was observed for samples with a nominal barrier thickness of 2 nm, which represents our experimental minimum for obtaining insulating transport characteristics. Already for slightly thicker STO barriers, resonant tunneling and hopping via a small number of localized states begins to dominate the transport behavior. For d>20 nm, a crossover to variable range hopping behavior is observed in the high-bias voltage regime as well as in the high temperature regime. A localization length of ∼0.46 nm indicating the spread of the localized states can be derived from these experiments. This value is close to the STO lattice constant and corresponds to a high density of localized states of nL∼6×1019(eV)−1 cm−3. In a free electron tunneling model, this corresponds to an average tunnel barrier height of ∼0.4 eV.

Reaction Paths in the Hydrogenolysis of Acetic Acid to Ethanol over Pd(111), Re(0001), and PdRe Alloys

Nonlocal density functional theory (DFT) calculations are used to examine alternative mechanisms for the hydrogenolysis of acetic acid to ethanol over Pd. The overall surface reaction energies, at low surface coverage, are computed for a number of different possible paths by which acetic acid may be converted to ethanol over Pd(111). Binding energies of the various oxygenated C2 intermediates formed along these paths are also reported. The overall reaction energies were used to propose a plausible mechanism for acetic acid hydrogenolysis. In the postulated mechanism, acetic acid dissociates to form an acetyl surface intermediate. The acetyl intermediate is then subsequently hydrogenated to ethanol via the formation of an acetaldehyde surface intermediate. Detailed reaction coordinate calculations were used to isolate the transition states and calculate activation barriers for acetic acid dissociation to acetyl (ΔEact=+142 kJ/mol) and acetyl hydrogenation to acetaldehyde (ΔEact=+66 kJ/mol) over Pd(111). Experimental observations and DFT calculations suggest that these two steps are likely to be rate determining in acetic acid hydrogenolysis. The barriers and overall reaction energies for these same steps are also computed on Re(0001) and pseudomorphic overlayers of Pd on Re (PdML/Re(0001)) as well. The results suggest that the C–OH bond-dissociation reaction is more favored over Re(0001) since it has a more open d band. However, bond-association reactions such as acetyl hydrogenation are more favored over PdML/Re(0001), which has an electronic d-band structure similar to that of a noble metal. The optimal balance may require a Pd/Re alloy. Calculations performed over a Pd0.66Re0.33 alloy demonstrate a nominal barrier for both C–OH bond breaking and C–H bond formation. This may be ideal for acetic acid hydrogenolysis to ethanol. Rhenium ensembles, however, should be avoided as they lead to acetic acid decarboxylation.

Signatures of multiphonon and neutron transfer couplings in the fusion of 36,32S+110Pd.

Fusion-evaporation cross sections have been measured for {sup 32,36}S+{sup 110}Pd to high accuracy in the energy range around the Coulomb barrier. The two extracted fusion barrier distributions display very similar structures with two well-resolved peaks below the nominal barrier, which are consistent with a model calculation including the coupling of the one-, two-, and three-phonon collective vibrations of {sup 110}Pd. A third lower-energy barrier, together with a strong relative cross section enhancement, is observed for {sup 32}S+{sup 110}Pd, and is reproduced by coupling the two-neutron pickup channel which has a large and positive ground state {ital Q} value (+5.1 MeV). For {sup 36}S+{sup 110}Pd, the calculated effect of the corresponding transfer channel (with negative {ital Q} value) is much smaller.

On vibrationally-assisted dissociation of H 2 at metal surfaces

Abstract A model potential energy surface describing strongly activated dissociation of a molecule at a metal surface is used to demonstrate that the vibrational-translational energy transfer in the entrance channel can have a very pronounced effect on dissociation probabilities. A continuous feeding of energy from the vibrational to translational coordinates allows the molecule to “bootstrap” its way up the potential hill in the entrance channel and dissociate even when its incident kinetic energy is much smaller than the nominal barrier height. This effect is discussed in connection with the dissociation of H 2 on copper surfaces.

On vibrationally-assisted dissociation of H 2 at metal surfaces

A model potential energy surface describing strongly activated dissociation of a molecule at a metal surface is used to demonstrate that the vibrational-translational energy transfer in the entrance channel can have a very pronounced effect on dissociation probabilities. A continuous feeding of energy from the vibrational to translational coordinates allows the molecule to “bootstrap” its way up the potential hill in the entrance channel and dissociate even when its incident kinetic energy is much smaller than the nominal barrier height. This effect is discussed in connection with the dissociation of H 2 on copper surfaces.

Influence of Dynamic Processes upon Complete Fusion of Heavy Nuclei at Subbarrier Energies

Excitation functions for complete fusion of $^{90}\mathrm{Zr}$+$^{90}\mathrm{Zr}$ and $^{90}\mathrm{Zr}$+$^{94}\mathrm{Zr}$ have been found to decrease gradually over a broad energy range below the nominal barrier. The effective barrier for $^{90}\mathrm{Zr}$+$^{94}\mathrm{Zr}$ was 10.5 \ifmmode\pm\else\textpm\fi{} 3.0 MeV higher than the nominal barrier implying a critical effective fissility of 31.5 \ifmmode\pm\else\textpm\fi{} 0.5. Although deep-interpenetration processes appear to limit fusion at near-barrier energies, weak-contact dynamics enable these cold, heavy nuclei to fuse with comparative ease at lower energies.